Gas welding, also called oxy-fuel or oxy-acetylene welding, uses a mixture of oxygen and acetylene to produce a flame hot enough to weld steel. Other gases like propane and hydrogen are sometimes used with oxygen to weld metals with lower melting points, but acetylene is the most common fuel gas. By varying the mixture of the acetylene and oxygen, flames with different temperature ranges and different chemical characteristics can be produced. The three distinct types of oxy-acetylene welding flames are neutral, reducing or carburizing, and oxidizing flames.

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Varying the oxygen-acetylene mixture produces different welding flame characteristics.

Neutral Flame

The neutral flame is used for most gas-welding applications, and has roughly equal amounts of acetylene and oxygen. To be more precise, the ratio of oxygen to acetylene for a neutral flame lies between 1.0 and 1.1. The neutral flame has a temperature between 5,600 and 5,900 degrees Fahrenheit and can be recognized by a light blue inner flame cone with a darker blue outer flame. The neutral flame takes its name from the fact that it produces very little or no chemical reaction in the molten metal. In fact, the neutral flame actually acts as a gas shield to protect the weld pool from chemical reactions with the atmosphere, much like the inert gas in TIG (tungsten inert gas) and MIG (metal inert gas) arc welding. It is used for welding mild and stainless steel, cast iron, aluminum, and copper.

Reducing or Carburizing Flame

The reducing, or carburizing, flame is produced by reducing the amount of oxygen in the mixture, producing an acetylene-rich gas. The carburizing flame has a temperature between 5,400 and 5,500 degrees Fahrenheit. It contains three distinct levels of color: a very light blue cone at the nozzle, surrounded by an envelope or feather of darker blue; both of those are surrounded by an outer envelope of even darker blue. It is a reducing flame because it does not oxidize the metal, and it is an oxidizing flame because it does not completely burn up the carbon and because the unconsumed carbon is forced into the metal. It is used for welding high-carbon steel and other metals that do not readily absorb carbon.

Oxidizing Flame

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Cast iron can be welded with an oxidizing flame.

The oxidizing flame is produced by increasing the oxygen in the mixture, producing an oxygen-rich gas. It has a temperature between 6,000 and 6,300 degrees Fahrenheit. The flame is shorter and bluer than both the neutral and carburizing flames, and the inner cone is more pointed. The excess oxygen from this flame will combine with the metal and form oxides, which are brittle and weaken the weld and the metal. Because of its oxidizing properties it is seldom used to weld steel, but is sometimes used for copper-based and zinc-based metals, as well as cast iron and manganese.