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Understanding the importance of pre weld preheating and post weld heat treatment!

  1. preheating before welding

Preheating before welding and heat treatment after welding to ensure the welding quality is very important. The important components of the welding, welding and thick parts of alloy steel, are required before welding. The following effects must be preheated before welding preheating:

(1) the preheating can slow cooling rate after welding, to escape the diffusion of hydrogen in weld metal, avoid hydrogen induced crack. And also reduce the weld and heat affected zone, the degree of hardening, improve the crack resistance of welded joints.

(2) the preheating can reduce the welding stress. Even local preheating or whole preheating can reduce the welding area work piece (also known as the temperature difference between the temperature gradient). In this way, on one hand reduces the welding stress, on the other hand, reduces the welding strain rate, to avoid the welding crack.

(3) preheating can reduce the restraint degree of welding structure, especially for reducing the restraint degree of fillet joint. With the increase of preheating temperature, the crack rate decreases

Preheat and interpass temperature selection of chemical composition and not only steel and electrode, and the welding structure rigidity, welding method, environmental temperature and so on, should consider these factors determined. In addition, the preheating temperature uniformity in the steel plate thickness direction and in the weld region uniformity, to reduce welding plays an important role in stress. Local preheating width should be welders according to the restraint of the circumstances, the general should be around three times the weld thickness, and not less than 150-200 mm. If the preheating is not uniform, not only reduce the welding stress, but will increase the welding stress the situation.

  1. post weld heat treatment

There are three purposes of post weld heat treatment: hydrogen removal, elimination of welding stress, improvement of weld microstructure and comprehensive properties

After welding hydrogen treatment, refers to the completion of welding, the weld is not cooling to 100 DEG, low temperature heat treatment. The general specification for heating to 200~350 Deg. C, 2-6 hours after welding. Heat dissipation main role is to accelerate the hydrogen treatment of weld and heat affected zone in the escape of hydrogen, to prevent low alloy steel welding welding crack effect is extremely significant.

In the welding process, due to non-uniform heating and cooling, and the component itself has binding or external restraint, the welding work after the end of the component will always produce the welding stress. The welding stress in the component exists, will reduce the actual area of welded joint bearing capacity, plastic deformation, severe that will lead to the destruction of components.

Stress relieving heat treatment is to make the welding workpiece in the condition of high temperature, the decrease of yield strength, to achieve the purpose of relaxation of welding stress. There are two common methods: one is the overall high temperature tempering, namely welding overall in the heating furnace, slowly heated to a certain temperature, then the temperature for a period of time. Finally, the cooling in the air or in the furnace. This method can be used to eliminate 80%-90% welding stress. Another method is the local high temperature tempering, only on and near the weld area for heating, and then slowly cooled, welding to reduce the peak stress, the stress distribution is relatively flat, played a part of stress objective to eliminate welding.

Some materials in alloy steel after welding, the welding joint will be quenched structure, the mechanical properties of the material deterioration. In addition, the quenched structure in the welding stress and the role of hydrogen, may lead to joint damage. If after heat treatment, the microstructure of the joints is improved, plastic, welded joint to improve the toughness, thus improving the comprehensive mechanical properties of the welded joint.

Source: heat treatment ecosphere

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