① Improve welding quality and strength. 

② Preheating can slow down the cooling rate after welding, which is conducive to the escape of diffusive hydrogen in the weld metal and avoid hydrogen induced cracks. At the same time, the hardness of weld and heat affected zone is reduced, and the crack resistance of welded joint is improved. 

③ Preheating can reduce welding stress. 

④ Preheating can reduce the restraint degree of welding structure, especially for the fillet joint. With the increase of preheating temperature, the crack rate decreases.