The 9% Cr steels were developed for ultra-supercritical (USC) power plants to meet the requirements of high operating temperature and pressure. These steels are produced to operate at high temperatures where impact toughness is not a concern; however, it becomes important for the welded joints to have good impact toughness at room temperature for manufacturing. The present work investigates the effect of the post-weld heat treatment (PWHT) parameters, i.e., temperature and time, on the impact toughness of multi-pass gas tungsten arc welded (GTAW) joints of ferritic/martensitic grade P92 steel. The microstructural evolution in welded joints given varying post-weld temperatures and times was studied. The lath martensitic structure of the weld metal for the as-welded joints resulted in high hardness and low impact toughness. The weld fusion zone toughness was 12 J, which was lower than the minimum specified values of 41 J (ASME standards) and 47 J (EN ISO 3580:2017). The PWHT temperature and time were found to have a significant effect on the impact toughness of the weld metal. A drastic increase in the impact toughness of the weld metal was noticed, which was attributed to lath break-up, reduction in dislocation density and reduction in solid solution hardening. The maximum impact toughness of 124 J was measured for PWHT temperature and time of 760 °C and 120 min, respectively. The effect of PWHT parameters on tensile strength was also investigated, and test results showed that the joint was safe for USC boiler application as it failed from the region of the P92 base metal. The variation in microstructural evolution along the weldments resulted in hardness variation. PWHT led to homogeneity in microstructure and, ultimately, reduction in hardness value. According to the study, the optimum temperature and time for PWHT of a GTAW joint of P92 steel were found to be 760 °C and 120 min, respectively.

The present study investigates the effects of multiple passes of equal channel angular pressing (ECAP) on magnesium alloy sheets with the assistance of an Inconel plunger along with a die setup having a channel angle of 120° and corner angle of 10° operating at a temperature of 200 °C followed by the required heat treatment processes. The microstructural analysis of the sheet samples at various stages of the multi-pass hot ECAP has shown evidence of ultrafine grain refinement (UFG) due to the occurrence of severe plastic deformation. X-ray diffraction analysis has also exhibited the presence of phases like MgZn and CeZn₃ which is supposedly responsible for the enhancement of the mechanical properties. As a result, the room temperature tensile and compressive strengths have improved by 6.12% and 6.63%, respectively, after the second pass, and 11.56% and 15.64%, respectively, after the fourth pass of ECAP. Additionally, the hardness of the sheets has increased by 6.49% and 16.64% after the second and fourth pass of hot ECAP, respectively, mainly attributed to the drastic decrease in grain size from 164 μm to 12 μm within four ECAP passes, all these with a negligible change in ductility. This success in the thermomechanical processing of Mg-RZ5 alloy sheets using a die channel angle of 120° with a minimal number of passes of hot ECAP under a controlled equivalent strain, further opens doors for incorporating optimizations and/or additional aspects so as to achieve even better grain refinements, and consequently, mechanical strength improvements thereby catering to the industrial needs of aerospace and construction areas.

To reduce costs and improve high-temperature performance in Advanced Ultra Super Critical (AUSC) boilers, it is necessary to weld austenitic steel to Inconel alloy. In this study, the autogenous tungsten inert gas (TIG) welding process was used to join Alloy 617 and an austenitic AISI 304H steel plate of thickness 5 mm. Microstructural analysis showed that the microstructure formation was uneven along the weldments, with columnar and cellular dendrites near the interface while the central area of the weld exhibited a combination of columnar, cellular, and equiaxed dendrites. The use of energy dispersive spectroscopy and electron probe micro-analysis unveiled the presence of an unmixed layer at the interface between the weld and AISI 304H steel. Furthermore, a notable variation in the concentration of alloying elements such as Fe, Cr, Ni, Co, and Mo was observed. Within the weld metal, inter-dendritic areas showed the presence of precipitates rich in Cr, Ti, and Mo. Meanwhile, the heat-affected zone (HAZ) of Alloy 617 exhibited the presence of phases like Cr and Mo-rich M₂₃C₆ as well as Mo-rich M₆C. Hardness tests showed non-uniform hardness along the weldments, with a hardness of 199 ± 6 HV in the weld metal and 225 ± 4 HV in Alloy 617 HAZ, and 207 ± 7 HV in AISI 304H HAZ. The Mo and Cr segregation in the inter-dendritic spaces led to a decline in the tensile properties of the welded parts and resulted in failure from the region of the weld metal.

