Ausforming Flashcards
Steps:
- austentize steel
- rapidly cool to an intermediate temp (500-600C) in metastable austenite bay region, and substantial plastic deformation of steel in austenitic state
- Quench to form martensite
- Temper
Mechanical Properties
> 30-50% higher strength than conventional without loss of ductility and toughness
fatigue life x5
Limitations
> very thick components can’t be ausformed
major equipment investment
ausforming steels has high amounts of alloys like Cr that increase cost
Composition of steel suitable for ausforming
> 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
Process parameters
> 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
Microstructural changes and strengthening mechanisms
> 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
What’s responsible for the increase in strength in ausformed steels?
> high dislocation density, fine carbide dispersion, and fine martensite plate size
Advanced high strength sheet steels (AHSS)
> 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
Effect of increasing carbon content
> CC^: strength ^, toughness v, DBTT ^, weldability v
Effect of Grain size on strength: Hall Petch equation
σy = σ0 + K d-1/2
> d: avg ferrite grain size
> σ0 and K constants