In the present work, dissimilar butt joints between a low-Ni, medium-Mn austenitic stainless steel, M-Mn SS, and a Ni-Cr austenitic stainless steel, Ni-Cr SS, were processed by utilizing the gas tungsten arc welding (GTAW) technique at different heat inputs. A filler metal of ER308 was employed in the welding process. The filler yields 480 MPa, which is equivalent to the yield strength of M-Mn SS. The microstructural analysis and mechanical performance (i.e., tensile strength and hardness properties) of the concerned joints were studied by using an optical microscope and uniaxial tensile tests, respectively. The results revealed that a duplex structure from austenite matrix and delta ferrite is promoted in the fusion zone (FZ) of the dissimilar joints processed with low and high energy inputs (0.486 kJ/mm and 0.558 kJ/mm). The FZ of the specimens welded at high heat input exhibited the lowest hardness value (151.2 HV) in comparison to heat affected zone (HAZ) (166.3 HV). Moreover, the joints exhibited a low tensile strength of 610 MPa. The achieved strength is significantly lower than the strengths of the base metals (BMs) M-Mn SS and Ni-Cr SS. This is mainly attributed to the inhomogeneous dendritic structure of the FZ with Cr-carbides precipitation.

The friction stir welding (FSW) process was recently developed to overcome the difficulty of welding non-ferrous alloys and steels. In this study, dissimilar butt joints between 6061-T6 aluminum alloy and AISI 316 stainless steel were welded by FSW using different processing parameters. The grain structure and precipitates at the different welded zones of the various joints were intensively characterized by the electron backscattering diffraction technique (EBSD). Subsequently, the FSWed joints were tensile tested to examine the mechanical strength compared with that of the base metals. The micro-indentation hardness measurements were conducted to reveal the mechanical responses of the different zones in the joint. The EBSD results of the microstructural evolution showed that a significant continuous dynamic recrystallization (CDRX) occurred in the stir zone (SZ) of the Al side, which was mainly composed of the weak metal, Al, and fragmentations of the steel. However, the steel underwent severe deformation and discontinuous dynamic recrystallization (DDRX). The FSW rotation speed increased the ultimate tensile strength (UTS) from 126 MPa at a rotation speed of 300 RPM to 162 MPa at a rotation speed of 500 RPM. The tensile failure occurred at the SZ on the Al side for all specimens. The impact of the microstructure change in the FSW zones was significantly pronounced in the micro-indentation hardness measurements. This was presumably attributed to the promotion of various strengthening mechanisms, such as grain refinement due to DRX (CDRX or DDRX), the appearance of intermetallic compounds, and strain hardening. The aluminum side underwent recrystallization as a result of the heat input in the SZ, but the stainless steel side did not experience recrystallization due to inadequate heat input, resulting in grain deformation instead.

In this work, 2 mm thick medium-Mn austenitic stainless steel (MMn–SS) plates were joined with austenitic NiCr stainless steel (NiCr–SS) and low-carbon steel (LCS) using the gas tungsten arc welding technique. A precise adjustment of the welding process parameters was conducted to achieve high-quality dissimilar joints of MMn–SS with NiCr–SS and LCS. The microstructural evolution was studied using laser scanning confocal and electron microscopes. Secondary electron imaging and electron backscatter diffraction (EBSD) techniques were intensively employed to analyze the fine features of the weld structures. The mechanical properties of the joints were evaluated by uniaxial tensile tests and micro-indentation hardness (H_IT). The microstructure of the fusion zone (FZ) in the MMn–SS joints exhibited an austenitic matrix with a small fraction of δ-ferrite, ~6%. The tensile strength (TS) of the MMn–SS/NiCr–SS joint is significantly higher than that of the MMn–SS/LCS joint. For instance, the TSs of MMn–SS joints with NiCr–SS and LCS are 610 and 340 MPa, respectively. The tensile properties of MMn–SS/LCS joints are similar to those of BM LCS, since the deformation behavior and shape of the tensile flow curve for that joint are comparable with the flow curve of LCS. The H_IT measurements show that the MMn–SS/NiCr–SS joint is significantly stronger than the MMn–SS/LCS joint since the H_IT values are 2.18 and 1.85 GPa, respectively.

The quality of welded joints of FSSW is mainly dependent on the processing parameters while the main disadvantage of this process is the creation of an exit hole. Process parameters, namely tool dimensions, tool rotational speed, and stir time were changed and their impact on bond dimensions and weld strength was investigated using 2024-T3 Al Alloy. Macro- and microstructures of the welded samples were examined; shear fracture loads were measured and the optimum set of operation variables was determined. To decrease the exit hole of the first stage the present paper proposes a modified two-stage weld-refill process employing the same welding machine. In this work, this two-stage process was referred to as reversed friction stir spot welding (ReFSSW). In the second stage, a smaller pin was used and the shoulder diameter was designed such that to force the metal of the upper plate to flow towards the exit hole of the first stage decreasing its dimensions. The metal flow in the second stage was evaluated by examining the microstructure of the metal that filled the exit hole of the first stage. Thin stir zone was found around the pin of the second stage followed by thermomechanically affected zone consisting of grains elongated in the vertical direction. The proposed process resulted in smaller exit hole dimensions and consequently higher mechanical properties compared with the conventional single-stage FSSW.

Steam boiler flange, made of steel ASTM A105 welded to spool pipe, experienced a failure near the welding area after six years of service. The operator company has reported that a sudden increase in operating pressure and temperature exceeded the normal operation conditions. As a result, the failed flange and welded spool pipe segments were received for further investigation using root cause analysis to determine the reason for the failure. The failed parts were visually inspected (VT) and tested for subsurface defects with a fluorescent wet magnetic particle test (MT). Chemical analysis, microstructure observations, and hardness tests are conducted for the failed flange and pipe. It is noticed that there were no abnormalities or indications of creep or overheating were found in the microstructure and hardness values. Upon examining the macro & microstructure of the connecting weld joint, several welding defects, such as non-metallic inclusions, few porosities, and lack of fusion between welded passes, were noticed. Scanning Electron Microscope (SEM) observations showed that fine striations as beach marks on the fractured surface provided strong evidence of fatigue failure. The identified crack originated from welding defects and progressed through the flange material due to fluctuating radial stresses due to the cyclic changes in working pressure during service. To prevent similar failures, it is recommended to apply proper welding procedure specifications (WPS) by qualified welders, conduct regular inspections and maintenance of the steam boiler, and keep working conditions stable.