JPH0317234A - Method for preventing integranular corrosion of stainless steel - Google Patents

Abstract

PURPOSE:To desensitize the sensitized region of the weld heat-affected zone of stainless steel and to prevent intergranular corrosion by melting the sensitized region to a specified depth by irradiation with laser beams, rapidly solidifying and cooling the molten region at a specified cooling rate. CONSTITUTION:The sensitized region (Cr-deficient layer) of the weld heat- affected zone of stainless steel is irradiated with laser or electron beams to melt the surface layer to >=50mum depth and the molten layer is rapidly solidified and cooled at 10 deg.C/sec cooling rate. The sensitized region can be desensitized and intergranular corrosion is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a method for preventing intergranular corrosion of, for example, austenitic stainless steel.

(Prior Art) Welded parts of austenitic stainless steel, such as SUS304 stainless steel, which has become sensitized during manufacturing by welding, are susceptible to intergranular corrosion in environments such as seawater and high-temperature pure water. In particular, it is necessary to eliminate the sensitization of the weld heat-affected zone on the surface in contact with corrosive fluid after welding, and to prevent intergranular corrosion.

Austenitic stainless steel, e.g. SUS304 steel,
According to the JIS standard, SUS316 steel contains 0.08% or less of C by weight. These stainless steels are approximately 550
When kept at a temperature between ℃ and 850℃ for a certain period of time, Cr23C6 carbide is precipitated at the grain boundaries, and Cr, which is normally contained at 18-20% by weight, near the grain boundaries is the same <1
A region (Cr-deficient layer) where the amount of Cr becomes 3% or less is generated. It is known that corrosion resistance in this region is poor, making corrosion along grain boundaries (intergranular corrosion) or cracking more likely to occur.
They also often experience damage due to this.

As mentioned above, the formation of Cr-depleted layers along grain boundaries, which makes intergranular corrosion more likely to occur, is called sensitization, and it is well known that this sensitization occurs in the weld heat-affected zone in austenitic stainless steel. be.

As a method to prevent damage caused by such sensitization of the weld heat affected zone, solution heat treatment (usually 105
After heating and holding at a temperature of 0°C to 1150°C, the carbide precipitated at the grain boundaries is re-dissolved by rapid cooling), or metals that do not become sensitized by welding are adjusted to the range that will be affected by heat before welding. Conventionally, methods have been developed to join these materials together.

(Problems to be Solved by the Invention) However, in the former case, these methods involve a step of removing oxidized scale that is deposited on the entire pipe by, for example, solid solution treatment.

In addition, distortion occurs due to solution heat treatment. Further, there are problems such as the inability to apply this method to welded structures having large or complicated shapes, and the cost also increases.

On the other hand, in the latter case as well, a step is included in which the laminates are joined in advance and the sensitization caused by joining is eliminated by solution heat treatment, which inevitably increases the cost significantly.

Furthermore, what is particularly important is that both of the above methods finish the production;
There are issues that cannot be applied to those that have been installed or put into service.

The present invention has been made to solve the above-mentioned problems, and is relatively inexpensive and can be applied even to products that have been manufactured and installed. The objective is to provide a method for preventing this.

[Structure of the Invention] (Means for Solving the Problems) The present invention melts the sensitized region of the weld heat affected zone of stainless steel by laser or electron beam irradiation at a depth of 50 am or more in the surface layer, and The above method is characterized by rapid solidification and cooling.

(Function) In the present invention, intergranular corrosion or similar damage occurs when the sensitized portion is directly exposed to a corrosive medium, that is, unless the surface layer exposed to the corrosive medium is sensitized. This is based on the knowledge that damage can be prevented and that sensitization can be eliminated by melting, solidifying, and rapid cooling instead of solution heat treatment.

That is, the present invention irradiates the surface of sensitized stainless steel with a laser or an electron beam to remove 501I of the surface layer.
By melting a region of m or more and solidifying it as it is, the sensitization of the surface layer is eliminated and intergranular corrosion is prevented.

Here, using a laser or electron beam as a melting heat source has a high incident energy density, resulting in rapid melting, solidification, and
This is because cooling is possible.

What is important here is that a uniformly melted and solidified layer of 50 μm or more is obtained, and that the cooling rate is high so that the area adjacent to the treated area becomes sensitized during the melting and solidifying process and during its cooling, as well as during the cooling of the solidified area. Don't do it. Practical heat input conditions can be determined by taking into account the thickness and temperature of the stainless steel to be treated. In this case, the cooling rate is 10°C/! on average up to 500°C in the solidified part and the adjacent base metal part. u
It is sufficient if it is equal to or higher than c.

On the other hand, the depth needs to be at least one grain or more deeper than the depth of the molten solidified part, or more than the surface irregularities caused by mirror dulling and pickling during stainless steel manufacturing. For a stainless steel pipe of several 100 μm, several 10 tw+ is sufficient.

The reason why the cooling rate was selected to be 10° C./Sec or more is as follows.

In other words, even if stainless steel is melted to eliminate the sensitized parts on its surface, if the cooling rate after melting is slow, Cr will regenerate during cooling.
Carbides may precipitate and cause sensitization. Usually, the temperature at which sensitization occurs is between 500 and 850°C, so the faster the cooling rate at 50 [1'C or higher during cooling, the better. The cooling rate required to prevent sensitization during cooling varies slightly depending on construction conditions such as plate thickness, temperature, and human heat, as well as the metallic structure of the material.

