The microstructure evolution, elements diffusion and fracture behavior of the Stellite 6 weld overlay, deposited on 10Cr9Mo1VNbN (F91) steel by the tungsten inert gas (TIG) cladding process, were investigated after long-time service. Obvious diffusion of Fe occurred from the steel and fusion zone to the Stellite overlay, resulting in the microstructure evolution and hardness increase in the coating, where hard Co–Fe phases, σ phases (Fe–Cr metallic compounds) and Cr-rich carbides (Cr_18.93Fe_4.07C₆) were formed. Besides, the width of the light zone, combined with the fusion zone and diffusion zone, increased significantly to a maximum value of 2.5 mm. The fracture of the Stellite coating samples mainly occurred in the light zone, which was caused by the formation and growth of circumferential crack and radial crack under high temperature and pressure conditions. Moreover, the micro-hardness values in the light zone increased to the maximum (470–680 HV) due to the formation and growth of brittle Co–Fe phases. The formation of these cracks might be caused by formed brittle phases and changes of micro-hardness during service.

This study investigates the high-temperature high-cycle-fatigue behavior of TIG-welded Haynes 230 alloy using smooth and notched specimens (Kt = 3) at 850–950 °C. Both elevated temperature and notch effect significantly degrade fatigue performance. As temperature increases, thermally activated crack initiation and propagation accelerate, shifting the S–N curves downward. Notched specimens exhibit substantially shorter fatigue lives due to stress concentration-induced multi-source crack initiation, dispersed propagation zones, and fragmented sudden fracture zones. The alloy shows low notch sensitivity across the test temperature range, with the lowest value at 900 °C. A unified fatigue life prediction model is established using a temperature correction factor exp[P × ()], demonstrating high fitting accuracy. M-G curves of notched specimens shift downward and leftward, revealing the aggravating effect of the notch on creep–fatigue interaction. Fractographic analysis shows that with increasing temperature, smooth specimens exhibit expanded sudden fracture zones and degraded striations, while notched specimens display multi-source initiation and a composite morphology of striations with fine dimples, consistent with macroscopic mechanical behavior. This work delivers comparative experimental data and mechanism-oriented insights for TIG-welded Haynes 230 joints within 850–950 °C; the proposed temperature-corrected model is valid for the investigated test window and provides reference for component performance analysis.