JPS6331287B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for straightening the end of a pipe using pressure generated by compressing an elastic body such as rubber in the axial direction. The present invention relates to a method suitable for accurately rounding a pipe with the amount of expansion.

A conventional method for straightening the end of a tube into a perfect circle is shown in FIGS. 1a and 1b. This method involves inserting several tube expansion pieces 1 and a tapered sleeve 2 into a tube 3, as shown in figure a, and then moving the sleeve 2 in the axial direction to press the pieces 1 against the inner surface of the tube, as shown in figure b. As shown, the tube is expanded and made into a perfect circle. Note that 4 is a piece holding jig. The disadvantage of this method is that the amount of pipe expansion is 3 to 10 times the pipe diameter.
%, and there is also the problem of lower reliability due to high residual stress after rounding and machining scratches remaining on the inner surface of the tube.

As a rounding method that compensates for the above-mentioned drawbacks, there is a method of expanding and rounding a soft elastic body such as rubber by utilizing the deformation force in the circumferential direction of the elastic body that is generated when a load is applied to the soft elastic body in the axial direction. The details of this method are shown in Figure 2 a~
Shown in b. The tube 5 to be straightened is often deformed into a substantially elliptical shape. Here, the initial shape of the tube is measured, and the most deformed portion, that is, the longer diameter portion of the ellipse, is restrained by an external load P to make the tube 5 into a temporary perfect circular shape. Thereafter, a soft elastic body such as rubber is inserted into the tube, a load F is applied to this elastic body, and internal pressure is applied to the tube by utilizing the expansion force of the elastic body in the circumferential direction. This internal pressure is applied up to 1.1 times the yield pressure P _y of the pipe 5, and at this point the external restraint load P is removed.
After that, the internal pressure is further increased and the yield pressure of the tube 5 is increased.
After applying up to 1.3 to 1.4 times P _y , the internal pressure is removed to complete the correction. Here, the reason why the long diameter part of the ellipse is restrained by the external load P and the internal pressure P is applied is as follows. In other words, when applying internal pressure to an elliptical shape,
Uniform correction cannot be achieved because a bending moment proportional to the internal pressure occurs at the major and minor diameters of the ellipse. On the other hand, if internal pressure is applied to a state in which the pipe is temporarily rounded by an external load P, a bending moment proportional to the external load P will similarly occur, but when the internal pressure increases and the pipe becomes almost in a yield state, the membrane stress due to the internal pressure becomes dominant. become.
Therefore, even if the internal pressure is removed, the residual stress distribution is uniform, and a perfect circle is maintained. Also, the reason why the external load P is removed at 1.1 times the yield internal pressure P _y of the pipe is that below 1.1P _y , when the internal pressure is removed, it returns to an elliptical shape, and above 1.1P _y , it becomes a gourd shape due to restraint. .
Furthermore, after removing the external restraint, the pressure is increased by 1.3 to 1.4 times in order to suppress the increase in the tube diameter after correction to within 1%.

On the other hand, in this roundness straightening method, the yield internal pressure P _y
is important. That is, the time to remove the external load and the final internal pressure are determined by the internal pressure P _y of the pipe.
Here, when the internal pressure P is applied to the thin cylinder, the circumferential stress σ generated in the tube is given by the following equation.

σ=Pr/t...(1) σ; Circumferential stress P; Internal pressure r; Inner diameter t of the tube; Thickness of the tube The yield internal pressure P _y of the tube required for the rounding method is:
In equation (1), σ is the yield strength σ _y , which is one of the material strengths of the pipe.
It can be found by substituting . For example, yield strength 38
When a plate-wound welded tube with a diameter of 406.4 mm and a thickness of 9.5 mm, weighing Kg/mm ² , was processed using the rounding method shown in Figure 2, the results were as follows.
Good roundness correction was achieved with a final pressure of 1.38P _y and an amount of tube expansion within 1.0mm (0.24%). In addition, since this method involves a small amount of tube expansion using a soft elastic material such as rubber, residual stress is small, and the inner surface of the tube can be processed without any damage, so reliability can be maintained sufficiently. However, there is a drawback that the yield strength (σ _y ) of the pipe to be straightened must be determined each time in order to set the yield internal pressure (P _y ). That is, since the material strength of the pipe to be straightened varies depending on its composition and specifications, it is necessary to obtain a tensile test piece from the pipe and find σ _y . In particular, plate-wound welded pipes have problems that differ from the strength of the material because they are bent and welded. Therefore, there was a need for a method to determine the pressure equivalent to P _y without cutting the pipe.

