Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001
I.Nucleation and Growth Transformations
A.Nucleation time - slow transformation as nuclei form
B.Growth time - rapid growth initially followed by slowing growth (Fig 10.1)
C.Rate of reaction generally taken as r = 1/t0.5
D.Nucleation process - fastest when old phase(s) most unstable
E.Growth rate - fastest at higher temperatures (diffusion)
F.Reactions on heating - faster as T increases
1.Nucleation faster as T decreases
2.Growth rate faster as T increases
1.Rate increases as T decreases - down to about 550 ° C
2.Higher nucleation rate at lower T gives finer Pearlite
3.Isothermal Transformation Diagrams - transformation from austenite to ferrite and carbide when
1.Homogeneous nucleation at lower T produces Bainite
2.Transformation rate decreases as T decreases due to slower growth
3.Rapid cooling required to form Bainite
1.Coarse carbide particles in a ferrite matrix
2.NOT produced directly from austenite
3.Reheating of some other ferrite and carbide structure to coarsen carbides
1.Rapid cooling of austenite to temperature where austenite is very unstable but transformation
rate to ferrite and carbide is very slow
2.Body centered tetragonal lattice produced (BCC ferrite with supersaturation of carbon)
3.Athermal transformation - not time dependent
4.Degree of transformation dependent on temperature (Ms & Mf)
1.Produced by reheating of Martensite
2.Martensite transforms to very fine ferrite and carbide
3.Structure similar to Bainite but easier to produce
4.Potential for warping and cracking in quench
F.Proeutectoid phase formation
1.Phases form only at higher temperatures where diffusion is fast
2.Proeutectoid ferrite for hypoeutectoid alloys
3.Proeutectoid carbide for hypereutectoid alloys
III.Isothermal Transformation Diagrams
1.Describe only the transformation of austenite when held at a fixed temperature
2.After austenite has been transformed, IT diagram no longer applies
3.Reheating will not reverse the transformation
a.a specific alloy composition
b.a specific austenitizing temperature and time
c.a specific austenite grain size
a.Pearlite produced at higher temperatures - becomes finer as transformation T decreases
b.Bainite produced at medium temperatures - becomes finer as transformation T decreases
c.Martensite produced when quenched
a.Proeutectoid ferrite produced if transformed above or just below eutectoid T
b.At lower T's, no proeutectoid regions
a.Proeutectoid carbide produced if fully austenitized and transformed above or just below
b.At lower T's, no proeutectoid regions
a.Ms & Mf decrease as carbon content increases
b.Higher carbon gives more strained M lattice
1.Alloying elements used to slow transformations (more diffusion required)
2.Pearlite transformation slowed more than bainite
3.Slower cooling and bainite formation possible
IV.Continuous Cooling Transformation Diagrams
A.CCT diagrams shifted to right and down from IT diagrams
B.Represent more realistic cooling in manufacturing
A.Effect of carbon content on mechanical properties
1.Higher C - higher strength and hardness
2.Higher C - lower ductility and toughness
B.Effect of microstructure on mechanical properties
1.Finer distribution of carbide - higher strength and hardness
2.Finer distribution of carbide - lower ductility and toughness