Phase transformation and flow simulation of weld pools at the crack tips of elbows under pulsed current

Objective In the base metal repair for elbows using pulsed currents, detour flow typically occurs at the crack tips, resulting in the formation of weld pools due to local temperature increases. The size, flow, and solidification behaviors of these weld pools, has a decisive effect on crack healing. Exploring the evolution patterns of weld pools near elbow cracks under the influence of pulsed currents, along with a further investigation into the healing mechanisms of crack tips and fronts, holds significant importance for the technological advancement of pipe crack repair utilizing pulsed currents.

Methods A finite element model for repairing 90° high-pressure elbows with cracks using pulsed currents was established, based on Comsol software, focusing on NiCr21Mo high-pressure elbows with cracks. This model incorporated a multi-physical field coupling approach that combines “two-phase flow and phase field” within an “electrical-thermal-structural” three-field coupling framework. This integration allows for the simulation of phase transformation and the flow of weld pools. Further analysis explored the evolution patterns of weld pools near cracks in elbows during pulsed current discharge, as well as the phase transformation patterns and flow characteristics of the liquid-phase metal after halting the discharge.

Results Both the duration and interval of pulse discharge were identified to be influential to the formation of molten pools associated with cracks.With a pulsed current density set at 2.4×10⁸ A/m², weld pools formed at the crack tips and fronts in U-shaped columns following current discharge exceeding 0.65 seconds. Beginning at 0.75 seconds of discharge, the melting effect extended from the regions around the cracks into the surrounding areas, and partial evaporation of the liquid-phase metal occurred, both of which are detrimental to crack healing. After the discharge was halted, the metal flow of the weld pools gradually slowed down, with crack healing efficiency peaking within the first 0.001 seconds, followed by complete solidification within 0.008 seconds. Consequently, a pulse interval greater than 0.008 seconds was identified as optimal to prevent adverse effects on the weld pool area during subsequent pulses, thereby enhancing repair performance.

Conclusion The application of pulsed currents results in the formation of U-shaped columnar weld pools at the crack tip, with an increasing volume of liquid-phase material near the cracks as discharge time progresses. To maximize repair effectiveness, it is essential to select an appropriate discharge duration that allows for sufficient phase transformation at the crack tips and fronts, resulting in the formation of weld pools while avoiding solid-liquid phase transitions in surrounding areas. Additionally, the pulse discharge interval should be longer than the solidification time of the liquid metal in the weld pools. Otherwise, repair efficiency may decrease due to smaller weld pool sizes. It is advisable to determine the pulse discharge power, duration, and interval through simulations prior to repairing microcracks in high-pressure elbows using pulsed currents, thereby improving both the accuracy and efficiency of actual elbow crack repairs. (20 Figures, 2 Tables, 22 References)