Evaluation of Post-Weld Heat-Treated 2.25Cr-1Mo Steel Joint Behaviors by Electromagnetic NDE Parameters

Abstract

A P22 (2.25Cr-1Mo) steel pipe is machined into two separate plates and then joined across with a V-groove by manual arc welding. The welded plate is further cut into three pieces and each piece is post-weld heat-treated (PWHT) at temperatures of 680 degrees C, 700 degrees C, and 720 degrees C for different holding times of 0.5, 1, and 2 h at each temperature. The welding area is typically categorized into three regions: base metal (BM), heat-affected zone (HAZ), and weld fusion (WF). As-weld and PWHT samples are characterized by destructive techniques of microscopy and hardness measurement and non-destructive evaluation (NDE) of magnetic behaviors. The WF region shows a bainitic lath structure, correlated to its high hardness and coercivity and low Barkhausen voltage. In contrast, the change in microstructure towards HAZ and BM decreases dislocation density, displaying a continuous decrease of hardness and coercivity and increasing Barkhausen voltage. This is endorsed by a decrease in magnetic and mechanical softness profiles, sequentially proceeding from WF towards HAZ and BM, while this softening behavior changes with PWHT temperatures and holding time. After PWHT at 720 degrees C for 2-h holding time, the magnetic softening along the welding region indicates an actual trend compared to mechanical softening. The electromagnetic parameters are considerable to identify the correct PWHT conditions necessary to maintain the accurate stress-relieving operation of weldments.