Chapter 10 Kinetics–Heat Treatment

1. Chapter10Kinetics–Heat Treatment

2. This Chapter is for Kinetics-Heat Treatment and deals the followings; 10.1 Time-The Third Dimension 10.2 The TTT Diagram 10.3 Hardenability 10.4 Precipitation Hardening 10.5 Annealing 10.6 The Kinetics of Phase Transformations for Nonmetals In Chapter 9, the various phase diagrams are introduced and it was told that they were under the thermal equilibrium i.e., temperature is changed slowly enough to maintain most stable phase transformation. In practice, materials processing is rushed, and time becomes an important factor. Also, having different cooling rates, the structures of materials become more diverse to give their characteristic mechanical properties. The practical aspect of this concept is heat treatment, and the fundamental basis of this is kinetics, which we shall define as the science of time-dependent phase transformations.

3. In this binary diagram, metal pure A is solid below Tmp. This is the case of thermal equilibrium. L Tmp L S Solidification S S However, if the melt is suddenly dropped to some temperature below Tmp, and let it stay at that temp. Even though this temp. is lower than Tmp, for some time period, the liquid state is maintained. time L Figure 10.1Schematic illustration of the approach to equilibrium. (a) The time for solidification to go to completion is a strong function of temperature, with the minimum time occurring for a temperature considerably below the melting point. (b) The Temperature–Time plane with a Transformation curve. We shall see later that the time axis is often plotted on a logarithmic scale. L L Equilibrium phase diagram • A 1 2 L S 3 4 5 6 7 TTT curve This time delay for the solidification is due to the kinetics. This diagram is called TTT curve or “C” curve. This is also called an “isothermal transformation curve.

4. All the steps require diffusion of atoms i.e., kinetics is involved. Figure 10.2(a) On a microscopic scale, a solid precipitate in a liquid matrix.The precipitation process is seen on the atomic scale in detail as(1) a clustering of adjacent atoms to form (2) a crystalline nucleus followed by (3) the growth of the crystalline phase. (1) a solid precipitate in a liquid matrix; in view point of microscopic scale a clustering of adjacent atoms (3) (2) a crystalline nucleus growth of the crystalline phase

5. When a solid nucleus is formed, there should be surface between solid and liquid. This surface has energy which increases system’s energy and proportional to r2. Figure 10.3Classical nucleation theory involves an energy balance between the nucleus and its surrounding liquid. A nucleus (cluster of atoms) as shown in Figure 10.2(c) will be stable only if further growth reduces the net energy of the system. An ideally spherical nucleus will be stable if its radius, r , is greater than a critical value, rc. SE∝ 4πr² Transformed solid has a volume, and this condensed phase makes the system more stable to reduce the system’s energy. VE ∝ 4/3πr³

6. 1). At high temperature ; Even though diffusion is fast enough for the fast nucleation, but the instability is small because the quenched temp. is not much differ from the Tmp. Therefore, the driving force for the nucleation is very low to retard the nucleation rate. T Figure 10.4The rate of nucleation is a product of two curves that represent two opposing factors ; instability and diffusivity. 2). At low temperature ; Due to big temp drop, instability is high to have large driving force for the nucleation. However, because of the low temp. diffusion is very slow to retard nucleation. • The fastest nucleationrate at mid temp. 3). Mid temperature; The product of the two rates makes fast nucleation rate.

7. • • 1). At high temperature ; (Growth rate dominant) Diffusion is faster at the higher temp. Therefore, once nucleated, the grow of the nuclei is becoming faster as the temperature is increased. Figure 10.5The overall transformation rate is the product of the nucleation rate, N, (from Figure 10.4) and the growth rate, G(given in Equation 10.1). This was shown in Fig. 10.4. 2). At low temperature ; (Nucleation rate dominant) As the temp. is lowered, the driving force for nucleation becomes larger.Therefore, the lower the temp., the faster the nucleation rate. But at the low temp., since diffusion is very slow the nucleation rate becomes slow. 3). Mid temperature; The product of the two rates makes fast nucleation rate.

