Microstructure and mechanical properties of 9Cr-3W-3Co steel welded joints by vacuum electron beam welding

Abstract

This paper aims to explore the microstructure and mechanical properties of 9Cr-3W-3Co steel welded joints. In the experiment, 9Cr-3W-3Co steel samples were welded by vacuum electron beam welding technology (VEBW) without any metal stuff, and all the welding joints were treated by high-temperature tempering at [Formula: see text]C for 8 h. The microstructure of welding joints was observed by OM, SEM and TEM; and the mechanical properties of welded joints were analyzed by microhardness test, room-temperature tensile, test impact test and high-temperature creep test. As a result, all the 9Cr-3W-3Co steel samples displayed the microstructure status as martensite under the Scheffler-Schneider prediction model, which conformed to the expectation. After high-temperature tempering, the grains of the welding zone were smaller than the base metal and the composition was tempered lath martensite only. Some of the lath martensite bundles even showed the incomplete polygonal transformation. The M[Formula: see text]C6 carbides and MX phase were distributed continuously along with the lath martensite interfaces, which showed a tendency for further aggregation. The microhardness of the weld zone was slightly higher than the base metal (mean of base metal: 240 HV[Formula: see text], mean of weld zone: 273 HV[Formula: see text] and mean of heat affected area: 274 HV[Formula: see text]. There was no softening phenomenon observed, and the welding joints maintaining the high intensity. Other mechanical properties like the tensile strength (mean: 750 MPa), yield strength (mean: 707 MPa) and impact toughness (mean of WM: 25.1 J and HAZ: 23.3 J) were also excellent. When the temperature parameter is constant, the time for creep fracture reduces significantly with the increase of the stress; whereas the time for creep fracture decreases significantly as the temperature increases, while the stress parameter is constant.