Module 36. Heat treatment of steel VI. Lecture 36. Heat treatment of steel VI

Transcription

1 Module 36 Heat treatment of steel VI Lecture 36 Heat treatment of steel VI 1

2 Keywords : Jominy end quench test, physical significance of hardenability & severity of quench, factors affecting hardenability, effect of tempering on toughness of steel, temper embrittlement Introduction This is the last module on heat treatment of steel. Hardening followed by tempering no doubt gives the best combination of strength and toughness in steel. However the ability of steel to harden depends on its section size, composition, austenitizing temperature, and quenching medium. It is best described by a term called hardenability. It gives an estimate of the depth of hardness. When red hot steel is quenched the cooling rate at its surface is no doubt going be very high but that at its centre will be relatively less. The difference between the two is a function of the section size of the component. Therefore even if you may have 100%M (martensite) at the surface the amount of M at its centre may be low. The distance of the region having 50%M and 50%FP (fine pearlite) from the periphery is defined as the depth of the hardened zone or the depth of hardness. The reason for the selection of such a criterion to determine the depth of hardness is its easy detection by the difference in the etching contrast of the two distinct zones: hard and soft, under an optical microscope. However the depth of hardness depends on several parameters like section size, shape and quenching medium (or cooling rate) for a particular grade of steel. It is not a material parameter or property. It is necessary to introduce the concept of an ideal quenching medium having infinite severity of quenching to define a material parameter which could represent hardenability. Ideal critical diameter (D I ) is such a parameter. It is the diameter of a cylindrical specimen of steel which on quenching in an ideal quenching medium develops a microstructure consisting of 50%M and 50%FP at its centre. A method of estimating the same was described in the last module. It is however a very tedious and time taking procedure to find out the hardenabilty of steel. You also need a very large number of samples. Let us now look at a much more convenient method of determining hardenability of steel. This is called Jominy end quench test. It uses gradient quench technique. Slide 1 describes the procedure with help of a set of diagrams. 2

3 Jominy end quench test (a) (b) Rc x (c) d CR is maximum at the bottom & decreases as you go up. Slide 1 25mm End quench by water & measure Rc hardness D I x (d) A single specimen gives hardenability Jominy end quench test: 3 It uses a cylindrical steel sample of standard dimension. It is 100mm (4inches) long and its diameter is 25mm (1inch). One of its ends either has a collar or a fixture so that it could be held vertically in a device so that its bottom face could be cooled by a stream of water flowing upwards from a tap at a specified distance. The specimen is austenitized at the recommended temperature for sufficiently long time so that it gets transformed into homogeneous austenite. It is then taken out of the furnace and placed vertically in a fixture so that its bottom face can be cooled by a stream of water flowing upwards. This is illustrated with the help of a sketch (a) in slide 1. A set of arrows pointing upwards represents the direction of the flowing water before striking the bottom of the specimen. The flowing water extracts heat from one end (the base or the lower face) of the red hot steel. This is the face having the highest rate of cooling whereas the far end has the lowest rate of cooling. The heat flow is predominantly unidirectional. It may take a few minutes to cool down. Once it is cold a part of the cylindrical face is ground off as shown in the sketch (b) of slide 1. This is to facilitate hardness measurement at regular intervals of distance from the quenched end. Rockwell hardness tester is used to measure hardness. There are fixtures to hold the Jominy test piece on the base of the hardness tester. This has facility to move the test piece axially in steps of 1/16 th of an inch. Hardness is measured in Rockwell C scale. Figure (c) in slide 1 shows a typical hardness versus distance plot. The quenched end has the highest hardness as expected. If %C is greater than 0.6 it is likely to be Rc64. The distance at which the hardness drops to Rc54, gives the depth of hardness. In this case it is x (see sketch c in slide 1). Higher the magnitude of x higher is the hardenability. Jominy depth has a direct correlation with ideal critical diameter. The sketch (d) in slide 1 gives a typical plot of D I versus x. This helps convert Jominy depth into ideal critical diameter. Thus it is possible to estimate the hardenability of steel and express the same in terms of DI from a single

