JPH0442462B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a Ni-based alloy member that improves stress corrosion cracking resistance without reducing the strength of a precipitation-hardened Ni-based alloy, and a heat treatment method for the same. Precipitation hardening Ni-based alloys are often used in springs, bolts, pins, springs, etc. for nuclear power plants.
However, when these alloys are subjected to normal heat treatment and used, stress corrosion cracking (hereinafter referred to as SCC) is likely to occur from the grain boundaries in a corrosive environment.
It turned out that it would happen. For example, among these alloys, the high-strength members for medium and high temperatures that are most frequently used for the above applications are American alloys with the chemical composition shown in Table 1.
UNS standard (Unified Numbering System for
Metals and alloys) NO7750 alloy (product name
Inconel X-750). This UNS−NO7750
The AMS standard (Aero Space Material
Specification). Conventionally, this alloy has been aged only at 1350〓 (732℃) x 16hAC of AMS5698, or after solution treatment at 955℃ and aging at 715℃~
Aging at 732°C for 16 hours, solution treatment at 982°C followed by heat treatment of 732°C x 8h → furnace cooling → 621°C x xh air cooling (8 ^h + furnace cooling + x ^h = 18 ^h ), and recently new It is used after solution treatment at a temperature of 1000 to 1250℃, which was proposed in The problem is that it is easy to cause

