Causes and solutions for cracks in stainless steel electric heating coils after welding

The reasons for cracks in stainless steel electric heating rings after welding are more complicated, involving material properties, welding process and other aspects. The following are some common reasons and corresponding solutions:
Material aspects
Improper material matching
Reason: The welding material and the parent material of the stainless steel electric heating ring do not match in chemical composition, mechanical properties and other aspects, resulting in insufficient bonding between the weld metal and the parent material, and easy cracking under the action of welding stress.
Solution: According to the specific material of the stainless steel electric heating ring, select welding materials with matching chemical composition and mechanical properties, such as welding rods, welding wires, etc. Before welding, carefully check the model and specifications of the welding material to ensure that it meets the requirements of the welding process.
High impurity content of the material
Reason: The impurity content in the parent material or welding material is too high, such as harmful elements such as sulfur and phosphorus, which will reduce the plasticity and toughness of the material and increase crack sensitivity.
Solution: Purchase stainless steel materials and welding materials of qualified quality, and require suppliers to provide quality certification documents for the materials to ensure that the purity and impurity content of the materials meet the standard requirements. Before welding, carry out necessary inspections and analyses of the materials, such as spectral analysis, to determine whether the composition of the materials is qualified.
Welding process
Welding heat input is too large
Reason: When welding, excessive welding current, too low welding speed or too long arc length are used, which makes the welding heat input too high, resulting in overheating of the metal structure of the weld and heat-affected zone, coarse grains, reduced mechanical properties of the material, and easy cracks during cooling.
Solution: Through welding process assessment, determine the appropriate welding parameters and reasonably control the welding heat input. Under the premise of ensuring good penetration and forming of the weld, try to use a smaller welding current and a faster welding speed, shorten the arc length, and reduce the welding heat input.
Irrational welding sequence
Reason: No reasonable welding sequence is formulated, resulting in the inability to effectively release the stress generated during the welding process, forming a large stress concentration in the weld, thereby causing cracks.
Solution: According to the structural characteristics of the stainless steel electric heating ring, formulate a reasonable welding sequence. For example, use symmetrical welding, segmented back welding and other methods to offset the welding stresses and reduce stress concentration. During the welding process, strictly follow the predetermined welding sequence.
Too fast cooling speed
Reason: After welding, the weld metal cools too fast, which will produce a larger hardened structure and increase the sensitivity of cracks. Especially when the ambient temperature is low or the weldment is thick, the cooling speed is faster and cracks are more likely to occur.
Solution: After welding, take appropriate slow cooling measures, such as covering the weld with asbestos cloth, or slow cooling in an insulation box to reduce the cooling speed of the weld. For thick plate welding, the preheating method can be used to increase the initial temperature of the weldment and reduce the temperature gradient during welding, thereby reducing the cooling speed.
Welding environment
The ambient temperature is too low
Reason: Too low ambient temperature will cause the weldment to dissipate heat too quickly during welding, resulting in a sharp increase in the cooling speed of the weld, resulting in greater welding stress, which is easy to cause cracks.
Solution: Before welding, preheat the weldment and welding materials to a suitable temperature, generally requiring the ambient temperature to be no less than 5℃. If the ambient temperature is too low, heating equipment can be used to preheat the weldment, such as a heating furnace, flame heater, etc. During the welding process, take effective insulation measures to prevent the weldment from losing heat too quickly.
Humidity is too high
Reason: If the ambient humidity is too high, the weld surface and welding materials will absorb moisture. During the welding process, the moisture decomposes to produce hydrogen, which enters the weld metal to form hydrogen pores and cracks.
Solution: Control the relative humidity of the welding environment to no more than 90%. When welding in a humid environment, dehumidification equipment can be used to reduce the ambient humidity. Dry the welding materials to remove moisture. For example, welding rods should be dried at the specified temperature and time before use.
Stress
The weld has a large degree of restraint
Reason: The weld is subject to greater restraint during the welding process and cannot shrink freely, resulting in increased welding stress. When the stress exceeds the strength limit of the material, cracks will occur.
Solution: When designing and assembling weldments, minimize the degree of restraint of the weld so that the weld can shrink freely during the welding process. For example, use a reasonable assembly process to reserve a shrinkage margin; during the welding process, loosen the clamp appropriately to allow the weld to have a certain amount of free expansion and contraction space.
Welding stress is not eliminated
Reason: The weldment is not treated to eliminate stress in time after welding, and the welding stress remains in the weldment. In the subsequent use process, due to the effect of stress, cracks may expand.
Solution: After welding, the stainless steel electric heating ring is treated to eliminate stress, such as using stress relief annealing process. Heat the weldment to a certain temperature, keep it warm for a period of time, and then slowly cool it to eliminate welding stress. For some weldments that cannot be heat treated, mechanical methods such as hammering the weld and vibration aging can also be used to reduce welding stress.