4 specimen test. This is the main advantage of Jominy end quenched test. This is why it has become a standard industrial practice to determine hardenability of steel. Figure 1 shows the effect of %C on the hardness versus distance plots of Jominy end quenched specimens. Rc 50%M + 50% ( +P) Effect of %C Hardenability Distance from quenched end Fig 1: Effect of %C on the hardenability of steel. Hardness of martensite increases with %C but beyond 0.6%C the increase is nominal. The hardness corresponding to 50%M + 50% ferrite pearlite too would increase initially and beyond 0.6 it may not change much. Hardenability or the Jominy depth increases with increasing %C. Physical significance of hardenabilty and severity of quenching: 4 Hardenability is a measure of the ability of steel to get hardened on quenching. It is expressed in terms of ideal critical diameter (D I ). It is the diameter of a cylindrical piece of steel which on quenching in an ideal medium having infinite severity of quenching (H) gives 50% martensite at its centre. H defines the capacity of the quenching medium to extract heat from the interface between the hot specimen and the surrounding medium. The problem becomes complex if the medium gets vaporized to form an insulating film on the interface. The heat extraction is not very effective unless the film breaks down into tiny bubbles which could easily float to setup convection current or agitation. This stage is known as nucleate boiling (NB). This is when the heat extraction rate becomes very high. Once the surface temperature drops below the boiling point of the medium formation of bubbles may cease but the temperature of medium at the interface still remains higher that further away from it. Heat would therefore continue to flow from the solid liquid interface by normal convection. Because of this complexity heat transfer rate is not expected to be constant. Nevertheless for simplicity Grossman assumed this to be constant. The effective heat transfer coefficient, apart from the three modes of heat transfer, conduction, convection and radiation, should depend on the capacity of the medium to absorb heat or its thermo physical properties. Slide 2 illustrates with the help of a set of sketches and the cooling curve at the interface, the three different stages of heat transfer during: Film boiling (FB), Nucleate boiling (NB) and Convection, during water quenching.

5 Heat transfer during water quenching NB convection T Steam blanket dq dt ht T k s f dt s dx s dt h T s T dx k s f Slide 2 time Heat transfer within steel takes place by conduction. The heat flux at QM / steel interface should be equal. Heat transfer within steel takes place by conduction. At any instant the heat ( ) flux at the interface between steel and the quenching medium should be equal. Note that: Heat flux at the interface due to conduction within steel = (1) Heat flux absorbed at the interface by the quenching medium = (2) T denotes the temperature of steel at a distance x from the interface, k is the thermal conductivity of steel, T S is the temperature of the quenching medium at the interface, T f is the average temperature of the quenching medium surrounding the interface, and h is the effective heat transfer coefficient at the interface. Equating the two you get the following expression: (3) 5 In spite of several simplifying assumptions (such as: h & k are constant at all times) equation 3 does give a quantitative insight into the cooling characteristics of samples of different dimensions in a wide range of quenching medium. Solution of equation 3 may not be that simple. This is because both T and Ts are functions of time and the equation is valid only at a given instant of time. We will not try to solve it but if we convert it in terms normalized temperature and normalized distance the effect of specimen size and quenching medium becomes much easier to interpret. Normalized temperature (U or sometimes called reduced temperature) and normalized distance (Z) are defined as follows:

