Ausforming

Question 1

Steps:

Answer 1

1. austentize steel
2. rapidly cool to an intermediate temp (500-600C) in metastable austenite bay region, and substantial plastic deformation of steel in austenitic state
3. Quench to form martensite
4. Temper

Question 2

Mechanical Properties

Answer 2

> 30-50% higher strength than conventional without loss of ductility and toughness
fatigue life x5

Question 3

Limitations

Answer 3

> very thick components can’t be ausformed
major equipment investment
ausforming steels has high amounts of alloys like Cr that increase cost

Question 4

Composition of steel suitable for ausforming

Answer 4

> high ferrite, pearlite, and bainite hardenability

> 0.3-0.4 wt% C, 4.5-5.0 wt% Cr, 1.5 wt% Mo, 0.4 wt% V

Question 5

Process parameters

Answer 5

> austenitizing temp should be low compatible with dissolution of all carbides
cooling from austenitizing temp to metastable austenite bay must be rapid to avoid formation of ferrite and pearlite
after deformation, cooling should be rapid enough to avoid formation of bainite
deformation temp should be low enough to avoid recovery and recrystallization of austenite, but high enouhgh to avoid formatoin of bainite

Question 6

Microstructural changes and strengthening mechanisms

Answer 6

> strength increases as deformation temp is decreased due to greater stain hardening in austenite
deformation should involve 25-30% reduction in thickness to achieve high strength
gives high dislocation density from heavy deformation and martensite transformation
fine alloy carbides precipitate in metastable austenite during deformation on dislocations
deformation of metastable austenite gives fine martensite plates after quenching

Question 7

What’s responsible for the increase in strength in ausformed steels?

Answer 7

> high dislocation density, fine carbide dispersion, and fine martensite plate size

Question 8

Advanced high strength sheet steels (AHSS)

Answer 8

> inexpensive, good strength, excellent weldability, high toughness, low DBTT
to increase strength, add alloys, but this will increase cost and hardenability which will affect weldability

Question 9

Effect of increasing carbon content

Answer 9

> CC^: strength ^, toughness v, DBTT ^, weldability v

Question 10

Effect of Grain size on strength: Hall Petch equation

Answer 10

σy = σ0 + K d-1/2
> d: avg ferrite grain size
> σ0 and K constants