I. What are welding hot cracks?
Welding hot cracks mostly occur at high temperatures near the solidus line, and are characterized by their distribution along grain boundaries. Sometimes they can also form along "polygonization boundaries" at temperatures below the solidus line. They usually occur within the weld metal, but may also occur in the heat-affected zone (base metal) adjacent to the welding fusion line. According to the mechanism, morphology, and temperature range of crack formation, welding hot cracks can be divided into four types: solidification cracks, liquefaction cracks, polygonization cracks, and ductility loss cracks.
II. What are the differences between hot cracks and cold cracks?
1. Different temperatures and times of formation
Hot cracks generally occur during the crystallization process of the weld. Cold cracks generally occur when the welded part cools to 200-300°C. Some appear immediately after welding, while others may be delayed for several hours to several weeks or even longer, so cold cracks are also called delayed cracks.
2. Different location and direction of formation
Most hot cracks occur in the weld metal; some are longitudinal, some are transverse, and sometimes hot cracks can extend into the base metal. Most cold cracks occur in the base metal or on the fusion line, most are longitudinal cracks, and a few are transverse cracks.
3. Different appearance characteristics
The fracture surface of hot cracks has a distinct oxidized color. The fracture surface of cold cracks is bright and has no oxidized color.
4. Different metallographic structure
Hot cracks are all intergranular cracks. Cold cracks penetrate the inside of the grains, i.e., transgranular cracking, although some are also intergranular cracks.
III. How are hot cracks formed?
First, segregation of low-melting point eutectic compounds: The low-melting point eutectic compounds formed by metallurgical reactions of sulfur (S) and phosphorus (P) impurity elements in steel exhibit serious macroscopic segregation, often forming a liquid film in the center of the weld.
Second, the influence of welding stress: The tensile stress generated by uneven heating and cooling during the welding process promotes the rupture and cracking of the liquid film. Third, some other factors include the different thermophysical properties of different materials, which result in different welding stresses; different welding methods and process parameters lead to different heat inputs; and the weld bead shape coefficient has a significant impact on segregation.
Fourth, how to prevent the formation of hot cracks?
1. Metallurgical Measures
Metallurgical measures mainly involve limiting the chemical composition of the weld. To reduce the tendency of low-melting-point eutectic formation in the weld, the content of sulfur (S) and phosphorus (P) should be limited as much as possible; the carbon content of the weld should be reduced, and the manganese content of the welding wire should be increased. Secondly, the weld microstructure should be altered. It is impossible to completely eliminate harmful impurities or prevent the formation of low-melting point eutectics altogether. Therefore, to prevent cracking under tensile stress, special alloying elements are often added to the weld metal to adjust its chemical composition, forming a dual-phase structure in the weld. This disrupts the crystallization direction of the weld metal, preventing the concentration of low-melting point eutectics and thus reducing the formation of hot cracks.
2. Process Measures
1) Reduce the fusion ratio, which means reducing the dilution. When welding the first layer of a multi-layer weld, the large proportion of base metal fused into the weld increases the carbon, sulfur, and phosphorus content, making it prone to hot cracking.
2) Limit overheating. Overheating of the molten pool easily promotes hot cracking. Reduce the heat input by using a small welding current and a low welding speed. Do not reduce the heat input by increasing the welding speed, while ensuring proper weld formation.
3) Choose a reasonable welding sequence and direction to reduce constraint. Generally, smaller convex welds can reduce crack sensitivity.
4) Use basic electrodes and fluxes. This is because the slag of basic electrodes and fluxes has strong desulfurization capabilities.
In summary, the fundamental way to prevent hot cracks is to reduce the amount of low-melting point eutectics, take appropriate measures to reduce welding tensile stress, reasonably control the weld bead shape coefficient, and reduce the welding current, thereby reducing the occurrence of hot cracks.