Creep damage mechanism of 20Cr32Ni1Nb steel at high temperatures

In the petrochemical industry, the centrifugally cast austenitic 20Cr32Ni1Nb stainless steel used in steam reformer furnace's hot outlet manifold experiences creep failure and microstructural degradation, impacting its structural integrity. Understanding the causes of these issues is crucial for addressing the associated challenges. This study examines the creep deformation behaviour, damage mechanisms, and microstructural evolution of 20Cr32Ni1Nb steel under stresses of 22–50 MPa at temperatures of 1163 K (890 °C) and 1223 K (950 °C). Analysis of creep and creep strain rate curves revealed the presence of both primary and tertiary creep stages, with the power law relationship governing the stress-dependent minimum creep rate and rupture life. The variance of creep damage tolerance factor was also analysed, with results showing that in high-stress regimes, loss of section and necking along with cavity growth may initiate tertiary creep, while in low-stress regimes, microstructural degradation, specifically G-phase formation, may initiate tertiary creep. As the duration of creep exposure increases, microstructures were observed to degrade, with the growth and coarseness of NbC and M₂₃C₆ carbides. The structure of the space group for the Ni–Nb–Si enriched G-phase in 20Cr32Ni1Nb steel was found to be Fm3‾m, with a lattice parameter of about 1.14 nm.