Influence of deformation on sensitization kinetics and its microstructural correlation in a nuclear grade 316 stainless steel

This paper deals with the influence of morphological features of grain boundary carbides on the sensitization behaviour in a nuclear grade 316 stainless steel. Sensitization kinetics was studied in the temperature range of 825 to 1175 K for different degrees of cold work, ranging from 5 to 25%. It was found that the temperature of the nose of the time-temperature-sensitization (TTS) diagrams at 1025 K, remained unaltered with cold work, whereas the rate of sensitization at 1025 K increased with deformation up to approx.= 15%, and remained constant thereafter. In order to understand the cause of the enhanced rate of sensitization with deformation, transmission electron microscopy was carried out. The morphology and the size of grain boundary M23C6 carbides, which are responsible for sensitization, were characterised for samples aged at 1025 K for ageing durations corresponding to the onset of sensitization. Transmission electron microscopy was employed to measure the average diameter anti D, the intercarbide spacing anti Λ, and the shape factor anti S (the ratio of length to width). These parameters were found to decrease with deformation, reaching a saturation at 15% cold work. The role of these microstructural variables in enhancing the sensitization kinetics with deformation, is discussed in the light of the well known chromium depletion theory. (orig.)