Microstructure Evolution of Fine-Grained Heat-Affected Zone of Gr.92 Steel Welded Joint During Creep
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OWING to their good creep strength and oxidation resistance, high Cr ferritic heat-resistant steels are widely used as main steam pipe and header in fossil-fired power plants operating at 600 °C or above.[1–7] However, one serious drawback of these steels is that the creep strength of their welded joints being inferior to that of the base metal. Moreover the formation of creep voids as well as cracking are significantly enhanced in the fine-grained heat-affected zone (FGHAZ) particularly at low stress and long-term creep deformation, which is knows as type IV failure.[8–13]
Y. LIU is with the Civil Aviation University of China, Jinbei Road 2898, Dongli District, Tianjin, P.R. China. Contact e-mail: [email protected] S. TSUKAMOTO, H. HONGO, M. TABUCHI, and F. ABE are with the National Institute for Materials Science, Sengen 1-2-1, Tsukuba, 305-0047, Japan. F. YIN is with the Hebei University of Technology, Xiping Road 5340, Beichen District, Tianjin, P.R. China. Manuscript submitted January 24, 2019. Article published online May 7, 2019 3080—VOLUME 50A, JULY 2019
The microstructure of high-Cr ferritic heat-resistant steels after heat treatment is very complicated microstructures, and includes high density of dislocations, various internal interfaces derived from the martensitic phase transformation, and different precipitates, such as M23C6 carbides and MX carbonitrides, which form during tempering. Subjecting the steels to welding thermal cycle further increases the complexity of their structure because it would undoubtedly cause microstructure changes which influence microstructure stability during creep. In the past decades, many studies have been performed to clarify how these microstructure changes affect the creep property and cause the occurrence of type IV failure in welded joint. The precipitation behavior of carbides and grain refinement of fine-grained heat-affected zone (FGHAZ) were considered to be potential causes for deteriorating the creep strength of high Cr ferritic heat-resistant steel welded joints.[9–16] In previous studies, we investigated the mechanism of type IV failure using 9Cr heat-resistant steel Gr.92 and revealed that type IV failure was mainly caused by the lack of precipitation strengthening at prior austenite grain (PAG) and block boundaries.[17,18] M23C6 carbides mainly distribute at PAG, packet, block, and lath boundaries of Gr.92 steel and stabilize its microstructure during creep. When Gr.92 steel is METALLURGICAL AND MATERIALS TRANSACTIONS A
heated to just above the Ac3 temperature during the welding thermal cycle, ferrite transforms into fine austenite via diffusional transformation. At equilibrium, M23C6 should be dissolved in the matrix, but some undissolved carbides are retained at the PAGs and block boundaries of Gr.92 steel owing to rapid heating of the heat-affected zone (HAZ) thermal cycle. Consequently, M23C6 carbides do not sufficiently precipitate at the newly formed fine-grained boundaries during post weld heat treatment (PWHT). The lack of precipitates at these boun
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