| Abstract Scope |
In-service welding is used on operational pipelines carrying high-pressure fluids. Unlike conventional welding, it is influenced by the thermal sink effect of the flowing fluid, significantly altering the welding thermal cycle. This method is essential for maintaining and repairing corroded pipelines and installing fittings without halting fluid transport. However, it presents challenges such as higher cooling rates, increased hydrogen levels, corroded thin walls, and the presence of flammable liquids. The main risks include hydrogen-induced cracking (HIC) due to rapid cooling, burn-through from intense heat on thin or corroded sections, and unstable chemical reactions between welding heat and the fluid.
This study explores pulsed gas metal arc welding (GMAW-P) with induction preheating to optimize heat input and regulate cooling rates. Welding qualification was performed on an API 5L grade B carbon steel pipe (11.75” OD, 3/8” wall thickness) in a laboratory setting, validated by multi-physical numerical modeling using finite element method (FEM). A water loop system simulated the high cooling rates of live pipelines, specifically for sleeve-type B repair configurations.
FEM simulations and experimental results validated the thermal profiles and microstructural evolution. Sleeve-type B welds showed hardness exceeding 400 HV in the coarse-grained heat-affected zone (CGHAZ) without preheating, indicating cracking susceptibility. FEM-predicted 200 ºC induction preheating reduced the cooling rate to ~41 ºC/s, producing a more refined, ductile microstructure, confirmed by metallographic analysis and hardness tests.
COMSOL Multiphysics was used for computational modeling, employing the Induction Heating and Non-Isothermal Flow modules. The internal fluid dynamics were simulated with the k-ε turbulence model to replicate the thermal sink effect. Without fluid flow, the induction heating system reached 400 ºC at the coil center in 7 minutes, demonstrating its efficiency. |