Welding metallurgical process

Welding Metallurgical Process
During arc welding, the interaction of liquid metal, slag, and gas constitutes a process of metal remelting. However, due to the unique characteristics of welding conditions, the chemical metallurgical process of welding has features different from general smelting processes.
Firstly, welding metallurgy involves high temperatures, large phase interfaces, and rapid reaction rates. When air is introduced into the arc, the liquid metal undergoes intense oxidation and nitridation reactions, and a large amount of metal evaporates. The water vapor in the air, as well as oil, rust, and water in the workpiece and welding materials, decompose into hydrogen atoms at the high temperature of the arc, which can dissolve into the liquid metal, leading to reduced plasticity and toughness of the joint (hydrogen embrittlement), and even cracking.
Secondly, the welding pool is small and cools rapidly, making it difficult for various metallurgical reactions to reach equilibrium. This results in uneven chemical composition in the weld, and gases and oxides in the molten pool do not have enough time to float out, easily forming defects such as pores and slag inclusions, and even cracks.
To ensure weld quality, the following measures are usually taken during arc welding:
(1) During the welding process, the molten metal is mechanically protected to isolate it from the air. There are three protection methods: gas protection, slag protection, and gas-slag combined protection.
(2) Metallurgical treatment of the welding pool is performed, mainly by adding a certain amount of deoxidizer (mainly ferromanganese and ferrosilicon) and a certain amount of alloying elements to the welding materials (electrode coating, welding wire, flux). This removes FeO from the molten pool during the welding process and compensates for the burn-off of alloying elements.