The effect of martensite on the sensitization of low carbon 304 stainless steel
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The Effect of Martensite on the Sensitization of Low Carbon 304 Stainless Steel C. L. B R I A N T A N D A. M. R I T T E R The applications of type 304 stainless steel have been limited by the fact that certain heat treatments can produce microstructures which are susceptible to intergranular corrosion, l For commercial alloys these detrimental treatments are usually produced by temperatures in the range of 600 to 750 ~ the times required at these temperatures to produce susceptible microstructures varying between approximately 0.25 and 5 h. Steels susceptible to integranular corrosion are defined as sensitized and the heat treatments which cause this susceptibility are referred to as the sensitization process. Bain and co-workers I suggested a theory to explain sensitization. They proposed that during sensitizing heat treatments chromium carbides nucleate and grow along the grain boundaries of the steel. Because of its high diffusion coefficient, carbon can easily diffuse from the matrix and enter the growing carbide. Chromium cannot diffuse as rapidly, and initially must be supplied from a volume of material very near the growing
C. L. BRIANT, Staff Metallurgist, and A. M. RITTER, Associate Staff, are with General Electric Company, Corporate Research and Development Center, P.O. Box 8, Schenectady, NY 12301. Manuscript submitted September 29, 1980. 910--VOLUME 12A, MAY 1981
carbide. This process substantially depletes the material near the carbide of chromium and thus lowers its resistance to corrosive attack. Since Bain put forth this chromium depletion model, many studies, both theoretical and experimental, have tested it. 2-12Generally, it has held up well. As the model would predict, steels with carbon levels below approximately 0.03 wt pct are quite resistant to sensitization and the model also nicely explains the fact that long aging treatments at 600 to 750 ~ cause steels of higher carbon content to regain their corrosion resistance. This process is referred to as healing, and presumably occurs because chromium diffuses from the matrix and replenishes the depleted zones. However, it has been recognized that variables besides the chromium and carbon concentration can affect the kinetics and degree of sensitization. 2,4,5:1-14One of these variables is cold work. Bain 1 reported that cold work greatly reduced the time required for healing. He reasoned that the slip bands and dislocations produced by the cold work provided many additional sites for carbide precipitation. Chromium depletion would then not be as severe at the grain boundaries and healing could occur more rapidly. Tedmon, Vermilyea, and Broecker 15showed that cold work prior to sensitization did cause carbide precipitation throughout the grains. They also suggested that the strain induced martensite formed by the cold work might accelerate corrosion. Other workers have examined the effect of martensite on corrosion and stress corrosion cracking and found that it increased not only the corrosion rate when the samples were held potentiostatically
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