Metallurgical investigations were carried out on a welded sample of 25% Cr super duplex stainless steel (UNS S32750) taken from a vessel that had been accidentally operated above 300-350°C (570-660°F) for at least six months, which had resulted in brittle fracture. As a result of microstructure observations, no abnormalities were found in the ferrite/austenite balance and no intermetallic phases were observed in the sample, while Charpy impact tests at room temperature revealed quite a low absorbed energy in both, the base metal and the weld metal. These results indicated the occurrence of 475°C (885°F) embrittlement in the steel. De-embrittlement heat treatment trials were also performed to find out the effective temperature range. The aim of this paper is to draw attention to the risk of inservice embrittlement of duplex stainless steels, highlighting the difference in susceptibility to 475°C (885°F) embrittlement of the base metal and the weld metal.
Duplex stainless steels have a two-phase microstructure, approximately 50% ferrite (a) and 50% austenite (?), and are now widely used for various industry applications because of their high strength and excellent corrosion resistance compared to 300 series stainless steels. However, these steels are prone to embrittlement when they are exposed to temperatures in the range of 315 – 540°C (600 – 1000°F) for longer than a certain period of time.1 Since this phenomenon most rapidly occurs at about 475°C (885°F), it is called “475°C (885°F) embrittlement.” This embrittlement results from decomposition of the original ferrite (a) phase in a ferrite (a)-austenite (?) mixture to an Fe-rich a’ phase and a Cr-rich a” phase.
Figure 1 2 shows typical time-temperature embrittlement curves of duplex stainless steel base metals. The lower nose (or “bay”) indicates the occurrence of 475°C (885°F) embrittlement. 25% Cr super duplex stainless steel (UNS S32750, marked “2507” in the figure) is more susceptible to this embrittlement than 22% Cr duplex stainless steel (UNS S32205, marked “2205” in the figure) is. It is known that 475°C (885°F) embrittlement tends to occur more rapidly in duplex stainless steel weld metals than in the base metals; however the embrittlement behavior of the weld metals has not been investigated in such detail as that of the base metals has been.