Welding cold crack

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Definition: Welding cold cracks refer to cracks that occur when the welded joint cools to lower temperatures (for steel, this is around the Ms point, That is, the temperature at which martensitic transformation begins). It is a type of delayed crack, some of which appear several hours, days, or even longer after welding. 1.1 Causes of formation: The welding process generates various types of stresses, including thermal stress, structural stress, and constraint stress. Thermal stress is caused by uneven local heating and cooling during welding ； Organizational stress is generated by volume changes resulting from structural transformations in the weld and heat-affected zone (such as the transformation of austenite to martensite) ； Confinement stress is the stress generated in a welded joint due to external constraints (such as clamping by fixtures or rigid constraints imposed by the structure itself). The accumulation of these stresses results in the welded joint being under a high stress condition during cooling. Cold cracks occur when the stress exceeds the strength limit of the welded joint at low temperatures. For example, when welding thick-walled pressure vessels, due to the high degree of restraint inherent in the welded parts, the welding stresses are also high, which increases the likelihood of the occurrence of cold cracks. Hydrogen is one of the key factors causing welding cold cracks. During the welding process, moisture in the welding materials, as well as oil and rust on the surface of the workpieces, all decompose to release hydrogen atoms under the high temperature of the arc. The radius of a hydrogen atom is very small, so it can easily diffuse through a metal lattice. During the cooling of the weld, hydrogen atoms diffuse and accumulate at stress concentration areas such as the fusion line. When the hydrogen concentration reaches a certain level, they combine to form hydrogen molecules. The volume of a hydrogen molecule is much larger than that of a hydrogen atom, which generates high pressure that causes localized brittleness in the metal, reduces its toughness, and thus leads to the formation of cracks. For example, in the welding of some low-alloy high-strength steels, hydrogen-induced cracks can easily occur if the weldments are not properly preheated and dehydrogenated. When the cooling rate of the weld and heat-affected zone is too fast, hardened structures such as martensite are likely to form. Martensite has high hardness and poor plasticity. For example, in the welding of high-strength steel, if the heat input during welding is too low, the cooling rate will be fast, resulting in the formation of a large amount of martensite in the heat-affected zone. Under the influence of welding stress, these martensitic structures are prone to cracking. Since the specific volume of martensite is greater than that of austenite, volume expansion occurs during the martensitic transformation, which generates significant mechanical stress in the surrounding tissue. When this stress combines with thermal stress, it leads to the formation of cracks. Formation of hardened microstructure, role of hydrogen, and preventive measures against welding stresses: Minimize external constraints on the welded parts as much as possible; for example, when designing welded structures, use appropriate joint types and structural shapes to avoid excessive constraint stresses. During welding, if conditions permit, the fixture can be loosened slightly to reduce constraint stress. Reasonably control process parameters such as welding current, voltage, and welding speed. For example, appropriately increasing the welding heat input can reduce the cooling rate of the weld and minimize the formation of hardened microstructures. However, the heat input cannot be too high, otherwise it will cause problems such as overheating of the weld metal and coarse grains. At the same time, the use of multi-layer and multi-pass welding allows for a more rational heat cycle in the weld and heat-affected zone, thereby reducing welding stress. Preheating involves appropriately heating the welded parts before welding. For materials prone to cold cracking, such as certain high-strength steels, preheating can reduce the cooling rate of the weld and heat-affected zone, thereby minimizing the formation of hardened structures. The preheating temperature is generally determined based on the material, thickness, and degree of restraint of the welded parts. Post-heating involves heating the welded parts immediately after welding is completed; its main purpose is to allow hydrogen present in the weld to diffuse and escape properly. The temperature for post-heating is usually around 200–350°C, while the duration depends on factors such as the thickness of the welded parts. Low-hydrogen welding materials should be selected; for example, when welding low-alloy high-strength steel, low-hydrogen electrodes or wires for argon-rich gas shielded welding should be used. These welding materials have a low water content, which reduces the sources of hydrogen during the welding process. Select appropriate welding materials, use preheating and post-heating, control welding process parameters, and reduce the constraint on the welded parts. Areas where this often occurs: near the weld line – the weld line is the boundary area between the weld metal and the base metal. At this location, cold cracks are likely to form due to the significant unevenness in chemical composition and mechanical properties. From a chemical composition perspective, the uneven distribution of alloying elements at the weld seam may lead to an increased tendency for localized hardening. For example, when welding low-alloy high-strength steel, the base metal portion at the weld seam may form a martensitic structure after cooling due to the thermal effects. At the same time, hydrogen atoms also tend to accumulate at the fusion line. Since the weld seam is an area with a large temperature gradient during welding, hydrogen atoms diffuse toward this region driven by the concentration difference. When the hydrogen content reaches a certain level, combined with the structural and thermal stresses at the weld seam, cold cracks are likely to form. The coarse-grained area in the heat-affected zone is a region prone to cold cracks. During welding, the heat acting on this area causes the grains to grow, resulting in coarsening of the microstructure. Taking steel welding as an example, the coarse-grained region in the heat-affected zone tends to develop a coarse martensitic structure after rapid cooling. This type of tissue has high hardness and poor plasticity; moreover, due to its coarse grains, the grain boundary area is relatively reduced, which weakens the inhibitory effect of grain boundaries on crack propagation. Furthermore, the heat-affected zone is also subject to hydrogen intrusion during welding; hydrogen atoms accumulate at the lattice defects in the coarse-grained region, and together with structural stresses and thermal stresses, they contribute to the formation of cold cracks in the heat-affected zone. Weld root: The weld root is a critical part of the welded joint; it is also prone to cold cracks. During the welding process, the welding quality at the root is affected by various factors. For example, if the penetration at the root is poor, defects such as lack of penetration may occur, and these defects can lead to stress concentration. At the same time, the cooling rate of the root may increase due to factors such as heat dissipation conditions, leading to the formation of a hardened structure. Moreover, hydrogen atoms also tend to accumulate at the root area, as the protective gas or slag used during welding may provide relatively poor protection there; external moisture and other hydrogen-containing substances can easily penetrate, thereby increasing the likelihood of cold cracks forming at the root of the weld.