Austenite --> martensite transformation Flashcards
No nuc and growth, it’s _____
diffusionless
Higher CC, _____ Ms (start temp)
lower
Alloys like Mn, Mo, Ni ____ start temp, but Co ____ start temp
decrease; increase
Vol fraction of martensite is a function of ____, not ___
temp; time
Vol fraction of retained austenite ____ with increase in CC
increases
How do you reduce the volume of retained austenite?
cool it to liquid nitrogen temperature so it will transform to martensite (sub zero treatment)
Temperature interval Ms-Mf _____ with increasing CC
increases
How do we increase Ms temp?
apply external stress while quenching
_______ _____ in plain carbon and low alloy carbon cannot be surpressed no matter how high quench rate
Martensitic transformation
During transformation, movement of Fe atoms change c/a ratio and _______ vol of unit cell
increase
<0.6wt% C
lath microstructure
> 1.0wt% C
tentricular plate microstructure
0.6-1wt% C
a mixed microstructure
A finer austenite grain leads to a finer martensite grain and ______ ______
higher toughness
Fe-C martensite is ____ & _____
hard; brittle
Higher CC in Fe-C, _____ hardness. _____ ___ depend on alloys.
higher; does not
Why is there no significant increase in hardness in Fe-C beyond 0.65wt%?
retained austenite (soft) increases at higher CC, which compensates for any additional hardness
Why is Fe-C martensite hard?
> it’s a supersaturated soln of interstitial C in BCT. C atoms»_space;> octahedral void space-lots of strain and soln hardening
vol change during transf = generation of dislocation/high dislocation density
fine microstructure
Why is Fe-C martensite brittle?
> BCT doesn’t have close packed planes or directions, slip is difficult
interstitial C and strain fields impede dislocation motion
Why isn’t steel used in its hardened quenched condition?
> very brittle, low toughness
retained austenite, no dimensional stability
contains residual stresses, warping and cracking
