Isothermal Transformation Diagrams (Time-Temperature-Transformation (TTT) Diagrams) Plot temperature on the y-axis Plot time on the x-axis (typically logarithmic scale) Maps of phase creation as a function time at temperature These are ONLY valid for isothermal (constant temperature) transformation Each diagram is ONLY valid for a specific composition

Consider Eutectoid Transformation … transformation (Fe-C): g Þ a + Fe3C 0.76 wt% C 0.022 wt% C 6.7 wt% C 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 1148°C T(°C) ferrite 727°C Eutectoid: Equil. Cooling: Ttransf. = 727ºC DT Undercooling by DTtransf. < 727C 0.76 0.022

Isothermal Transformation Diagrams • Fe-C system, Co = 0.76 wt% C • Transformation at T = 675°C. 100 T = 675°C y, % transformed 50 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

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) g g

Hypoeutectoid TTT Diagram T(°C) d L g +L g 1148°C L+Fe3C g +Fe3C a 727°C Fe3C a+Fe3C 1 2 3 4 5 6 6.7 (Fe) Co , wt%C Af represents highest temperature ferrite can form As is the eutectoid temperature MS is the martensite start temperature Isothermal transformation diagram for 0.35% C, 0.37% Mn Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

Hypereutectoid TTT Diagram Fe3C 1 2 3 4 5 6 6.7 L g g +L g +Fe3C a+Fe3C L+Fe3C d (Fe) Co , wt%C 1148°C T(°C) a 727°C Af represents highest temperature ferrite can form As is the eutectoid temperature MS is the martensite start temperature Isothermal transformation diagram for 1.13%C, 0.30% Mn Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

Metastable Phase Transformations Where on this diagram is martensite shown? How about bainite? How about spheroidite? This is the EQUILIBRIUM Phase Diagram for Fe-C system Metastable phases are temporary phase which are intermediate between the initial and equilibrium states

Spheroidite Spheroidite: -- a grains with spherical Fe3C d L g a a 1 2 3 4 5 6 6.7 L g g +L g +Fe3C a+Fe3C L+Fe3C d (Fe) Co , wt%C 1148°C T(°C) a 727°C 60 m a (ferrite) (cementite) Fe3C Equilibrium phase diagram tells us that the stable phase distribution in the two phase field is: a + Fe3C – typically as lamellar microstructural constituent pearlite Is pearlite the lowest energy state – NO! With lamellar structure pearlite has a lot of interfacial energy If we anneal a pearlite microstructure we will get a transformation to a new phase distribution that minimizes the energy of the system when atomic mobility is activated Spheroidite: -- a grains with spherical Fe3C

Martensite Transformation Formed by rapid quenching of an alloy Occurs in several alloy systems (indium-thallium, titanium, nickel-iron, gold-cadmium, and Steel) Shear driven atomic realignment – similar to deformation twinning only more complex Militaristic transformation – diffusionless transformation Only driven by changes in temperature -- DG New lattice is formed around a habit plane (plane shared between parent and daughter phases) Form lens-shaped shear plates during transformation – speed of transformation can approach speed of sound in the material Congruent phase change

Martensitic Transformation Bain distortion: conversion of one lattice into another by expansion or contraction along crystallographic axes Lattice parameters change as we increase the amount of carbon in solution Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

Martensite Effects Change in volume associated with formation of martensite For a 1% carbon steel we see a volume increase of 4% The shear transformation and the volume change combine to create a high density of dislocations Lath martensite has internal dislocation density on the order of 1015 – 1016 /m2 Very fine microstruture of cell boundaries and laths

Mechanical Properties Source: H. K. D. H. Bhadeshia, Bainite in Steels, 2nd Edition, Cambridge Press, 2001.

Bainite • Bainite: • Isothermal Transf. Diagram TE T(°C) time (s) --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% bainite 100% pearlite 5 mm

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. 10.33, 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

Alloying Additions Effect of adding other elements Cr, Ni, Mo, Si, Mn Change transition temp. Cr, Ni, Mo, Si, Mn retard    + Fe3C transformation 4340 steel (alloyed steel)

Cooling Curve plot temp vs. time