MATERIALS MODIFICATIONS AFTER FORMING

Transcription

1 Vysoká škola báňská Technical University of Ostrava MATERIALS MODIFICATIONS AFTER FORMING Lecture notes doc. Ing. Richard Fabík, Ph. D. Ostrava 2013

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3 Materials modifications after forming PREREQUISITES The subject has no prerequisities AIM OF SUBJECT After study of these notes, students should be able to: describe properties of scales, their generation mechanisms and possibilities of descaling explain processes occurring in steels during heat treatment characterize individual products of austenite transformation, describe and characterize heat treatment technologies, explain theoretical basis of hot-dip galvanizing and other metal coating processes, describe and characterize technologies of steel metal coating

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5 1. HEAT TREATMENT 240 minutes After study of this chapter students will be acquired with basic characterization of heat treatment, revise basic information about structure and behavior of steels, be acquired with mechanism of overcooled austenite transformation, understand transformation diagrams and know factors influencing locations of curves of phase transformations starting and finishing, revise products of non-overcooled austenite transformation,be acquired with individual types of annealing and tempering and their physical bases 1.1. Basics of heat treatment According to ČSN , heat treatment is a process during which a solid state body or its part is subjected to one or more heat cycles in order to achieve required material properties. Each heat treatment process consists of three parts: heating to temperature, dwell at temperature and cooling. Heat treatment processes are usually depicted in temperature time coordinates. The main factor influencing properties of steels during heat treatment is temperature. Structural transformations occurring during heating and cooling of steels during heat treatment basically proceed according to Fe - Fe 3 C diagram. Heat treatment processes can be characterized according to process temperature and aim, see table 2.1. Table 2.1 Characterization of heat treatment processes (HT) Annealing Quenching Tempering Surface quenching with no structural transformation with no structural transformation combined direct discontinuous thermal isothermal, isothermal refining soaking, refining (quenching + tempering) flame, induction to reduce stress recrystallization soft counter-flake to eliminate brittleness after pickling normalization homogenization isothermal

6 1.2. Structures of steel during rapid cooling Martensitic transformation Martensitic (MA) transformation occurs during significant overcooling of austenite to low temperatures, at which diffusion of substitution elements and even carbon is no longer possible. The process has a character similar to plastic deformation and is thus denoted as shear transformation. The product of this diffusionless transformation occurring within the temperature range between M s and M f is martensite, a disequilibrium metastable solid solution of carbon in iron. MA is a typical structure component of quenched steels crystallizing in tetragonal body centered lattice (Fig. 2.2.). Lattice parameters and its tetragonality ( c a ratio) increase in dependence on carbon content. For concentrations lower than 0.2% C, a majority of carbon atoms segregates on dislocations or structure boundaries of MA needles, the tetragonality of which is hardly measurable. For higher contents, carbon atoms are located in interstitial octahedral locations of the body centered lattice. Fig. 2.2 Relation of martensitic and austenitic lattices Fig Schematics of martensitic transformation; a,b) homogenous deformation, c) inhomogeneous slip deformation, d) inhomogeneous deformation by twinning, e) deformation of free sample surface after formation of martensitic plate (needle) The mechanism of MA generation lies in homogenous deformation, which results in formation of a transformed volume (crystal) of MA (Fig a) b)). Resistance offered by the surroundings of the transformed volume results in generation of stress leading to second inhomogeneous deformation occurring only in MA lattice. These phenomena are connected to development of surface relief (Fig. 2.3 e)). The inhomogeneous (additional) deformation is executed by slip (Fig. 2.3 c) or twinning (Fig. 2.3 d)) and forms internal structure (substructure) of MA, according to which two MA types can be characterized. Dislocation MA with needle(lath)-type morphology is with no internal twins and consists of a dense dislocation mesh (strong and more ductile). Twin(plate)-type MA consists of plates divided by internal twins (stronger and less ductile)

7 Factors primarily influencing generation of MA are not totally clear. Needle-type MA forms in low-carbon steels and steels rich in substitution alloying elements, while plate-type MA forms in carbon and alloyed steels with medium and high carbon contents (Fig. 2.4.). Considering martensite substructure, a contribution to strengthening caused by high lattice defect density has to be pointed out. According to current knowledge, MA transformation proceeds by nucleation and growth. One of the most important ideas of MA nuclei nucleation was formulated by Cohen et al. They Úpravy materiálu po tváření Fig. 2.4 Influence of carbon on M s temperature and on needle and plate martensite types ratio supposed that a suitable arrangement of dislocation results in generation of regions, the energy of which approaches the energy of MA. Interstitials, the presence of which restricts dislocation movement, accumulate in the vicinities of dislocation interfaces. By this, the existence of a large energetic barrier between austenite and MA can be explained. MA growth is executed by movement of semi-coherent austenite-martensite interfaces, which is energetically demanding and can occur only at a sufficient overcooling. Semi-coherent interface is however well movable and MA growth rate can reach great values (speed of sound). As has been shown, length of plates is limited by obstacles, which offer a significant resistance to dislocations (grain boundaries, boundaries of formed martensitic plates, precipitates etc.). The overall MA formation rate thus depends primarily on nucleation rate, which is significantly dependent on transformation temperature. After a rapid cooling to a temperature between M s and M f, austenite rapidly transforms to martensite. During further temperature decrease growth of MA structures does not proceed, the transformation continues via formation of new MA structures and the range of transformation is thus actually only a function of temperature (thermic behavior). MA content increases with decreasing temperature but it never increases to 100%, since a certain content of residual austenite is always maintained in the structure (Fig. 2.5.). During a dwell within the temperature range between M s and M f MA transformation ceases almost immediately. After the dwell is finished, it does not continue immediately with further cooling, but after a certain temperature decrease (austenite stabilization). By this the remaining portion of transformation curve shifts to lower temperatures and thus to higher portions of remaining austenite. The cause for austenite stabilization can be either relaxation of stress in crystal lattice, or blocking of dislocations by interstitials at austenite-martensite interfaces. In the temperature interval between pearlitic and MA transformations, i.e. at medium overcooling, bainitic (BA) transformation of overcooled austenite occurs. The temperature region of BA transformation overlaps with pearlitic area for carbon steels, while both the regions are divided by a temperature region of stable austenite for steels alloyed with carbide-formative elements

8 Needle-type martensite in 50Cr1 steel; 680 x magnification Thin foil of quenched HS C high-speed steel, needle-type martensite with cementite inside needles and residual austenite, x magnification Thin foil of X37CrMoV5-1 anticorrosion steel for elevatedtemperature applications, tempered needle-type martensite with dispersion M 4 C 3 and M 4 C carbide precipitates, x magnification Thin foil of HS high-speed steel after quenching from 1240 C and tempering at 540 C, tempered martensite, x magnification Plate-type martensite in FeNi33 alloy, 200 x magnification Fig Martensitic microstructures Thin foil of FeNi33 alloy, platetype martensite with twins, x magnification Bainitic transformation Transformation of overcooled austenite at temperatures 550 C to M s results in formation of nonlamellar ferrite-carbide mixture denoted as bainite (BA). The basics of BA transformation are the transformation of face centered lattice of iron to body centered lattice of iron, change in carbon

9 distribution and formation of carbide phase. From conditions of BA transformation for steels (interstitial diffusion is possible, diffusion of Fe and substitution elements is negligible) ensues that no distribution of substitute elements between solid solution and carbides occurs during transformation (no alloyed cementite or special carbides generate). The character of BA transformation is special; some factors are similar to martensitic (shear) transformation, while some are similar to pearlitic (diffusion) transformation. Among the characteristics similar to martensitic transformation are e.g. generation of surface relief, which accompanies growth of the basic phase, i.e. BA ferrite (more or less oversaturated solid solution of carbon in iron), crystallographic correspondence of BA ferrite lattice and original austenite, and morphology of BA ferrite (plate-like, needle(lath)-like). The range of isothermal BA transformation depends on transformation temperature (BA curve); BA transformation can stop when a certain amount of non-transformed austenite is still present. On the other hand, generation of BA is similar to pearlitic transformation primarily in small BA growth rate (approx. 10 orders lower than martensitic plate growth rate) and existence of incubation period between the isothermal BA transformation starts. Due to complexity and variability of BA transformation, any unambiguous idea about BA formation mechanism has not been formed so far. It is usually supposed that BA ferrite generates from austenite by shear mechanism (similarly as martensite), but its growth is driven primarily by the rate of transport of carbon atoms from moving BA ferrite-austenite interphase interface, i.e. by carbon diffusion Isothermally generated BA BA structure changes significantly with transformation temperature and chemical composition of austenite. Two basic types of bainite can usually be categorized - upper and lower BA. For steels with carbon contents of 0.6 %, upper BA generates at temperatures higher than 350 C and lower BA forms in the temperature region of 350 to M s. Nucleation of upper and lower BA occurs primarily at austenite grain boundaries and on already existing BA plate (needle) structures. Nuclei of BA ferrite grow in the form of needles (laths) from austenite grain boundaries along particular crystallographic austenitic planes. Growth of BA ferrite results in enrichment of surrounding austenite areas with carbon and cementite particles precipitate at the - interphase interface in a certain distance from the needle front. The structure of upper BA (Fig. 2.6.) consists of beams of coarser needles (plates) of BA ferrite with longitudinally arranged cementite particles, which are segregated primarily on needle surfaces; inclination of austenite/ferrite planes is close to {111} /{110}. Lower BA (Fig. 2.7.) forming at lower temperatures consists of thin plates (three-dimensional scheme of plates and needles shapes is in Fig. 2.8.) of BA ferrite, which also grow along certain preferential austenite planes (close to {111} ), primarily from grain boundaries. BA ferrite is more oversaturated with carbon and precipitation of fine carbide particles occurs primarily inside plates. In the primary stage of transformation, the carbidic phase is carbide, which is later replaced with cementite. The structure of lower BA is formed by beams of thin BA ferrite plates containing large amounts of fine carbides precipitated along certain planes. During transformation, the so far untransformed austenite is enriched with carbon. The range of changes in carbon concentration depends, among temperature, also on contents and type of alloying elements. The largest enrichment has been found for hypo-eutectoid steels with Si, Al, V; lower influences have Cr, Ni and Mn