8. As it has been shown in Fig. 10.2, the growth of the crystalline phase is involved the clustering of atoms which is the diffusional in nature. This means that the growing process is one of the thermally activated processes, and the growth rate (G ) can be expressed in terms of the Arrhenius Expression ; • • G = Ce-Q/RT (10.1) In Fig 10.5, The growth rate(G) and the nucleation rate (N) are shown in one figure to show the temperature dependence of the phase transformation rate. This figure shows that, for phase transformation is possible, first, there has to be nuclei and then growing of them. Therefore, temperature dependence of the growth rate has to be influenced by both rates. Because of this reason, the transformation rate is maximum at the intermediate temperature range. • •

9. Examples and Practice Problems : Students are asked to review the “Example” of 10.1 and solve the “Practice Problem” of 10.1 in the text.

10. 10.2 The TTT Diagram In Fig. 10.1, the TTT diagram was already introduced. However, in this figure, the time necessary for 100% completion of transformation was plotted. In Fig. 10.6 (next slide), it is shown how the transformation (i.e., solidification in this particular case) progresses with time for the completion of the transformation. In Fig. 10.7, solid state isothermal phase transformation is illustrated using plain carbon steel, for eutectoid reaction. This reaction is a typical diffusional transformations (a change in structure due to the long-range migration of atoms) in solids. Explain how the TTT curves are obtained for solid state.

11. The diagram is said to be for “solidification reaction” but it is also good for the solid state phase transformation case. liquid Figure 10.6A time–temperature–transformation diagram for the solidification reaction of Figure 10.1 with various percent completion curves illustrated. solid liquid

12. DIFFUSIONAL TRANSFORMATIONS γ Explain : Pearlite, Coarse & fine, Bainite, Microstructure (Fig. 10.9) Mechanical property. γ Figure 10.7TTT diagram for eutectoid steel shown in relation to the Fe–Fe3 C phase diagram (see Figure 9.38). This diagram shows that, for certain transformation temperatures, bainite rather than pearlite is formed. In general, the transformed microstructure is increasingly fine grained as the transformation temperature is decreased. Nucleation rate increases and diffusivity decreases as temperature decreases. The solid curve on the left represents the onset of transformation (~1% completion). The dashed curve represents 50% completion. The solid curve on the right represents the effective (~99%) completion of transformation. This convention is used in subsequent TTT diagrams. ~1% ~99% α + Fe3C (α+FexC) 50% Martensitic transformation temp. range ! This will be explained later, Fig. 10.11

13. Continuous cooling Figure 10.8A slow continuous cooling path that leads to coarse pearlite formation is superimposed on the TTT diagram for eutectoid steel. This type of thermal history was assumed, in general, throughout Chapter 9. Coarse pearlite by isothermal transformation Coarse pearlite Fine pearlite by isothermal transformation. Bainite by isothermal transformation. Fine pearlite Explain the differences in mechanical properties. Bainite

14. FexC : x < 3 (why?) Carbide shape is different from that of pearlite. → tough, strong. Figure 10.9The microstructure of bainite involves extremely fine needles of carbide in continuous ferrite, in contrast to the lamellar structure of pearlite (see Figure 9.2), 250×. Isolated ferrite by planar carbide Pearlite Figure 9.2

15. Figure 10.10The interpretation of TTT diagrams requires consideration of the thermal history “path.” For example, coarse pearlite, once formed, remains stable upon cooling. The finer-grain structures are less stable because of the energy associated with the grain-boundary area. (By contrast, since phase diagrams represent equilibrium, it identifies stable phases independent of the path used to reach a given state point.) Since the most stable coarse pearlite is already formed at high , temperature no change happens during cooling. In case of finer pearlite, compared to the most stable coarse pearlite, because of the more unstable grain boundaries, it is less stable.

16. DFFUSIONLESS (MARTENSITIC) TRANSFORMATIONS γ Figure 10.11A more complete TTT diagram for eutectoid steel than was given in Figure 10.7. The various stages of the time-independent (or diffusionless) martensitic transformation are shown as horizontal lines. Ms represents the start, M50represents 50% transformation, and M90represents 90% transformation. One hundred percent transformation to martensite is not complete until a final temperature (Mf) of −46°C. If an eutectoid steel is que-nched below 250oC, the temperature is so low that no diffusion is possible, i.e., since diffusional transfo-rmation is not possible, the FCC structure is elastically distorting to be martensite. Quench directly below 250oC Martensitic transformation This Ms is time independent line and the temp is different for the different steels. As C increases, Ms decreases.