6 (4) (5) D is the diameter of the cylindrical sample, T i is the initial temperature (austenitizing temperature) and T f is the final temperature (or the temperature of the quenching medium. Substitute equation 4 & 5 in equation 3 to get the following expression: (6) Recall that the severity of quenching = H = (7) Equation 6 can be integrated and the constant of integration can be shown to be zero because at Z = 0, U=1. This gives: (8) Therefore all cylindrical components of different diameters quenched in different media should have similar thermal gradient at any instant if the product HD happens to be the same. Note the product HD is a dimensionless parameter. It is popularly known as Biot number. It is an indicator of the relative importance of conduction and convection during heating or cooling of a body by convection at the surface. It may also be defined as the ratio of the resistance to heat transfer due to conduction within the body and that due to convection in the surrounding medium. Figure 2 gives a set of plots for normalized temperature versus normalized distance for a set of different Biot numbers. These have been generated using equation 8. Biot number corresponding to air cooling is very low (~0.0025). Therefore for all practical purposes the temperature within the body may be assumed to be uniform. 6

7 Normalized temperature Normalized distance from quenched end Fig 2: Shows a set of curves representing the temperature profile set up within a body due to heat flow from its surface to the cooling medium surrounding it. The Biot numbers, describing the relative importance of conductive heat transfer within the body and the convective heat transfer in the surrounding are given in the legend. The thermal gradient increases with increasing severity of cooling. 7 Figure2 shows the combined effect of the severity of cooling (H) and the size of the sample (D) on the temperature gradient that develops within steel on quenching. A large thermal gradient means a longer time gap between the transformations occurring at the surface and the centre. It is likely to give a large difference in hardness too. For example take a thin steel wire of diameter 0.001in and quench it from its austenitic state in a medium whose H = The product HD = (This in fact is the Biot number). From fig 2 it appears that there would hardly be any temperature gradient within the wire as it cools. Therefore the structure within the wire should be uniform. Take the case of a 1in diameter steel rod. Quench it from its austenitic state in iced brine with agitation. Assume H to be 5. The product HD = 5. Look at the curve corresponding to HD=5 in fig 2. The gradient is sharp. The time gap between the transformations at the surface and the centre would be significant. This is further explained with the help of a set of cooling curves for a thin wire and a thick rod in fig 3. The surface would always cool faster. Every point within the body would pass through the same temperature at different points of time. The gap between the two is a function of the size of the specimen for a given quenching medium. It is very small in the case of a thin wire. Figure 3 also includes the CCT diagram of eutectoid steel. In the case of the thin wire the two cooling curves (labeled as S & C) completely avoid the lines representing pearlitic transformation but intersect the line

8 denoting M s temperature within a short time gap. The microstructure of the wire is therefore expected to be the same from its surface to its centre. A 1 T log t + P Coarse Pearlite S C Fine Pearlite Surface Centre Thin wire M S + M Thick rod M f M Log (t) Fig 3: Shows the effect of HD on the cooling curves at the centre and the surface of test specimen or products. The thin wire is quenched in oil. HD is low. The thick wire is quenched in water. HD is high. Note that in the case of thin wire the temperature difference between the two locations is small whereas it much larger in the case of the thick rod. This has significant effect on the evolution of microstructure within the two samples. Thin wire has uniform structure. It is totally martensitic. The thick rod would consist of 100%M at the surface but 100%fine pearlite at its centre. 64 Thin wire HRC Thick Rod Fig 4: Hardness versus distance plots across the sections of the thin wire and the thick rod. The magnitudes of HD for the wire and the rod are and 5.0 respectively S Normalized distance from the surface C However in the case of the thick rod (see fig 3) only the cooling curve of its surface completely avoids the lines representing pearlitic transformation. It intersects the line denoting M s

