Role of precipitate in strain induced martensitic transformation during cold deformation of SS 321

Austenitic stainless steels are extensively used in many areas of application because of their superior corrosion resistance and satisfactory mechanical properties. Amongst various grades of stainless steel, titanium stabilized SS 321 is chiefly used for high temperature structural application due to its extraordinary stability against sensitization at higher temperature. Titanium tends to form carbides/carbonitrides, thereby reducing the carbon content in the austenitic matrix. This helps preventing sensitization problem. The stability of austenitic phase at room temperature is key factor to govern the propensity of strain induced martensitic transformation during cold deformation of austenitic stainless steels. Titanium is a strong ferrite stabilizer. The effect of dissolved titanium content in austenite/the percentage precipitation of titanium on the stability of austenite were studied. Solutionized and as-received specimens were considered for experiments. Solutionizing was done at 1100°C for 30 mins and then water quenched. One of the solutionized specimens was heated at a slow rate of approx. 5°C/minute from room temperature to 1100°C, while the other one was rapidly heated upto the solutionizing temperature. As received structure showed austenitic matrix along with coarse titanium carbide precipitates. Tensile tests on samples were carried out and the volume percentage of the strain induced martensite in each specimen was estimated with a calibrated ferritscopic measurement. Optical microscopy, SEM and EDAX were also performed to find out grainsize, morphology and composition of titanium rich precipitates and regions of austenitic matrix. The results showed a higher propensity of strain induced martensitic transformation in case of solutionized samples. The effect of strain rate on the transformation was also realized through carrying out tensile tests at different strain rates on as received specimens. Afaster test was seen to hinder martensitic transformation, quite expectedly, possibly because of adiabatic heating.