17. Intersticial voids and size in FCC and BCC crystals : Octahedron in FCC is the largest void (0.414R). Carbon dissolves in this void. 0.414R 0.225R Octahedron in BCC is the smallest void (0.154 R). 0.154R 0.291R

18. Octahedral void in FCC Octahedral void in BCC Figure 10.12For steels, the martensitic transformation involves the sudden reorientation of C and Fe atoms from the fcc solid solution of γ-Fe (austenite) to a body-centered tetragonal (bct) solid solution (martensite). In (a), the bct unit cell is shown relative to the fcc lattice by the 100α axes. In (b), the bct unit cell is shown before (left) and after (right) the transformation. The open circles represent iron atoms. The solid circle represents an interstitially dissolved carbon atom. This illustration of the martensitic transformation was first presented by Bain in 1924, and while subsequent study has refined the details of the transformation mechanism, this diagram remains a useful and popular schematic. α 0/√2 carbon α0 α0 γ α0 BCT Quenching (No diffusion but elastic lattice distortion) BCT (α’) FCC (γ) Before quenching : C locates in the largest void, octahedron in FCC (γ). After quenching : C does not move so is remained the same site. But this site is the smallest void of octahedron in BCC. Martensite formation : Supersaturated carbons distort the BCC lattice to form the metastable martensitic structure, extremely hard and brittle. VBCT > VFCC → initiates surface crack and distortion.

19. How the white and black parts are differ? Figure 10.13Acicular, or needlelike, microstructure of martensite 100×.

20. During continuous cooling, the specimen has been kept higher temp range compared to the isothermal cooling of same time period. So it took longer time for the nucleation. Therefore, the curve for the transformation begins shift to the right hand side, longer time. Figure 10.14A continuous cooling transformation (CCT) diagram is shown superimposed on the isothermal transformation diagram of Figure 10.11. The general effect of continuous cooling is to shift the transformation curves downward and toward the right. (After Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, Metals Park, OH, 1977.) (Some fine pearlite and martensite) (Less coarse pearlite) (Coarse pearlite)

21. γ proeutectoid γ + (Fe3C)proeut Figure 10.15TTT diagram for a hypereutectoid composition (1.13 wt % C) compared with the Fe–Fe3 C phase diagram. Microstructural development for the slow cooling of this alloy was shown in Figure 9.39 (next slide). Ms is lower than that of the eutectoid steel. (Fig. 10.14)

22. (a) Carbon atom position in fcc and bcc Fe , (b) lattice parameter change in Fe-C martensite with carbon content(c) Hardness of fully hardened martensitic plain carbon steel as a function of carbon content (shaded area indicates possibility of loss of hardness due to formation of retained austenite)

23. L Figure 9.39Microstructural development for a slowly cooled hypereutectoid steel or high carbon steel(of composition 1.13 wt % C). γ γ Primary Fe3C γ + Fe3C primary (Fe3C)primary γ Primary or γ • (Fe3C is the continuous phase) (727+1)oC 727oC αeut (727-1)oC (Fe3C)eut 727oC eutectoid α Primary or αeut + Fe3Cpri+eut αeut + (Fe3C)eut Pearlite structure by eutectoid reaction Secondary or eutectoid Fe3C As C ↑, Fe3Cpri+eut ↑→ stronger & brittle Amount of phase calculation is important because it makes the understanding of the mechanical property of the alloy. Explain the property of hypereutectoid steel & with carbon. 0.77 Fe3C

24. Figure 10.16TTT diagram for a hypoeutectoid composition (0.5 wt % C) compared with the Fe–Fe3C phase diagram. Microstructural development for the slow cooling of this alloy was shown in Figure 9.40. By comparing Figures 10.11, 10.15, and 10.16, one will note that the martensitic transformation occurs at decreasing temperatures with increasing carbon content in the region of the eutectoid composition. Proeutectoid alpha. α + γ How does the microstructure look like? Ms is higher for this low C-steel.

