For welded steel pipe production, the technique means that heating the whole of the weldment or local welding area before welding is known as preheating. For those welds with higher weld strength, hardening tendency, particularly good heat-conducting property and big thickness, and when the environment temperature of welding zone is too low, it is necessary to preheating the weldment before welding. The main purpose of preheating is reducing the cooling rate and preheat temperature of welded joint. Preheating can reduce cooling rate but basically has impact on time of staying in high temperature, that is so prefect. So when welding the steel with hardening tendency, the main processing measure to reduce cooling rate and decrease hardening tendency is preheating, rather than increase the heat input.