Low temperature impact properties of phosphorus and sulfur doped and sensitized type 304 stainless steel
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INTRODUCTION
T Y P E 304 stainless steel undergoes a dramatic reduction in aqueous corrosion resistance and toughness and an increased susceptibility to intergranular stress corrosion cracking (IGSCC) when heat treated at 450 to 850 ~ ~incidental to the corrosive environment and stress are the microchemical changes that occur in the material and lead to a reduction in mechanical properties. This reduction in properties, referred to as sensitization, is generally attributed to the diffusion of carbon and the formation of chromium-rich carbides at grain boundaries and a chromium depletion region adjacent to the carbidesfl The propensity for carbide formation and the extent of the chromium depletion region into the grain matrix varies with the bulk composition of the alloy. The grain boundaries become susceptible to enhanced corrosion as the chromium level in the depleted region falls below 12 wt pct, and this susceptibility varies with carbon level. A number of models, based on carbon and chromium diffusion, have been used to explain this behavior, 3-6 and the ASTM has designated several aqueous corrosion tests by which the degree of sensitization may be obtained. There has been evidence in recent years that carbide precipitation and chromium depletion in Type 304 stainless steel may not be the sole contributing cause to susceptibility to IGSCC. For example, several authors have reported that weldments in piping were susceptible to intergranular cracking of the heat-affected zones even when the bulk compositions were identical.7's Microchemical analyses of grain boundaries in the heat-affected zones of random heats as well as from sections that were furnace sensitized showed sulfur concentrations in excess of 10 to 100 times the bulk level. 9 Other researchers have also shown that commercial heats of Type 304 stainless steel may have grain boundary compositions substantially different from the grain interiors.~~ For example, phosphorus as well as sulfur has been found to segregate to grain boundaries as well as free surfaces in these steels.'~ The grain boundary structure may STEVEN DANYLUK, Associate Professor, and IRA WOLKE, Graduate Student Assistant, are with the Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago, Chicago, IL 60680. Manuscript submitted October 23, 1984.
METALLURGICAL TRANSACTIONS A
also contribute to a variability in extent of segregation, v These changes in grain boundary composition are believed to contribute to the formation of inte~ranular cracks when the steel is subjected to corrosion environments and stresses. In this paper we present results of a study of the low temperature impact properties of a systematically doped and sensitized Type 304 stainless steel. The intention of this work was to develop a link between the impact properties, bulk composition, and thermal treatment. This paper is a summary of part of a broader study of the influence of sulfur and phosphorus additions on the grain boundary composition of a commercial heat of Typ
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