25. Various TTT curves for major industrial steels.

26. Various TTT curves for major industrial steels (continued).

27. HEAT TREATMENT OF STEELS Heat treatment of metals and alloys is one of the most powerful techniques for modifying the mechanical properties through the microstructural change. Among the metals and alloys, carbon steels is the most widely applied for heat treatment. Because the interesting combination of iron and carbon gives very many varieties of the heat treatment processes. Since the heat treatment of steels is important not only for science and technology but commercially and industrially, most of the engineers have to understand the basic concepts related with the non-equilibrium phase transformation, heat treatment of steels. We have studied how martensite is formed with extremely brittle property. In fact 100% martensite can not be used in practice. A common approach to fine-tuning the mechanical properties of martensite is careful reheating to a temperature where transformation to the equilibrium phases of α and Fe3C is possible. (Heating temp and time are important for the controlled properties)

28. By reheating for a short time at a moderate temperature, a high-strength, low-ductility product is obtained. (Much more martensite is still retained.) By reheating for a longer times, greater ductility obtained . (Less martensite.) Figure 10.17Tempering is a thermal history [T = f n(t)] in which martensite, formed by quenching austenite, is reheated. The resulting tempered martensite consists of the equilibrium phase of α-Fe and Fe3 C, but in a microstructure different from both pearlite and bainite (note Figure 10.18). (It should be noted that the TTT diagram is, for simplicity, that of eutectoid steel. As a practical matter, tempering is generally done in steels with slower diffusional reactions that permit less-severe quenches.)

29. Temperaing : Heating martensite to get bainite for improved properties. Why bainite, explain. Tranforms to Bainite : Still strong and very tough. (Explain the structure,Fig.10.18 ) Bainite formation range Large instability → big driving force for transformation, → small but many sphere-carbide nuclei, → high strength and toughness.

30. Figure 10.18The microstructure of tempered martensite, although an equilibrium mixture of α-Fe and Fe3C, differs from those for pearlite (Figure 9.2) and bainite (Figure 10.9). This micrograph produced in a scanning electron microscope (SEM) shows carbide clusters in relief above an etched ferrite. pearlite (Figure 9.2) Carbide shape & distribution → more tough than bainite. bainite (Figure 10.9)

31. Figure 10.19In martempering, the quench is stopped just above Ms . Slowcooling through the martensitic transformation range reduces stresses associated with the crystallographic change. The final reheat step is equivalent to that in conventional tempering. Bainite Quench just above Ms temp Ms Slow cooling

32. Advantages : Since quenching below Ms is skipped, cracking or distortion is prevented. Also, since “reheating” step is avoided, economically benificial. Figure 10.20As with martempering, austempering avoids the distortion and cracking associated with quenching through the martensitic transformation range. In this case, the alloy is held long enough just above Ms to allow full transformation to bainite. Bainite Disadvantages : Takes long time, to get bainite compared to “quench-reheating” process because of low driving force.

33. So far the eutectoid steel has been used for the introduction of steel heat treatment which is relatively simple in phase transformation kinetics. In practice, there are many different alloy with different alloying elements. Therefore, the heat treatment mechanisms may be quite different from the eutectoid carbon steel. For instance, high alloy steels, the austempering process is not feasible because some alloying elements retard the phase transformation rate, the bainitic transformation takes really long time, which is not applicable in the industries. Also the time gap at the knee is very short, it is not possible to quench the steels to make 100% martensite. For this difficulty, special alloying elements are added in C-steel to retard the beginning of the phase transformation , ex, 4340 steel.

34. Examples and Practice Problems : Students are asked to review the “Example” of 10.2 ~ 10.5 and solve the “Practice Problem” of 10.2 ~ 10.5 in the text.

35. 10.3 Hardenability As we can see in the TTT diagram, for a given steel, the faster the cooling rate, the more the martensite transformed and higher the hardness. However, the commercial steels have the enormous range of compositions to have all different hardenability, i. e., for a given heat treatment condition, every steel has different hardness. It is called hardenability, “relative ability of a steel to be hardened by quenching” of the steel. A relative simple experiment has become standardized for industry to provide such a systematic comparison, “Jominy end-quench test”, shown in the next slide.

