The better the weldability, the easier it is to weld. However, weldability is a complicated property, as it encompasses the metallurgical compatibility of the metal or alloy with a specific welding process, its ability to be welded with mechanical soundness, and the capacity of the resulting weld to perform satisfactorily under the intended service conditions.
Before attempting to weld any material, it is essential to know how easy it is to weld and to be aware of any problems that might arise. One of the main problems likely to be encountered when welding carbon and alloy steels is hydrogen cracking. For hydrogen cracking to occur, it is necessary to have a supply of hydrogen to the weld and a heat affected zone (HAZ), a susceptible hardened microstructure, and tensile stress. If any one of these three components is eliminated, then hydrogen cracking will not happen. Solidification cracking and lamellar tearing are other potential problems associated with welding steel.
The main problem when welding steel is hardenability. As long as the steel contains sufficient carbon when it is cooled rapidly from high temperature, a phase transformation takes place. The phase transformation from austenite to martensite causes the material to harden and become brittle. It is then liable tocrack on cooling, due to restraint, or later under the action of hydrogen.