Phase transformations during tempering of quenched Fe−N alloys

It was found that tempering processes are similar in quenched steel and nitrided iron — in the first stage of tempering (20–180°) the martensite with nitrogen transforms, with formation of metastable F16N2 and temper martensite; in the second stage (180–300°) the retained austenite decomposes and the Fe16N2 →Fe4N transformation occurs; in the third stage (300–550°) the number of lattice defects decreases and the Fe4N particles coalesce. After quenching and tempering at 500–600° the alloy consists of a ferrite—nitride mixture of the type of temper sorbite in carbon steel.