JPS5841617A - Residual stress improving method of pipe or tubular vessel - Google Patents

Abstract

PURPOSE:To reduce tension residual stress, and to prevent stress corrosion cracking, by applying local external pressure onto several place of the outside circumferential surface of a tubular vessel, moving it in order along the outside circumferential surface, and causing plastic deformation by low pressure. CONSTITUTION:In several places of a tubular vessel 1, an external pressure 2 is applied toward the inside from the outside. Subsequently, the external pressure 2 is removed, the vessel is rotated by some angle, and the external pressure 2 is applied again. This operation is repeated in order, and to the whole outside circumferential surface of the vessel 1, the external pressure 2 is applied. In this way, plastic deformation is generated under the low pressure, tension residual stress is reduced, and stress corrosion cracking is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reducing the tensile residual stress on the inner surface of a pipe or a tubular container by applying an external pressure load or improving it to a compressed state, and in particular, to stably improve the stress at a small pressure. This is related to the method by which this can be done.

Residual stress on the inner surface of plant piping or tubular containers near welded joints is usually in a tensile state. For this reason 5
There were disadvantages such as the occurrence of CC (stress corrosion cracking).

Conventionally, several methods (1) have been proposed to improve this problem and reduce the tensile residual stress on the inner surface of the tube, and even bring it into a compressed state. However, most of these methods involve thermal methods such as creating a temperature gradient in the thickness direction of the plate, making it difficult to control the temperature distribution and having the disadvantage that it is difficult to obtain a stable effect.

Therefore, in order to obtain a more stable effect, we mechanically applied pressure uniformly from the circumferential direction of the outer surface of the tube to cause plastic deformation.
A method has been proposed in which the residual stress on the inner surface of the tube is brought into a compressed state by this effect (see Japanese Patent Application No. 56-29970). However, this method has the drawback of requiring a fairly large device in order to apply a uniform external pressure in the circumferential direction.

The present invention provides a mechanical method for improving residual stress that provides the above-mentioned stable effects and does not require generation of a large pressure.

In other words, the present invention does not apply external pressure uniformly in the circumferential direction of the outer surface of a pipe or tubular container, but locally applies external pressure at several points on the circumference and (2) sequentially moves the external pressure in the circumferential direction. The present invention relates to a method for producing plastic deformation in a pipe or a tubular container to improve residual stress. According to the method of the present invention, since pressure is not applied uniformly, the pressure required to cause plastic deformation is relatively small, and the apparatus can be made smaller.

Further, as will be explained in detail later, since local pressure is sufficient, processing can be performed using a roller, and work efficiency can be improved.

Hereinafter, the method of the present invention will be explained in detail with reference to the accompanying drawings.

FIG. 1 (A+) (a) to (d) and (B) are diagrams showing an embodiment of the method of the present invention.

Figure 1 (AI (al, (b), (C), (d) shows the operation order. First, the tube or tubular container (hereinafter abbreviated as tube) 1 is placed at two locations indicated by 2 in the figure. The operation of applying force (external pressure) from the outside to the inside [Fig. 1 (A) (a) '], then removing the external pressure, rotating it by the specified angle, and applying external pressure again [Fig. 1 (AHb)) Repeat 1111! times [Figure 1 (AHCJ-(d)), outside tube 1 (5
) External pressure 2 is applied to the entire circumferential surface.

Figure 1 (Bl is Figure 1 (AH21), (b), (C1-
(This shows a vertical cross-sectional view of dl.

FIG. 2 is a diagram showing another embodiment of the method of the present invention, in which external pressure is applied to five locations (portions indicated by 5 in FIG. 2),
The external pressure is applied by a roller 5 and rotated while being applied, unlike the method shown in FIG. Note that 6 in the figure is a casing that holds the roller 2.

FIG. 5 and FIG. 4(A) are schematic diagrams showing an example of implementing the uninvented method using an actual machine.

Fig. 5 shows what is carried out using the hydraulic ram 5, Fig. 4 (A)
, (B) are carried out by the roller 5.

In the case of Figures 4 (A) and (B), if the roller 5 and the tube 4 outside it are tapered, they will look like a rotating wedge, and a large (4) external pressure can be obtained with a small rotational force. can. Furthermore, the central axis of the roller 6,
If the center axes of the tube 1 and the tapered tube 4 are not parallel but are made at an angle α, when the tapered tube 4 is rotated in one direction, the tapered tube 4 will slide in the axial direction and the pushing force will increase. A self-feeding effect is obtained. 6 rollers 5 in the diagram
Fig. 4 (A) is a casing that holds the
Bl I″i No. 4
It is a cross-sectional view taken along the line 1-1 in Figure (A).

Next, the functions and effects of the method of the present invention will be explained.

Figures 5(A) to 5(D) show how the stress distribution changes on the inner and outer surfaces of the tube when force (external pressure) is applied to n points on the tube from the outside to the inside.

In Fig. 5 (Ai to (D)), (a) is a chart showing the stress distribution on the inner surface and tb+ is the stress distribution on the outer surface, where the vertical axis is the stress σ or strain εp1, and the horizontal axis is the angle β. As shown in Figure 5 (E), this is the angle from the external pressure load point ■.

As shown in (a) and (b) of Figure 5 (A), external pressure load (
5) Initially, on the inner surface (a), the load point is + (tension) near the load point, - (compression) at the intermediate position of the load point, and on the outer surface (bl
Then, the stress distribution will be the opposite.

If the external pressure load is increased beyond a certain level, yielding will begin at places with high stress, and the initial plastic strain distribution will be as shown in Figure 5 (
The outer surface of (a) in B) becomes as shown in (b) of FIG. 5(B).

As shown in Figure 1.2 above, if the external pressure is controlled appropriately while moving the external pressure load location, the final plastic strain distribution will be as shown in Figure 5 (C1 (a) for the inner surface and Figure 5 for the outer surface). (C)
(b). That is, the plastic strain is + (elongation) on the inner surface (a) and - (shrinkage) on the outer surface (b). Therefore, the final stress distribution (residual stress distribution) on the inner and outer surfaces
As shown in (a) and (b) of Fig. 5 tD), the inner surface (a) is - (compression), and the outer surface (1)) is + (tension).

As explained above, the method of the present invention does not require a large external pressure and can stably reduce tensile residual stress or improve it to a compressed state.

(6)

[Brief explanation of the drawing]

FIG. 1 (A+(a) to (d) is a diagram showing the operation sequence of one embodiment of the method of the present invention, FIG. 1(B) is a longitudinal sectional view thereof,
FIG. 2 is a diagram showing another embodiment of the method of the present invention, FIG. 6 and FIGS. ) to (DJ are diagrams showing changes in stress distribution obtained by the method of the present invention, and the fifth
Figure (A) is the stress distribution at the initial stage of external pressure loading, Figure 5 (Bl is the initial plastic strain distribution, Figure 5 ((l is the final plastic strain distribution, Figure 5 (D) is the final stress distribution (residual stress distribution) , and FIG. 5 (A1-(Dl) (al is the inner surface, (bl is the outer surface)) shows the stress distribution in the two notations, and FIG. 5 (E) is an explanatory diagram of the external pressure load location. 1) Meifuku agent Ryo Hagiwara - (7) Figure 1 Figure 3

Claims (1)

[Claims] 1. A method for improving residual stress in a pipe or tubular container, comprising locally applying external pressure to several locations on the outer circumferential surface of the pipe or tubular container, and sequentially moving the locations loaded with external pressure along the outer circumferential surface.