Abstract
9CrMoW steel tubes were welded in multiple passes by gas-tungsten arc welding. The reheated microstructures in the Gr. 92 weld metal (WM) of a multiple-pass weld were simulated with an infrared heating system. Simulated specimens after tempering at 760 °C/2 h were subjected to constant load creep tests either at 630 °C/120 MPa or 660 °C/80 MPa. The simulated specimens were designated as the over-tempered (OT, below AC1, i.e., WT-820T) and partially transformed (PT, below AC3, i.e., WT-890T) samples. The transmission electron microscope (TEM) micrographs demonstrated that the tempered WM (WT) displayed coarse martensite packets with carbides along the lath and grain boundaries. Cellular subgrains and coarse carbides were observed in the WT-820T sample. A degraded lath morphology and numerous carbides in various dimensions were found in the WT-890T sample. The grain boundary map showed that the WT-820T sample had the same coarse-grained structure as the WT sample, but the WT-890T sample consisted of refined grains. The WT-890T samples with a fine-grained structure were more prone to creep fracture than the WT and WT-820T samples were. Intergranular cracking was more likely to occur at the corners of the crept samples, which suffered from high strain and stress concentration. As compared to the Gr. 91 steel or Gr. 91 WM, the Gr. 92 WM was more stable in maintaining its original microstructures under the same creep condition. Undegraded microstructures of the Gr. 92 WM strained at elevated temperatures were responsible for its higher resistance to creep failure during the practical service.
Highlights
Increasing the steam pressure and operating temperature of a boiler in a coal-fired power plant can reduce the CO2 emissions and save energy
As reported in international journals, the microstructural evolution of an advanced ferritic steel weld generally focuses on the creep failure of the simulated heat-affected zone (HAZ), and less attention is paid to the weld metal (WM)
The results indicate that the WM had very high brittleness in the AW condition, whereas the WM tempered at 760 ◦C exhibited the same resistance to impact fractures as the base metal (BM)
Summary
Increasing the steam pressure and operating temperature of a boiler in a coal-fired power plant can reduce the CO2 emissions and save energy. After creep of Gr. 92 steel, the laths and M23C6 carbides increase in size and the dislocation density decreases near the crack tip, as compared with those of the virgin material [3]. Creep failure in the HAZ of advanced ferritic steel welds is identified as Type IV cracking [5,6,7,8]. Among the different zones of the P92 weld, the fine-grained HAZ (FGHAZ), heated just above the temperature of AC3, shows the highest degree of creep damage [14]. As reported in international journals, the microstructural evolution of an advanced ferritic steel weld generally focuses on the creep failure of the simulated HAZ, and less attention is paid to the WM. The impacts of welding thermal cycles on the microstructures of the Gr. 92 WM were related to creep rupture life
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