Question

1. Martensite is on the TTT diagram but is not on the phase diagram of steel. How do you explain the formation of martensite in steel?

Answer 1

On TTT Diagram, Martensite is formed in steels when the cooling rate from austenite is sufficiently fast ( which can not occur in Phase diagram involving slow cooling process). It is a very hard constituent, due to the carbon which is trapped in solid solution. Unlike decomposition to ferrite and pearlite, the transformation to martensite does not involve atom diffusion, but rather occurs by a sudden diffusionless shear process. The term is not limited to steels, but can be applied to any constituent formed by a shear process which does not involve atom diffusion or composition change. The martensite transformation normally occurs in a temperature range that can be defined precisely for a given steel. The transformation begins at a martensite start temperature (Ms), and continues during further cooling until the martensite finish temperature (Mf) is reached. Ms can occur over a wide range, from 500°C to below room temperature, depending on the hardenability of the steel. The range Ms to Mf is typically of the order of 150°C. .