JPS62167838A - Ni-based alloy and its manufacturing method - Google Patents

Abstract

PURPOSE:To obtain Ni base alloy having a high strength, further superior stress corrosion cracking resistance in high temp. and pressure water, by specifying metallographic structure and contents of C, Si, Mn, Fe, Cr, Mo, Ti, Al, Nb, rare earth element, Mg, Ca in Ni base alloy. CONSTITUTION:The Ni base alloy contains, by weight <=0.08% C, <=0.15% Si, 0.1-1% Mn, <=15% Fe, 20-30% Cr, <=10% Mo, <=3.5% Ti, <=2% Al, <=7% Nb and <=0.1% one kind or more among respective rare earth element, Mg and Ca and the balance Ni, has gamma' phase and/or gamma'' phase in gamma matrix, and M23C6 are semicontinuously and preferentially pptd. at grain boundaries. For obtaining the alloy, soln. heat treatment composed of heating, holding at 980-1,200 deg.C and cooling, then further aging treatment at one time or more of heating and holding at 550-850 deg.C are applied to alloy having the compsn. In case of cold and hot workings, working by >=10% percentage reduction in area and that at 850-1,250 deg.C by >=20% draft are applied after and before soln. heat treatment, respectively.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to fastening members such as pins, bolts, screws, etc., plate springs, coil springs, etc. used for internal structural members and fuel elements of light water reactors or new type converter reactors. The present invention relates to a high-strength Ni-based alloy with excellent stress corrosion cracking resistance suitable for use in spring members, bolts for turbines, support structure members for heat exchangers, heat exchanger tube materials, etc., and a method for producing the same.

[Conventional technology]

Conventionally, Inconel x-7 has been used as a material for light water reactors, etc.
50 (product name) Ni≧72%, Cr14~1
7chi, Fe 6-9%, Al, Ti, Nb 1-2
% precipitation-strengthened Ni-based alloys are often used.

[Problem that the invention seeks to solve]

However, depending on the heat treatment conditions, Inconel X-750 is highly susceptible to stress corrosion cracking in the usage environment of the above-mentioned applications, and stress corrosion cracking may occur in the above-mentioned fastening members and the like. Moreover, it is generally said that the higher the 0.2% proof stress or tensile strength of a high-strength material, the poorer the stress corrosion cracking resistance. There was no material suitable for use as a member that requires excellent stress corrosion cracking resistance in water.

The present invention was made in view of the above-mentioned problems with the conventional alloys, and an object of the present invention is to provide a Ni-based alloy that has high strength and excellent stress corrosion cracking resistance in high-temperature, high-pressure water.

[Means for solving problems]

As a result of intensive research, the present inventors discovered that the conventional Inconel χ-
750 fasteners, the metal structure changes due to the chemical composition, heat treatment conditions, processing conditions, etc., making the material more susceptible to stress corrosion cracking. We have developed a Ni-based alloy with a composition and metal structure, and a method for producing the same.

Therefore, the present invention first focuses on (1) weight ratio of 0.08% or less, SLo, 15% or less, Mn 0.1-1%, Ni 45-75%, Cr20
~30%, 1010% or less, Ti3.5 or less, Al2
Nb or less, Nb7 or less, rare earth elements, Mg and Ca
Contains at least one type of each having a coefficient of 0.1 or less and the remainder Fe, has at least one type of γ1 phase and γ11 phase in the γ base, and M23C6 precipitates preferentially in a semi-continuous manner at the grain boundaries. The first invention is a high-strength Ni-based alloy that has excellent stress corrosion cracking resistance in high-temperature, high-pressure water.

As a manufacturing method for obtaining this Ni-based alloy, the following inventions (2) to (3) were designated as the second to fourth inventions.

