Abstract: 20Cr1Mo1VTiB bolt steel is widely used as a high-temperature fastener in thermal power plants, where it operates under complex stress conditions. However, this steel undergoes significant strength-toughness degradation after prolonged high-temperature service, and the microstructural mechanisms responsible for this degradation under real service conditions remain unclear. In this work, the microstructure evolution of this steel served after 1.30×10⁵ h was systematically analyzed. We found that long-term exposure promoted the continuous precipitation of M₆C carbides along grain boundaries, and facilitated the co-precipitation of Laves phases, accompanied by Ostwald ripening-induced coarsening of VC carbides, grain coarsening, and a reduction in dislocation density. Correspondingly, the yield and tensile strengths decreased from 809 ± 8 and 890 ± 6 MPa to 720 ± 7 and 833 ± 12 MPa, respectively. Especially, severe impact toughness deterioration was also observed, with absorbed energy sharply dropping from 132 to 34 J and the fracture mode shifting from ductile to brittle. These precipitation clusters lower the critical stress for cracking, concentrate boundary stress, and weaken strain compatibility, promoting crack propagation along prior-austenite grain boundaries. This work provides microstructural evidence and a mechanistic understanding of this steel's degradation under true service conditions, offering guidance for service life assessment and alloy optimization of high-temperature fasteners.

Key words: Bolt steel, Long-term high-temperature service, Laves phase, Carbides, Performance degradation, Failure mechanism