Microstructure and brittle of 35CrMoA steel in laser quenching nitriding compound modified layer

The phase composition, microstructure, micro-hardness and brittleness of gas nitriding layer and laser quenching-nitriding layer have been studied. The results show that the content of ε-Fe₃N in bright layer of laser quenching-nitriding layer increases from 14.74% to 69.45%, while the content of crisp and hard ξ-Fe₂N drops from 79.95% to 25.03%. The curve of nitrogen concentration in nitriding layer decreases gently with the decrease of the nitrogen content in the surface layer. The total diffusion flux of nitrogen and the thickness of nitriding layer increase. After laser quenching-nitriding, the crystal grain is tiny with cementite distributing in strip lower bainite. The number of high hardness Cr₂N particles enchances, which obviously improves the micro-hardness of bright layer. Brittle fracture critical pressure of nitriding layer increases from 3 N of gas nitriding layer to 6 N of laser quenching-nitriding layer.