Influence of rare earths on the gas nitriding kinetics of 32CrMoNiV5 steel at low temperature

ABSTRACT This work aims to study the influence of rare earth on nitrogen diffusion kinetics and on surface properties (surface hardness and diffusion depth) of 32CrMoNiV5 steel containing various additions of rare earth (Ce and La). Gas phase nitriding treatments based on ammonia, nitrogen, and hydrogen were carried out by thermogravimetry at 520 °C for 48 h with a constant KN nitriding potential. Several methods of measurement, analysis and observation were used to characterize the nitrided alloys: X-ray diffraction, microhardness, optical and scanning microscopes, and electron microprobe. All the results show that rare earth have a significant influence on the nitrogen diffusion rate: improvement of surface hardness, diffusion depth and gradients of hardness, and nitrogen concentration in the diffusion layer. Secondary electron images, X-ray images and X-ray mappings performed on different particles showed the presence of cerium and lanthanum oxides in the diffusion layer in spheroidal form and of elongated platelets, of size 0.50 to 15 µm, distributed along the grain boundaries and in the grains of the matrix.