Abstract
The post-weld heat treatment (PWHT) cracking susceptibility of a coarse grain heat-affected zone (CGHAZ) in SA213-T23 (2.25Cr-1.6W steel) that was used for boiler tubes employed in thermal power plants was investigated using a Gleeble thermal cycle simulator. The PWHT cracking susceptibility test was performed at 650 °C, 700 °C, and 750 °C, and it can be judged that the lower the reduction of the area, the more susceptible it is to PWHT cracking. The results of the test also showed higher cracking susceptibility at 650 °C and 700 °C, which mostly involved intergranular fracture, while at 750 °C, transgranular fracture was exhibited. Therefore, the PWHT cracking susceptibility is considered to be closely related to grain boundary. The microstructure of the simulated CGHAZ and PWHT at 650 °C, 700 °C, and 750 °C was observed after etching with nital and alkaline sodium picrate etchants. Alkaline sodium picrate-etched microstructures showed a white band at the grain boundary at 650 °C, 700 °C, and 750 °C, which did not appear in nital etching. An analysis of the white band using an electron probe micro-analyzer (EPMA), TEM, and nanoindentation revealed that it was intergranular ferrite depleted with C, W, and Cr as compared with that in the matrix. Based on these results, we investigated the mechanism of intergranular ferrite formation during PWHT and its effect on PWHT cracking susceptibility at 650 °C, 700 °C, and 750 °C.
Highlights
In order to achieve a higher efficiency of thermal power plants, pipes and tubes used therein are increasingly required to sustain higher temperatures and pressures [1,2]
Result of the electron probe micro-analyzer (EPMA) and TEM analysis indicated that the line at the grain boundary of coarse grain heat-affected zone (CGHAZ) was result of the analysis indicated that thethe line atat the the result of the analysis indicated that line thegrain grainboundary boundaryofofCGHAZ
post-weld heat treatment (PWHT) cracking susceptibility and microstructural characteristics of CGHAZ in SA213-T23 were investigated at various PWHT temperatures
Summary
In order to achieve a higher efficiency of thermal power plants, pipes and tubes used therein are increasingly required to sustain higher temperatures and pressures [1,2]. (2.25Cr-1.6W steel), which has better high-temperature creep strength than SA213-T22 (2.25Cr-1Mo steel), has been developed as a candidate material. To develop SA213-T23 (2.25Cr-1.6W steel), SA213-T22 (2.25Cr-1Mo steel) was modified by the addition of tungsten (1.6%), a reduction of molybdenum (0.2%) and carbon content (0.04–0.10%), and small additions of vanadium, nitrogen, and boron [3,4,5,6,7]. In the case of T23 material, PWHT cracking (reheat cracking) has been reported in the process of post-weld heat treatment (PWHT) to mitigate the residual stress in the weld metal [8,9,10,11,12,13,14,15,16]. Much research has been conducted to analyze the cause of the formation of reheat cracks in. Nawrocki et al explained that the cause of reheat cracking is due to the weakening
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