10 Fig Upper BA (Ni-Cr-Mo steel) 1 000x Fig Lower BA (Ni-Cr-Mo steel) 1 000x Fig D view of BA ferrite plates and needles shapes. Mechanical properties of BA structures are influenced primarily by BA plates (needles) sizes, carbide particles dispersion, dislocation density in BA ferrite and interstitial C atoms dissolved in BA ferrite. Decrease in temperature causes decrease in thickness of BA ferrite particles, increase in dislocation density and C concentration in solid solution (see Fig. 2.9) and refinement of carbide dispersion, increase in strength and hardness of BA. Hardness of BA is not as high as for martensite, its ductility is however significantly higher. When compared to lamellar structures of pearlitic type, strength properties of BA are higher [20]. Fig Quantitative description of trends of microstructural factors as functions of transformation temperature

11 Continuously cooled BA Úpravy materiálu po tváření It is difficult to practically distinguish lower and upper BA, since practical cooling does not proceed isothermally and thus does not proceed according to IRA diagrams. Cooling is usually performed with a constant cooling rate and austenite transformations proceed according to ARA diagrams. Continuously cooled BA is significantly more complex and also other BA forms, such as globular (granular) BA (Fig. 2.9.), exist. Due to a large amount of BA variations, a new classification has been introduced. According to contemporary knowledge, BA morphology within a wide range of carbon contents (0.05 % to eutectoid Fig Globular BA, 1 000x. content) can be categorized into three groups [18]: B 1 (Fig a) BA with needle-type ferrite and precipitation of carbides inside ferritic plates (lower BA). B 2 BA with needle-type ferrite with precipitated carbides (Fig b) or a martensite residual austenite film at ferritic plates interfaces (carbideless BA, also upper BA belongs here, Fig c). These steels exhibit good wear resistances, comparable to or better than pearlitic steels with identical hardnesses, at high contact pressures. B 2 structure is typical for low-carbon BA (up to 0.2% C). For higher carbon contents, a presence of Si (above 0.5%) to reduce carbide formation and improve plastic properties is necessary to achieve higher strengths. B 3 (Fig d) BA with needle-type ferrite and discretely precipitated martensitic islands with residual austenite (granular BA). Fig Schematics depicting various Ba morphologies (a) B 1 BA (lower BA) with carbides inside ferritic plates, (b) B 2 BA (upper BA) with carbides at ferritic plates boundaries, (c) B 2 (carbideless) BA with residual austenite at ferritic plates boundaries, (d) B 3 (globular) BA with discrete islands of residual austenite and/or martensite at ferritic plates boundaries

12 Since microstructures of the first two types of BA are very fine, their characterization only using optical microscope is very difficult and electron microscope needs to be used. For example, Fig represents B1 BA, Fig B2 BA and Fig B3 BA. Electron microscope also needs to be used to differentiate B1 BA from tempered lath-type martensite. The morphology of lath-type martensite is shown in Figs and Contrary to lower BA, carbides occur in three different planes of crystal lattice (in Figures shown as three different angles). Fig B 1 BA, thin foil. Fig B 2 BA, thin foil. Fig B 3 BA, thin foil. Fig Tempered lath-type martensite, thin foil. Fig Schematics depicting carbides in three planes within a tempered martensite lattice

13 1.3. Annealing Annealing is a process of heat treatment, during which products are heated to a certain temperature followed by a dwell and subsequent (usually retarded) cooling. The aim is to achieve a certain equilibrium state (stable structure). Aims of annealing can be various and depend not only on steel types, but also on previous production operations. They can be e.g. elimination of structure inhomogeneity and its refinement, equalization of differences in chemical composition (homogenization), achievement of low hardness, decrease in residual stress, recovery of deformed grains etc. Annealing is often used not as the final heat treatment operation, but as an intermediate operation, i.e. operation included within a technological process in order to prepare structure (properties) for subsequent operations. Some annealing procedures require transformation of the original ferritic-pearlitic structure to austenite, while some procedures are performed without any phase transformation (temperature is above A c1 ) (Fig ). We thus distinguish: Annealing with no structural transformation Annealing with structural transformation Fig Annealing temperatures regions in Fe-Fe 3 C equilibrium diagram Since the treatment usually involves slow cooling, possibly low overcooling, proeutectoid and pearlitic transformations have significant influences. For low austenite overcoolings, the transformation starts with ferrite precipitation in hypo-eutectoid steels, while in hyper-eutectoid steels it starts with

14 precipitation of secondary cementite. Common features of formation of both the proeutectoid phases are generally applicable rules of nucleation and growth and morphology of both the phases. From morphological viewpoint, there are three basic types of proeutectoid phases: allotriomorphic particles, Widmanstätten plates and idiomorphic particles. Formation of various types of proeutectoid phases depends primarily on carbon content and presence of other elements in steel, austenitic grain size, transformation temperature and steel purity. Some of the formed proeutectoid structures significantly increase brittleness and thus their occurrence needs to be prevented. The difference between allotriomorphic and idiomorphic ferrite is depicted in Fig Allotriomorphic particles nucleate on boundaries of austenite grains, grow primarily along them and thus they progressively form a more or less continuous network. An example of allotriomorphic ferrite network at boundaries of pearlitic colonies is shown in Fig Fig shows a microstructure of steel with idiomorphic ferrite. Fig : Difference between allotriomorphic and idiomorphic ferrite Fig : Allotriomorphic ferrite (white locations) and pearlite (dark locations) Fig : Idiomorphic ferrite in material partially transformed to, remaining transformed to martensite after quenching Widmanstätten particles of proeutectoid phases generate at a great oversaturation of a solid solution with relevant component. Widmanstätten ferrite plates (Fig ) most often occur in steels with low carbon contents and with coarse austenite grains, at certain favorable overcoolings. Precipitation of Widmanstätten cementite plates is similar, although it is less common. Nucleation of Widmanstätten particles is always executed heterogeneously, on austenite grain boundaries (primary plates Fig ) and on previously formed allotriomorphic particles (secondary plates Fig ). Formation of a Widmanstätten plate features formation of a relief (likewise to martensite and BA), which imparts the conclusion that slip mechanism also contributes to its growth. Fig Difference between primary and secondary Widmanstätten ferrite Fig Microstructure of primary Widmanstätten ferrite, martensite matrix Fig Microstructure of secondary Widmanstätten ferrite and pearlite

15 Annealing with no structural transformation During this annealing, the temperature usually does not exceed Ac1 temperature. The following primarily occurs in steels changes of dissolubility of carbon and nitrogen, spheroidization and coagulation of carbide particles, dissociation of non-equilibrium phases and possible softening processes development (recovery and recrystallization). Annealing at temperatures below Ac1 actually only impart structural changes in steels (spheroidization, coagulation of carbide particles, recovery, recrystallization, grain growth) Stabilization annealing The aim of this treatment is to achieve the best possible dimensional stability, especially for calibers and gauges. Changes of dimensions of precise gauges and changes of magnetic properties of permanent magnets can be prevented by intentional long-time tempering of martensite and by efforts to dissociate residual austenite completely. The treatment itself (for low-alloyed steels) consists of heating to temperature, usually 120 C, dwell on this temperature for approximately 200 h and subsequent cooling. For steels with higher contents of alloying elements, temperatures up to 160 C and dwells up to 500 h are used. To this group of heat treatment belongs also stabilization of antifriction bearings, for which long-time tempering at temperatures of 170 to 200 C is performed after quenching with freezing Artificial steel ageing The aim of this annealing is to fasten the process of nitrogen and carbon precipitation from oversaturated alfa solid solution. It has meaning only for low-carbon steels (up to 0.2% C). For higher carbon contents, changes in properties caused by precipitation are eliminated by the presence of pearlite. Precipitation of nitrogen is the most intensive at the temperature of 150 C, while carbon precipitation is best at 250 C. Parameters of annealing are best to be obtained via tests of artificial ageing of a given material Annealing to reduce stress The aim is to reduce internal stress developed as a result of non-uniform cooling, local heating (welding), cold forming etc., with no intentional structure change and no substantial changes of original mechanical properties. The procedure consists of a slow heating (100 to 200 C.h -1 ) to the annealing temperature, dwell at this temperature (usually 1 to 2 h after perfect equalization of temperature across cross-section) and subsequent slow cooling (30 to 50 C.h -1 ) to temperature of 200 to 250 C. A proper selection of annealing temperature, which is usually between 450 and 650 C for constructional steels, has the crucial importance. The lower limit is given by the decrease in yield strength necessary to decrease stress by plastic deformation. For thermally refined steels, the lower limit should be by 20 to 30 C lower than the tempering temperature and by the same value higher than possible operating temperature of the component. Internal stress represented within a product by elastic lattice deformation is at such temperatures decreased by local plastic deformation. To prevent further development of internal stress, subsequent cooling is very slow (in furnace). Low-alloyed weldable steels with increased yield strengths, for which precipitation strengthening occurs during heating, are annealed at temperatures of 530 to 580 C. Annealing temperatures before final heat treatments of highly-alloyed steels (e.g. high-speed steel) are higher and can achieve 780 C. On the other hand, annealing to reduce stress is performed at temperatures as low as 150 to 200 C for surface-quenched components. In this

