Improvement of corrosion resistance of sensitized austenitic stainless steel by laser surface modification

The effect of the laser surface melting on the corrosion resistance of sensitized austenitic stainless steel was investigated relating to the post laser-melting heat treatment. Type 304 stainless steel with high amount of carbon content sensitized by a heat treatment was used for the examination of improvement of corrosion resistance by laser beam irradiation. The continuous wave type of CO2 laser equipment and the pulse wave type of YAG laser equipment were used. The corrosion resistance was examined by the ASTM262 practice-E test (Strauss test). Two types of specimens, that is, the as-laser-melted specimen and the PLHT (Post Laser-melting Heat Treated at 773 K for 24h) specimen were prepared for the corrosion test. The corrosion resistance of the sensitized specimen was remarkably improved by laser surface melting. Surface-melted zones had high corrosion resistance on all laser process condition for as-laser-melted specimens. The corrosion resistance of the surface melted zone depended on the thermal hysteresis by laser beam irradiation for the PLHT specimens. As the cooling rate becomes rapid, the corrosion resistance gets higher. The effect of the microstructure on the corrosion resistance was investigated. The primary-austenite microstructure formed at the cooling rate high than 105 K/s had a marked corrosion resistance even for the PLHT specimens. It is suggested that the corrosion resistance in the surface melted zone is related to the precipitation behavior of chromium carbide. (author)