EFFECT OF GRAIN-BOUNDARY PHENOMENA ON THE SOFTENING OF 316L-AUSTENITIC STEEL AT HIGH-TEMPERATURES

In this paper we present results of tests concerning changes in the fraction of grain boundaries (GBs) free from trapped lattice dislocations (TLDs) as a function of the heating temperature. 316L austenitic stainless steel was used. The tests were performed both on thin foils and on bulk samples. The hardening coefficient for uniaxial tension and low cycle fatigue (LCF) was tested. The relationship between the hardening of the tested material and the presence of TLDs on boundaries was investigated. It was found that the kinetics of trapped lattice dislocation spreading in GBs in thin foils, measured in in situ experiments, is controlled by boundary diffusion. In bulk samples, in ex-post investigations the kinetics of dislocation trapping by GBs can control whether or not the boundaries are free from dislocations, after heating the sample at a given temperature. During uniaxial tension at a strain rate epsilon = 2.8 X 10(-5) s-1 an increase in the fraction of GBs free from dislocations is directly proportional to the decrease in the value of the hardening coefficient as a function of temperature. This means that, under the described conditions of straining, GB migration and GB sliding play a decisive role in the softening of the tested steel. When the strain rate epsilon is 2.8 X 10(-4) s-1, GB sliding and GB migration play a smaller role in the softening of the tested steel. During LCF, changes in the hardening coefficient are strongly affected by strain aging. The effect of strain aging of the tested steel on the changes in the hardening coefficient as a function of strain temperature is much more intensive during LCF than during uniaxial tension.