[Table] An object of the present invention is to provide a Ni-based alloy member and a heat treatment method thereof that eliminate the drawbacks of the conventional member and heat treatment method described above and improve SCC resistance without significantly reducing the strength level. That is, the SCC-resistant Ni-based alloy member of the present invention is
Weight ratio: C: 0.08 or less, Mn: 1% or less, Cr: 14
% or more and 42% or less, Fe: 5% or more and 33% or less, Ti:
2% or more and 30% or less, Nb: 0.5% or more and 28.6% or less,
Al: Contains 0.4% or more and 28.5% or less, the remainder is substantially
In precipitation-hardened Ni-based alloy members that are made of Ni and precipitate γ′ through aging treatment, the grain boundaries on the metallographic structure are formed into a zigzag shape, and the precipitates aligned with the crystal grains are formed in the convex portions of the grain boundaries. The gist is that it was deposited in a continuous manner. In addition, semi-continuous precipitation means that, as shown in the schematic diagram of the precipitation state in Figure 1a, if the major axis of the precipitates 2 is A, and the length of the grain boundary 1 between the precipitates 2 is B, then It refers to a precipitation form where B/A≦1, and a precipitation form where B/A>1 as shown in FIG. 1b is called discontinuous precipitation. Making the grain boundaries zigzag, as shown in FIG. 2b, means making the grain boundaries uneven as shown in the text. Note that 3 and 3' in FIGS. 1 and 2 indicate the parent phase of the alloy, and the arrows in FIG. 2 indicate matching crystal grains of the precipitates. In the present invention, the following heat treatment method for forming the structure of the member into the above structure is the second invention. That is, in the heat treatment method of the present invention, C: 0.08 or less, Mn: 1% or less, Cr: 14% or more and 42% or less,
Fe: 5% or more and 33% or less, Ti: 2% or more and 30% or less,
Nb: 0.5% or more and 28.6% or less, Al: 0.4% or more and 28.5%
For precipitation-hardened Ni-based alloy members containing the following, the remainder of which is essentially Ni, and which precipitates γ' through aging treatment, the complete solid solution temperature is below T℃ and (T-150).
After holding the part at a temperature of ℃ or higher for 30 minutes or more per 25 mm of the maximum wall thickness of the part, it is subjected to a liquefaction treatment that rapidly cools it.
The gist of the process is to further hold the material at 675 to 760°C for 10 to 100 hours, and then perform an aging treatment by water cooling, oil cooling, or air cooling. Here, the rapid cooling performed in the solution treatment includes water cooling and oil cooling. The reason why the present invention has such a configuration will be explained below. The present invention is not about discovering an alloy composition;
The alloy composition itself is a known precipitation hardening type Ni-based alloy. Although its composition is as described above, the reason for having such a composition will be explained below in general terms. Ni: Ni is a base metal for Ni-based alloys and has the property of stabilizing the austenite phase and improving relagation properties. Cr: 14% or more and 42% or less Cr is effective in improving corrosion resistance, and for that purpose, it needs to be 14% or more. Also, from the whole
When the minimum required content of other elements other than Cr is subtracted, it is impossible for Cr to exceed 42%. Fe: 5% or more and 33% or less Fe is effective in stabilizing the structure of the Ni-based alloy, and for that purpose, it needs to be 5% or more. Furthermore, if the minimum required content of other elements other than Fe is subtracted from the total, it is impossible for Fe to exceed 33%. Ti: 2% to 30% Ti, together with Ni and Al, is γ′ (gamma prime)
It forms and precipitates a phase, which is effective in improving the strength of the member, but for this purpose it needs to be 2% or more. In addition, when the minimum required content of other elements other than Ti is subtracted from the total, Ti is 30%
It is impossible to exceed. Nb: 0.5% or more and 28.6% or less Nb forms and precipitates the γ' phase together with Ni and is effective in improving the strength of the member, but for this purpose it needs to be 0.5%. Also, from the whole
Subtracting the minimum required content of other elements other than Nb, it is impossible for Nb to exceed 28.6%. Al: 0.4% or more and 28.5% or less Al is effective in forming and precipitating the γ′ phase together with Ni and improving the strength of the member, and for this purpose, it needs to be 0.4% or more. Also, from the whole
Subtracting the minimum required content of other elements other than Al, it is impossible for Al to exceed 28.5%. C: 0.08% or less C is the main element that forms the precipitation of M ₂₃ C ₆ that affects corrosion resistance, but if it is contained in excess of 0.08%, TiC
and NbC were precipitated and added to obtain strength.
This is not preferable because it consumes Ti and Nb. Mn: 1% or less Mn is an element that stabilizes austenite, but if it is contained in an amount exceeding 1%, it is not preferable because it deteriorates the relagation properties. In general, in precipitation hardening Ni-based alloys such as UNS-NO7750 alloy, the inside of the grain is extremely strengthened due to the precipitation of γ' inside the grain, but on the contrary, the grain boundary is weakened. For this reason, SCC is likely to occur at grain boundaries, but in the present invention, in order to strengthen the grain boundaries and their vicinity, the grain boundaries are made into a zigzag shape and precipitates consistent with the crystal grains are precipitated at the convex portions of the grain boundaries. There is. When the grain boundaries are configured in this way, first, the strong precipitates located in the convex portions of the grain boundaries and consistent with the matrix exhibit strong deformation resistance against the shear force acting on the grain boundaries. Furthermore, grain boundaries are a type of planar defect and are extremely weak against tensile stress perpendicular to the plane, but compared to this tensile strength, it is thought that the shear strength is still stronger. Therefore, if the grain boundaries are not linear but have an uneven shape, even if tensile stress is applied to the grain boundaries, it will act as shear stress depending on the location of the grain boundaries, so grain boundary cracking will be resistant. is expected to increase. Furthermore, by making the grain boundaries uneven, the propagation of cracks is suppressed at the bending points of the grain boundaries. Normally, when a crack extends to such an inflection point, this crack may even extend beyond the inflection point and penetrate into the grain.
