Analysis of the influence of non-metallic inclusions on the properties of seamless steel pipes

Non-metallic inclusions in steel primarily refer to oxides, sulfides, silicates, and nitrides. These compounds generally lack metallic properties and are mechanically incorporated into the steel’s microstructure. While their effect on steel’s strength is minimal, they significantly impact fatigue performance, impact toughness, and ductility.

Fatigue Performance: The presence of inclusions can reduce steel’s fatigue performance. This is because the physical and mechanical properties of inclusions, such as elasticity, plasticity, and thermal expansion coefficient, differ significantly from those of the base metal. Under load, inclusions cannot deform with the base metal. The more deformed iron undergoes plastic flow around the less deformed inclusions. Stress distribution at these junctions is uneven, leading to stress concentration and a sharp increase, which in turn causes microcracks. These weak zones create weak points for material failure and accelerate the plastic fracture process.

Impact Toughness: Inclusions can also reduce steel’s impact toughness. When subjected to impact loads, the presence of inclusions can easily lead to stress concentration near the inclusions, inducing microcracks and reducing the steel’s fracture toughness.

Plasticity: Inclusions also negatively impact the plasticity of steel. During plastic deformation, inclusions hinder dislocation motion, increase deformation resistance, and thus reduce the steel’s plasticity.

Cold Working Performance: Inclusions can affect the internal quality of stainless steel and the surface finish of the plate, reducing the product’s corrosion resistance. In China, product scrap rates at stainless steel mills due to inclusions can reach as high as 20% or even higher, significantly reducing production efficiency and economic benefits.

Machinability: Inclusions also negatively impact the machinability of steel. For example, while sulfides can improve steel’s machinability, excessive sulfides can reduce the steel’s plasticity and toughness.