An object of the present invention is to provide a method that can straighten the roundness of the end of a pipe without determining the material strength of the pipe to be straightened.

The method of straightening the end of a pipe into a perfect circle according to the present invention is a method of expanding the pipe using the deformation force in the circumferential direction of the elastic body that occurs when an axial load is applied to the elastic body, and straightening the end of the pipe into a perfect circle. In this process, a displacement meter is installed in the circumferential direction of the end of the pipe to be straightened, and the initial shape of the elliptical long-diameter part is restrained by an external load and the pipe is expanded after being temporarily rounded. Determine the yield load of the pipe from the displacement of the pipe and
After unloading the external restraint load at the time of 1.1 times,
It is characterized by expanding the pipe to 1.3 to 1.4 times the yield load and then unloading it.

In the present invention, when a load is applied to the elastic body and the deformation force of the elastic body at that time is used to straighten the tube, the load F to be applied to the elastic body is given by the following equation.

F=p×s/η …(2) s; Cross-sectional area of the elastic body η; Efficiency of transmitting the pressure of the elastic body p; Generated internal pressure Therefore, the load F is calculated by first finding the pressure p necessary for straightening the roundness. Must be. The internal pressure p varies depending on the shape of the pipe, and its value can be found from equation (1). In other words, even if the shape is the same, if the yield stress of the material differs, the required internal pressure will change significantly. For this reason, it is necessary to obtain an actual tensile test piece from the pipe and confirm the yield stress value.

Generally, when a metal material is subjected to a tensile test, the stress-strain diagram shown in FIG. 3 is drawn. Here, in the elastic region, σ=E・ε...(3) E: longitudinal elastic modulus ε: strain holds, and the distance between AB in Figure 3 is theoretically (3)
It shows linearity according to Eq. further increasing the stress,
Equation (3) no longer holds true. In other words, the critical stress at which the strain does not return to point A when the stress is unloaded is called the yield stress, but this yield stress varies depending on the history of the material, and for example, as shown in Figure 3, various yield stress values σ _y2 , σ Indicates _y3 .

The principle of the present invention is to compare the strain that actually occurs on the pipe surface when internal pressure is applied to the pipe and the strain obtained from the theoretical calculation shown in equation (3), and determine the yield strength from the difference between the two. . The load F for applying internal pressure is given by equation (2), and can be determined from equations (1) and (3), which are the relationships between internal pressure P and strain ε. Therefore, the relationship between load and strain is as follows.

F=α・ε …(4) α; s/η・t/r・E Constant determined by the shape of the tube and the shape of the rubber This relationship is essentially the straight line AB part of the stress-strain diagram in Figure 3. will be the same. Therefore, the strain that actually occurs on the tube surface is in the elastic region.
When formula (4) is satisfied and the tube begins to yield, it becomes larger than the theoretical value. In other words, the yield load of the tube is the point at which the actually occurring strain and load lose their direct proportionality to each other. In practice, this is managed by methods such as drawing the relationship between generated strain and load. A specific method will be explained using FIG. 4.

The basic structure of this method consists of a tube expansion section for correcting the roundness and a section for controlling the displacement of the tube during tube expansion and the tube expansion load. The rounding process is the same as that shown in FIG. 2, in which a cylinder 7 equipped with an elastic body 6 such as rubber is inserted into the temporarily rounded tube. Here, a calculation section 9 receives signals from sensors 8 disposed at four locations in the circumferential direction, and a recording section 1 records the initial shape of the tube.
0 remembers. Next, the piston 12 is gently pulled by the hydraulic power source 11. Since the deformation of the elastic body 6 is restrained by the back-up spring 13, an internal pressure p is generated in the tube 14. During this period, the load F and the displacement of the tube 14 resulting from the movement of the piston 12 are recorded by the calculating section 9 in the recording section 10 in the relationship between load and displacement.
As this operation is repeated, as the internal pressure p increases, there comes a time when the load F and the displacement of the tube 14 lose their constant direct proportionality to each other. This indicates the point at which the tube 14 begins to yield, and the load is defined as the yield load F _y . Next, the piston 12 is moved slowly to reach the yield load.
Apply a load up to 1.1 times F _y . At this time, the internal pressure P _y of the pipe 14 is 1.1 times the yield pressure. Therefore, at this point, the external load P restraining the maximum deformation portion of the tube 14 is removed. Thereafter, the cylinder load is further increased to 1.3 to 1.4 times the yield load F _y to complete the pipe expansion, and then the load P is removed, and the pipe 14 becomes a perfect circle.

