Effect of High Temperature Dynamic Strain on Grain Boundary Sensitization in Type 304 Stainless Steels

In order to investigate the controlling factors in grain boundary sensitization of type 304 stainless steels during welding, grain boundary corrosion and carbide precipitation were examined using optical and transmission electron microscopy after thermal or thermomechanical cycles. Thermomechanical treatment, which simulated high temperature dynamic strain during welding, enhanced carbide precipitation on the grain boundary and sensitization of SUS304 steel. This behavior occurred only when the maximum temperature was below the solution temperature of the carbide. Straining in heating was more effective for sensitization and carbide precipitation than that in cooling. High temperature dynamic strain was more effective for sensitization and carbide precipitation than cold working before thermal treatment. Thermomechanical treatment sensitized SUS304L steel as well, while the degrees of grain boundary corrosion and carbide precipitation were lower than those in SUS304 steel. Grain boundary migration was observed only in SUS304L steel after thermomechanical cycles when the maximum temperature was below the solution temperature of the carbide. Ratio of sensitization increased with increasing grain size.