Materials Engineering – Day 13 More About Steel

Steel and Carbon Conent 1015 steel – plain carbon – 0.15%C 1090 steel – plain carbon – 0.90%C What happens as carbon content increases? In general, we see more and more pearlite in slow cooled steels. More and more cementite available in all steels. Strength  up. Ductility  down. BUT, AT A GIVEN CARBON CONTENT, WIDELY VARYING PROPERTIES ARE AVAILABLE DEPENDING ON PROCESSING.

Chapter 10: Phase Transformations ISSUES TO ADDRESS... • Transforming one phase into another takes time. Fe g (Austenite) Eutectoid transformation C FCC Fe3C (cementite) a (ferrite) + (BCC) • How does the rate of transformation depend on time and T ? • How can we slow down the transformation so that we can engineering non-equilibrium structures? • Are the mechanical properties of non-equilibrium structures better?

The Austenite to Pearlite Transformation This takes time. Two things must happen. Nucleation of Pearlite. (Best nucleation sites are in or near grain boundaries.) Nuclei are baby crystals. The thermodynamics of nucleation get stronger with supercooling. Growth of Pearlite. This takes diffusion. Carbon atoms must diffuse away from the g to the Fe3C layers which are starting. Diffusion is stronger at high temperatures and weakens as the temperature decreases. These two aspects of the transformation work against each other.

Eutectoid Transformation Rate • Growth of pearlite from austenite: Adapted from Fig. 9.15, Callister 7e. g a pearlite growth direction Austenite (g) grain boundary cementite (Fe3C) Ferrite (a) Diffusive flow of C needed a g • Recrystallization rate increases with DT. Adapted from Fig. 10.12, Callister 7e. 675°C (DT smaller) 50 y (% pearlite) 600°C (DT larger) 650°C 100 Course pearlite  formed at higher T - softer Fine pearlite  formed at low T - harder

C-Curves Think of some austenite, lowered suddenly to a temperature below 727C and allowed to transform at that temperature. At high temperature, atoms can diffuse rapidly BUT, nucleation rates are very low due to only slight undercooling. Therefore, the overall transformation tends to be lengthy. At low temperature, nucleation is very speedy, but diffusion is slow. Therefore the transformation tends to be lengthy. At some intermediate temperatures there must be an optimum. Thus we get a C-shaped curve.

Nucleation and Growth • Reaction rate is a result of nucleation and growth of crystals. % Pearlite 50 100 Nucleation regime Growth log (time) t 0.5 Nucleation rate increases with T Growth rate increases with T Adapted from Fig. 10.10, Callister 7e. • Examples: T just below TE Nucleation rate low Growth rate high g pearlite colony T moderately below TE g Nucleation rate med . Growth rate med. Nucleation rate high T way below TE g Growth rate low

TTT Curves for a Euctectoid Steel Note the axes: Time – always log scale. Temperature Note the curves: Start time 50% Completion (dashed) End time. (after crossing this there is no more austenite left to transfform.

Isothermal Transformation Diagrams • Fe-C system, Co = 0.76 wt% C • Transformation at T = 675°C. Note the axes: Time – always log scale. Temperature 100 T = 675°C y, % transformed 50 Note the curves: Start time 50% Completion (dashed) End time. (after crossing this there is no more austenite left to transform. 2 4 1 10 10 time (s) 400 500 600 700 1 10 2 3 4 5 0%pearlite 100% 50% Austenite (stable) TE (727C) Austenite (unstable) Pearlite T(°C) time (s) isothermal transformation at 675°C

The Pearlite Spectrum The kind of pearlite that you get depends on the transformation temperature. High temperature. Low nucleation. Higher growth. Larger coarser more separated layers in the pearlite.  Not as strong, more ductile. Low temperature. (Nearer to the “nose.”) High nucleation. Little diffusion. Finer thinner closer spaced layers in the pearlite.  Stronger, less ductile. Intermediate temperatures – an intermediate pearlite.

Two Pearlites. Which formed at the higher Temperature? Which is stronger? Which is more ductile?

