Oxy-Acetylene Gas Welding, Cutting and Brazing
What this domain tests
Flame adjustment and identification, cutting procedure and parameters, brazing versus soldering, and safe handling of gas equipment.
Core rules
- Flame types. Neutral is roughly 1:1 oxygen to acetylene, about 3200 °C, with a sharply defined rounded white inner cone. Oxidising (excess oxygen) is hotter, with a short pointed cone and a hiss; it oxidises steel, giving porosity and a brittle weld. Carburising (excess acetylene) has a ragged, feathery secondary cone, is cooler, and adds carbon.
- Cutting. Preheat the edge to dull cherry red, about 870–900 °C, then open the cutting-oxygen lever. Oxygen purity of at least 99.5% is needed for a clean kerf; a stand-off of 1.5–3 mm and steady travel prevent drag and slag adhesion.
- Brazing and soldering. Brazing uses a filler melting above 450 °C but below the base metal, and relies on capillary action into a lap clearance of about 0.05–0.15 mm. Fluxes: borax/boric acid for steel, fluoride-chloride mixes for aluminium. Soldering works below 450 °C.
- Hose and thread identification. Oxygen fittings are right-hand thread, acetylene left-hand; in Indian practice cylinders are black for oxygen and maroon for acetylene.
- Backfire versus flashback. A backfire is a pop as the flame retreats into the nozzle and relights; a flashback travels back through the torch into hose or regulator - close the cylinder valves at once and check the arrestors.
Worked example
A welder on 3 mm mild steel reports a hissing flame with a short, pointed, pale inner cone; the bead is porous and shows a grey oxide scale. Reading the cones: pointed and short with a hiss and a shortened outer envelope indicates excess oxygen, i.e. an oxidising flame. Correct setting: reduce the oxygen until the inner cone becomes clearly rounded and distinct with no acetylene feather - the neutral flame - then re-adjust the welding pressure.
Traps
Calling a feathery cone 'neutral'; confusing backfire with flashback; assuming brazing melts the parent metal; using a right-hand-thread acetylene nut.