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.

The effect of the post-weld heat treatment on the microstructures and mechanical properties of the dissimilar joint of G115, a novel developed martensite heat resistant steel, and CB2 steel, currently used in an ultra-super-critical power unit, was investigated. The results indicate that the quenched martensite underwent decomposition and transformation, and the amount of dislocations were sharply decreased in the weld metal after post-weld heat treatment (PWHT). Many nano-scale M₂₃C₆ precipitates present in the weldment were distributed on the grain and grain boundary in a dispersed manner with PWHT. The average microhardness of the weldment decreased from about 400 HV to 265–290 HV after PWHT and only a slight decrease in the microhardness of CB2 steel was detected after PWHT at 760 °C. In contrast to the case of the as-received joint, the tensile strength of the joint was improved from 630 MPa to 694 MPa and the fracture location moved from the weld metal to the base metal after PWHT. The fracture surface consisted of a cleavage fracture mode without PWHT, whereas many dimples were observed on the fracture surface with PWHT.

The dissimilar joining of solid-solution-strengthened superalloys and precipitation-strengthened superalloys enables complementary performance synergy, holding significant application potential in the aerospace industry. This study investigated the transient liquid phase bonding of Hastelloy X and IN738 using a BNi-2 interlayer, focusing on the effects of bonding temperature and time on interfacial microstructure evolution and mechanical properties. The results demonstrated that achieving complete isothermal solidification is paramount for joint properties, a process governed by the synergistic control of bonding temperature and time. At lower temperatures (e.g., 1050 °C), the joint centerline contained an athermal solidification zone (ASZ) rich in hard and brittle Cr-rich (∼15.9 GPa) and Ni-rich borides, which served as the failure initiation site. As the ASZ was progressively eliminated with increasing temperature, a fully isothermal solidified zone (ISZ, ∼52 μm wide) consisting of γ-Ni formed at 1100 °C. Concurrently, Cr-Mo borides (∼9.8 GPa) precipitated within the diffusion-affected zone (DAZ) on the Hastelloy X side, becoming the new potential sites for crack initiation. Prolonging the holding time at 1100 °C not only ensured complete isothermal solidification but also promoted Mo diffusion, which improved the plasticity of the Cr-Mo borides and their interfacial bonding with the γ-Ni matrix (∼5.9 GPa). This synergistic optimization resulted in a significant increase in joint shear strength, achieving a maximum value of 587 MPa under the optimal condition of 1100 °C/40 min.

Repairing is considered as a practical and economic scheme to effectively extend the lifetime of components made of Inconel 738 (IN 738), which has been widely used in hot sections of jet engines and industrial gas turbines. In this study, the feasibility of repairing IN 738 substrates was investigated using InterPulse tungsten inert gas (TIG) welding with Inconel 625 (IN 625) and Pmet 838 as feeding materials. It was shown that single-pass deposition achieved sound metallurgical bonding under three different TIG parameters, with no cracks observed in any of the specimens. Under multi-pass repairing, γ ′ dissolution was observed in the heat-affected zone (HAZ), and cracks were observed neither in the multi-pass deposited layers nor the HAZ. The Pmet 838 deposited layer was quasi-homogeneous and entirely devoid of γ ′; the IN 625 multi-pass deposited layer, however, exhibits pronounced mesoscopic segregation with significantly lower hardness than the IN 738 substrate. The Pmet 838 deposited layer demonstrates considerably higher hardness than that of the IN 738 substrate, primarily attributed to the existence of dense cellular structures. The hardness in the HAZ of both specimens decreases with increasing the distance from the fusion line to the base metal, which may be associated with gradient variations of the cellular structures. After annealing at 920 °C for 150 h, the average Vickers hardness of the Pmet 838 multi-pass deposited layer decreased from 486.1 ± 13.9 HV _0.2 to 378.2 ± 6.2 HV _0.2 , even though copious precipitation of γ ′ was observed in it, which is attributable to the disappearance of the cellular structure; in contrast, the IN 625 deposited layer exhibited heterogeneously distributed γ ′ precipitates and needle-like δ phase, corresponding to the aforementioned mesoscopic segregation, resulting in an increase in the average Vickers hardness from 252.1 ± 6.6 HV _0.2 to 317.6 ± 17.4 HV _0.2 .

Inconel 625 and Hastelloy X are two commonly used solid‐solution‐strengthened superalloys, and the connection of them is inevitable in high‐temperature applications. Brazing is a commonly accepted joining method. In this study, the two alloys were vacuum brazed by sandwiching a BNi‐2 adhesive filler into them. A brazing temperature‐dependent cracking phenomenon, where the cracking ratio decreased from 50.73% to 7.09% when the brazing temperature was elevated from 1030°C to 1070°C, was observed at the junctions. It is found that the microstructures of the junctions are not uniform and are categorized into two main types, both consisting of Si‐denuded, Si‐rich, and medium Si regions. The cracks predominantly form in Type II microstructures in which the well‐aligned Si‐denuded zones are fully constrained by the surrounding Si‐rich regions. The formation of the unique microstructures is attributed to deep undercooling induced by fast diffusion of B, as supported by composition characterization and thermodynamic calculations. The cracking susceptibility and the proportion of each microstructure type are governed by the brazing temperature, which influences the extent and likelihood of deep undercooling. Critical discussion indicates that the crack initiation most likely occurs within the Si‐denuded regions rather than at the eutectic regions.

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.