MATERIALS INFORMATION SERVICE

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1 MATERIALS INFORMATION SERVICE The Materials Information Service helps those interested in improving their knowledge of engineering materials and highlights the national network of materials expertise. This Profile is one of a series produced by the Materials Information Service. For advice relating to your particular materials problem, you can contact the MIS at: The Materials Information Service The Institute of Materials, Minerals and Mining Danum House, South Parade Doncaster DN1 2DY Tel: Fax: MIS Profiles are produced by IOM Communications Ltd, a wholly owned subsidiary of the Institute of Materials, Minerals & Mining

2 CORROSION Ref: 3/95 Introduction Steels are undoubtedly the most successful group of engineering materials. They enjoy widespread use throughout most industries and can attain an unrivalled range of affordable mechanical properties. The current world production of steel is about 780 million tonnes per annum, and is predicted to rise to about 930 million tonnes by the year The Profile concentrates upon the mass-market engineering steels, and indicates the general effects of adding various amounts of carbon and other elements to iron. Steels contain carbon in amounts ranging from very small (about wt% in ultralow carbon, vacuum degassed steels), to a maximum of 2.00 wt% in the highest carbon tool steels. Iron containing over 2.00 wt% carbon is classified as cast iron and will be dealt with in a future Profile. Carbon profoundly alters the microstructure and properties in steel. Generally, carbon content is kept low in steels that require high ductility, high toughness and good weldability, but is maintained at higher levels when high strength, high hardness, fatigue resistance and wear resistance are required. The performance of a steel component can be as highly dependent upon its thermal treatment as its alloying additions. This review refers to thermal treatment and cold working effects but does not attempt to explain the theories in great detail. The interested reader is referred to the section Useful Texts. Structure of Steel The component elements in steel can be categorised in terms of their crystal structures. At least a basic knowledge of the practical implications of these crystal arrangements is essential to understand the performance of steel in service. The structures are dependent upon the concentrations of each element, the fashion in which the steel is cooled from furnace temperatures, and the amount of cold work performed on the steel. Crystal Structures Ferrite, α, is the crystal arrangement for pure iron. This form exists as part of the structure in most steels and can usefully absorb carbides of iron and other metals by diffusion in the solid state. Ferrite takes a body centred cubic (bcc) form and is soft and ductile.

3 Austenite, γ, is a solid solution, that is, the component elements are arranged as if in solution (it also exists as an allotrope of pure iron). All steel exists in this form at sufficiently high temperatures (see Figure 1). Some alloy steels stabilise this singular phase and it is present even at room temperatures. The crystal arrangement is face centred cubic (fcc) and, like ferrite, it is soft and ductile. Cementite is iron carbide (Fe 3 C). When carbon atoms can no longer be accommodated in solution in ferrrite and austenite (due to an increase in carbon content or reduction in temperature), cementite forms, as it can accommodate more carbon in its crystal structure. Like other carbides, it is hard and brittle. Pearlite is a phase mixture consisting of alternating platelets of ferrite and cementite (α+ Fe 3 C) which grows by conversion from austenite. A steel containing 0.77 wt% carbon can consist solely of pearlite if cooled sufficiently slowly from austenite (see Figure 1). Under the microscope it can have an iridescent mother-of-pearl appearance, hence the name. Fig.1: Part of the equilibrium phase diagram for the Fe-C system Martensite is commonly found in steel that has been rapidly cooled ( quenched ) from austenite (see Thermal Treatment below). It is a particularly hard, brittle arrangement. Essentially it forms because any carbon in solid solution in austenite at high temperatures does not have enough time to be incorporated into cementite when cooled rapidly. The austenite crystals undergo a transformation involving the shearing of atom planes over each other. Martensite does not appear on the phase diagram (Figure 1) as it is not an equilibrium phase. The strain energy involved in the martensitic reaction is enormous and a large undercooling is necessary. In low and medium carbon alloys, the martensite tends to form in lath-shaped crystals that are generally too fine to resolve in the light microscope. In high carbon steels, plate

