Gas welding is a method of joining two metal pieces (similar or dissimilar) together by melting and fusing their edges at the joint. It involves applying intense concentrated flame on the metal pieces (at the joint) until an area (of both pieces) under the flame becomes molten and forms a liquid puddle such that the molten puddle of one metal piece mixes up and run together with the molten puddle of another metal piece. The welding puddle on cooling and solidification results into a strong joint. The flame for melting the metal pieces is produced by burning various fuel gases.

Fuel gases use in gas welding include: acetylene, hydrogen, city gas, natural gas of liquefied petroleum gas (LPG). The gases (oxygen and fuel gas) are mixed in proper proportion in a welding torch which carries two regulators - one for controlling the quantity of oxygen and the other for controlling the quantity of fuel gas. The mixture of oxygen and acetylene is most popularly use for gas welding and produces temperature within  range  of  3200'C  to  3300'C,  which  makes  it  possible  to  melt  and  weld  all common metals. A filler rod (that also melts during welding) makes the joint stronger on solidification.