Module 38. Structural steel I. Lecture 38. Structural steel I

Transcription

1 Module 38 Structural steel I Lecture 38 Structural steel I 1

2 Keywords : Killed, semi killed & rimming steel, properties of structural steel, steel specification, strong structural steel, mechanism of strengthening, strain hardening, grain refinement, solid solution strengthening, precipitation hardening, cold & hot working Introduction By now you have a fairly good idea on the basic concepts of physical metallurgy. Next few modules will be devoted to the applications of physical metallurgy. The main focus will be to illustrate with examples how the basic principles that you have learnt so far have been used to improve the performance of a variety of engineering materials. Let us begin this module with structural steel, the most widely used and abundantly available material of construction. Bulk of steel produced all over the world comes under this category. These are used as load bearing components. These are available in various forms and shapes like plates, rods, sheet, I section, T section, channels, angles, rods, wires, wire ropes, bars, pipes, and tubes. It is the main material of construction of buildings, sheds, ware house, hangers, bridges, towers, power plants, refineries, automobiles, commercial vehicles, ships, railway carriages, storage vessels, pipe lines, and varieties of other applications. The main features or characteristics of structural steel are as follows: Cheapest material of construction (next to cement and concrete) Accounts for 90 95% of world steel production %C < 0.25 virtually plain carbon steel Residual elements present: S, P, Mn & Si Microstructure consists of mostly ferrite and a little pearlite Fairly good strength and ductility Good hot and cold workability Poor hardenability Good weldabilty 2 The last three decades have seen significant improvement in the strength and toughness of structural steel. This has led to a substantial reduction in the weights of the load bearing structures of automobiles commercial vehicles and rolling stocks. This has made these more fuel efficient. This has been achieved primarily by thermo mechanical processing with the addition of exceedingly small amounts of alloying elements having strong affinity for carbon. The improvement in properties is mainly due to grain refinement. This is the only strengthening mechanism that can improve both the strength and the toughness of steel. It needs a much more precise control on the processing parameters during every stages of steel making. The concurrent development of equipment and facility to maintain these within close limits were also crucial for the successful implementation of such basic concepts into practice. The quality of steel depends to a great extent on the processing route and the ability to control harmful elements (S, P, O, N, H, etc) within a specified limit. The bulk of steel today comes from the BF

3 (Blast Furnace), BOF (Basic Oxygen Furnace) and CC (Continuous Casting) route or the DRI (Direct reduced Iron), EAF (Electric Arc Furnace), and CC route. A major trend today is the increased use of recycled steel scrap. This is often a source of several harmful elements (Sn, Cu, etc) which must be removed during steel making. Introduction of large scale vacuum degassing, stream degassing, ladle treatment and vacuum arc re melting has also played a major role in the improvement in the quality of steel being produced today. Oxygen is used to remove carbon from molten iron. A part of it remains in solution when the amount of carbon in iron goes down. Unless it is removed it would be impossible to produce defect free casting or to make alloy addition. Let us first look at the basic principle involved in the control of the amount of oxygen present in steel with respect to ingot route of steel making which is still practiced for certain grades of steel. Killed, semi killed & rimming steel: Oxygen is used to remove the impurities (unwanted solutes) present in steel. The list of impurities includes C, S, P, etc. Once these are removed to the desired level, a part of the oxygen remains within the molten iron. At a given temperature there is an inverse relationship between the concentration of dissolved oxygen [O] and carbon [C] in the molten iron. This is shown with the help of a set of %C versus %O plots at two different temperatures in a diagram in slide1. The solubility product defined as ([C] [O]) is a function of temperature. It decreases as the temperature of the molten iron drops as it is cools in an ingot mold. The dissolved oxygen in steel makes it brittle. It is removed effectively by adding elements having very strong affinity for oxygen in the form of Ferro Alloys. These are known as de oxidizers. Elements like Si or Al reacts with dissolved oxygen to form solid particles of oxides. Some of these may float up and get removed while the steel is still molten but a substantial part may remain entrapped within the dendritic channels during solidification. These are retained as inclusions within the steel even after it has solidified. Fully de oxidized steel is known as killed steel. It solidifies within the mold quietly without much agitation. Solidification of steel is accompanied by shrinkage. Every attempt is made to impose the condition of directional solidification so that it begins at the bottom and the top is the last to solidify. It is often necessary to use hot top or to add exothermic mixture at the top of the solidifying steel. Slide 1 also shows the solidification behavior of molten steel with a set of ingots under three different conditions. 3 Killed steel: It is characterized by a pronounced shrinkage cavity. This is often referred to as pipe. A part containing the pipe has to be cropped and scrapped. Therefore the yield is low. This is why ingot route for killed steel has now been replaced by continuous casting for bulk steel production. Killed steel has little dissolved oxygen. It is ductile it has low DBTT. However it is not so clean. It has inclusions.

