When cracks appear in welding parts, analyzing the causes from a process perspective requires considering multiple dimensions, including material properties, welding parameters, operating procedures, and environmental conditions. The essence of welding cracks is the formation of new interfacial gaps due to the breakdown of the atomic bonding forces of metals under the combined effects of welding stress and material embrittlement factors. The main types of cracks include hot cracks, cold cracks, reheat cracks, and lamellar tears. The causes of different crack types are closely related to process control.
Hot cracks often occur in the weld metal or the base metal near the fusion line, and their formation is directly related to the control of welding parameters. If the welding current is too high, the voltage is unstable, or the welding speed is too fast, the weld metal will experience hindered shrinkage when solidifying near the solidus line. The residual liquid metal cannot fill the gaps in time, and cracks will open along the austenite grain boundaries under tensile stress. For example, if the content of impurities such as sulfur and phosphorus in low-alloy steel welds is high, low-melting-point eutectics will form, which liquefy along the grain boundaries at high temperatures. Upon cooling, shrinkage stress triggers liquefaction cracks. Furthermore, in multi-layer welding, incomplete interlayer cleaning can lead to slag inclusions or oxides becoming crack initiation sites, exacerbating the tendency for hot cracking.
Cold crack formation is closely related to welding residual stress, hydrogen content, and the material's hardening tendency. When welding parameters are inappropriate (e.g., insufficient heat input or excessively rapid cooling), brittle and hard structures such as martensite easily form in the heat-affected zone, reducing the material's plasticity. Simultaneously, if the welding rod is not dried, the bevel is damp, or the ambient humidity is too high, the weld metal will absorb excessive hydrogen, which accumulates at grain boundaries or defects under stress, forming hydrogen-induced cracks. For example, when welding medium- and high-carbon steel or low-alloy high-strength steel, if preheating or slow cooling after welding is not performed, the combined effect of residual stress and hydrogen diffusion can easily induce delayed cracks in the weld toe, weld root, or heat-affected zone.
Welding operation specifications are equally crucial for crack control. Improper electrode angle can lead to insufficient penetration, resulting in a weak bond between the weld and the base metal. Incomplete interlayer cleaning can leave slag inclusions, creating stress concentration points. Failure to fill the crater during weld completion can easily lead to crater cracks. For example, in manual arc welding, excessive electrode oscillation can cause uneven molten pool temperature, leading to shrinkage stress in localized areas due to rapid cooling, which can then cause cracks. Furthermore, in thick plate welding, failure to use symmetrical welding or segmented back-welding sequences can result in unilateral stress accumulation, increasing the risk of deformation cracks.
Inadequate weld structure design can exacerbate stress concentration, becoming a cause of cracking. If a part has sharp corners, notches, or abrupt changes in cross-section, stress will concentrate at these points during welding, exceeding the material's plastic deformation capacity and causing cracks. For example, if T-joints or cross-joints lack a transition fillet, the weld root is prone to cracking due to stress concentration. In addition, improper welding sequence can lead to stress superposition; for example, welding welds with large shrinkage before welds with small shrinkage will subject the already welded welds to additional tensile stress, increasing the risk of cracking.
The influence of environmental conditions on welding cracks cannot be ignored. Low temperatures accelerate weld cooling, increasing thermal stress; high humidity causes welding rods or workpieces to absorb moisture, leading to increased hydrogen content in the weld; and excessive wind speeds without protection accelerate weld cooling, resulting in hardened structures. For example, in winter, outdoor welding without preheating can cause rapid cooling and significant residual stress, easily leading to cold cracking.
From a process perspective, preventing welding cracks requires optimizing parameters, standardizing operations, improving design, and controlling the environment for different crack types. By rationally selecting welding materials, adjusting current and voltage, controlling welding speed, optimizing bevel design, adopting symmetrical welding sequences, rigorous interpass cleaning, and preheating before welding and slow cooling after welding, the risk of cracking can be effectively reduced, improving the quality and reliability of welding parts.
