Causes of Reheating Cracks in Welded Joints and Preventive Measures

Reheat crack is that the strength in the grain is high and the strength at the grain boundary is weak. The deformation during the stress relaxation period is concentrated on the grain boundary. When the strength limit of the grain boundary is exceeded, it will lead to cracking and crack.

1.Internal cause of reheat crack formation

During welding, the heat affected zone is heated to 1200 ℃. After multiple heating of thick plate, the grain is coarse and the precipitation of strong carbide is slow. During submerged arc welding, due to large linear energy and coarse grain, during post welding SR treatment, trace elements and low melting point impurities are segregated at the grain boundary, weakening the plasticity of the grain boundary. When the strain exceeds the plasticity of the grain boundary, cracking is formed.

2.External causes of reheat crack formation

External factors should be considered from two aspects: welding residual stress and expansion stress.

During post weld stress relief heat treatment, the welding residual stress is reduced through creep deformation. In the welding area, due to the existence of low melting point compounds and coarse-grained embrittlement zone, the grain boundary toughness and strength are insufficient, so it can not resist creep expansion deformation, resulting in failure during crack.

Precautions:

1.Strictly control raw materials and strong carbide forming elements in steel, which have a great impact on reheat cracks. It is also one of the more effective measures to solve the internal causes of reheat cracks.

The welding material with low sensitivity to hot crack shall be selected, and the lower limit of weld metal strength shall be taken.

2.Formulate reasonable welding specifications:

Reducing the weld energy and controlling the preheating temperature have a great impact on the metallographic structure of the weld area. Generally speaking, using multi-channel and multi-layer small line energy and appropriately increasing the cooling rate of the weld area is conducive to improving the metallographic structure, improving the impact toughness and preventing hot cracks. However, too low interlayer temperature is not conducive to the escape of hydrogen and there is a risk of cold cracks. Therefore, Controlling the cooling rate and obtaining fine grains should focus on controlling the line energy.

Proper preheating measures can soften the hardness and improve the toughness and crack resistance of the hardened layer.

Controlling the welding process, reducing the amount of micro defects and deposited metal, and using narrow gap welding can also effectively control reheat cracks.

Through the above discussion, these minor defects are the stress concentration points. During the release process of heat treatment, there is the cause of stress superposition, resulting in reheat cracks. Therefore, these defects should be controlled.

Control the welding residual stress. Under the creep expansion force of heat treatment, the stress concentration point of the weld and the grain boundary after the precipitation hardening zone produced by carbide cannot resist the cracking caused by strain. Therefore, reducing the residual stress can also reduce the occurrence of reheat cracks before heat treatment. Half pass intermediate heat treatment and high frequency ultrasonic impact method can reduce welding residual stress.

Post weld heat treatment: control the heating and cooling rate during post weld heat treatment to make it expand and contract slowly and evenly, so as to reduce the occurrence of reheat cracks.