Effects of rare earth on nitriding microstructure and properties of a La-Ce micro-alloyed Q235RE steel

Accelerating nitriding is an interesting topic in chemical heat treatment of ferrous parts. In order to have a deeper understanding of the catalysis principles of rare earth in nitriding, a Q235 carbon steel and a Q235RE steel microalloyed with rare earth elements La and Ce were nitrided respectively by plasma nitriding and gas nitriding. The microstructure, phase constitution, and composition of the nitrided specimens were analyzed. The hardness profile of nitriding layer and the brittleness of surface compound layer were measured. The results demonstrate that the Q235RE specimens obtained a higher hardness and deeper effective hardening layer than the Q235 specimens without rare earth microalloying. The rare earth catalysis effects were presented as the acceleration of nitrogen diffusion and the inhibition of nitride precipitation in the diffusion layer of the Q235RE specimens. In addition, the surface compound layer of the Q235RE specimens showed higher toughness and better adhesion to the matrix than that of the Q235 specimens, which is considered to be strongly related to the rare earth elements diffused into the compound and the enhanced residual compressive stress in the layer.