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, 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 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 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 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 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, 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, resulting in 475°C embrittlement.
Post time: Sep-01-2023