Hot cracks are generated at high temperatures during the welding process, also known as high temperature cracks. Thermal cracks generally appear in the weld, sometimes in the heat-affected zone, and can occur inside the material or on the surface of the material. It is a defect that must be avoided in welding.
The microscopic feature of hot cracking is cracking along austenite grain boundaries. According to the shape, temperature range and main causes of cracks, thermal cracks can be divided into crystallization cracks, liquefaction cracks and polygonal cracks.

1. The influence of alloying elements and impurity elements
The crystallization temperature range of the material expands with the increase of the alloy element content, and at the same time the range of the brittle temperature zone also increases, so the sensitivity to thermal cracks (here mainly refers to crystal cracks) also increases.
Crack initiation depends on the material's own ability to deform during solidification. The solidification of the weld undergoes a transition from liquid-solid (liquid-dominated), to solid-liquid (solid-dominated), 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 alloy elements. This crystal 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 boundary or the center of the weld. When there are relatively many crystal grains that have been solidified, the low-melting phase remaining at the grain boundary has not yet solidified, and spreads on the surface of the crystal grains in the state of a liquid film, cutting off some connections of the crystal grains. Under the action of tensile stress caused by cooling shrinkage, the liquid film cannot withstand this tensile stress, and it separates and forms (crystallization) cracks at the grain boundaries.

2. The influence of crystal structure
The coarser the grain size of the primary crystallization structure of the weld, the stronger the crystallization direction, the easier it is to promote the segregation of impurities, and it is easier to form a continuous liquid eutectic film after crystallization, which increases the tendency of hot cracks. Add some grain refining elements, such as Mo, V, Ti, Nb, Zr, Al, RE, etc., to the weld or base metal, on the one hand to refine the grains, increase the grain boundary area, and reduce the concentration of impurities; on the other hand On the one hand, it can disrupt the crystallization direction of columnar crystals and destroy the continuity of the liquid film, thereby improving the crack resistance.
If the primary crystallization structure is a single-phase austenite γ that is roughly consistent with the crystallization axis direction, the crystallization crack tendency is very large. If the primary crystallization structure is ferrite δ, or a dual-phase structure in which γ+δ exists simultaneously, the tendency of crystallization cracks can be reduced.

3. Mechanical factors
The low plasticity or brittleness of materials in the brittle temperature zone is only one of the conditions for the formation of thermal cracks. If there is no strain caused by tensile stress and reaches a certain amount of 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. Measures to prevent thermal cracks
The causes of thermal cracks are discussed above, so the preventive measures are also obvious, mainly including:
1) Control C, S, P and other harmful impurity elements; transition Mn, Ti, Zr and other elements through welding materials to overcome the adverse effects of S.
2) Basic 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 (selection) can improve crack resistance and corrosion resistance.
4) Control the shape of the weld seam
Surface surfacing and butt welds with shallow penetration have good crack resistance, while butt welds with large penetration and fillet welds have poor crack resistance, because the shrinkage stress of the latter two welds is basically perpendicular to the accumulation of impurities The crystalline interface has a greater tendency to thermal cracks.
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 promote grain growth and increase the tendency of segregation, which is counterproductive.
6) To reduce the stiffness and restraint of joints, specific measures include reducing the thickness of the structure in design, rationally arranging weld seams, and rationally arranging assembly and welding sequences.
7) For the welding of thick plates, multi-layer welding can be used, and the crack tendency 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 misalignment, meat bite, and incomplete penetration) is also an effective way to reduce the tendency of cracks.