Abstract
Various water cooled cutting and grinding operations were found to promote sensitization to intergranular attack of AISI 304 stainless steel in two ways. There may be precipitation of chromium carbides, one to two grains deep, at grain boundaries near the surface, even during water cooled cutting with an abrasive wheel. Of greater importance is the sensitization observed when cut material is heated at relatively low temperatures, 300 or 500 C for 24 h to 19 days. The resulting sensitized surface layer is up to seven grains (0.5 mm) deep in a steel containing 0.069% C, without any effect at these temperatures on the bulk material below this layer. In both types of sensitization, stress appears to be a powerful promoter of carbide precipitation. During cutting with a water cooled abrasive wheel, the stress applied by the wheel accelerates carbide precipitation in a thin layer during very brief exposures at the relatively high temperatures generated by the cutting action. During prolonged isothermal heating at relatively low temperatures, precipitation of chromium carbides appears to be promoted by the residual stresses left in a layer which may be as much as 1.3 mm deep.
In contrast, machining and cold rolling, which result in plastic deformation, concentrate chromium carbides at slip lines during prolonged heating at low temperatures (300 and 500 C). These findings made it possible to resolve an apparent discrepancy reported in the literature between measurements of sensitization with an electrochemical potentiokinetic reactivation (EPR) test and the tests in ASTM A 262.
