Precautions when welding spiral steel pipe

It is inevitable to weld and cut the spiral steel pipe structure in the application of spiral steel pipe. Due to the characteristics of the spiral steel pipe itself, compared with ordinary carbon steel, the welding and cutting of the spiral steel pipe have their particularity, and it is easier to produce various defects in its welded joints and heat-affected zone (HAZ). The welding performance of the spiral steel pipe is mainly manifested in In the following aspects, the high-temperature crack mentioned here refers to the crack related to welding. High-temperature cracks can be roughly divided into solidification cracks, micro-cracks, HAZ (heat-affected zone) cracks, and reheating cracks.

Low-temperature cracks Sometimes low-temperature cracks occur in spiral steel pipes. Because the main reason for its generation is hydrogen diffusion, the degree of restraint of welded joints and the hardened structure in it, the solution is mainly to reduce the diffusion of hydrogen during welding, properly perform preheating and post-weld heat treatment, and reduce the degree of restraint. The toughness of welded joints To reduce the susceptibility to high-temperature cracks in spiral steel pipes, 5%-10% ferrite is usually left in the composition design. However, the presence of these ferrites leads to a decrease in low-temperature toughness.

When the spiral steel pipe is welded, the amount of austenite in the welded joint area decreases, which affects the toughness. In addition, with the increase of ferrite, the toughness value tends to decrease obviously. It has been proved that the toughness of welded joints of high-purity ferritic stainless steel is significantly reduced because of the mixing of carbon, nitrogen, and oxygen. Oxygen-type inclusions are born after the oxygen content in the welded joints of some steels increases, and these inclusions become the source of cracks or the way of crack propagation to reduce the toughness. In some steels, because air is mixed in the protective gas, the nitrogen content in it increases to produce lath-like Cr2N on the cleavage surface {100} of the matrix, and the matrix hardens and the toughness decreases.

Sigma phase embrittlement: Austenitic stainless steel, ferritic stainless steel, and duplex steel are prone to sigma phase embrittlement. Because a few percent of the α phase is precipitated in the structure, the toughness is reduced. The "phase" is generally precipitated in the range of 600-900°C, especially at around 75°C. As a preventive measure to prevent the "phase", the content of ferrite in austenitic stainless steel should be reduced as much as possible. Embrittlement at 475°C, when kept at 475°C (370-540°C) for a long time, the Fe-Cr alloy is decomposed into α solid solution with low chromium concentration and α' solid solution with high chromium concentration. When the chromium concentration in the α' solid solution is greater than 75%, the deformation changes from slip deformation to twin deformation, thus embrittlement at 475 °C occurs.