What Happens as we Transform at Lower and Lower Temperatures? T (C) 727 Coarse P Fine P ??? Log time

The Answer is “Bainite” Bainite is relatively rare. The austenite is changed to pearlite before the bainite transformation can start in most practical cooling processes. But it is good stuff. Strong, hard, some ductility. Upper (above 300)  Lower (below 300)  Upper B. Needles of ferrite with strips of cementite Lower B. Very thin ferrite plates. Ultra thin cementite rods

What Happens if we Transform at an even lower Temperature? The answer is that we get a phase known as martensite. Martensite is BC tetragonal with C interstitially. It forms instantaneously. No nucleation or diffusion is required. Martensite is one of the hardest substances known to man. It is also totally brittle like a ceramic. The production of martensite is an important step, but not the final step, in the production of high strength steels. (Quench and tempered)

Here is the TTT Curve for a Euctectoid Steel. Coarse P Fine P Upper B Lower B M (Martensite)

Martensite: Fe-C System --g(FCC) to Martensite (BCT) Martensite needles Austenite 60 m x potential C atom sites Fe atom sites (involves single atom jumps) (Adapted from Fig. 10.20, Callister, 7e. • Isothermal Transf. Diagram 10 3 5 time (s) -1 400 600 800 T(°C) Austenite (stable) 200 P B TE 0% 100% 50% A M + A 90% (Adapted from Fig. 10.21, Callister, 7e. (Fig. 10.21 courtesy United States Steel Corporation.) Adapted from Fig. 10.22, Callister 7e. • g to M transformation.. -- is rapid! -- % transf. depends on T only.

Martensite Formation  (FCC)  (BCC) + Fe3C slow cooling quench M (BCT) tempering M = martensite is body centered tetragonal (BCT) Diffusionless transformation BCT if C > 0.15 wt% BCT  few slip planes  hard, brittle

Tempering Martensite • • • reduces brittleness of martensite, • reduces internal stress caused by quenching. YS(MPa) TS(MPa) 800 1000 1200 1400 1600 1800 30 40 50 60 200 400 600 Tempering T (°C) %RA TS YS Adapted from Fig. 10.34, Callister 7e. (Fig. 10.34 adapted from Fig. furnished courtesy of Republic Steel Corporation.) Adapted from Fig. 10.33, Callister 7e. (Fig. 10.33 copyright by United States Steel Corporation, 1971.) 9 mm produces extremely small Fe3C particles surrounded by a. • • decreases TS, YS but increases %RA

Mechanical Prop: Fe-C System (1) • Effect of wt% C Pearlite (med) Pearlite (med) C ementite ferrite (soft) (hard) Co < 0.76 wt% C Co > 0.76 wt% C Adapted from Fig. 9.30,Callister 7e. (Fig. 9.30 courtesy Republic Steel Corporation.) Adapted from Fig. 9.33,Callister 7e. (Fig. 9.33 copyright 1971 by United States Steel Corporation.) Hypoeutectoid Hypereutectoid 300 500 700 900 1100 YS(MPa) TS(MPa) wt% C 0.5 1 hardness 0.76 Hypo Hyper wt% C 0.5 1 50 100 %EL Impact energy (Izod, ft-lb) 40 80 0.76 Hypo Hyper Adapted from Fig. 10.29, Callister 7e. (Fig. 10.29 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.) • More wt% C: TS and YS increase , %EL decreases.

Mechanical Prop: Fe-C System (2) • Fine vs coarse pearlite vs spheroidite 80 160 240 320 wt%C 0.5 1 Brinell hardness fine pearlite coarse spheroidite Hypo Hyper 30 60 90 wt%C Ductility (%AR) fine pearlite coarse spheroidite Hypo Hyper 0.5 1 Adapted from Fig. 10.30, Callister 7e. (Fig. 10.30 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.) • Hardness: fine > coarse > spheroidite • %RA: fine < coarse < spheroidite

Mechanical Prop: Fe-C System (3) • Fine Pearlite vs Martensite: 200 wt% C 0.5 1 400 600 Brinell hardness martensite fine pearlite Hypo Hyper Adapted from Fig. 10.32, Callister 7e. (Fig. 10.32 adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p. 1776.) • Hardness: fine pearlite << martensite.

