A mixture of fuel gas, usually
acetylene, and oxygen are mixed in a blow pipe and burnt to provide a high
temperature flame, approximately 3200’C. An oxygen-acetylene flame can be set
to neutral, oxidizing or carburizing condition depending upon the metal to be
welded and the type of filler metal to be used. In an oxygen-acetylene neutral
flame the gases are mixed, with the result that the oxygen burns up the carbon
and hydrogen in the acetylene to release heat and gases, the products of
combustion. When welding mild steel these gases reduce oxide protect the molten
pool and welding zone.
- Neutral
- Oxidizing
- Carburizing
THE NEUTRAL FLAME
The oxygen-acetylene neutral
flame is produced when approximately equal of oxygen and acetylene are mixed in
the welding blow pipe. It is recognized by the sharply defined white or
luminous inner core, that is the inner part of the flame adjacent to the tip of
the blow pipe nozzle. The combustion of gases is completed just beyond the
extremity of the inner cone.
A Neutral Flame
To ensure that a strictly neutral
flame is being obtained, the neutral flame should fired be adjusted to give
smooth and rounded inner cone. The oxygen supply is then slightly reduced to
give just the hint of a white haze of acetylene ( feather ) barely visible at
the tip of the inner cone. When the neutral flame is set in this condition it
can easy be seen when the flame is becoming oxidizing by the disappearance of
this white haze. This is particularly important because as welding proceeds the
flame will tend to become slightly oxidizing.
THE OXIDIZING FLAME
An oxidizing flame is the flame
containing an excess of oxygen the flame is first set to a neutral condition
and the acetylene value is then turned down to reduce the amount of acetylene
giving an excess of oxygen. A harsh sounding flame is produced, the inner cone
is sharply defined but is slightly shorter and more pointer than in a neutral
flame. The amount of excess oxygen will depend upon the metal being welded of
filler metal to be used.
A Oxidizing Flame
Slightly oxidizing flame give a
higher flame temperature than a neutral flame the top of the inner cone being
the hottest part of the flame.
THE CARBURIZING FLAME
A carburizing flame is a flame
containing an excess of acetylene which gives up carbon when it is applied to
heated steels. An oxygen-acetylene carburizing flame is often reffored to as a
reducing flame it is three distinct flame section, the inner cone which is not
sharply defined, surrounded by a quantity of unburned gas known as a feather
all in the outer envelope of the flame.
The flame is first set to neutral
condition and the acetylene supply increased until the end of inner cone lose
its sharp outline with the appearance as a feather, between the inner cone and
the outer envelope. For varying the welding techniques the correct amount of
excess acetylene is obtained by adjustment to all give a feather inner cone.
A Carburizing Flame
A carburizing flame give a
slightly lower flame temperature than a neutral flame.
The Neutral Flame
A Neutral flame is used for the
fusion of mild steel, copper, aluminium, stainless steel, cast iron.
The Oxidizing Flame
The Oxidizing flame is in most cases undesirable, mild steel, weld made with an oxidizing flame will be oxidized giving a weak and porous.
The Carburizing Flame
A Carburizing flame is use when
depositing hard facing and wear resistant materials, onto the surface of the
steel and cast iron it is also frequently used for the flame brazing of
aluminium.
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