Electric welding includes electric welding and gas welding. Electric welding uses arc discharge to generate high temperatures to melt iron; gas welding is gas combustion to produce high-temperature molten metal.
The principle of electric welding is to use an electric arc to ignite chemical agents instantly to release high temperatures. The center temperature can reach 5000 degrees Celsius. Common ones are arc welding and submerged arc welding.
The principle of gas welding is to accelerate the combustion of combustible gas through combustion aids to generate high temperatures. Common ones are ethane welding, hydrogen welding, and carbon dioxide cooling welding.

Two-shielded welding is the common name for "carbon dioxide gas shielded welding", and is collectively called "gas shielded welding" with argon gas shielded welding (commonly known as "argon arc welding") and other gas shielded welding.
The advantages of two-shielded welding are open arc welding, easy-to-achieve all-round semi-automatic and automatic welding, general use bare welding wire, arc heat concentration, small heat-affected zone, welding deformation, and weld cracking tendency are small. Therefore, the welding operation is convenient, the weld quality is high, and the comprehensive cost is only half of the manual arc welding and submerged arc welding.

Since it is difficult to prepare pure carbon dioxide, the impurities such as water and nitrogen in it often cause large smoke and splash, making the weld appearance not smooth enough, generally a dryer must be installed in the gas path; another disadvantage is that carbon dioxide is easy to decompose at high temperature to generate carbon monoxide and oxygen, causing a certain amount of weld metal oxidation and pore inclusions (which is also one of the reasons for the molten pool splash).

Two-layer welding is mainly used for welding low-carbon steel and low-alloy steel structural parts with a thickness of less than 25mm and is also often used for surfacing repair of worn parts and repair welding of cast iron parts.

According to national and industry standards, the porosity of welded joints should generally not exceed 3%. If the porosity exceeds 3%, the mechanical properties and corrosion resistance of the welded joint will be affected, resulting in fatigue fracture and damage to the weld. Therefore, when conducting welding quality inspections, the porosity of welded joints needs to be strictly controlled to ensure that it meets the standard requirements.

1. Improper quenching temperature
The quenching temperature has a very important influence on the structure of steel parts. Too low or too high a quenching temperature will lead to the presence of residual austenite in the quenched steel. The too-low quenching temperature will lead to the incomplete transformation of martensite, while the too-high quenching temperature will lead to the re-precipitation of austenite.
2. During the quenching process, the martensite that has been transformed first hinders the transformation of the untransformed austenite to martensite.