9 Temperature much before any transformation begins at the centre of the rod. The microstructure of the thick rod would therefore consist of 100% martensite. The cooling curve at the centre of the rod intersects the pearlite start and finish lines before crossing M s & M f temperatures. Therefore the microstructure of the centre of the thick rod should be 100% pearlite. In between there will be regions consisting of martensite and pearlite in varying proportions. The hardness at its surface is expected to be much higher than that at its centre. Figure 4 shows the expected hardness versus distance plot in the case of both the thick and the thin rods. In other words the hardness profile may look very much similar to that of the temperature versus distance plots. Factors affecting hardenability of steel: Three most important factors that affect hardenability of steel are austenitic grain size, %C and the presence of additional alloy elements. Hardenability is closely connected with the ease with which diffusion controlled transformation can be suppressed. Diffusion control transformation takes place by nucleation and growth. Grain boundaries are the preferred sites for the nucleation of pro eutectoid / eutectoid constituents of steel. Grain boundary area per unit volume decreases as the size of austenite grain increases. It is much easier to suppress the nucleation of ferrite and pearlite in coarse grained steel. Therefore it is expected to have higher hardenability. This is illustrated in slide 3. Factors affecting hardenability Austenite grain size Carbon Alloy addition #6 #7 D I #8 n=2 N-1 N: grain size no. ASTM n: no. grains / sq in at 100X Higher no. : finer grain & lower hardenability Slide 3 %C 9

10 PAGB Pearlite Nodules Martensite Fig 5: Dotted lines in the two microstructures represent prior austenitic grain boundary (PAGB). It cannot be seen at room temperature since is replaced by nodules of pearlite. Quenching after partial transformation makes it visible. Austenitic grain size of steel is represented by ASTM grain size number (N). If n is the number of grains of austenite per square inch seen under an optical microscope at a magnification of 100X, the ASTM grain size number is given by: 2 (9) If N = 6, n = 32. It means its microstructure at 100X should have 32 austenite grains in every square inch. It also suggests that higher the grain size number higher is the number of grains per unit area. In other words higher N denotes finer grain size. Slide 3 has a set of plots showing the effect of %C and ASTM grain size number on the ideal critical diameter of steel (D I ). Note that for a given %C, D I increases with decreasing N. It means coarser the austenite grains gives higher hardenability. 10 How do we find austenite grain size? Austenite is not stable at room temperature. It transforms into a structure consisting of ferrite and pearlite. A grain of austenite in the case of eutectoid steel may transform into several nodules of pearlite as shown in fig 5. It may therefore be difficult to identify the prior austenite grain boundaries. Figure 5 also suggests a method of marking by controlled isothermal transformation at temperature a little below A 1 for a specific period before final quenching. Pearlitic transformation is controlled by nucleation and growth. Grain boundaries are the most preferred sites for nucleation. Figure 5 shows how it helps mark the prior austenite boundaries. Do not allow the nodules to become very large. Avoid impingement, but there should be enough nodules to mark the boundary. Usually 15 20% transformation may be sufficient. The balance austenite transforms into martensite on quenching. The final structure would consist of primarily martensite with several nodules of

11 pearlite along the prior austenite boundaries. Such a structure facilitates estimation of austenitic grain size. There are also special etching reagents that can help identify PAGB in a fully martensitic structure. A 3 A 1 A 3 A 1 + P + P T Effect of increasing austenite grain size T Effect of increasing % C M s M s + M + M M f M Log (t) M f Log (t) Fig 6: Effect of increasing austenite grain size and increasing % C on the CCT diagram of hypoeutectoid steel. This is closely related to the hardenability of steel. Figure 6 shows the effect austenite grain size on the CCT diagram of hypo eutectoid steel. Grain boundaries are the preferred sites for the nucleation of pro eutectoid ferrite and pearlite. Average length of grain boundaries / unit area or average grain boundary area / unit volume is a measure of the number of nucleation sites. Clearly the number of such sites increases with increasing grain size number. Therefore it would be more difficult to suppress formation of ferrite and pearlite. In other words fine grain means low hardenability whereas coarse grain means high hardenability. However it is not a preferred method to increase hardenability. This is because coarse grain structure has poor toughness and therefore it is more susceptible to quench cracking or failure. 11 The second sketch in fig 6 shows the effect of increasing %C on the CCT diagram of hypoeutectoid steel. It affects A 3, M s and M f temperatures. All of them decrease with increasing %C. However what affects hardenability is the effect of %C on the position of the curves representing the starting and the finishing points for the precipitation of ferrite and pearlite. Figure 6 suggests that it slows down the precipitation of both ferrite and pearlite. Therefore hardenabilty would increase with increasing %C. All alloy elements with the exception of Co improves the hardenability of steel. The presence of additional alloy elements makes all diffusion controlled transformation sluggish. The critical

