In the production process of welded steel pipe (such as spiral welded pipe), there are three purposes of post-weld heat treatment: hydrogen, eliminate welding stress, improve weld organization and overall performance.
After the welding hydrogen treatment, refers to the welding is completed, the weld has not cooled to 100 ℃ below, the low temperature heat treatment. General specifications for heating to 200 ~ 350 ℃, insulation 2-6 hours. The main effect of post-weld hydrogen reduction treatment is to accelerate the escape of hydrogen in the weld and heat affected zone, which is very effective for preventing welding cracks in the welding of low alloy steels.
In the welding process of welded steel pipe, due to heating and cooling of the non-uniformity, as well as the component itself is bound or imposed binding, welding work in the end, the components will always produce welding stress. The existence of welding stress in the component will reduce the actual carrying capacity of the welded joint area, resulting in plastic deformation, serious, but also lead to damage to components.
Consumable stress heat treatment of welded steel pipe is to make the welding of the workpiece in the high temperature state, the yield strength decreased, to achieve the purpose of relaxation of welding stress. Commonly used methods are two: First, the overall high temperature tempering, that is, the whole weldment into the heating furnace, slowly heated to a certain temperature, and then heat for some time, and finally in the air or furnace cooling. With this method can eliminate 80% -90% of the welding stress. Another method is the local high temperature tempering, that is, only the weld and its vicinity of the area of heating, and then slow cooling, reducing the peak stress, so that the stress distribution is relatively gentle, play a part of the purpose of eliminating welding stress.
Some alloy steel materials in the welding, the welded joints will appear hardened, so that the mechanical properties of materials deteriorated. In addition, this hardened structure in the welding stress and the role of hydrogen, may lead to joint damage. If after heat treatment, the microstructure of the joint is improved, the plasticity and toughness of the welded joint are improved, thus improving the comprehensive mechanical properties of the welded joint of welded steel pipe.