The Influence of Inclusions on the Fatigue Strength of Seamless Steel Pipe Materials

Factors affecting the fatigue strength of seamless steel pipe materials: inclusions.

The inclusion itself or the hole created by it is equivalent to a tiny gap, which will produce stress concentration and strain concentration under the action of alternating load, which will become the source of fatigue fracture and have a negative impact on the fatigue performance of the material. The influence of inclusions on fatigue strength not only depends on the type, nature, shape, size, number and distribution of inclusions, but also depends on the strength level of the material, the level and state of applied stress and other factors.

Different types of inclusions have different mechanical and physical properties, and the difference between the properties of the base metal and the impact on the fatigue performance are also different. Generally speaking, easily deformable plastic inclusions (such as sulfides) have little effect on the fatigue performance of steel, while brittle inclusions (such as oxides, silicates, etc.) have greater harm.

Inclusions with a larger expansion coefficient than the matrix (such as sulfides) have little effect due to compressive stress in the matrix, while inclusions with a smaller expansion coefficient (such as alumina) in the matrix have a greater influence due to tensile stress in the matrix.

The tightness of the combination of inclusions and base metal will also affect the fatigue strength. Sulfides are easy to deform and are tightly bonded to the base material, while oxides are easy to separate from the base material, causing stress concentration. It can be seen that from the type of inclusions, sulfides have less influence, while oxides, nitrides and silicates are more harmful.

Tips: ASTM A53 covers welded and seamless steel pipe with nominal wall thickness. The surface condition is usually black and hot-dipped galvanized. ASTM A53 is produced mainly for pressure and mechanical applications, and is also used for transport of steam, water, gas line pipes.