JPS5820393A - Decreasing method for residual stress - Google Patents

Abstract

PURPOSE:To improve tensile residual stress in the inside surface of the pipe of a welded joint relatively easily to a decreased or compressed state by chipping the outside surface of the pipe in the welded joint part of pipes down to a constant depth then subjecting the joint part to build-up welding while cooling the inside surface of the pipe to restore the same state as the original shape of reinforcement of weld. CONSTITUTION:The outside surface of the weld zone 2 of a joint to be improved of the residual stresses in the inside surface of an existing or newly constructed pipe 1 is chipped down to a constant depth by a mechanical means such as a grinder. While the inside surface of the pipe 1 is cooled with tapping water, static water or the like, the weld zone is subjected to build-up welding 3 to restore the original shape. The thermal stresses shown by a solid line curve 2 are generated by the temp. difference, shown in the graph, generated during execution of the welding 3, and tensile plastic deformation arises in the inside surface and compressive plastic deformation in the outside surface respectively. When a uniform temp. distribution is attained by cooling after the welding 3, the stress state shown by a broken line curve beta is obtained, wherein the inside surface has compression and the outside surface tension. As a result, the parts near the welded joint have higher resistance to stress corrosion cracking and cor rosion fatigue.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for easily reducing or reducing the tensile stress remaining on the inner surface of a newly installed pipe or a tubular container near a welded joint to a compressed state.

Generally, residual stress is in a tensile state on the inner surface of a pipe near a welded joint of a plant pipe or a tubular container. Therefore, inconveniences such as the tendency to cause stress corrosion cracking (SCC) are alleviated. .

Under these circumstances, various methods have been proposed to improve the residual stress state, reduce the tensile residual stress, and further bring the material into a compressed state.

For example, there is a method of improving the residual stress state by high-frequency heating the entire tube from the outside and cooling the inside with water. However, this method is difficult to implement because it requires large-capacity high-frequency equipment.

The present invention enables pipe drawing of existing or new welded joints.
The object of the present invention is to provide a method for relatively easily reducing residual stress or improving the compressed state.

In other words, the present invention involves lifting off the outer surface of a joint to a certain depth to improve the residual stress on the inner surface of the tube, and then applying overlay while cooling the inner surface of the tube to bring it into the same state as the original IIM shape. The present invention relates to a residual stress reduction method characterized by the following.

In this way, the method of the present invention aims to improve the residual stress inside the tube by creating a humidity difference between the inner and outer surfaces by welding the outer surface while cooling the inner surface with water, and the resulting plastic deformation. Flash welding while cooling the inner surface with water without stripping can also have the effect of improving residual stress. but,
'In the latter case, the original bead shape is deformed by the build-up, which deteriorates the ultrasonic flaw detection performed for plant maintenance during the use period, and the build-up becomes too large, resulting in shape discontinuity. This results in an increase in mass, and disadvantageously causes stress concentration and changes in vibration response when external force is applied. On the other hand, in the case of the former method, that is, the method of the present invention, it is lifted once and then overlay welded to the original shape.
The above-mentioned disadvantages do not occur, and the residual stress on the inner surface of the tube can be easily improved.

The method of the present invention will be described in more detail below with reference to the accompanying drawings. FIG. 1 is a schematic sectional view showing an example of a welded joint in which internal residual stress is to be improved. In the figure, 1 is a pipe and a double welded part.

The outer surface of the material shown in Figure 1 is removed by grinding, milling, other mechanical means, or gouging, as shown in Figure 2.

After that, as shown in Figure 3, the inner surface was soaked with running water, still water,
Overlay welding while cooling with various other cooling methods 3
to return it to its original shape as shown in Figure 1.

When performing the overlay welding shown in FIG. 5, a temperature difference occurs between the inner and outer surfaces as shown in FIG. 4.

Due to this temperature difference, thermal stress as shown by the solid curve α in FIG. 5 is generated during the overlay welding 5, and tensile plastic deformation occurs on the inner surface and compressive deformation on the outer surface.

After overlay welding 5, when it cools down and becomes uniform in temperature distribution,
The stress state is as shown by the broken line curve β in FIG. 5, with the inner surface being in compression and the outer surface being in tension.

As explained above, according to the method of the present invention, it is possible to reduce the tensile residual stress on the inner surface of the pipe near the welded joint and improve it to a compressive state, thereby improving stress corrosion cracking resistance and corrosion fatigue resistance. Can be done. Furthermore, according to the method of the present invention, after the outer surface of the bead is once removed, overlay is applied thereto.
The rfff is the same as the initial bead reinforcement shape, and as a result, plant maintenance can be performed satisfactorily without impairing flaw detection during the period of use performed on existing plants.

[Brief explanation of the drawing]

1 to 5 are schematic cross-sectional views showing one embodiment of the method of the present invention in the order of construction, and FIG. 4.5 is a chart for explaining the state of stress distribution that occurs during construction of the method of the present invention. Sub-Agents 1) Meikoku Agent Ryo Hagiwara - , Figure 11, Figure 2, Figure 3

Claims (1)

[Claims] A method for reducing the tensile residual stress on the inner surface of a tube and improving it to a compressive state is to first remove the outer surface of the tube and then perform overlay welding while cooling the inner surface of the tube. Mitigation method.