JP4298825B2 - High corrosion resistance stainless steel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high corrosion resistance stainless steel used in an environment affected by radiation, and more particularly to a high corrosion resistance stainless steel used in a nuclear environment such as a fusion reactor or a nuclear reactor or in a space environment.
[0002]
[Prior art]
Until now, SUS304 stainless steel, SUS316 stainless steel, and the like, which have high corrosion resistance, have been used as structural materials for nuclear reactors. In recent years, low carbon steel SUS304L stainless steel and SUS316L stainless steel have been used from the viewpoint of corrosion due to thermal sensitization.
[0003]
However, in a radiation environment, particularly in an environment where neutrons are irradiated for a long period of time, Cr at the grain boundary is deficient, and a decrease in corrosion resistance due to this may cause grain boundary cracking.
[0004]
[Problems to be solved by the invention]
Taking nuclear reactors as an example, nearly 30 years have passed since the first nuclear reactor was built in Japan. During this period, many nuclear power plants were built, and now about 50 nuclear reactors are in operation. It has come to bear about 30%.
[0005]
The longer a nuclear power plant can be used, the more it can save on the cost of rebuilding and new construction, and it will be very advantageous from an economic point of view. Therefore, extending the life of nuclear reactors, which are the heart of nuclear power plants and are exposed to corrosive environments, is an important issue.
[0006]
In view of the above, an object of the present invention is to provide a stainless steel that can maintain high corrosion resistance over a long period of time even in a radiation environment.
[0007]
[Means for Solving the Problems]
The gist of the present invention that achieves the above object is as follows.
[0008]
[1] A highly corrosion-resistant stainless steel characterized in that a concentrated layer of Mo is formed at grain boundaries.
[0009]
[2] A highly corrosion-resistant stainless steel in which a concentrated layer of Mo is formed at grain boundaries by heat treatment.
[0010]
[3] A highly corrosion-resistant stainless steel in which a concentrated layer of Mo is formed at grain boundaries by heat treatment at a temperature of 550 to 800 ° C. for 5 minutes or more.
[0011]
[4] The high corrosion resistance stainless steel, wherein the mechanical properties of the material in which the Mo enriched layer is formed at the grain boundaries are substantially the same as the mechanical properties before the formation.
[0012]
[5] The high corrosion resistance stainless steel as described above, wherein the Mo concentrated layer formed at the grain boundary has a width of 20 nm or less.
[0013]
[6] The high corrosion resistance stainless steel as described above, in which precipitates due to Mo and C do not exist at grain boundaries.
[0014]
[7] The high corrosion resistance stainless steel as described above, wherein the Mo composition ratio is within a range defined by the standard of SUS316L stainless steel.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The reason why the corrosion resistance of stainless steel is reduced in a radiation environment, particularly in an environment where neutron irradiation is applied, is because Cr related to corrosion resistance is deficient from the grain boundary due to irradiation.
[0016]
Therefore, in order to suppress the grain boundary Cr deficiency under irradiation, Mo is concentrated at the grain boundary of stainless steel. That is, by applying heat treatment to stainless steel having Mo as one of its compositions, a Mo concentrated layer is formed at the grain boundaries. The heat treatment conditions depend on the amount of Mo added, the temperature, and the time, and remain at the stage before the Mo precipitate is formed.
[0017]
As shown in FIG. 1, the stainless steel 1 exhibits high corrosion resistance because the surface is covered with a Cr passive oxide film 2 which is a passive film. Normally, even if this coating breaks for some reason in a corrosive environment, Cr is replenished from the inside of the stainless steel and instantly regenerated.
[0018]
However, under a radiation environment, particularly in an environment where neutron irradiation is applied for a long time, when the Cr is depleted from the grain boundary 3 due to irradiation and the Cr-depleted layer 4 is formed near the grain boundary, the grain boundary reaches the surface. When the film breaks at the portion where the film is formed, Cr is not replenished and the passive oxide film is not regenerated, Fe ions 5 which are main constituent elements are eluted, and intergranular corrosion 6 occurs.
[0019]
This phenomenon will be described at the atomic level with reference to FIG. The neutron 7 incident on the stainless steel 1 generates a force scale 8 and excessive lattice defects such as vacancies (V) and interstitial atoms. Among these, annihilation occurs due to recombination 9 of some vacancies (V) and interstitial atoms, and the rest exist as excessive lattice defects. This state is thermally unstable.
