Microstructural Progression and Softening Mechanisms in the Heat-Affected Zone of P22 Steel: A Systematic Investigation of Post-weld Heat-Treatment Duration Effects

In this work, the evolution of microstructure and microhardness in the heat-affected zone (HAZ) of P22 heat-resistant steel under varying post-weld heat-treatment (PWHT) durations was systematically investigated. The results show that prolonged PWHT transforms the heterogeneous HAZ microstructure into ferrites and carbides, progressively reducing both the average HAZ hardness and the interzonal hardness gradient. Concurrently, quantitative analysis demonstrates that ferrite grain growth (from 5.56 μ m to 8.47 μ m and finally to 13.06 μ m), carbide coarsening (from 0.27 μ m to 0.47 μ m and to 0.50 μ m), and alloying element segregation collectively drive the spatial shift, progressive widening, and pronounced softening of the soft zone, where the hardness decreases from 194.1 HV0.2 to 187.9 HV0.2 and to 152.2 HV0.2, as the PWHT duration is extended from 6 h to 48 h and further to 480 h.