16 case, maintaining of hardness of the quenched layer is more important than a radical decrease in internal stress Recrystallization annealing The aim of recrystallization annealing is restoration of plastic properties of steels after cold forming. Thus, (according to ČSN ) elimination of elongated grains and strengthening caused by cold forming and simultaneous nucleation of new ferritic grains without any deformation features and restoration of the ability of steel to be plastically deformed. The procedure consists of heating to temperature minimum recrystallization temperature (usually 550 to 700 C) a short dwell (approx. 1 h) and subsequent cooling. The value of temperature depends on deformation degree; it however must not exceed A c1 temperature to prevent structure transformation. If structure refinement is to be achieved after a lower deformation degree, a lower annealing temperature needs to be applied. More advantageous (especially for inhomogeneous imposed strains for which local grain coarsening can occur) is to select normalization annealing (possibly patenting), which ensures final fine-grained structure within the whole product. The recrystallization temperature is approx. 800 C for nonpolymorphic ferritic steels, while the temperature of solution annealing is usually selected for austenitic steels. On the other hand, grain coarsening, which is required e.g. for transformer sheets steels, can be achieved by annealing at higher temperatures after a lower than critical cold deformation (5 to 12 %). The beginning of recrystallization and recrystallization temperature are influenced by various factors, but especially by previous reductions. Fig Structure changes during recrystallization annealing of steels, from left to right: deformed structure (strengthened state), finishing of static recrystallization (restored structure portion X = 1), grain coarsening after static recrystallization. Steels after cold forming recrystallize during subsequent heating. At temperatures below the transformation point, nuclei of new crystallites form. The higher is the deformation degree, the more new crystallites develop, recrystallization starts at lower temperatures and grains are finer. If heating continues after recrystallization has been finished, or if heating temperature is too high, some grains start to grow at the expense of others and structure coarsens (Fig ). For deformation degrees between 8 and 16 % and heating to temperatures of 650 to 850 C, significant grain coarsening occurs. A coarse-grained structure of steel wire has a tendency to exhibit brittle fracture, which can be observed after a very low number of wire bendings. The wire can crack during passing through a straightening device. The most significant grain coarsening occurs at the total wire deformation of Q c 10 %. The danger of grain coarsening for low-carbon steel wires drawn with critical strains decreases with increasing carbon contents and is eliminated in steels with carbon contents of

17 Task 4: Describe kinetics of static recrystallization of St24 (DIN ) steel in dependence on recrystallization temperature (minimum 3 temperatures) using laboratory experiment. You have a wire of the given steel cold drawn from d 0 = 5,5 to d n = 2 mm. Start from knowledge which you acquired in the subject Metallurgical formability. Elaborate a plan of experiment and discuss it with lecturer Soft annealing The aim of this type of annealing is usually to transform lamellar pearlite to globular (Fig ). For steels with carbon contents above 0.4 %, eutectoid and hyper-eutectoid steels, decrease in hardness and consequent improvement of cold machinability occurs. The effort is also to achieve better structure homogenization suitable for subsequent quenching. The process of soft annealing consists of heating to temperature just below or inside the transformation temperature interval (Fig ), dwell at the temperature, possible oscillation around the temperature and subsequent controlled cooling, usually inside a furnace or under a protective cover. The simplest soft annealing is performed for hypo-eutectoid low-alloyed steels heating to temperature just below Ac1 Fig Sferoidized cementite in ferritic matrix Fig Recommended soft annealing temperatures region and dwell at this temperature for several hours. For steels with higher contents of elements, which decrease diffusion rate of carbon in ferrite or stabilize cementite, annealing time has to be increased up to dozens of hours. If the original structure is BA or martensite, relatively rapid precipitation and subsequent coagulation of carbides occur. If the original structure is lamellar pearlite, spheroidization of lamellae process slowly and it is the slower the thicker they are. Coarse globular pearlite forms by this way from thick lamellar pearlite (similarly to too long dwell). Carbide network, i.e. hyper-eutectoid carbides precipitated at austenite grain boundaries, is not eliminated by this type of annealing. For hyper-eutectoid steels, high-alloyed, high-speed, and chromium steels, formation of globular pearlite is accelerated by annealing at temperatures above A c1 with subsequent continuous cooling. If lamellar pearlite is present in the original structure, this type of annealing is the only possible type to achieve globular pearlite. Annealing temperature must not be too high to prevent complete dissolution of cementite within steel (austenite homogenization) in order to keep a sufficient number of nuclei, at which fine and homogenously distributed carbides precipitate during cooling. The higher is the carbon content within the steel (or the more the steel is alloyed), the longer time is needed for the carbides to dissolve and thus the higher is usually the annealing temperature (e.g. annealing temperature is approx

18 730 C for steels with 0.9 % C, while it is approx. 750 C for 1.2 % C). Cementite globule form during subsequent slow cooling (rate 10 to 15 C.h -1 ). The slower is cooling through critical temperatures, the coarser are carbides and the lower is hardness. On the contrary, high cooling rates can result in formation of lamellar pearlite. Slow cooling is necessary not only to the temperature of C, but also lower to prevent generation of residual heat stress. The annealing process can be slightly accelerated by oscillation of temperature around the A1 point /Fig. 5. 9c, d/. The region of annealing temperatures and cooling rates is very small and thus soft annealing of large batches in chamber furnaces is not ideal. Non-uniform heating of batch occurs already at low heating rates. This results in different austenite homogenizations and various cooling rates, which influence the final cementite spheroidization. By the same reasons, such processing conditions are not suitable for acceleration of annealing by temperature oscillation, since changes in temperature influence only the batch surface. More suitable is usage of continuous furnaces, in which a more homogenous temperature distribution, but also required changes (e.g. higher temperature or overcooling) and dwells, can be achieved. Changes in properties of annealed alloys depend on spheroidization degree (ratio of content of spheroidized cementite to the overall cementite content). Decrease in hardness in dependence on time during isothermal annealing below temperature A c1 can be expressed by a relation derived from analyses of coagulation processes within microstructure: HV 1 HV K t 3 1 K c 1, 3 HV K2 f 0 (06) HV hardness of ferritic matrix, f c volume fraction of cementite, for mean value (radius) of particles according to Lifšic-Slyozov-Wagner applies: 1 3 r K 2 t (07) K 2 involves the influence of annealing temperature suitable size 2.r = 3 μm Spheroidization of carbides can unfavorably influence austenitization by retarding their dissolving (section 3. 2.). Thus a compromise between improvement of mechanical machinability, inflammability and hardenability, needs to be selected e.g. a lower spheroidization degree. Soft annealed low-carbon steel wires are used to produce chains, connection components for production of components by cold bulk forming and for general applications. Soft annealed high-carbon steel wires are suitable for example for production of needles, balls and bearing rollers etc Annealing in vacuum and under protective atmosphere To ensure lustrous surfaces of annealed steel wires, materials need to be protected against oxidation, decarburization and carburization during annealing. This protection is ensured by annealing of steel wires in vacuum or under protective atmospheres. Furnace or protective atmosphere within an annealing furnace does not directly influence the technological properties of annealed wires. However, the influence of furnace atmosphere can result in oxidation, decarburization or carburization of wire surface, or in change of grain size in the wire surface layer, which can influence mechanical properties. Usually, this influence is not substantial and can be noticed only for technological properties (bendings, twists), which are influenced by wire surface quality. For steel wires with decarburized surfaces, ultimate strength, yield strength, ductility and contraction slightly decrease, while numbers of bendings and twists increase slightly. However, wire

19 fatigue limit can decrease but also increase; the different influence of surface decarburization on fatigue limit is given by different character of the decarburized layer. The influences of various protective gas types on carbon contents in wire surface layers vary. Annealing of wires in vacuum was developed especially to anneal alloyed steel wires to achieve lustrous surfaces and ensures full protection against oxidation. The vacuum is needed to be lower than 0.1 torr, if possible, since only a negligible content of oxygen leads to surface decarburization and to occurrence of dull surface during annealing of alloyed steel wires. Technical argon is a fully inert protective gas. The common purity of argon is %, but this does not provide complete protection against slight oxidation and decarburization of wire surface. The mostly applied protective gases in wire drawing mills are mixtures of CO 2, CO, H 2 and N 2 with additions of small amounts of methane Annealing with structural transformation During annealing above A c1, complete, or at least partial, transformation of the original ferriticpearlitic structure to austenite occurs. Hypo-eutectoid steels are annealed at temperatures above A 3, hyper-eutectoid steels at temperatures above A cm Homogenization (diffusion) annealing The aim of homogenization annealing is the best possible homogenization of non-uniform chemical composition of steels via diffusion. Zonal heterogeneity (liquation) depends on the shape of cast or formed semi-product and concerns especially impurities and gases. Inter-dendritic heterogeneity (segregation) depends on morphology of dendrites, which influences distribution of alloying and additional elements. In formed semi-products, dendrites as well as liquation regions are preferentially elongated in a certain direction, which results is development of banded structure. This is characterized by spacing between bands and concentration profile in the direction perpendicular to bands; the spacing between bands decreases with increased forming degree. The process of homogenization annealing consists of heating to a temperature significantly higher than A c3 or A cm (to ensure sufficient diffusivity), thus a temperature between 1000 and 1200 C, a sufficiently long dwell at this temperature (6 to 15 h) and slow cooling from this temperature according to the shape of casting. Selection of homogenization annealing conditions should always be performed considering diffusivity of segregating elements and allowable segregation degree (it concerns decrease or elimination of interdendritic heterogeneity, not zonal one). The influence of a long dwell at a high temperature results in chemical composition as well as structure homogenization (improvement of steel microstructure), but also in higher oxidation and decarburization of surface and significant austenitic grain growth (structure coarsening deterioration of microstructure). Homogenization annealing is an expensive operation and it is thus efficient to connect it with heating before subsequent forming. In this case, the dwell time on the rolling or forging temperature is increased. Refinement of austenite grains can then occur via recrystallization in dwells between subsequent forming operations. For non-formed products, grain refinement is achieved by subsequent operation steps, such as normalization (combined annealing). Homogenization annealing is used the most for large castings or forged-products from alloyed steels Normalization annealing The aim of normalization annealing is especially the achievement of fine-grained and homogenous structure usually consisting of a mixture of ferrite and pearlite. The process of

