JPS58153731A - Method of reducing residual weld stress - Google Patents

Abstract

PURPOSE:To reduce residual stress in a welded joint without the formation of the sigma phase, by welding martensitic steel pieces together, melting the parent metal at the weld heat-affected zone of the welded joint and its vicinity, and then slowly cooling it. CONSTITUTION:The parent metal 1 of martensitic steel is welded by an austenitic welding material 2. The parent metal 1 at the weld heat-affected zone and its vicinity in about 2-3mm. apart from the weld beads is then melted by heating it over the width of about 10mm. along the weld line to transform it into austenite, in a manner such that the weld metal 2 is not melted. In succession, the parent metal is slowly cooled, so that residual stress in the welded joint along a direction making a right angle with the weld line is reduced without the formation of the sigma phase in the weld metal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a welded joint using a martensitic steel base material, and particularly to a method for reducing welding residual stress suitable for a welded joint using an austenitic steel as the weld metal material.

In martensitic steel, welded joints are often manufactured using austenitic weld metal materials from the viewpoint that weld cracking can be prevented with a low preheating temperature.

Austenitic weld metal materials have a larger coefficient of thermal expansion than ferritic and martensitic materials, and furthermore, no expansion occurs due to transformation during the cooling process.

For this reason, in a bath welded joint in which a martensitic steel base material is joined with an austenitic welded metal material, a large amount of residual stress is generated in the direction perpendicular to the weld line on the tensile side, which has a disadvantageous effect on the strength of the joint. A conventional method for reducing this welding residual stress is stress relief annealing heat treatment. However, for austenitic unwelded metals, δ is applied for the purpose of preventing hot cracking.
Ferrite is included, and if the weld metal ingot is exposed to a high temperature of 650°C or higher for a long time during stress relief annealing heat treatment, the δ ferrite changes to the σ phase and the weld metal becomes brittle. Ta.

An object of the present invention is to provide a method for reducing the residual stress in the direction perpendicular to the application tangent of a welded joint in which a base metal of martensitic steel is joined with an austenitic weld metal material without generating a σ phase in the weld metal. It is about providing.

That is, to summarize the present invention, in a welded joint in which a martensitic steel base material is joined with an austenitic weld metal material, after welding, the weld heat affected zone of the welded joint and the base metal in the vicinity The present invention relates to a method for reducing welding residual stress, characterized in that the welded joint is melted along the weld line direction to undergo austenite transformation, and then slowly cooled.

In welded joints in which martensitic steel is joined with austenitic weld metal material, bath welding residual stress increases, but welding residual stress in the direction perpendicular to the weld line, especially at the toe of reinforcement, reduces fatigue strength. Therefore, it becomes a problem. but,
The present invention focuses on the property that martensitic steel as a base metal expands greatly due to transformation during cooling after solidification by welding, and after joining the base metal with the weld metal material to form a welded joint, The weld heat-affected zone of the welded joint and the base metal in the vicinity thereof are melted along the weld line direction of the welded joint and transformed into austenite, and then gradually cooled to remove excess heat due to the expansion that occurs due to the transformation during cooling. This is designed to reduce the tensile residual stress in the direction perpendicular to the weld line near the weld toe.

Furthermore, when applying the present invention, as a method of melting and slowly cooling the base metal in the weld heat affected zone and its vicinity, two It is better to move two heat sources in the direction of the welding line, and to further melt a portion including at least a portion of the portion melted and solidified by the preceding heat source by the following heat source, and then slowly cool the melted and solidified portion. The cooling rate of the solidified portion can be slowed down, which is desirable from the viewpoint of improving toughness.

Next, the present invention will be specifically explained based on the accompanying drawings.

FIG. 1 is a schematic perspective view of a welded joint after conventional welding.

In FIG. 1, 1 indicates a base material and 2 indicates a weld metal.

FIG. 2 is a schematic perspective view of a welded joint treated according to one embodiment of the method of the present invention. In FIG. 2, 1 and 2 have the same meanings as in FIG. 1, and 3 indicates a weld bead for reducing residual stress.

In the process of the method of the present invention, as shown in FIG.
Heats the heat affected zone approximately 2-3 mm away from the bath weld bead without causing melting of the weld metal. As the heating method, a method using a tungsten electrode, a method using high frequency heating, etc. may be selected as appropriate. The weld bead shown in FIG. 2 may be formed on one side or only on one side depending on the shape of the weld joint, and its width may be about 10 mm.

The treatment according to the present invention is advantageous in that residual stress can be removed to a greater extent than in the case of conventional stress-relieving annealing heat treatment, and the operation is simple and costs can be reduced.

Hereinafter, the present invention will be explained in more detail with reference to Examples.
The present invention is not limited thereto.

