Steels- Reaction Characteristics and Properties of Martensite

Question 1

Characteristics of the martensitic reaction

Answer 1

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

Question 2

What supports the view that the α’/γ interface consists of an array of dislocations?

Answer 2

Transformation involves shear.
Highly mobile interface over wide temperature range.
Mobility influenced by plastic deformation.

Question 3

%α’ vs T graph

Answer 3

Flat at 100% up to Mf. Then concave curve down to Ms. Or less steep curve down to higher Md

Question 4

Effect of alloying elements on Ms

Answer 4

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

Question 5

What does untransformed austenite eventually transform into if not martensite?

Answer 5

Lower bainite

Question 6

What does martensite look like?

Answer 6

Lots of needles. Can have lath martensite and plate martensite

Question 7

How does hardness vary with wt% carbon in martensite?

Answer 7

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

Question 8

How does volume of retained austenite vary with wt% C?

Answer 8

Increases exponentially. Although 0% volume retained until about 0.4 wt% C

Question 9

How does volume of lath martensite vary with wt% C?

Answer 9

At 0wt% is 100% lath. Decreases with concave curve as wt% C increases until around 0.8wt% then levels off to zero after

Question 10

What gives martensite its strength?

Answer 10

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

Question 11

What uses martensite in its as-quenched form? Describe it

Answer 11

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.

Question 12

How are dual phase steels fabricated?

Answer 12

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.

Question 13

How to design a dual phase steel with 50% α and 50% α’ so that the composition of α’ is 0.6wt% C

Answer 13

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