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When welding stainless steel tees with different wall thicknesses, in addition to following the general stainless steel tee welding precautions, you also need to pay special attention to the following points:
Before welding
Bevel design
For the case of large differences in wall thickness, special bevel forms should be used, such as gradient bevel or stepped bevel, in order to ensure that the heat is evenly distributed during welding and the root of the weld can be well fused. For example, when the branch pipe of the tee wall thickness is thinner, while the main pipe wall thickness is thicker, a transition area can be machined at the main pipe bevel, so that the bevel angle from the main pipe side to the branch pipe side gradually becomes smaller, so that the welding arc can better act on the thicker pipe wall, while avoiding overheating of the thinner pipe wall.
Calculate the dimensions of the bevel, including angle, depth and width, etc., to ensure that the quality of the weld seam can be guaranteed during the welding process, but also minimize the amount of welding filler material and improve welding efficiency.
Pre-heating
If the wall thickness difference is large and the material is high-strength stainless steel or stainless steel sensitive to cold cracking, it may be necessary to preheat the weldment. The thicker part of the heat dissipation is slow, the thinner part of the heat dissipation is fast, preheating can reduce the temperature difference during welding, reduce welding stress and prevent cracks and other defects.
Pre-heating temperature should be determined according to the material of stainless steel, wall thickness and welding process evaluation, generally around 100 - 200 ℃. Heating to ensure that the overall temperature of the weldment is uniform, heating furnace, heating tape and other equipment can be used for heating.
Welding process
Welding parameter adjustment
Due to the different wall thicknesses, the absorption and conduction of welding heat is also different. For thick-walled parts, a larger welding current and voltage is required to ensure sufficient depth of fusion; for thin-walled parts, the current and voltage should be appropriately reduced to prevent burn-through. For example, welding a thick wall of 10mm stainless steel tee with a thin wall of 3mm pipe, thick-walled welding current may be set to 150 - 200A, thin-walled adjusted to 80 - 120A.
The welding speed should also be adjusted according to the wall thickness. In the thick-walled welding speed can be appropriately slower, so that the weld metal has enough time to fill and fusion; thin-walled to speed up the welding speed, reduce heat input, to avoid overheating.
Multi-layer multi-pass welding
For thick-walled parts, the use of multi-layer multi-pass welding is necessary. This can control the thickness of each layer of the weld, to avoid excessive thickness of a single weld resulting in internal defects, but also conducive to improving the organizational properties of the weld. The thickness of each layer of weld is generally controlled at about 3 - 5mm.
In welding multi-layer weld, pay attention to the control of temperature between layers, the temperature between layers should not be too high, otherwise it will make the weld metal grain size, reduce the mechanical properties of the weld. Usually the interlayer temperature should be controlled between 150 - 250 ℃.
After each weld, it is necessary to clean up the slag and spatter on the surface of the weld in a timely manner, to check whether there are defects in the weld, if there are defects should be repaired in a timely manner, and then proceed to the next weld welding.
After welding
Stress relief
Different wall thickness of stainless steel tee after welding, due to the difference in wall thickness will produce large welding residual stress, the need for stress relief treatment. For some important structures or occasions with strict requirements for deformation, heat treatment can be used to eliminate stress, such as high-temperature tempering treatment, the temperature is generally in the 600 - 700 ℃ or so, the holding time according to the thickness of the weldment and the material to determine.
For large components can not be heat treatment, can also be used to reduce the welding residual stress, vibration aging and other methods to reduce the welding residual stress, by applying a certain frequency of vibration to the weldment, so that the weldment internal residual stress is released and homogenized.
Weld seam inspection
Strengthen the inspection of the weld, in addition to the conventional appearance inspection and non-destructive testing, for the parts with large differences in wall thickness, we should focus on the transition area between the weld and the base material, where stress concentration and welding defects are prone to occur. Methods such as ultrasonic testing or radiographic testing can be used to conduct a comprehensive inspection of the interior of the weld to ensure that the quality of the weld meets the requirements.
According to the test results, the defects found are analyzed and treated. If the welding defects are caused by wall thickness differences, such as unfused, slag, etc., the welding process parameters should be adjusted in a targeted manner, and rework welding should be carried out until the defects are eliminated.