Improvement of sensitisation of austenitic stainless steel weld metals by laser surface melting treatment: Study of low-temperature sensitisation of austenitic stainless steel weldments and improvement by laser surface melting treatment (3rd report)

Abstract

Summary This paper describes an investigation of the application of laser surface melting treatment to sensitised SUS308 stainless steel to improve its intergranular corrosion resistance. The treatment was performed by a CO2 laser processor with a rated power of 2.5 kW. Argon was used as the shielding gas during the laser treatment. The degree of sensitisation was measured by the EPR method. Before laser treatment, the specimens were sensitised at 773 K × 30 ksec to simulate the conditions causing low-temperature sensitisation of stainless steel weld metals. The results show that the sensitised weld metal fully recovers its intergranular corrosion resistance by laser treatment at the higher laser travel speed, whereas an insufficient improvement effect is obtained at the lower speed. The SEM microstructural analysis results suggest that heavily corroded regions occur in the heat affected zone (HAZ) of the following pass during laser treatment at the lower laser travel speed. The TEM microstructural analysis results suggest that chromium carbides causing loss of corrosion resistance in the HAZ of the following laser treatment pass are precipitated at the δ/γ grain boundaries in laser-treated weld metals. These results indicate that chromium carbides are precipitated in the HAZ of the following laser treatment pass at a low cooling rate and that resensitisation occurs through chromium carbides being precipitated during laser treatment. When the laser-treated zone is heat-treated again after laser treatment, resensitisation takes place within a relatively short time. It is concluded that, when δ ferrite occurs in the laser-treated zone, δ ferrite promotes the precipitation of chromium carbides during a thermal cycle of a low cooling rate and heat treatment after laser treatment.