The creep behavior of austenitic stainless steels with various dissolved C and N contents was investigated at room temperature. It was found that, although the additions of both C and N increased the proof stress, their effects upon creep behavior were different. Thus, C had little effect upon the creep strain, whereas N increased it. Because N additions increased the strain rate dependence of the flow stress in tensile tests, N had the effect of enhancing the thermally activated movement of dislocations, and thereby increasing the creep strain. On the basis of transmission electron microscopic observations of dislocation structures after creep testing, it was found that N-containing steels contained marked planar dislocation structures and many dislocation pile-ups. These dislocation structures were clearly different to those of a C-containing steel, or of a high-purity steel. As the effect of N upon creep was detected at the high dislocation densities that were produced by high applied stresses, it was concluded that the dislocation pile-ups in the planar structure produced stress concentrations which enhanced the thermally activated movement of dislocations. Also, the activation volumes which were calculated from the coefficients in the logarithmic creep law, and were estimated from the stress changes in strain-rate change tests, were markedly reduced by N additions. These were considered to be apparent reductions that were caused by the effective stress; as increased locally by dislocation pile-ups.

M.Tendo, T.Takeshita, T.Nakazawa: Journal of the Japan Institute of Metals, 1995, 59[6], 620-6