UNIT IV: HEAT TREATMENT OF STEEL SUBJECT:ENGINEERING METALLURGY (S.E) MR.ANIKET BHANUDAS KOLEKAR

1. UNIT IV: HEAT TREATMENT OF STEEL SUBJECT:ENGINEERING METALLURGY (S.E) MR.ANIKET BHANUDAS KOLEKAR ASST. PROF DEPARTMENT OF MECHANICAL ENGINEERING DYPIEMR,AKURDI,PUNE

2. Definition of Heat Treatment • Phase transformations to form Pearlite, Hypo-eutectoid and Hyper-eutectoid steel structures with change in carbon composition in plain steels are obtained during slow cooling • By rapid cooling entirely different results are obtained since sufficient time is not available for phase reaction to occur. The properties of steels thus can be altered by controlling heating and cooling of different plain carbon as well as alloy carbon steels. The process is Heat Treatment of Steels. • The chemical composition, the rate of heating & rate of cooling (Time), Temperature and Transformations to form typical microstructuresare important aspects to understand various Heat Treatment processes designed and developed to greatly influence mechanical properties such as strength, hardness, ductility, toughness, and wear resistance.

3. Significances of Heat Treatment • Heat treatment achieves the desired changes in structure and properties • Various types of HT processes are used to meet design requirements for mechanical strength, corrosion, wear, and so on. • The effect of HT on component design are: • Control on microstructures • Increase in strength, toughness or perhaps creep resistance • Relieving of residual stresses and prevention of cracking • Control on hardness (and softness) • Improvement of machinability • Improvement of corrosion resistance or wear resistance

4. Phase Transformations Transformation rate Kinetics of Phase Transformation Nucleation: homogeneous, heterogeneous Free Energy, Growth Isothermal Transformations (TTT diagrams) Pearlite, Martensite, Bainite Continuous Cooling Mechanical Behavior Precipitation Hardening

5. Phase Transformations • Diffusion is the net movement of a substance (e.g., an atom, ion or molecule) from a region of high concentration to a region of low concentration. This is also referred to as the movement of a substance down a concentration gradient. • Phase transformations – change in the number or character of phases. • Simple diffusion-dependent • No change in # of phases • No change in composition • Example: solidification of a pure metal, allotropic transformation, recrystallization, grain growth • More complicated diffusion-dependent • Change in # of phases • Change in composition • Example: eutectoid reaction • Diffusionless • Example: metastable phase - martensite

6. Fe3C Fe g Eutectoid (cementite) transformation + (Austenite) a C (BCC) FCC (ferrite)

7. Martensite T Martensite bainite Strength Ductility fine pearlite coarse pearlite spheroidite General Trends Possible Transformations c11f37

8. Pearlite [1] [1]  →  + Fe3C • Nucleation and growth • Heterogeneous nucleation at grain boundaries • Interlamellar spacing is a function of the temperature of transformation • Lower temperature → finer spacing → higher hardness [1] Physical Metallurgy for Engineers by Donald S Clark and Wilbur R Varney (Second Edition) Affiliated EastWest Press Pvt. Ltd., New Delhi, 1962

9. Pearlite (high diffusion rate) and (b) fine pearlite c11f15

10. Bainite •  →  + Fe3C** • Finely mixture of ferrite and cementite • Acicular, accompanied by surface distortions • Banitic transformation stars by nucleation of ferrite • very fine distribution of ferrite and cementite • Ferrite in Bainite plates possess different orientation relationship relative to the parent Austenite than does the Ferrite in Pearlite

11. Upper bainite • At the higher end of the temperatures (350-550°C) upper bainite is observed • the microstructure consists of feathery like apperance • Ferrite nucleated at the grain boundary and grown into one of the grains with cemetite precipitates between the ferrites. • The ferrite formed is Widmanstatten; it has orientation relationship with the austenite grain into which it is growing; • It is in this respect, namely the orientation relationship between the ferrite/cementite and the austeinite grain in which they grow, that the the bainite differs from pearlite.

12. Lower Banite At low enough temperatures(350°C to 250°C ), the bainitic microstructure changes to that of plates of ferrite and very finely dispersed carbides; since the diffusion of carbon is very low at these temperatures (especially in the austenite phase as compared to ferrite)The carbides precipitate in ferrite (and, with an orientation relationship). These carbides that precipitate could be the equilibrium cementite or metastable carbides (such as ε-carbide, for example).A schematic of lower bainite plate that is formed is shown in figure.