4 Rimming steel: De oxidation can also be carried out by allowing the residual carbon to react with the dissolved oxygen in molten steel as it cools within an ingot mold. The excess carbon and oxygen reacts to form bubbles of CO. The bubbles float up due to the difference in density causing severe agitation within the molten metal. This is called rimming action. The product is known as rimming steel. The main characteristic of this process is the absence of shrinkage cavity as it is offset by the presence of a large number of tiny CO gas bubbles. The pores in the ingot get welded during subsequent rolling / hot working. The steel is relatively clean as there are no solid oxide inclusions. The yield too is high. The rim of the ingot is extremely pure. However rimming steel does have some amount of dissolved oxygen. Therefore DBTT of rimming steel is higher than that of the killed steel. Killed, semi killed & rimming steel Bulk steel production: impurities in hot metal are removed by oxidation. It picks up oxygen. The above classification is based on how it is removed. %C T Slide 1 %O C + O = CO [C] [O] = k : f(t) killed SK rimming Ingots As T decreases this rimming reaction (CO bubbles) occurs. Semi killed steel: This is where de oxidation is partly due to addition of ferroalloys and partly due to the rimming action. As a result the size of the pipe that forms due to shrinkage is relatively small. The yield is therefore a little better than that of the killed steel. Continuous casting (CC): 4 Bulk of steel today comes from either BOF/CC or Electric Arc Furnace / CC route. Molten steel from either BOF or EAF comes to the casting unit in ladle. A number of operations like deoxidation and alloy additions before casting are done while it is in the ladle. Figure 1 gives a

5 sketch showing different parts of a continuous casting unit. Liquid steel from the ladle is transferred into a tundish where there is a protective layer of slag. It prevents pickup of impurities from the surrounding. Liquid steel from the tundish enters into a water cooled copper mold through a submerged entry nozzle. A layer of molten and solid flux cover is maintained over the liquid within the mold. As solidification begins from the mold wall, excess solute present in steel gets rejected into the liquid where some these might react to form inclusions that float up and get captured by the flux. This is how a relatively homogeneous continuously cast billets or slabs are produced. Although the setup is expensive the cost of steel produced by this route is significantly less because of higher yield, high productivity, lower rejection and fewer downstream processing steps. Above all the quality of steel is far superior. However it needs more precise control of temperature and composition of liquid steel. Some of these are achieved through vacuum degassing and ladle refinement. During this stage some of the harmful elements are removed and some useful elements are added. This is often referred to as secondary steel making. It has been covered well by Ahindra Ghosh in his book on Secondary Steelmaking: Principles & Application, CRC Press (2000). Protective slag layer Liquid steel Mold flux Tundish Submerged entry nozzle Copper mold Impurities move into the liquid Billet Fig 1: Shows a schematic representation of different parts of a continuous casting (CC) unit. Solubility of impurities in solid is less. Therefore atoms of such elements move into the liquid during solidification. This helps minimize the level of segregation in the cast billet or slab. 5 Properties of structural steel: Microstructure of structural steel consists of ferrite and pearlite. Ferrite is soft whereas pearlite is hard. The amount of pearlite increases as %C increases. The strength of structural steel

