Effects of tempering time on microstructure and residual stress of surfacing weldments of thin tube sheets

Abstract

Strip electrode electroslag welding was applied to deposit a weld overlay on Q345R thin tube sheets with dimensions of Φ600mm × 20 mm.After welding, four specimens were subjected to heat treatment with varying tempering durations.Post-welding, the matrix microstructure of the EQ309MoL weld overlay consists of austenite and interdendritic ferrite with a dendritic morphology at grain boundaries, while the Q345R base metal near the weld exhibits a microstructure of bainite and acicular ferrite.Following heat treatment, significant carbide precipitation was observed at the austenite grain boundaries in the weld overlay. Moreover, the extent of carbide precipitation increased with prolonged tempering time, and the precipitates were identified as M23C6.The thin tube sheet weldments exhibited the highest residual stress in the as-welded condition, with a magnitude approaching their yield strength (469 MPa). As tempering time increased, the residual stress gradually decreased; after 3 h of tempering, it stabilized at approximately 100 MPa.In the as-welded state, the weldments showed the highest dislocation density, with numerous dislocations accumulating at grain boundaries and within grains. This led to the formation of dislocation tangles and internal strains, resulting in further dislocation accumulation and stress field concentration within grains and near grain boundaries.After tempering, elevated temperatures provided the driving force for dislocation glide, reducing dislocation accumulation and intersections, decreasing dislocation density, and promoting uniform dislocation distribution. This alleviated dislocation accumulation and stress concentration at grain boundaries and within grains, thereby reducing residual stress in the weld overlay.The maximum grain size was 990304 μm² in the as-welded state, 1122848 μm² after 1 h of tempering, 1274160 μm² after 2.5 h, and 1544880 μm² after 3 h.Grain growth was accompanied by homogenization of grain size distribution, which reduced deformation incompatibility caused by variations in grain size and thus lowered the risk of stress concentration.