Cause analysis and preventive measures of welding hot crack

Hot crack is produced at high temperature during welding, so it is called hot crack. The morphology, temperature zone and main reason of thermal crack are different according to the material of welding metal. Hot cracks occur frequently at high temperatures close to the solid phase line and are distributed along grain boundaries (see interface). But it can sometimes form along the polygonal boundary at temperatures below the solid line. Hot cracks usually occur in the weld metal, but may also form in the weld metal (the base metal) near the weld fusion line. According to the characteristics of its forming process, it can be divided into liquefaction cracks and polylateral cracks.

Under the action of peak temperature of welding thermal cycle, the high temperature liquefaction crack occurs remelting between the heat affected zone and the multilayer welding, and the crack is produced under the action of stress. The liquefaction cracks mainly occur in the base metal near the fusion line of 鏠 welding, and sometimes in the preweld pass of multilayer welding. The formation is due to the local melting along the grain boundary in the outer metal of the whole welding line under the action of welding heat, and the cracking of the liquefied layer along the grain boundary caused by the subsequent cooling contraction. There are two reasons for this kind of crack: one is that there are more materials with low melting point at the crystal boundary; The other is that some alloying elements are enriched or even eutectic in local grain boundaries due to the decomposition of some metal compounds by rapid heating without diffusion. The principle of preventing this kind of crack is to strictly control the impurity content, rationally select the welding material and minimize the effect of welding heat.

Polylateral cracks are formed at temperatures below the solid phase line. They are distributed along polygonal boundary and have no obvious relation with primary crystal boundary. Easy generation in single-phase austenitic metal, this phenomenon can be interpreted as due to the high temperature of welding overheating and imbalance of crystallization conditions make inside the crystal form of vacancy and dislocation, under the required temperature, stress is arranged in the grain boundary (polygon boundary), when some grain boundary and harmful impurities enrichment region overlap, tend to form micro cracks. Some of these defects can be eliminated by adding alloying elements that increase polygon activation. For example, W, Mo, Ta and so on are added to Ni-Cr alloy; On the other hand, it can reduce overheating and welding stress.