For high-temperature industries operating at nearly 750 °C (advanced ultra-super critical boilers), dissimilar welding between Inconel alloys and austenitic stainless steel (ASS) are commonly adopted. The high-temperature resistive properties of Inconel and ASS alloys are highly qualified for high-temperature applications. In this experimental study, dissimilar autogenous laser beam welding (LBW) between Inconel 718 and ASS 304L is investigated. This paper explains the detailed study on the microstructural and mechanical behavior of the LBW dissimilar joint. The microstructural study indicates the presence of laves phases in the weld zone. Additionally, the weld zone shows heterogeneous microstructural formation, owing to the non-uniform welding heat in the different areas of the weld zone. The optical images show the presence of mixed dendrites, i.e., equiaxed, cellular, and columnar morphology, in the weld zone and in the fusion zones of either side. The energy-dispersive spectroscopy (EDS) results show the presence of segregated elements (Nb, Mo, Cr, and Ti) at the weld center. These segregated elements are the reason for the occurrence of the laves phases in the weld zone. The presence of Nb and Mo may form the laves phase (Fe, Ni, Cr)₂ (Nb, Mo, Ti) along with Fe, Ni and Cr. The presence of an unmixed zone is observed in the HAZ of the Inconel 718, whereas the HAZ of the ASS 304L shows the presence of an unmixed zone (UZ) and a partially mixed zone (PMZ), as observed on the optical and SEM images. To obtain the mechanical properties of the laser weld, the tensile test, microhardness test, and impact test were measured at room temperature. The tensile specimens show a brittle failure at the ASS 304L side, which was initiated from the weld top, with average tensile stress of 658.225 MPa. The reason for the ASS 304L fracture is because of the presence of UZ and PMZ, and the lower hardness value of the ASS side. The UZ and PMZ lead to the fracture of the tensile specimen along the ASS 304L side’s HAZ. The measurement of microhardness carried out along the transverse length indicates an average microhardness of 214.4 HV, and the value is 202.9 HV along the weld depth. The mixed morphology of the microstructure promotes the variation in hardness in both directions. The hardness along the length shows a high hardness value in the weld zone and uniformly decreases along the base materials. The Charpy impact test of the weld zone shows the brittle fracture of the impact specimens. From the microstructural and mechanical results, the LBW dissimilar weld between Inconel 718 and ASS 304L is qualified for safe use in high-temperature end applications, such as AUSC power plants.

The dissimilar combination of 2.25Cr-1Mo (P22) and modified 9Cr-1Mo (P91) obtained using Gas Tungsten Arc Welding (GTAW) process employing the Ni-based superalloy filler ERNiCr-3 (IN82) and ERNiCrMo-3 (IN625) have been investigated for microstructure evolution and mechanical properties. The butt weld joint was produced using single bevel groove geometry. The structural integrity of the welded joint was measured in respect of tensile strength, impact toughness and hardness. The alloying elements’ segregation at the inter-dendritic areas of the weld metal was witnessed while using the IN82 and IN625 filler. The impact test trials showed the mixed mode of fracture with an impact toughness of 82 ± 6 J and 70 ± 5 J for IN82 and IN625 filler, respectively, ensuring that the welded joint was safe for the end boiler application. The tensile test coupons were fractured from the P22 base metal in all the trials and for both the fillers which confirmed the negligible effect of the filler composition on the tensile properties. The hardness plots showed the inhomogeneity in hardness value, which was also supported by the microstructure evolution along the weldments. The average hardness of the IN82 filler was measured lower than the IN625 filler.