What should be paid attention to in the welding process of stainless steel ventilation pipeline?

When welding stainless steel ventilation pipes, it is necessary to pay attention to relevant matters before, during and after welding to ensure welding quality and pipe performance, as follows:

Before welding

Material inspection: Strictly inspect the quality of stainless steel plates and welding materials to ensure that they meet the design requirements and relevant standards. Check whether the thickness of the plate is uniform, whether there are any defects such as scratches and cracks on the surface, and whether the model and specification of the welding material should match the parent material, and there are no problems such as dampness and rust.

Groove machining: according to the wall thickness of the pipeline and the requirements of welding process, groove machining is carried out on the pipeline interface. The angle of groove and the size of blunt edge shall conform to the specifications. Generally, the angle of V-groove is about 60-70, and the blunt edge is 1-2mm, so as to ensure that the weld root can be fully fused during welding.

Pipeline assembly: assemble the pipeline to be welded to ensure the coaxiality and butt clearance of the pipeline. Coaxiality deviation should be controlled within a small range, generally not exceeding 1% of the outside diameter of the pipeline, and the butt clearance is generally 2-4mm depending on the wall thickness.

Surface cleaning: use mechanical or chemical methods to remove oil, rust, scale and other impurities on the surface of pipeline welding parts to prevent defects such as porosity and slag inclusion during welding. The cleaning range is generally 20-30mm on both sides of the groove.

Protective measures: Welding personnel should wear protective articles, such as protective glasses, gloves, work clothes, etc., to prevent the strong light and splash generated during welding from causing harm to human body.

During the welding process

Selection of welding process parameters

Current: Choose the appropriate welding current according to the wall thickness and groove form of stainless steel pipeline. Generally speaking, for thin pipes, the current can be controlled at 80-120 A; For thicker pipes, the current can be appropriately increased to 120-200A A.

Voltage: Voltage and current match each other, usually between 20-30V. Too high voltage will lead to defects such as excessive weld width and undercut, while too low voltage may lead to insufficient weld penetration.

Welding speed: the welding speed should be moderate, too fast may lead to problems such as incomplete weld and incomplete fusion, too slow will overheat the weld and affect the performance of stainless steel. The general welding speed is 15-30cm/min.

Welding technique

Conveying strips: adopt appropriate methods of conveying strips, such as straight line, crescent, zigzag, etc. In the welding process, the bars should be transported evenly, avoiding fast and slow, left and right, and ensuring the forming quality of the weld.

Arc starting and arc closing: When starting the arc, pay attention to arc starting in the groove to avoid arc scratch on the pipeline surface. When the arc is closed, the arc pit should be filled to prevent cracks in the arc pit.

Gas protection: When using gas shielded welding, ensure the purity and flow of shielding gas. Argon gas is generally used as shielding gas, and the purity should be no less than 99.99%. The flow rate is adjusted according to the welding current and welding speed, usually 8-15L/min.

Interlayer temperature control: During multi-layer and multi-pass welding, the interlayer temperature should be well controlled to avoid the coarse weld structure caused by excessive interlayer temperature and affect the performance of welded joints. The interlayer temperature is generally controlled at 100-150℃.

After welding

Welding seam cleaning: After welding is completed, clean up impurities such as slag and spatter on the surface of the welding seam in time, and check the appearance quality of the welding seam for defects such as undercut, porosity, cracks and incomplete welding.

Weld inspection: according to the design requirements and relevant standards, nondestructive inspection, such as X-ray inspection and ultrasonic inspection, is carried out to detect whether there are defects in the weld.

postweld treatment

Heat treatment: For some important stainless steel ventilation pipes, post-weld heat treatment may be needed to eliminate welding residual stress and improve weld microstructure and properties. The heat treatment temperature is generally 850-950℃, and the holding time is determined according to the wall thickness and size of the pipeline.

Surface treatment: surface treatment is carried out on the welded parts, such as pickling and passivation, to improve the corrosion resistance of stainless steel pipes.