Importance of preheating before welding and annealing after welding
The welding of important components, alloy steel and thick parts requires preheating before welding. The main functions of preheating before welding are as follows:
(1) Preheating can slow down the cooling rate of post-welding annealing, which is conducive to the escape of diffused hydrogen in weld metal and avoid hydrogen-induced cracking. At the same time, the hardening degree of weld and heat affected zone is reduced, and the crack resistance of welded joint is improved.
(2) Preheating can reduce welding stress. The temperature difference (also known as temperature gradient) between the parts being welded in the welding area can be reduced by uniform local preheating or overall preheating. In this way, on the one hand, the welding stress is reduced, on the other hand, the welding strain rate is reduced, which is conducive to avoid welding cracks.
(3) Preheating can reduce the constraint of the welded structure, especially the constraint of the Angle joint. With the increase of preheating temperature, the crack incidence decreases.
The selection of preheating temperature and interlayer temperature is not only related to the chemical composition of steel and electrode, but also related to the rigidity of welding structure, welding method, environmental temperature, etc., should be determined after taking these factors into consideration. In addition, the uniformity of preheating temperature in the thickness direction of the steel plate and in the weld area has an important effect on the reduction of welding stress. The width of local preheating should be determined according to the constraint of the welder, which should generally be three times the wall thickness around the weld area, and not less than 150-200 mm. If the preheating is not uniform, the welding stress will not be reduced, but will increase the welding stress.
