JPS58927B2 - Bending method for austenitic stainless steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for bending an austenitic stainless steel pipe.

Austenitic stainless steel pipes are widely used in nuclear power plants, thermal power plants, petrochemical plants, and other plants, but there are five problems in that the welded parts of the pipes, especially the welded parts at bent parts, are easily corroded by high-temperature, high-pressure steam. .

In order to solve this problem, conventionally, 1) Perform local solution treatment.

2) Special heat treatment for the weld, for example, igniting heat from the outside while cooling the inside, creating a sharp temperature gradient inside and outside the wall thickness, leaving compressive stress in the weld and the inner surface of the tube in the vicinity. Perform heat treatment.

3) Welding is performed using a special welding method, such as a method in which a special alloy is overlaid on the inner surface of the tube and then welded.

Countermeasures such as these have been taken, but in order to be completely safe, it is necessary to perform the processes 1) and 2) or 3) and 2), which results in extremely high costs.

The reason for the high cost of austenitic stainless steel piping is that when a curved section is formed in the piping,
This is because curved pipes are manufactured by welding, and conventionally, it has been impossible to bend relatively large-diameter, thick-walled stainless steel pipes to a small relative radius without reducing the wall thickness and while improving the metallographic structure. Considering the circumstances, it was unavoidable.

However, if the bending process becomes possible, it would be extremely useful industrially.

In view of the above, the present invention provides a relatively large-diameter, thick-walled stainless steel pipe with a small relative radius and without reducing the wall thickness.
However, the purpose of this method was to provide a method that could be bent while improving the metallographic structure, and the method consisted of forming an austenitic stainless steel pipe into a narrow annular zone using heating means such as induction heating or a gas burner. In a method of bending by applying a bending moment to the zone while heating the tube uniformly and gradually moving the heating zone relatively slowly in the longitudinal direction of the tube, the steel tube is attached to the steel tube before the bending. After removing foreign matter that promotes thermal embrittlement, a compressive force is applied to the portion of the steel pipe to be bent that is sufficient to cause zero or substantially zero wall thickness reduction at the outermost part of the steel pipe with respect to the neutral axis of bending. By applying a force in the opposite direction to the moving direction of the steel pipe to the tip side of the steel pipe, the heating zone is cooled from the inner surface of the steel pipe so that compressive stress remains on the inner surface of the steel pipe. The device is characterized by the following: a heating device using induction heating or a gas burner, etc. that heats the austenitic stainless steel pipe to be bent into a narrow annular zone; a device for applying a bending moment to the heating zone of the tube and bending the tube; and a device for applying a bending moment to the heating zone of the tube and bending the tube; A device for applying a compressive force sufficient to reduce the heating zone to zero or substantially zero, and a cooling device for quenching the heating zone annularly and continuously from the inner surface of the tube. It is something.

It is known that by bending austenitic stainless steel pipes while rapidly heating and cooling them using induction heating, etc., it is possible to perform solution treatment at the same time as bending, but this has the problem of thermal brittle cracking. .

This thermal brittle cracking is a phenomenon in which foreign substances such as aluminum, zinc, copper, etc. attached to the surface of the tube penetrate into the base metal under high heat when the tube is heated, causing minute cracks.To prevent this, A method has been considered in which the surface of the tube is polished in advance to remove foreign matter and prevent it from spreading into the base metal, as well as to remove minute scratches and eliminate stress concentration before bending. A patent application is pending as No. 127607.

This method is extremely effective as it can almost avoid the thermal brittle cracks mentioned above, but in rare cases cracks may still occur, so further research revealed that thermal brittle cracks are It was confirmed that plastic deformation does not occur near or inside the neutral axis of the tube, but is limited to the range where plastic deformation occurs under tensile force exclusively on the outside of bending, and strong compressive force is applied to the heating zone of the tube from the front and back They discovered that thermal brittle cracking can be prevented by bending while adding a

Therefore, applying a strong compressive force to the heating zone as described above has a double benefit since it can prevent the wall thickness from decreasing, and also allows solution treatment to be performed at the same time by rapidly cooling the heating zone. be able to.

Furthermore, in the bending method of the present invention, if the heating zone of the tube is cooled from its inner surface, compressive stress can remain on the inner surface of the tube, thereby increasing the corrosion resistance of the product against high-temperature water and steam.

Next, an apparatus for carrying out the method of the present invention will be explained with reference to the drawings.

