The effects of cold work and low-temperature sensitization heat treatment of non-sensitized austenitic stainless steels have been investigated and related to the cracking in nuclear power reactors. ...Types 304, 304L and 304LN developed martensite after 15% cold working. Heat treatment of these cold worked steels at 500 °C led to sensitization of grain boundaries and the matrix and a desensitization effect was seen in 11 days due to fast diffusion rate of chromium in martensite. Types 316L and 316LN did not develop martensite upon cold rolling due to its chemical composition suppressing the martensite transformation (due to deformation) temperature, hence these were not sensitized at 500 °C. The sensitization of the martensite phase was always accompanied by a hump in the reactivation current peak in the double loop electrochemical potentiokinetic reactivation test, thus providing a test to detect such sensitization. It was shown that bending does not produce martensite and therefore, is a better method to simulate weld heat affected zone. Bending and heating at 500 °C for 11 days led to fresh precipitation due to increased retained strain and desensitization of 304LN due to faster diffusion rate of chromium along dislocations. The as received or solution annealed 304 and 304LN with 0.15% nitrogen showed increased sensitization after heat treatment at 500 °C, indicating the presence of carbides/nitrides.
Because of their austenitic-ferritic microstructures, duplex stainless steels offer a good combination of mechanical and corrosion resistance properties. However, heat treatments can lower the ...mechanical strength of these stainless steels and render them susceptible to intergranular corrosion (IGC) and pitting corrosion. In this study, a low-carbon (0.02%) duplex stainless steel is subjected to various heat treatments at 450-950 deg C for 30 min to 10 h. The heat-treated samples then undergo ASTM IGC and pitting corrosion tests, and the results are correlated with the microstructures obtained after each heat treatment. In the absence of Cr sub 23 C sub 6 precipitation, sigma -phase precipitates render this duplex stainless steel susceptible to IGC and pitting corrosion. Even submicroscopic sigma -phase precipitates are deleterious for IGC resistance. Longer-duration heat treatments (at 750-850 deg C) induce chromium diffusion to replenish the chromium-depleted regions around the sigma -phase precipitates and improve IGC resistance; pitting resistance, however, is not fully restored. Various mechanisms of sigma -phase formation are discussed to show that regions adjacent to sigma -phase are depleted of Cr and molybdenum. The effect of chemical composition (pitting resistance equivalent) on the pitting resistance of various stainless steels is also noted.