4 martensite forms. For certain steels, the rapid cooling necessary to produce a martensitic structure (e.g. water or brine baths) introduces large surface tensile stresses and may cause quench cracking. However, when medium carbon steels are alloyed with elements such as nickel, chromium and molybdenum, the development of equilibrium phases is suppressed and martensite can be formed with less drastic cooling, such as oil quenching. If the steel is cooled such that the formation of pearlite by the short range diffusion of iron atoms is not possible, bainite can be produced. The bainite that forms at temperatures just below those at which pearlite forms is termed upper bainite (see Figure 2). At lower temperatures, lower bainite forms. Both lower and upper bainite consist of aggregates of platelets or laths of ferrite, separated by regions of residual phases consisting of untransformed austenite or of phases such as martensite or cementite. Thermal Treatment Conditioning of steel by thermal or heat treatment relies on the different mechanical properties which are exhibited by the structures above. Figure 1 illustrates the equilibrium structures present at different temperatures with changing carbon content for the iron-carbon system. Figure 2 demonstrates the effect of chill rate upon final structure, and is called a time-temperature-transformation or TTT diagram. Essentially, when cooling from the melt or high temperature phases, there is an incubation period below the equilibrium melting point or transformation temperature (723 0 C in the case of the steel shown) before the transformation occurs. This undercooling provides the driving force for the transformation. During a furnace cool (i.e. slow cooling rate) the austenite will start to transform to ferrite and cementite after sufficient undercooling, resulting in a microstructure of coarse pearlite. With a high cooling rate, such as experienced with a water quench, it is possible to miss the nose of the TTT curve altogether. Martensite is produced, starting at about C for the composition shown. The finish temperature of the martensite reaction for certain alloys can be below room temperature, so that at room temperature some unstable austenite is present.

5 Fig.2: The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at C The design of steels and cooling conditions to produce required amounts of martensite has its own branch of technology, hardenability. In plain carbon steels, the nose of the TTT curve occurs at very short times, hence fast cooling rates are required to produce martensite. In thin sections of steel, a rapid quench can produce distortion and cracking. In thick plain carbon steels, it is not possible to produce an all martensitic structure. All common alloying elements shift the nose of the TTT diagram to longer times, thus allowing the development of martensite in thick sections at slower cooling rates. Thermal treatment of steels will be dealt with in a future Profile. Work Hardening Resistance to continuing plastic flow as a metal is worked is termed work hardening. When work is performed below hot-working temperatures (i.e. below about 0.5T m, where T m is the melting point), and the crystal structure is forced to deform to accommodate the strain, microscopic shearing (or slip) occurs along definite crystalline planes. Discontinuities in the crystal structure, present in all metals and known as dislocations, increase in density during plastic flow and those moving on intersecting slip planes tangle and pile up. This means that an everincreasing shear stress is required for deformation, increasing the yield stress. Eventually the stress required to move dislocations is high enough for a crack to

6 initiate and subsequently propagate, and the material breaks. Fig.3 demonstrates the effect of work hardening during a tensile test. Fig.3: Loading and unloading cycles in a tensile test demonstrating work hardening Most steels with appreciable alloy content possess a complex crystal structure resulting in numerous potential slip planes and intersection points, consequently most engineering steels are highly susceptible to work hardening. Work hardening improves tensile strength, yield strength and hardness at the expense of reduced ductility (see Table 1). These effects can only be removed by annealing or normalising.

7 Table 1: The effect of heat treatment and condition on the properties of plain carbon steels Plain Carbon Steels Carbon steels are supplied in the as-rolled, normalised, or hardened and tempered condition, with the best properties developed by hardening and tempering. The effect of carbon content on the tensile strength, elongation to failure and hardness of annealed plain carbon steel is shown in Fig.4.

8 Fig.4: The effect of carbon content and heat treatment on the typical properties of plain carbon steel Very low carbon content (up to 0.05%C) These steels are ductile and have properties similar to iron itself, they cannot be modified by heat treatment. They are cheap, but engineering applications are restricted to non critical components and general panelling and fabrication work. Low carbon content (0.05% to 0.2%C) e.g. 080M15, 150M19, 220M07, AISI 1006, AISI 1009, AISI 1020 Such steel cannot be effectively heat treated, consequently there are usually no problems associated with heat affected zones in welding. Batches which are free of tramp elements such as chromium are ductile with good forming properties, as little work hardening is exhibited. However, chromium as low as 0.1% and vanadium and molybdenum contents as low as 0.05% can have a dramatic effect on hardenability. Surface properties can be enhanced by carburising and then heat treating the carbon-rich surface. High ductility results in poor machinability, although these steels can be machined if high spindle speeds are employed. More commonly sulphur and lead are added to form free-machining inclusions. Low quality steels with high quantities of sulphur and phosphorus will have better machinability than good quality steels which are clean and free from oxides and slag inclusions. This group represents the bulk of the market for general purpose steel, finding usage in car bodies, ships and domestic appliances. Stainless steels and aluminium alloys compete with these steels in certain areas.