6 depends on the amount of pearlite and its lamellar spacing. Finer the pearlite higher is the strength. Usually structural steel is used in normalized condition (air cooled). The degree of fineness is therefore dependent on the section size of the component. Slide 2 shows the effect %C on the mechanical properties of steel. It follows the rule of mixture. Note that the increase in strength is accompanied by loss of ductility. The slide also includes a table giving the lower and the upper limits of strength and ductility. Note that P stands for pearlite and CR denotes cooling rate in this slide. Effect of % C on properties of + P steel %RA %El BHN UTS YS %C BHN UTS YS %El %RA % C 0.8 Strengthening follows rule of mixture. It increases with increasing % P. CR depends on section size. Slide 2 Steel specification: 6 The properties of steel depend primarily on its composition and processing. The bulk of structural steel are hot rolled and air cooled. Besides C it also has certain amounts of impurities (unwanted elements like S & P) and some amount of intentionally added Si (as deoxidizer) and Mn (added primarily to reduce the harmful effects of S on the properties of steel). As a result the phase diagram and the CCT (or thettt) diagram of such steel are likely to be different from those of the plain carbon steel. With increasing concentration of alloying element %C in Pearlite decreases. It means that the effective concentration of C in steel increases as the concentration of alloying elements increases. Often to interpret the structure of steel the contribution of each alloying element is expressed in terms of its carbon equivalent. The expressions for carbon equivalents are derived empirically. These are extremely useful in predicting the structure, hardenabilty and HAZ (Heat Affect Zone) of steel. One of the earliest and still popular expressions is the one introduced by the International Institute of Welding (IIW). This is as follows: (1)

7 It was found so useful that it was incorporated either as it is or with a little modification in several material standards. For example AWS (American Welding Society) introduced an additional factor, Si/6 to include the effect of Si which is invariably present in almost all grades of steel. ISS AISI- SAE BS old BS 970 UNS Steel specification C07: %C ~ 0.07; C15: %C ~ 0.15; 15Mn1: 0.15C 1Mn; 25Mn1S14 (?) xx25: %C~0.25; xx=10 carbon steel, 11: resulfurized; 13: Mn steel; 40: Mo steel; 41: CrMo; 43:NiCrMo; 51:0.8Cr; 52:1.45Cr; 61: CrV, 92: SiMn: Example: C steel; 52100: ball bearing steel 1.45Cr1C En 24 ~ SAE4340: 1.8Ni0.8Cr0.25Mo0.4C xxx (A/M/H) xx: A: analysis; M: mechanical property; H: hardenability; last digits: C Unified numbering system: G: carbon steel; S: stainless; T: tool steel; UNS G (?) Slide 3 7 Although it is possible to make steel of various compositions by a variety of processing routes it is necessary to classify these into different groups and grades for the benefit of the steel producer and the user. The need was felt ever since the time steel became available in bulk for the construction of variety products and structures. One of the earliest and well documented systems of describing the types of steel was that developed by AISI (American Iron and Steel Institute) and SAE (Society of Automobile Engineers). A numerical index system consisting of 4 digits was introduced to represent a specific grade of wrought steel. The numbers are used in such a manner that it gives at least a partial description of the composition of steel. Alphabets are also used to denote the presence elements like B (Boron) or L (Lead). Table 1: Numerical Index System used by AISI / SAE for the designation of wrought steel Designation Type of steel Remarks 10xx Plain carbon steel (S & P < 0.05, Mn < 1.65) 1030 means %C is xx Resulphurized carbon steel S added to improve machinability 12xx Resulphurized & rephosphorized steel 13xx 1.75% Mn steel 23xx 3.5%Ni steel 3 denotes % Ni 31xx 1.25%Ni 0.65%Cr steel 1 denotes % Ni 40xx 0.25%Mo steel 0 denotes %Mo = 0.25