(2) Weight ratio c 0.08% or less, st 0.15
% or less, Mn 0.1-1%, Ni 45-75%, Or
20-30%, IA010% or less, Ti3.5% or less, Al2% or less, 1ib7% or less, and rare earth elements,
An alloy containing a small amount of 0.1 or less of each of Mg and Ca (one or more of them and the balance Fe) is heated and held at 980 to 1200 °C, cooled at a cooling rate faster than air cooling, and further heated to 550 to 85 °C.
A method for producing a high-strength Ni-based alloy having excellent stress corrosion cracking resistance in high-temperature, high-pressure water, the method comprising performing aging treatment at least once by heating and holding the alloy at 0°C.

(3) Weight ratio: C: 0.08% or less, Si: 0.15% or less, Mn: 0.1-1, Ni: 45-75%, C: 20-
30%, 1010% or less, Ti 3.5% or less, Al2 or less, Nb 7 or less, and at least one of rare earth elements, Mg and Ca, each 0.1% or less, and the balance Fe, 980 to 1200 It is characterized by heating and holding at a temperature of 550 to 850 degrees Celsius, cooling it at a cooling rate higher than that of air cooling, cold working with a cross-section reduction ratio of 10 or more, and then aging treatment at a temperature of 550 to 850 degrees Celsius at least once. A method for producing a high-strength Ni-based alloy with excellent stress corrosion cracking resistance in high-temperature, high-pressure water.

(4) Weight ratio: c 0.08'A or less, Sl, 15% or less, Mn 0.1-1%, Ni 45-75%, Cr20
~30%, Mo10% or less, Ti3.5 or less, Al2
7% or less, Nb 7% or less, rare earth elements, Mg and Ca
After hot working at 850 to 1250°C with a reduction rate of 204 or more to an alloy containing at least one type of each of 061% or less and the balance Fe, the alloy is heated and held at 980 to 1200°C at a cooling rate faster than air cooling. Cool at 550-850℃
A method for producing a high-strength Ni-based alloy having excellent resistance to stress corrosion cracking in high-temperature, high-pressure water, the method comprising performing aging treatment at least once by heating and holding the alloy.

[Reason for numerical limitation]

C: C combines with Or to form Or carbide M23C6 at the grain boundaries, and serves to increase the grain boundary bonding strength of the crystal grains. However, when C exceeds 0.08%, it combines with Nb and Ti to form NbC and TIG, and reduces the γ' and Ti1 phases produced when Nb and Ti combine with Ni, resulting in a decrease in strength. Therefore, C was set to 0.08 inch or less.

Si: Sl has the effect of removing oxygen as an impurity in the alloy, but on the other hand, if it exceeds 0.15%, it inhibits the semi-continuous precipitation of M, C, in the grain boundaries, and reduces stress corrosion cracking resistance. decrease. Therefore, sl was set to o and 15 inches or less.

Mn: Mn is an element that promotes the semi-continuous precipitation of M23C6 at grain boundaries, and must be contained in an amount of 0.1% or more, but if it is contained in excess of 1%, the precipitation of a brittle phase impairs ductility. encourage Therefore, Mn is 0.1~
It was set at 1%.

Fe: Fe is an element that improves the stability of the structure during casting or plastic working, but if it is contained in an amount exceeding 15%, it impairs ductility. Therefore, the Fe content was set to 15% or less.

Or: Cr is the most important element in maintaining stress corrosion cracking resistance and must be contained at 20% or more, but if it is contained in excess of 30%, solidification segregation will be significant and not only will it be difficult to forge, but it will also become homogeneous. It is difficult to have a comfortable ear foot. Therefore, the angle was set at 20 to 3 degrees.

Mo: Mo improves pitting corrosion resistance and crevice corrosion resistance, but if it is contained in an amount exceeding 10 inches, grain boundary precipitation of M23C6 is suppressed, and stress corrosion cracking resistance decreases. Therefore, M
o was set to 10 inches or less.

Ti: Ti combines with the former to precipitate γ', which is Ni3Ti, increasing the strength.
Phases precipitate and stress corrosion cracking resistance decreases.