36. Test procedures : 1). The specimen is heated at γrange. 2). It is quickly set in the tester and at the same time, water is sprayed from the bottom. 100mm Figure 10.21Schematic illustration of the Jominy end-quench test forhardenability. 25mmΦ Having done this quenching, the cooling rate becomes differ along the specimen length. →Using one specimen, various cooling rates can be obtained.

37. Figure 10.22The cooling rate for the Jominy bar (see Figure 10.21) varies along its length. This curve applies to virtually all carbon and low-alloy steels. 700oC It is seen from the data that the cooling rate at the middle of the bar is almost negligible.

38. The max hardness of martensite is ~65. Figure 10.23Variation in hardness along a typical Jominy bar.

39. Increasing hardenability Figure 10.24Hardenability curves for various steels with the same carbon content (0.40 wt %) and various alloy contents. The codes designating the alloy compositions are defined in Table 11.1. Why? Knowing the cooling rate, one can predict the hardness, on the other hand, if the hardness is known then the cooling rate of the part can be estimated.

40. Examples and Practice Problems : Students are asked to review the “Example” of 10.6 and 10.7 and solve the “Practice Problem” of 10.6 and 10.7 in the text.

41. 10.4 Precipitation Hardening Small second-phase precipitation particles hinder the motion of dislocations to strengthen (or harden) metals. For some alloys, if a solid solution is cooled to RT crossing the solvus line, the solutes precipitate to form the second phase. In this case, by controlling the cooling procedures, one can have the precipitates with different size and distri-bution mode. In Fig. 10.25 ~ 28, next slides, precipitation mechanism of Al-4.5%Cu alloy is explained.

42. Figure 10.25Coarse precipitates form at grain boundaries in an Al–Cu (4.5 wt %) alloy when slowly cooled (under equilibrium) from the single-phase (κ) region of the phase diagram to the two-phase (θ + κ) region. These isolated precipitates do little to affect alloy hardness. κ The θ phase may seal GB to make the structure hard and less ductile. θ : brittle phase κ (Inhomogeneous precipitation)

43. Figure 10.26By quenching and then reheating an Al–Cu (4.5 wt %) alloy, a fine dispersion of precipitates forms within the κ grains. These precipitates are effective in hindering dislocation motion and, consequently, increasing alloy hardness (and strength) with significant toughness. This process is known as precipitation hardening, or age hardening. Big driving force for pptn → homogeneous nucleation of 2nd phase (θ) in κ grains. (Homogeneous pptn) Taging Explain the effects of aging t & T. Super saturated κ. Degree of super saturation

44. Figure 10.27(a) By extending the reheat step (aging time), precipitates coalesce and become less effective in hardening the alloy. The result is referred to as overaging. (b) The variation in hardness with the length of the reheat step. Optimum aging time with highest hardness

45. (a) Schematic aging curve( strength or hardness vs. time) at a particular temperature for a precipitation–hardenable alloy(b) Correlation of structure and hardness of Al-4%Cu alloy aged at 130 and 190 degree C.

46. Figure 10.28(a) Schematic illustration of the crystalline geometry of a Guinier–Preston (G.P.) zone. This structure is most effective for precipitation hardening and is the structure developed at the hardness maximum shown in Figure 10.27b. Note the coherent interfaces lengthwise along the precipitate. The precipitate is approximately 15 nm × 150 nm.

47. S = 4πr2 , V = ¾ πr3 . S/V ∝ 1/r If r is small → S/V ↑, r is large → S/V ↓. Therefore, at the beginning of pptn, surface energy has more effect on increasing the system energy. → coherent pptn. Sequence of structures produced during precipitation

48. Microstructures of aged Al-4%Cu alloys aged at 130 degree C. (a) aged 16hrs (GP zone formed as disk shape, 100 A in diameter and a few atomic thickness) (b) aged 1 day ( coherent GP 2 zone, notice strain field) (c) aged 3 days at 200 degree C ( formation of meta stable theta prime phase)

49. Incoherent pptn : low YS, high hardening rate Coherent pptn : high YS, low hardening rate strength Pure aluminum YS elongation

50. Examples and Practice Problems : Students are asked to review the “Example” of 10.8 and solve the “Practice Problem” of 10.8 in the text.