Three reasons for post weld heat treatment and two commonly used methods

There are three main reasons for post-weld heat treatment: eliminating hydrogen, eliminating weld stress, and improving weld microstructure and overall performance.
The hydrogen treatment after welding is after the low temperature heat treatment, the weld is not cooled to below 100 ° C, generally heated to 200 ~ 350 ° C, and kept for 2-6 hours.
The main function of hydrogen removal after welding is to accelerate the hydrogen evolution of the weld, which is of great significance to prevent the occurrence of weld cracks in low alloy steel.
During the welding process, due to uneven heating and cooling, and the components themselves are constrained or applied, welding stress is always generated in the components after the welding work. The existence of the welding stress of the component can reduce the actual bearing capacity of the weld zone, cause plastic deformation, and cause damage to the component in severe cases. The stress dissipation heat treatment is to reduce the yield strength of the welded workpiece at high temperatures, thereby achieving the relaxation welding stress.
There are two common methods: First, the overall high temperature tempering, that is, the entire welded part enters the furnace, slowly heated to the temperature, and then kept for a while, cooled in the air or in the furnace. In this way, 80%-90% of the welding stress can be eliminated. Another method is local high temperature tempering, that is, only the weld and its vicinity heat, and then slowly cooling, reducing the peak welding stress, making the stress distribution relatively flat, thereby partially eliminating the welding stress.
After welding some alloy steel materials, the welded joints will have hardened structure, which will deteriorate the mechanical properties of the material. In addition, the hardened structure causes damage under welding stress and hydrogen. After heat treatment, the joint structure is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.