Effects of shielding gas on the improving effect of corrosion resistance in the laser surface melted region — Study on low temperature sensitisation in weldments of austenitic stainless steels and its improvement by laser surface melting treatment (Report 4)

Abstract

The effect of nitrogen addition into the laser treated region on sensitization behavior was investigated in this study. The laser surface melting treatment was performed by a CO2 laser by using nitrogen and argon for the shielding gas. Specimens for the laser treatment were heat-treated at 773K for 30ks to imitate the condition to cause low temperature sensitization in weld metals of austenitic stainless steels. The EPR method was adopted for evaluation of sensitization in the melted region. EPR test results indicated that in the case where δ-ferrite was contained in the laser-treated region, improvement of sensitization was insufficient at lower laser traveling velocity. By contrast, striking improvement was obtained even at the lower laser traveling velocity when nitrogen was used for the shielding gas. This is because that re-sensitization in the melted region heat-treated after laser treatment with nitrogen gas shielding was suppressed. Micro-structural analysis revealed that the primary phase at solidification in the melted region by the laser treatment changed from δ to γ, resulting in the decrease in the amount of δ-ferrite at the lower laser traveling velocity by using nitrogen for the shielding gas. Such microstructural change seemed to be responsible for the fact that sufficient improvement was obtained even at the lower laser traveling velocity, because theoretical analysis revealed that sensitization at γ/γ boundary occurred much longer time than that at δ/γ boundary. Thus, re-sensitization in the laser-treated region was prevented by using nitrogen shielding gas in the laser remelting treatment.