20 normalization annealing consists of the quickest possible heating to a temperature of 30 to 50 C above A c3 or A cm, a short temperature equalization across the whole cross-section and subsequent cooling, usually on still air (approx. 100 to 200 C.h -1 ). Normalization annealing is mostly applied for castings and forged-pieces, smaller welded-pieces (for larger pieces the danger of deformation under their own weight occurs) and cold-processed stampings. The final structure of steels after normalization annealing is not unambiguously defined and depends on chemical composition of a given steel and size of annealed product. Hypo-eutectoid steels are usually normalized, while hyper-eutectoid steels are usually not normalized. If normalization annealing is to eliminate carbide network on austenite grains boundaries developed during slow cooling from forming temperatures, normalization temperature of approx. 30 C above A cm followed by quick cooling to a temperature below 700 C is suitable. Further cooling has to be slow. To refine grains for these steels, heating only above Ac1 temperature followed by cooling on air is performed. Normalization annealing is not used only to refine grains and homogenize structure, as a preparation for subsequent heat treatment or possibly before subsequent forming, but also as a final heat treatment operation for less loaded machine components. To eliminate internal stress, decrease strength and improve plastic properties, products after normalization are further annealed at temperatures of 500 to 600 C. Normalization annealing is important for steel castings, for which coarse and non-uniform grains, developed during slow cooling from casting temperatures, refine after normalization annealing; plastic properties improve as well Annealing for grain growth The aim of this type of annealing is to increase the austenite grain size. The process of annealing for grain growth consists of austenitization at temperatures higher than A c3 (950 to 1000 C) and subsequent cooling, usually in furnace. This procedure is used in special cases, such as for steels wires for electrotechnics, steels for cementation etc. For transformer or dynamo sheets, grain coarsening limits loses during magnetic reversal (magnetic hysteresis). It can also be applied for steels with low carbon contents, which are too soft and thus hardly machinable. Such materials feature long machining chips and consequently low-quality surfaces. Coarse-grained structure improves machinability Basic categorization of annealing furnaces The following types of annealing furnaces are used to anneal steel wires. Foreign producers also offer various types of this equipment. For a better orientation, the following categorization is used. Furnace types according to construction Continuous furnaces stripper type for wires in cores continuous type for wire coils special continuous type (unreeled coils) Discontinuous furnaces shaft type (Fig ) bell type (Fig and ) line bell type

21 Furnace types according to energetic sources gas electric liquid fuel furnaces Furnace types according to material protection with no protective atmosphere with protective atmosphere annealing in vacuum Úpravy materiálu po tváření Fig Schematics of gas bell annealing furnace with convective heat transfer Fig Electric bell annealing furnace, cooling cover take off after annealing finish, annealing cover at the back on the left Table 2.3 Average energy consumption, average heat efficiency of selected annealing furnaces types and scaling of annealed steel wires Annealing furnace Stripper gas muffle multi-core furnace Continuous gas furnace for wire coils protective atmosphere Shaft gas furnace for black annealed wires Shaft gas furnace Grünewald construction Shaft electric furnace Grünewald construction Bell electric furnace Herdickerhoff; double vacuum and inert atmosphere Bell electric furnace Ebner. protective atmosphere Wire diameter Power Wire temperatu re Energy Efficienc consumption y Atmosphere Scaling [mm] [kg.h -1 ] [ C] [MJ.t -1 ] [%] - [g.m -2 ] oxidative to inert < to oxidative max oxidative oxidative inert 0 inert

22 Questions to chapter 2 1. Define the basic difference between quenching and tempering. 2. Define the difference between recrystallization and phase transformation. 3. Characterize allotropic modification of a metal. 4. Shortly explain the role of carbon in steels. 5. Draw the iron iron carbide diagram and describe individual phases and temperatures. 6. Describe the principle of martensitic transformation. 7. Explain the principle of BA transformation. 8. Describe the difference between lower and upper bainite. 9. Characterize globular bainite. 10. Explain, how IRA and ARA diagrams can be constructed. Define the difference between them. 11. Describe the influence of chemical composition of steel, grain size and residual deformation on the locations of curves of transformation start and finish. 12. Characterize the basic morphological types of ferrite. Draw them. 13. Describe the principle of annealing to reduce stress. 14. Describe the principle of recrystallization annealing. Characterize kinetics of recrystallization. 15. Describe the principle of soft annealing. 16. How is a glossy surface of wire ensured during annealing? 17. Describe the principle of diffusion annealing. 18. Describe the principle of normalization annealing. 19. When coarse grains are artificially invoked? 20. Characterize transformations occurring during tempering. 21. Define sorbite. 22. In which cases to use tempering at low and when at high temperatures? 23. Define the cause of temper embrittlement. 24. Characterize the basic types of annealing furnaces categorization. 25. Characterize the principle of bell furnace. 2. PATENTING 100 minutes After study of this chapter students will be able to define the process of patenting, be able to describe microstructural factors of pearlite, be acknowledged with basic physical phenomena occurring during patenting, understand factors influencing microstructure of steel during patenting, be able to characterize technological devices for patenting,

23 2.1. Introduction Patenting is a special case of isothermal refining. Austenitized wires achieve very ductile and strong structures consisting of fine lamellar pearlite during passing through a lead bath with temperature about C, which enables significant reductions of their cross-sections during subsequent forming. Mechanical properties of wires after patenting are considered to be better not only their absolute values but also their homogeneity. It is used as initial heat treatment of rolled wires (if the wires were not processed with controlled cooling), or as inter-operation heat treatment of steels with carbon contents from 0.3 to 1.0 %. The development is focused on two areas: 1. Conventional patenting method (into Pb, salts, polymers, by compressed air, possibly using fluidized beds). 2. Development of various heat treatment systems with similar effects (controlled cooling of rolled wires, Easy Drawing Continuous EDC of SKF Steel company in Bofórs, Sweden, Kobe Kakogawa Patenting KKP of Kobe Kakogawa company from Japan) Austenitization heating Austenitization of carbon steels proceeds quite rapidly. When heating rate v o < 10 C.s -1, complete dissolution of carbides and austenite homogenization are ensured already at the temperature of 900 C, with no dwell at this temperature. Austenite grain growth rate is controlled by diffusion. The process usually features activation energy of 460 kj.mol -1. Equivalent austenitization regimes, i.e. heating times at various temperatures leading to identical austenite grains sizes, can be calculated for this value via application of the principle of equivalence of temperature and time of diffusion controlled processes. This calculation ensues from the presupposition of mutual equivalence of two different heat treatment regimes during which processes controlled by diffusion occur, if the following relation applies: Q Q 1 exp 2 exp (08) R T1 R T2 1 and 2 - heating time to temperature (s) T 1 and T 2 - temperature (K), Q - apparent activation energy of the controlling process (J.mol -1 ) R - universal gas constant (J.mol -1 K -1 )

24 = 190 W.m -2.K -1, v o = 9.8 C.s -1 = 80 W.m -2.K -1, v o = 3.7 C.s -1 Fig Heating and cooling temperatures for wires with carbon contents 0.7 % in various patenting furnace types Increase in temperature has a significantly more important influence than increase in heating time at a given temperature. For example, two minutes long heating at the temperature of 900 C has the same influence on grain size as 12 s dwell at the temperature of 960 C. Thus when compared to heating to the temperature of 900 C, the necessary dwell time at the temperature is decreased to one tenth of the original value via increasing the temperature by 60 C. Selection of a higher heating temperature is thus from this viewpoint more suitable for increasing grain size than a longer heating time. Nevertheless, heating temperatures above 950 C cause rapid oxidation of wire surfaces if the furnace is not operated with protective atmosphere. Although a thin layer of oxides is advantageous for heating of drawn wires with originally lustrous surfaces since it increases emissivity from 0.5 to 0.8 and thus increases heat transfer coefficient, excessive oxidation is undesirable. Scales restrict effective cooling within lead and moreover cause lead take out from the bath. The influence of austenitization temperature on austenite grain size is depicted in Fig Size of the original austenite grains of pearlitic steels is determined via quenching of samples after austenitization with a rate just slightly lower than the critical rate. This results in formation of pearlite at boundaries of the original grains, which subsequently transforms to a mixture of BA and martensite

25 Průměr austenitického zrna (mm) Úpravy materiálu po tváření y = 0,0449e 0,0069x R 2 = 0, Austenitizační teplota ( C) Fig top) influence of austenitization temperature of austenite grain size, bottom) microstructures corresponding to the particular temperatures Task 5: Determine activation energy for austenite grain growth for steel depicted in Fig using the Excel software. Necessary data is depicted in Table 3.1. Table 3.1. Data for calculation of task 5 Austenitization temperature ( C) grain size (μm) Decomposition of austenite Contrary to austenitization conditions, physical-metallurgical and technological conditions for cooling within patenting devices are more unambiguous. Optimum conditions of transformation of austenite to fine pearlite are achieved at temperatures around 550 C for carbon steels. This temperature is only marginally influenced by carbon content and corresponds to the temperature of pearlitic nose in IRA diagrams. At this temperature, fine lamellar pearlite with inter-lamellar distance less than 80 nm generates. Simultaneously, formation of ferrite at boundaries of original austenite grains is suppressed, which enables the possibility of achievement of purely pearlitic structures even for hypoeutectoid steels

