Oxyfuel Cutting and Heating
This domain tests setup and manipulation of the oxyfuel torch: flame adjustment, tip selection, cutting technique, kerf and drag, and the safety devices that separate a normal backfire from an emergency flashback.
Flame adjustment
An oxyacetylene torch uses a neutral flame-equal oxygen and acetylene-for steel cutting and most heating. A carburizing flame has excess acetylene and adds carbon; an oxidizing flame has excess oxygen and can decarburize or oxidize the steel. Light with a striker, adjust the fuel gas first, then add oxygen. Acetylene line pressure stays at or below 15 psig.
Cutting action
Preheat the steel to its ignition temperature, then press the cutting oxygen lever. The oxygen jet oxidizes iron and the gas stream blows slag through the kerf. Tip size follows plate thickness; a tip that is too small cannot cut through, while one that is too large overheats the edges and widens the kerf. Hold the tip about 1/8–1/4 in above the plate after piercing. Travel speed controls drag: too fast leaves a lagging bottom edge and incomplete cut; too slow melts the edges.
Backfire vs flashback
A backfire is a pop as the flame returns into the torch and usually goes out; a flashback travels into the hose or regulator and is an emergency. Shut off the torch and cylinder valves, then inspect before relighting. Flashback arrestors on the regulators and torch help prevent hose explosions.
Worked example: Cut a 6 in × 4 in rectangular part with a 1/8 in kerf. The cut path removes 1/16 in on each side of the line. To hold the outside dimensions, offset the path 1/16 in outward; if the operator follows the scribed part line, the finished part is 1/8 in undersize in both directions.
Traps: Backfire and flashback are not interchangeable; a carburizing flame is not correct for cutting; a loud pop does not mean the torch is ready to relight without inspection.