Causes and prevention measures of welding "hot cracks"

Thread Content

Hot cracks are produced at high temperatures during the welding process, also known as high-temperature cracks. Hot cracks generally appear in welds and sometimes in heat-affected zones. They can occur inside the material or on the surface of the material. They are a defect that must be avoided during welding. The microscopic characteristic of hot cracking is cracking along austenite grain boundaries. According to the shape of the crack, the temperature range and the main cause, thermal cracks can be divided into crystal cracks, liquefaction cracks and polygonal cracks. 1. Influence of alloying elements and impurity elements The crystallization temperature range of materials expands as the content of alloying elements increases, and the range of the brittle temperature zone also increases, so the sensitivity to hot cracks (here mainly refers to crystallization cracks) also increases. The occurrence of cracks depends on the deformation ability of the material itself during the solidification process. The solidification of the weld goes through a transition from liquid-solid state (mainly liquid phase), to solid-liquid state (mainly solid phase) and then to complete solidification. During solidification, the part that crystallizes first is relatively pure, and the part that crystallizes later contains more impurities and alloying elements. This crystallization segregation causes the inhomogeneity of the chemical composition of the weld metal. In the middle and late stages of solidification, impurities will be continuously repelled to the grain boundaries or the center of the weld. When there are relatively many solidified grains, these low melting phases remaining at the grain boundaries have not yet solidified, and are spread on the surface of the grains in a liquid film state, cutting off some connections between the grains. Under the action of the tensile stress caused by cooling shrinkage, the liquid film cannot withstand this tensile stress and separates at the grain boundaries to form (crystalline) cracks. The influence of impurities or alloying elements on the hot crack sensitivity of materials can be judged by the following two formulas:: In the picture, C is easily segregated and forms a low-melting eutectic with other elements, which is the main element that aggravates the tendency of hot cracking. ； S and P can also easily cause crystallization segregation. At the same time, S and P can also form a variety of low-melting substances. ； Mn has a desulfurization effect, which can improve the distribution pattern of sulfides and reduce the tendency of crystal cracks. ； Si is a ferrite-forming element. A small amount of Si is beneficial to improving the crack resistance. However, when Si>0.4%, silicate inclusions will be formed to reduce the crack resistance. ； The melting point of Ni and Ni3S2 eutectic is only 645°C, which can cause thermal cracks. ； Rare earth elements such as Ti, Zr, RE, etc. can form high melting point sulfides, which are beneficial to eliminating crystal cracks. 2. Influence of crystal structure The coarser the grain size of the primary crystal structure of the weld, the stronger the crystallization directionality, the easier it is to promote the segregation of impurities, and it is easy to form a continuous liquid eutectic film after crystallization, which increases the tendency of hot cracks. Adding some grain-refining elements, such as Mo, V, Ti, Nb, Zr, Al, RE, etc., to the weld or base metal can, on the one hand, refine the grains, increase the grain boundary area, and reduce the concentration of impurities. ； On the other hand, it can disrupt the crystallization direction of columnar crystals and destroy the continuity of the liquid film, thus improving the crack resistance. If the primary crystallographic structure is single-phase austenite γ that is roughly consistent with the direction of the main axis of crystallization, the tendency of crystallization cracks is very high. If the primary crystallographic structure is ferrite δ, or a dual-phase structure in which γ+δ coexists, the tendency of crystal cracks can be reduced. 3. Mechanical factors The low plasticity or embrittlement of materials in the brittle temperature zone is only one of the conditions for the formation of hot cracks. If there is no strain caused by tensile stress and reaches a certain strain, no cracks will occur. These stresses are mainly caused by the uneven heating and cooling process of welding, such as thermal stress, tissue stress and restraint stress. 4. Preventive Measures for Hot Cracks The causes of hot cracks have been discussed above, so the preventive measures are also obvious, mainly including: 1), control harmful impurity elements such as C, S, P, etc. ； The adverse effects of S can be overcome by transferring elements such as Mn, Ti, and Zr through welding materials. 2) Alkaline welding rods or fluxes should be used for important welding structures because they have strong desulfurization capabilities. 3) Adding grain-refining elements to the weld metal or base metal (selected materials) can improve crack resistance and corrosion resistance. 4) Control the weld shape. Surface surfacing and butt welds with shallower penetration have better crack resistance, while butt welds and fillet welds with larger penetration have poorer crack resistance, because the shrinkage stress of the latter two welds is basically perpendicular to the crystallization interface where impurities gather, and the tendency of hot cracks is greater. 5) If the cooling rate is too fast, the strain rate of the weld metal will increase (the plastic deformation of the material cannot keep up), and cracks will easily occur. For this reason, slow cooling measures should be adopted, and preheating can slow down the cooling rate. In addition, slow cooling cannot be achieved by increasing the welding heat input, because excessive welding heat input will cause grain growth and increase the tendency of segregation, which is counterproductive. 6) Reduce the stiffness and restraint of joints. Specific measures include reducing the structural thickness in design, rationally arranging welds, and rationally arranging assembly and welding sequences. 7) For thick plate welding, multi-layer welding can be used. The tendency of cracks is less severe than that of single-layer welding, but attention should be paid to controlling the penetration depth of each layer. In addition, avoiding stress concentration at welded joints (such as stress concentration caused by defects such as misaligned edges, meat bites, incomplete welding, etc.) is also an effective way to reduce the tendency of cracks.