Unexpected Charpy Impact Toughness Spike of 1.25Cr-0.5Mo Steel After Simulated Postweld Heat Treatment
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Unexpected Charpy Impact Toughness Spike of 1.25Cr-0.5Mo Steel After Simulated Postweld Heat Treatment YANG SHEN and CONG WANG The present study elaborates on the effect of simulated postweld heat treatment (SPWHT) on microstructure and mechanical property variations of 1.25Cr-0.5Mo steel. With the increase in SPWHT holding time, lath ferrite tends to morph into blocky ferrite, and carbides aggregate along grain boundaries. Although tensile strength keeps decreasing, Charpy impact toughness undergoes a spike, followed by a sharp decrease. Such an interesting phenomenon is explained by the competition between high-angle grain boundary population and carbide segregation, and bodes well to the pressure vessel industry for optimized PWHT. https://doi.org/10.1007/s11663-019-01519-x The Minerals, Metals & Materials Society and ASM International 2019
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INTRODUCTION
1.25CR-0.5MO steel is one of the most versatile Cr-Mo ferritic heat-resistant grades, and has been extensively employed for boilers and heat exchanger seamless tubes, due to its high-temperature stability, excellent creep strength, and eminent corrosion resistance.[1,2] Such heat-resistant grades are generally welded, and the entire welded joints, including the weld metal, heat-affected zone, and base metal, need to be postweld heat treated, to alleviate residual stress.[3] To a large extent, PWHT is equivalent to high-temperature tempering treatment,[4] and mechanical properties of the base metal will be negated to a certain level after long periods of PWHT.[5] In order to guarantee the stability and reliability of the base metal during long-term services experiencing harsh environment, it is necessary to estimate the performance of the as-received steels by applying simulated postweld heat treatment (SPWHT). Up to now, numerous studies have been focused on the effects of PWHT on microstructure and mechanical properties of the weld metal and heat-affected zone. Yang et al.[6] reported that short-time PWTH improved the corrosion resistance of the welded joint and more austenite was formed in fusion zone compared with the as-welded state. Silwal et al.[7] suggested that a 1033 K for 2 hours PWHT can significantly increase the
YANG SHEN and CONG WANG are with the School of Metallurgy, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] Manuscript submitted November 16, 2018.
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cross-weld toughness of the heat-affected zone. Olabi et al.[8] undertook a series of studies to evaluate the effect of PWHT on mechanical properties of welded joints of low-carbon structural steel and found that with a slight reduction in the tensile strength and the hardness, the toughness improved by 15 pct, and the residual stress reduced by about 70 pct after applying PWHT. Pandey et al.[9] and Wang et al.[10] revealed that Vickers hardness of the weld metal and heat-affected zone in 9Cr-1Mo steel exhibited significant reduction after PWHT at 1033 K for 2 hours. Nevertheless, only a few of the studies are concen
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