Things to note when welding SSAW steel pipes

Welding and cutting of SSAW steel pipe structures are inevitable in their applications. Due to the inherent characteristics of SSAW steel pipes, the welding and cutting processes exhibit particularities when compared to ordinary carbon steel. These processes are more likely to result in various defects in the welded joints and the heat-affected zone (HAZ). The welding performance of SSAW steel pipes is primarily characterized by the following aspects.

High-Temperature Cracks:
High-temperature cracks refer to cracks associated with welding. These cracks can be divided into several types: solidification cracks, microcracks, HAZ cracks, and reheating cracks.

Low-Temperature Cracks:
Low-temperature cracks occasionally occur in SSAW steel pipes. The main causes include hydrogen diffusion, the degree of constraint in the welded joint, and the hardened structure within. Solutions to this issue mainly involve reducing hydrogen diffusion during the welding process, appropriately performing preheating and post-weld heat treatment, and minimizing the degree of constraint.

Toughness of Welded Joints:
The toughness of welded joints in SSAW steel pipes is typically designed to contain 5% to 10% ferrite to reduce susceptibility to high-temperature cracks. However, the presence of these ferrites can lead to a decrease in low-temperature toughness. When welding SSAW steel pipes, the amount of austenite in the welded joint area is reduced, which affects the toughness. Additionally, as the ferrite content increases, the toughness value significantly decreases. It has been proven that the toughness of welded joints in high-purity ferritic stainless steel decreases markedly due to the mixing of carbon, nitrogen, and oxygen.

Oxide-Type Inclusions:
Increased oxygen content in the welded joints of some steels can generate oxide-type inclusions. These inclusions serve as crack initiation sites or propagate cracks, causing a reduction in toughness. In some cases, when air is mixed with the protective gas, the nitrogen content increases, leading to the formation of lath-like Cr2N on the cleavage plane {100} of the matrix. This results in a harder matrix and a reduction in toughness.

σ Phase Embrittlement:
Austenitic stainless steel, ferritic stainless steel, and duplex steels are susceptible to σ phase embrittlement. This occurs when a small amount of the α phase precipitates in the structure, significantly reducing toughness. The σ phase generally precipitates within the temperature range of 600–900°C, with the most significant precipitation occurring around 750°C. To prevent σ phase embrittlement in austenitic stainless steel, the ferrite content should be minimized.

Embrittlement at 475°C:
At 475°C (range: 370–540°C), Fe-Cr alloys may decompose into an α solid solution with low chromium concentration and an α’ solid solution with high chromium concentration. When the chromium content in the α’ phase exceeds 75%, deformation shifts from slip deformation to twin deformation, leading to embrittlement at 475°C.