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Diffusion welding of heat-resistant alloys has attracted interest in the manufacturing of components with complex configurations. Controlling secondary precipitates along the interface is necessary to enhance the quality of the diffusion welding. Surface treatment to increase the solubility product (Ksp) of Ti-rich carbide is proposed to accomplish such an enhancement. The reduction of secondary precipitates along the interface induced grain boundary migration across the interface. The chemical compositions at/near the interface satisfied the material specifications. The mechanical properties of the diffusion weldment were similar to those of Alloy 800H that underwent the same thermo-mechanical processes in the range of 25–700 °C. At 25 °C, the tensile strength was 553 MPa, which satisfied the minimum specified tensile strength described in ASTM B: 409-22. The location of failure was random in the gauge section, and dimples, the evidence of the macroscopic plastic deformation, were observed on the fracture surface. [ABSTRACT FROM AUTHOR]

Changes in the material properties of alloys subjected to diffusion welding need to be analyzed to increase the adoption of compact heat exchangers in American Society of Mechanical Engineers standards. Hastelloy X was diffusion-welded in this study following a procedure recently developed by the authors. A metallographic study showed that the interface was discernible with some grains migrated across the interface. Satisfactory tensile properties were obtained at 25 °C, but the tensile samples were fractured at/near the interface with ∼40 % of tensile elongation in the tensile tests. The minimum strength reduction factors of the yield strength and tensile strength were 0.799 and 0.843, respectively. Dimples were extensively observed on the fracture surfaces, but the fracture surfaces were predominantly flat owing to the remnant Mo-rich M6C carbides (stringers). [ABSTRACT FROM AUTHOR]

Tension and creep rupture behaviors of Alloy 617 after a year of thermal ageing at 900 °C were investigated in comparison with those of unaged (virgin) material. Tension tests were carried out under a controlled strain rate of 5.85 × 10−4 (1/s) from room temperature to 950 °C, and creep tests were conducted with different stresses at 900 °C. Tensile and creep rupture properties were obtained, then creep microstructures and the oxidation layer were measured and analyzed. The mechanical properties of Alloy 617 aged for a year showed effects of thermal damage. The mechanical properties were reduced in terms of tensile and creep strengths compared with the unaged. However, the tensile and creep rupture elongation increased. The reduction of creep life (or strength) in the aged material was caused by the rapid creep strain rate, which resulted from material softening due to thermal aging. In the micro-hardness values, the aged material decreased by approximately 26 % compared with the unaged one. The aged material formed a Cr2O3 layer of approximately 20 µm thickness on its outer surface. The outer and internal oxide-layer thickness revealed a typical trend of a parabolic increase. [ABSTRACT FROM AUTHOR]

• Diffusion welding is performed for Alloy 617 following ASME Section IX. • Al-rich oxide formed at interface interrupts the grain boundary migration. • Satisfactory tensile strength is achieved at room temperature. • The slope of curve in Monkman-Grant relationship is a negative 0.77. The development of a compact-type heat exchanger is one of the critical issues for realizing an efficient gas-to-gas heat exchanging system in a very high-temperature gas-cooled reactor (VHTR). The nickel-based superalloy Alloy 617 (Ni-Cr-Co-Mo, UNS N06617) is considered as the leading material for this application owing to the excellent thermal stability and strength in high temperature ranges. Sixty sheets having dimensions of 200 × 200 mm2 were diffusion-welded under uniaxial compressive pressure of 14.7 MPa at 1423 K (1150 °C) in a high vacuum condition. After 2 h of hot pressing, a post-weld heat treatment (PWHT) was applied to enhance the constituent atomic diffusion across the interface. To evaluate the high-temperature mechanical properties, tensile and stress-rupture tests were employed in this study. In tensile tests, the yield strength of the diffusion weldment is 318 ± 4 MPa at room temperature, and superior yield strengths over the ASME code requirement are maintained up to 1223 K (950 °C). Similarly, the tensile strength reaches 730 ± 15 MPa with ductility of 37.1 ± 1.4% at room temperature. However, both the tensile strength and ductility drop rapidly at elevated temperatures (≥973 K (700 °C)). During stress-rupture testing at a fixed temperature of 1173 K (900 °C) in an ambient air environment, the diffusion weldment shows lower creep properties. A plot of log time-to-rupture vs. log minimum creep rate is illustrated on the basis of the Monkman-Grant (M–G) relationship. [ABSTRACT FROM AUTHOR]