Lighting and Adjusting the Torch  Different Flames Neutral Flame Oxidizing Flame Soft or Harsh Flames Carburizing Flame Handling of the Torch With the proper working pressures set for the acetylene and  oxygen, open the torch acetylene valve a quarter to a half turn.  Direct the torch away from the body and ignite the acetylene  gas with the flint striker. Open the acetylene valve until the  black sooty smoke disappears from the flame. The pure  acetylene flame is long, bushy, and has a yellowish color.  Open the torch oxygen valve slowly and the flame shortens  and turns to a bluish-white color that forms a bright inner  luminous cone surrounded by an outer flame envelope. This  is a neutral flame that should be set before either a carburizing  or oxidizing flame mixture is set. The three types of flame commonly used for welding are  neutral, carburizing, and oxidizing. Each serves a specific  purpose.  [Figure 5-22] The neutral flame burns at approximately 5,850 °F at the tip  of the inner luminous cone and is produced by a balanced  mixture of acetylene and oxygen supplied by the torch. The  neutral flame is used for most welding because it does not  alter the composition of the base metal. When using this  flame on steel, the molten metal puddle is quiet and clear,  and the metal flows to give a thoroughly fused weld without  burning or sparking. The carburizing flame burns at approximately 5,700 °F at  the tip of the inner core. It is also referred to as a reducing  flame because it tends to reduce the amount of oxygen in the  iron oxides. The flame burns with a coarse rushing sound,  and has a bluish-white inner cone, a white center cone, and  a light blue outer cone. The flame is produced by burning more acetylene than  oxygen, and can be recognized by the greenish feathery tip at  the end of the cone. The longer the feather, the more acetylene  is in the mix. For most welding operations, the length of the  feather should be about twice the length of the inner cone.  The carburizing flame is best used for welding high-carbon  steels, for hard facing, and for welding such nonferrous alloys  as aluminum, nickel, and Monel. The oxidizing flame burns at approximately 6,300 °F and  is produced by burning an excess of oxygen. It takes about  two parts of oxygen to one part acetylene to produce this  flame. It can be identified by the shorter outer flame and the  small, white, inner cone. To obtain this flame, start with a  neutral flame and then open the oxygen valve until the inner  cone is about one-tenth of its original length. The oxidizing  flame makes a hissing sound, and the inner cone is somewhat  pointed and purplish in color at the tip. The oxidizing flame does have some specific uses. A slightly  oxidizing flame is used for bronze welding (brazing) of steel  and cast iron. A stronger oxidizing flame is used for fusion  welding of brass and bronze. If an oxidizing flame is used  on steel, it causes the molten metal to foam, give off sparks,  and burn. With each size of tip, a neutral, carburizing, or oxidizing  flame can be obtained. It is also possible to obtain a soft or  harsh flame by decreasing or increasing the working pressure  of both gases (observing the maximum working pressure of  15 psi for acetylene gas). For some work, it may be desirable to have a soft or low  velocity flame without a reduction of thermal output. This  can be achieved by reducing the working pressure using a  larger tip and closing the torch valves until the neutral flame is  quiet and steady. It is especially desirable to use a soft flame  when welding aluminum to avoid blowing holes in the metal  when the puddle is formed.  It should be cautioned that improper adjustment or handling  of the torch may cause the flame to backfire or, in rare  cases, to flashback. A backfire is a momentary backward  flow of gases at the torch tip that causes the flame to go out.  A backfire may be caused by touching the tip against the  work, overheating the tip, by operating the torch at other than  recommended pressures, by a loose tip or head, or by dirt or  slag in the end of the tip, and may cause molten metal to be  splattered when the flame pops. Figure 5-22.  Oxy-acetylene flames. A. Neutral flame B. Carburizing (reducing) flame C. Oxidizing flame 5-13