1. Hardenability
Problem:
Suppose want to full harden a shaft – i.e., make MAR all the way through shaft
center cools slowly
surface cools rapidly

2. Hardenability
So - this could happen:

3. Hardenability
Solution to problem: increase quench rate
But rapid quenching leads to distortion & cracking
Generally quench as gently as possible

4. Hardenability
So….is there a solution??
OK, how to do that?
What controls position of TTT curves?

5. Hardenability
So….is there a solution??
OK, how to do that?
What controls position of TTT curves?
a. % C – more C to move…slower P growth
b. add alloy elements…slows C diffusion
c. increase AUS grain size. P nucleates on AUS
grain boundaries. Increases diffusion distance

6. Hardenability

7. Hardenability

8. Hardenability
Shift curves to right: Mn Mo Cr V Nb Si Ni W

9. Tempering
Fresh MAR too brittle - infrequently used
Brittle materials can be used in compression
Ex: fresh MAR in ball bearings

10. Martensite (MAR)

11. Tempering
Tempering conditions
200°C < T < 700°C
days sec
What happens?
carbide formation starts - very fine closely spaced (makes material harder)
carbon depletion in MAR (makes
material softer)
net change about 0

12. Tempering
Tempering does not form P - get discontinuous particles

13. Tempering
What else can happen?
stress relief
carbide formation
transition Fe carbides (not Fe3C)
alloy carbides (ex: Cr7C3, Mo6C, VC)
start FE formation

14. Austenitizing
Hypoeutectoid steels: about 25°C above a3
no FE

15. Austenitizing
Hypereutectoid steels: betwn a3 & acm
undissolved carbides present - not
particularly detrimental

16. Tempering temperature/time
Depend upon desired props, application
Low T gives high hardness,
low toughness - wear
resistance
Hi T gives high toughness,
reduced hardness

17. Plain carbon steels alloy steels Low carbon steels high carbon steels
Types of steels
Plain carbon steels alloy steels
Low carbon steels high carbon steels
Stainless steels
Cr-0.15C ksi(ht treated)
440A Cr-0.5Ni-0.7C ksi(ht treated)
440C Cr-0.5Ni-1.2C ksi(ht treated)
Cr-0.5Ni-0.08C 60ksi(annealed)
Cr-0.5Ni-0.12C 65ksi(annealed)
Cr-10Ni-0.08C 85ksi(annealed)
304L Cr-10Ni-0.03C 80ksi(annealed)
Cr-12Ni-0.08C 85ksi(annealed)
Cr-0.15C cutlery
440A Cr-0.5Ni-0.7C cutlery
440C Cr-0.5Ni-1.2C ball bearings
Cr-0.5Ni-0.08C machine parts
Cr-0.5Ni-0.12C acid tanks
Cr-10Ni-0.08C food equipment
304L Cr-10Ni-0.03C food equipment
Cr-12Ni-0.08C chemical tanks
420
440A
440C
405
430
304
304L
316
Cr-0.15C
440A Cr-0.5Ni-0.7C
440C Cr-0.5Ni-1.2C
Cr-0.5Ni-0.08C
Cr-0.5Ni-0.12C
Cr-10Ni-0.08C
304L Cr-10Ni-0.03C
Cr-12Ni-0.08C

18. Types of steels
Stainless steels