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Flattening Tests of Welded Steel Pipes

Posted: 10/13/2022 11:07:11 Hits: 18

The analysis and conclusion of inspection of flattening tests of welded steel pipes
2. Analysis and discussion
The material and mechanical properties of the samples meet the requirements of the standard. The fracture of the sample is a wood-grain layered fracture, and there are many longitudinally arranged cracks on the fracture surface. After analysis, there are many longitudinally distributed strip-shaped sulfide inclusions, and the sulfide inclusions are very serious. Sulfide inclusions are endogenous inclusions, which are formed by the chemical reaction of Fe and other elements in the steel with S in the liquid state and solidification process. When the liquid steel solidifies, the sulfide cannot float and is embedded in the steel, mainly FeS, MnS and their solid solution. Sulfide inclusions have good plasticity and belong to plastic inclusions. During rolling, their deformation index is close to 1. After rolling, they deform together with the metal matrix along the rolling direction, and extend into strips, spindles or line segments. It is an important reason for the anisotropy of properties of steel.

After the steel pipe is welded, expansion caused by heat and contraction caused by cold occur near the welding seam and the heat-affected zone. Because the thermal expansion coefficient of sulfide is different from that of the matrix, under the action of expansion caused by heat and contraction caused by cold, micro-cracks are generated between the long-striped sulfide inclusions and the matrix, and each slender sulfide inclusion is a micro-crack source. Under the action of a small load, stress concentration happens due to the inclusion as a crack source. When the stress increases to a certain value, cracks are initiated at the crack source, which seriously reduces the lateral performance of the steel pipe. When the steel pipe is subjected to the flattening test, the steel pipe is mainly affected by the lateral stress and deforms laterally, and the micro-cracks generated by the sulfide inclusions gradually grow up until they become small macroscopic cracks along the longitudinal direction of the steel pipe. The steel pipe cracks with the increase in deformation.

3. Conclusion and improvement measures
After the steel pipe is welded, microcracks are formed at the interface between the elongated sulfide inclusions and the matrix under the action of expansion caused by heat and contraction caused by cold due to the existence of many sulfide inclusions distributed in longitudinal strips in the steel, which destroys the continuity of the steel pipe’s matrix. Under the action of external force, stress concentration happens, and cracks are initiated. With the increase in deformation, the steel pipe finally cracks. These longitudinally distributed elongated sulfide inclusions are the main reason for the failure of the flattening test of the steel pipe.

Controlling sulfide inclusions in steel is an effective measure to improve the flattening process of steel tubes. The control of sulfide inclusions mainly refers to controlling the sulfur content in the steel and ensuring that the sulfides are spherical after rolling. There are many methods for refining and desulfurization outside the molten steel furnace, and the most used method is the synthetic slag method with argon stirring. The most important way to change the morphology of sulfide inclusions is to add certain elements into the ladle, such as Ca and Re in the tapping process, so that the new sulfides formed have a higher melting point than the original MnSm, more stable thermodynamic properties and no deformation during processing, which is an effective method to improve the mechanical properties of steel and reduce anisotropy.

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