Welding and cutting of spiral steel pipe structures are unavoidable in applications. Due to the inherent characteristics of spiral steel pipes, their welding and cutting have unique characteristics compared to ordinary carbon steel, making them more prone to various defects in the weld joints and heat-affected zone (HAZ). The welding performance of spiral steel pipes is mainly reflected in the following aspects: High-temperature cracks, referred to here as welding-related cracks, can be broadly classified into solidification cracks, microcracks, HAZ (heat-affected zone) cracks, and reheat cracks.
Low-temperature cracks sometimes occur in spiral steel pipes. The main causes are hydrogen diffusion, the degree of constraint of the weld joint, and the presence of hardened structures. Therefore, the solutions mainly involve reducing hydrogen diffusion during welding, appropriate preheating and post-weld heat treatment, and reducing the degree of constraint.
Weld joint toughness in spiral steel pipes: To reduce susceptibility to high-temperature cracking, the composition design typically includes 5%–10% residual ferrite. However, the presence of these ferrites leads to a decrease in low-temperature toughness.
During the welding of spiral steel pipes, the reduction in austenite in the weld joint region affects toughness. Furthermore, with the increase of ferrite, the toughness value tends to decrease significantly. It has been proven that the significant decrease in toughness of weld joints in high-purity ferritic stainless steel is due to the incorporation of carbon, nitrogen, and oxygen.
In some steels, increased oxygen content in the weld joint leads to the formation of oxide inclusions. These inclusions become crack initiation sites or pathways for crack propagation, resulting in decreased toughness. In other steels, the introduction of air into the shielding gas increases the nitrogen content, producing lath-like Cr2N on the {100} cleavage plane of the matrix, hardening the matrix and thus decreasing toughness.
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 precipitates in the microstructure, toughness decreases significantly. Ferrite phases typically precipitate within the temperature range of 600–900℃, with the highest precipitate around 75℃. As a preventative measure, the ferrite content in austenitic stainless steel should be minimized.
475℃ embrittlement occurs when Fe-Cr alloys are held at 475℃ for an extended period (370–540℃) to decompose into a low-chromium-concentration α-solid solution and a high-chromium-concentration α’-solid solution. When the chromium concentration in the α’-solid solution exceeds 75%, the deformation changes from slip deformation to twinning deformation, resulting in 475℃ embrittlement.
Post time: Dec-15-2025