JPS60162728A - Improvement of residual stress of pipe - Google Patents

Abstract

PURPOSE:To generate residual compressive stress to the inside surface of a pipe and to prevent generation of stress corrosion cracking by heating inductively the pipe which contacts with a corrosive material by plural high frequencies of different frequencies in the thickness direction to heat the inside and outside surfaces of the pipe at different temps. then cooling quickly the pipe. CONSTITUTION:An induction heating coil 2 is disposed on the outside circumference of a metallic pipe 1 and the current A having the modulating wave consisting of two kinds of high frequencies of different frequencies or the current B superposed with two high frequencies of different frequencies is passed therein to heat inductively the pipe 1 at different depths in the thickness direction thereof thereby heating the pipe at the temp. difference given to maintain the inside surface temp. of the pipe lower than the outside surface temp. The pipe is then cooled to generate residual tensile stress to the outside surface of the pipe and residual compressive stress to the inside surface of the pipe. Even if a corrosive material is brought into contact with the inside of the pipe, generation of stress corrosion cracking to the inside surface of the pipe is obviated owing to the residual compressive stress.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a method for improving residual stress in pipes, and particularly to a method for improving residual stress suitable for use in pipes that are brought into contact with corrosive substances.

"Prior Art" Generally, it is known that stress corrosion cracking rapidly progresses in steel materials when tensile stress and corrosive substances coexist.

Therefore, in the past, in order to solve the above-mentioned problems, the residual tensile stress remaining on the surface of the pipe on the side where the corrosive substance is present must be removed, or the residual compressive stress must be actively applied to that surface. As a specific example, there are methods for improving residual stress shown in FIGS. .

The conventional residual stress improvement method mentioned here is to generate a positive compressive stress Knn on the inner surface of the chain pipe when a corrosive substance is present inside the pipe.

That is, an induction heating coil 2 is disposed around the outer circumference of the pipe 1 as shown in FIG. By applying force to cool the inner surface, the distance K between the inner and outer surfaces of the tube 1 is reduced.
By applying a temperature difference T as shown in the mJ diagram, as shown in Figure 3, a compressive thermal curl σ of more than the yield point is produced on the outer surface of the tube l, and a tensile stress of more than the yield point is produced on the inner surface. A thermal stress +d1 is generated. Next, by stopping the induction heating and cooling the tube 1, as shown in Fig. '.

However, such conventional residual stress improvement methods - if any, tube IK3! In order to provide the temperature difference necessary to improve the retention stress, a large amount of coolant W is required, and the planning and work tend to become complicated, as it requires special rounding equipment to feed the coolant W. There are still issues that need to be improved.

``Object of the Invention'' The present invention has been made in view of the circumstances of the prior art described above, and is a method of creating a tube that can provide the temperature difference necessary for improving residual stress between the inner and outer surfaces of the tube without using a coolant. The purpose of this invention is to provide a method for improving residual stress.

"Structure of the Invention" In order to achieve the above-mentioned object, the present invention generates high-frequency waves by induction heating a tube from one side of the tube at different depths in the tube thickness direction using multiple high-frequency waves with different frequencies. By superimposing the heat generated on each other to create a temperature difference between the inner and outer surfaces of the tube at a temperature lower than the transformation temperature and generating a thermal stress higher than the yield point, and then cooling the tube, the other side of the tube is cooled. It is characterized by generating residual compressive stress on the surface side.

"Embodiment" The present invention will be described below based on an embodiment shown in FIGS. J to 1D.

This embodiment is intended to create residual compressive stress on the inner surface of a tube transporting a corrosive substance.

That is, an induction heating coil 2 is disposed around the outer periphery of the tube 1, and the induction heating coil 2 is heated by a type of high frequency wave having one different frequency. A current A having a modulated wave as shown in the figure or a type of high frequency wave with different frequencies is superimposed, and a current B having a superimposed wave as shown in the figure is passed through the tube 1 at different depths in the tube thickness direction. Heat by induction.

The depth of the induction heating is given by the following equation in relation to the frequency of the radio frequency.

S = (πJ10''''μr6f)-1...
...(1) However, S: Depth of induction heating σ: Electrical conductivity of the conductor (pipe) f: Frequency of high frequency μr: Effective magnetic permeability By performing such induction heating, high frequency , As shown in FIGS. 7(a) and 7(b), heating is performed in a shallow heating region depth Sl, and a temperature distribution shown by temperature distribution curve 1 is given to the tube 1, and,
As shown in Figures (&) and (b), the low frequency high frequency heats the deep heating region depth S2, and provides the tube 1 with a temperature distribution shown by the temperature distribution curve I2, and Both temperature distributions are superimposed. As a result, a temperature distribution having a large temperature gradient can be generated in the tube 1, as shown by the temperature distribution curve I3 in FIG.

The heating operation described above is continued until a temperature difference T necessary for improving the residual stress is generated between the inner and outer surfaces of the tube l, and
As shown in the figure, a tensile thermal stress +σ1 higher than the yield point is generated on the inner surface of the tube 1, and a compressive thermal stress −σ1 higher than the yield point is generated on the outer surface of the tube 1. The time required to generate the temperature difference T can be extremely short because the gradient of the temperature distribution within the tube 1 can be increased as described above.

Next, by stopping the induction heating and cooling the tube 1, a residual tensile stress σ2 is generated on the outer surface of the tube 1, and a residual tensile stress σ2 is generated on the inner surface of the tube 1, as shown in FIG. Residual compressive stress −σ2 is generated.