9 Medium carbon content (0.2% to 0.5%C) e.g. 070M20, 080M40, 216M44, AISI 1023, AISI 1030, AISI Heat treatment and work hardening are now effective methods for modifying mechanical properties. Hardenability increases in proportion to carbon content. Welders must now take note of the hardening effects in the heat affected zone and take precautions against excessive energy input, as increased hardenability results in an increased likelihood of brittle structures forming. All common alloying elements increase the hardenability and hence a carbon equivalent scale has been devised as an approximate guide to weldability (see below). In the normalised condition, machinability is improved compared with low carbon steels due to their lower ductility and it can be further enhanced with the addition of sulphur or lead if special free-machining properties are required. Ductility and impact resistance is, however, reduced. The corrosion resistance of these steels is similar to low carbon steel, although small additions of copper can lead to significant improvements when weathering performance is important. Most steels in this category contain some silicon and manganese, added as de-oxidising and de-sulphurising elements during manufacture (see below). While the quantities present are not considered to effect mechanical properties, an indication of the quality of the steel is given by the phosphorus and sulphur content, where the lower the content, the higher the quality. This category represents medium strength steels which are still cheap and command mass market. They are general purpose but can be specified for use in stressed applications such as gears, pylons and pipelines. Medium-high carbon content (0.5% to 0.8%C) e.g. 070M55, 080M50, AISI 1055, AISI 1070 These steels are highly susceptible to thermal treatments and work hardening. They easily flame harden and can be treated and worked to yield high tensile strengths provided that low ductility can be tolerated. For example, spring wire in this category can have an ultimate tensile strength (UTS) >2GPa. Clearly, welders must take care to prevent heat affected zone (HAZ) cracking with these steels, and specialist advice should always be obtained. The carbon equivalent can be used to evaluate potential welding problems. Although high strengths and hardnesses are attainable, impact strengths are poor. These steels are not normally used in stressed applications subjected to shock. They are used where hardness is valued, such as for blades, springs, collars, etc. High carbon content (>0.8%C) e.g. 050A86, 080A86, AISI 1086, BS 1407 Cold working is not possible with any of these steels, as they fracture at very low elongation. They are highly sensitive to thermal treatments. Machinability is good, although their hardness requires machining in the normalised condition. Welding is not recommended and these steels must not be subjected to impact loading. These steels can have UTSs greater than 1 GPa, and care needs to be taken to avoid hydrogen embrittlement following electroplating. Advice should be sought from

10 the plating-shop. As with the medium-high plain carbon steels, steel with >0.8%C is used for components requiring high hardness such as cutting tools, blades, etc. Alloy Steels By the addition of alloying elements such as chromium, nickel, molybdenum and vanadium it is possible to increase the hardenability of a carbon steel, along with other properties, such as corrosion resistance and fatigue strength. The general trend of improved response to heat treatment and cold working in proportion to carbon content, described for plain carbon steel, applies equally to alloy steels. However, final properties are sensitive to the alloying additions as well. The number of alloy steels available makes the choice of a steel for any given application difficult, and in most cases there will be a number of steels that would meet the requirements. Chromium (Cr) can improve general high temperature properties, and also corrosion and oxidation resistance. It forms carbides with the available carbon in preference to iron which aids carburisation. It also slows down metallurgical reactions, thus increasing hardenability. Chromium results in larger grained structures which can cause problems as a result of the associated poorer mechanical performance. (See below for high chromium content steels) Nickel (Ni) lowers critical heat treatment temperatures and generally allows for easier conditioning. Nickel strengthens and toughens steel by dissolution into the ferritic matrix. It is particularly valued in low temperature service where impact strengths can be maintained at sub-zero temperatures. Vanadium (V) is a very good carbide former, although it is also useful as a deoxidiser. Vanadium carbides are particularly fine and evenly distributed, and they provide the best grain refining properties, which generally improves mechanical properties. Vanadium carbide is very hard and has a stabilising effect on other carbides (notably chromium carbide) which might otherwise precipitate, causing grain growth and brittleness during heat treatment. Vanadium forms nitrides and consequently is often present in nitriding steels. Molybdenum (Mo) like vanadium, usefully yields a fine grain structure with consequent improvements in overall strength. This fine structure is a result of the stable, even distribution of molybdenum carbide. These carbides also serve to stabilise steels with nickel and chromium additions which can otherwise show temper brittleness due to carbide precipitation. Molybdenum also enhances corrosion resistance in stainless steels. Tungsten (W) forms carbides which are exceptionally hard. These carbides are beneficial in a similar fashion to molybdenum carbides, although far greater concentrations are required. Tungsten is valued in steels requiring hardness with stability at high temperatures, for example, tool steels.

