Recently, demand for stainless steels have increased continuously in desulfurization facilities, unclear power plant cooling facilities and petrochemical industries. High corrosion resistance materials such as duplex stainless steels are required in various industrial fields. In this study, gas tungsten arc welding (GTAW) and CO2 laser beam welding (LBW) processes were carried out without filler materials. The corrosion behaviors of GTAW and LBW on super duplex stainless steel tube with or without post weld heat treatment (PWHT) and different PWHT condition (950, 1000, 1050, 1100 oC for 5 and 30 min). Electrochemical potentiodynamic polarization and critical pitting temperature measurements were carried out for pitting corrosion resistance.
The critical factors affecting the degradation of pitting corrosion resistance of as-welded condition (GTAW, LBW) were unbalanced phase fraction and Cr2N of precipitation. The improvement for the corrosion resistance of as-welded specimen depends on the PWHT conditions. There are many Al-Mg-Ca oxides in the GTAW weld metal compared to LBW weld metal as a result of the slow cooling rate. With or without PWHT, oxides existed in weld metal, the pitting corrosion was generated on GTAW weld metal near the oxide. In case of LBW, weld metal showed inferior pitting corrosion resistance caused by sigma phase after PWHT below 1000 oC. Sigma phase grew long time and low temperature of PWHT, corrosion damage increased in weld region and base metal. The austenite phase stable temperature was about 950~1000 oC and the ferrite phase stable temperature was over 1050 oC. As the PWHT temperature and duration increased after welding, the equal phase fraction in weld metal, but the base metal phase fraction was greatly changed. The pitting was generated in the base metal region by ferrite phase growth of ferrite stable temperature