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Abstract
The present investigation concerns plasma-keyhole welding of UNS S32760 super-duplex stainless steel. Apart from Argon, a mixture of argon with 2 vol% nitrogen was used in both plasma and shielding gas, in order to improve the austenite/ferrite balance. Keyhole welding with argon provides with a narrow fusion zone with a satisfactory microstructure, which is further improved by the addition of nitrogen in the plasma and protective gas. The mechanical properties, in terms of hardness and impact strength, have been studied and compared for both operating gases used. It has been shown that better mechanical properties of the weldments are obtained when using an argon-2vol% nitrogen mixture. All welded specimens exhibit satisfactory impact toughness with regard to standard requirements. Ductile to brittle transition takes place in the temperature range from −40°C to −65°C. Nitrogen addition to the plasma gas affects positively the impact toughness of the weldments above the transition temperature, but below this temperature the effect of nitrogen becomes negative. This is interpreted in terms of the austenite-to-ferrite ratio and of the possibility of ferrite to deform by twinning even in the subcritical temperature range.