Things to note when welding spiral steel pipes

welding and cutting of spiral steel pipe structures are inevitable in spiral steel pipe applications. due to the characteristics of the spiral steel pipe itself, the welding and cutting of the spiral steel pipe has its particularities compared with ordinary carbon steel. it is more likely to produce various defects in its welded joints and heat-affected zone (haz). the welding performance of the spiral steel pipe is mainly reflected 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, microcracks, haz (heat-affected zone) cracks, and reheating cracks.

low-temperature cracks low-temperature cracks sometimes occur in spiral steel pipes. because the main causes are hydrogen diffusion, the degree of constraint of the welded joint, and the hardened structure therein, the solution is mainly to reduce the diffusion of hydrogen during the welding process, appropriately perform preheating and post-weld heat treatment, and reduce the degree of constraint.

the toughness of welded joints in spiral steel pipes is usually designed to contain 5% to 10% ferrite to reduce the susceptibility to high-temperature cracks. however, the presence of these ferrites leads to a decrease in low-temperature toughness.

when spiral steel pipes are welded, the amount of austenite in the welded joint area is reduced, which affects the toughness. in addition, as the ferrite increases, its toughness value has a significant downward trend. it has been proven that the toughness of welded joints of high-purity ferritic stainless steel decreases significantly because of the mixing of carbon, nitrogen, and oxygen.

the increase in oxygen content in the welded joints of some steels generates oxide-type inclusions. these inclusions become the source of cracks or the path for crack propagation, causing the toughness to decrease. some steels are mixed with air in the protective gas, and the nitrogen content increases, producing lath-like cr2n on the cleavage plane {100} of the matrix. the matrix becomes hard and the toughness decreases.

σ phase embrittlement: austenitic stainless steel, ferritic stainless steel, and duplex steel are prone to σ phase embrittlement. because a few percent of the α phase is precipitated in the structure, the toughness is significantly reduced. the "phase" generally precipitates in the range of 600 to 900°c, especially around 75°c. as a preventive measure to prevent the occurrence of the " phase, the ferrite content in austenitic stainless steel should be reduced as much as possible.

embrittlement at 475°c, and when kept at 475°c (370-540°c) for a long time, the fe-cr alloy decomposes into an α solid solution with a low chromium concentration and an α’ solid solution with a 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, resulting in 475°c embrittlement.