Welding Basics: Arc Welding and Gas Welding For students.

Welding is the process of joining two metal pieces by melting their edges and letting them fuse into one solid piece, often with extra filler metal added. Once cooled, the joint is as strong as the parent metal itself, which is why welded structures hold up bridges, ships and buildings.

The two most common methods are arc welding, which melts metal with an electric arc, and gas welding, which burns oxygen and acetylene. Both are the backbone of modern fabrication.

Key Concepts

• Electric arc: a sustained spark between an electrode and the workpiece, reaching about 6000°C — hot enough to melt any common metal instantly.
• Electrode: the consumable rod in arc welding that carries the current, melts into the joint as filler, and is coated with flux that shields the molten metal from air.
• Oxy-acetylene flame: the gas welding heat source at about 3200°C. The neutral flame suits most work; the carburising flame (excess acetylene) adds carbon to steel; the oxidising flame (excess oxygen) is for cutting and brazing brass.
• Weld joints: butt (edge to edge), lap (overlapping), T-joint, corner and edge joints — the five basic configurations.
• Weld defects: porosity (trapped gas bubbles), cracks, slag inclusion, incomplete fusion and distortion from uneven heating.
• Safety: arc rays burn eyes and skin like severe sunburn, so a welding helmet, gloves, apron and ventilation are mandatory.

Formulas

• Heat input: H = (V × I × 60) / (1000 × S) kilojoules per mm, where V is arc voltage in volts, I is current in amperes, and S is travel speed in mm per minute. It measures how much heat each millimetre of weld receives.
• Effect of heat input: too little heat causes lack of fusion; too much causes burn-through and distortion. The formula assumes steady arc conditions.
• Current selection: as a rough guide, use about 30 to 40 amperes per millimetre of electrode diameter for mild steel — a 4 mm electrode runs at roughly 140 to 170 A.

Worked Example

A butt weld in mild steel is made with arc voltage 24 V, current 150 A and travel speed 300 mm/min. Calculate the heat input and judge it.

1. Apply the formula: H = (24 × 150 × 60) / (1000 × 300) = 216000 / 300000 = 0.72 kJ/mm.
2. For a single-pass weld on 6 mm plate, 0.72 kJ/mm is a moderate heat input — enough for full penetration without excessive distortion.
3. Check the electrode: at 150 A a 4 mm electrode is appropriate (within the 140–170 A guide range).
4. After welding, inspect the bead: it should be uniform ripples about 8–10 mm wide with no visible porosity or undercut at the toes.

Common Misconceptions

• "A bigger weld bead is always stronger." An oversized bead wastes electrode and can hide defects — correct size with full fusion beats a fat bead.
• "Gas welding can handle any thickness." Gas welding is slow and best for thin sheet (up to about 6 mm). Thick sections need arc welding or the heat spreads and warps the whole part.
• "You can weld in any position the same way." Overhead and vertical welding fight gravity — they need lower current, shorter arc and different electrode angles than flat welding.

Real-World Applications

• Construction: structural steel frames of buildings and bridges are welded on site and in fabrication shops.
• Shipbuilding: entire hulls are assembled from welded steel plates.
• Pipelines: oil, gas and water pipelines are joined by arc welding, then X-ray inspected.

Welding fuses metals with intense heat — 6000°C from an arc, 3200°C from oxy-acetylene. Controlling heat input through voltage, current and travel speed decides weld quality.