Analysis of Cracks in Heat Affected Zone of X52 High Frequency Longitudinal Welded Steel Pipe

erw steel pipeThe composition of rare-earth sulfide and rare earth oxide layer in the steel were determined by microanalysis of inclusions. Inclusion is the main reason for the formation of cracks, band-shaped inclusions in the high temperature and the role of welding pressure within the separation cracks.

In recent years, the rapid development of China's oil industry, especially in the western development / West-East Gas Pipeline project implementation, extensive use of low-alloy high-strength steel, welding production of large diameter oil or gas pipeline, in order to ensure a high pipeline Shear breaking capacity, requiring high resistance to transverse tear steel plate capacity. How to improve the purity of steel and the deformation of sulfide is not deformed, as the steel production enterprises of the core technology issues. X52 pipeline steel is currently China's oil and gas transmission commonly used steel welding steel, generally take calcium to reduce the sulfur content of steel, phosphorus and other impurities, reduce the total amount of inclusions in steel; or add rare earth in steel to achieve Spheroidized sulfides to prevent the formation of long strip MnS inclusions, the two production processes have achieved good results.

X52 test coil edge along the rolling direction of double-sided magnetic particle inspection, the results did not find an alarm phenomenon. The results show that there are no cracks and other defects before the high frequency welding. The cracks in the heat affected zone of the welded pipe occur during the welding process, and the possibility of cracking from the X52 test coil is eliminated. The microstructure and crack morphology of X52 test steel pipe were sampled horizontally after sampling. The X52 erw steel pipe and coil were sampled longitudinally and inclusions were graded.

The welding crack is located in the weld heat affected zone of the inner wall of X52 pipe, about 15 ~ 210mm away from the weld, the crack trend is consistent with the metal flow line of the welded joint, showing a slight bending state, there are inclusions in the crack, indicating the crack generation and welding squeeze Pressure and inclusions. Samples were taken from the visible part of the crack and pressed along the crack to produce an electron microscope fracture specimen. Crack fracture is cracked from the crack side of the force to expand the fracture and the fracture can be seen on the old and has oxidized black crack source area, and sample preparation of tearing fibrous expansion zone two obvious areas. The depth of the crack source area is generally about 110 ~ 210mm, the length of some reach 100mm, some only 310 ~ 510mm, different lengths.