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Causes and Preventive Measures for Welding Cracks in Customized Structural Steel

Sep. 29, 2019

As a Steel Structure Accessories Factory, let's share with you the causes and precautions for the occurrence of weld cracks in steel structures.

(a) Hot crack

The hot cracks in Steel Structures for Grand Stands refer to cracks generated at high temperatures, also known as high-temperature cracks or crystal cracks. They are usually found inside the weld and sometimes in the heat-affected zone. The expressions are longitudinal cracks, transverse cracks. , root crack crater crack and heat-affected zone crack.

The reason for this is that there is segregation in the crystallization process of the weld pool, and the low-melting eutectic and impurities exist in the form of a liquid interlayer during the crystallization process to form segregation, and the strength after solidification is also low when the welding stress is sufficiently large. Then, the liquid interlayer or the solid metal which has just solidified is pulled apart to form a crack.

In addition, if low-melting eutectic and impurities are also present on the grain boundaries of the base material when the welding tensile stress is sufficiently large, it is also pulled apart. In short, the generation of hot cracks is the result of a combination of metallurgical and mechanical factors.

For its causes, its preventive measures are as follows:

(1) Limit the content of easily segregated elements and harmful impurities in the base metal and welding materials (including welding rod, welding wire, flux and shielding gas), especially the content of sulfur and phosphorus and the reduction of carbon content, which are generally used in welding steel. The content of sulfur should not exceed 0.04 5%, and the content of phosphorus should not exceed 0.055%. In addition, the farther the carbon content of steel is, the worse the welding performance is. The thermal crack sensitivity is generally controlled when the carbon content in the weld is below 0.10%. Can be greatly reduced.

(2) Adjust the chemical composition of the weld metal, improve the weld structure, refine the weld grain to improve its plasticity, reduce or disperse the degree of segregation, and control the harmful effects of low melting point products.

(3) Use alkaline electrode or flux to reduce the inclusion of impurities in the weld and improve the degree of segregation during crystallization.

(4) Appropriately increase the shape factor of the weld, and adopt the multi-layer multi-pass welding method to avoid centerline segregation and prevent centerline cracks.

(5) Adopting a reasonable welding sequence and direction, using a small welding line can be super, overall preheating and hammering method, filling the arc pit and other process measures.

Customized Structural Steel

Customized Structural Steel

(b) Cold crack

Cold cracking of Customized Structural Steel generally refers to the fact that the temperature of the weld is reduced to the martensite transformation temperature (below 300-200 °C) during the cooling process, which may occur immediately after welding or maybe longer after welding. Time only occurs, so it is also called delayed crack.

There are three basic conditions for the formation: the welded joint forms a hardened structure; the presence and concentration of diffused hydrogen; there is large tensile stress of welding.

For its causes, its preventive measures are as follows:

(1) Select reasonable welding specifications and line energy to improve the microstructure of the weld and heat-affected zone, such as preheating before welding, the temperature of the control layer, slow cooling after welding or after heat to accelerate the escape of hydrogen molecules.

(2) Use a basic electrode or flux to reduce the diffusion of oxygen content in the weld.

(3) The electrode and flux should be dried in strict accordance with the specified requirements before use (low hydrogen electrode 300 ° C ~ 3 50 ° C insulation lh; acid electrode l 00 ° C ~ l50 ° C insulation lh; flux 200 ° C ~ 250. C insulation 2h), carefully clean the groove and wire to remove dirt such as oil, moisture, and rust to reduce the source of hydrogen.

(4) Heat treatment in time after welding. First, an annealing treatment is carried out to eliminate internal stress, tempering the quenched structure and improving its toughness. Second, it is subjected to hydrogen elimination treatment to fully escape hydrogen from the welded joint.

(5) Improve the quality of steel and reduce layered inclusions in the steel.

(6) Take various technological measures that can reduce welding stress.