Causes of welding \"heat cracks\" and preventive measures

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Thermal cracks occur at the high temperatures during the welding process; they are also known as high-temperature cracks. Hot cracks generally occur in the weld seam, and sometimes in the heat-affected zone as well; they can form either inside the material or on its surface, and are a defect that must be avoided during welding. The microscopic characteristic of thermal cracking is cracking along the austenite grain boundaries. Based on the morphology of the cracks, the temperature range, and the main causes, thermal cracks can be classified into crystalline cracks, liquefaction cracks, and polygonalization cracks. I. Effects of alloying elements and impurity elements: The crystallization temperature range of the material increases as the content of alloying elements rises; simultaneously, the range of the brittle temperature zone also expands. As a result, the sensitivity to thermal cracks (referring here mainly to crystallization cracks) increases as well. The formation of cracks depends on the deformation ability of the material itself during the solidification process. The solidification of a weld goes through a transition from liquid-solid state (with the liquid phase dominating), to solid-liquid state (with the solid phase dominating), and finally to complete solidification. The portion that crystallizes first during solidification is purer, while the portion that crystallizes later contains more impurities and alloying elements; this crystallization segregation leads to unevenness in the chemical composition of the weld metal. In the later stages of solidification, impurities are continuously pushed to the grain boundaries or the center of the weld. When there are relatively many already solidified grains, these low-melting phases remaining at the grain boundaries have not yet solidified; they exist in a liquid film form distributed on the surface of the grains, breaking some of the connections between those grains. Under the tensile stress caused by cooling contraction, the liquid film cannot withstand this stress, and thus separates at the grain boundaries to form (crystalline) cracks. Among them, C is highly prone to segregation and forms low-melting eutectics with other elements; it is the main element that exacerbates the tendency to thermal cracking ； S and P also easily cause crystalline segregation; furthermore, S and P can form various low-melting compounds ； Mn has a desulfurization effect, can improve the distribution pattern of sulfides, and can reduce the tendency to form crystalline cracks ； Si is a ferrite-forming element; a small amount of Si helps improve crack resistance, but when Si > 0.4%, silicate inclusions are formed which reduce crack resistance ； The eutectic melting point of Ni and Ni3S2 is only 645°C, which can cause thermal cracking ； Rare earth elements such as Ti, Zr, and REs can form sulfides with high melting points, which is beneficial for eliminating crystalline cracks. II. Influence of the crystalline structure: The coarser the grain size of the primary crystalline structure in the weld, and the stronger the directionality of crystallization, the more likely it is that impurities will become concentrated. After crystallization, a continuous liquid eutectic layer is likely to form, increasing the tendency for thermal cracks. By adding elements that refine grains to the weld or base metal, such as Mo, V, Ti, Nb, Zr, Al, RE, etc., the grain size is reduced, the grain boundary area is increased, and the concentration of impurities is decreased ； On the other hand, it can disrupt the crystal growth direction of the columnar crystals and break the continuity of the liquid film, thereby improving crack resistance. If the microstructure after single crystallization is single-phase austenite γ that is roughly aligned with the crystal axis direction, the tendency for crystalline cracks is high. If the microstructure after single crystallization is ferrite δ, or a biphasic structure with both γ and δ present, the tendency for crystalline cracks to form can be reduced. III. Mechanical factors: In the brittle temperature range, the low plasticity or embrittlement of materials is only one of the conditions necessary for the formation of thermal cracks; cracks will not occur without strain induced by tensile stress and without reaching a certain strain level. These stresses are mainly caused by the uneven heating and cooling processes during welding, such as thermal stress, structural stress, and restraint stress. IV. Measures to Prevent Thermal Cracking Having discussed the causes of thermal cracking, the measures to prevent it are also obvious, mainly including: 1) Controlling harmful impurity elements such as C, S, and P ； Elements such as Mn, Ti, and Zr are introduced through welding materials to overcome the adverse effects of S. 2) For important welded structures, alkaline electrodes or fluxes should be used, as they have a strong desulfurization capacity. 3) Adding grain-refining elements to the weld metal or base material (material selection) can improve crack resistance as well as corrosion resistance. 4) Control of weld shape: Surface surfacing welds and butt welds with a shallow penetration have good crack resistance, whereas butt welds and fillet welds with a greater penetration have poorer crack resistance, as the contraction stresses in these latter types of welds are essentially perpendicular to the crystal interfaces where impurities accumulate, leading to a higher tendency for thermal cracking. 5) An excessively fast cooling rate increases the strain rate of the weld metal (the material’s plastic deformation cannot keep up), which makes cracks more likely to form. Therefore, slow cooling measures should be adopted, and preheating can help reduce the cooling rate. Furthermore, slow cooling cannot be achieved by increasing the welding heat input, as too high a welding heat input promotes grain growth and increases the tendency for segregation, which has the opposite effect. 6) Reduce the stiffness and restraint of joints; specific measures include reducing the structural thickness in the design, arranging welds properly, and planning the assembly and welding sequence appropriately. 7) For thick plate welding, multi-layer welding can be employed, which reduces the tendency to cracking compared to single-layer welding; however, care must be taken to control the penetration depth of each layer. Furthermore, avoiding stress concentration at welding joints (such as that caused by defects like misalignment, undercutting, and incomplete penetration) is also an effective way to reduce the tendency for cracks.