The GTD-111 alloy is a nickel base superalloy designed to work at high temperatures which is used in manufacturing gas turbine blades. The welding of these alloys is associated with fundamental problems such as weld cracks and heat affected zone. In this research, the effect of three basic parameters, i.e. filler metal, heat treatment, and heat input, were evaluated to eliminate hot cracks. Initially, the airfoil and root microstructure were investigated according to which it was determined that the airfoil structure was degraded due to high temperature and stress tolerance, but the root structure has not been significantly changed and can be considered as the GTD-111 superalloy reference microstructure. Then, welding operations were performed on extracted samples from blade roots with two IN625 and IN617 filler metals. With microstructure examinations, using optical microscope, scanning electron microscope, and calculating the crack length mean, the IN625 filler metal was selected as the optimum filler. The samples were then subjected to three types of heat treatments and then the welding operations were performed. Microstructure examinations by optical microscope, EDS-equipped electron microscope, Vickers hardness test, and software calculations of precipitate mean size and the final crack length mean indicated that the samples under solution-treat operations at 1180° C for 2 hours had no cracks the cause of which is the dissolution of the secondary phase particles and lack of formation of grain boundary liquid film. Investigating welded samples with different heat inputs; it was found that increasing the heat input increases the cracks, except for the heat treatment samples at 1000° C in which the cracks decrease with increasing heat input. Heat treatment samples exhibited the highest resistance against cracks at 1180° C and with a heat input of 3.7 kj/cm. In order to extend the extracted results of welding root samples to airfoil, solution-treated operation was proposed at 1180° C for 3.5 hours.

 In this research, the dissimilar welding of hastelloy nickel-base superalloy C276 to austenitic stainless steel 316 L was performed with Argon shielding gas tungsten arc welding (GTAW). In order to evaluate the effect of filler metal, three filler metals including Inconel 625 (ERNiCrMo-3), hastelloy C276 (ERNiCrMo-4) and austenitic stainless steel 309L (ER309L) were used. The effect of inlet heat on the microstructure of the weld metal and the adjacent area for welding Inconel 625 was studied. After performing welding operations, microstructure of different areas for each joint, including base metals, welding metals, heat affected zones, and the intersections using an optical microscope and field emission scanning electron microscope (FESEM) equipped with chemical analysis (EDS) were evaluated. The weld metal ferrite was measured by the ferrite device. To study the mechanical properties of the weld, tensile, hardness, micro-hardness, and impact tests were conducted. The fracture surfaces of samples were studied using a scanning electron microscope (SEM). The results showed that the filler metal of Inconel 625 has better mechanical properties than the other filler metals. The results of studying the inlet heat for welding Inconel 625 showed that increasing the inlet heat from 530 to 600J/mm improved the mechanical properties (impact energy, hardness,) and the further increasing the inlet heat from 600 to 700J/mm reduced the mechanical properties of the weld metal.

 

Abstract : The aim of this study was to investigate the effect of casting parameters on the residual stress and strain caused in cast iron components. In order to investigate the effects of casting parameters (such as risering and chilling) on the residual stress accumulated in hypo, hyper and eutectic cast irons with lamellar and spherical graphite, a rectangular shape component was selected. The model dimensions were determined based on the Taguchi method which is one of the well known methods to optimize the parameters of the experiments. The mentioned model was employed to study and simulate the influence of metallurgical parameters using a commercial finite element called ProCAST software. Solidification process of the cast component in mold casting process was simulated using a combined solution of heat and tension. The results showed that due to the difference in cooling rate of various sections of the component, residual stress begins to be accumulated in the cast component. Moreover, the results showed that the accumulated stress in the cast component decreases as the percentage of carbon increases and also the mentioned stress decrease when a piece of chill locates in the mold. It was also found that the level of the residual stress is graphite shape dependent so that the stress increases as the shape of the graphite varies from lamellar to spherical. Simulation results were also affected by mechanical and thermal model used by the software. The behavior of the cast irons used in this study, was considered to be elastoplastic and the mechanical behavior of the mold was not considered in the software, due to its very low strength (compared to cast iron). Cutting method as a destructive method for residual stress measurement, was employed to measure the amount of residual stress in the cast components and to verify the simulated results. Owing to the availability and relatively low cost of the cutting method. Results showed that the central bar's diameter, side bar's diameter and the length of the bars have a decreasing trend on the residual stress level, respectively. In addition, the optimal values for the mentioned dimensions were 19, 12 and 125 mm, respectively. Regarding residual stress level of the optimized lattice, the results showed acceptable concordance between simulation and practical values.