8 43xx 1.8%Ni, Cr, 0.25%Mo steel 4340: %C = xxx %Cr means %C is xx Cr, V 81xx 0.3Ni 0.4Cr 0.12Mo 92xx 0.85Ni 2.0Si 93xx 3.25Ni 1.25Cr0.12Mo Similar designations have evolved ever since by several countries and organizations. A set of alphanumeric digits is used to represent different grades of steel. The numbering system often provides some idea about the composition, processing or the yield strength of steel. For example ASTM A27 U60 30 represents casting grade of plain carbon steel. The number 60 denotes its UTS in ksi and 30 = YS. The numbering system does not give any idea about its composition. You need to look at the full specification to get the details. However from the magnitude of YS you get an estimate of its %C. It should be around 0.25 if the same level of YS is to be obtained without any expensive heat treatment process. By and large the designation of steel has evolved over the years by the manufacturers, buyers, society or countries more by convention rather than any scientific basis. Some of the more popular and widely used designation systems for steel and other engineering materials are as follows: ASTM (American Society of Testing Materials) BS (British Specification: OLD / NEW) UN (ISO) UNS (Unified Numbering System: USA) DIN (Germany) AFNOR (France) UNI (Italy) JIS (Japan) GOST (Russia) IS (India) The designation systems that are followed by users / buyers and manufacturers are numerous. Therefore it is necessary to have conversion tables from one system to another. An example of the same is given in table2. 8 Table 2: An example of equivalent grades of plain carbon steel under different designation system ASTM BS UNS DIN AFNOR UNI JIS GOST LC A15 G10150 Ck15 XC15 C15 S15C A20 G10200 C22 C20 C21 S20C MC A30 G10300 C30 XC32 C30 S30C M50 G10500 C50 XC50 C50 S50C HC A78 G10800 Ck75 XC75 C

9 Table 3 gives an idea about the Indian Standard (IS) specifications used for the designation of different types of steel. This is primarily based on the composition of steel. However apart from composition the properties of steel depend on its section size / shape and the processing route. The details are given in the specification. Table 3: An example of the designation system adopted by the Indian Standard (IS) Designation Composition Type C C Plain carbon steel 55Mn1 0.55C 1Mn Medium carbon steel 05Mn C 0.75Mn Low carbon steel 25Mn1S C 1Mn 0.14S Free cutting steel (good machinability) T C Tool steel T105Cr1Mn C 1Cr 0.6Mn Low alloy tool steel T75W18Cr4V1 0.75C 18W 4Cr 1V High speed tool steel Strong structural steel: 9 The strength of steel depends on its composition and the way it has been processed. The simplest way to improve its strength is to increase its %C. This is certainly the cheapest of all alloying elements used to improve its strength. There are several benefits that may accrue from the use of high strength structural steel. It makes structures lighter resulting in significant savings in the transportation and the material handling costs during its entire life cycle. If it forms a part of automobile, aircraft, or railway carriage the impact is much more. It significantly brings down the running cost by making these more fuel efficient. Ever since the fuel crisis of the 1970 s there have been concerted efforts from various sectors to improve the strength of steel without adversely affecting other useful properties of steel. This is where the growing knowledge of physical metallurgy proved to be useful. The four basic methods of improving the strength of metals are as follows: Work (Strain) hardening Grain refinement Solid solution strengthening Precipitation hardening The metals are ductile even though their melting points and elastic modulus are high. This is primarily because of the presence of crystal defects known as dislocations. It is extremely difficult to produce dislocation free metal crystals by conventional processing routes. Extremely thin filaments or whiskers of Fe are known to have been produced by vapor deposition technique. The widths of such whiskers are too small to contain a dislocation. The strength of such whiskers is of the order of 10GPa. This is quite close to the theoretical strength of defect free crystal. However this cannot be a feasible route for the production of ultra high strength steel in tonnage scale. Nevertheless by suitable combination of alloying and thermo mechanical processing steels having nearly the same orders of strength are routinely being manufactured. A few of these are listed below:

10 Cold drawn 1%C steel ~ 3 GPa Bainitic steel 0.8C ~ 1.5GPa Low alloy Martensitic steel: ~ 2GPa Work (Strain) hardening: Metals we use are mostly polycrystalline. Plastic deformation alters the shape of the grains or the individual crystals of which the metal is made. The deformation of individual grains occurs by dislocation glide. The distance through which it can move is governed by its grain size. It cannot cross a grain boundary to move over to a neighboring grain. Every new dislocation that is generated from the dislocation sources would remain within the grain. This results in an increase in dislocation density. The increase in shear strength is proportional to the square root of dislocation density. Slide 4 illustrates with the help of a set of diagrams the nature of stress strain plots of single and polycrystalline metal. Strain hardening I II III SC PC Slide 4 I: Easy glide: interaction with forest dislocation 0 Gb II: Linear hardening: Duplex slip :Cottrell -Lomer locks & jogs III: Softening: cross slip Dislocation density increases with cold work 10 The basic principles were already explained in module 9 & 13. The deformation behavior of SC (a single crystal) depending on its orientation may consist of three distinct stages. The type of dislocation interactions that takes place are indicated in the above slide. Recall that denotes resolved shear stress on a slip plane along a specific slip direction and is the corresponding resolved shear strain. During stage I slip occurs only on one slip system. Therefore the magnitude of strain hardening is negligible. Significant stain hardening occurs during the stage II. This is because of dislocation glide occurring on multiple slip planes and the formation of Cottrell Lomer locks and dislocation pileups. This imposes severe restriction on the average glide distance of a dislocation leading to significant strain hardening. The stage III sets in when the dislocations get stuck up at obstacles (Grain boundary or Cottrell Lomer lock) move over

