Formation Mechanism of Type IV Failure in High Cr Ferritic Heat-Resistant Steel-Welded Joint
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INCREASING requirements for reducing CO2 emission and improving the thermal efficiency of fossil-fired power plants promote worldwide development of high Cr ferritic heat-resistant steels with superior creep properties. However, one of the problems for these steels is reduced creep lives of welded joints compared to the base metal, especially in low stress and long-term ranges. The welded joints are fractured at the finegrained heat-affected zone (FGHAZ), where the peak temperature reaches around Ac3 during the weld thermal cycle.[1–5] This phenomenon is known as type IV failure. We have revealed that the multiaxial stress condition in the HAZ with lower creep strength, resulting from the mechanical constrain effect by the surrounding weld metal and base metal with higher creep strength, is essential for the formation of creep voids and brittle Y. LIU, formerly Postdoctoral Researcher with Structural Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan, is now Researcher with R&D Center, Tianjin Pipe Corporation, Tianjin, P.R. China. S. TSUKAMOTO and F. ABE, Researchers, are with the Structural Materials Center, National Institute for Materials Science. Contact e-mail: [email protected] T. SHIRANE, formerly Visiting Researcher, with Structural Materials Center, National Research Institute for Materials Science, is now Researcher with Mitsubishi Heavy Industries, Ltd., Hiroshima 7338533, Japan. Manuscript submitted January 15, 2013. Article published online May 24, 2013 4626—VOLUME 44A, OCTOBER 2013
type IV fracture in the HAZ of conventional 9 to 12 Cr steel welded joint.[6] However, the cause of reduced creep strength in FGHAZ has not yet been clarified. Many works have been done in the past decade to clarify the main metallurgical cause of type IV failure.[7–27] Several important factors were proposed to be taken into account. One is HAZ softening induced by the weld thermal cycle. However, a study on HAZ simulated P122 indicated that the shortest creep rupture time was observed at the peak temperature of around Ac3, whereas the minimum hardness was found near the peak temperature of around Ac1.[2] This discrepancy suggests that the HAZ softening cannot thoroughly explain the occurrence of type IV failure. Behavior of precipitates is another potential cause of type IV failure. It includes rapid coarsening of M23C6,[3–5] MX,[4,5] or Laves phase[8–10,12,13] during postweld heat treatment (PWHT) and creep. Distribution of the precipitates is also an important factor for stabilization of the microstructure during creep. The third important factor is the grain refinement taking place during the HAZ thermal cycle at the peak temperature of around Ac3.[6] Even though these potential causes are proposed to account for the occurrence of type IV failure, a specific mechanism and a dominant metallurgical factor of type IV failure have not been well understood, because the effect of each factor on the creep property has not been analyzed separately. An objective of the present stu
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