Machining Of Pipe End Of LSAW Steel Pipe

The process of threading, sizing, thickening, strengthening heat treatment and chamfering on the pipe end of LSAW steel pipe.

1. Steel pipe end thread
Referred to as the pipe end wire. The straight seam steel pipe produced by the steel pipe factory usually has a single length of less than 12 m, and the length of the steel pipe in practical applications often exceeds this length. For example, in geological drilling and oil drilling, the drilling depth is usually above 1000m and the deepest is 7000m; the conveying pipeline for transporting oil, natural gas, water, alkali, minerals, etc. is several kilometers or even hundreds of kilometers. To do this, it is necessary to thread the ends of the steel pipe and connect the steel pipes with the corresponding couplings. Threaded connections require high joint strength and good air tightness to prevent breakage of the steel pipe and leakage of the transport medium. In order to meet the needs of various purposes, various countries have developed various forms of thread, but mainly use the following three types:
(1) Dome triangle thread. It is characterized by simple threading. However, due to the uneven force and deformation of the thread, the joint strength is low, and only the side of the thread is sealed, and the sealing performance is poor.
(2) Partial trapezoidal (Boutres) threads. The utility model has the advantages of high thread precision, high processing requirement and high joint strength; and the sealing performance is relatively high by relying on the side of the thread and the top pressing seal.
(3) Direct connection type thread. Its characteristic is that it does not need to take over, but the pipe end needs to be thickened, the external thread of one end of the car and the internal thread of the other end are difficult to process. High thread precision and high joint strength; relying on the top surface and one side seal for good sealing performance.
   
The machining usually takes place at the end of the SSAW steel pipe in front of the wire, including: pipe cutting, outer circle, boring, chamfering and end face machining. These processes are generally completed on one dedicated wire machine, and some can be done on two machines. In order to ensure that the thread size meets the standard requirements, the oil drill uses the pipe end thread of the pipe. It needs to be strictly tested by various instruments at the factory. The test items include screw moment, tooth height, tooth shape, thread length, taper, tight moment and so on.

2. Tube end sizing
The purpose is to reduce the ellipticity of the steel pipe and ensure the dimensional accuracy of the steel pipe after machining, mainly for oil casing. After the end of the sizing of the sleeve, the number of black buckles at the end of the wire is small, which can increase the yield. The pipe end sizing adopts the cold deformation process, and the commonly used sizing methods include the punch expansion diameter and the punch expansion diameter + sizing ring compression. The effect of reducing the ellipticity of the steel pipe when the punch is expanded is largely determined by the uniformity of the wall thickness of the steel pipe. For a hot-rolled pipe with a severe wall thickness unevenness, such as a steel pipe rolled by a periodic pipe mill, the surface quality of the pipe end deteriorates after the diameter of the punch is expanded.

Therefore, in order to eliminate the influence of uneven wall thickness and reduce the ellipticity of the steel pipe, the punch diameter expansion + sizing ring compression method should be used. Due to the cold sizing, the pipe end not only has elastic deformation but also a small amount of plastic deformation. After sizing, there may be different degrees of residual stress in the transition zone between the expanded diameter and the non-expanded diameter. At the same time, internal and external surface scratches caused by the punch and the sizing ring may occur, resulting in slight stress corrosion. Therefore, the dimensional accuracy of the hot-rolled steel pipe should be increased as much as possible, and the method of sizing the pipe end should be avoided.

3. Tube end thickening
Mainly used in oil pipes and geological pipes. Due to the thinning of the pipe wall behind the pipe end, the threaded connection becomes a weak link in the strength of the entire pipe. In order to increase the strength of the threaded joint portion, the pipe end thickening method is used to increase the pipe end wall thickness. In addition, the drill pipe uses the pipe end and the tool butt weld connection, the weld and its heat affected zone are the weak points of strength. In order to increase the strength of the weld zone, the pipe end is also generally thickened. The thickened form of the pipe end has inner thickening, outer thickening and inner and outer thickening.
   
The process of thickening the steel pipe is: pipe end heating (in the slot furnace or induction heater) → thickening (with horizontal forging machine or hydraulic forging machine) → cooling → inspection. Depending on the size of the thickened tube, the thickening form and the amount of thickening compression, the number of heatings used and the number of thickening passes are also different, one heating once thickened, or one heating twice thickening. In order to eliminate the uneven performance of the thickened end and the pipe body caused by the thickening, the steel pipe is usually subjected to an overall heat treatment after being thickened.

4. Tube end strengthening heat treatment
The strength of the end portion of the tube end is higher than the strength of the tube by heat treatment. The tube end strengthening heat treatment process includes tube end heating, quenching and tempering.

5. Pipe end chamfering
Pipes used to transport minerals, gases, oils, water, etc., are usually welded together by steel pipes. In order to facilitate the welding between the tubes, the outer corners of the tube ends are chamfered. According to the use of steel pipes, the angle of α angle and the value of blunt edge are also different, generally α=30. . In order to ensure the accurate size of α and ζ after chamfering of the pipe end, it is required that the steel pipe is not rounded and the wall thickness unevenness is as small as possible. When the steel pipe factory is processing at the pipe end, the pipe end chamfering and the steel pipe cutting are performed on the same machine tool. The internationally advanced pipe cutting machines have the function of chamfering. The processing of the oil pipe coupling is completed on a special machine tool, and the other processes are similar to the pipe end processing except for the cut length of the coupling material.