In the present invention, by precipitating precipitates that are consistent with the crystal grains at the convex portions of grain boundaries, a coherent strain field is formed in the matrix around the precipitates, which strengthens the matrix itself and prevents cracks from propagating within the grains. is hindering. Also,
Even if a crack were to occur within the crystal grains, it can be expected that the hard precipitates would directly suppress the progress of the crack. If the precipitates are mismatched with the crystal grains, cracks will easily propagate on the surface of the precipitates, which is undesirable. Note that by making the precipitation form semi-continuous, that is, by increasing the density of precipitated grains at the grain boundaries, the grain boundaries and their vicinity are strengthened to a high density. The solution temperature is T (complete solid solution temperature) ~ (T-150)
As °C. At temperatures above T, carbides dissolve into solid solution,
Recrystallization becomes active and grain boundary movement becomes noticeable. As a result, grain boundaries become straight, semi-continuous grain boundaries as shown in Figure 2b no longer occur, and the bonding strength of grain boundaries decreases, resulting in SCC. resistance becomes lower. On the other hand, (T
Cr ₂ (C,N) has low SCC resistance below -150)℃
and non-coherent M ₂₃ C ₆ precipitation, resulting in SCC
Since it does not result in a semi-continuous M ₂₃ C ₆ coherent precipitation with high resistance, no improvement in SCC resistance can be expected. That is (T-
At temperatures below 150)°C, the amount of solid solution of Cr and the amount of solid solution of C and N are not balanced well, so it is thought that coherent precipitation of M ₂₃ C ₆ does not occur. The solution holding time should be set to ensure sufficient soaking.
Requires holding time of 30 minutes or more per 25 mm of wall thickness at the maximum wall thickness of the part. Here, "30 minutes or more per maximum wall thickness of 25 mm" means that a single member has thinner and thicker parts, but heat treatment is performed based on the thickness of the largest part. The purpose is to calculate time. For example, if the thickest part of the part is 25 mm, it should be heated for at least 30 minutes, but if it is 50 mm, it will need to be heated for at least 1 hour. Aging treatment is performed by holding at 675 to 760°C for 10 to 100 hours and then cooling at a cooling rate of air cooling or water cooling. When the aging temperature is lower than 675°C, the amount of M ₂₃ C ₆ precipitated is small, the grain boundaries are not zigzag but straight, and the bonding strength of the grain boundaries is reduced. On the other hand, if it exceeds 760℃
Since agglomeration of M ₂₃ C ₆ tends to occur, the precipitation form becomes discontinuous rather than semi-continuous, and the bonding strength of the crystals decreases, making it impossible to obtain sufficient SCC resistance. If the aging time is less than 10 hours, the amount, shape, and distribution of γ' precipitation will be inappropriate, making it impossible to obtain the desired mechanical strength. On the other hand, if the aging time exceeds 100 hours, M ₂₃ C may aggregate, resulting in discontinuous precipitation rather than semi-continuous precipitation. The present invention will be explained below with reference to Examples. The experimental data presented here was obtained by heat-treating the various test materials in Table 2 under various conditions and performing a U-bending test corresponding to JIS standard G0576 at high temperatures of 280 to 360°C with or without deaeration. These are the results of a stress corrosion cracking test conducted by holding in high pressure water for 1200 hours. Table 3 shows the relationship between the precipitation form of precipitates and SCC resistance when the precipitation-hardening Ni-based alloys having the chemical compositions in Table 2 were subjected to solution treatment and aging treatment under various conditions. . In addition, Fig. 3 shows the relationship between various heat treatment temperatures and the distribution shape of grain boundary precipitates, Fig. 4 shows the relationship between heat treatment temperature and the consistency of precipitates, and Fig. 5 shows the relationship between the heat treatment temperature and the consistency of precipitates. is the relationship between heat treatment temperature and SCC resistance. Note that the experimental data in the figure includes data other than those in Table 3. From Table 3 and Figures 3 to 5, semi-continuous consistency M ₂₃ C ₆ on the zigzag grain boundary
(M: Carbide forming element such as Cr, Mo, V, Ti
It can be seen that the SCC resistance is improved if the metal structure is made of precipitated metals. Furthermore, from Table 3, it can be seen that the SCC resistance is improved by water cooling than by air cooling after solution treatment. In FIG. 6, the vertical axis represents the solution treatment temperature, and the horizontal axis represents the amount of carbon contained in the alloy, and shows their influence on the SCC resistance. Note that the reason why the amount of carbon is plotted on the horizontal axis is that the amount of carbon has a large influence on the solid solution temperature, and the reason for the limitations of the present invention becomes clearer. According to FIG. 6, it can be seen that an alloy with high SCC resistance can be obtained by performing solution treatment in the temperature range from the complete solid solution temperature T to (T-150)°C. As described above, according to the member of the present invention, the crystal grain boundaries are formed in a zigzag shape, and the precipitated particles are precipitated in the convex portions of the crystal grain boundaries in a semi-continuous manner in alignment with the crystal grain boundaries.
A product with high SCC properties and strength can be obtained. Furthermore, according to the heat treatment method of the present invention, a certain amount of chromium carbide is typically formed at the grain boundaries by optimal solution treatment.
Grain growth is promoted by leaving M ₂₃ C ₆ and holding it at that temperature, but undissolved carbides are pinned on grain boundaries, resulting in carbides remaining at grain boundaries. . By rapidly cooling this, the grain boundaries become semi-continuous, improving the strength near the grain boundaries. By further aging, M ₂₃ C ₆ that is consistent with the matrix precipitates at the grain boundaries, improving the SCC resistance near the grain boundaries, and γ' precipitates within the grains, increasing the strength of the member. improve. Therefore, the member of the present invention is a spring for nuclear power use,
It is ideal for use as corrosion-resistant, high-strength members such as bolts, bottles, and springs.

