レーザ表面溶融処理によるステンレス鋼溶接金属の鋭敏化改善

オーステナイト系ステンレス鋼溶接部の低温鋭敏化とレーザ表面溶融処理によるその改善(第3報)

抄録

Laser surface melting treatment was applied to the sensitized Type 308 stainless steel weld metal in order to improve its intergranular corrosion resistance in the sensitized Type 308 stainless steel weld metal. The treatment was performed by a CO₂ laser generator with the maximum power of 2.5 kW. Argon was used for the shielding gas in the laser treatment. The degree of sensitization was examined by the EPR method. Prior to the laser treatment, the specimens were sensitized at 773 K for 30 ks in order to imitate the condition to cause low temperature sensitization in stainless steel weld metals. Experimental results indicated that the sensitized weld metal restored a complete corrosion resistance to intergranular corrosion by the laser treatment at the higher laser traveling velocity, whereas the improving effect was insufficient at the lower laser traveling velocity. Owing to the microstructural analysis by a SEM, serious corroded regions were found at the heat affected region of following pass in the laser treatment with lower laser traveling velocity. The microstructural analysis by a TEM revealed that chromium carbide which caused the deterioration of corrosion resistance precipitated at δ/y boundary in laser treated weld metal. These results indicated that chromium carbides precipitated in the HAZ of following laser treating pass with the low cooling rate, and resensitization occurred by the precipitation of chromium carbides during the laser treatment. Consequently, when laser treated region was heat-treated again aftr laser treatment, re-sensitization took place relatively early. These results suggest that, in the case of δ-ferrite existing in the laser treated region, the precipitation of chromium carbides during the thermal cycle of low cooling rate and the haet-treatment after the laser treatment was promoted by δ-ferrite.