Sensitization of Austenitic Stainless Steels, I. Controlled Purity Alloys

Abstract

Abstract This paper presents a study of the effects of carbon, nitrogen, molybdenum, and manganese on the sensitization of high-purity austenitic stainless steels of composition similar to Type 304. The modified Strauss test (ASTM-A262-E), the oxalic acid etch test (ASTM-A262A) and analytical electron microscopy were used to determine the degree and nature of sensitization in the steels. The alloy compositions are considered in terms of effective chromium content, and from plots of this parameter versus sensitization time a strong effect of carbon content is seen. Additions of nitrogen, molybdenum, and manganese are found to delay sensitization at any given carbon concentration. In the case of nitrogen, the amount of improvement depends on both the carbon content and the sensitization temperature. Strong evidence is presented that nitrogen acts to retard the nucleation and/or growth of carbides at grain boundaries and hence increase the time necessary for sensitization. Molybdenum appears to increase the ease with which the steel passivates; thus more chromium depletion is required before sensitization will be detected. The combination of molybdenum plus nitrogen is found to be particularly effective in retarding nucleation and/or growth of carbides; molybdenum alone does not have this effect. Only limited experimental evidence is presented concerning the role of manganese. Beneficial effects of this element are primarily seen at low sensitizing temperatures, where manganese assists nitrogen in slowing carbide formation.