Gas Welding and Cutting Questions and Answers

Answer: C

The flame associated with cutting is the oxidising flame.

Oxy-fuel cutting works by oxidation: the steel is heated to its ignition temperature and then a jet of pure oxygen burns it, the molten oxide being blown out of the cut. The cutting operation is therefore an oxygen-rich, oxidising process, which is the textbook basis for this answer. In practice the small preheating flames are often set close to neutral, but the cut itself is made by an excess of oxygen.

A carburising flame, with excess acetylene, adds carbon and is used for hard-facing and some non-ferrous work, not cutting. The neutral flame is the general welding flame for steel.

Answer: B

An oxidising flame is used for copper and brass.

An oxidising flame has excess oxygen. When brass is welded, its zinc tends to boil off; the slight excess of oxygen forms a thin oxide film on the pool that holds the zinc in. The textbook list for the oxidising flame is therefore brass, bronze and copper-based alloys.

Steel is welded with the neutral flame, because excess oxygen would burn the steel and make the weld porous and brittle. The metals in the last option are those listed for the carburising (reducing) flame, which keeps oxygen away from metals that oxidise easily.

Answer: B

The paper's answer is in liquid form.

Acetylene cannot safely be stored as a compressed gas: above about 15 psi (1 bar gauge) it can decompose explosively. Cylinders are therefore packed with a porous filler soaked in liquid acetone, and the acetylene is dissolved in the acetone, which can hold many times its own volume of the gas. This dissolved acetylene, held in a liquid, is what the key means by liquid form.

Storing it as free compressed gas is exactly what must be avoided, and it is never stored as a solid, so the remaining options are wrong.

Answer: A

The neutral flame is used to weld steel.

A neutral flame burns oxygen and acetylene in roughly equal proportions. It neither adds carbon to the metal nor oxidises it, which is what mild steel and most ferrous work need, so it is the standard welding flame.

Copper and brass go with the oxidising flame. The list in the last option, aluminium, stainless steel, zinc die casting, nickel and monel, is the textbook list for the carburising (reducing) flame, and carburised steel is not the neutral flame's typical application.

Answer: D

Backfire can be caused by any of the above.

Backfire is a sharp pop at the torch tip, with the flame going out and perhaps relighting. It happens when the gas mixture leaves the tip more slowly than the flame burns back towards it, or when the tip is disturbed. Each listed cause does this: gas pressure set too low makes the gas flow too slow, an overheated nozzle ignites the mixture inside the tip, and carbon deposits blocking the orifice disturb the flow.

Because all three are recognised causes, no single option is complete on its own.

Answer: C

The highest temperature comes from oxy-acetylene.

An oxy-acetylene flame reaches about 3,100 °C to 3,300 °C, the hottest of the common fuel-gas flames, which is why it is the standard gas for welding steel. Acetylene releases a great deal of heat on burning because of its triple bond, and pure oxygen avoids diluting the flame with nitrogen.

Oxy-hydrogen (about 2,800 °C) and oxy-LPG (about 2,800 °C to 2,900 °C) are cooler. Air-acetylene is much cooler still, because the nitrogen in the air absorbs heat.

Answer: D

A gas regulator is used to set the working pressure.

Gas cylinders store oxygen at very high pressure. The regulator reduces this cylinder pressure to a steady, much lower working pressure suited to the torch and keeps it constant as the cylinder empties.

Mixing the gases in the right proportion happens in the blowpipe (torch). The type of flame is adjusted with the torch valves, which also control the volume of gas flowing; the regulator's job is the pressure.

Answer: B

The nozzle is selected according to the pipe wall thickness.

In gas welding the nozzle (tip) size sets how much gas, and so how much heat, the flame delivers. The heat needed depends on how much metal must be melted through, which for a pipe is its wall thickness: a thicker wall needs a larger nozzle.

The groove angle affects the joint preparation and the amount of filler, and the pipe diameter affects how the joint is welded around, but neither sets the flame size the way the wall thickness does. So 'All of the above' is wrong.

Answer: A

The fault is corrected by reducing the pressure of the gases.

A flame that burns away from the tip (lifting off) shows that the gas is leaving the nozzle faster than the flame can burn back to it. The usual cause is too high a gas pressure for the nozzle size, so lowering the pressure brings the flame back onto the tip.

Cooling the torch and cleaning the orifice are remedies for backfire and flashback, where the flame burns back into the tip. Reducing only the oxygen would change the flame's character rather than its speed.