[0020]
Many of the interstitial atoms are bonded in a loop before flowing into the grain boundary, and a large number of interstitial dislocation loops 10 are formed. The vacancies (V) flow into the grain boundaries 3 of the sink. In order for the vacancies (V) to diffuse and flow into the grain boundary, the position must be exchanged with other atoms. On the grain boundary, Cr atoms larger than the solvent atoms are selectively vacant (V) due to the size effect. ), And the so-called reverse Kirkendall effect is generated, which diffuses in the direction opposite to the diffusion direction of the holes (V). The grain boundary Cr deficiency 11 due to the size effect and the reverse Kirkendall effect causes Cr is deficient.
[0021]
When the stainless steel 1 is placed in a corrosive environment, even if the passive film on the surface is damaged for some reason, a new film is instantaneously formed. When the ratio is less than about%, the film cannot be easily regenerated, resulting in a decrease in corrosion resistance.
[0022]
Therefore, in order to suppress such grain boundary Cr deficiency under irradiation, in the present invention, a Mo concentrated layer is formed at the grain boundary by heat treatment. Mo is an element that improves the corrosion resistance and is added to the SUS316 stainless steel in an amount of 2 to 3% by weight.
[0023]
However, as shown in FIG. 3, since Mo is an oversized atom like Cr, the position of the grain boundary Mo due to the size effect and the inverse Kirkendall effect is exchanged with the vacancies (V) under irradiation as with Cr. The deficiency 12 moves in the direction of deficiency from the grain boundary.
[0024]
The atomic number balance at the grain boundary is as shown in equation [1]:
[0025]
[Expression 1]
J _v = J _Cr + J _Mo (1)
The number J _{Cr of} Cr atoms flowing out from the grain boundary and the number J _{Mo of} Mo atoms flowing out of the grain boundary are equal to the number J _v of holes (V) flowing into the grain boundary from the principle of the inverse Kirkendall effect. If the number of Mo atoms at the grain boundary is large, the number J _{Cr of} Cr atoms flowing out from the grain boundary is relatively reduced.
[0026]
Therefore, if a Mo enriched layer is formed at the grain boundary according to the present invention, the lack of Cr from the grain boundary due to irradiation can be suppressed, and as a result, a stainless steel that can maintain high corrosion resistance over a long period of time. Obtainable.
[0027]
Heat treatment is used as a method for concentrating Mo to the grain boundaries. In stainless steel to which a certain concentration of Mo is added, Laves phase or χ phase, which is an intermetallic compound of Mo and Fe, precipitates at grain boundaries when heat treatment is performed at a temperature of 550 ° C to 800 ° C for a certain time (more than 5 minutes). To do.
[0028]
When such precipitation occurs, the corrosion resistance is lowered. Therefore, the temperature and time are adjusted in consideration of the Mo concentration so that the heat treatment is kept at the precursor stage where precipitation occurs and Mo is only concentrated at the grain boundary.
[0029]
【Example】
Examples of the present invention will be described below.
[0030]
[Example 1]
In order to form a concentrated layer of Mo at the grain boundaries of SUS316L stainless steel, the sample (10 mm × 10 mm × 5 mmt) was heat-treated in the atmosphere using a pine-full electric furnace.
[0031]
First, the temperature of the electric furnace was raised to 650 ° C., and after the temperature had stabilized for a certain time, the sample was put in. After 1 hour, the sample was taken out of the electric furnace and rapidly cooled in water.
[0032]
In order to confirm that Mo is concentrated in the grain boundary of the above sample, the sample is processed into a transmission electron microscope sample, and the particles are analyzed using a field emission transmission electron microscope and an energy dispersive X-ray spectroscopic analyzer attached thereto. The boundary composition was analyzed. Processing into the transmission electron microscope sample was performed according to the following procedure.
[0033]
First, a thin plate having a thickness of 0.5 mm was cut out from the sample after the heat treatment with a low-speed precision cutting machine. This was mechanically polished to reduce the thickness to 0.2 mm. Thereafter, a 3 mmφ sample was punched with a punching jig. The sample of 3 mmφ × thickness 0.2 mm thus produced was thinned by electrolytic polishing (twin jet method) to obtain a transmission electron microscope sample.
