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Hardenability. center cools slowly. surface cools rapidly. Problem: Suppose want to full harden a shaft – i.e., make MAR all the way through shaft. Hardenability. So - this could happen:. Hardenability. Solution to problem: increase quench rate.

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Hardenability

center cools slowly

surface cools rapidly

Problem:

Suppose want to full harden a shaft – i.e., make MAR all the way through shaft


Hardenability

So - this could happen:


Hardenability

Solution to problem: increase quench rate

But rapid quenching leads to distortion & cracking

Generally quench as gently as possible


Hardenability

So….is there a solution??

OK, how to do that?

What controls position of TTT curves?


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




Hardenability

Shift curves to right:

Mn

Mo

Cr

V

Nb

Si

Ni

W


Tempering

Fresh MAR too brittle - infrequently used

Brittle materials can be used in compression

Ex: fresh MAR in ball bearings



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


Tempering

Tempering does not form P - get discontinuous particles


Tempering

  • What else can happen?

stress relief

carbide formation

transition Fe carbides (not Fe3C)

alloy carbides (ex: Cr7C3, Mo6C, VC)

  • start FE formation


Austenitizing

  • Hypoeutectoid steels: about 25°C above a3

  • no FE


Austenitizing

  • Hypereutectoid steels: betwn a3 & acm

  • undissolved carbides present - not

  • particularly detrimental


Tempering temperature/time

  • Depend upon desired props, application

Low T gives high hardness,

low toughness - wear

resistance

Hi T gives high toughness,

reduced hardness


Types of steels

Plain carbon steels alloy steels

Low carbon steels high carbon steels

Stainless steels

420 13Cr-0.15C

440A 17Cr-0.5Ni-0.7C

440C 17Cr-0.5Ni-1.2C

405 13Cr-0.5Ni-0.08C

430 16Cr-0.5Ni-0.12C

304 19Cr-10Ni-0.08C

304L 19Cr-10Ni-0.03C

316 17Cr-12Ni-0.08C

420

440A

440C

405

430

304

304L

316

420 13Cr-0.15C cutlery

440A 17Cr-0.5Ni-0.7C cutlery

440C 17Cr-0.5Ni-1.2C ball bearings

405 13Cr-0.5Ni-0.08C machine parts

430 16Cr-0.5Ni-0.12C acid tanks

304 19Cr-10Ni-0.08C food equipment

304L 19Cr-10Ni-0.03C food equipment

316 17Cr-12Ni-0.08C chemical tanks

420 13Cr-0.15C 230ksi(ht treated)

440A 17Cr-0.5Ni-0.7C 260ksi(ht treated)

440C 17Cr-0.5Ni-1.2C 285ksi(ht treated)

405 13Cr-0.5Ni-0.08C 60ksi(annealed)

430 16Cr-0.5Ni-0.12C 65ksi(annealed)

304 19Cr-10Ni-0.08C 85ksi(annealed)

304L 19Cr-10Ni-0.03C 80ksi(annealed)

316 17Cr-12Ni-0.08C 85ksi(annealed)


Types of steels

Stainless steels