Continuous cooling and low temperature sensitization of AISI types 316 SS and 304 SS with different degrees of cold work

This paper presents the results of investigations carried out to study the sensitization behaviour of AISI Types 316 SS and 304 SS with various degrees of cold work ranging from 0 to 25%. Initially Time-Temperature-Sensitization (TTS) diagrams were established using ASTM standard A262 Practice A and E tests. From these diagrams it was found that the rate of sensitization and overall susceptibility to intergranular corrosion increases up to 15% cold work and above that starts decreasing. Desensitization was observed to be faster for higher levels of cold work, especially in the higher sensitization temperature range. From the TTS diagrams, the critical linear cooling rate below which sensitization occurs was calculated. From these data, Continuous Cooling Sensitization (CCS) diagrams were established. The results show that as the degree of cold work increases up to 15%, time needed for sensitization decreases and hence faster cooling rates must be used in order to avoid sensitization. At temperatures sufficiently below the nose temperature of the TTS diagram, log t versus 1/T plots follow a linear relationship where t is the time needed for the onset of sensitization at temperature T. From the slope, the apparent activation energy for sensitization was estimated. The validity of extrapolating these linear plots to lower temperatures (725 to 775 K) (which lie in the operating temperature regime of fast reactors) has been verified by experiment. The effect of heat treatment and microstructure on the Low Temperature Sensitization (LTS) behaviour was investigated. The results indicate that carbides of optimum size and distribution are the essential pre-requisites for LTS and cold work enhances susceptibility of stainless steels to LTS. (orig.)