12 cooling rate to avoid pearlitic transformation decreases. Therefore it becomes much easier to avoid transformation of austenite into a mixture of ferrite and carbide. Figure 7 shows the effect of alloy addition on the CCT diagram of steel. Alloying elements are often classified into two categories: austenite or ferrite stabilizer. A 3 A 1 T + P Effect of increasing ferrite stabilizer A 3 A 1 T + P Effect of increasing austenite stabilizer M s M s + M + M M f M Log (t) M f Log (t) Fig 7: Effect of increasing alloy addition on the CCT diagram of hypo eutectoid steel. Irrespective of the alloy being either a ferrite or an austenite stabilizer all of these increase the hardenability of steel (Exception cobalt. It decreases hardenability). This is because alloy addition makes diffusion controlled transformation slow. It decreases critical cooling rate to avoid transformation of austenite to ferrite carbide structure. Irrespective of whether an alloy element is a ferrite or an austenite stabilizer it makes diffusion controlled transformation slow. Therefore the curves denoting the starting of the finish points of such transformation shift to the right (or towards longer times) of the CCT diagram. Effect of alloy addition may affect the pearlitic and bainitic transformations differently. An example is given in fig 8. 12

13 900 C A 3 A 1 T M s B s s P s + B + + P + + P B P f B f HRC + M M 0 1 Log (t, sec) 10 6 M f 60 Fig 8: Shows the transformation diagram of low alloy medium carbon steel having Ni, Cr, and Mo. Elements like Mo & B affect pearlitic and bainitic portions of the C curve differently. Here is an example where the transformation of austenite to pearlite is slower than that of austenite to bainite. In such steels it is possible to get bainite even on continuous cooling. Recall that you can get bainite in plain carbon steel only by isothermal transformation. The sketch is not to scale. Numbers have been included to give an approximate idea about the time scale, the temperature and the hardness of the transformation product obtained under different cooling conditions. In order to get martensite at the surface of low alloy steel it should be cooled from austenitic state at a rate so as reach M s temperature within 100sec. 13 Hardness of steel depends only on its carbon content. However, plain carbon steel can be fully hardened only if its section size is less. For components like thin wire, hack saw or razor blade hardening is not at all a problem. Such components can be hardened at moderate severity of quenching. If the section size becomes large very high quenching severity may be needed to get the same hardness. This would setup a large thermal gradient between the surface and the centre of the component. The transformation stresses that may develop under such a situation would make the component highly susceptible to quench crack and / or distortion. Addition of alloying elements makes steel harden able at moderate cooling rates. In other words the main reason for the addition of alloying elements to steel is to increase its hardenability. The effectiveness of a particular element in increasing the hardenability is expressed in terms of a multiplying factor. Slide 4 gives a partial list of such factors for a number of common alloying in a tabular form. The first row gives the headings of each column. The first column gives the weight % of a particular alloy element (X). The numbers under the column C#6 give the multiplying factors for austenite grain size corresponding to ASTM grain size number (N) 6 as a