In addition, FIG. 3 shows the thermal expansion curve of the base material used in Example 1,
FIG. 4 is a graph showing the thermal expansion curves of the base material used in Example 2 in relation to temperature (° C.) (horizontal axis) and thermal expansion strain (vertical axis).

Example 1 Table 1 shows the chemical components of the base material and weld metal materials used in this example.

The base material is martensitic 1 containing approximately 5% N1 by weight.
It was made of 3Cr stainless steel and had a plate thickness of 50 mm. The deposited metal material contains approximately 12% N1 and approximately 22% O by weight.
It is an austenitic material containing r. FIG. 3 shows the thermal expansion curve of the base material. When the base material reaches about 180° C. during cooling, it starts to transform and expands significantly.

Table 2 lists the welding elements applied to make the welded joints.

The preheating/bath temperature was 50°C. The welding method uses a rod diameter of 4m.
A covered arc welding method was used using a welding rod of 1.5 m. Two welded joints were prepared and the effect of reducing residual stress by the method of the present invention was investigated. One of the two welded joints has base metal 1 joined with weld metal 2 as shown in Figure 1, and the other has base metal 1 joined with weld metal 2 as shown in Figure 2. Welding bead 3 for reducing residual stress, which is processed by the residual stress reducing method of the present invention after joining with weld metal 2, and welding bead 3 for reducing residual stress is formed on the front and back surfaces of the weld joint. was formed in an inert gas atmosphere at a preheating temperature of 50°C by arranging two tungsten electrodes with a diameter of 32 mm in the direction of the bath tangent to the welded joint and keeping the distance between them constant at 15 mm. . The processing conditions shown in Table 3 were applied to all electrodes. Table 4 shows the results of measuring the residual stress in the direction perpendicular to the weld line in the weld heat affected zone near the toe of reinforcement of the welded joint using an electrical resistance strain gauge.

Measurements were performed at three points each at the center of the welded joint.

From the results in Table 4, the residual stress before treatment with the method of the present invention reaches 30 to 40 kgf/mm2, but the residual stress after treatment with the method of the present invention can be less than 10 kgf/mm2. confirmed.

Regarding the welded joint shown in the same Figure 1, stress relief annealing was performed by heating at 590°C ± 25°C for 3 hours, and the residual stress was measured in the same manner.
and 24 kgf/mm2, or about 20 kgf/mm
Two measurements were obtained.

Therefore, the residual stress due to the method of the present invention is 10 kgf/mm.
A value of 2 or less indicates a significant effect.

Example 2 Table 5 shows the chemical components of the base material and weld metal materials used in this example.

The base material was martensitic 13Cr stainless steel containing about 1% N1 by weight, and also had a plate thickness of 50 mm. The welded metal material is approximately 13% N1 by weight.
and an austenitic material containing about 24% Cr. FIG. 4 shows the thermal expansion curve of the base material, and the base material during cooling starts to transform when it reaches about 330°C, and Example 1
It expands considerably, although not as much as the base material.

Two welded joints were made in the same way, and the welding conditions were adjusted. The processing conditions for the residual stress reduction method of the present invention and the method for measuring welding residual stress were carried out in accordance with Example 1.

However, the preheating and interpass temperature for joint welding was 250°C, and the preheating during residual stress reduction treatment was 250°C. Table 6 shows the results of measuring the residual stress in the direction perpendicular to the weld line in the weld heat shadow near the toe of the welded joint.

As a result, it was confirmed that the welding residual force was 35 to 40 kgf/mm2 before being treated by the method of the present invention, but it was reduced to about 10 kgf/mm2 after being treated by the method of the present invention.

As is clear from the above examples and comparative examples, the method of the present invention has a remarkable effect in that the residual stress after welding can be significantly reduced with a simpler operation than the conventional method.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view of a welded joint after conventional welding. FIG. 2 is a schematic perspective view of a welded joint treated according to one embodiment of the method of the present invention. FIG. 3 is a graph showing the thermal expansion curve of the base material used in Example 1, and FIG. 4 is a graph showing the thermal expansion curve of the base material used in Example 2 in relation to temperature and thermal expansion strain. . 1: Base metal, 2: Weld metal, 3: Weld bead for reducing residual stress

Claims (1)

[Claims] 1. In a welded joint in which a martensitic steel base material is joined with an austenitic weld metal control material, after welding,
A method for reducing welding residual stress, which comprises melting the weld heat affected zone of the welded joint and the fJ area in the vicinity thereof along the weld line direction of the welded joint to transform it into austenite, and then slowly cooling it. 2. As a method of melting and slowly cooling the base metal in the weld heat affected zone and its vicinity, two heat sources that are lined up one after the other in the direct line direction of the weld joint and kept at a constant distance are moved in the weld line direction. The method for reducing welding residual stress according to claim 1, wherein at least a portion of the portion melted and solidified by the preceding heat source is further melted by the subsequent heat source, and then slowly cooled.