Weldability refers to the difficulty of obtaining excellent welded joints by metal materials under certain welding process conditions, including welding methods, welding materials, welding specifications and welding structure forms. If high-quality joints can be obtained by ordinary and simple welding process, it is considered that metals have good welding performance. The weldability of steel mainly depends on its chemical composition. Among them, carbon is the most influential element, that is to say, the carbon content of metal determines its weldability. Most other alloying elements in steel are also unfavorable for welding, but their influence is generally much smaller than that of carbon. With the increase of carbon content in steel, the hardening tendency increases, but the plasticity decreases, which is easy to produce welding cracks. Usually, the sensitivity of cracks in metal materials during welding and the change of mechanical properties in welded joint area are the main indexes to evaluate the weldability of materials. Therefore, the higher the carbon content, the worse the weldability. Therefore, the carbon content in steel is often regarded as the main symbol to judge the weldability of steel. Low carbon steel and low alloy steel with carbon content less than 0.25% have excellent plasticity and impact toughness, and the plasticity and impact toughness of welded joints after welding are also good. There is no need for preheating and post-weld heat treatment during welding, and the welding process is simple and has good weldability. With the increase of carbon content, the tendency of welding cracks increases greatly. Therefore, steel with carbon content greater than 0.25% should not be used to manufacture pressure-bearing parts of boilers and pressure vessels.

The weldability of metal materials can be verified by calculating carbon equivalent, oblique Y-groove welding crack test, highest hardness test in heat affected zone, thermal simulation test, high temperature creep test and aging test.