Study on the microstructural degradation and brittle fracture mechanism of S20910 stainless steel bolts during long-term high-temperature service

This study investigates the brittle fracture mechanism of S20910 stainless steel bolts after long-term service at 670 °C in sulphur-bearing flue gas. Through macro-fractography, mechanical testing, and multi-scale microstructural characterisation (metallography, EBSD, EDS), failure is attributed to continuous σ-phase precipitation along austenite grain boundaries. Quantitative EBSD reveals a σ-phase content gradient along the fracture path: 16.1% in initiation zone, 13.1% in propagation zone, and 6.9% in final fracture zone. This network embrittles grain boundaries and induces severe Cr depletion, enabling preferential ingress of S and O and forming a characteristic three-layer corrosion structure. The material exhibits dramatic loss of high-temperature ductility (15.5% elongation at 670 °C) and significant hardening (348–378 HV10), leading to intergranular brittle fracture under impact loading. EBSD-based strain analysis (KAM, GND density) confirms strain localisation at σ-phase interfaces. This work establishes a complete failure chain from composition to microstructure, properties, corrosion, and fracture.