Suppression of grain refinement in heat-affected zone of 9Cr-3W-3Co-VNb steels

Prompt phase transformations make grains in the heat-affected zone (HAZ) smaller during welding of 9% chromium (9Cr) heat-resistant steels leading to premature failure under creep conditions, which is well known as a type IV fracture. Because the type IV fracture shortens the creep lifetime of the steels, suppressing the fracture is an urgent task in the energy industry. The present study shows that boron addition and nitrogen reduction inhibit grain refinement after welding because of a change in the morphology of the precipitate at prior austenite grain boundaries. In conventional 9Cr steel (ASME Gr. 92 steel), a high amount of MX was unable to pin interface migration of the phase transformation and generated fine grains in the HAZ. In the new B-added steels, B-stabilized M23C6 became the dominant precipitate and showed a larger pinning effect of the phase transformation than MX, which resulted in coarse grains in the HAZ. This suggests that designing stabilized M23C6 forms a superior welded microstructure and results in a longer creep lifetime of 9Cr steels.