Embrittlement of steel by nitriding

Abstract

Nitriding of AISI 316 stainless steel leads to the formation of a multilayered case and the distribution of the phases through the nitrided layer depends on the reaction temperature and cooling rate. Optical microscopy, electron microscopy and X-ray techniques are used to identify the phases formed. Nitriding kinetics are studied between 5500 and 800°C and the results are explained in terms of the phase distribution. Internal nitriding theory is valid at 800°C but at temperatures lower than 650°C abnormally high nitriding rates are observed due to the formation. of a thick "white layer" of massive y '-Fe4N with CrN. Three-point bend tests of sheets of nitrided AISI 316 are described and it is shown that the extent. of brittle behaviour depends on the nitriding temperature, the thickness, hardness and composition of the nitrided layer and also on the metallurgical state of the core. Bending of hollow nitrided tubes of the same steel in four-point bend leads to deformation of the material at the level of the anvils of the jig while the mechanical properties of filled nitrided tubes depend on the nature of the filling material.