Pre welding preheating and post weld heat treatment
Welding of important components, welding of alloy steel and welding of thick parts all require preheating before welding. The main function of preheating before welding is as follows:
(1) preheating can slow down the cooling rate after welding, which is beneficial to the escape of diffusion hydrogen in weld metal and avoid hydrogen induced cracking. At the same time, the hardened degree of weld and heat affected zone is reduced, and the crack resistance of welded joint is improved.
(2) preheating can reduce the welding stress. The uniform local preheating or overall preheating can reduce the temperature difference between the weldment parts in the welding area (also known as the temperature gradient). Thus, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is conducive to avoiding welding cracks.
(3) preheating can reduce the binding degree of the welding structure, and the binding degree of the angle joint is particularly obvious. With the increase of the preheating temperature, the crack rate 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, welding method and ambient temperature of the welding structure, and the factors should be taken into consideration. In addition, the uniformity of the thickness of the steel plate and the uniformity of the weld area have an important effect on reducing the welding stress. The width of the partial preheating should be determined according to the restraint degree of the weldment parts, generally three times the thickness of the surrounding area of the weld area, and shall not be less than 150-200 mm. If the preheating is uneven, not only does not reduce the welding stress, but it will increase the welding stress.
There are three purposes for post weld heat treatment: hydrogen elimination, elimination of welding stress, improvement of weld microstructure and comprehensive properties.
Post weld hydrogen elimination treatment means low temperature heat treatment when the weld has not cooled to below 100 degrees after welding. The general specification is to heat up to 200~350 C for 2-6 hours. The main effect of post welding dehydrogenation is to accelerate the escape of hydrogen in the weld and heat affected zone, which is very significant to prevent welding cracks in low alloy steel.
In the welding process, due to the inhomogeneity of heating and cooling, and the binding or external constraint of the component itself, the welding stress will always be produced in the component after the end of the welding. The existence of welding stress in the component will reduce the actual carrying capacity of the welded joint area and produce plastic deformation. In severe cases, the failure of the component will also occur.
The stress relieving heat treatment is to make the welded workpiece at high temperature and decrease its yield strength, so as to achieve the purpose of relaxation welding stress. There are two commonly used methods: one is the overall high temperature tempering, that is, the welding parts are put into the heating furnace, slowly heating to a certain temperature, and then holding the heat for a period of time, and finally cooling in the air or in the furnace. The welding stress of 80%-90% can be eliminated by this method. The other method is the local high temperature tempering, that is to heat only the weld and its vicinity, then slow down the cooling, reduce the peak of the welding stress, make the stress distribution relatively gentle, and partially eliminate the welding stress.
Some alloy steels will harden their welded joints after welding, which will deteriorate the mechanical properties of the materials. In addition, the hardened structure may result in joint failure under the action of welding stress and hydrogen. If after heat treatment, the microstructure of the joint is improved, the plasticity and toughness of the welded joint are improved, thus the comprehensive mechanical properties of the welded joint are improved.