The effects of postweld heat treatment and isothermal aging on T92 steel heat-affected zone mechanical properties of T92
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o Dlouhý Institute of Physics of Materials, Academy of Sciences of the Czech Republic, CZ 616 62 Brno, Czech Republic (Received 7 September 2015; accepted 21 March 2016)
Cross-weld hardness profile, notch-tensile strength, and impact toughness of T92 steel heataffected zone (T92 HAZ) of dissimilar T92/TP316H welds were studied in dependence of their postweld heat treatment (PWHT) and subsequent long-term aging. Two weldments series were individually subjected to either “single-step” tempering PWHT or a modified “two-step” renormalizing and tempering PWHT. Subsequently, the welds were isothermally aged at 625 °C for durations from 500 up to 11,000 h. The “single-step” PWHT preserved sharp hardness gradient of T92 HAZ, whereas the “two-step” PWHT led to the hardness values equalization. The T92 HAZ of the weldment after the “two-step” PWHT exhibited initially lower strength and higher toughness, compared to the weldment after the “single-step” PWHT. However, after long-term aging a more suitable combination of T92 HAZ mechanical properties i.e., the higher toughness and acceptable strength exhibited the weldments processed by “single-step” PWHT.
I. INTRODUCTION
Ni-based transition weldments between tempered martensitic and austenitic creep-resistant steels are frequently used in power engineering for construction of terminal parts of superheaters in supercritical steam boilers. The use of Ni-based weld metal (Ni WM) for fabrication of dissimilar welds is related to its medium coefficient of thermal expansion and thus the ability to moderate residual welding stresses. In addition, it prevents diffusional migration of carbon between the dissimilar base materials.1 However, it has been generally accepted that fusion weldments of tempered martensitic power-plant steels require the application of postweld heat treatment (PWHT). Conventional “single-step” PWHT of such welds is commonly carried out by their subcritical tempering with regard to the corresponding steel Ac1 temperature. Besides its stress-relieving effect, it also contributes to thermal stabilization of initial microstructures with a positive effect on the weldments toughness.2 However, a persisting problem of tempered martensitic steels weldments during operation in creep conditions is related to the occurrence of premature “type IV failure” within their heat-affected zone (HAZ).3,4 Recently, Abe et al.5 revealed that the welded joints of newly developed
Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.134
9Cr–3W–3Co–0.2V–0.05Nb steel with 160 ppm boron and 85 ppm nitrogen exhibited no “type IV failure” during creep exposure as a result of specific HAZ microstructure modification. Nevertheless, literature information about the effects of such increased boron contents in advanced tempered martensitic steels welds on their toughness is rather scarce.6,7 Another approach assuring “type IV failure” suppression and thus creep strength enhancement of tempered martensitic steels weldments is
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