Post-heating refers to the process measures of heating or insulating the whole (or part) of the weldment immediately after welding, so that it can be cooled slowly. The purpose of post-heating is to reduce the content of diffused hydrogen in welded joints, especially in the heat affected zone, so it is also called dehydrogenation treatment, which is an important technological measure to prevent delayed cracks when welding some low-alloy structural steels. The post-heating temperature is generally 200~350℃, and the time is not less than 30 minutes.
Post-weld heat treatment is mainly to eliminate residual stress.
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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 after welding, which is beneficial to the escape of diffused hydrogen in weld metal and avoid hydrogen-induced cracks. At the same time, it also reduces the hardening degree of weld and heat affected zone, and improves the crack resistance of welded joint.
(2) Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also called temperature gradient) between the welded parts in the welding area. On the one hand, it reduces the welding stress, on the other hand, it reduces the welding strain rate, which is beneficial to avoid welding cracks.
(3) Preheating can reduce the constraint of welding structure, especially fillet weld. With the increase of preheating temperature, the incidence of cracks decreases.
The selection of preheating temperature and interlayer temperature is not only related to the chemical composition of steel and covered electrode, but also related to the rigidity of welded structure, welding method and environmental temperature, which should be determined after comprehensive consideration of these factors. In addition, the thickness direction of steel plate and the uniformity of preheating temperature in weld zone have important influence on reducing welding stress. The width of local preheating should be determined according to the constraint degree of the workpiece to be welded. Generally, it should be three times the wall thickness around the weld area and not less than 150-200mm. If the preheating is uneven, the welding stress will not decrease, but will increase.
Post-weld heat treatment has three purposes: eliminating hydrogen, eliminating welding stress and improving weld microstructure and comprehensive properties.
Dehydrogenation after welding refers to the low-temperature heat treatment when the weld has not cooled to below 100℃ after welding. The general specification is to heat to 200~350℃ and keep the temperature for 2-6 hours. The main function of hydrogen elimination treatment after welding is to accelerate the escape of hydrogen in weld and heat affected zone, which is very effective in preventing welding cracks of low alloy steel.
In the welding process, due to the non-uniformity of heating and cooling, as well as the constraints of the component itself or external constraints, after the welding work, welding stress will always occur in the component. The existence of welding stress in components will reduce the actual bearing capacity of welded joint area, produce plastic deformation, and even lead to the destruction of components in serious cases.
Stress relieving heat treatment is to reduce the yield strength of welded workpiece at high temperature, thus relaxing welding stress. There are two common methods: one is overall high-temperature tempering, that is, the whole weldment is put into a heating furnace, slowly heated to a certain temperature, then kept for a period of time, and finally cooled in the air or in the furnace. This method can eliminate 80%-90% welding stress. Another method is local high temperature tempering, that is, only the weld and its vicinity are heated, and then slowly cooled, so as to reduce the peak value of welding stress, make the stress distribution more gentle, and partially eliminate welding stress.
After some alloy steel materials are welded, hardened structures will appear in the welded joints, which will worsen the mechanical properties of the materials. In addition, this hardened structure may lead to the failure of the joint under the action of welding stress and hydrogen. After heat treatment, the metallographic structure of the joint is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.