26 Temperature of the bath has to be modified according to wire diameter, since with increasing wire diameter cooling rate in sub-surface layers decreases and thus austenitic transformation occurs at higher temperatures than bath temperatures (to 60 C for wires with diameters d = 15 mm). It is thus not isothermal cooling any more, but austenite decomposition during continuous cooling. Incubation time for carbon steels in the region of pearlitic nose (approx. 600 C) of an IRA diagram is on the order of seconds see table 3.2. Table 3.2 Incubation time and structure of pearlitic steels in dependence on austenite grain size Carbon content Grain Time of transformation start at 600 C Content of free ferrite [wt. %] ASTM [s] [vol. %] 0,53 7 to to ,63 7 to to Influence of carbon content on location of pearlitic nose The influence of carbon content on location of pearlitic transformation region and courses of accelerated cooling curves are depicted in Fig The longest time and lowest temperature of pearlitic nose have steels with eutectoid compositions, increase in carbon content leads to shifting to lower times and higher temperatures. Fig Influence of carbon content on location of pearlitic nose Influence of grain size Generation of pearlite and ferrite from austenite proceed via heterogeneous reactions, localized primarily at austenitic grains boundaries. Structures with larger grains have lower total areas of grain boundaries and nucleation of pearlite and ferrite is there slower. Curves in IRA diagrams shift to longer times and incubation time elongates. Larger grains thus enable to better approach the temperature of lead before austenite decomposition starts and thus to improve isothermality of pearlitic transformation

27 A larger number of nuclei could be within structures with residual deformation, but there is no residual deformation present in structures of wires during patenting Kinetics of phase transformation and its consequences Pearlitic transformation of austenite within carbon steels proceeds in the region of pearlitic nose by the rate of approx to l0-6 m.s -1, which means that decomposition of austenite with grain size on the order of dozens of micrometers is finished within several seconds. By this reason, the first ten seconds of wire cooling in lead are critical for final structure and properties after patenting. Therefore, the inlet part of lead bath has to be very carefully controlled. In the inlet part of the bath, heating of lead by the heat of heated wires occur. It is thus favorable to cool the inlet part of the bath either by indirect cooling of bath by compressed air, of by lead circulation. Effective, but technologically relatively demanding improvement of isothermal method of transformation, especially for large wire diameters, are various systems of double-step patenting. Double-step patenting uses the incubation time of pearlitic transformation for rapid cooling to the temperature of approx. 100 to 200 C above the martensite start temperature. The transformation temperature is subsequently approached from below by the influence of equalization of wire center and surface temperatures Influence of alloying elements Alloying elements strongly influence kinetics of austenite decomposition. They thus usually occur in low concentrations within steels for patenting. Alloying elements also decrease pearlite growth rate, which results in transformation time increase. In decreasing order of efficiency of individual alloying elements on decreasing incubation time and pearlite growth rate, the common alloying elements can be grouped as follows: Mo, Cr, Mn, Ni, Si. Fig depicts the influence of individual alloying elements on incubation time of isothermal austenite transformation in a steel with carbon content of 0.8 Fig Influence of alloying elements on isothermal % at the temperature of 627 C. In the dependence in transformation finish at 627 C for eutectoid steels Fig. 3.4., y axis depicts the ratio of finish time of isothermal transformation for steel with a given content of alloying element (t j )x to finish time of isothermal transformation for eutectoid steel with no alloying elements (t j )

28 Microstructural factors of pearlite Úpravy materiálu po tváření Fig schematically depicts basic factors defining microstructures of pearlitic steels. These are: size of pearlitic colony D PC, inter-lamellar distance and volume fraction of cementite V nter-lamellar distance of patented wires is among the limit of 100 nm, which is the limit of industrial production (further decrease would be possible by micro-alloying of steels by carbide-nitride-forming elements V, Ti, Zr) Inter-lamellar distance Fig Schematics of pearlitic Inter-lamellar distance λ changes with cooling microstructure rate in the pearlitic region and with alloying. Interlamellar distance increases with increasing austenitization temperature. Alloying elements, especially Mn, Cr, Mo, V, Nb, significantly decrease inter-lamellar distance and thickness of cementite lamellas. Steels alloyed with niobium moreover have higher strengths at identical inter-lamellar distances. Ultimate strength and yield strength increase with decreasing inter-lamellar distance (Fig ), steel A has eutectoid composition, steel B is micro-alloyed with niobium. Fig Dependence of yield and ultimate strengths on inter-lamellar distance for two austenitization temperatures

29 Inter-lamellar distance has only marginal influence on ductility, especially for small original austenite grain sizes. Initiation of fatigue cracks depends only on inter-lamellar distance, not on yield or ultimate strengths. The following equation describes the influence of inter-lamellar distance and other microstructure factors on offset yield strength of steels alloyed with vanadium and titanium: R 311,6 11,4 0,6 D PC 234,3 f [MPa] (09) where R 0. 2 is offset yield strength, D PC f is inter-lamellar distance of pearlite is size of pearlitic colony is volume fraction of precipitates (determined by chemical or metallographic analysis), Austenitic grain size Via its direct relationship with size of pearlitic grain after transformation, it primarily influences plastic properties. Fig shows the dependence of elongation during tensile test on austenite grain size for two steels, eutectoid (0.8 wt. % C) and hyper-eutectoid (0.9 wt. % C). Elongation for the steel with 0.9 wt. % C is lower than for the steel with 0.8 wt. % C, although experiments proved that values of elongation corresponding to eutectoid steels can be achieved for hyper-eutectoid steels via refinement of austenite grains. Fig Ductility in dependence on size of original grains for eutectoid and hyper-eutectoid steels. Strength increases with increasing size of original austenite grains at constant inter-lamellar distance. Fine primary austenite grains increase notch toughness, shifts the temperature dependence (transit curve) to lower temperatures and increases fracture toughness. Besides cooling rate, significant grain refiners are especially nitride and carbide-nitride precipitates

30 Parameters influencing patenting process Among the basic parameters influencing structure of wires after patenting is austenitization temperature, which directly influences grain size and thus size of pearlitic colony D PC. Another parameter is wire diameter, which influences heating and cooling rates and consequently all microstructure factors of pearlite. Cooling rate is also an important parameters influencing primarily inter-lamellar distance and for hyper-eutectoid steels also volume fraction of cementite in pearlite. If cooling rate is not sufficiently rapid, cementite precipitates as networks along grain boundaries. The last important parameter is chemical composition of steel, which influences kinetics of austenitization and subsequent phase transformations and fractions of individual structure components. If the individual basic patenting parameters are controlled, structure can be controlled as well, like in the following example. Fig illustrates the effect of microstructure controlling on strength (a) and ductility (b). The original microstructure, used in this experiment, was a common eutectoid steel. In this case, increase in cementite volume fraction from 41 % to 50 % resulted in increase in strength by 4.3 % and ductility by 6.5 %, while decrease in lamellar distance from 350 nm to 110 nm increased strength by 30 %. As already mentioned, 100 nm is a theoretical limit achievable by heat treatment. Further increase in strength by 16 % can be achieved via refinement of pearlitic colony size from 150 μm to 50 μm. Fig Improvement of a) strength and b) ductility by controlling of pearlite microstructure Characterization of patenting devices Various types of devices are used for patenting. The selection depends on required quality of patented wires, volume and assortment of produced wires, technology and economy of production. Patenting devices are for better lucidity categorized in several groups, see Table 3.4. Constructional solutions of new types of patenting devices are generally oriented towards increasing quality of patented wires, i.e. towards providing optimum structures of materials for given application purposes, decreasing deviations in strength and technological properties and increasing wire surface quality. Simultaneously, the trends are is focused on increasing efficiency and utilization of devices, increasing energetic efficiency, improvement of regulation level and control of patenting furnace, improvement and simplification of operation and maintenance, increasing work productivity, hygiene, elimination of physically demanding work and decreasing in consumption of used types of resources and materials (Pb, H 2 SO 4, HC1, water, Zn etc.)

31 Table 3.4 Basic categorization of patenting devices Construction Stripper type - wires in cores Type of heating Gas Heat transfer during heating Direct heating by exhaust gas Protective atmosphere No PA Cooling medium Chamber wires in coils Electric Indirect muffle type With PA Salts Special loop Other Indirect electric (tube furnaces) With possibility to control oxygen content Lead Compressed air Liquid medium Fluidized bed 1) Direct resistant Combined 1) Fluidized bed consists of particles of Al 2 O 3, SiO 2, ZrO 2 etc. whirling in a stream of gas. Combination of high heat transfer coefficient, great thermal capacity and uniformity of temperatures makes the whirling layer a suitable replacement for lead bath. Selected types of patenting devices are characterized in Fig and Fig It is supposed that technologies of patenting and relevant technological equipment will not significantly change in the near future. The rates and numbers of wires in continuous patenting, patenting-pickling and patentingmetal coating plants will also probably not change substantially in the nearest future. Constructional solution of individual equipment for patenting is oriented towards automation of individual production operations. The whole lead bath is covered with an insulation layer, which decreases heat loses and also prevents lead oxidation. Reaction of PbO with iron results in oxidation of iron. To prevent lead take out from the bath in the form of drops stuck on wire surfaces, the surface of lead bath is covered with graphite pellets in the location of wire outlet from the bath. Lead oxides are toxic and easily stick on operators hands, from where they can easily get into body, e.g. during smoking. Lead oxides thus have to be removed whenever possible. By these reasons, alternatives to lead are searched. A perspective possibility is replacement of lead baths by fluidized beds. Fig Gas patenting furnace with wire surface treatment; 1 wire, 2 uncoiler, 3 heating furnace part, 4 bath for cooling of wire in lead, 5 equipment for wire surface treatment, 6 coiler

32 Fig Patenting furnace with heating of wire by self-electric resistance; 1 wire, 2 uncoiler, 3 contact lead bath, 4 heater field, 5 lead cooling bath (second contact bath), 6 coiler, 7 transformer Fig Chamber patenting furnace for wire coils, Berg company Task 6: Using TTSteel software perform simulation of patenting of wires with diameters: 1.5; 3; 6; 10; 15 mm and design technological procedure of patenting to achieve the maximum possible ductility of wires and maximum homogeneity of wire microstructure. Select one of the following steels: C86D, C50D, C110U. Chemical composition of individual steels can be gained from the Forge 2005 software database. For determination of heat transfer coefficient via inverse method use the dependence in Fig Fig The course of temperature during cooling of wire from C70 steel, diameter 1.5 mm, in fluid bed (2) and lead bath (1); Pb - temperature of lead bath, F. l. temperature of fluid bed.