11 to the cross slip plane. As a result there is some amount of strain softening. The shape of the stress strain plot is a strong function of the orientation of the crystal with respect to its tensile axis. For example the stage I may be totally absent. The deformation behavior of a polycrystalline (PC) metal would depend on the orientation and the number of individual grains in a unit volume. A typical tensile stress () versus strain () plot is shown in slide 4. Unlike multiple stages in the case of SC it has only two stages: elastic and plastic. This suggests that because of the constraints imposed by the neighboring grains deformation occurs in PC only when there is slip taking place on multiple slip systems in every grain. Note that the strength of PC increases with increasing strain beyond its yield point. The yield strength in tension is approximately twice the yield strength in shear. In short the increase in strength due to work hardening may be given by: (2) The dislocation density in annealed metal may be in the range per mm 2. It may increase to around per mm 2 as a result of cold work. It short the dislocation density can increase by four orders of magnitude due to cold work. This suggests that the increase in strength due to strain hardening can be 100 folds. Therefore in principle the strength of low carbon steel could be increased from 100MPa to around 10GPa. However this may be of theoretical interest. Cold work makes steel brittle. Such a material may not be of any use. Grain refinement: Grain boundaries act as dislocation barriers. The total plastic deformation () is proportional to dislocation density (). Recall that, where b is Burgers vector and x is the average distance a dislocation could move by glide. With increasing strain the number of dislocations in the pileups at the grain boundaries increases. When it exceeds a certain number, the dislocation at the tip of the pileup overcomes the barrier to allow further deformation to take place. Using this concept the following relation between the yield strength and grain size (d) can be obtained. This is known as the Hall Petch equation (see module 13). (3) Thus the increase in yield strength due to grain refinement is given by: / (4) 11 The effect of grain refinement is illustrated with the help of a set of diagrams in slide 5. A decrease in grain size from 1mm to 1m can result in a six fold increase in its yield strength. This is the only strengthening mechanism that has no adverse effect on ductility and toughness.

12 Grain refinement Because of back stress pile up can have a limited no. of dislocations Hall Petch relation k 0 d 1mm 1m 600 YS Slide / d Solid solution strengthening: 12 When a dislocation moves in a pure metal it has to overcome only the resistance offered by a periodic array of identical atoms. This is known as the friction stress ( 0 ). In a solid solution however apart from the friction stress it has to overcome the resistance due to the stress fields surrounding the solute atoms present in the matrix. The solid solution strengthening is therefore primarily due to the interaction between the stress fields of a solute atom in the lattice with that of a nearby dislocation. The magnitude of the stress field is a function of the difference in the atomic diameter of the matrix and that of the solute. If the solute atoms are smaller than that of the solvent, the nature of the stress field around it is tensile. Therefore the preferred sites for smaller atoms are the upper half of the slip plane of an edge dislocation. This is because the stress field within this region is compressive. The same logic can be extended to the case of larger atoms. These are surrounded by compressive stress field. The preferred locations of such atoms are beneath the slip plane. Thus every dislocation in a solid solution is likely to have an atmosphere of solute atoms. This is often referred to as Cottrell atmosphere. Slide 6 gives a schematic representation of a dislocation lying on a slip plane where there are a few solute atoms. The solute atoms can be visualized as points where the dislocations are pinned or anchored. Let F be the force needed to unpin the dislocation from an anchor and d be the average distance between two anchors or solute atoms. Higher the concentration of

