Alloy steel has good high temperature oxidation resistance, structural stability and high high temperature strength. However, its weldability is poor and its hardening tendency is large. During welding, the following problems may occur in the weld and heat affected zone: (1) After welding, hard brittle hardening structures appear in the weld and heat affected zone. If there is more internal stress, cold cracks will appear, and toughness and plasticity will decrease. (2) There is a tendency of crater cracks. (3) Some alloying elements, especially steels containing vanadium and boron, tend to crack after welding and reheating. In order to avoid and eliminate the above defects, alloy steel needs to be preheated before welding and heat treated after welding. The main purpose of preheating before welding is to improve the weldability of materials. In the process of heating and maintaining a certain temperature, the temperature difference in the welded joint area can be reduced, and the hardening tendency of the heat affected zone can be weakened. In the welding process, it is beneficial to the escape of hydrogen, reduce the hydrogen content in the weld, prevent the occurrence of cold cracks and improve the plasticity and toughness of the welded joint. The post-heat treatment method is generally high-temperature tempering, and its main purpose is to (1) eliminate or reduce the internal stress generated in the welding process and prevent cracks in the weld and heat affected zone. (2) Improve the mechanical properties of weld and heat affected zone, that is, improve its plasticity and toughness, and improve the tendency of hard and brittle. (3) Improve the metallographic structure of weld and heat affected zone. Generally, carbon steel has good weldability, so preheating and heat treatment are not needed for welding. However, for weldments with high carbon content and high stiffness, preheating and heat treatment are also needed because of the large residual stress after welding, such as carbon steel pipes with wall thickness greater than 30 mm