33 Questions to chapter 3 1. Define patenting. Define its aim. 2. Characterize common methods of patenting. 3. Do you know other technologies which can substitute patenting? 4. Define steps of which patenting consists. 5. What happens with steels during austenitization heating? 6. Define the dependence of austenite grainsize on temperature. 7. How austenite grain size can be determined for pearlitic steels? 8. What happens with microstructure during cooling after austenitization heating? 9. Define parameters on which depend pearlitic nose coordinates in ARA diagram. 10. Explain the principle of double-step patenting. 11. How would you define microstructure parameters and quantitatively describe pearlitic structure? 12. Define the influence of inter-lamellar distance of cementite within pearlite on strength of steel. 13. Define the influence of austenite grain size on ductility of steel. 14. Define parameters influencing patenting process and their mutual relations (draw them). 15. Characterize and categorize patenting devices. 3. METAL COATING OF STEELS 180 minutes After study of this chapter students will have basic information about the issue of corrosion, be acknowledged with the technology of zinc dipping (galvanization), know the basics of theory of galvanization, be able to orient in the most common types of metal-coating of steels, Customers often require steel wires to be in corrosion resistant states, even when the wires are semi-products. This requirement is in accordance with the efforts to prevent economic and ecological loses caused by corrosion, which are estimated to be 1 to 4 % of annual production in industrial countries. In former Czechoslovakia, the costs for surface treatments and loses, which are annually caused by corrosion of metals in electro-technical devices, were estimated to be 15 billion CZK. However, loses caused by power shut offs, limitations in productions and power supplies and breakdowns, which are caused by corrosion, are probably far higher

34 Wire-production industry has basically three possibilities for production of corrosion resistant steels wires: cover wire surfaces with metal coatings generate organic or inorganic surfaces on wires fabricate wires from anticorrosive alloyed steels The most developed and wide-spread method of protection of steel wires against corrosion, verified by a long-time practice, is metal coating, galvanizing in particular Overview of modifications of steel wires before metal coating Metal coating of steel wires is usually categorized according to the type of applied metal and method of metal application on wire surfaces as follows: 1. Zinc coating hot dipping (in melt) electrolytical brushing 2. Aluminum-coating hot dipping (in melt) 3. Hot-dipping in Zn-Al alloy coating Galvalume (55% Al, 43.5 % Zn, 1.5 % Si) Galfan (95% Zn, 5% Al, lanthanum, cerium) Trigalva (84.5% Zn, 15% Al, 0.5 % Sn) 4. Tin-coating hot dipping (in melt) electrolytical 5. Copper-coating electrochemical with no current electrochemical with external current source 3.2. Corrosion of steel wires Generally, corrosion of metals is a spontaneous process of deterioration of metal materials resulting from their chemical and electrochemical reactions with aggressive surroundings. In dependence on the type of reaction, the following can be distinguished: electrochemical corrosion, which proceeds in electric-conductive water-based solutions, salts, acids and alkalis, thus in electrolytes. chemical corrosion, which proceeds in electrically non-conductive environments, e.g. in gases and non-electrolytes. Steel wires and products from steel wires are most often affected by atmospheric corrosion. Aggressiveness of an atmosphere and the influence of its components are for example specified in ČSN According to this standard, the aggressiveness of an atmosphere is categorized in 5 grades. In the conditions of central Europe, the decisive factors are especially time of exposure of wire surface to humidity and concentration of corrosion stimulant SO 2. Examples of atmospheres for the individual degrees of aggressiveness are depicted in Table 4.1. Corrosion rates for some technical metals in atmospheres of various aggressivenesses related to corrosion rate of Fe in atmosphere 1 are given in Table

35 Table 4.1 Examples of corrosion aggressiveness for the region of Central Europe. Degree Time of exposure to humidity SO 2 content [h.year -1 ] [mg.m -2.d -1 ] Examples - closed areas with modified atmospheres (heating, air-conditioning) - residential, administrative, storage, production areas. - closed areas without modified atmospheres; - unheated storages, parts of production halls, internal areas of agricultural facilities. - external polluted atmospheres with full exposure to all factors or climates with frequent condensation as 3, in industrial atmospheres max as 3, under extremely polluted conditions (inter-climates of chemistry, metallurgical, pickling plants etc.) Table 4.2 Corrosion rate for some metals at various degrees of aggressiveness of atmosphere Metal Relative corrosion rate [-] Degree of corrosion aggressiveness of atmosphere Fe Zn Cu Al Hot dipping of steel wires Introduction Hot-dip metal coating is a widely applied method of protection against corrosion, especially for steel constructions, machines and devices operating in aggressive corrosion environments. The most wide-spread process of metal coating by dipping for iron alloys is zinc-dipping. The method of zinc coating by dipping was invented by French chemist Malouin in He described a method of zinc coating, processing analogically to phenomena proceeding simultaneously, consisting in dipping of steel or cast iron components into melted zinc. This method was however not applicable in practice, since no sufficiently economic method for preparation of iron alloys component surfaces was available at that time. Hot-dip zinc coating (galvanizing) in industrial scale was introduced in 1836 in Paris by Stanislav Sorel, who invented a method of pickling of iron alloys surfaces. This zinc coating process, consisting in anticorrosion protection of products by their individual dipping, has been used till present. Zinc coating by dipping is the most efficient and most economic method of protection of steels and cast iron against corrosion. Hot-dip galvanizing is usually performed within the temperature range of C [1]. Application of this bath temperature range is limited by substantial dissolubility of iron within the temperature interval of C (low-temperature zinc coating, LT-HDG low temperature hot dip galvanizing), in which is the danger of accelerated wear of steel baths, which are mostly used for storage of melted metal, occurs [2]

36 The intensity of iron dissolubility decreases to values only slightly higher than at temperatures below 480 C at temperatures higher than 520 C [1]. Analysis of literature imparts that increase in processing temperature above 520 C should cause increase in process productivity. The final coatings are characterized by widened diffusion layer with no occurrence of phase, which improves resistance of the coating against corrosion [2, 3]. Protective coatings are formed continuously or by individual dipping of formed metallurgical products, cast-products or constructional components, with the purpose of their anticorrosion protection Influence of chemical composition of bath Zinc with content of impurities of 0.1 % and lower is suitable for zinc coating of steel wires. Among the alloying elements, aluminum influences growth and structures of coatings the most. The Al 5 Fe 2 intermetallic compound preventing reactions of iron and zinc bath precipitates at first at the interface of the steel and zinc bath. After its dissolution, a normal formation of alloy compounds occurs; the process is faster than without aluminum. Thus, a presence of aluminum suppresses formation of the coating for a certain time. With increasing concentration of aluminum, the incubation time increases. With increasing concentration of aluminum within the range of to 0.05 %, appearance of the coating improves. Coating is lustrous with patterns. High concentrations increase the danger of generation of surface oxides. Since formability of coatings decreases with decreasing fraction of alloy phase (Fe Zn) within the coating, aluminum influences favorably its formability up to the concentration of 0.3 % Al. Zinc coating with aluminum within the range of % is prone to formation of white rust, i.e. unsticking coating of zinc hydroxide. With decreasing content of zinc the content of impurities, especially lead, increases, but decreases the cost. Analyses of zinc never determine ZnO contents despite the fact that the contents are quite important for evaluation of zinc suitability. Presence of ZnO prevents sinking of lead to bottom of zinc bath even for larger Pb contents. Addition of lead causes decrease in fluidity of melted bath. Zinc coating is then brittle. Another common unfavorable admixture is cadmium. Its contents in very pure zinc are usually up to %. Contents of copper and tin in pure zinc should be max %. Experiments have proven that tin decreases fluidity of zinc. Fe is also an unfavorable admixture. Pure zinc contains only approx % of Fe. In work bath, melted zinc is enriched with iron by reaction of steel bath with wire, which moreover introduces Fe salts from foregoing operations. A high content of Fe in zinc has unfavorable influence on plasticity of zinc coatings Skimming of zinc Considering the exiting angle of the wire from melt during hot-dip galvanizing of steel wire, standard zinc coating and thick zinc coating can be distinguished. During standard hot-dip galvanizing of steel wire, the wire exits the zinc melt under an angle and the layer of pure zinc is almost completely mechanically skimmed from wire surface. Zinc coating consists of an iron-zinc layer and its determined minimum amount on the wire surface in dependence on wire diameter is depicted in Table 4.3. During thick hot-dip galvanizing of steel wire, the wire exits the zinc melt perpendicularly upwards from the melt, so the layer of pure zinc solidifies on wire surface. Zinc coating consists of an iron-zinc layer and a layer of pure zinc. The determined minimum amount of zinc on the wire surface in dependence on wire diameter is depicted in Table

