The U.S. Bureau of Mines has studied the effects of cold working and subsequent heat treatments on the susceptibility of a high-nitrogen stainless steel to localized corrosion. The degree of sensitization (DOS) of a cold worked high-nitrogen stainless steel was measured using procedures specified in ASTM G108 and compared to DOS values measured for the nanworked steel. Cold working was done at the 20% level and heat treatments ranged from 600 to 800°C for up to 100 hours. Sensitization of high-nitrogen stainless steels occurs in a tmannar similar to that of carbon-containing stainless steels with the exception that chromium nitrides are predominantly responsible for the sensitization. Micrographs comparing the effect of cold working showed that the formation of both grain boundary and intragranular Cr2N precipitates occurred more rapidly for the cold worked samples. The effect of heat treatment on the non-worked high-nitrogm stainless steels is to increase the DOS with increasing aging time at both 600 and 700°C. At 800°C, however, the DOS decreases or “heals” for aging times greater than 30 hours. The healing process has been related to the chromium depletion of the matrix material. Cold working prior to heat treatment increases the DOS values but also decreases the aging time necessary to heal the high-nitrogen stainless steel. Thus at 700% aging of cold worked material causes healing for times greater than 30 hours while the non-worked samples showed no healing in similar time periods. Also, at 800°C healing of the old worked samples occurs in 10 hours compared to 30 hours for the non-worked samples.
Keywords: cold work, corrosion, nitrogen stainless steel, sensitization, stainless steel