Abstract: Mechanical properties and microstructure of Q690D steel butt joints under different post-weld heat treatment(PWHT) processes, namely, 580 ℃×2 h, 450 ℃×8 h, 450 ℃×4 h, 450 ℃×2 h, and 250 ℃×2 h, were studied and compared with those under welded state. The results show that PWHT has a little effect on strength, but has a significant influence on impact properties. When the heating temperature is higher than the tempering temperature(480 ℃) of the Q690D steel, the impact properties of the weld seam and heat affected zone(HAZ) decrease significantly, exhibiting quasi cleavage fracture. When the heating temperature is lower than the tempering temperature and after holding for 2 h, the impact properties of the weld seam and HAZ are significantly improved, showing ductile fracture. When the heating temperature is 30 ℃ lower than the tempering temperature, the holding time has a small impact on impact properties of the HAZ. However, an excessive holding time (8 h) will significantly reduce the impact properties of weld seam, so the holding time should not exceed 4 h. There are hardening and softening zones in the HAZ of welded joint, after heat treatment, the hardening effect is eliminated but the softening effect is slightly aggravated. The microstructure of weld seam after post-weld heat treatment is mainly composed of acicular ferrite and granular bainite. With the increase of heating temperature or the prolongation of holding time, obvious carbides precipitate at the grain boundaries, and the average size of acicular ferrite increases.

Key words: Q690D steel, post-weld heat treatment, butt joint, microstructure, mechanical properties