Austenite --> martensite transformation

Question 1

No nuc and growth, it’s _____

Answer 1

diffusionless

Question 2

Higher CC, _____ Ms (start temp)

Answer 2

lower

Question 3

Alloys like Mn, Mo, Ni ____ start temp, but Co ____ start temp

Answer 3

decrease; increase

Question 4

Vol fraction of martensite is a function of ____, not ___

Answer 4

temp; time

Question 5

Vol fraction of retained austenite ____ with increase in CC

Answer 5

increases

Question 6

How do you reduce the volume of retained austenite?

Answer 6

cool it to liquid nitrogen temperature so it will transform to martensite (sub zero treatment)

Question 7

Temperature interval Ms-Mf _____ with increasing CC

Answer 7

increases

Question 8

How do we increase Ms temp?

Answer 8

apply external stress while quenching

Question 9

_______ _____ in plain carbon and low alloy carbon cannot be surpressed no matter how high quench rate

Answer 9

Martensitic transformation

Question 10

During transformation, movement of Fe atoms change c/a ratio and _______ vol of unit cell

Answer 10

increase

Question 11

<0.6wt% C

Answer 11

lath microstructure

Question 12

> 1.0wt% C

Answer 12

tentricular plate microstructure

Question 13

0.6-1wt% C

Answer 13

a mixed microstructure

Question 14

A finer austenite grain leads to a finer martensite grain and ______ ______

Answer 14

higher toughness

Question 15

Fe-C martensite is ____ & _____

Answer 15

hard; brittle

Question 16

Higher CC in Fe-C, _____ hardness. _____ ___ depend on alloys.

Answer 16

higher; does not

Question 17

Why is there no significant increase in hardness in Fe-C beyond 0.65wt%?

Answer 17

retained austenite (soft) increases at higher CC, which compensates for any additional hardness

Question 18

Why is Fe-C martensite hard?

Answer 18

> it’s a supersaturated soln of interstitial C in BCT. C atoms&raquo_space;> octahedral void space-lots of strain and soln hardening
vol change during transf = generation of dislocation/high dislocation density
fine microstructure

Question 19

Why is Fe-C martensite brittle?

Answer 19

> BCT doesn’t have close packed planes or directions, slip is difficult
interstitial C and strain fields impede dislocation motion

Question 20

Why isn’t steel used in its hardened quenched condition?

Answer 20

> very brittle, low toughness
retained austenite, no dimensional stability
contains residual stresses, warping and cracking