13. Feathery appearance Niddle like appearance [1] Bainite formed at 348oC Bainite formed at 278oC [1] Physical Metallurgy for Engineers by Donald S Clark and Wilbur R Varney (Second Edition) Affiliated EastWest Press Pvt. Ltd., New Delhi, 1962

14. Martensite Possible positions of Carbon atoms Only a fraction ofthe sites occupied FCC Austenite Bain distortion During shear transformation C along the c-axis obstructs the contraction FCC Austenite Alternate choice of Cell In Pure Fe after the Matensitic transformation c = a Tetragonal Martensite 20% contraction of c-axis 12% expansion of a-axis Austenite to Martensite → 4.3 % volume increase Refer Fig.9.11 in textbook

15. Martensite forms in steels possesses a body centered tetragonal crystal structure with carbon atoms occupying one of the three interstitial sites available. • This is the reason for characteristic structure of steel Martensite instead of general BCC. • Tetragonal distortion caused by carbon atoms increases with increasing carbon content and so is the hardness of Martensite. • Austenite is slightly denser than Martensite, and therefore, during the phase transformation upon quenching, there is a net volume increase. • If relatively large pieces are rapidly quenched, they may crack as a result of internal stresses, especially when carbon content is more than about 0.5%.

16. Martensite • The martensitic transformation occurs without composition change • The transformation occurs by shear without need for diffusion • The atomic movements required are only a fraction of the interatomic spacing • The shear changes the shape of the transforming region → results in considerable amount of shear energy → plate-like shape of Martensite • The amount of martensite formed is a function of the temperature to which the sample is quenched and not of time • Hardness of martensite is a function of the carbon content → but high hardness steel is very brittle as martensite is brittle • Steel is reheated to increase its ductility → this process is called TEMPERING

17. 60 Harness of Martensite as a function of Carbon content Hardness (Rc) → 40 20 % Carbon → 0.4 0.6 0.2

18. Microstructure Properties Bainite • High strength and toughness • Considerably high ductility • Moderate high hardness • Tempering is not required • Not stable at high temperature Martensite • Hard but brittle • High wear resistance • Martensite hardness depends on %C • Not stable at high temperature • Requires tempering to release internal stresses and or sub-zero heat treatment for martensite complete reaction

19. Nucleation and Growth transformation complete maximum rate reached – now amount unconverted decreases so rate slows rate increases as surface area increases & nuclei grow • Iso thermal transformation curve Fixed T Fraction transformed, pearlite,y 0.5 t0.5 Logarithmic cooling time (t)

20. Isothermal Transformation Diagrams

21. T-T-T plot

22. Time-Temperature-Transformation TTT Diagrams • The equilibrium Iron-Iron Carbide Phase Diagram is not adequate enough to explain • Non-equilibrium structures • Phases present in rapidly cooled parts • TTT Diagram used to determine when transformation begins and end for an isothermal (constant temperature) heat treatment of a Previously austenitized alloy. • Cooling rates in order of Increasing severity are • furnace cooling, air cooling, oil quenching, liquid salts, water quenching, and brine. • If these cooling curves are superimposed on the TTT diagram, the end product structure and the time required to complete the transformation can be found out.

23. The area on the left of the transformation curve represents the austenite region (A). Austenite is stable at temperatures above LCT but unstable below LCT. • Left curve indicates the start of a transformation and right curve represents the finish of a transformation. • The area between the two curves indicates the transformation of austenite to different types of crystal structures. (Austenite to pearlite, austenite to martensite, austenite to bainite transformation.) • Isothermal Transform Diagram shows that γ to transformation (a) is rapid -; (b) the percentage of transformation depends on temperature only.

24. Isothermal Transformation Diagram • Iron-carbon alloy with eutectoid composition. • A: Austenite • P: Pearlite • B: Bainite • M: Martensite

25. Factors affecting TTT diagram • Composition of steel- (a) carbon wt%, (b) alloying element wt% • Grain size of austenite • Heterogeneity of austenite TTT diagram for eutectoid steel (a) TTT diagram for hypoeutectoid steel (b) TTT diagram for hypereutectoid steel

26. Cooling curve ABCD in T-T-T plot for eutectoid reaction

27. A rapid quenching process is interrupted (horizontal line represents the interruption) by immersing the material in a molten salt bath and soaking at a constant temperature followed by another cooling process that passes through Bainite region of TTT diagram. • The end product is Bainite, which is not as hard as Martensite. • As a result of cooling rate D; more dimensional stability, less distortion and less internal stresses are created.

