Tungsten - Arc Gas Shielded Welding - Overhead _Pipe_ — Skills and Education Group Awards Vocationally-Related Qualification Manufacturing & Engineering Revision
This subtopic addresses the advanced skill of Tungsten-Arc Gas Shielded (TIG) welding on low carbon steel pipe in the overhead position, integrating safe w
Topic Synopsis
This subtopic addresses the advanced skill of Tungsten-Arc Gas Shielded (TIG) welding on low carbon steel pipe in the overhead position, integrating safe working practices, an understanding of consumables and parameters, and the execution of procedures that control distortion. It equips learners with the ability to produce code-quality welds through practical application, inspection techniques for defect identification, and corrective actions, while also comprehending the process's industrial applications and limitations.
Key Concepts & Core Principles
- Welding Processes: Understanding the principles and applications of MIG (Metal Inert Gas), TIG (Tungsten Inert Gas), and MMA (Manual Metal Arc) welding, including parameter selection and defect prevention.
- Material Properties: Knowledge of ferrous and non-ferrous metals, their mechanical properties, and how heat treatment affects weldability and strength.
- Joint Configurations: Proficiency in preparing and welding various joint types (butt, fillet, lap, T-joint) and positions (flat, horizontal, vertical, overhead) to BS EN ISO standards.
- Fabrication Techniques: Skills in marking out, cutting (using plasma, oxy-fuel, or mechanical methods), forming, and assembling components with precision.
- Health and Safety: Compliance with COSHH regulations, use of personal protective equipment (PPE), and safe handling of welding equipment and gases.
Exam Tips & Revision Strategies
- For the overhead practical assessment, position your body comfortably and brace your arms to achieve smooth, steady torch and filler wire manipulation; perform a dry run to synchronise movements before striking an arc.
- In written or oral questioning, always relate the choice of DCEN polarity to the need for deep penetration and a focused arc for the root pass on carbon steel, and mention the effect of electrode tip geometry on arc shape.
- Before submitting the test piece, conduct a critical self-inspection using the acceptance criteria provided; pay special attention to the stop/start areas for crater cracks, and the root bead for consistent penetration.
- Refer explicitly to the provided WPS and, where permitted, verify your intended parameters with the assessor before welding; demonstrate your understanding by explaining how each parameter influences the weld outcome in the overhead position.
Common Misconceptions & Mistakes to Avoid
- Misjudging shielding gas flow rate: too low results in porosity and oxidation, while too high causes turbulence that draws in atmospheric contamination.
- Contaminating the tungsten electrode by dipping it into the weld pool or contacting the filler rod, which causes arc instability and necessitates stopping to re-grind the tip.
- Incorrect torch angle in the overhead position, leading to arc wandering, incomplete sidewall fusion, or excessive filler metal sagging due to gravity.
- Insufficient cleaning of the pipe surface and filler wire to remove mill scale, oil, or moisture, leading to hydrogen-induced porosity or oxide inclusions.
- Inconsistent travel speed causing uneven bead edges, variable root penetration, or insufficient throat thickness; often accompanied by overly long arc length.
- Overlooking the importance of pipe fit-up: excessive root gap or misalignment complicates the root pass and may lead to burn-through or lack of fusion.
Examiner Marking Points
- Award credit for demonstrating a thorough risk assessment prior to welding, identifying hazards such as UV radiation, inert gas asphyxiation, and electric shock, and implementing appropriate control measures.
- Evidence of correct selection and preparation of tungsten electrode (type, diameter, tip geometry) and filler rod grade/diameter for the specified low carbon steel pipe, with justification aligned to the welding procedure specification (WPS).
- Ability to set welding parameters (amperage, voltage, travel speed, gas flow rate, and pre/post-flow times) accurately as per WPS, with rationale for adjustments made for the overhead pipe position.
- Demonstrated technique: maintaining a short, consistent arc length, correct torch angle (approximately 90° to pipe axis with a 10–15° push angle), and controlled addition of filler wire to achieve full root penetration and a flat bead profile.
- Completed weld meets acceptance criteria per a recognised standard (e.g., ISO 5817 Level B) upon visual inspection: uniform bead width and reinforcement, no cracks, lack of fusion, undercut, porosity, or excessive spatter.
- Application of effective distortion control methods: correct tack weld sequence and size, use of chill blocks or pre-setting if applicable, and symmetrical welding to minimise residual stress and distortion.
- Ability to identify common TIG weld defects (e.g., tungsten inclusions, lack of root fusion, crater cracks) from samples or photographs, and describe corrective actions such as electrode re-grinding, parameter adjustment, or improved cleaning.
- Accurate explanation of the TIG process's applications (e.g., root passes on pipe, thin-gauge materials, high-integrity joints) and limitations (low deposition rate, sensitivity to draughts and poor gas coverage, slower travel speeds).