Metallurgical aspects on the fatigue of solution-annealed austenitic high interstitial steels

Austenitic stainless steels have been used for over 100 years for their combination of strength and ductility. In order to further improve the mechanical and chemical properties of austenitic high nitrogen steels (AHNS) were developed. Ni reduces the solubility of N and, therefore, was substituted by Mn in order to allow for up to 1 weight-% N to be alloyed. AHNS show an even higher strength for the solution annealed state, which can be increased further by cold working. Unfortunately the endurance limit did not follow this trend as it is known to for cold-worked Ni-containing steels. The solution annealed Ni-containing austenites allow for wavy slip and the generation of dislocation cells while the Mn-alloyed AHNS only show planar slip with twins and stacking faults. While the stacking fault energy was thought to be the main reason for planar slip, early results showed that there must be other near-field effects. The density of free electrons, which is mainly influenced by the sum and the ratio of C and N, might be responsible. Strain-controlled fatigue tests were carried out in CrMn-alloyed austenitic steels with different sums (C + N: 0.65–1.2) and ratios (C/N: 0.13–∞) of C and N. Manson–Coffin analyses revealed distinct differences in the fatigue behaviour to CrNi-alloyed C + N steels investigated earlier. This contribution presents these differences and discusses them in relation to microstructural characteristics as well their alterations under cyclic loading.