How to avoid cracks when welding the insulation layer of a sealed straight stainless steel electric heating coil?

To avoid cracks when welding the insulation layer of the sealed straight stainless steel electric heating ring, measures can be taken from the aspects of welding material selection, pre-welding preparation, welding process control and post-welding treatment, as follows:
Choose suitable welding materials
Ensure that the material of the welding rod or welding wire matches the stainless steel electric heating ring and the insulation layer material. For example, for the common austenitic stainless steel electric heating ring, the corresponding austenitic stainless steel welding rod or welding wire can be selected to ensure that the chemical composition and performance of the weld are similar to those of the parent material, reducing the tendency of cracks caused by composition differences.
Choose welding materials with good toughness and strong crack resistance. These materials can withstand stress changes during welding and reduce the possibility of cracks.
Make good preparations before welding
Weldment cleaning: Thoroughly remove impurities such as oil, rust, and moisture on the surface of the weldment to prevent these substances from generating gas during welding, resulting in defects such as pores and cracks. Mechanical cleaning can be performed using tools such as sandpaper and wire brushes, or chemical cleaning methods can be used.
Preheating treatment: According to the material and thickness of the electric heating ring, the weldment should be properly preheated. Preheating can reduce the cooling rate of the welded joint, reduce welding stress, avoid the formation of hardened structure, and thus reduce crack sensitivity. For example, for thicker stainless steel electric heating rings, the preheating temperature can be controlled between 100-200℃.
Control welding process
Optimize welding parameters: select appropriate welding current, voltage and welding speed. Too large current can easily lead to overheating of the weld, coarse grains, and increase crack tendency; too small current may cause defects such as incomplete penetration, which will also affect the quality of the weld. The welding speed should be moderate. Too fast will cause poor weld formation, and too slow will cause the weld to be heated for too long, resulting in greater welding stress. Generally speaking, for the welding of straight stainless steel electric heating rings, the welding current can be selected according to the diameter of the electrode or wire. For example, for a welding rod with a diameter of 3.2mm, the welding current can be between 100-130A; the voltage is around 22-24V; the welding speed is controlled at 15-30cm/min.
Adopt a reasonable welding sequence: For the welding of the insulation layer of the straight-tube stainless steel electric heating ring, a reasonable welding sequence should be formulated to minimize the accumulation of welding stress. For example, the segmented symmetrical welding method can be used to weld one end of the straight tube first, and then weld the other end, to avoid continuous welding from one end to the other end, so that the welding stress distribution is uneven.
Control the interlayer temperature: When welding multiple layers, control the interlayer temperature to avoid the interlayer temperature being too high or too low. If the interlayer temperature is too high, the weld structure will be overheated, reducing the toughness and crack resistance of the weld; if the interlayer temperature is too low, the welding stress will increase. Generally, the interlayer temperature should be controlled within a range close to the preheating temperature.
Perform post-welding treatment
Slow cooling treatment: After welding, let the weldment cool slowly in the insulation material, or take other slow cooling measures, such as covering with asbestos cloth, to reduce the cooling rate, reduce welding stress, and prevent cracks.
Stress relief treatment: For some electric heating rings with high requirements for welding quality, stress relief treatment can be performed after welding, such as using heat treatment methods to heat the weldment to a certain temperature (such as 550-650℃) and keep it warm for a period of time, and then slowly cool it to eliminate the residual stress generated during the welding process.
Appearance inspection and non-destructive testing: After welding is completed, the weld is visually inspected to check for defects such as cracks, pores, and undercuts. At the same time, non-destructive testing methods such as penetration testing, magnetic particle testing, or radiographic testing can be used to conduct a comprehensive inspection of the weld, timely discover possible internal defects such as cracks, and take appropriate measures to deal with them.