JPH0424421B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is applicable to high-temperature environments subject to neutron irradiation, such as fast breeder reactors (FBR) and nuclear fusion reactors (FER).
Regarding heat-resistant alloys used as structural materials for nuclear reactor pressure vessels, this paper proposes an advantageous manufacturing method for super-heat-resistant alloys that have particularly excellent resistance to neutron irradiation embrittlement. (Prior Art) Conventionally, heat-resistant alloys containing large amounts of Cr, Ni, etc. have been used as structural materials for nuclear reactors such as FBR and FER. However, it is known that such heat-resistant alloys contain 2 to 3 ppm of boron B as an alloying component (even if the addition is not intended, it is inevitably mixed in as an impurity during the normal steelmaking process). It is being Such unavoidably mixed boron or boron added as an alloy component causes the following problems under neutron irradiation conditions. In other words, natural boron contains two radioactive isotopes, ¹⁰ B
and ¹¹ B, of which ¹⁰ B
When irradiated with neutrons, ¹⁰ B (n, α) ⁷ Li nuclear reaction occurs, and ¹⁰ B decays to produce He gas. the result,
It is known that when He gas is generated from boron compounds, which tend to be unevenly distributed at grain boundaries, the bonding strength of the grain boundaries is weakened, causing so-called creep embrittlement. “Journal of Nuclear Materials”
Nuclear Materials) 16〈'65〉68-73'' Several techniques have been proposed to prevent the above-mentioned creep embrittlement, for example, Japanese Patent Application Laid-Open No. 53-88499.
No. 103 of the Journal of Materials proposes reducing the absolute amount of ¹⁰ B in alloys.
104 (1981) p845'' proposes a method to trap the generated He and prevent its aggregation. (Problems to be Solved by the Invention) The problems to be solved by the present invention are the problems faced by the ^above -mentioned prior art. However, the disadvantages are that the cost increases and the manufacturing process becomes complicated (Japanese Patent Application Laid-open No. 53-88499), and that it is difficult to put it into practical use (He trap method). When ^heat -resistant alloys containing
α) The purpose of the present invention is to propose an advantageous method for manufacturing products that can effectively avoid He embrittlement due to nuclear reactions at low cost and in a practical manner without going through complicated processes. (Means for solving the problem) The present inventors investigated the influence of boron using a heat-resistant alloy obtained through a normal manufacturing process (in which a predetermined amount of boron is unavoidably mixed). . As a result, they discovered that even if a heat-resistant alloy contains ¹⁰ B, it will not become brittle unless it exists in solid solution at the grain boundaries. We came up with the idea of precipitating and dispersing it within the interior. Moreover, if the location of B is dispersed within the grain at an early stage, even if the above nuclear transmutation causes He
Even if gas is generated, it occurs within the grains, so the adverse effect on high-temperature strength is small. Of course, there is a possibility that the generated He moves to the grain boundaries and aggregates, but the speed of this movement is much slower than in the normal grain boundary type.
As a result, it was predicted that the creep embrittlement phenomenon would shift to the long-term or heavy irradiation side, and the high-temperature strength would be improved. Therefore, regarding a heat-resistant alloy with a predetermined composition,
We investigated the grain boundary segregation and precipitation behavior of boron and focused on heat treatment techniques to finely disperse boron within the grains, and found that the following method was suitable. In other words, as a result of studying the heat treatment conditions for heat-resistant alloys having compositions within the following range, we found that casting,
After forging or rolling, or during forging or rolling,