Figure 1 shows a patent application published in Japanese Patent Application No. 52-73337 that applies a sufficiently strong compressive force to the heating zone of a tube to bend it while preventing wall thinning.
This is an example of the device of the present invention, which utilizes one of the embodiments of the device for which a patent is being applied for under No. 1, and is equipped with a device that cools the heating zone of the bendable tube from the inner surface of the tube.

That is, in the figure, 1 is an austenitic stainless steel pipe to be bent, 2 is a bent part thereof, 3 is a clamp that grips one end of the pipe 1, and 4 is a pipe that fixes the clamp 3 and bends around the central axis 5. It is a rotatable bending arm, and chain wheels 6 are fixed on both sides of the shaft 5.

Reference numeral 7 indicates a heating device consisting of an annular inductor, and 8 indicates a high-frequency transformer; however, an annular gas burner may also be used for the heating device 7.

Reference numeral 9 denotes a clamp for gripping and fixing the other end of the tube 1, and is fixed on the truck 10, but if necessary, it may be connected by a pivot perpendicular and horizontal to the traveling direction of the truck 10.

Reference numeral 11 denotes hydraulic cylinders arranged on both sides of the pipe 1. One end of a chain 13 is connected to these piston rods 12, and the other end of the chain 13 is wound around the chain wheel 6. It is designed to pull strongly.

14 is the wheel of the trolley 100, 15 is a rail on which the wheel 14 is placed, and 16 is a base that supports a water pipe 17 with a cooling jacket 18 attached to its tip so that the jacket 18 is in an appropriate position. It also serves to supply cooling water to the water pipe 17.

An example of the device of the present invention is configured as described above. In this device, the center of the shaft 5 is the center axis of the O9 pipe 1, the radius of the pitch circle of the chain wheel 6 is r, and the straight line XX from the center 0 is ′
Let the distance to the hydraulic cylinder 11 be R, , R-r=l.
is located at a distance l from the straight line X-X'.

The above-mentioned device heats the tube 1 at a high temperature in a narrow width from the outside by the heating device 7, while pulling the chain 13 by the hydraulic cylinder 11, and injects water from the cooling jacket 18 into the heating zone of the tube 1. The pipe 1 is bent while being cooled with cold water. During this bending process, the pipe 1
If P is the tensile force to cause bending, then Pl=M.

Here, M is the moment required to cause bending in the heating zone of the tube 1, and from the above equation, it becomes as follows.

That is, as the diameter of the chain wheel 6 is increased so that r becomes closer to R, P becomes an extremely large force, and the compressive force completely reduces the wall thickness on the outside of the bending of the pipe 1. Conventionally, the heating zone of the tube 1 was cooled from the outside of the tube 1, so tensile stress remained on the inner surface of the tube 1. By doing so, the stress remaining on the inner surface of the tube 1 becomes compressive stress, so that the corrosion resistance of the product against high-temperature water and steam can be increased as described above.

In steel pipes such as general carbon steel, chrome steel, and molybdenum steel, even if the heating zone is cooled from the inside of the pipe during the bending process, the residual stress on the inside does not necessarily become compressive stress.
Depending on the thermal history, it becomes tensile stress or compressive stress, and this is due to the magnitude relationship between the volume change due to transformation and martensite formation and the volume change due to heat.

However, since austenitic stainless steel does not undergo transformation even if it is rapidly cooled, if it is rapidly cooled from the inner surface, the stress remaining on the inner surface will inevitably become compressive stress.

Therefore, the effect of the compression mentioned above is not only to prevent the reduction in wall thickness, but also to extremely minimize the flattening phenomenon that occurs in the hot cracking of the pipe as described above. It is noteworthy that it alleviates thermal brittleness, which is one of the serious drawbacks of austenitic stainless steels.

In other words, as mentioned above, this thermal brittleness is caused by foreign substances such as aluminum, zinc, and steel attached to the pipe penetrating the austenitic stainless steel base metal at high temperatures and causing minute hair cracks. During bending, tension will never occur in parts that are not subjected to force, so when bending an austenitic stainless steel pipe, thoroughly polish the pipe in advance to remove the foreign matter, and during bending, compressive force is applied to the pipe as described above. It was confirmed that thermal embrittlement cracking in products can be completely prevented by applying heat.

Note that the above compressive force is applied in the axial direction of the tube,
Naturally, it acts as a compressive force in the circumferential direction, and also relieves the tensile stress in the circumferential direction.