13 solute atoms shorter is the distance between the anchors. If x f denotes the number of solute atoms / unit area of the slip plane then d is given by 1/. Therefore the force needed to unpin a dislocation from an anchor (or a solute atom) is equal to F/d =. If denotes the applied resolved shear stress, the force per unit length of the dislocation is equal to b. Thus:. The increase in strength due to the addition of an alloying element is given by (5) Solid solution strengthening Solute atoms having different size exerts a force / unit length F/d = b also d 1/ x f Interstitial atoms produces symmetric distortion in fcc but asymmetric in bcc. Offers higher effect as it blocks both edge & screw dislocations G x f Slide 6 F x f G x 0.83b f Substitutional atoms The proportionality constant k 3 in equation 5 depends on the difference between the diameters of the two atoms. The higher the difference higher is the strengthening. However a large difference in diameter puts a restriction on the amount of solute that can be accommodated within the lattice. The constant k 3 also depends on the type of the solute element and the crystal lattice of the parent metal. Interstitial atoms like C & N in iron are more effective in pinning dislocations. The effect is more pronounced in ferrite because it has BCC structure where the interstitial sites are asymmetric. The stress field surrounding such sites in a crystal has hydrostatic as well as shear stress field. Therefore it can effectively pin both edge and screw dislocations. 13

14 Precipitation strengthening: CRSS f Particle looping vs cutting cutting f 1/2 r 1/2 r Looping f 1/2 /r Smaller precipitate: cutting more likely Initial stage: small coherent particles form & f keeps increasing till its limiting value. Strength increases due to both f & r Slide 7 Once f reaches its limit particles coarsen & become large enough to allow looping The strength of steel can also be significantly improved by precipitation hardening (strengthening). Precipitates act as obstacles to the movement of dislocations. There are two ways a dislocation can move beyond a precipitate. It could move through the precipitate by cutting it into two halves. This mode of movement is called cutting. Alternatively it may move beyond the precipitate by leaving dislocation loops surrounding the precipitates. This is known as looping. The mechanism of hardening was explained in detail in module 30. The factors determining the magnitude of strengthening are the volume fraction (f) of the precipitate and its size (radius r). Slide 7 shows the effect of f and r on the shear strength of steel in the presence of precipitates. The figure in this slide suggests that for a given f the stress needed to cut the precipitate increases with r. It reaches a peak when it intersects the stress at which a loop can form around the precipitate. If r increases beyond this, the strength decreases. In order to get the highest strength f should be as high as possible and r should be just large enough to resist cutting and small enough to allow looping. Thus the increase in strength due to precipitation strengthening ( ps ) is given by: (6) The resultant strength of steel due to all the four mechanisms is thus given by: / / (5) 14 You may wonder why martensitic transformation has not been included as a separate mechanism of strengthening. This is because martensite owes its strength due to a combination of several factors. These are solid solution strengthening, fine grain size and high dislocation density.

15 Strength versus ductility (or toughness): Strength vs. ductility Brittle ductile %El TS YS DBTT Hardness % C, %, % Pearlite Slide 8 Grain refinement is the only mechanism that improves YS, ductility, toughness & lowers DBTT 15 High strength is not the only criterion for the selection of materials for structural applications. Often there is an inverse relationship between strength and ductility. You may be able to achieve significant strengthening in steel by increasing its carbon content, by cold work, by giving special heat treatment, by precipitation hardening or by grain refinement. However strengthening is often associated with loss of ductility. Hard drawn steel wires are brittle. Try to bend it. It would crack. Slide 8 shows the nature of plots of ductile and brittle forms of steel. Elastic modulus of steel is insensitive to processing. The initial parts of the two plots are identical. Ductile material is characterized by high ductility (for example annealed steel) whereas brittle material may fail much before plastic deformation sets in (for example hard drawn steel wire). Structural steels normally are not used in hardened and tempered conditions. The most common way of increasing its strength is either by adding carbon or by cold work. Slide 8 shows the effect of these on the strength (TS & YS), ductility (%El) and ductile to brittle transition temperature (DBTT). As you increase %C in steel the amount of pearlite in the microstructure increases. Pearlite being a fine mixture of soft ferrite and hard cementite has much higher strength than that of ferrite. This is why with increasing %C, the strength (TS & YS) of structural steel increases. However %El decreases as YS or TS continues to increase. With increasing strength the DBTT too increases. High DBTT means the material is susceptible to brittle failure. It is certainly undesirable. For most structural applications DBTT should be below 0 C. As against these, grain refinement can improve the strength of steel without any loss of ductility or an increase in its DBTT. This is the current trend in improving the strength of structural steel. This can be achieved by controlled thermo mechanical processing.

