Mechanical Engineering - Workshop Technology - Discussion

Neutral Flame.

As the supply of oxygen to the blowpipe is further increased; the flame contracts and the white cone becomes clearly defined, assuming a definite rounded shape. At this stage, approximately equal quantities of acetylene and oxygen are being used and the combustion is complete, all the carbon supplied quantities of acetylene is being consumed and the maximum heat is given out. The flame is now neutral, and this type of flame is the most extensively used by a welder, who should make himself thoroughly familiar with it appearance and characteristics.

The neutral flame has a one-to-one ratio of acetylene and oxygen. It obtains additional oxygen from the air and provides complete combustion. It is generally preferred for welding. The neutral flame has a clear, well-defined, or luminous cone indicating that combustion is complete.

Neutral welding flames are commonly used to weld:

Mild steel.
Stainless steel.
Cast Iron.
Copper.
Aluminum.

The welding flame should be adjusted to neutral before either the carburizing or oxidizing flame mixture is set. There are two clearly defined zones in the neutral flame. The inner zone consists of a luminous cone that is bluish-white.

Surrounding this is a light blue flame envelope or sheath. This neutral flame is obtained by starting with an excess acetylene flame in which there is a "feather" extension of the inner cone. When the flow of acetylene is decreased or the flow of oxygen increased the feather will tend to disappear. The neutral flame begins when the feather disappears.