Welding Inspection & Metallurgy API ICP Self Study Notes

Martensite
Martensite is commonly found in steel that has been rapidly cooled
('quenched') from austenite. It is a particularly hard, brittle arrangement.
Essentially it forms because any carbon in solid solution in the austenitic
phase at high temperatures does not have enough time to be incorporated
into cementite when cooled rapidly. The austenite crystals undergo a
transformation involving the shearing of atom planes over each other.
Martensite does not appear on the phase diagram (figure 1), as it is not an
equilibrium phase.