37 Table 4.3 Determined minimum amount of zinc on steel wire surface for normal and thick zinc coating and for wire after galvanizing drawn according to CSN Nominal wire diameter [mm] Minimum zinc amount [g.m -2 ] Galvanizing type standard thick in melt with additional drawing through up to in melt galvanizing to to to to to to to to to to to to Technological devices The arrangement of the process of continuous hot dip galvanizing of steel wires is generally identical for all types of wires. The difference is only in the already mentioned types of exiting of wires from melts and methods of skimming. The process of hot-dip galvanization in a continuous line includes uncoiling, alternatively patenting, annealing or degreasing, pickling and application of fluxing agent, drying, galvanizing and coiling. According to the steel grade and application purpose, some technological steps can be omitted. Fig schematically depicts alternatives of individual parts of hot dip galvanization lines for steel wires. Fig Schematic depiction of alternatives of individual parts of hot dip galvanization lines for steel wires

38 Galvanizing furnaces Galvanizing furnaces are important components of lines for hot dip zinc galvanization of wires. Galvanizing furnaces are constructed with steel or ceramic baths, i.e. working tanks for zinc melt. Gas, possibly fuel oil or electricity, is mostly used to heat the steel and ceramic baths. A progressive method of gas heating of steel baths is the method with forced atmosphere circulation. Combustion of gas is performed in a separate chamber. Exhaust gas is driven with high velocity around the bath by a fan. High velocity of streaming enables heat transfer by radiation, as well as by convection. Important during heating is to limit the exhaust gas temperature to ensure the maximum temperature on the internal wall of the steel bath to be lower than 470 C. One of the main issues of hot dip galvanizing is the lifetime of steel baths, which is according to /10/ influenced especially by: bath material, method of heating heating homogeneity, ratio of zinc mass in the bath and mass of zinc-coated wire in a time unit, ratio of heated area to zinc level. Laws of dissolubility of steel wires in melted zinc are certainly applicable also for steel baths. From this ensues that the bath should be fabricated from the purest iron with minimum amount of silicon, phosphorus and carbon. The material mostly used abroad for steel baths is ARMCO technically pure iron. For heat transfer via walls of a bath considering their height applies that at least 75 % of the overall supplied heat has to be transferred through the upper half of the bath. The reason for this is a larger demand for heat to cover heat loses by radiation of zinc level. Heat loses by radiation of zinc level for the temperature of 450 C: pure zinc level 62.8 GJ.m -2.h -1, level covered with ash 33.5 to 41.9 GJ.m -2.h -1, level covered with insulations cover 11.9 GJ.m -2.h -1. An important factor influencing lifetime of baths is the ratio of zinc mass to mass of zinc-coated wire passed through the zinc bath in one hour. Generally, zinc mass in the bath should be at least twenty times higher than the production of zinc-coated wire in an hour Balance of zinc consumption The overall zinc consumption during production of zinc coated steel wires is a significant item from the viewpoint of costs. It can be expressed by the relation: M Zn,c = M u + M p + M z [kg.t -1 ] (10) where: M u zinc consumption to ensure zinc take on by wire according to ČSN, M p zinc consumption due to exceeding of specified zinc take on by wire according to ČSN, M z zinc loses. Zinc loses during galvanization can be expressed by the relation: M z = M tz + M zp + M zo [kg.t -1 ] (11)

39 where: M tz losses from generation of hard zinc M zp loses from generation of zinc ash M zo other loses. Úpravy materiálu po tváření Hard zinc generates from introduced iron salts, by reaction of liquid zinc with iron from wire surfaces and by reaction of steel walls of bath and armature with zinc melt. It contains approx % of pure iron, 8 5 % of Fe, the rest are impurities. Zinc ash generates from oxygenation of zinc level, among this it contains burned covering material from zinc Fig Zinc consumption during standard level and rests of fluxing agent. Zinc ash contains approx. hot dipping of steel wires % of zinc. 1 - Overall consumption Other loses M zo generate by outflow of zinc from 2 - Consumption of zinc ash 3 - Consumption of hard zinc bath, zinc spray-out etc. These loses are relatively small. Consumption of zinc during standard zinc coating of steel wires is depicted in Fig Electrolytic zinc coating 3.5. Copper coating Copper is a reddish metal with the density of 8.95 g.cm -3 and melting point of 1084 C. It has the best electric and thermal conductivity after silver and is not magnetic. According to galvanic (electropotential) series, copper is nobler than iron and it ensures protection of steel wires against corrosion already at low thicknesses. Copper coating of wires is a technological step of surface treatment of wires performed after patenting, before drawing. Copper metal coatings decrease friction between wires and drawing dies, facilitate the process of drawing and ensure smooth and clean surfaces of drawn wires. Copper coating with thicker coper layers is used as a final technological operation in order to achieve lustrous and clean wire surfaces especially in production of wires for mattress springs and in production of welding wires for automatic welding in CO 2 atmosphere or under a fluxing agent. Coating with copper is performed on wires with large and medium diameters up to 1.6 mm, wires with smaller diameters are usually phosphated. A high-quality steel wire surface conversion coated with copper has to contain (independently of wire diameter) min. approx. 2 g of Cu on one square meter (which corresponds to 0.22 mm thickness of Cu layer). Table 4.4 shows calculated factors of conversion copper coating for 2 gm -2 Cu take on during continuous process

40 Table 4.4 Calculated factors of conversion copper coating for 2 gm -2 Cu take on continuous process. drátu Povrch drátu Potřebné množství Cu Spotřeba CuSO 4.5H 2 O při jejím využití na. Objem mědící lázně (l) počáteční obsah CuSO 4.5H 2 O (gl -1 )* mm m 2 kg -1 gt % 90 % poměděné množství drátu (kg) bez ztráty kvality 1,0 0, ,0 4, ,5 0, ,3 3, ,0 0, ,5 2, ,5 0, ,0 1, ,0 0, ,7 1, ,5 0, ,4 1, ,0 0, ,3 1, Fundamentals for selection of parameters for copper coating devices Fundamentals for selection of parameters for copper coating devices can be summarized as follows: Optimum Cu take on is almost independent on wire diameter. The last draw before continuous copper coating has to be under wet conditions. Copper coated wire is eventually drawn with a small reduction final draw (after rinsing behind the bath and immersion into emulsion). The wire drawing machine should have relatively lower or possibly controllable drawing speed. Copper coating device should provide the wire in the bath with a sufficient immersion time (which is given also by the speed of the drawing machine), at least 5 seconds. For this purpose, the bath can be equipped with reversible trolleys. Advantageously, the number of immersed lengths can be adjustable Fundamentals for selection of concentrations of components within copper coating bath and for copper coating operation 1. A sufficient Cu take on by the wire and its stability in time is dependent on the content of copper sulphate. For continuous process (short exposure times), high concentrations of copper sulphate are needed. 2. Conversion copper coating cannot generate copper coatings with arbitrary thicknesses, copper starts to strip off above the critical level of approx. 3 gm Excessive intensity of Cu deposition can be controlled by: diluting the bath with water (no danger of losing copper coating ability, although Cu concentration decreases). acidifying of the bath (relatively small amounts of acids, adhesiveness of the layer increases as well, but the danger of losing copper coating ability of the bath occurs). a combination of the above mentioned methods. double-step copper coating (1 st section with low Cu concentration, higher acidity weak adhesive coating; 2 nd section with higher Cu concentration, lower acidity coating strengthens significantly). 4. The mentioned fundamentals are applicable for new baths and roughly also for partially used baths. If deposition of Cu from the solution on the wire results in loss of a portion of Cu 2+ content, it needs to be supplemented by adding of concentrated CuSO 4 solution. Simultaneously, loss of each 1 gl -1 of Cu 2+ from the solution results in generation of 0.88 gl -1 of Fe salt, which is

41 a contaminant. The limiting highest content of Fe for long-time usage of a bath can be determined only empirically with regard to the particular immersion time of wire in the bath and real Cu take on. 5. The content of free acid in copper coating baths loses during coating quite slowly (Fe +2H - Fe salt). 6. When the Fe content in the solution is high after having performed increase in CuSO 4 concentration repeatedly, the bath needs to be exploited (for this step, the possibility of reduction of wire drawing machine speed would be applicable) and then disposed. Before disposing the waste copper coating bath by a specialized company, the rest of Cu 2+ from the solution gathered in a special tank should be completely segregated via a longer immersion of a waste wire (till the Cu 2+ content decreases to zero). The considered factor is the difference in disposal costs of solutions with and without Cu 2+ contents (disposal of solutions containing copper ions can be supposed to be more expensive). 7. Possibilities of checking of wires coated with copper conversely with one draw after coating. Among visual evaluation of copper coated wires, customers usually do not require values of copper take on. If testing of this parameter was necessary, it would be performed by double weighting of a wire sample before and after coating removal. Mechanical properties are usually required and met in accordance with standards Controlling of copper coating process Based on the previous information, copper coating process can be controlled by eye. A technically more accurate method is however to know concentrations of limiting components in the bath and supplement the components and replace the whole bath on the basis of knowledge of concentrations of individual ions. The following components are needed to be monitored within a bath: concentration of copper (Cu 2+ ) - must not decrease below 25 gl -1 concentration of iron (Fe ) - must not exceed 40 gl -1 ph - must be always < 1 furthermore, monitoring of SO 2-4 concentration is advised There are two methods of controlling the copper coating process: a) Analytical method This method comes out from the supposition of regular taking of bath samples. These samples are to be regularly analyzed in maintenance laboratory for contents of Fe, Cu, possibly SO 4. On the basis of the results, the bath is then supplemented with copper, possibly after exceeding concentration of Fe, SO 4, the operator decides to replace the bath. b) Analytical-empirical method If the average copper take on for a given device is known, the amount of copper sulphate necessary for copper coating of 1 ton of a wire of given diameter can be calculated. Based on the calculation, the consumption of CuSO 4.5H 2 O can be estimated and then regularly added into the bath. To verify the dependence, approx. one month observation is recommended (see point a)). If observation verifies the foregoing calculations, they can further be used as directional and bath can be checked only once or two times a week. In case of significant differences between the calculation and analytically observed reality, long-time observation and evaluation of copper take on in dependence on wire diameter and all technological factors is necessary