Figure 2 shows that not only the bent portion 2 of the pipe 1 but also the straight portions before and after the bent portion 2 are continuously and uniformly heat-treated, resulting in disadvantages such as sensitization of the metal structure at the boundary between the bent portion and the straight portion. This figure shows an apparatus and its working process for carrying out a method of bending so that compressive stress does not occur and compressive stress remains on the inner surface of straight pipe parts.

c and d indicate the start of heat treatment, the start of bending, the end of bending, and the state of the heat treatment path, respectively. In the figure, the same reference numerals as in Figure 1 indicate those that perform the same function, and the figures are made easier to understand. In order to do this, the hydraulic cylinder 11 in FIG. 1 is omitted, and
Instead, an arrow 13 indicates that the aforementioned tensile force P is acting on the chain 13.

Furthermore, in FIG. 2, the clamp 9 on the rear end side of the tube 1 is slidably and securely engaged with a fixing base 20 by a guide 19, and is fixed from the start of heat treatment to the end of bending, and after the end of bending, heat treatment is performed. If it continues, it is released from its fixation and can be moved back.

The bending arm 4 is rotatably attached to a base 21 by a shaft 5, the base 21 is placed on a rail 22 and is movable toward the fixed base 20, and the heating device 7 is connected to a hydraulic cylinder (not shown). The receiver can be moved in parallel with the movement of the platform 21, while the water pipe 17 to which the cooling jacket 18 is attached is moved by the platform 16a.
The trolley 16a is mounted on the rail 2 in parallel with the heating device 7.
It is now possible to move above 2.

The operation of the apparatus shown in FIG. 2 will be explained step by step as follows.

A shows the state where the heat treatment has started, and at this time the heating device 7 and the cooling jacket 18 are gradually moving to the left in the figure.
The clamp 3 has not yet reached the tip of the tube 1.

As the movement progresses from this state and reaches the state b where the clamp 3 reaches the position where it grips the tip of the tube 1, the clamp 3 grips the tip of the tube 1 and begins bending. Since the clamp 3 is pulled by the chain 13 with lever length 1=R-r (same as in Figure 1), a bending moment is generated, and as a result of bending in the heating zone of the tube 1, the chain wheel 6 is moved clockwise. Rotate and cradle 2
1 moves to the left at a speed greater than the pulling speed of the chain 13.

The clamping force applied to the pipe 1 at this time is the same as in the case shown in FIG. It goes around the left clamp 9, hydraulic cylinder, piston rod, and chain 13, and is completely balanced, so the shaft 5
Since the receiver does not act on the stand 21, the receiver and the stand 21 smoothly move to the left as the bending progresses.

When the bending is completed and state C is reached, the hydraulic cylinder is stopped and the pressure is gradually reduced to zero.
The heating device 7 and the cooling jacket 18 continue to move them at the same speed as described above to continue the heat treatment.

As this heat treatment gradually progresses and the heating device 7 and cooling jacket 18 approach the clamp 9, loosen the guide 19 that supports and guides the clamp 9, move the clamp 9 backward, and as shown in d, the heating device 7 and the cooling jacket 18 approach the clamp 9. Jacket 18 is tube 1
The heat treatment is finished when it can be removed to the left.

In the figure, 23 is a support roller that supports one end of the pipe 1 at the beginning of bending, moves to the left during bending, and moves to C at the end of bending.
The tube 1 is lowered as shown in FIG.

Although the above device is the most preferred example for carrying out the method of the present invention, a polygonal guide may be used in place of the chain wheel shown in FIG. It could be something like this.

Furthermore, the radius of the chain wheel may be made larger than the bending radius, and the center axis 5 may be pulled parallel to the pipe 1 at the same time as it is pulled from the fixed end.

Further, as a method of applying compressive force to the heating zone of the tube 1, as shown in FIG. A tension member may be tensioned between the ends of the tube 1 as shown.

That is, in FIG. 3, 24 is a brake hydraulic cylinder with a chain 13 connected to the tip of its piston rod 25, 26 and 27 are rollers that receive reaction force during bending, and 28 is a piston fixed to the bending arm 4. Money 30 at the tip of rod 29
When pressing and bending the chain pipe 1 by applying pressure P' from the rear end of the pipe 1, the hydraulic cylinder 24 pulls the chain 13 and applies a compressive force pb to the pipe 1.
The hydraulic cylinder 28 presses the bent portion 2 of the pipe 10 toward the center O via the stopper 30, and the core target portion 2 receives a strong pressure P'.
This is to prevent excessive deformation, and it is desirable that this stopper 30 is also attached to the apparatus shown in FIGS. 1 and 2.

In addition, in FIG. 4, a tail clamp 32, which is fixed to the truck 10 and supports the rear end of the tube 1, is attached to the clamp 31.
The screwdriver 33 loosely inserted into the screwdriver is a nut that is screwed into the screwdriver 32,
A chain 34 is stretched between the tip of the screw punch 32 and the bending arm 4, and when bending the chain 34 is tightened by tightening the nut 33.
make me nervous.