28. TTT Diagrams • The cooling rates A and B indicate two rapid cooling processes. • In this case curve A will cause a higher distortion and a higher internal stresses than the cooling rate B. The end product of both cooling rates will be martensite. • Cooling rate B is also known as the Critical Cooling Rate, which is represented by a cooling curve that is tangent to the nose of the TTT diagram. • Critical Cooling Rate is defined as the lowest cooling rate which produces 100% Martensite while minimizing the internal stresses and distortions OR • The rate of cooling necessary to just to suppress the diffusion transformation or to avoid nose of IT diagram. Factors affecting Critical Cooling Rate • %of Carbon • %of alloying element

29. c11f23 • Example 11.2: • Iron-carbon alloy with eutectoid composition. • Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments: • Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austeniticstructure. • Treatment (a) • Rapidly cool to 350˚C • Hold for 104 seconds • Quench to room temperature Bainite, 100%

30. c11f23 • Example 11.2: • Iron-carbon alloy with eutectoid composition. • Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments: • Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austeniticstructure. • Treatment (c) • Rapidly cool to 650˚C • Hold for 20 seconds • Rapidly cool to 400˚C • Hold for 103 seconds • Quench to room temperature Austenite, 100% Almost 50% Pearlite, 50% Austenite Bainite, 50% Final: 50% Bainite, 50% Pearlite

31. c11f23 • Example 11.2: • Iron-carbon alloy with eutectoid composition. • Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments: • Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austeniticstructure. • Treatment (b) • Rapidly cool to 250˚C • Hold for 100 seconds • Quench to room temperature Austenite, 100% Martensite, 100%

32. Continuous Cooling Transformation Diagrams(CCT) c11f26 • Isothermal heat treatments are not the most practical due to rapidly cooling and constant maintenance at an elevated temperature. • Most heat treatments for steels involve the continuous cooling of a specimen to room temperature. • TTT diagram (dashed curve) is modified for a CCT diagram (solid curve). • For continuous cooling, the time required for a reaction to begin and end is delayed. • Transformation curve shifted down and to the right – (i.e. lower temperature and increased time) • Normally bainite does not form when an alloy is continuously cooled to room temperature; austenite transforms to pearlite before bainite has become possible.

33. c11f27 • Moderately rapid and slow cooling curves are superimposed on a continuous cooling transformation diagram of a eutectoid iron-carbon alloy. • The transformation starts after a time period corresponding to the intersection of the cooling curve with the beginning reaction curve and ends upon crossing the completion transformation curve.

34. c11f28 • For continuous cooling of a steel alloy there exists a critical quenching rate that represents the minimum rate of quenching that will produce a totally martensitic structure. • This curve will just miss the nose where pearlite transformation begins

35. Different cooling treatments Eutectoid steel (0.8%C) 800 723 600 M = Martensite 500 P = Pearlite Water quench Full anneal 400 T → Normalizing 300 Oil quench 200 Coarse P 100 Fine P + M M P 103 102 104 1 0.1 10 105 t (s) →

36. Quenching media • Brine (water and salt solution) • Water • Oil • Air • Turn off furnace

37. Annealing Annealing refers to a wide group of heat treatment processes and is performed primarily for homogenization, recrystallization or relief of residual stress in typical cold worked or welded components. Depending upon the temperature conditions under which it is performed, annealing eliminates chemical or physical non-homogeneity produced of phase transformations. Few important variants of annealing are • Full annealing • Isothermal annealing • Spheroidise annealing • Bright annealing • Box annealing Subcritical annealing: • Recrystallization annealing • Stress relief annealing

38. Full annealing (conventional annealing) • Full annealing process consists of three steps. • First step is heating the steel component to above A 3 (upper critical temperature for ferrite) temperature for hypoeutectoid steels and above A1 (lower critical temperature) temperature for hypereutectoid steelsby 30-50 °C

39. The second step is holding the steel component at this temperature for a definite holding (soaking) period of at least 20 minutes per cm of the thick section to assure equalization of temperature throughout the cross-section of the component and complete austenization. • Final step is to cool the hot steel component to room temperature slowly in the furnace, which is also called as furnace cooling. The full annealing is used • to relieve the internal stresses induced due to cold working, welding, etc, • to reduce hardness and increase ductility, • to refine the grain structure, • to make the material homogenous in respect of chemical composition, to increase uniformity of phase distribution • to increase machinability.

