Causes and Solutions of Welding Hot Cracks

Welding cracks are relatively dangerous welding defects, which seriously affect the performance and safety of welded structures. Many damage accidents of welded structures are caused by welding cracks. In addition to reducing the strength of welded joints, cracks will also cause serious stress concentration, promote the development of cracks and damage of welded structures, The weld metal and heat affected zone metal are cooled to the high temperature zone near the solidus to produce welding cracks.

Formation of hot cracks

The hot crack in weld metal is also called solidification crack. Most of the welding materials are alloy. The solidification of alloy is carried out in a suitable temperature range from beginning to end, which is the basic reason of hot crack. The solidification temperature of many impurities in the weld metal is lower than that of the weld metal, so the first solidified weld metal pushes the impurities with low melting point to the grain boundary of solidification crystallization, forming a layer of liquid film. Because the cooling rate of weld pool is very fast during welding, the weld metal shrinks during the cooling process, resulting in tensile stress. When the tensile stress pulls the solidified weld metal apart along the grain boundary, and there is not enough liquid metal to supplement it, tiny cracks will form. As the temperature continues to decrease, the tensile stress increases and the crack continues to expand, which is called solidification crack.

When the weld metal contains a lot of low melting point impurities, the weld metal is easy to produce solidification cracks. The base metal and welding materials contain harmful impurities, especially sulfur. Sulfur combines with iron in steel to form ferrous sulfide (FES), and ferrous sulfide reacts with iron to form a eutectic material. The solidification temperature is 988 ℃ °， So sulfur is the main element that causes solidification cracks in weld metal. In addition, when the carbon content in steel is high, it is conducive to sulfur enrichment at the grain boundary, which is also the reason for the formation of solidification cracks. Therefore, the use of welding materials with low carbon content is conducive to preventing the formation of solidification cracks.

Measures to prevent hot crack

1.Manganese has the function of desulfurization. If the carbon content of base metal and welding material is high, but the manganese content is insufficient, it is easy to produce hot cracks. Generally, the sulfur content of base metal, welding rod and welding wire should not exceed 0.04%, and the carbon content of welding rod and welding wire for low carbon steel and low alloy mesh should not exceed 0.12%, Using qualified welding rod is an important measure to prevent hot crack.

2.For the weldment with high cutting property, the vertical deformation is small during welding, which leads to the increase of welding stress and the generation of hot cracks. When welding, select the appropriate welding specification, take preheating and slow cooling measures, and arrange the welding direction and welding sequence to reduce the welding stress.

3.Adjusting the alloy composition of weld metal, such as welding Cr Ni stainless steel, appropriately increasing the chromium content of weld metal can significantly increase the hot crack resistance of weld metal. Adding elements that can refine the grain in weld metal is conducive to removing the concentrated liquid film and preventing the generation of hot crack.

4.The hot crack plate is easy to produce in the crater, that is, the crater crack. It is necessary to pay attention to fill the crater during the shielded metal arc welding. In the argon tungsten arc welding without filler wire, when the arc is closed, the welding current should be gradually reduced, and then when the welding volume is reduced to a very small size, the welding current should be cut off. When welding the materials that are difficult to remove the crater crack, the lead out plate should be used to lead out the crater.

Basic knowledge about welded pipe