The effects of deformation induced martensite on the sensitization of austenitic stainless steels
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THE fact that certain heat treatments make nonstabilized austenitic stainless steels susceptible to corrosion greatly limits the applications of these alloys. The heat treatments which produce this susceptibility or sensitization involve isothermally heating in, or slowly cooling through, the range 600 to 800 ~ The corrosion which results is most often intergranular. Its occurrence has been attributed to the precipitation of chromium carbides along the grain boundaries during these sensitizing heat treatments and the concomitant depletion of chromium around the growing carbide.~ This chromium depletion model gains support from the facts that intergranular chromium carbides do form during sensitizing heat treatments, that lowering the chromium content makes the steel more susceptible to corrosion, and that long time heat treatments in the sensitizing temperature range do restore the excellent corrosion properties to the material. This latter phenomenon is called healing and presumably occurs because chromium has sufficient time to diffuse to the depleted boundary. Throughout the years examples have shown that prior cold work changes the sensitization process. Bain and co-workers ~ showed that cold work led to rapid healing. They suggested that this rapid healing occurred because the carbides could nucleate both along the grain boundaries as well as along the slip bands within the grains. With the higher density of carbides, less chromium depletion would occur at any point. Tedmon, Vermilyea, and Broecker2 found that cold work produced rapid healing in 304 stainless steel if applied before sensitization, but increased the corrosion rate if applied after sensitization. They suggested that this increase could result from rapid corrosion of the martensite which formed during the deformation.
C. L. BRIANT and A. M. RITTER are Staff Metallurgist and Associate Research Staff, respectively, General Electric, Corporate Research and Development, P.O. Box 8, Schenectady, NY 12301 Manuscript submitted May 13, 1980.
Other workers 3-9 have specifically studied the effects of deformation induced martensite on the corrosion of 304 stainless steel. SeveraP -~ have found that its presence increased both the corrosion rate of samples held potentiostaticaUy in the active range and the critical current density required for passivation. Vermilyea6 showed that austenitic steels containing martensite were more susceptible to stress corrosion cracking in aqueous solutions than were solution annealed samples, and Honkasalo, e t a l 7 found that the martensite was preferentially corroded in U-bend tests in sulfate solutions. Finally, Hahin and coworkers a reported that deformation by cold upset forging of sensitized samples greatly increased the corrosion rates of type 304 and 304L stainless steel in the highly oxidizing Huey test once the deformation was sufficient to induce martensite. Deformation did not increase the corrosion of 316 stainless steel, nor did it induce martensite in the alloy. Recently, we showed that deformation prior to
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