Precipitation of γ′ nitrides in N-saturated ferrite at high temperature and its effect on nitriding

Abstract The precipitation behavior of the γ′ phase during ferritic nitriding has ever been unclear although a lot of attentions have been drawn to shortening the incubation period of γ′ nitrides and improving the nitriding efficiency. In the present work, the precipitation of γ′ nitrides at high temperature and the effect of early formed γ′ nitrides on nitriding were systematically investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and hardness test. The results indicate that, once nucleation occurs, the γ′ phase is formed quickly via the phase transformation of α-Fe → α N → γ′ at high temperature. The early-formed nitrides on the surface of samples accelerate the penetration of nitrogen in the α-Fe grains and improve the nitriding efficiency. The decrease of temperature during nitriding induces the formation of the γ′ nitrides in advance, shortens the incubation time and further increase the nitriding rate. A new nitriding process, involving a short term nitriding at high temperature and a short term nitriding with temperature drop, is proposed to shorten the incubation time and improve the nitriding efficiency.