Question

Question 3

Discuss the martensitic crystal structure and why martensite is very hard and brittle.

Question 4

1.Explain why fine pearlite forms under moderate cooling of austenite through the eutectoid temperature, whereas coarse pearlite is the product of slow cooling rates.

2.Which is more stable, the pearlitic or the spheroiditic microstructure? Justify your answer.

3.What is the function of alloying elements in tool steels?

4.Describe one problem that might occur with a steel weld that was cooled to rapidly.

Answer 1

The hardness in iron carbon martensite comes actually due to carbon present in the octahedral positions in austenite(FCC) that does not let FCC to become BCC and thus we get BCT structure because it is a diffusionless transformation and at lower temperature after quenching the FCC is not stable phase and it has to transform into some other phase stable at that temperature so it tries to transform into BCC but carbon present in the octahedral position do not allow ‘c’ axis to shrink(in order to compensate shrink in ‘a’ axis) and thus what we get is due to shear is BCT structure .whereas in iron nickel martensites you don’t have carbon there so you get a BCC martensite , thats why its soft . There are many reason that contribute towards hardness of martensite -

1. Dispersion hardening-fine scale precipitation of carbon results in hardening.
2. Distortion dipoles are created due to different dilatometric changes in a axis and c axis. So this distortion field interacts with dislocations and raises hardness.These distortion dipoles do not exists in iron nickel martensites.
3. Supersaturated solid solution of carbon in austenite is martensite so naturally solid solution hardening do occurs
4. BCT structure with carbon entrapped in between edges are prime cause for high hardness, and high stress fields associated with it.
5. Martensite is highly dislocated with extensive dislocation density so naturally average distance between dislocations decreases and thus they restricts or blocks each other movement resulting in hardening