35 and 36 are chain wheels arranged on the frame F. A chain 13 is wound around these chain wheels, and both ends of the chain 130 are fixed to both sides of the cart 100, and the chain wheels are driven to rotate clockwise. Then, thrust is applied to the tube 1.

In this figure, the chain 34 is placed on the outside of the bend of the pipe 1.
Since the tube is stretched, the heating device 7 is equipped with an integral cooling nozzle so that the tube 1 is heated and cooled from its inner surface, and when an inductor is used for the heating device 7, A conductor 37 from a high frequency transformer 8 is built into the water pipe 17.

Incidentally, the chain 13 may be pulled by a hydraulic cylinder as in the case of FIGS. 1 and 2.

In the above device, the chain wheel 35 is driven to apply thrust to the pipe 1 to propel the chain pipe 1 as described above, while the chain 3
Since the bending process is performed while applying compressive force to the chain pipe 1 using the mechanism 4, in combination with the inner surface cooling by the cooling nozzle attached to the heating device 7, the bending process is performed while applying compressive force to the chain pipe 1. bending process can be performed.

In the heating method of the present invention, if the tube is thin, it may be heated from either the inside or outside, and if the tube is thick, it may be better to heat from both the inside and outside.

In addition, in order to maintain appropriate cooling rates for solution treatment and residual stress treatment, it is desirable to cool the outer surface at a slightly delayed position from the inner surface cooling position to adjust the deviation.

The present invention is as described above, and the austenitic stainless steel pipe is heated uniformly or approximately uniformly in a narrow annular zone by a heating means and is gradually moved relatively in the longitudinal direction of the chain pipe in the heating zone. When bending is performed by applying a bending moment to the zone while bending, a compressive force is applied to the heating zone sufficient to reduce the wall thickness of the outermost part of the tube to zero or substantially zero with respect to the neutral axis of bending. As a result, the bent austenitic stainless steel pipe has almost no decrease in wall thickness on the outside of the bend, and can be bent without defects such as thermal brittle cracking. If the stress remaining on the inner surface of the tube is turned into compressive stress by cooling it from the inner surface of the tube, the corrosion resistance against high-temperature water or steam will increase, and furthermore, not only the bent section but also the straight section can be heated and cooled in the same way as above. By heat-treating the entire tube, solution treatment is performed, resulting in a tube that has high corrosion resistance against high-temperature water or steam as a whole.

Therefore, the method of the present invention is suitable as a method for bending austenitic stainless steel pipes used in high-temperature, high-pressure parts such as atomic couplants, thermal power plants, etc., and the apparatus thereof is relatively lightweight and easy to manufacture.

[Brief explanation of the drawing]

Figure 1 is a front view of an example of the apparatus of the present invention, and Figure 2 is a front view of the same separate apparatus in the order of steps, where a is at the start of heat treatment, C is at the start of bending, C is at the end of bending, and d is at the end of bending. 3 and 4 are explanatory diagrams showing different methods of applying compressive force to the tube during bending. DESCRIPTION OF SYMBOLS 1... Austenitic stainless steel pipe to be bent, 2... Bent part, 3... Clamp, 4... Bending arm, 5... Medium water shaft, 6... Chain wheel, 7... ... Heating device, 9... Clamp, 10... Dolly, 11... Hydraulic cylinder, 12... Piston rod, 13... Chain, 14... Wheel, 15... Rail, 16 ...Base, 16a...Dolly, 17...Water pipe, 18...Cooling jacket, 19...Guide, 20...Fixing stand, 21...Support stand, 22... ...Rail, 23...The receiver is a roller.

Claims (1)

[Claims] 1. An austenitic stainless steel pipe is heated by induction heating or a heating means such as a gas burner to form a narrow annular shape. In a method of bending by applying a bending moment to the zone while heating the zone uniformly or approximately uniformly and gradually moving the heating zone relatively in the longitudinal direction of the colored tube,
After removing foreign matter promoting thermal embrittlement adhering to the steel pipe before bending, compression is sufficient to cause zero or substantially zero wall thickness reduction at the outermost part of the steel pipe with respect to the neutral axis of bending. A force is applied to the heating zone by applying a force in the opposite direction to the moving direction of the steel pipe to the tip side of the portion of the steel pipe to be bent, and the heating zone is cooled from the inner surface of the steel pipe. A method for bending an austenitic stainless steel pipe, characterized in that compressive stress remains on the inner surface of the steel pipe.