14 function of %C. The numbers under the columns Mn, Si, Ni, Cr, Mo give the multiplication factors for the respective elements as a function of its composition. Note that the numbers listed under these are all greater than 1. It signifies the addition of alloying elements increases the harden ability of steel. You already know that harden ability is expressed in terms of ideal critical diameter. Look at the figures entered under the columns C#6, C#7 & C#8 against the row corresponding to 0.30%. These are 0.20, 0.19 and The sequence suggests that the ideal critical diameter of 0.3%C steel decreases as its ASTM grain size number increases from 6 to 8. The multiplying factors given in such tables are obtained from experimental data. ASM Metals Handbook is an excellent source of tables giving harden ability multiplication factors. It can be used to estimate harden ability of steel from its composition using the expression given in slide 4. For example; the ideal critical diameter of 0.15%C, 0.35%Cr, and 0.25% Mo steel having ASTM grain size 7 is equal to 0.13 x 1.76 x 1.75 = Note that such a small addition of Cr & Mo to steel increases the magnitude of D I of 0.15%C steel from 0.13 inch to 0.4. Effect of alloy element % C#6 C#7 C#8 Mn Si Ni Cr Mo Slide 4 D I = f1 x f2 x f3 x f4 x f5 xf6 Effect of tempering on the toughness of steel: 14 The hardness of steel can be increased significantly by quenching from its austenitic state. However this is accompanied by a significant loss of ductility and toughness. Hardened steel is often brittle and prone to cracking. This makes it unusable. Therefore hardening heat treatment is always followed by tempering. It was discussed in connection with the heat treatment of plain carbon steel. During tempering steel may be heated to a temperature below A 1 for around an hour. Tempering temperature and time may however vary depending on its application.

15 Depending on its composition hardened steel may consist of martensite, retained austenite and cementite (if it is hyper eutectoid steel). Both martensite and austenite are unstable at room temperature. On heating both may transform into an extremely fine mixture of ferrite and carbide. As a result its hardness decreases but its ductility as well as its toughness increases. The kinetics of decomposition of martensite and austenite is best followed by hardness versus temperature or hardness versus time plots. See fig 9. t 1 > t 2 > t 3 T 1 > T 2 > T 3 HRC HRC Temp. Temp. (a) t 2 t 2 t 1 Time (b) T 3 T 2 T 1 Fig 9: Show the effect of temperature and time on the hardness of steel. HRC decreases with increasing temperature and time. The transformations that take place during tempering have been discussed in module 34 & 35. With alloy addition the kinetics of transformation becomes slow. The presence of a few alloy elements adversely affects the toughness of steel. Hardness plot does not reveal that. Drop in hardness does not always indicate increase in toughness. Although the main purpose of tempering is to improve the ductility and toughness of steel, sometimes you may find that the toughness or ductility of steel decreases on tempering. The phenomenon is known as embrittlement. This cannot be detected by hardness measurement. Izod / Charpy V notch Impact test or fracture toughness test is used to detect the susceptibility of steel to embrittlement. There are two distinct kinds of situations 15 TME: tempered martensitic embrittlement: 250 / 350 C also known as OSTE (One Stage Temper Embrittlement) : occurs once TE: temper embrittlement: high temp tempering ( C) + slow cooling: TSTE (Two Stage Temper Embrittlement) reappears if tempered again in the same temp range.

16 Embrittlement is accompanied by loss of toughness and an increase in its ductile to brittle transition temperature (DBTT). It is also characterized by inter granular fracture. The main features of the two types of embrittlement are explained with help of sketches in slide 5 & 6. Embrittlement is always associated with the formation of brittle precipitates or film along the grain boundaries. The presence of trace elements increases susceptibility too embrittlement. Harmful trace elements in steel are As, P, Sn, and Sb. Presence of such elements in exceedingly small amount (~0.01%) may cause TME. Transformation of retained austenite at lath boundaries may also be responsible for TME. The diagram in slide 5 shows the effect of tempering temperature on hardness (R C ) and impact toughness (CVN). Hardness decreases with increasing temperature. CVN versus tempering temperature plot too increases with increasing temperature except for a slight drop in its magnitude around 250 C. It occurs only once. Effect of tempering on toughness: TME Rc P Segregation on prior GB (As, Sn, Sb) amt ~0.01% CVN 250 Temp (high carbon) at lath boundary converts to + thin carbide plates Slide 5 Loss of toughness due to TME 16

