JPS5935831A - Hot bending method of pipe - Google Patents

Abstract

PURPOSE:To obtain a bend pipe having large corrosion strength, by executing hot working of a pipe, and also generating compressive residual stress on the inside surface of the pipe of the bend part. CONSTITUTION:A straight pipe fixed to a rotary arm 7 by a clamp 8 is heated from the outside surface of a pipe by a high frequency heating coil 2, a pipe 1 is pushed from the rear by operating a load P, and a bend pipe 1 whose rotation center is a pivot 9 is obtained. In case of this working, forced cooling is executed to the heating bend-working end part successively by a cooling agent 6 from the inside surface of the pipe. As for the cooling agent 6, air or water, etc. are used. In this way, all the residual stress on the inside surface of the pipe 1 can be made residual stress of compression, and a bend pipe having high corrosion strength is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to hot bending of pipes, and more particularly to a pipe bending method that simultaneously performs bending and improves residual stress.

In conventional pipe hot bending using the arm rotation method using high-frequency heating, the pipe is cooled naturally after processing, or it is forced to cool from the outside surface of the pipe. The disadvantage is that tensile residual stress is generated.

An object of the present invention is to provide a dog-bent pipe with high corrosion resistance by hot bending the pipe and simultaneously generating compressive residual stress on the inner surface of the pipe at the bent portion.

In the present invention, during hot bending by high-frequency heating, out of the residual stress generated in the bending part due to thermal stress during heating and plastic deformation during bending, all residual stress on the inner surface of the pipe is converted into compressive residual stress. By sequentially forcibly cooling the end portion of the tube from the inner surface of the tube, compressive residual stress is generated on the inner surface of the tube by utilizing cooling thermal stress. This is a pipe hot bending method characterized in that heat treatment for improving residual stress during hot bending of pipes is simultaneously performed efficiently.

To manufacture the bent tube 1 shown in FIG. 1 by bending a straight tube, there is a hot bending method using an arm rotation type bending device using high-frequency heating as shown in FIG. Clamp 8
A straight pipe fixed to a rotary arm 7 is heated from the outer surface of the pipe by a heating coil 2, and a load P' is applied to the pipe 1 from the rear to produce a nine-bend pipe 1 with the push pivot 9 as the center of rotation.

As shown in FIG. 3, the present invention uses a method and a device for forcedly cooling the heated bending end portion with a coolant 6 sequentially from the inner surface of the tube in the arm rotation type bending process using high-frequency heating. Air or water is used as the coolant 6. According to the present invention, all residual stress on the inner surface of the pipe can be made into compressive residual stress. Fourth
The figure schematically shows the stress/strain behavior of the inner surface of the bent tube 10 at the bent portion when manufactured by this method. A, B, and C correspond to the positions shown in FIG. During the high solid wave heating process, the stress changes from (a) to (b), a compressive stress state, and during the subsequent bending process, the stress changes from (b) to (c). Further, during the cooling process from the inner surface of the tube according to the present invention, the stress changes from (c) to (), and all stress on the inner surface of the tube becomes a compressive residual stress state. Piping made by the method of the present invention has a great effect on improving corrosion strength. Further, according to the present invention, it is possible to efficiently perform the bending process of the pipe and the heat treatment for improving residual stress at the same time.

FIG. 5 shows another embodiment of the invention. The difference from FIG. 3 is that the heating coil is also installed on the inner surface of the tube, and the cooling of the heating coil and the tube are performed in the same system. This method not only brings the inner surface of the pipe into a state of compressive residual stress, but also has the effect of simplifying the cooling system.

According to the present invention, since the inner surface of the bent portion of the bent pipe is brought into a state of compressive residual stress, a bent pipe with high corrosion resistance can be produced.

Furthermore, since hot bending and heat treatment are performed simultaneously, it is extremely efficient and economical.

[Brief explanation of the drawing]

Fig. 1 is a state diagram of a bent pipe according to an embodiment of the present invention, Fig. 2 is a diagram of a conventional apparatus for hot bending of a pipe using high-frequency heating, and Fig. 3 is a configuration diagram of the pipe hot bending method of the present invention. , FIG. 4 is a stress behavior diagram of the inner surface of a bent portion of a bent pipe, and FIG. 5 is a structural diagram showing another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Bend pipe, 2... Heating coil, 3... Cable, 4... Takakawa wave heating transmitter, 5... Cooling pipe, 6...
... Coolant, 7... Rotating arm, 8... Clamp,
9...Pivot. 3rd / 3rd concave 4th roC+″″B8 (c) / i -'Ron--()\ncao■/--・-(lsu~(ni)nshi蔚ji!Lgit-to

1ε/ / / /'＜=)11;' 7, Mu,

Claims (1)

[Claims] 1. A pipe hot bending method using an arm rotation method using high-frequency heating, which is characterized in that the heated bending part or the heated bending end part is bent while being cooled from the inner surface of the pipe.