This study aims to investigate the suitability of requirement for post-weld heat treatment(PWHT) temperature when different P-No. materials are welded, which is defined by ASME Sec. III Code. For SA-516 Gr. 60 and SA-106 Gr. B carbon steels that are typical P-No. 1 material, simulated heat treatment were conducted for 8 h at 610℃, 650℃, 690℃, and 730℃, last two temperature falls in the temperature of PWHT for P-No. 5A low-alloy steels. Tensile and Charpy impact tests were performed for the heat-treated specimens, and then microstructure was analyzed by optical microscopy and scanning electron microscopy with energy-dispersive spectrometry. The Charpy impact properties deteriorated significantly mainly due to a large amount of cementite precipitation when the temperature of simulated heat treatment was 730℃. Therefore, when dissimilar metal welding is carried out for P-No. 1 carbon steel and different P-No. low alloy steel, the PWHT temperature should be carefully selected to avoid significant deterioration of impact properties for P-No. 1 carbon steel.

The influence of thermo-mechanical cycling on the microstructure and strength in the weld coarse-grained heat affected zone (CGHAZ) of Ti-Nb added low carbon HSLA steel was explored through Vickers hardness tests, nanoindentation experiments, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Undeformed and deformed CGHAZs were simulated using Gleeble simulator with different heat inputs of 30kJ/cm and 300kJ/cm. At high heat input of 300kJ/cm, the CGHAZ consisted of ferrite and pearlite and then their grain sizes were not affected by deformation. At low heat input of 30kJ/cm, the CGHAZ consisted of lath martensite and then the sizes of prior austenite grain, packet and lath width decreased with deformation. In addition, the fraction of particle increased with deformation and this is because the precipitation kinetics was accelerated by deformation. Meanwhile, the Vickers and nanoindentation hardness of deformed CGHAZ with 30kJ/cm heat input were higher than those of undeformed CGHAZ, which are due to the effect of grain refinement and precipitation strengthening.

The effect of post-weld heat treatment (PWHT) temperature on the mechanical properties of the heat-affected zone (HAZ) of P-No. 3 low-alloy steel was investigated by taking into account the changes in microstructure. SA-508 Gr. 3 Cl. 1 forged steel was employed, and the specimens taken from the steel were thermally cycled using a Gleeble simulator to simulate coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), intercritical HAZ (ICHAZ), and subcritical HAZ (SCHAZ). The respective base metal and simulated HAZ specimens were heat treated in a furnace at 610, 650, 690, and 730℃ for 8 hours. Before the heat treatment, the ICHAZ, FGHAZ and CGHAZ exhibited high hardness and poor impact toughness. However, when the PWHT was applied, the hardness decreased and the impact toughness improved; the impact toughness was highly dependent on the PWHT temperature. The impact toughness of the ICHAZ and FGHAZ increased significantly when the PWHT temperature was 650℃ or below, while that of CGHAZ improved significantly when the PWHT temperature was 650℃ or higher. The microstructural changes due to the PWHT were observed by scanning electron microscopy and correlated with the mechanical properties.r composed of iron-based composition system had similar characteristics to the material to be repaired.

The effects of weld heat input modeling and the material hardening model on gas tungsten arc welding were evaluated by implementing different input models in a finite element analysis (FEA). For the comparison with FEA results, an experiment was conducted on an SA-516 Gr.70 plate specimen with a part-through wall groove. The comparison with the experimental results confirmed that the shape of the heat distribution during the welding process was changed according to each heat input method. For heat input models based on heat flux, the heat distribution of the weld was dependent on the volume and shape of the finite element model. For the welding deformation results, the FEA results obtained when using the temperature boundary condition method differed from the experimental data. The difference in the FEA results of deformation and residual stress according to the heat input method and material kinematic hardening model was analyzed as a complex phenomenon in which the distribution of the equivalent plastic deformation during the welding process and the hardening behavior can be treated differently.

Additive manufacturing (AM) is an interesting technology with the potential to replace general fabrication techniques such as machining, forming and casting. AM provides two main benefits: greater freedom of part design and lower material consumption. To improve quality, many researchers have investigated the mechanical characteristics of AM products and additive parameters. In the present study, tensile and fatigue tests were conducted on stainless steel (SS) 316L manufactured using powder bed fusion (PBF) technology. These tests were carried out using two different building directions, along the Z and X axis, to investigate the effect of additive direction. In addition, the effects of hot isostatic pressing (HIP) related to fatigue properties, which can influence the volume density and porosity of the specimen, was studied. Based on the test results, changes in the mechanical characteristic and fatigue strength due to the HIP treatment were investigated.