JPS5941425A - Improvement in residual stress of hollow body - Google Patents

Abstract

PURPOSE:To obtain the method which certainly exhibits the effect to improve residual stress with a heating means having small heat capacity, by forming a temp. distribution along an axial direction in the proper domains of the butt- welded circumferential joint of a hollow body at both sides of a weld line, and then spontaneously cooling them. CONSTITUTION:Specified positions in response to the size of pipes at both sides of a butt-welded circumferential joint 2 between pipes 1 and 1, e.g. domains in a distance of 10-30mm. or about 20-40mm. apart from a weld line in case of pipes made of steel SUS 304 having an outer diameter of about 100mm. and thickness of about 6mm., are selected. Said domains are annularly simultaneously heated by a heating means 3 such as a high-frequency induction heater to form high-temp. parts and a temp. distribution along its axial direction, and then spontaneously cooled while stopping the heating. Said heating may be performed to an extent enough to form the temp. distribution without the need to perform forcible cooling such as water cooling after or during the heating. Thus, the heating means is managed with small capacity.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for improving residual stress in lap welded joints.

Tensile residual stress exists in both the circumferential and axial directions on the inner surface of butt circumferential welds of hollow bodies such as piping or tubular containers, and the presence of this inner surface tensile residual stress affects fatigue strength and stress corrosion cracking resistance. Therefore, it is desired to reduce the degree of tensile residual stress or bring it into a compressed state.

Various residual stress improvement methods have been proposed to achieve this purpose.The outer surface heating/inner surface cooling method heats the outer surface and simultaneously cools the inner surface with water to create a temperature difference between the inner and outer surfaces. This method attempts to make the residual stress state of the inner surface into a compressive state by thermal stress, but this method requires cooling the inner surface, so it cannot be applied in places where cooling is difficult. There are disadvantages such as the need for large capacity.

In addition, the Linde method heats both sides of the weld line and cools the weld line immediately afterwards, processing the weld line sequentially, but this method also has the disadvantage of requiring more cooling. In addition, the effect is uncertain because heating and cooling are performed sequentially in the weld tangent direction.

All of these methods require heating and cooling at the same time, but among the previously proposed methods, one method that does not require cooling is to place weld beads on both sides of the weld line and utilize the resulting residual stress distribution. There is a method that attempts to improve the residual stress distribution in the desired part 1, but this method has the disadvantage that material changes occur due to welding and, like the Linde method, it lacks reliability because it does not heat at once. Furthermore, there is a fundamental flaw in that the residual stress in the axial direction is improved by 7 points.

The present invention does not have the drawbacks of conventional methods as mentioned above.
In other words, the purpose is to provide a highly reliable method that does not require cooling after or during processing, requires a small capacity heating device, and has a reliable and highly reliable effect on improving residual stress. A hollow body characterized by applying uniform annular heating to an appropriate area according to the dimensions of the hollow body on both sides of the weld line of a circumferentially welded joint, creating a temperature distribution in the axial direction, and then stopping the heating and allowing it to cool. We propose a method to improve residual stress in the body.

An embodiment of the method of the present invention will be described with reference to the drawings.

First, in FIG. 1, a specific position depending on the pipe dimensions on both sides of the butt circumferential weld joint 2 of the pipe 1.1, for example S U S 3
04 In the case of a copper pipe with an outer diameter of approximately 100 m and a plate thickness of 6 mm, the distance from the weld line is 10 to 30 mm.

Alternatively, a region of 20 to 40 mm is simultaneously heated in an annular manner using a heating device 3 such as a gas flame or high-frequency induction heating to create a high temperature section, and after creating an axial temperature distribution, the heating is stopped and allowed to cool. Heating is sufficient to produce temperature distribution, and after heating, there is no need for forced cooling such as water cooling during heating.

By heating in this manner, the welded structure of the tube 1.1 exhibits a temperature distribution in the axial direction as shown in FIG. 2, and as a result deforms into the shape shown in FIG.

