Fri, 27 Jul 2018 02:29:40 +0000
The purpose and method of heat treatment after welding
The residual stress of welding is caused by the non-uniform temperature distribution of welding parts caused by welding, the thermal expansion and cold shrinkage of weld metal, etc., so the residual stress will inevitably be generated along with welding construction.
The most common method to eliminate residual stress is high temperature tempering, the weldment in heat treatment furnace heated to a certain temperature (below Ac1) and heat preservation time, using the material yield limit under high temperature is reduced, the internal stress of high local produce plastic flow, elastic deformation gradually reduce, plastic deformation gradually increase and reduce stress.
The influence of post-welding heat treatment on tensile strength and creep limit of metal is related to heat treatment temperature and heat preservation time. The effect of post-weld heat treatment on the impact toughness of weld metal is different with different types of steel.
1. Purpose of post-welding heat treatment
2. Stabilize the shape and size of the structure to reduce distortion. 3. Improve the properties of the base material and welded joints, including a. improve the plasticity of weld metal. B. Reduce the hardness of the thermal impact zone. C. improve fracture toughness. D. Improve fatigue strength. E. restore or increase the reduced yield strength during cold forming. 4. Improve the ability to resist stress corrosion. 5. Further release harmful gases in weld metal, especially hydrogen, to prevent the occurrence of delayed cracks.
2. Selection of heat treatment methods
The heat treatment after welding usually adopts single high temperature tempering or normal tempering. The gas welding joint is heat treated by normalizing and high tempering. This is because the welding seam of gas welding and the grain size of the heat-affected zone is large and needs to be refined. However, a single normal fire cannot eliminate the residual stress, so it is necessary to add high temperature tempering to eliminate the stress.
Single medium temperature tempering is only applicable to the assembling and welding of large ordinary low carbon steel containers assembled on site. The purpose is to partially eliminate residual stress and dehydrogenation. In most cases, a single high temperature tempering is used. Heat treatment should not be heated and cooled too fast to achieve uniform inner and outer wall.