(Example) An example in which the surface of sensitized SUS304 stainless steel was melted and solidified by YAG laser irradiation will be described.

Stainless steel is weight%, C: 0.058%, Cr: l
8.6%, Ni: 8.7%, the balance contains Fe and some unavoidable impurities, and the plate thickness is 5+
It is nm. This stainless steel was heat-treated at 1100° C. for 15 minutes, cooled to room temperature by water cooling, further heated and held at 620° C. for 24 hours, and then heat-treated by cooling in the atmosphere to prepare a test plate. After irradiating this test plate with a YAG laser under predetermined conditions, it was cut into the dimensions shown in Figures 2 (a) and (b), and the cross section was polished.
Electrolytic etching was performed in a % oxalic acid solution, and the metal structure of the test piece 1 and the depth of the molten solidified layer 2 were examined using an optical microscope.

As a result, it was confirmed that the structure of the unmelted/solution-treated portion 3 of the test piece 1 was the same as that of the weld heat affected zone, and was that of sensitized stainless steel.

Further, FIG. 1 shows the relationship between the depth of the molten solidified layer 2 and the heat input conditions by laser irradiation. As the amount of human heat generated by the laser beam increases, the depth of the molten solidified layer also increases. In addition, test pieces 1 were taken from near the surface of these test plates as shown in Figure 2 (0 and (b)), and JISGO5
The intergranular corrosion test specified in 75 was conducted. That is, test piece 1 was mixed with a test solution (100 ml of sulfuric acid and 1 ml of copper sulfate in deionized water).
00g dissolved to make 1000ml),
Boiling continued for 16 hours. After this, the test piece {is taken out and bent with a radius of 4 so that the surface of the molten solidified layer 2 on the surface is on the outside.
Bending with mlI1, bent outer surface (melted solidified layer on the surface)
The presence or absence of cracks due to intergranular corrosion was examined using an optical microscope (IGO ~ 2 fl [l times)]. The results are shown in the table.

The table shows the heat input 1 molten solidified layer depth and intergranular corrosion test results for test specimens 1 to 7.

Note that test piece 1 shows an example in which no laser irradiation was performed.

As is clear from the table, the specimen without surface melting and solidification treatment (
1) and a test piece (2) with a melted solidified layer depth of 3G + 1111
), cracks were observed, indicating susceptibility to intergranular corrosion.

In contrast, the treatment depth for test pieces (3) to (7) was 5 lb.
In the case of +m or more, there was no cracking, and it was recognized that the susceptibility to intergranular corrosion on this surface had been resolved.

In this example, Ar gas was used as a shielding gas for the surface melting and solidification treatment by laser 1, and the laser output was set to +3QOW.

The key point of the present invention is to eliminate sensitization through rapid melting and solidification treatment of the surface, so it can be applied not only to 304 stainless steel but also to SUS316, SUS347, SUS32
1 steel, such as NCF600.
The same applies to base alloys.

In addition, the present invention is effective not only at welded parts of pipes made of stainless steel, but also at liquid-contacted parts of containers and welded parts between containers and pipes, and as shown in FIG. It is particularly suitable for application to the sensitized part of the inner surface of a tube in a structure in which one hole is welded.

That is, FIG. 3 shows a longitudinal section of a structure in which a stainless steel pipe 5 passes through the wall of a container 4 and is attached to the container 4 by a weld metal 6.

In such a structure, the stainless steel pipe close to the weld metal 6 becomes sensitized and forms a sensitized region.

When the inner surface of the stainless steel tube 5 comes into contact with seawater or high-temperature pure water, the sensitized region 7 may undergo intergranular corrosion or intergranular stress corrosion cracking.

Therefore, a molten solidified layer 8 is formed on a portion of the inner surface of the stainless steel pipe 3 corresponding to the sensitized region 7 by laser irradiation.

This molten solidified layer 8 can prevent intergranular corrosion or intergranular stress corrosion cracking. As is clear from this example, the method of the present invention can be applied to products that have been manufactured and installed, or to products that have begun to be put into service.

[Effects of the Invention] According to the present invention, sensitization of the weld heat affected zone of stainless steel can be eliminated and intergranular corrosion can be prevented. Furthermore, intergranular corrosion of the sensitized portion of the stainless steel can be prevented even after welding of the stainless steel structure or after the start of service.

[Brief explanation of drawings]

1 to 3 show examples of the method according to the present invention. 2(b) is a longitudinal sectional view showing the length direction of the test piece, and FIG. 3 is a partial sectional view showing an example of welding a stainless steel pipe to a container.1. ...Test piece 2...Melted solidified layer 3...Unmelted/solutionized portion 4...Container 5...Stainless steel tube 6...Molten metal 7...Sensitized region 8...Surface Melt solidification treatment layer

Claims (1)

[Claims] The sensitized area of the weld heat affected zone of stainless steel is
A method for preventing intergranular corrosion of stainless steel, which is characterized by melting by laser or electron beam irradiation at a depth of .mu.m or more and rapidly solidifying and cooling at 10.degree. C./Sec or more.