Abstract
Pipes are generally used to transport oil and gas and the growing need for clean natural gas has increased the production of High Strength Steel (HSS) lately. However, both the fluid and the environment have a severe corrosive impact on the pipes, especially at the weld joints. While several welding techniques exist and have been deployed into the welding of these pipes, there seems to be a gap in the literature as to how the parameters can be combined and optimised for effective welding operation. This study investigates the modeling and optimisation of the evolving mechanical and microstructural properties of High Strength Steel X65 pipe welded using Tungsten Inert Gas (TIG), Metal Inert Gas (MIG), and a hybrid TIG-MIG welding process. The aim is to evaluate and compare the performance of these welding techniques under varying process parameters.
A weld‐assembly model was built in SolidWorks and subjected to static analysis to quantify deformation, Von Mises stress, and displacement under representative TIG and MIG loads. Experimentally, a full factorial design (3×3 levels) generated 27 runs each for TIG (70–90 V; 30–50 mm/s), MIG (150–170 A; 50–80 mm/s), and hybrid scheme combining TIG and MIG settings. The welded samples were subjected to mechanical testing, post weld heat treatment, corrosion analysis, and microstructural characterisation to determine optimal welding conditions.
Results showed that tensile strength across the welds reached a maximum of 300 MPa, with hybrid TIG-MIG welds achieving the highest strength values due to improved fusion and filler distribution. Microhardness in the weld zone ranged from 40 BHN to 68 BHN, with TIG welds displaying more uniform hardness profiles. Impact test values varied from 60 J/mm2 to 110 J/mm2, with the hybrid process consistently outperforming individual techniques by producing tougher joints at moderate heat input.
iii
Corrosion testing in a simulated CO2 corrosive environment revealed that the hybrid TIG -MIG welds had the lowest corrosion rates (2.65 mm/year), followed by MIG welds (5.07 mm/year), and TIG welds (5.60 mm/year). Scanning Electron Microscope/Energy-dispersive X-ray spectroscopy revealed significant microstructural variation among the processes: TIG welds exhibited fine-grained polygonal ferrite and pearlite; MIG welds showed a mix of bainite and acicular ferrite, while the hybrid welds demonstrated a refined and well-balanced microstructure with reduced heat-affected zone width. The findings confirmed that hybrid TIG-MIG welding combined the benefits of both TIG and MIG, offering a balance of strength, toughness, and corrosion resistance.
This study introduces a novel methodology for modeling and optimising the mechanical and corrosion performance of TIG and MIG welded HSS X65 steel pipes by employing empirical techniques based on weld joint efficiency and manual experimentation guided by an L27 Taguchi Orthogonal array. Uniquely, the research excludes statistical and AI-based modeling and does not incorporate thermal analysis within SolidWorks simulations. Instead, structural validation was achieved through mechanical testing and CO₂ environment for corrosion testing, presenting a cost-effective and reproducible strategy for enhancing weld integrity in pipeline applications.
This work provides a practical foundation for selecting and optimising welding techniques for HSS X65 pipeline applications, particularly in the oil and gas sectors demanding high-performance and durability under harsh conditions.