Macroscopic Analysis of Cracking
The fracture direction of the casing is horizontal, and the fracture propagates along the circumference with a circumference of about 460 mm. Fluid can flow into the casing < 24415mm from the outer wall of the casing along the middle of the fracture, indicating that part of the fracture has penetrated the thickness of the casing wall.
bushing
The main process of casing production is as follows:
Furnace Refining Continuous Casting Ring Furnace Heating Perforation Rolling Walking Furnace Sizing Sawing Quenching and Tempering Straightening Physicochemical Inspection Inspection Thread Processing Coupling Tightening Hydraulic Pressure Test Measuring Length Weighing, Marking Packaging Storage.


The billet is fed into the annular furnace for about 2.5 hours, then punched at about 1250 C, and then rolled continuously at the sizing temperature of 950 C and the finishing rolling temperature of 900 C. The tube can be used to determine that the chemical composition of the tube is not the cause of crack formation. From the metallographic and energy spectrum analysis results, it can be seen that the crack tip and middle contain iron oxide inclusions. The crack is a non-quenched crack, which should occur before heat treatment, probably in the process of steel pipe piercing and hot rolling. In the process of piercing, because of the non-uniformity of temperature and the large additional stress of non-uniform deformation, the steel billet is prone to crack in the process of piercing; moreover, if the heating temperature is too high, the intergranular bonding force and crack will be reduced; if the heating temperature is too low, because of the decrease of plasticity, the deformation resistance will increase, and the piercing under the occlusion condition will occur. The deterioration will lead to cracks. In addition, in the process of piercing, due to additional deformation, the surface layer will produce longitudinal shear stress and transverse shear stress. Therefore, there are large lateral and longitudinal additional stresses on the outer surface of the steel tube. When the heating of the tube is not uniform, when the overlap of the basic transverse stress and the additional tensile stress exceeds the intergranular bonding force, the cracks will be elongated and extended along the rolling direction in the subsequent rolling and sizing process, forming the final cracks. Therefore, in the future production, special attention should be paid to the monitoring and control of perforation temperature, sizing temperature and terminal temperature, and the fluctuation range of temperature should be controlled within (+20). If necessary, the inspection of pipe end should be strengthened to increase the length of pipe saw.


Cracks occur at the end of casing in the process of piercing and rolling of steel pipe. When the billet is perforated, the heating temperature is not uniform, and cracks are formed and expanded under lateral and longitudinal additional stresses. It is suggested that the process parameters of pipe piercing and hot rolling should be adjusted in time, and the heating temperature of pipe should be controlled to make the heating temperature fluctuate within a reasonable range.