Low Temperature Sensitization in the Weld Metal of Type 308 Stainless Steel and Its Improvement by Laser Surface Melting Treatment.

Abstract

Low temperature sensitization (LTS), which is a sensitization for intergranular corrosion at temperatures below the normal isothermal sensitization range owing to forming of chromium carbide nuclei at grain boundaries already, in the weld metal of Type308 stainless steel and its improvement by laser surface melting treatment were investigated in this study.Three kinds of SUS308 stainless steels which were varied in carbon contents were used. TIG welding method was adopted to make the weld metals. The degree of sensitization in the sensitized weld metals was evaluated by the EPR test and the Strauss test.Experimental results revealed that LTS in weld metals occurred at much shorter time than in HAZ, which was attributed to the accelerated precipitation of chromium carbides at δ/γ grain boundaries in the weld metals.Surface melting treatment by a CO2 laser generator was found to improve the corrosion resistance in sensitized weld metals. Especially on the conditions with high cooling rates, the corrosion resistance was markedly improved, because re-sensitization during cooling could be suppressed. Such effects were considered to be attributed to the dissolution of chromium carbides with this treatment, which results in annihilation of the chromium depleted zone adjacent to grain boundaries