The absorption of gas from the arc or flame into the weld-pool causes gas-metal reaction (since both the metal and the gas are at higher temperatures). There are two types of such reactions. In the first type the gas may be just dissolved in the liquid metal. In the second type, the gas and liquid metal may chemically react to form stable chemical compounds. In case this chemical compound is soluble it may cause embrittlement of the welded joint.
An insoluble reaction product may produce surface scale or slags and thus physically interferes with the formation of the weld pool. In this case the excess gas is either prevented or a flux is used to dissolve or disperse the reaction product.
When the gas is dissolved in the liquid weld pool, the gas evolves during cooling as its solubility decreases with fall of temperature. Gas bubles are formed. If these bubles are trapped, the weld becomes porous and of low quality. This defect is common in metals whose oxides are easily reducible by hydrogen, and can be avoided by the addition of a suitable deoxidant in the filler metal.
Another important gas-metal reaction is the diffusion of the gas into the parent metal from the weld pool. When the temperature of the thermal cycle is high, this diffusion process may be quite fast. The diffusion of hydrogen into the HAZ may again cause an embrittlement of the welded joint.
5.2.2 Liquid-Metal Reactions
During welding, non-metallic liquid phases are produced that interact with the weld metal. These may be slag layers formed by the melting of flux in SMAW, SAW, etc. They may also be produced as a result of reactions occuring in the molten weld-pool and remain in or on top of the weld metal after welding.
The flux layers used in SMAW or SAW etc. processes are designed to absorb deoxidation products produced in the arc and molten metal. They usually float to the surface of the weldpool forming part of the slag, but some may remain in the metal as inclusions.
Another important effect of liquid solid interaction is hot cracking, which occurs during solidification. The interdendritic liquid, the last region to freeze, has a substantially lower freezing temperature than the bulk dendrite. The shrinkage stresses produced during solidification act upon this small liquid region and produce interdendritic cracks. These cracks occur at temperatures close to bulk solidification temperature, therefore, they are called hot cracks.