Summary: Processing Options Austenite (g) Bainite (a + Fe3C plates/needles) Pearlite (a + Fe3C layers + a proeutectoid phase) Martensite (BCT phase diffusionless transformation) Tempered (a + very fine Fe3C particles) slow cool moderate rapid quench reheat Strength Ductility T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends Adapted from Fig. 10.36, Callister 7e.

Effect of Cooling History in Fe-C System • Eutectoid composition, Co = 0.76 wt% C • Begin at T > 727°C • Rapidly cool to 625°C and hold isothermally. 400 500 600 700 0%pearlite 100% 50% Austenite (stable) TE (727C) Austenite (unstable) Pearlite T(°C) 1 10 2 3 4 5 time (s) Adapted from Fig. 10.14,Callister 7e. (Fig. 10.14 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) g g

Transformations with Proeutectoid Materials CO = 1.13 wt% C TE (727°C) T(°C) time (s) A + C P 1 10 102 103 104 500 700 900 600 800 Adapted from Fig. 10.16, Callister 7e. Adapted from Fig. 9.24, Callister 7e. Fe3C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) +L +Fe3C a L+Fe3C d (Fe) Co , wt%C T(°C) 727°C DT 0.76 0.022 1.13 Hypereutectoid composition – proeutectoid cementite

Non-Equilibrium Transformation Products: Fe-C • Bainite: --a lathes (strips) with long rods of Fe3C --diffusion controlled. • Isothermal Transf. Diagram Fe3C (cementite) a (ferrite) 10 3 5 time (s) -1 400 600 800 T(°C) Austenite (stable) 200 P B TE 0% 100% 50% pearlite/bainite boundary A 100% pearlite 100% bainite 5 mm (Adapted from Fig. 10.17, Callister, 7e. (Fig. 10.17 from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.) Adapted from Fig. 10.18, Callister 7e. (Fig. 10.18 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

Spheroidite: Fe-C System (Adapted from Fig. 10.19, Callister, 7e. (Fig. 10.19 copyright United States Steel Corporation, 1971.) 60 m a (ferrite) (cementite) Fe3C • Spheroidite: --a grains with spherical Fe3C --diffusion dependent. --heat bainite or pearlite for long times --reduces interfacial area (driving force)

Phase Transformations of Alloys Effect of adding other elements Change transition temp. Cr, Ni, Mo, Si, Mn retard    + Fe3C transformation Adapted from Fig. 10.23, Callister 7e.

Cooling Curve plot temp vs. time Actual processes involves cooling – not isothermal Can’t cool at infinite speed Adapted from Fig. 10.25, Callister 7e.

Dynamic Phase Transformations On the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures: 42% proeutectoid ferrite and 58% coarse pearlite 50% fine pearlite and 50% bainite 100% martensite 50% martensite and 50% austenite

Example Problem for Co = 0.45 wt% 42% proeutectoid ferrite and 58% coarse pearlite first make ferrite then pearlite course pearlite  higher T A + B A + P A + a A B P 50% 200 400 600 800 0.1 10 103 105 time (s) M (start) M (50%) M (90%) T (°C) Adapted from Fig. 10.29, Callister 5e.

Example Problem for Co = 0.45 wt% 50% fine pearlite and 50% bainite first make pearlite then bainite fine pearlite  lower T A + B A + P A + a A B P 50% 200 400 600 800 0.1 10 103 105 time (s) M (start) M (50%) M (90%) T (°C) Adapted from Fig. 10.29, Callister 5e.

Example Problem for Co = 0.45 wt% 100 % martensite – quench = rapid cool 50 % martensite and 50 % austenite A + B A + P A + a A B P 50% 200 400 600 800 0.1 10 103 105 time (s) M (start) M (50%) M (90%) c) T (°C) d) Adapted from Fig. 10.29, Callister 5e.