17 Temper embrittlement: TSTE T Time ~600 NE E CVN TSTE TME Temp Q AC Long term service exposure to C also results in such embrittlement. It can be de-embrittled by retempering & fast cooling. Reasons: P, Sn, Sb, As, N segregation & their interaction with Cr, Mn, Ni. Mo helps avoid such embrittlement. Slide 6 The sketch on the left of slide 6 shows heat treatment cycle during hardening and tempering. After tempering at 600 C, embrittlement takes place only if the job is cooled slowly through the temperature range C. Embrittlement can be avoided by oil quenching after tempering. This suggests that it is due to precipitation. The sketch on the right shows the effect of tempering temperature on CVN. It drops significantly on slow cooling (air cooling) from 600 C. The best way to avoid this is to adopt fast cooling. Alloy steels are more prone to TSTE. It is associated with the segregation of trace elements like P, Sn, Sb, As, & N to grain boundaries. It is more common in alloy steel having Cr, Ni & Mn. Long thermal exposure in the temperature range of C also makes steel brittle. The only way to make it ductile again is to harden and temper the job at 600 C followed by quenching. Avoid tempering steel in the temperature range which makes it brittle. Presence of Mo in steel prevents temper brittleness. 17

18 Temper embrittlement: DBTT FC after tempering CVN SC after tempering Slide 7 Test temp Slide 7 shows the effect of test temperature on the CVN impact toughness of steel after tempering. It has two plots. One of these corresponds to slow cooling after tempering whereas the other represents the effect of fast cooling after tempering. Note that sharp drop in toughness occurs at temperatures shown by the two vertical lines. These are known as ductile to brittle transition temperature (DBTT). Temper embrittled steel has higher transition temperature. Sometimes it can be as high as 70/80 C. This makes steel prone to brittle failure. Summary: 18 In this module we have learnt about Jominy end quench test. This is a simple method of determining harden ability of steel. Although Grossman method gives the physical significance of harden ability it is a rather long drawn process involving heat treatment of multiple test samples. Jominy test needs only one standard test piece. This is why it is the most commonly used technique to determine hardenability. It is necessary to have an idea about the mechanism of heat transfer during quenching. It helps one understand the physical significance of term called severity of quench (H). It gives the power of the quenching medium to extract heat from red hot steel. The product HD is a dimensionless parameter. It is known as Biot number. Its role in the selection of appropriate quenching medium has been explained with illustrations. We also looked at the factors that affect the hardenability of steel. Although coarse austenite grain size gives higher hardenability it is not usually preferred. The best way to improve harden ability is the addition of alloy element. The role of alloy addition in improving harden ability of steel is expressed in terms of a set of multiplying factors. This helps in estimating the hardenability of steel if its composition and ASTM grain size number were known. Effect of alloy addition on tempering of steel has been revisited. Alloy addition slows

19 down tempering kinetics. It may also induce brittleness in steel. It is known as temper embrittlement. There are two kinds of embrittlement one occurring at a relatively lower tempering temperature and the other occurring at a higher tempering temperature. The reason for their occurrence and methods to overcome the problem has been discussed. Exercise: 1. Give example of a shallow hardening and a deep hardening steel. 2. Martensite in Fe 30% Ni alloy is reversible but in most common grades of steel it is not reversible. Explain why it is so. 3. Hardness of a quenched and tempered steel is reported to be Rc 35. What additional tests will you recommend to know that it has indeed been given this heat treatment? 4. It is apprehended that a hardened and tempered steel has become brittle. Suggest a suitable test to check if it is so. 5. Microstructure of a steel consists of 100% coarse pearlite. Its carbon content is reported to be 0.65%. Is this possible? 6. Diameter of a hardened 1.0% carbon steel rod is found to have increased on tempering. What will you infer from this? 7. A hardened steel has become embrittled on tempering. Can this be de embrittled? Answer: 1. Most low carbon steels are examples of shallow hardening. Most alloy steels for tools and dies are examples of deep hardening steel. Many of these exhibit air hardening characteristics. 2. Martensitic transformation takes place through shear when temperature goes below Ms. There is no diffusion or change in composition. Therefore if the temperature is again raised beyond Ms it should go back to austenitic state. Most commercial steel has carbon. Martenste in these steel is susceptible to tempering if temperature is raised. Carbon precipitates as carbide. As a result composition changes. Therefore question of reverse martensitic transformation does not arise. However Fe 30Ni has little carbon. Here there is no precipitation. This is why it exhibits reverse martensitic transformation where it regains its original shape. Such alloys are known as shape memory alloys. This is shown schematically in the following schetch. 19