After processing with an equivalent strain of 5% or more in a temperature range of 900℃ or less, hold the material at a temperature of 700 to 900℃ for 5 minutes or more to precipitate B nitride, and then cool it down or not. If the material is then heat treated at a temperature of 800 to 1000°C, which is higher than the B nitride precipitation treatment temperature, for 1 minute or more, it is possible to disperse boron within the grains and reduce the amount of B at the grain boundaries. It turned out that. The composition of the heat-resistant alloy to which the present invention is applied is as follows:
0.002wt%≦C≦0.5wt%, Si≦2.0wt%, Mn≦
2.0wt%, 9wt%≦Cr≦30wt%, Ni≧20wt%,
Ti≦0.5wt%, Al≦0.5wt%, Fe≦50wt%, P≦
0.04wt% and S≦0.01wt%, and for N and B, in relation to Ti, Al and C, when Ti≦4・C, N≧Al/2 B≦N−Al/2 Ti> 4.When C, N≧
Al/2+(Ti-4・C)/4 B≦N-Al/2-(Ti-4・
C)/4. Note that the above heat treatment (position control of B) for the B-containing alloy according to the present invention is not related to ¹⁰ B or ¹¹ B, so ¹⁰ B, which causes creep embrittlement, is
Naturally, it acts effectively in precipitating and dispersing the particles within the grains. (Function) First, the range of the composition of the heat-resistant alloy to which the present invention is applied and the reason for the limitation will be described. C: The content of C is 0.002wt% (hereinafter simply "%")
), it is difficult to obtain the necessary high-temperature strength as a heat-resistant material. Also,
The higher the amount of C, the higher the high temperature strength, but the effect is saturated even if it is added in excess of 0.5%. Therefore, the range was set to 0.002% or more and 0.5% or less. Si: Si is an element that improves oxidation resistance, but 2
When added in excess of %, intermetallic compounds such as σ phase tend to precipitate. Therefore, the upper limit was set at 2%. Mn: Mn is an element that stabilizes austenite and is used as a substitute for Ni. However, when added in excess of 2%, intermetallic compounds such as σ phase tend to precipitate. Therefore,
The upper limit was set at 2%. P; P is an element that inhibits hot workability, and is 0.04
%, hot working such as forging and rolling becomes difficult. Therefore, the upper limit was set at 0.04%. S: Like P, S is an element that inhibits hot workability, and when it exceeds 0.01%, hot working such as forging and rolling becomes difficult. Therefore, the upper limit was set at 0.01%. Cr: Cr is an element that improves oxidation resistance,
If it is less than 9%, there is no effect, and if it is added in excess of 30%, the effect is saturated and the precipitation of the σ phase becomes easy. Ni: Ni is an element that stabilizes austenite, and if its content is low, a ferrite layer will appear and the high temperature strength will decrease. Therefore, it was set at 20% or more. Fe; Fe is a metal that reduces high temperature strength, and 50
If it is added in excess of %, the strength will drop significantly. Therefore, it was set to 50% or less. Al: Al is usually added as a deoxidizing agent, but 0.5%
If added in excess of this amount, the amount of N required to precipitate as BN will decrease, reducing the effect of the present invention, so the upper limit was set at 0.5%. Ti: Ti is added to fix carbon, but
When added in excess of 0.5%, the amount of N required to precipitate as BN decreases, reducing the effect of the present invention. Next, regarding the amount of N and B, Ti and Al
In connection with this, it is limited as follows. When Ti≦4・C…N≧Al/2 B≦N−Al/2 When Ti>4・C…N≧
Al/2+(Ti-4・C)/4 B≦N
-Al/2-(Ti-4・C)/
4. The upper and lower limits for the amount of N are such that N is an element that easily combines with Al and Ti to form nitrides. Therefore, if the amount is less than the above amount in relation to Ti and Al, the effective amount of N to be combined as BN will be insufficient. Note that Ti is an effective element for fixing N, and the remaining Ti that has become TiC becomes TiN. Accordingly