11 Boron (B) is able to improve hardenability in concentrations as low as 0.001%. This particularly sensitive behaviour is only effective with low to medium carbon steels. Despite increasing hardenability, these steels are still easily welded and are often specified where controlled hardenability in the weld is required. It is not known for steel to use boron as the sole alloying element, but it is frequently found in conjunction with other elements such as vanadium, chromium and molybdenum, as it also increases their hardening effect. Copper (Cu) generally enhances corrosion resistance, although if it is present as a tramp element, possibly due to poorly separated scrap, it can be disastrous, causing grain segregation during hot working. Cobalt (Co) is never present alone, but always as an addition to alloy steels. It is not a carbide former but dissolves in the ferrite matrix, like nickel and silicon. Additions of up to 30% cobalt to ferrous alloys have a significant effect on the materials magnetic properties. Cobalt can not only strengthen the ferrite, but also appears to stabilise the carbides and maintains their properties to much higher temperatures. Titanium (Ti) forms very stable carbides, combining with carbon in preference to iron and chromium. For the titanium to combine with all the carbon, a minimum of eight times as much titanium as carbon is used, resulting in titanium stabilised weldable austenitic steels. Aluminium (Al) is a good deoxidiser, but alumina (aluminium oxide) is a brittle material which can be a damaging inclusion in steel. Aluminium can, however, increase the ability of the steel to nitride and has some grain refining properties. Manganese (Mn) is a useful deoxidiser and desulphuriser as the oxides and sulphides are particularly ductile and harmless. It is found in almost every steel for these reasons. In fact, because it is such a common addition, it is often omitted from specifications unless it is present in quantities >2%. Manganese lowers heat treatment temperatures and can give a wholly austenitic steel in concentrations greater than about 15%. These steels are non-magnetic. Manganese also strengthens steel and can yield high carbon steels that are tough and workable. It should be noted that manganese tends to increase the likelihood of quench cracking. Silicon (Si) is a cheap and harmless deoxidiser found almost without exception in steels. It raises heat treatment temperatures and forms graphites which are useful for decarburising. At levels above about 0.5%, silicon can increase corrosion resistance and fatigue strength, although not to the same extent as other alloying elements. High Chromium Steels Additions of chromium in excess of 12% gives rise to a stable surface film of chromium oxide, the stability of the film increasing with increasing chromium content.