16 Cold working: 250 Cold working & annealing of steel Re-crystallization LC steel mostly BHN Recovery Grain growth 0 Temp Annealing temp %C BW CW Recryst FG Cold working increases S/V. It means more nucleation sites Pearlitic area remains elongated Slide 9 16 Slide 9 illustrates with the help of a set of sketches how extremely fine grains can be produced in low carbon steel by cold work and annealing. Low carbon steel is made of mostly grains of ferrite. During cold work the grains get elongated. The surface area to volume ratio (S/V) increases. The dislocation density within the grains is much higher. Therefore the net stored energy per unit volume (G v ) too is much higher. The state of cold work is thermodynamically unstable although it could remain in that state indefinitely at room temperature. In principle cold worked steel could be used as it is unless ductility or toughness is an important criterion of acceptance. The cold worked steel can be annealed at a temperature below A 1 (the lower critical temperature). Normal annealing time is 1 hour. Slide 9 shows how the hardness is expected to change with increasing annealing temperature. There may be an initial increase in hardness. This is due to strain ageing. During cold working dislocations move away from the solute atmosphere. With a little thermal activation during low temperature annealing the solute atoms move towards dislocations and occupy positions of lowest energy. This makes the dislocations immobile. Therefore the strength increases. Simultaneously the dislocations too rearrange themselves into a low energy configuration. On the whole during the recovery stage there may not be any significant loss of hardness. However for grain refinement the next stage of annealing called re crystallization is more important. This is the stage where the elongated grains are replaced by a new set of strain free grains. The large S/V ratio and high dislocation density provides more numbers of favorable sites for the nucleation of such grains. Recall the expressions for the critical nucleus size, activation barrier, degree of cold work (stored energy) and nucleation rate:

17 : As cold work (G v ) increases r* decreases (6) : As cold work increases the activation barrier decreases (7) : As cold work increases nucleation rate increases (8) The key to get fine grain structure is to have high prior cold work, high S/V ratio, high nucleation rate, and a relatively lower annealing temperature. A high annealing temperature may lead to excessive grain growth. Therefore it should be avoided. The annealing at a temperature below A 1 does not have any effect on the pearlitic areas. These too get elongated get elongated during cold work. This shape is retained even after annealing. This is because the morphology of cementite cannot undergo any change at such a low temperature and a short annealing time. The microstructure of low carbon steel after cold work and annealing may consist of elongated pearlitic regions in a matrix of ferrite. Such a structure is likely to exhibit anisotropic behavior. The only way to avoid this is to anneal above A 1 or above A 3 temperatures. The ferrite grain size after such treatment may not be that fine. Hot working: 17 Deformation processing of metals above its re crystallization temperature is known as hot working. The temperature of re crystallization depends on the microstructure, the amount of cold wok, the strain rate and the duration of thermal exposure. By and large it is around 0.5 times its melting point in K. The strain rate during hot working is quite high and the time gap between different stages of deformation may be short. Therefore hot working of most metals is done at T m. The temperature at which the last stage of deformation is given is of considerable importance in all thermo mechanical processing. It determines the final microstructure and the properties of the product. During hot working recovery, recrystallization and grain growth may occur. These are thermally activated processes. At higher temperatures the re crystallization time is short. The material softens quickly. Therefore the load on the hot rolling mill is never too high. However in order to get a fine grain structure it may be necessary to increase the amount reduction per pass, lower the finish rolling temperature, and adopt a relatively faster cooling so that the grain growth can be suppressed. The prior austenite grain size also affects the properties of steel after hot working. If the initial grain size is less the final grain size is going to be even finer. If the initial grain size is coarse the final grain size is unlikely to be fine. This is illustrated with the help of a set of diagrams in slide 10. Fine austenite grains provide more nucleation sites. Therefore the final structure is still finer.

