EFFECTS OF SOLUTE CARBON AND NITROGEN ON ROOM-TEMPERATURE CREEP OF AUSTENITIC STAINLESS-STEELS

Creep behavior at room temperature has been investigated for austenitic stainless steels (18%Cr-14%Ni) with different contents of dissolved carbon and nitrogen. Creep strains of all the specimens satisfied a logarithmic creep law. Though the additions of both carbon and nitrogen increased the proof stress, the effects on the creep behavior were different between them. Comparing the creep strain at the normalized stress (applied stress/0.2% proof stress of specimens), carbon had little influence, whereas nitrogen increased the creep strain. Since the nitrogen addition increased the strain rate dependence of the flow stress for a tensile test, nitrogen had an effect of enhancing the thermally activated movements of dislocations and increased the creep strain. From the observation of dislocation structures after creep tests using a transmission electron microscope, the nitrogen-containing steels showed marked planar dislocation structures and many dislocation pile-ups. These dislocation structures were obviously different from those of the carbon containing steels and a high purity steel. Since the nitrogen effect on the creep was recognized at the high dislocation density produced by high applied stress, it is concluded that the dislocation pile-ups in the planar structure made stress concentrations, which enhanced the thermally activated movements of dislocations. Moreover, the activation volumes, calculated from the coefficients in the logarithmic creep law and measured from the stress changes in the strain-rate change tensile tests, were remarkably reduced by nitrogen addition. These were considered as an apparent reduction caused by the effective stress increased locally by the dislocation pile-ups.