질소 보호 가스 첨가가 하이퍼 듀플렉스 스테인리스 밀봉용접재의 마모부식 저항성에 미치는 영향

Abstract

Duplex stainless steels with nearly equal fraction of the ferrite(<TEX>${\alpha}$</TEX>) phase and austenite(<TEX>${\gamma}$</TEX>) phase have been increasingly used for various applications such as power plants, desalination facilities due to their high resistance to corrosion, good weldability, and excellent mechanical properties. Hyper duplex stainless steel (HDSS) is defined as the future duplex stainless steel with a pitting resistance equivalent (PRE=wt.%Cr+3.3(wt.%Mo+0.5wt.%W)+30wt.%N) of above 50. However, when HDSS is welded with gas tungsten arc (GTA), incorporation of nitrogen in the Ar shielding gas are very important because the volume fraction of <TEX>${\alpha}$</TEX>-phase and <TEX>${\gamma}$</TEX>-phase is changed and harmful secondary phases can be formed in the welded zone. In other words, the balance of corrosion resistance between two phases and reduction of <TEX>$Cr_2N$</TEX> are the key points of this study. The primary results of this study are as follows. The addition of <TEX>$N_2$</TEX> to the Ar shielding gas provides phase balance under weld-cooling conditions and increases the transformation temperature of the <TEX>${\alpha}$</TEX>-phase to <TEX>${\gamma}$</TEX>-phase, increasing the fraction of <TEX>${\gamma}$</TEX>-phase as well as decreasing the precipitation of <TEX>$Cr_2N$</TEX>. In the anodic polarization test, the addition of nitrogen gas in the Ar shielding gas improved values of the electrochemical parameters, compared to the Pure Ar. Also, in the erosion-corrosion test, the HDSS welded with shielding gas containing <TEX>$N_2$</TEX> decreased the weight loss, compared to HDSS welded with the Ar pure gas. This result showed the resistance of erosion-corrosion was increased due to increasing the fraction of <TEX>${\gamma}$</TEX>-phase and the stability of passive film according to the addition <TEX>$N_2$</TEX> gas to the Ar shielding gas. As a result, the addition of nitrogen gas to the shielding gas improved the resistance of erosion-corrosion.