42 3.6. Aluminum-coating Úpravy materiálu po tváření Aluminum is a metal with the density of 2.7 g.cm -3, softening at the temperature of 600 C and the melting point is 658 C. According to galvanic (electro-potential) series, it is less noble than Fe. When compared to zinc coatings on steel, aluminum coatings with identical thickness have 2.5 to 3 times higher longevity in seaside, urban and industrial atmospheres. Aluminum alloys also exhibit high corrosion resistances. Aluminum coating has up to 3 times higher longevity than zinc coating in industrial atmospheres. Aluminum coating is mostly performed in melts (melting point of Al = 658 C) in the following technological steps: uncoiling, degreasing, pickling, fluxing agent application, aluminum coating, cooling, coiling. It is performed in Al 99.5 melt at temperatures around 700 C with application of fluoride fluxing agents. At such temperatures, recrystallization of steel can occur and thus hot dip galvanizing is suitable for coating of wires, which are required to be in softened states (nets, meshes, barbed wires etc.). Development of unwanted FeAl 3 intermetallic phase can be reduced by alloying with Si. Coatings of Zn-Al alloys are highly resistant against corrosion, and thus they are often used as alternatives to zinc coatings in industrial practice (Table 4.5). Table 4.5 Properties of Zn-Al alloys coatings on steel wires Coating (production process) Chemical composition Specific mass Melting temperature Corrosion resistance in atmosphere Drawability, area reduction Adhesiveness Coating hardness [%] [g.cm -3 ] [ C] [%] [HV] Zinc Zn Pb Fe Cd good, to 85 good, decreases with increasing zinc thickness layer of Zn 35 layer of Fe Zn 93 Galvalume Zn 43.5 Al 55.0 Si times higher than zinc good, corresponding to properties of Zn and Al Trigalva (production process) Zn 84.5 Al 15.0 Sn 0.5 Zn 95.0 Al 5.0 Ce traces Le traces to 6 times higher than zinc excellent, 90 excellent Galfan more than 2 times higher than zinc excellent, 90 excellent layer of Zn/Al 56, layer of Fe/Zn/Al nonmeasureable 3.7. Tin-coating Tin is a soft silver-like metal with the density of 7.28 g.cm -3 and melting point of C. According to galvanic (electro-potential) series, it is nobler than Fe. Tin is important for protection of other metals against corrosion. The ability to form resistant films of SnO and SnO 2 oxides causes tin to be very stable in atmosphere and in water. Regarding its low mechanical properties and high cost, tin is applied especially in the form of coatings on other metals, especially on iron, cast iron and copper

43 Tin coating of wires is, contrary to tin coating of sheets, not so widespread. Coating with tin is used especially in food processing industry for its non-toxic properties (meshes and filters), and in electrotechnics for its good solderability (bandaging of electric devices). In special applications it is used for its ductility and slipperiness. Tin coating is performed: 1. By hot dipping, immersing into melted tin 2. Electrolytically Hot-dip tin coating The technology of preparation of tin coating on steel with probably the longest history is hot-dip tin coating performed by immersion of a steel component into a bath with melted tin (hot dip tinning), or wiping of a steel component heated above the melting temperature of tin with tin ash (wipe tinning), in cases when the tin coating has to be applied only from one side. An important technological step during hot-dip tinning is thorough cleaning of surfaces of to be tinned steel or iron components, especially thorough degreasing and removal of all possible residuals of corrosion products. Hot-dip tinning is performed as follows: steel sheets enter a bath with melted tin (temperatures of 310 C and 245 C to 310 C) through a layer of fluxing agent (melt of ZnCl 2, NaCl and NH 4 Cl) and exits through a layer of palm oil and pass through rolls, which limit the thickness of the coating to be between and mm. A thin layer of intermetallic phase generates between the steel surface and tin coating. This phase was identified by X-rays to be FeSn 2 (see diagram Fe-Sn, Fig ). Anticorrosion resistance is ensured only by a layer of pure tin with the coating quality of 30 to 40 g.m Electrolytic tin coating Acid or alkaline baths can be used for continuous electrolytic tinning of wires. The surface of a deposited coating is dull. The mass of the coating is in the range of 10 to 15 g.m -2. For deposition of tin from alkali baths, a solution of sodium stannate in water with addition of sodium hydroxide to improve conductivity and decrease corrosion of anodes is used. Acid baths apply the solution of fluoride-boride stannate and boric acid. Acid baths result in segregation of diatomic tin Passivation of tin coatings At low temperatures, tin covers with a layer of SnO, while at higher temperatures it covers with a SnO 2 coating. At normal temperatures, the increment of this layer is approx. 2 Ǻ (0.2 nm) in a week and 5 Ǻ n a year. However, at the temperature of 200 C, it is 20 Ǻ and the coating has gold-like color. This natural passivation is applied for protection against black spots, which are results of mapping of tin by reaction with sulphur contained in food. Natural passivation is not uniform; the final coating does not have uniform color. This can be solved via chemical passivation consisting in immersion of a component into a solution (e.g. of trisodium phosphate 9 g/l, sodium dichromate 3 g/l, sodium hydroxide 20 g/l fluxing agent 3 g/l) with temperature of 85 C for approx. 30 s. Rinsing and drying follow

44 3.8. Chromium coating Fig Fe-Sn binary diagram Decorative chromium coating is one of the most impressive galvanic processes from the viewpoints of appearance, as well as physical properties. Chromium coatings are highly resistant against atmospheric corrosion at normal and elevated temperatures, they are also very hard and resistant against mechanical wear and exhibit low friction coefficient. Lustrous chromium coatings exhibit high reflectivity of light, which results in highly lustrous final surfaces of coated products Analysis of metal layer Measurement of thicknesses of layers and coatings are performed on metallographically prepared cuts or by grinding of a dimple into the surface (Calotest method). If the layer of coating is very thin, it is hardly analyzable on a perpendicular cut, especially via optical microscopy observations. Samples cut under a certain angle, on which the coating layer appears thicker, are prepared in such cases. The real thickness of layer or coating can then be calculated from the sloped cut if the cutting angle is known. Observations of microstructures of layers and coatings are performed using optical, scanning or transmission electron microscopies. Optical microscopy uses visible light to display structures and thus it has quite low resolution. Its applicability for observations of layers and coatings is thus significantly limited. Scanning electron microscopes have better resolution. To display structures, these devices use an electron beam generated by the effect of high voltage (usually 5-30 kv) on a heated tungsten filament or so called auto-emissive jets crystals with the ability to easily release electrons. For their operation they need vacuum. Electro-static or electro-magnetic lens (coils) are used to focus electron beams. This set is used to deflect electron beam to gradually move across the surface of the sample. The affected area of the sample then reflects secondary and back-scattered electrons. Secondary electrons (SE) provide information about surface relief, similarly to optical microscope, while back-scattered

45 electrons (BSE) give information about chemical composition of material and provide so called material contrast. Contact signals from individual points are digitally processed and displayed on TV screen or computer. Interaction of electrons with atoms in the vicinity of sample surface results also in radiation of characteristic x-rays. The rays provide information about chemical composition of the observed area. Analysis of characteristic x-rays can be performed using EDS (energy-dispersive) or WDS (wavelength dispersive) spectrometry. Common scanning electron microscopes are equipped with EDS chemical composition analyzer. Using these devices, chemical microanalysis can be performed. Questions to chapter 4 1. Describe corrosion. 2. How can steel products be protected against corrosion? 3. How is atmosphere aggressiveness defined? 4. Define the difference between mechanism of anticorrosion protection of copper and aluminum. 5. Describe the principle of hot-dip galvanizing. Define processing temperatures. 6. Characterize the preparation of initial product before metal coating. 7. Define the course of reaction between iron and zinc. 8. Characterize individual phases of zinc coating. Define the difference between low-temperature and high-temperature zinc coating from the viewpoint of phases. 9. Define the influence of silicon on zinc coating structure. 10. Define impurities commonly occurring in zinc bath and their influences. 11. Define the difference between standard and thick zinc coating. 12. Design from a to z your own wire galvanizing line. Focus on the best possible economy of the process while ensuring the best possible quality of zinc coating. 13. How do you calculate how much zinc is needed for coating of a given amount of wire? 14. Define the basic difference between two commonly used technologies of electrolytic zinc coating application. 15. Define the difference between zinc coating generated via electrolytic and hot-dip methods. 16. Explain the principles of conversion copper coating. 17. How do you calculate the amount of copper, which is needed to coat a given amount of wire? 18. How can the amount of deposited copper be controlled? 19. Define the process of hot-dip galvanization. Compare corrosion resistances of Zn and ZnAl coatings. 20. When tinning is used? 21. Characterize metals passivation (explain using the example of Sn passivation). 22. Define the contemporary possibilities of evaluation of metal layer quality