Therefore, the Ti content was set to 3.5% or less.

Al: Al combines with Ni to precipitate γ', Ni3Al, increasing the strength (however, if it exceeds 2%, the stress corrosion cracking resistance decreases. Therefore, the Al content was set to 2 or less.

sb: Nb combines with Ni to precipitate the γ′l phase or δ phase of Ni3Nb, increasing the strength, but when the coefficient exceeds 7, the stress corrosion cracking resistance decreases. Therefore, the Nb content was set to 7% or less.

Rare earth elements, Mg, Ca: Rare earth elements such as Hf and Y, Mg and Ca, not only can remove 0 as impurities in the alloy, but also precipitate at grain boundaries and increase grain boundary bonding strength, but each of them If it exceeds .1 inch, the pitting corrosion resistance will deteriorate. Therefore, at least one rare earth element, Mg, and Ca each contained 0.1- or less.

In addition, the heat treatment conditions are 980 to 1200 in order to maintain high strength and high stress corrosion cracking resistance.
After solution treatment in which the material is heated and held at 550 to 850°C and cooled at a cooling rate higher than air cooling, it is necessary to perform an aging treatment at least once in which the material is heated and held at 550 to 850°C.

Note that the heat treatment time is 5 minutes to 5 hours for solution treatment;
It is preferable that the aging treatment be performed for about 1 to 150 hours.

Generally, in the case of cast materials, only the above-mentioned solution treatment and aging treatment are sufficient, but when further treatment, cold working, and hot working are performed, they are carried out under the following conditions.

That is, in the case of cold working, in order to obtain excellent stress corrosion cracking resistance, the workpiece is uniformly worked at a high working rate of 10 inches or more after the solution treatment.

According to the above cold working conditions, in addition to excellent stress corrosion cracking resistance, 02% yield strength is 90 kg/1 m1 or more,
It becomes a high-strength material with a tensile strength of 100 Kq/- or more.

In addition, in the case of hot processing, it prevents cracking due to hot processing,
In addition, the processing temperature was set at 85% to prevent excessive grain growth.
It is carried out at a temperature of 0 to 1250°C, and uniformly carried out at a rolling reduction of 20 inches or more in order to maintain excellent stress corrosion cracking resistance.

In addition, according to the above hot working conditions, in addition to excellent stress corrosion cracking resistance, the 0.2 inch proof stress at room temperature is 70 exposures or more,
It becomes a high-strength material with a tensile strength of 90 Kl- or more.

〔Example〕

1) Stress corrosion cracking test In order to evaluate stress corrosion cracking resistance when the Ni-based alloy of the present invention is used for fastening members, bellows, etc. in a light water reactor environment, a table simulating secondary water in a pressurized water reactor Under the environment shown in 1, the U-bend test piece shown in FIG. 1 was immersed, and a stress corrosion cracking test was carried out on each specimen under high stress for up to 4000 hours to investigate the presence or absence of cracks.

2) Test material Table 2 shows the chemical composition of the test material used in this test, and Table 3 shows examples of heat treatment and processing conditions for the test material.

The impurities contained p and s at a maximum of about 0.101%, Cu at a maximum of about 0.07%, and N at a maximum of about 0.01%.

3) Test results are as shown in Table 3 and Figures 2 to 10. As shown in Table 4, the white symbols in the figures indicate those without cracks, and the black symbols indicate those with cracks. It is something.

In addition, when we observed the metallographic structure of these test materials that did not cause cracking, we found that rI phase or i11 was present in the r base.
The phase was dispersed, and Mtscs was preferentially precipitated in a semi-continuous manner at the grain boundaries. Typical examples are shown in Table 5.

Each component element, heat treatment conditions, and presence or absence of cracks are shown in Figure 2 (al
(This is as shown in FIGS. 1 to 4), and it can be seen that the stress corrosion cracking resistance is better than that of the others if the composition range and heat treatment conditions are within the range of the present invention.

