Microstructures and properties of a novel 115 mm thick 08Cr9W3Co3VNbCuBN heat-resistant steel tube joints welded by shielded metal arc welding

Abstract

A novel heat-resistant steel tube, 08Cr9W3Co3VNbCuBN (G115), is the primary choice for crucial components of ultra-supercritical (USC) thermal power plants. In this paper, the 115 mm thick G115 steel tube was welded successfully by shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW), and the microstructures and mechanical characteristics of joint were studied. The results indicated that the microstructure of the weld zone (WZ) was mostly ferrite and martensite, and the content of ferrite at the weld root was 19.9% lower than that at the weld center. However, at the weld toot the grain size was reduced by about 20%, and the content of M23C6 was increased by 30.9%. Previous austenite grain boundaries (PAGBs) and martensite occurred in the heat-affected zone (HAZ) because to varying heat input, and a tiny quantity of fine Cu-rich phase precipitated from the root of HAZ. The hardness distribution of joint from the base metal (BM) to WZ demonstrated a low to high phenomena, with the average hardness of the BM being 235 HV and the average hardness of WZ being enhanced by 1.16 times. The fine secondary phases precipitated and diffused in the WZ, therefore reinforcing the martensite and improving its mechanical properties. The average impact absorption energy of the weld zone was 43.89 J, and the average impact energy of the HAZ was 54.57 J, both of which were sufficient to fulfill the actual standard (>31 J). The fracture morphology revealed a complex fracture mechanism with dimples and cleavages on the surface of joint, and the toughness was positive. The causes of the decrease in hardness and the rise in toughness might be the Cu-rich phase precipitated from the root of HAZ. Furthermore, the corrosion resistance of the welded joint (WZ: Icorr = 10.542 μA cm−2, HAZ: Icorr = 11.241 μA cm−2) was lower than that of the BM (Icorr = 9.4844 μA cm−2).