Characterization of Duplex Stainless Steel Weldments

Abstract

Duplex Stainless Steels (DSS) have found widespread use in a range of industries, because of their high strength with high resistance to stress corrosion cracking and resistance to sensitization-induced intergranular corrosion. The more high alloyed grades [super DSS] are resistant to oxygenated or chlorinated seawater. Consequently, due to the stable mechanical and corrosion properties after thermal aging for long term service at temperatures below 400°C, duplex stainless steel is used in pipes and components in the primary cooling systems in the PWRs (Pressurized Water Reactors). The investigated material was DSS of grade {UNS S31803 (ASTM A790, ASTM 2205)} in the form of pipe. Each weldment specimen was heat treated at one of the following temperatures: 475°C, 600°C, 700°C, 850°C, 1050°C and 1150°C for 1 hour. The ferrite content and hardness tests were performed along the weldment. The impact test was performed according to the ASTM E23 and the examination of intergranular corrosion susceptibility was performed (ASTM A262 - Practice B). The microstructure investigation was carried out by using the light optical microscopy for etched specimens. The SEM with EDAX was used to detect the microstructural changes due to the heat treatment and after corrosion as well as within the fracture surface of the impact specimens. Slight changes were observed at 475 and 600 °C/1 h conditions as compared to the as received condition, while a little loss in ferrite content was observed at 700°C aging condition. This may be the result of precipitation of different types of secondary phases such as χ-phase, ζ-phase, carbides and nitrides. At 850°C, the observed great loss in ferrite content may be attributed to precipitation of σ-phase with large volume fraction. The solution treatment at 1050 °C/1h condition improved the ferrite content values as a result of decomposition of σ-phase. But the ferrite % value does not return back to its original value at the as received condition. On the other hand, when the solution treatment was done at 1150 °C/1h condition, the ferrite content continued to increase to the as received condition as a result of recovery of δ-ferrite inside the structure.