Dissimilar joining of nickel-based superalloys to stainless steel is widely used in strategic and important industries such as aerospace, nuclear, and oil and gas. In this research, joining of Hastelloy 276C nickel-based superalloy to 316 austenitic stainless steel using the transient liquid phase process at two temperatures of 1050 and 1150 °C and at times of 5, 30, 60 and 90 minutes using BNi-2 interlayer was examined under a vacuum of 10-5 mbar. Microstructure of the joints was eva‎luated using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive analysis (EDS) and X-ray diffractometery. Mechanical properties of the joints were eva‎luated using shear strength and microhardness tests. In addition, fracture surfaces of the samples obtained from the shear strength tests were studied by scanning electron microscopy. The results of microstructural studies showed that the joint area consisted of several regions with different microstructures, including isothermal solidification, athermal solidification, and diffusion affected zones. The athermal solidification zone contained nickel-rich boride compounds and chromium and nickel-rich silicides. In the isothermal solidification zone, only nickel-rich γ solid solution phase was present. The diffusion affected zone of the Hastelloy nickel base superalloy contained boride compounds rich in chromium-molybdenum, nickel-chromium-molybdenum and nickel-chromium-molybdenum-tungsten, but in the diffusion affected zone of austenitic stainless steel, boride compounds rich in iron, iron-chromium and iron-chromium-nickel were observed. At 1050 and 1150 °C, isothermal solidification took place in 30 and 5 minutes, respectively. The results of the shear strength test at 1050 °C indicated that with the increase in the holding time, the shear strength of the joints increased and the highest shear strength of 535 MPa was achieved after 90 minutes. Study of the fracture surfaces indicated the occurrence of brittle fracture in the interface containing the athermal solidification zone and occurrence of ductile fracture in the joints containing the isothermal solidification zone. The highest hardness value in the sample whose 1050 °C and 5 min isothermal solidification zone was not completed belonged to the athermal solidification zone. In other samples, where isothermal solidification was completed, the highest hardness value belonged to the diffusion affected zone of nickel-based superalloy.

مرتضي شمعانيان اصفهاني، بهزاد نيرومند

حسين مستعان

مسعود عطاپور، علي اشرفي