Ti-4·C is the amount of Ti that fixes N. Therefore, in the present invention, when Ti-4・C becomes negative, there is virtually no Ti to fix N, so the effects on N and B are different, so we considered them separately as above. . Similarly, the amount of B is also limited as described above, since if it exceeds the above amount, it will not be fixed as BN and will tend to segregate as B alone at grain boundaries. Next, heat treatment conditions, which are a feature of the present invention, will be explained. The reason for performing processing of 5% or more at a temperature of 900℃ or less is to first perform boron nitride precipitation treatment at a temperature of 700 to 900℃, and then process it at a temperature of 800℃ to 800℃ to move the grain boundaries.
Heat treatment is performed at a temperature of 1000℃, but if the temperature is
If the temperature exceeds 900°C, grain boundary movement will be small even if processing is applied, and grain boundary movement will be small even if the processing amount is less than 5%. This is because as a result, the effect of fixing BN within the grains becomes smaller. Therefore, the initial heat treatment was performed at a temperature of 900° C. or less and a processing of 5% or more. Next, the alloy that has undergone initial heat treatment has a 700 to 900
℃ temperature for 5 minutes or more. This processing temperature and time limit is outside these ranges.
BN does not precipitate, especially when held for more than 5 minutes,
This is because BN does not precipitate in less than 5 minutes even in the above temperature range. Note that the BN precipitation treatment temperature is desirably lower than the grain boundary migration temperature. This is because if heat treatment is performed below the grain boundary migration temperature, BN will precipitate at newly formed grain boundaries, reducing the effect. Finally, heat treatment is performed at a temperature of 800°C to 1000°C for more than 1 minute because B is an element that tends to segregate at grain boundaries. There is. Therefore, grain boundaries are reliably moved by performing heat treatment at 800°C to 1000°C for 1 minute or more. At 800°C for less than 1 minute, grain boundary movement does not occur, and at temperatures exceeding 1000°C, BN decomposes and segregates at grain boundaries again. Therefore, 800℃~1000℃,
Limited to 1 minute or more. (Example) For the three types of heat-resistant alloys shown in Tables 1 and 2, 100 kg small ingots were melted in a vacuum melting furnace, and 25 mm
Hot rolled to a thickness of , final rolling pass temperature 850℃,
The rolling amount at that time was set to 2.5% and 5% as equivalent strain amounts at 1000°C, and the precipitation treatment shown in Table 3 was immediately performed, followed by a reheat treatment for grain boundary movement. In addition, in order to capture the changes in the location ^{of 10} B due to heat treatment, we used the fission tracking method (FTE method). The presence of ¹⁰ B was imaged using a method for analyzing the position of ¹⁰ B, and the ratio of the amounts of images within the grain to the grain boundaries was quantified using the area ratio. As can be seen from Table 3, ¹⁰ B present at grain boundaries was reduced only in the production method of the present invention. Therefore, in the alloy obtained by this method, even under a neutron irradiation environment, the amount of He gas generated at the grain boundaries due to the nuclear reaction of ¹⁰ B(n, α) ⁷ Li is small;
It was confirmed that embrittlement could be reduced. Furthermore, as a result of identifying B precipitates within grains,
These turned out to be BN.

【table】

【table】

[Table] (Effects of the Invention) As explained above, according to the present invention, a heat-resistant alloy that does not undergo creep embrittlement even when subjected to neutron irradiation can be produced relatively easily and at low cost. .

Claims (1)

[Claims] 1 0.002wt%≦C≦0.5wt%, Si≦2.0wt%, Mn
≦2.0wt%, 9wt%≦Cr≦30wt%, Ni≧20wt%,
Ti≦0.5wt%, Al≦0.5wt%, Fe≦50wt%, P≦
0.04wt% and S≦0.01wt%, and for N and B, in relation to Ti, Al and C, when Ti≦4・C, N≧Al/2 B≦N−Al/2 Ti> 4.When C, N≧
Al/2+(Ti-4・C)/4 B≦N-Al/2-(Ti-4・
C)/4 is processed at a temperature range of 900°C or lower with an equivalent strain of 5% or more, held at a temperature of 700 to 900°C for 5 minutes or more, and then processed at a temperature of 800 to A method for producing a heat-resistant alloy with excellent neutron irradiation embrittlement resistance, characterized by performing heat treatment by heating to a temperature of 1000°C and holding it for 1 minute or more.