| IN 625 superalloy is a solid solution strengthened nickel-base superalloy. It has good stress rupture properties, creep properties and corrosion resistance properties. This alloy was originally designed for applications in gas turbines and steam generator. Recently years, cast IN 625 alloy has been studied by many countries as a candidate material of 700℃ ultra-supercritical thermal power unit. Some trial products of IN625 alloy were produced in Europe, America and Japan. In China, a little work on cast IN 625 alloy have been done for using on the 700 ℃ultra-supercritical thermal power unit. The microstructure and property of cast IN 625 alloy were not clearly known. This paper has studied the microstructure, solution treatment, tensile property and thermal stability of IN 625 alloy. The microstructure of IN625 alloy was observed by SEM, and the identification of precipitation phases was carried out by TEM and EDS. The main conclusions are as follows:The results show that the solidification structure of IN 625 alloy is dendrite structure.The alloy has obvious primary and secondary dendrite arms. And no tertiary dendrite arms can be observed. Precipitated phases in IN 625 alloy are primary MC carbides enriched with Nb. The addition of 0.2% Al and 0.2%Ti has little influence on the dendrite structure and precipitated phases of IN 625 alloy. While the spacing of secondary dendrites are increased by the 0.04%B addition.The suitable solution treatment system for IN 625 alloy was confirmed by studying the effect of solution parameters. The suitable solution treatment system for IN 625 alloy is 1200 ℃/1 h / WQ. The precipitated phases in IN 625 alloy are still MC carbides enriched with Nb element after solution treatment. NO.1 and NO.3 alloy have more carbides than NO.2 and NO.4 alloy. The tensile property of the alloy after solution treatment was tested. The results show that the addition of 0.2%Al, 0.2%Ti and 0.04% B elements has little influence on tensile strength of IN 625 alloy either tested at room temperature or tested at 700℃. However the addition of B element can improve the ductility of IN 625 alloy tested at room temperature and 700 ℃.An investigation was carried out to study the structure stability of IN 625 alloy. The results show that M23C6 and γ″ phases formed at both 700 and 750 after aging ℃ ℃for 300 h. M23C6 carbides which have net structure formed on grain boundries, while γ″ phases with disc-shaped structure precipitated in the interdendritic. Moreover, there is much stacking fault in the γ″ phases. M23C6 carbides are stable during long-term exposure at both 700 and 75℃ 0 up℃ to 3000 h, while γ″ phases are relatively stable at 700℃.γ″ phases would quickly turn into δ phases when exposed at 750℃. δ phases begin to form around the grainboundary and MC after aging for 300 h. MC carbides transformed into M23C6 during the long-term exposure and the degeneration process can be described as MC+γâ†'M23C6. Moreover, Adding 0.2%Al and 0.2% Ti elements can increase the stability of γ″ phases.The microstructure of IN 625 alloy is relatively stable aging at 700℃,and very unstable aging at 750℃.750℃ obviously beyond the using temperature range of IN 625 alloy. |
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