In addition, Figure 5 (al l (bl) shows the relationship between cold working rate, solution temperature, and the presence or absence of cracking, and all of them show that under the conditions of the present invention, stress corrosion cracking resistance is better than others. I know it's excellent.

In FIGS. 6 and 7, Ti. The effect of the amount of Al and the amount of Nb on the stress corrosion cracking resistance is shown, and it can be seen that if both are within the range of the present invention, the stress corrosion cracking resistance is excellent compared to the others.

Figure 8 shows the relationship between mechanical properties and cold working rate.
As shown in FIG. 5 (al l (bl), within the scope of the present invention, despite having excellent stress corrosion cracking resistance,
It also has excellent yield strength and tensile strength.

Figures 9 and 10 show the relationship between the chemical composition and mechanical properties of an alloy that has been hot worked at a rolling reduction of 30%. However, it has excellent mechanical properties.

In addition, in each figure, some ta+ and fb1 figures are data divided according to the presence or absence of Mo, and (at
is the one without Mo, (bl is the one with Mo.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain a Ni-based alloy that satisfies both mechanical strength and stress corrosion cracking resistance.
It can be used extremely safely and with a long life as a spring member, etc.

[Brief explanation of drawings]

FIG.
Figures TA+, FBL to FIG. 10 are diagrams showing experimental results regarding the embodiments of the present invention.

Claims (4)

[Claims] (1) Weight ratio: C0.08% or less, Si0.15% or less, Mn0.1-1%, Fe15% or less, Cr20-30
%, Mo 10% or less, Ti 3.5% or less, Al 2% or less, Nb 7% or less, and at least one of rare earth elements, Mg and Ca 0.1% or less, and the balance Ni,
Stress corrosion resistance in high-temperature, high-pressure water characterized by having at least one of the γ' phase and γ'' phase in the γ base, and M_2_3C_6 preferentially precipitating semi-continuously at the grain boundaries. High strength Ni-based alloy with excellent crackability. (2) Weight ratio of C0.08% or less, Si0.15% or less, Mn0.1-1%, Fe15% or less, Cr20-30
%, Mo 10% or less, Ti 3.5% or less, Al 2% or less, Nb 7% or less, and at least one or more of rare earth elements, Mg and Ca 0.1% or less, and the balance Ni, at 980 to 1200°C. A high-strength Ni-based alloy with excellent stress corrosion cracking resistance in high-temperature, high-pressure water, characterized by being heated and held at 550 to 850°C, cooled, and then subjected to at least one aging treatment of heating and holding at 550 to 850°C. Manufacturing method. (3) Weight ratio: C0.08% or less, Si0.15% or less, Mn0.1-1%, Fe15% or less, Cr20-30
%, Mo 10% or less, Ti 3.5% or less, Al 2% or less, Nb 7% or less, and at least one or more of rare earth elements, Mg and Ca 0.1% or less, and the balance Ni, at 980 to 1200°C. After heating and cooling,
Cold working with a cross-sectional reduction ratio of 10% or more, and further 550
A method for producing a high-strength Ni-based alloy having excellent stress corrosion cracking resistance in high-temperature, high-pressure water, the method comprising performing aging treatment at least once by heating and holding the alloy at ~850°C. (4) Weight ratio: C0.08% or less, Si0.15% or less, Mn0.1-1%, Fe15% or less, Cr20-30
%, Mo 10% or less, Ti 3.5% or less, Al 2% or less, Nb 7% or less, and at least one or more of rare earth elements, Mg and Ca 0.1% or less, and the balance Ni, at 850 to 1250°C. After hot working at a reduction rate of 20% or more, the process is heated and held at 980 to 1200°C, cooled, and further subjected to aging treatment at least once at 550 to 850°C. A method for producing a high-strength Ni-based alloy with excellent stress corrosion cracking resistance in high-pressure water.