Straightening Principle of Straight Seam Steel Pipe

The straightening of steel pipes is an important process in the production of straight seam steel pipes. Especially for the high quality API standard petroleum casing and oil and gas pipes, special pipes for machinery and equipment, these steel pipes not only have strict requirements on the steel grade and weld quality, but also have high straightness of the steel pipe. Requirement, because the deviation of straightness is directly related to the processing and connection of the pipe end threads and pipe collars of the oil casing and conveying pipe, as well as the distortion of the pipeline during use. There are two processing forms of pipe end wire turning-pipe rotation and tool rotation. Most of the wire processing uses pipe rotation, which requires higher straightness of the steel pipe.

When it comes to steel pipe straightening, we must first figure out why the steel pipe bends during the production process? Many people will think: Is this problem still necessary? In fact, it is really not easy to find out why the steel pipe bends. There are many reasons for the bending of the steel pipe, such as the thermal effect of the weld during welding, the eccentricity during forming, and the imbalance of pressing force and bending force, etc. But fundamentally speaking, bending is the effect of the internal stress of the steel pipe. Simply put, bending is the imbalance of stress. So, does the straight steel pipe have no internal stress? It's not. Straight steel pipes also have internal stress, but the internal stress of straight pipes is smaller.

What is internal stress?

Internal stress is a term of physical mechanics. Its essence is a kind of interaction force between molecules when the material is deformed by the influence of temperature and external force. When steel pipes are formed and welded, they are also affected by external forces such as welding temperature and forming and bending, resulting in internal stress. The cross section of the steel pipe is a ring, and two basic stresses are generated on this ring area: the force parallel to the ring and the force perpendicular to the ring. Parallel stress will make the tube out of round; vertical stress will make the tube bend. Therefore, there is a cold expansion process in the production process of the straight seam steel pipe, the purpose is to eliminate the internal stress of the steel pipe and increase the use strength of the straight seam steel pipe.

Let's take a look at the working principle of the hydraulic six-roll straightening machine:

The pipe that needs to be straightened is fed from the feeding device at the left end (or right end) of the machine to the lower roller of the straightening machine, and the upper roller moves downward to make it press the pipe until it reaches the corresponding position and stops. The upper and lower rollers are respectively inclined at a certain angle with the axis of the straightened pipe, and the hyperbolic generatrix of the roller matches the outer diameter of the pipe and is in an envelope shape. The three upper rollers are pressed on the pipe under the action of their respective hydraulic cylinders, and the two lower rollers are respectively driven to rotate by their respective hydraulic motors to drive the pipe to rotate around the axis and move in the axial direction. Changing the direction of rotation of the hydraulic motor can change the direction of rotation and axial movement of the pipe to achieve reversible straightening. Then the straightened pipe is sent to the material rack through the discharge device at the other end. For pipes with large local bending deformations, the punches at both ends can be used for local straightening first, and then overall straightening.

Anyone who has studied elementary mathematics knows that three points form a line. However, if we straighten the pipe with three points in a line, the pipe will not be straightened, and it will bounce back if it is straightened by you. This is due to the internal stress of the steel pipe. When straightening steel pipes, we can only straighten steel pipes by overcorrecting methods, that is, only by applying a force greater than the stress but in the opposite direction to the bent pipe, can the original stress be completely eliminated and the pipe straightened. The straightening machine makes the pipe straighten the pipe through multiple elastic-plastic bending deformations. There are two basic ways of straightening steel pipes: linear straightening and rotary straightening. These two methods essentially straighten the pipe through the principle of eliminating internal stress. However, there are differences between the use and the straightening. There is a big difference in the way. In the production of ERW steel pipes, these two methods are generally configured in the production equipment, linear straightening uses Turkish head and linear straightening machine, and rotary straightening uses roller straightening machine. Each of these two straightening methods has its own uses, but their role objects and working principles are very different.

Straight seam steel pipe is a steel pipe whose weld seam is parallel to the longitudinal direction of the steel pipe. Straight seam steel pipes can be divided into high-frequency straight seam steel pipes (ERW Pipe) and submerged arc welded straight seam steel pipes (LSAW) according to the production process. Straight seam welded pipe has simple production process, high production efficiency, low cost and rapid development.

ASTM A53 ERW steel pipe is a typical carbon steel pipe. It is largely used to convey fluids at low / medium pressures such as oil, gas, steam, water, air and also for mechanical applications.

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