Steels- Martensite Flashcards
Characteristics of the martensitic reaction?
Discrete volumes of matrix are sheared
Homogeneous distortion and lattice invariant deformation
No thermal activation required (diffusionless)
Only a function of T not t
α’/γ interface is highly mobile most likely consisting of an array of dislocations.
α’ starts forming at a specific temperature (Ms)
Plastic deformation at any T
What support the view that the α’/γ interface most likely consists of an array of dislocations?
Transformation involves shear
Highly mobile interface over wide T range
Mobility influenced by plastic deformation
What does the quantity of martensite α’ depend on?
How far below Ms the specimen is cooled
Graph of % α’ vs temperature with Mf, Ms and Md
Up to Mf is 100% α’. Then concave curve down to 0 at Ms. Curve stretches further to higher temperatures for Md
What do alloying elements do to the Ms temperature?
Most decrease the Ms temperature. Exception is Co and Al. Interstitials like C and N are the most effective. For multiple alloying elements in a steel, can have a formula for Ms as a function of the concentrations of each element. Negative coefficients mean decrease the Ms
What happens to untransformed austenite when some has gone to martensite?
The untransformed austenite is not stable at low temperatures so eventually transforms to lower bainite
What does martensite look like?
Lots of needles and heavily dislocated. Can have lath martensite or plate martensite
Graph of hardness vs wt% C for martensite
Steep curve with ever decreasing gradient. As get towards 0.8wt% theory predicts higher hardness than reality. In reality there is incomplet γ transformation if only quenching to room temperature. Volume of retained austenite increases as wt% C increases
Things that contribute to the strength of martensite
Substitutional/interstitial solid solution strengthening.
Dis location strengthening (work hardening).
Grain size (Dγ determines max size of α’ plates.
Fine twins
Segregation of C atoms
Precipitation of Fe-carbides
What uses as-quenched martensite?
Dual phase steel
Describe dual phase steel
Consists of islands of martensite (hard and brittle) in a ferrite matrix (soft and ductile). Results in composite-type behaviour. Gives rise to high rates of work hardening and excellent formability. The islands could also be a combination of α’ and untransformed γ.
How are dual phase steels fabricated?
Using an intercritical annela. Means they are heat treated in the two phase (α+γ) region. The temperature in this phase field fixes the amount of α formed (lever rule) and the composition of both α and γ (from ends of tie line). Fraction of γ which remains can form α’ on quenching to RT. Normally only want 20-30% martensite.
How to design a dual phase steel with 50% α and 50% α’ such that α’ has 0.6wr% C
Means γ must have 0.6wt% C. Draw the tie line for this. This is at temperature 750C. Need a steel composition (X) so that there is 50%α at 750C. Use lever rule:
0.5=(0.6-X)/(0.6-0.02)
X=0.31 wt% C