12 This oxide film confers corrosion resistance and is the basis on which the stainless steel family is built. Low carbon steels containing 12-30% chromium are the ferritic stainless steels (e.g. 430, 409) which are not heat treatable. Increases in mechanical properties can only be achieved by cold working. The corrosion resistance of this group is significantly better than the high carbon-high chrome steels. High carbon-high chrome steels are heat treatable as a consequence of the higher carbon content, and are known as martensitic stainless steels (e.g. 410, 416). They do, however, exhibit lower corrosion resistance due to chromium depletion of the oxide film. They exhibit good strength and oxidation resistance up to C, although their creep strength above C is poor. Austenitic stainless steels (e.g. 302, 316) result from additions of nickel (usually between 10-20%) to low carbon steels containing 18-25% chrome. These steels exhibit superior corrosion resistance in a wide range of environments. The properties can only be modified by cold work. They are also significantly more expensive than the straight chromium grades. When mention is made of stainless steel it is generally these non-magnetic steels that are being referred to. While the thermal expansion of these steels is similar to that of copper, their thermal conductivity is less than that of alumina at room temperature. Precipitation-hardening stainless steels (e.g PH, PH 13-8 Mo) are chromium-nickel alloys containing precipitation hardening elements such as copper, aluminium or titanium. The alloys are of two general types: semi-austenitic, requiring a dual heat treatment to achieve final strength properties. The main advantage of these alloys is the low temperature heat treatment required to achieve final strength, which can be as high as 2 GPa, resulting in minimal scaling and distortion, thus enabling parts to be finished machined prior to final heat treatment. It should be noted that chromium has a tendency to migrate to grain boundaries at elevated temperatures, where it forms chromium carbide. This is a series problem in the heat affected zones of welds. This effect is known as weld decay and causes failure due to corrosion along grain boundaries where there is a depletion of chromium. For welding, a carbon content <0.03% is specified to avoid significant carbide formation. Alternatively, the steel can be stablised with the addition of titanium or niobium which form carbides in preference to chromium. Although stainless steels are more corrosion resistant than other steels, they are subject to specific corrosion mechanisms, such as weld decay. Advice must be sought for particular applications. Selected Special Steels High strength-low alloy (HSLA) steels are a group of low-carbon steels that utilise small amounts of alloying elements to attain yield strengths in excess of 275 MPa in the as-rolled or normalised conditions. These steels have better mechanical properties than as-rolled carbon steels, largely by virtue of grain refining and

13 precipitation hardening. Because the higher strength of HSLA steels can be obtained at lower carbon levels, the weldability of many HSLA steels is at least comparable to that of mild steel. They can allow more efficient designs with improved performance, reductions in manufacturing costs and component weight reduction. Applications include oil and gas pipelines, automotive beams, offshore structures and shipbuilding. Maraging steels differ from conventional steels in that they are hardened by a metallurgical reaction that does not involve carbon. These steels are strengthened by intermetallic compounds such as Ni 3 Ti and Ni 3 Mo that precipitate at about C. These steels typically have very high nickel, cobalt and molybdenum contents while carbon is essentially an impurity and its concentration is kept as low as possible in order to minimise the formation of titanium carbide, which can adversely affect mechanical properties. Ultra-high strengths may be obtained with these steels, and weldability is good. Toughness is superior to all low alloy carbon steels of similar strength, particularly the low temperature toughness. Although they are expensive, they are easy to machine and heat treat, so that some economies result in component production. The Editor thanks John Donlon of MIS Midland Region and Ian Marr of MDP Associates for their help and advice in the preparation of the Profile. Useful Addresses Engineering Steels While British Steel do not currently operate a Technical Advisory Service for general enquiries, the product-based advisory services may be able to help, e.g.: Strip Products Advisory Service Tel: x 4090 Plates Advisory Service Tel: x231 Structural Steel Advisory Centre Tel: UK Stockholders National Association of Steel Stockholders Gateway House High Street Birmingham B4 7SY Tel: Fax:

14 Stainless Steel Stainless Steel Advisory Centre c/o The Institute of Materials The Innovation Centre 217 Portobello Sheffield S1 4DP Tel: Fax: Nickel Development Institute European Technical Information Centre The Holloway Alvechurch Birmingham B48 7QB Tel: Fax: Specifications World Metal Index Sheffield Libraries & Information Services Central Library Surrey Street Sheffield S1 1XZ Tel: Fax: Useful Texts Properties & Selection: Irons, Steels & High Performance Alloys ASM Metals Handbook, Vol 1 (1990) ASM International Materials Selector N A Waterman & MF Ashby (1994) Chapman & Hall Bainite in Steels HKDH Bhadeshia (1992) The Institute of Materials Metals Databook Colin Robb (1988) The Institute of Materials Metallic Materials Specification Handbook Robert B Ross (1992) Chapman & Hall Iron & Steel Specifications 1989 British Steel plc Worldwide Guide to Equivalent Irons & Steels 1993 ASM International