Comparison of the microstructure, corrosion resistance, and hardness of 321 and 310s austenitic stainless steels after thermo-mechanical processing

Abstract

In this study, the effects of microstructural changes after thermo-mechanical processing (TMP) on the corrosion resistance and hardness of 321 and 310 s austenitic stainless steels (ASS) were investigated and the corrosion behavior of these two alloys were compared. First, the as-received sample was subjected to 90% thickness reduction through cryo-rolling at liquid nitrogen temperature (-196 °C), and then, annealing treatment was performed at 850 °C for 5 and 15 min. To evaluate the corrosion properties of the samples, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were performed. The microstructure of the as-received samples consisted of equiaxed grains, annealing twins, and a small amount of delta ferrite. Due to annealing for 5 min after cryo-rolling, a combination of fine and ultrafine grains was observed. By increasing the annealing time to 15 min, grain growth and secondary recrystallization (abnormal grain growth) occurred in 310 s and 321 ASS, respectively. The results of potentiodynamic polarization and EIS tests confirmed each other and both indicated an improvement in the corrosion resistance of cryo-rolled and annealed specimens compared with as-received samples. It is postulated that the development of fine-grained and ultrafine-grained structures in TMP specimens leads to the reinforcement of the passive layer of chromium and improvement of re-passivation. Moreover, optical microscope images before and after the corrosion test showed pitting corrosion. The differences in the corrosion behavior of 321 and 310 s ASS were also discussed. The results of the hardness test showed hardness amplification in TMP specimens which were confirmed by the Hall-Petch relationship.