On the other hand, in the process of cooling the tube 1 described above, a phenomenon occurs in which the temperature distribution of the tube 1 is averaged and then further reduced as shown by the chain line in FIG. Due to this phenomenon, a uniform compressive thermal stress -σ3 is generated between the inner and outer surfaces of the tube 1 as shown in Fig. 1, and as the tube 1 is further cooled, as shown in Fig. 1J. A uniform residual tensile stress +σ3 is generated between the inner and outer surfaces of the tube 1, and is superimposed on the aforementioned residual stress +σ2−σ2.

As a result, the residual stress +σ
2. A residual stress is generated which is the sum of −σ2 and the residual tensile stress +σ3.

As mentioned above, the residual stress generated in the cooled tube 1 is caused by the short time required to generate the temperature difference T between the inner and outer surfaces of the tube 1, and the increase in the average temperature is suppressed as much as possible and the temperature difference T can be sufficiently generated, so that the absolute value of the residual compressive stress -σ2 can be made larger than the absolute value of the residual tensile stress +σ3. Therefore, residual compressive stress -σ4 can be generated on the inner surface of the tube 1 after cooling, as shown in FIG. 14c.

Here, an example of various conditions in the case where the above-mentioned embodiment is applied to a stainless steel pipe having a pipe thickness of Jagm at an ambient temperature of 10° C. is as follows.

A) Frequency of high frequency: f1=YujKIlz, fz=! o
OHzB) Depth of heating area; S1=10wx,,Sg
= KoOwIC) vs induction heating D output; i o o o
kwd) Heating time; No. 05-30 When the stainless steel pipe is treated under the conditions of A) to n), approximately
A temperature difference of 0°C to 300°C is created, and the inner surface after cooling is -! A residual compressive stress of less than kg/- can be generated.

On the other hand, FIG. 1j shows another embodiment of the present invention,
By widening the range of induction heating along the length of the tube 1, the influence of the residual tensile stress σ3 caused by the temperature averaging during cooling of the tube 1 can be suppressed, and the The residual compressive stress -σ4 generated on the inner surface is increased.

That is, the average radius of the pipe l is R, and the pipe thickness is t.
When the induction heating range is set to 394i or more, the heated portion of the tube 1 is thermally deformed so that the diameter of the tube becomes larger, as shown in FIG.
The restraining force against deformation along the length of is reduced. tf
t1 Various restraints at both ends of the heated portion can be prevented from reaching the center of the heated portion as much as possible.

Therefore, it is possible to reduce the uniform compressive thermal stress -σ3 caused by temperature averaging during the cooling process, and also to reduce the residual tensile stress +σ3 after cooling. As a result, the final residual compressive stress -σ4 generated on the inner surface of the tube 1 can be increased.

In the above embodiments, an example in which one induction heating coil 2 performs induction heating using modulated wave A or superimposed wave B is explained. ``Effects of the Invention'' As explained above, according to the method for improving residual stress in a pipe according to the present invention, induction heating may be performed by passing high frequencies of different frequencies through each of the induction heating coils. Excellent nine effects to Qin.

■ By heating the tube by performing multiple induction heatings at different depths, it is possible to create a temperature distribution with a large gradient in the tube, thereby increasing the temperature required to improve residual stress between the inner and outer surfaces of the tube. The difference can be applied sufficiently and quickly to eliminate the need for coolant.

■ By selecting and combining multiple high-frequency waves, it is possible to easily adjust the depth of the heating area and the gradient of temperature distribution, creating the optimal temperature distribution in response to changes in pipe thickness, etc. can be done.

(2) Due to the synergistic effect of the above effects, the residual stress in the pipe can be improved reliably and efficiently.

[Brief explanation of drawings]

In the drawings, Figures 1 to 3 show an example of a conventional method for improving residual stress in pipes, Figure 7 is a schematic side view for explaining the heating and cooling operations of the pipe, and Figure 1 shows Figure 3 is a simulated diagram showing the thermal stress generated between the inner and outer surfaces of the tube. Figure 1 is a simulated diagram showing the residual stress generated between the inner and outer surfaces of the tube. Figures 7 to 74C show an embodiment of the present invention. Figures 1 and 7 are mock diagrams showing high frequency waves, and Figures 7 and 7 show the depth of the heating area and the tube thickness by each high frequency wave. A simulated diagram showing the temperature distribution generated between the inner and outer surfaces, the final KI
10 and 1J are simulation diagrams showing the thermal stress generated between the inner and outer surfaces of the tube. Figure 1 is a simulated diagram showing the residual stress that is generated. Figure 1 is a simulated diagram that shows the residual stress that is finally generated between the inner and outer surfaces of the tube. Figure 1j is a schematic diagram for explaining another embodiment of the present invention. It is a schematic diagram showing a heating method. l...tube, 2...spun heating twill, A
, B... Current, S s * S 2...
・Heating region depth, Ix*Ize13...Temperature distribution curve. Applicant Ishikawajima Harima Heavy Industries Co., Ltd.eeeeooeeee. Figure 2 Figure 3 Figure 4 Figure 8 Crime -α' Figure 5 Figure 6 Figure 7 (0) (b) Figure 8 (0) (b) Figure 9 Figure 1O Figure 1 Figure 12 Figure 13 Figure 14

Claims (1)

[Claims] By induction heating the tube at different depths in the thickness direction from one side using multiple high-frequency waves with different frequencies, the inner and outer surfaces of the chain tube are heated at a temperature lower than the transformation temperature and above the yield point in different directions. giving a temperature difference that causes a thermal stress of
A method for improving residual stress in a tube, characterized in that the induction heating is then stopped and the tube is cooled to generate residual compressive stress on the other side of the tube.