18 Effect of austenite GS on hot worked structure Coarse Fine T Fine grain Coarse grain Fine Finer Slide 10 time CCT diagram Finer initial GS finer is the final structure The grain size of austenite also affects the CCT diagram. The diffusion controlled processes like the formation ferrite and pearlite get suppressed. Therefore coarse grain austenite is likely to have relatively more amount of pearlite. Summary 18 In this module we learnt about structural steel and how these are designated. The properties of such steel are determined by its composition and processing. Steel is primarily an iron carbon alloy. %C in bulk of the structural steel is less than Apart from carbon it has Si and Mn. The former is added as a de oxidizer whereas the latter is added to fix S which is invariably present in steel. The total amount of Si & Mn may not exceed 2%. P is the other unwanted element present in steel. It makes steel brittle. S & P should be within 0.05%. We also looked at the ways the strength of steel can be improved. Steel is amenable to all the four mechanisms of strengthening. However, strengthening due to increased carbon or other alloy addition, cold work or precipitation hardening is accompanied by loss of ductility and toughness. Such steel may also become prone to brittle failure because all the three strengthening processes increase DBTT. Grain refinement is the only mechanism that improves strength without any adverse effects on its ductility and toughness. We also saw how by deformation processing grain size can be controlled.

19 Exercise: 1. Microstructure of a cold rolled annealed mild steel sheet shows elongated pearlitic region in equiaxed ferritic matrix. Comment on the processing route that will give this type of microstructure. 2. Name three common grades of steel produced by ingot route. Which of these has higher yield? 3. Why does compressive stress develops at the surface in a case carburized and subsequently case hardened steel? 4. Why low carbon content is preferred in most structural application? Is high strength not a major criterion? 5. What is the composition of the steel designated as 25Mn1S14? What is the role of sulfur in this steel? Answer: 1. Elongated pearlite is a sign of cold work whereas equiaxed ferrite means annealed structure. It looks that the steel was cold worked when both pearlite and ferrite got elongated. Later on annealing just below eutectoid temperature ferrite has recrystallized but pearlite remains untransformed. To modify pearlite shape it must be heated above the eutectoid temperature. 2. Steel produced by ingot route is classified as killed, semi killed and rimming steel. This is based on the way the dissolved oxygen in molten steel is removed. Rimming steel has the highest yield. The oxygen gets removed by the reaction between dissolved carbon and oxygen with evolution of CO bubbles as the steel cools in mold. This gives the rimming action. Formation of solid crust at the top of the ingot does not allow the gas to go out. It remains entrapped as porosity and balance the shrinkage associated with solidification. There is no shrinkage pipe. Therefore the entire ingot without cropping can go for hot rolling. The pores get welded during hot working This is illustrated with the help of following diagram. Here we have high carbon steel with lower Ms & Mf temperature at the surface whereas core is a low carbon steel with higher transformation temperatures (Ms & Mf). Therefore transformation occurs first in core with expected expansion when surface is still soft austenite and therefore it can allow the core to expand (due to transformation) by plastic deformation. When martensite starts forming at the surface accompanied by volume expansion the core which is now strong will inhibit this. This is why there is a residual compressive stress at the surface.

20 T Ms2 Mf2 Ms1 Mf1 S C tc ts core surface Note that transformation would start at tc within core although it is at a higher temperature whereas when transformation starts at surface at ts transformation at core is complete. 4. Often structural steel is supposed to have additional properties like weldability & formability apart from high strength. Low carbon content gives better formability & weldability. Loss in strength due to lower carbon is made up by grain refinement through controlled thermo mechanical processing with addition of micro alloy elements like Nb, V, & Ti. They prevent grain growth during hot rolling. 5. This is a free cutting steel having 0.25% carbon, 1% Mn and 0.14% S. In presence of Mn, S is present as MnS inclusion. This improves machinability. It is mostly used for making nuts & bolts. If Mn is not present then S forms a low melting eutectic between Fe & FeS. This segregates along austenite grain boundary leading to hot shortness. This is undesirable. This is why in most steel S is less than 0.05%. 20