20 T Ms + M Mf 3. Examine its microstructure to see if it has tempered martensitic structure. Fine pearlite can also give similar hardness. 4. Best test to know if it has been embrittled to find out is the Charpy V notch impact test. If it has become brittle its CVN value will be low and fractured face will be predominantly cleavage with little notch root contraction. 5. Only eutectoid steel can have 100% pearlitic structure. Carbon content of eutectoid steel is a function of other alloying elements present in steel. It must be an alloy steel. Relatively fast cooling or isothermal trasformation could give 100% pearlite. This possibility is ruled out since the pearlite is coarse. 6. 1% carbon steel is austenitized at 760 C and subsequently quenched during hardening. Since it has high carbon content it is likely that its Mf temperature is lower than room temperature. Therfore it would have some amount of retained austenite having a much more close packed structure than martensite. On tempering retained austenite would decompose to martensite. This transformation is accompanied by volume expansion. This is the most likely reason for increase in diameter. 7. Certain alloy steel on tempering at C may become embrittled. This is known as temper embrittlement. To de embrittle the steel it must again be hardened and tempered at 600 C followed by quenching. Fast cooling will help avoid exposure to the temperature range susceptible to such embrittlement. 20

21 Appendix Temperatures at which phase transformation takes place in steel depend on its composition. You can get these from iron carbon phase diagram. This is valid for plain carbon steel only. All commercial grades of steel have several other alloying elements. Some of these raise the critical temperatures whereas others lower them. The critical temperatures representing solid state transformation exhibit hysteresis. This is because of the difference in heating and cooling rates used to determine these. The critical temperatures estimated during the heating cycle are denoted as Ac whereas that obtained during cooling is represented as Ar. The average of the two may be assumed to the equilibrium temperature (Ae). There are three critical temperatures. Ae1 denotes the lower critical temperature. Ae3 is the upper critical temperature. Apart from these there is an additional critical temperature denoted as Ae2. This is the temperature at which ferrite in steel undergoes a transformation from ferromagnetic to paramagnetic state. For heat treatment of steel it important to have an idea about Ae1 & Ae3. This helps one decide the temperature to which a particular grade of steel must be heated for annealing, normalizing and hardening heat treatment. Besides these it is also important to have an idea about Bs & Ms temperature as well. All of these are functions of composition. Several empirical equations have evolved over the years. These are valid for a limited range of composition. Some of these are listed here. These have been taken from the following website site: Ref. Andrews Austenite formation temperature ( C): Lower critical temp: Ae1 = Ni Si +6.38W 10.7Mn +19.9Cr +290As Upper critical temp: Ae3 = C Si 15.2Ni Mo + 104V W 30.0Mn Cr +20.0Cu 700P 400Al 120As 400Ti Both of these are valid for low alloy steel having less than 0.6%C. Alloy contents are in wt%. Ref. Kirkaldy Bainite start temperature ( C): 21 Bs = C 35Mn 75Si 15.3Ni 34Cr 41.2Mo Alloy contents are in wt%

22 Martensite start temperature ( C): Andrews: Ms = C 30.4Mn 17.7Ni 12.1Cr 11.0Si 7.0Mo Alloy contents are in wt% This is valid for low alloy steel having less than 0.6C, 4.9Mn, 5.0Cr, 5.0N, and 5.4Mn Mf temperature is approximately 215 C less than that of Ms. (Stevens & Haynes) 22