DEFORMATION EFFECTS ON INTRAGRANULAR CARBIDE PRECIPITATION AND TRANSGRANULAR CHROMIUM DEPLETION IN TYPE-316 STAINLESS-STEELS

Grain matrix or transgranular (TG) chromium depletion in type 316 stainless steels (SS) has been observed to be strain and heat treatment dependent, yielding C-curve behavior on a time-temperature-strain-TG corrosion map. The strain effect dominates above 16 to 20 percent showing increased TG corrosion and decreased time to develop TG depletion with increasing strain level. Higher temperatures also reduce the time to develop grain matrix attack, while isothermal holding for longer times produces higher amounts of TG corrosion in SS. Transgranular depletion is caused by precipitation of carbides within grain-matrix regions. Specifically, defect sites created during straining are favored locations for intragranular precipitation in SS. Regions of high dislocation density, deformation twin-stacking fault planes and twin-fault intersections were identified to be preferred sites for grain matrix precipitation during transmission electron microscopy (TEM) of 16 to 35 percent deformed SS.