Hot cracks are generated at high temperatures during the welding process, also known as high-temperature cracks. Hot cracks generally occur in welds and sometimes also in the heat affected zone, which can occur inside the material or on the surface of the material. They are a defect that must be avoided in welding.

The microscopic feature of hot cracking is cracking along austenite grain boundaries. According to the morphology, temperature range, and main causes of cracks, hot cracks can be divided into crystallization cracks, liquefaction cracks, and polygonal cracks.

1、 The influence of alloy elements and impurity elements

The crystallization temperature range of materials expands with the increase of alloy element content, and the range of brittle temperature zone also increases, so the sensitivity of hot cracks (mainly referring to crystallization cracks here) also increases.

The generation of cracks depends on the deformation ability of the material itself during the solidification process. The solidification of the weld seam undergoes a transition from liquid solid (mainly liquid phase) to solid liquid (mainly solid phase) and then to complete solidification.

During solidification, the part that crystallizes first is relatively pure, while the part that crystallizes later contains more impurities and alloy elements. This crystallization segregation causes uneven chemical composition of the weld metal.

In the middle and later stages of solidification, impurities will be continuously expelled to the grain boundaries or the center of the weld seam. When there are relatively many solidified grains, these residual low melting phases at the grain boundaries have not yet solidified and are scattered on the surface of the grains in a liquid film state, cutting off some of the connections between the grains. Under the tensile stress caused by cooling shrinkage, the liquid film cannot withstand this tensile stress and forms (crystalline) cracks at the grain boundaries.

2、 The influence of crystal structure

The coarser the grain size of the primary crystallization structure of the weld seam, the stronger the directionality of crystallization, and the easier it is to promote impurity segregation. After crystallization, it is easy to form a continuous liquid eutectic film, increasing the tendency for hot cracking. Adding some refined grain elements such as Mo, V, Ti, Nb, Zr, Al, RE, etc. to the weld or base metal can refine the grains, increase the grain boundary area, and reduce impurity concentration; On the other hand, it can disrupt the crystallization direction of columnar crystals, disrupt the continuity of the liquid film, and thus improve crack resistance.

If the primary crystalline structure is single-phase austenite roughly aligned with the main axis direction of crystallization γ， The tendency for crystal cracks is significant. If the primary crystalline structure is ferrite δ， or γ+δ The dual phase structure that exists simultaneously can reduce the tendency for crystal cracks.

3、 Mechanical factors

The low plasticity or embrittlement of materials in the brittle temperature zone is only one of the conditions for forming hot cracks. If there is no strain caused by tensile stress and a certain strain is reached, cracks will not occur. These stresses are mainly caused by the uneven heating and cooling process of welding, such as thermal stress, structural stress, and restraint stress.

4、 Preventive Measures for Hot Cracks

The causes of hot cracks have been discussed above, and its prevention measures are also obvious, mainly including:

1) Control harmful impurity elements such as C, S, and P; Overcoming the adverse effects of S by transferring elements such as Mn, Ti, Zr, etc. to the welding material.

2) Important welding structures should use alkaline welding rods or fluxes because they have strong desulfurization capabilities.

3) Adding refined grain elements to the weld metal or base metal (selected material) can improve crack resistance and corrosion resistance.

4) Control the shape of the weld seam

Surface overlay welding and butt welds with shallow penetration have good crack resistance, while butt welds and fillet welds with larger penetration have poor crack resistance because the shrinkage stress of the latter two welds is basically perpendicular to the crystal interface where impurities gather, and the tendency for hot cracking is greater.

5) If the cooling speed is too fast, it will increase the strain rate of the weld metal (material plastic deformation cannot keep up), which is prone to cracking. Therefore, slow cooling measures should be taken, and preheating can slow down the cooling speed. In addition, slow cooling cannot be achieved by increasing the welding heat input, as excessive welding heat input can promote grain growth and increase segregation tendency, which is counterproductive.

6) To reduce the stiffness and restraint of joints, specific measures include reducing structural thickness in design, arranging welding seams reasonably, and arranging assembly and welding sequences reasonably.

7) For thick plate welding, multi-layer welding can be used, and the crack tendency is milder than 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, undercutting, and incomplete penetration) is also an effective way to reduce the tendency for cracks.