Matters needing attention when welding spiral steel pipe

Welding and cutting of the spiral steel pipe structure are inevitable in the application of the spiral steel pipe. Because of the characteristics of the spiral steel pipe itself, compared with ordinary carbon steel, the welding and cutting of the spiral steel pipe have 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, high temperature cracks The high-temperature cracks mentioned here refer to cracks related to welding. High-temperature cracks can be roughly divided into solidification cracks, micro-cracks, HAZ (heat-affected zone) cracks, and reheat cracks.

Low-temperature cracks sometimes occur in spiral steel pipes. Because the main reason for its occurrence is hydrogen diffusion, the degree of restraint of the welded joint, and the hardened structure therein, the solution is mainly to reduce the diffusion of hydrogen during the welding process, appropriately preheat and post-weld heat treatment, and reduce the degree of restraint. To reduce the high-temperature crack sensitivity in the spiral steel pipe, the toughness of the welded joint is usually designed so that 5%-10% of ferrite remains in it. But 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 has a significant downward trend. It has been proved that the reason why the toughness of the welded joint of high-purity ferritic stainless steel is significantly decreased is due to the mixing of carbon, nitrogen, and oxygen. The increased oxygen content in the welded joints of some of these steels resulted in the formation of oxide-type inclusions, which became sources of cracks or pathways for crack propagation and reduced toughness. For some steels, the increase of nitrogen content in the protective gas results in the formation of lath-like Cr2N on the {100} surface of the matrix cleavage plane, and the matrix becomes hard and the toughness decreases.

σ-phase embrittlement: Austenitic stainless steel, ferritic stainless steel, and dual-phase steel are prone to σ-phase embrittlement. Because of the precipitation of a few percent of the α phase in the structure, the toughness is significantly reduced. The "phase is generally precipitated in the range of 600-900 °C, especially at about 75 °C. It is the most likely to precipitate. As a preventive measure to prevent the" phase, the content of ferrite in austenitic stainless steel should be minimized.

Embrittlement at 475 °C, when kept at 475 °C for a long time (370-540 °C), 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 twinning deformation, resulting in 475 °C embrittlement.