Neutral carburising and oxidising flames, and choosing a cutting tip by plate thickness and oxygen purity

Every oxy-acetylene job begins with the same thirty seconds at the torch valves, and the flame set in those thirty seconds decides the quality of the weld or the cut that follows. There are three flames, each correct for some jobs and wrong for others. Cutting adds a second decision, the tip, and a third that most shops never think about, the purity of the oxygen. This page covers all three with their sources.

IS 1323:1982, the Indian code for oxy-acetylene structural welding, puts it in four lines in its Appendix C: for steel the flame is neutral, with the inner cone sharply defined and a very slight haze or flicker; an oxidising flame is necessary for welding brass; a carburising flame, with a small excess of acetylene, is used for stelliting and hardfacing; and flame size is regulated preferably by changing the nozzle rather than by altering the gas pressure.

The 1965 welding literature, as compiled by S. A. Miller, adds the reasons. In the neutral flame the inner cone carries out the primary reaction, acetylene plus oxygen to carbon monoxide and hydrogen, and the envelope burns those products with oxygen from the air; the weld pool is neither carburised nor oxidised. With excess acetylene, free carbon carburises the weld metal; that is exactly what a hardfacer wants, because it lowers the surface melting point so the deposit sweats on, and exactly what a structural welder does not. With excess oxygen the pool oxidises; in brass that is useful, because a zinc oxide film holds the volatile zinc in the pool, and in steel it is a defect. The same literature records the audible cues: a flame that is too long sounds harsh and blows metal out of the pool, and an oxidising flame hisses.

US Bureau of Mines equilibrium calculations by Edwards, Smith and Brinkley give the flame temperature of oxygen-acetylene mixtures at 1 atm as 3,387 K, about 3,114 °C, for the equimolar mixture, against 3,359 K (3,086 °C) for the fully stoichiometric 2.5:1 mixture and 3,214 K (about 2,941 °C) for the fuel-rich mixture with half a volume of oxygen per volume of acetylene. The maximum sits at about one volume of oxygen per volume of acetylene, which is the neutral welding flame. Without the dissociation of carbon dioxide and water at these temperatures, the same calculation would give about 7,000 °C; dissociation sets the ceiling. In air the calculated maximum is 2,604 K, about 2,331 °C. The thermochemistry is on the heat of formation and flame temperature page.

Temperature alone does not explain acetylene's place in welding. The 1965 literature argues that the better measure is combustion intensity, the product of burning velocity and the heating value of the mixture, on which acetylene leads hydrogen, methane and propane by a wider margin than on temperature, with most of its advantage in the secondary envelope. That is the honest basis of the comparison with LPG, set out on acetylene vs LPG cutting.

In oxygen cutting the chemistry is done by the oxygen. The acetylene flame raises the steel to its ignition temperature and keeps the top face of the cut hot; the jet of pure oxygen then burns the iron, and the molten oxide, which melts below the iron, is blown out of the kerf. Spraragen and Claussen's heat balance for cutting 25 mm mild steel (1939) found only 28 to 29 % of the heat input coming from the oxy-acetylene flame, the rest from burning iron. Continuous preheat is kept because heating the metal ahead of the cut makes the cut much faster.

The Linde procedure of 1955 for manual cutting is still the right one: set the preheat flames neutral with inner cones 0.2 to 0.25 in (5 to 6 mm) long, measured with the cutting oxygen valve open; hold the nozzle vertical with the cone tips close to but not touching the plate; preheat a spot to bright red, just starting to melt; press the cutting-oxygen lever slowly; and travel at the speed where the spark stream leaves the underside vertically. If travel is too fast the cut stops; release the oxygen, reheat and restart. With some nozzles a slightly oxidising preheat is preferred.

Each cutting tip has a cutting-oxygen bore sized for a range of thickness, and the nozzle maker's chart gives the tip and the pressures. What the chart is doing can be seen in the British Oxygen Co.'s 1960 machine-cutting tables for mild steel (Technical Information Booklet No. 8), converted here to metric:

Three things follow. Acetylene use barely triples from 25 to 300 mm, while cutting oxygen rises nearly sevenfold; a thick-plate cutter is an oxygen customer first. Preheat time grows faster than anything else, which is where a hot, intense preheat flame earns its keep. And the tip must be big enough to deliver that oxygen volume at a velocity that holds its shape through the full depth; the 1965 literature notes that little is gained above about 9 atm head pressure. Too small a tip for the plate gives an incomplete cut; too large wastes oxygen and widens the kerf. The tip should be chosen from the chart, not by trial.

Sloman and Roper's Oxygen Cutting (1951) and Roper and Kiernan's 1952 foundry work record the effect of oxygen purity: steel cannot be cut at all below about 80 %; below about 95 % the cut degenerates into a melt-and-wash action of poor quality; and a fall of a single point, from 99.5 to 98.5 %, cuts speed by about 25 % and raises oxygen consumption by about 25 %. Going the other way, from 99.5 to 99.8 %, bought only 6 to 10 % more speed, not enough to pay for guaranteeing it. The mechanism is a stagnant boundary layer of impurity at the burning face that the oxygen must diffuse through. The practical standard is IS 309:2023 Grade 2 (N2.5), minimum 99.5 % oxygen, which is the grade of KIGL's industrial oxygen.

Drag lines, the curved striations on the cut face, show the cut's quality. The 1965 cutting literature, as compiled by S. A. Miller, treats drag of up to 10 % of plate thickness as the mark of a high-quality cut needed for profile work, and about 20 % as often acceptable for straight commercial cuts; German work of the early 1960s (Zorn 1961; draft DIN 2310, 1963) built a fuller cut-quality classification on the same measures. More drag than that means travel is too fast, there is too little cutting oxygen for the tip, purity is low, or the tip is damaged. Slag sticking to the underside usually means travel is too slow or there is too much preheat. A rounded top edge means the preheat cones are too close or too large.