Due to this deformation, the circumferential stress becomes tensile stress near the weld line as shown in FIG. 4, and becomes compressive stress in the heated area as shown in FIG. 5. On the other hand, in the axial direction, since the circumferential surface is heated at the same time, thermal expansion causes a difference in the size of the diameter in the axial direction, resulting in a bending moment, and as a result, the axial stress is
Near the weld line, as shown in Figure 6, the outer surface of the tube is in a compressive stress state, the inner surface of the tube is in a tensile stress state, and in the heating area, a seventh
As shown in the figure, the outer surface of the tube is under compressive stress, and the inner surface of the tube is under tensile stress.

However, after such a stress distribution occurs during heating, when the thermal stress is unloaded by cooling after heating, these stresses decrease or reverse, and the tensile residual stress decreases or compresses near the weld. It becomes stress.

The following is a numerical experiment (simulation) using the thermoelastic-plastic finite element method conducted to confirm the effects of the method of the present invention.
The results will be explained with reference to FIGS. 8 to 18.

The specimen is made of SUS 304 steel and has an outer diameter of 114.3 m.
A butt-circumferential welded joint of a tube with a wall thickness of 6 mm was used.

FIG. 8 shows the outer surface residual stress distribution as it is after welding, and FIG. 9 shows the inner surface residual stress distribution.

Figure 10 shows a gas flame of 0.1 cal/mm3X6
The inner surface temperature distribution during heat treatment when heating for 0 sec is applied to a region 10 to 30 mm from the weld line is shown. 11th
The figure shows the outer surface residual stress distribution after the heat treatment, and FIG. 12 similarly shows the inner surface residual stress distribution.

Next, Fig. 13, Fig. 14, and Fig. 15 show the gas flame
．． Inner surface temperature distribution during heat treatment when heating at 1 cal/mm3x 5Q eθC is applied to an area 20 to 40 mm from the weld line, outer surface residual stress distribution after heat treatment,
and the inner surface residual stress distribution are also shown.

Furthermore, Fig. 16, Fig. 17, and Fig. 18 show that the gas flame is 0.
．． The inner surface temperature distribution during the heat treatment, the outer surface residual stress distribution after the heat treatment, and the inner surface residual stress distribution when heating at 1 cal/mm3x6Q sec is applied to a region 40 to 60 mm from the weld line are shown.

From the diagrams shown above, it can be seen that after welding in Figure 8, the internal residual stress was in a tensile state of 20 kgf 7 mm2 or more in both the circumferential and axial directions, but by heating the appropriate heating area, it can be reduced only in the circumferential direction. It can be seen that the residual stress in the axial direction is also largely removed. That is, as shown in FIG. 18, if a region of 40 to 60 mm from the weld line is heated, and if a region of 10 to 30 mm is heated as shown in the figure, there is a residual stress removal effect. Among them, 20 to 40 mm is most appropriate for this pipe diameter.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of an embodiment of the method of the present invention, Figures 2 and 3 are explanatory diagrams of temperature distribution and deformation states in the same as above, Figures 4 to 7 are diagrams, and Figure 9 is welding. Residual stress distribution diagrams, Figures 10 to 12 are 10 to 30 mm from the weld line.
Temperature distribution diagrams and residual stress distribution diagrams in the case of heating in the area of
FIG. 1B is a temperature distribution diagram and a residual stress distribution diagram of heating in a region of 40 to 60 mm from the weld line. 1: Pipe, 2: Butt circumferential weld joint, 3: Heating device Fig. 1 Fig. 2 Fig. J lock, ■ Collection 3 Fig. 4- M5 Fig. O Fig. 7

Claims (1)

[Claims] Uniform heating is applied in an annular manner to an appropriate area according to the dimensions of the hollow body on both sides of the weld line of the butt circumferential weld joint of the hollow body,
A method for improving residual stress in a hollow body, which is characterized by generating a temperature distribution in the axial direction, then stopping heating and allowing it to cool.