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This refers to the case where oxygen (O₂) and acetylene (C₂H₂) are mixed in equal amounts and burned at the tip of the welding torch. A short inner cone and a longer outer envelope characterize a neutral flame (Figure 1.9a). The inner cone is the area where the primary combustion takes place through the chemical reaction between O₂ and C₂H₂, as shown in Figure 1.10. The heat of this reaction accounts for about two‐thirds of the total heat generated. The products of the primary combustion, CO and H₂, react with O₂ from the surrounding air to form CO₂ and H₂O. This is the secondary combustion, which accounts for about one‐third of the total heat generated. The area where this secondary combustion takes place is called the outer envelope. It is also called the protection envelope since CO and H₂ here consume the O₂ entering from the surrounding air, thereby protecting the weld metal from oxidation. For most metals, a neutral flame is used.

Figure 1.10 Chemical reactions and temperature distribution in a neutral oxyacetylene flame.