Duplex stainless steels (DSS) due to their high corrosion resistance combined with good mechanical properties are suitable for critical applications such as oil and gas industries. These properties of duplex stainless steels are attributed to the dual phase microstructure where austenite and ferrite exist in equal-volume fraction and also their high pitting resistant equivalent number.
Further, stress corrosion cracking (SCC) resistance of DSS is superior as compared to austenitic stainless steel because of their dual phase microstructure. However, fusion welding which is an essential operation for joining many of the structural components disturbs this advantage. Moreover, fusion welding of DSS results in the formation of intermetallic phases like chromium nitride and sigma phase respectively, which in turn depletes the key elements like Cr and Mo from the matrix and thereby making DSS prone to localized corrosion.
Friction stir welding (FSW), an advanced solid-state welding technique, can overcome some of the crucial problems like deleterious phase precipitation and phase ratio balance associated with the conventional fusion welding. However, achieving excellent strength without altering the dual phase microstructure of DSS during friction stir welding is a challenge. Further, it is important to study the effect of friction stir welding parameters on SCC resistance of DSS to ensure its applicability.
FSW was performed on UNS S32750 with parameters 600 rpm rotational speed and 25 mm/min welding speed by using W-25%Re tool. It has been observed that grain size decreased significantly in the weld zone as compared to the base metal while the ratio of austenite to ferrite is not affected in the weld zone.
The friction stir welded and base metal UNS S32750 were investigated in 26 wt% NaCl of pH 2 solution through potential dynamic polarizations well as slow strain rate tests (SSRT) to study corrosion and SCC behaviour, respectively.
It is found from SSRT test results that the SCC resistance of friction stir weld joints of UNS S32750 super duplex stainless steel is similar to that of the base metal.
This article is shared by E. Ajay, A. G. Rao, A.P. Singh, V.P. Deshmukh and V S Raja of Department of Metallurgical Engineering and Materials Science, IIT Bombay