Welding and Thermal Cutting
What the domain tests
Welding and Thermal Cutting tests process selection, safe setup, joint preparation and defect recognition. Expect MMA, MIG, TIG and oxy-acetylene terms, current and polarity, flame types, distortion control and cutting technique.
Core rules
- MMA uses a flux-coated electrode; the electrode and current range must suit the steel thickness and position. E6013 is a general-purpose electrode for mild steel.
- MIG uses a continuous wire electrode and shielding gas; TIG uses a non-consumable tungsten electrode and separate filler. Both need clean metal and correct gas flow.
- Oxy-acetylene welding uses a neutral flame for steel. A carburizing flame has excess acetylene; an oxidizing flame has excess oxygen.
- Prepare joints: clean, bevel thick plate, remove rust and oil, and tack before welding. Use the shortest arc that maintains control.
- Prevent distortion by balanced welding, tacking, sequencing and allowing cooling between passes. Fit flashback arrestors at the regulators.
Worked example
A 6 mm mild steel butt joint is to be welded by MMA. Bevel the edges to form a V, clean to bare metal and tack at both ends. Select a 3.2 mm E6013 electrode and a current in the manufacturer’s range, typically about 90–130 A, then run a root pass and a cover pass. Too high a current causes undercut and spatter; too low a current causes lack of fusion.
Traps
Do not use water on an acetylene fire, and never use oil on oxygen fittings. Common welding defects include slag inclusion, porosity, undercut and lack of penetration. A pretty bead does not prove fusion; inspect the joint.