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The welding heat-affected zone (HAZ) of C-HRA-2 nickel-based alloy at different welding peak temperatures (T _p ) (850 to 1450 °C) was obtained through welding thermal simulation. The microstructure of the different simulated HAZs was characterized by an optical microscope (OM), a scanning electron microscope (SEM), and a transmission electron microscope (TEM). The microhardness of simulated HAZs was examined using a microhardness tester. The results indicated that fine and dispersed M _23 C _6 carbides were identified along the grain boundaries in the simulated HAZs at 1050 and 1150 °C. In the simulated HAZs above 1250 °C, eutectic microstructure of γ matrix bonded with M _23 C _6 carbides was identified close to the grain boundaries due to constitutional liquation. The liquefaction of γ matrix and M _23 C _6 carbides primarily accounted for the simulated HAZ liquation of C-HRA-2 alloy during continuous welding heating. The content of the liquefied phase was increased as T _p was increased. The liquefied phase was converted into the eutectic microstructure of γ matrix and M _23 C _6 carbides in the cooling process. Besides 1050, 1150, and 1450 °C, the microstructures under other T _p were similar to the microstructure of the Base Metal (BM). The microhardness distribution of simulated HAZs tended to be higher in the middle and lower at both ends with the increase of T _p . Furthermore, the result indicated that the microhardness of the simulated HAZ was the closest to BM at 1050 °C.

We studied the microstructure and mechanical properties of the simulated welding heat-affected zone (HAZ) for modified austenitic medium manganese steels (MAMMS) in the welding peak temperature range of 750 °C to 1050 °C. The precipitation behavior of cementite and VC particle was sensitive to the welding thermal cycle in this temperature range. When the peak temperature increased from 750 °C to 850 °C, the intergranular cementite and VC particle became coarser, leading to deterioration of toughness. However, when the peak temperature raised to 1050 °C, the intergranular cementite disappeared. The sensibility of tensile strength to heat input at high peak temperature was higher than that at low temperature, which implied that it was affected by combined effects of the two factors of heat input and peak temperature, rather than a single factor. This variation tendency was highly related with the coarsening behavior of VC particle depending on the interaction effect of these two factors. The growth rate of VC particle with peak temperature of 850 °C was ∼4 times as high as that with peak temperature of 750 °C. However, it only increased to ∼1.3 times when the peak temperature increased from 850 °C to 1050 °C.

The relevance of the researchinthe field of welded joints strength and failure is associated with the necessity of ensuring high operational reliability and safety running hazardous production facilities. Internal stresses and their relaxation as an independent cause of destruction are widely recognized nowadays. The final form of the maximum permissible state is cracks appearance, however the signs of the appeared maximum permissible state of the node or structure are not clearly defined. The aim of the researchis to establish the signs of weld zone prefracture and fracture under redistribution conditions of internal stresses during thermal relaxation. The subject of the research iswelded superheater units made of dissimilar steels. Research methods: physical modeling of operating conditions by thermal cycling of samples in the MIMP-10UE electric furnace, X-ray dosimetry of the samples, internal structural stresses evaluation on DRON- type X-ray diffractometers, morphological analysis using the «Resource S7» metallographic analyzer, including an inverted Olympus GF41 microscope with the SIAMS Photolab software, cracks morphology with a PEN SCKOPE microanalyzer, microhardness analysis using a PMT-3 microhardness tester. Results. According to the hypothesis of crack formation correlation with internal stress relaxation, crack appearance and growth and stress relaxation are interrelated processes. As a result of thermal fluctuation relaxation, the same qualitative regularities (which lead to destruction) for a single-phase and two-phase system are found. Natural aging processes occurring in the sample with an initial crack in the absence of external loads and deformations are controlled only by internal stresses that activate all mechanisms of destruction, including phase decay. Under the conditions of internal stresses thermofluctuation relaxation, the temperature limit of the stress state region is set, which determines the reliable operating temperature.