Effect of Postweld Heat Treatment Conditions on Microstructure of 9Cr-1Mo-V Steel Welds for Pressure Vessel

We investigated the effects of post-weld heat treatment conditions on the microstructure of the multi-pass submerged arc weld metal of 9Cr-1Mo-V steel used in pressure vessels. The microstructural properties were analyzed under three conditions (as-weld, Larson-Miller parameter (LMP) = 21.38 × 10³, and LMP = 21.99 × 10³). The precipitation behavior was observed using scanning electron microscopy, and the difference in precipitation behavior in the “as-welded” and “reheated” regions of the prepared multi-pass weld metal was clarified at the different LMP values. The precipitate was analyzed using scanning transmission electron microscopy. An oxide and two types of precipitates were identified, and a dislocation network pinned by MX-type carbides was visualized under the low-LMP condition. The effects of LMP on the effective grain size and dislocation amount were also evaluated using electron back-scattering diffraction. All microstructural change along the LMP had a positive effect on the toughness of weld metal.