Abstract
The effects of the post-weld treatment on the impact performance, microstructure, and carbide precipitation behavior of pressure vessel steel were evaluated under simulated post-weld conditions. The continuous cooling transformation and isothermal transformation curves of undercooled austenite for the steel were constructed based on the expansion curve, serving as a guide for the potential heat treatment of the steel plates. A more detailed study was conducted on the simulated post-weld process with an insulation temperature of 690 ℃ and an insulation time of 24 h, based on the delivery status of the steel plate. The microstructure was characterized using transmission electron microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, and electron probe microanalysis. The Charpy V-notch impact test was used to assess the impact performance of the steel plates. The results showed that refining the microstructure to 50% bainite and 50% ferrite, along with a high proportion of large-angle grain boundaries and large-angle misorientation grains at half the thickness of the steel plate, contributed to enhanced low-temperature impact toughness in its delivered state. Additionally, the steel predominantly consists of chromium-containing carbides. In the as-delivered state, the carbide size was measured at 110 nm. However, after post-weld heat treatment (PWHT), the carbide size significantly increased to 360 nm, reflecting a 227% growth. This coarsening is observed along the grain boundaries and through intragranular aggregation. Additionally, there was a change in carbide type from Cr7C3 in the as-delivered state to Cr23C6 following the heat treatment. This transformation was accompanied by a significant reduction in impact toughness, as evidenced by the impact energy dropping from 116 J to an unacceptable 43 J.