Steels- Martensite

Question 1

Characteristics of the martensitic reaction?

Answer 1

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

Question 2

What support the view that the α’/γ interface most likely consists of an array of dislocations?

Answer 2

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

Question 3

What does the quantity of martensite α’ depend on?

Answer 3

How far below Ms the specimen is cooled

Question 4

Graph of % α’ vs temperature with Mf, Ms and Md

Answer 4

Up to Mf is 100% α’. Then concave curve down to 0 at Ms. Curve stretches further to higher temperatures for Md

Question 5

What do alloying elements do to the Ms temperature?

Answer 5

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

Question 6

What happens to untransformed austenite when some has gone to martensite?

Answer 6

The untransformed austenite is not stable at low temperatures so eventually transforms to lower bainite

Question 7

What does martensite look like?

Answer 7

Lots of needles and heavily dislocated. Can have lath martensite or plate martensite

Question 8

Graph of hardness vs wt% C for martensite

Answer 8

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

Question 9

Things that contribute to the strength of martensite

Answer 9

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

Question 10

What uses as-quenched martensite?

Answer 10

Dual phase steel

Question 11

Describe dual phase steel

Answer 11

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.

Question 12

How are dual phase steels fabricated?

Answer 12

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.

Question 13

How to design a dual phase steel with 50% α and 50% α’ such that α’ has 0.6wr% C

Answer 13

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