885°F Embrittlement of Super Duplex Stainless Steel Impeller in Hot Off Gas Blower

Abstract During Start-up of Hot off Gas blower, catastrophic brittle fracture was observed on the final stage impeller of 4 stage Hot off gas blower. The metallurgy of the impeller is Super Duplex stainless Steel (UNS S32750) and the 4th stage of the blower was continuously operated above 325°C (615°F). Metallurgical failure analysis was carried out using Optical Microscopy, Chemical Composition analysis, Ferrite measurement, Hardness measurement, Impact test and Fractography analysis by Scanning Electron Microscopy. In Microstructure analysis no abnormalities in the Ferrite/austenite balance and no intermetallic were observed. However, the room temperature impact test revealed very low observed energy (2J) suggesting possibility of 475°C (885°F) Embrittlement. To ascertain the same De-embrittlement heat treatment was carried out at 550°C & 600°C and subsequent impact test energy values confirmed the in-service embrittlement. ASME BPV Codes restricts the use of Duplex stainless steel above 315°C (600°F), and it is followed in the pressure vessel components. However, in the case of rotary equipment internals and valve internals this restriction is not considered by some of the vendors due to high mechanical strength advantage. The aim of the paper is to draw attention towards metallurgical selection for high temperature internal components for pumps, compressors, valves and blowers, which are typically packaged items and follow vendor specifications.