On the M23C6-Carbide in 2205 Duplex Stainless Steel: An Unexpected (M23C6/Austenite)—Eutectoid in the δ-Ferritic Matrix

The study of sigma and carbide in cast austenitic stainless-steel grade HH after 24 years of high-temperature service

Cast austenitic stainless steel (CASS) has been widely used for long-term service periods in high-temperature applications. Nonetheless, the behavior of CASS after high-temperature and long-term service is insufficiently researched alongside the microstructural analysis for sigma phase and carbide. Here, intermediate pipe support made from CASS grade HH was investigated after 24 years of service at 700 °C and compared with the solution-treated specimen at 1100 °C for 2 h. The chemical composition was analyzed by optical emission spectroscopy (OES), while the microstructure was observed using optical microscopy and scanning electron microscope (SEM). The confirmation of phase composition and lattice parameters were further analyzed by energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction spectroscopy (XRD). The mechanical properties were assessed by tensile tests, hardness tests, and impact tests, whereas the corrosion property was analyzed using potentiodynamic polarization. Based on experiment and analysis, the effects of 24 years of exposure on the sigma phase are spheroidization and Cr depletion, and even partial transformation to ferrite. The sigma phase significantly increases the hardness, but Cr depletion slightly reduces the hardness to 96.86 HRB. Meanwhile, the carbide will increase in quantity and develop an irregular interface at long-term high-temperature exposure. Microstructure evolution of the sigma phase and carbide decreased tensile strength to 46%, elongation to 3%, and impact value to 1.6% of the minimum specifications, respectively, while the corrosion rate increased 10 times (about 7.35 μm per year).