【table】

【table】

【table】

【table】

【table】 [Brief explanation of drawings]

Figure 1 is a schematic diagram showing the morphology of precipitation on grain boundaries, where a shows semi-continuous precipitation, b shows discontinuous precipitation, and second
The figure is a schematic diagram showing semi-continuous precipitation, where a shows conventional linear precipitation and b shows coherent precipitation at the convex portions of zigzag grain boundaries according to the present invention. Furthermore, the second
Arrows in the figure point to coherent grains. Figure 3 is a diagram showing the relationship between various heat treatment temperatures and the distribution shape of grain boundary precipitates, Figure 4 is a diagram showing the relationship between various heat treatment temperatures, grain boundary precipitates, and consistency. FIG. 6 is a diagram showing the relationship between various heat treatment temperatures and SCC resistance. FIG. 6 is a diagram showing the influence of solution treatment temperature and C content of the alloy on SCC resistance. 1... Grain boundary, 2... Precipitate, 3... Parent phase.

Claims (1)

[Claims] 1. C: 0.08 or less, Mn: 1% or less in weight ratio,
Cr: 14% or more and 42% or less, Fe: 5% or more and 33% or less,
A precipitation-hardening Ni-based alloy that contains Ti: 2% to 30%, Nb: 0.5% to 28.6%, Al: 0.4% to 28.5%, the remainder is essentially Ni, and γ′ is precipitated by aging treatment. SCC-resistant Ni-based alloy member 2, characterized in that the grain boundaries on the metallographic structure are formed in a zigzag shape, and precipitates aligned with the crystal grains are precipitated semi-continuously in convex portions of the grain boundaries. Weight ratio: C: 0.08 or less, Mn: 1% or less,
Cr: 14% or more and 42% or less, Fe: 5% or more and 33% or less,
Contains Ti: 2% or more and 30% or less, Nb: 0.5% or more and 28.6% or less, Al: 0.4% or more and 28.5% or less, the remainder is essentially Ni, and is a precipitation hardening type Ni group that precipitates γ′ by aging treatment. In the heat treatment method for alloy members,
For the Ni-based alloy member, the complete solid solution temperature T of the alloy
After holding the maximum wall thickness of the part at a temperature of less than ℃ and above (T-150) ℃ for 30 minutes or more, the material is subjected to solution treatment by oil cooling or water cooling, and then the temperature is 675 to 760℃.
SCC resistance characterized by aging treatment by water cooling, oil cooling, or air cooling after being held at ℃ for 10 to 100 hours.
Heat treatment method for Ni-based alloy parts.