40. Bright annealing • Annealing of steel component is carried out using some protective medium to prevent oxidation and surface discoloration. • Such type of annealing keeps the surface bright and hence called bright annealing . • Surface protection is obtained by the use of an inert gas such as argon or nitrogen or by reducing atmosphere. • Box annealing • Annealing is carried out in sealed container under conditions that minimize oxidation. • The component packed with cast iron chips, charcoal or clean sand and annealed in a same way to full annealing . • It is also called as black annealing , close annealing or pot annealing

41. Isothermal annealing • Isothermal annealing consists of four steps. The first step is heating the steel components similar as in the case of full annealing. • The second step is slightly fast cooling from the usual austenitizing temperature to a constant temperature just below A 1 . • The third step is to hold at this reduced temperature for sufficient soaking period for the completion of transformation. • the final step involves cooling the steel component to room temperature in air.

42. Isothermal annealing has distinct advantages over full annealing which are given below, refer to ferrite, austenite, pearlite, pearlite starting and pearlite finish, respectively. • Reduced annealing time, especially for alloy steels which need very slow cooling to obtain the required reduction in hardness by the full annealing. • More homogeneity in structure is obtained as the transformation occurs at the same time throughout the cross section. • Improved machinability and surface finish is obtained after machining as compared to that of the full annealed components. • Isothermal annealing is primarily used for medium carbon, high carbon and some of the alloy steels to improve their machinability.

43. Spheroidise annealing • Spheroidise annealing is one of the variant of the annealing process that produces typical microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix of ferrite. The following methods are used for spheroidise annealing • Holding at just below A1 • Holding the steel component at just below the lower critical temperature (A1 Thermal cycling around A) transforms the pearlite to globular cementite particles. But this process is very slow and requires more time for obtaining spheroidised structure. • A – Normalising • B – Annealing or Hardening • C – Spheroidising or Process Annealing • D - Tempering

44. Thermal cycling around A1: • In this method, the thermal cycling in the narrow temperature range around A 1 transforms cementite lamellae from pearlite to spheroidal. • Figure 4.7.4 depicts a typical heat treatment cycle to produce spheroidised structure. • During heating above A 1, cementite or carbides try to dissolve and during cooling they try to re-form. This repeated action spheroidises the carbide particles.

45. Spheroidised structures are softer than the fully annealed structures and have excellent machinability. • This heat treatment is utilized to high carbon and air hardened alloy steels to soften them and to increase machinability • This reduces hardeningtime. • This reduces socking time required during hardening, subsequently reduces oxidation and decarburization • Hence utilized in hardening of thin sections such as safety razor blades and needles to reduce decarburization.

46. Recrystallization annealing • Recrystallization annealing process consists of heating a steel component below A 1 temperature i.e. at temperature between 625°C and 675°C (recrystallization temperature range of steel), holding at this temperature and subsequent cooling. • This type of annealing is applied either before cold working or as an intermediate operation to remove strain hardening between multi-step cold working operations. • In certain case, recrystallization annealing may also be applied as final heat treatment. The cold worked ferrite recrystallizes and cementite tries to spheroidise during this annealing process. • Recrystallization annealing relieves the internal stresses in the cold worked steels and weldments, and improves the ductility and softness of the steel. • Refinement in grain size is also possible by the control of degree of cold work prior to annealing or by control of annealing temperature and time.

47. Stress relief annealing • Stress relief annealing process consists of three steps. The first step is heating the cold worked steel to a temperature between 500°C and 550°C • The second step involves holding the steel component at this temperature for 1-2 hours. The final step is to cool the steel component to room temperature in air. • The stress relief annealing partly relieves the internal stress in cold worked steels without loss of strength and hardness i.e. without change in the microstructure. • It reduces the risk of distortion while machining, and increases corrosion resistance. • Since only low carbon steels can be cold worked, the process is applicable to hypoeutectoid steels containing less than 0.4% carbon. • This annealing process is also used on components to relieve internal stresses developed from rapid cooling and phase changes.

48. Normalizing • Normalizing process consists of three steps. The first step involves heating the steel component above the A 3 temperature for hypoeutectoid steels and above A cm (upper critical temperature for cementite) temperature for hypereutectoid steels by 30°C to 50°C . • The second step involves holding the steel component long enough at this temperature for homogeneous austenization. • The final step involves cooling the hot steel component to room temperature in still air.

49. Due to air cooling, normalized components show slightly different structure and properties than annealed components. • The properties of normalized components are not much different from those of annealed components. • However, normalizing takes less time and is more convenient and economical than annealing and hence is a more common heat treatment in industries. • Normalizing is used for high-carbon (hypereutectoid) steels to eliminate the cementite network that may develop upon slow cooling in the temperature range from point A cm to point A 1. • Normalizing is also used to relieve internal stresses induced by heat treating, welding, casting, forging, forming, or machining. • Normalizing also improves the ductilitywithout reducing the hardness and strength.

50. Table 4.7.1 The variation in the properties of the annealed and normalized