[0034]
The concentration distribution in the vicinity of the grain boundary of this sample was analyzed using a field emission transmission electron microscope and an attached energy dispersive X-ray spectrometer. The diameter of the electron beam probe at the time of analysis is approximately 1 nm. The analysis results are shown in FIG. For comparison, the result of the same analysis is also shown for the sample before heat treatment (unheat-treated material).
[0035]
As is apparent from the figure, Mo was significantly concentrated at the grain boundaries of the heat-treated material (650 ° C. × 1 h) as compared to the unheat-treated material sample.
[0036]
It can be seen that the concentration of Mo at the grain boundary at this time is about 13% by weight with respect to about 5% by weight in the grain, and has a concentration of about 260%. In addition, the width of the concentrated layer at this time was very narrow at 10 nm or less. As a result of observation with a transmission electron microscope, formation of precipitates at the grain boundaries was not observed.
[0037]
Example 2
In order to verify that grain boundary Mo concentration can suppress grain boundary Cr deficiency due to neutron irradiation and maintain high corrosion resistance up to high irradiation dose, the change in grain boundary concentration with respect to neutron irradiation dose was calculated.
[0038]
In the case of SUS316L stainless steel, a better correlation is obtained when comparing the corrosion resistance with the sum of the Cr concentration and the Mo concentration at the grain boundary than when comparing only with the Cr concentration at the grain boundary. Therefore, the change in the grain boundary concentration with respect to the irradiation amount was performed by the sum of the Cr concentration and the Mo concentration.
[0039]
Grain boundary Cr + Mo concentration when the initial grain boundary Mo concentration is three types of 0 wt%, 10 wt%, and 15 wt%, and the irradiation amount is 1 × 10 ²⁴ to 1 × 10 ²⁶ (n / m ² ). Is calculated by an irradiation-induced segregation program, and the results are shown in FIG.
[0040]
The black circle is the initial concentration of 15% by weight of Mo at the grain boundary, the white circle is the initial concentration of 10% by weight of Mo at the grain boundary, and the black square is the initial concentration of Mo at the grain boundary. It is a result when there is no.
[0041]
Under any of the calculation conditions, the grain boundary Cr + Mo concentration decreased with increasing dose. However, it was found that the higher the initial Mo concentration, the higher the grain boundary Cr + Mo concentration even when the irradiation amount was increased. As described above, since the grain boundary Cr + Mo concentration shows a good correlation with the corrosion resistance, the higher the initial grain boundary Cr + Mo concentration, the higher the effect of suppressing the decrease in the corrosion resistance accompanying the increase in the dose.
[0042]
【The invention's effect】
According to the present invention, it is possible to obtain stainless steel that keeps high corrosion resistance for a long period of time compared to conventional materials in an environment exposed to radiation over a long period of time. By using this, it is possible to extend the life of a nuclear reactor. The economic effect is very large because it can be done and the cost of renewal and new construction of nuclear power plants can be saved.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic cross-sectional view illustrating how corrosion resistance is reduced due to grain boundary Cr deficiency.
FIG. 2 is an explanatory diagram of the behavior of atoms under irradiation.
FIG. 3 is an explanatory diagram of an effect of suppressing Cr deficiency due to grain boundary Mo concentration.
FIG. 4 is a graph showing a result of Mo concentration to grain boundaries by heat treatment.
FIG. 5 is a graph of the amount of Mo enrichment affecting the change in grain boundary concentration.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Stainless steel, 2 ... Cr passive state oxide film, 3 ... Grain boundary, 4 ... Cr deficient layer, 5 ... Fe ion, 6 ... Grain boundary corrosion, 7 ... Neutron, 8 ... Cascade, 9 ... Vacancy (V) And recombination of interstitial atoms, 10 ... interstitial dislocation loop, 11 ... depletion of grain boundary Cr due to size effect and inverse Kirkendall effect, 12 ... depletion of grain boundary Mo due to size effect and inverse Kirkendall effect.

Claims (2)

Stainless steel used in an environment where neutrons are irradiated. Heat treatment is performed at a temperature of 550 to 800 ° C. for 5 minutes or more and then rapidly cooled in water, so that there is no precipitation of Laves phase and χ phase at the grain boundary, and Mo concentration high corrosion resistance stainless steel, characterized in Rukoto such than SUS316L stainless steel layer is formed. The high corrosion-resistant stainless steel according to claim 1, wherein the Mo concentrated layer formed at the grain boundary has a width of 20 nm or less .

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