Definition of welding: welding is a machining technique that USES heat or pressure, or heating and pressure, to join two pieces of metal together, with or without filler metal, to form a permanent connection.Classification of welding is divided into: pressure welding, fusion welding and brazing.What we say in daily life is electric welding.Gas welding mainly includes gas welding, manual arc welding, submerged arc welding, argon arc welding, CO2 gas shielded welding, plasma welding, electroslag welding, electron beam welding and laser welding.In the following, we explain the definition of welding mainly by describing manual arc welding and CO2 gas shielded welding.Manual arc welding (SMAW) is a method of welding with a manually operated electrode.The energy of manual arc welding is the energy produced during discharge in gas medium.Generally speaking, welding and gas welding are protected by gas.Welding does not involve gas protection.