Next, specific examples of the present invention will be described.

Example 1 Carbon steel tubes for boilers and heat exchangers (STB42) with a nominal outer diameter of 127 mm, wall thickness of 4.5 mm, and length of 60 mm were used.
The tube before straightening had an elliptical shape, and the difference between the major axis and the minor axis was 1.8 mm. The method shown in FIG.
The roundness was corrected using the process shown in Figure 2. At this time, the load used for tube expansion and the circumferential strain generated in the tube were recorded and the results are shown in FIG. At the stage where the load is small, the relationship between load and strain becomes almost a straight line. When the generated strain is 0.09%, the direct proportional relationship between load and strain breaks down. The load at this time is the yield load
When the load is increased to 1.1 times F _y ,
The load restraining the maximum deformation part was removed, and the tube expansion load was further applied to 1.31 times, and then the load was removed. After this, the distribution of tube diameters was measured, and as a result, the difference between the maximum diameter and the minimum diameter was 0.15 mm, and extremely good roundness was obtained. The yield load at this time was F _y =14.2 tons, and the amount of tube expansion after rounding was increased by 0.9 mm.

Example 2 A plate-wound welded steel pipe having a nominal outer diameter of 406.4 mm, a thickness of 9.5 mm, and a length of 5 m was straightened by the same method as in Example 1. The tube before roundness correction had an elliptical shape, and the difference between its major axis and minor axis was 7.6 mm. In this case, when 48.5 tons was applied as the tube expansion load, linearity was lost due to the relationship between the generated strain and the load, so this load was taken as the yield load, and subsequent corrections were made. As a result, the difference between the maximum and minimum diameters was 0.38 mm. Not only was extremely excellent roundness obtained, but the amount of tube expansion was also 0.8 mm.

Example 3 Using another plate-wound welded steel pipe having the same shape as in Example 2, rounding was performed in the same manner as in Example 1. The tube before roundness correction had an elliptical shape, and the difference between its major axis and minor axis was 7.75 mm. In this case, the linear relationship between pipe strain and pipe expansion load was lost due to pipe expansion load.
65.0 tons, and this load was taken as the yield load. Therefore, the external load restraining the maximum deformation area was removed at a tube expansion load of 72 tons, and the tube expansion was completed at a tube expansion load of 88 tons (1.35 F _y ). At this time, the difference between the long and short diameters of the tube after straightening is
The diameter was 0.5mm, and the amount of tube expansion was 0.9mm, making it possible to achieve excellent roundness correction with a small amount of tube expansion.

As explained above, according to the present invention, even if the material strength of the pipe to be straightened is unknown, it is possible to straighten the pipe with extremely high accuracy, thereby eliminating the need for collecting strength test pieces from conventional solid pipes and testing them. Therefore, the time required for the process can be significantly reduced.

[Brief explanation of the drawing]

Figure 1 is a vertical cross-sectional view of the conventional roundness correction method;
The figure shows a roundness straightening procedure using an elastic body, Figure 3 is a stress-strain curve during a tensile test of the material, Figure 4 is an explanatory diagram of an apparatus for carrying out the roundness straightening method of the present invention, and Figure 5 The figure is an explanatory diagram of the relationship between circumferential strain and load in an embodiment of the present invention. 6...Elastic body, 7...Cylinder, 8...Sensor, 9...
Arithmetic unit, 10... Recording unit, 12... Piston, 13...
Backup spring, 14...tube.

Claims (1)

[Claims] 1 In a method of expanding a pipe and straightening the end of a pipe to a perfect circle by using the deformation force in the circumferential direction of the elastic body that occurs when an axial load is applied to the elastic body, the circumferential direction of the end of the pipe to be straightened is A displacement meter is installed at the tube, and the initial shape of the ellipse is restrained by an external load to temporarily make it circular, and then the tube is expanded.During the expansion, the yield load of the tube is determined from the tube expansion load and the displacement of the tube. A method for straightening the end of a pipe into a perfect circle, characterized by removing an external restraining load at a time point of 1.1 times the yield load, then expanding the pipe to 1.3 to 1.4 times the yield load, and then removing the load.