Thermal Surface hardening techniques

Question 1

What is thermal surface hardening?

Answer 1

selective heating of surface to austenitizing temp, then rapid cooling and tempering giving hard surface and soft core

Question 2

What are the three types of thermal surface hardening?

Answer 2

induction, laser, and electron beam

Question 3

Induction Hardening

Answer 3

> I(x) = Io Exp {-βx}
^ frequency, ^ β, v eddy current I(x)
I(x) decreases exponentially with depth x from surface
effective depth of induction heating d = K1 {ρ /(µf)}^(½)
–> p: resistivity, µ magnetic permeability, K1 constant
additional depth of heating da = K2t^(1/2)
total effective depth of steel that’s heated: d + da

Question 4

What happens as a result of the non-uniform heating in induction hardening?

Answer 4

lots of small spots reach austenitizing temp before the rest of the surface does, giving higher austenite nucleation rate and finer grains

Question 5

What causes super hardness in induction hardened steels?

Answer 5

Fine austenite grains mean fine martensite grains, so hardness and toughness is higher than conventionally hardened

Question 6

Why do induction hardened steels have good fatigue resistance?

Answer 6

They have compressive residual stresses in their surface

Question 7

What happens if the initial microstructure contains coarse carbides in induction hardened steel?

Answer 7

It requires higher time for their dissolution in austenite

Question 8

What happens if the austenitizing temp is too high during induction hardening?

Answer 8

it may give coarse austenite grains, so an initial microstructure with finely distributed carbides or fine pearlite is best

Question 9

What kind of steels is induction hardening performed on?

Answer 9

Plain carbon (quenced after induction heating) and ow alloy (air cooled after induction heating)

Question 10

Laser Hardening

Answer 10

> no separating quenching medium required
surface must be coated with enegy absorbing coating to generate intense localized heat
surface gets austenitized during heating, then rapid cooling gives martensitic surface
high capital and low efficiency

Question 11

Electron beam hardening

Answer 11

> no surface coating required

> for effective self quenching, thickness of part must be at least five times that of the treated layer