Steels- Reaction Characteristics and Properties of Martensite Flashcards
Characteristics of the martensitic reaction
Discrete volumes of matrix sheared (producing orientation relationships between parent and product crystals).
Homogeneous distortion (Bain) and lattice invariant deformation.
No thermal activation required (diffusionless), only f(T) not f(t).
α’/γ interface highly mobile most likely consisting of array of dislocations.
α’ starts forming at Ms, quantity depends how far below Ms is cooled to.
Plastic deformation at and T under Ms increases amount of martensite and deformation above Ms will cause the formation of martensite up to Md
What supports the view that the α’/γ interface consists of an array of dislocations?
Transformation involves shear.
Highly mobile interface over wide temperature range.
Mobility influenced by plastic deformation.
%α’ vs T graph
Flat at 100% up to Mf. Then concave curve down to Ms. Or less steep curve down to higher Md
Effect of alloying elements on Ms
Most decrease the Ms temperature, with the exception of Co and Al. Interstitials like C and N are most effective. Can have a formula for Ms in terms of the wt% of each alloying element. Negative coefficients mean they decrease the Ms
What does untransformed austenite eventually transform into if not martensite?
Lower bainite
What does martensite look like?
Lots of needles. Can have lath martensite and plate martensite
How does hardness vary with wt% carbon in martensite?
Steep increase with decreasing gradient. Around 0.8wt% theory is greater than reality because there will be incomplete γ transformation if only quenching to room temperature
How does volume of retained austenite vary with wt% C?
Increases exponentially. Although 0% volume retained until about 0.4 wt% C
How does volume of lath martensite vary with wt% C?
At 0wt% is 100% lath. Decreases with concave curve as wt% C increases until around 0.8wt% then levels off to zero after
What gives martensite its strength?
Substitutional/interstitial SS strengthening
Dislocation strengthening
Grain size (Dγ determines maximum size of α’ plates)
Fine twins
Segregation of C atoms
Precipitation of Fe-carbides
What uses martensite in its as-quenched form? Describe it
Dual phase steel. Microstructure consisting of islands of martensite (hard and brittle) in a ferrite matrix (soft and ductile). Result in composite-type behaviour and gives rise to high rates of work hardening and excellent formability. Islands could be a combination of martensite and untransformed austenite. Normally only want about 20-30% martensite.
How are dual phase steels fabricated?
Using an intercritical anneal. Means they are heat treated in the two phase (α+γ) region. The temperature fixes the amount of α formed (lever rule) and the composition of both α and γ (from the needs of the tie line). Fraction of γ which remains can then form α’ upon quenching to RT.
How to design a dual phase steel with 50% α and 50% α’ so that the composition of α’ is 0.6wt% C
Means γ must have had 0.6wt% C. Draw tie line that indicates this in the two phase region (750C). Need to select composition (X) so that have 50% α at 750C.
α=0.5=(0.6-X)/(0.6-0.02)
X = 0.31 wt% C