JPH03138343A - Nickel-base alloy member and its production - Google Patents

Abstract

PURPOSE:To produce an Ni-base alloy member excellent in SCC resistance by subjecting an Ni-base alloy having a specific composition consisting of Cr, Mo, Nb, Ta, Fe, and Ni to specific solid solution heat treatment, to cooling, and then to plastic working at specific draft. CONSTITUTION:An Ni-base alloy having a composition consisting of, by weight, 18-25% Cr, 5-12% Mo, 2-6% Nb and/or Ta, <=10% Fe, and the balance Ni with inevitable impurities is subjected to solid solution heat treatment at 950-1150 deg.C, followed by temporary cooling. It is preferable to perform the above cooling by means of rapid cooling to prevent the formation of Cr carbide. After cooling, the above alloy is subjected to plastic working at a draft of >=10%, preferably of 10 to about 30%. By this method, the Ni-base alloy member improved in SCC resistance as well as in mechanical properties can be obtained.

Description

Detailed Description of the Invention [Objective of the Invention] (Industrial Application Field) The present invention relates to a nickel-based alloy, and in particular to a nickel-based alloy with stress corrosion cracking resistance (
The present invention relates to a Ni-based alloy member with excellent SCC resistance (hereinafter referred to as SCC resistance) and a method for manufacturing the same.

(Prior Art) Conventionally, spring materials such as lantern springs and finger springs for nuclear reactors have been manufactured from a Ni-based alloy of Inconel X-750, which has relatively excellent spring properties. In this case, in the conventional method, Inconel
After melting and manufacturing 50, it was manufactured by subjecting it to solution heat treatment and further age hardening heat treatment.

(Problems to be Solved by the Invention) Alloy members manufactured from conventional Inconel alloys have relatively excellent properties, and are suitable for spring materials used under harsh conditions including high-temperature water such as in nuclear reactors. It is suitable as a plant member such as, and by performing an age hardening treatment during the manufacturing process, properties such as mechanical strength required for a nuclear reactor member can be made even better.

However, members using conventional Inconel alloys as described above do not always have sufficient SCC resistance, and are particularly suitable for members used in harsh high-temperature water environments, especially springs. In addition to strength and spring characteristics, improving SCC resistance is an important issue for materials.

The present invention has been made in view of the problems associated with the prior art described above, and an object of the present invention is to provide a method for manufacturing a nickel-based alloy member with improved SCC resistance.

[Structure of the invention]

(Means and effects for solving the problem) The method for manufacturing a nickel-based alloy member of the present invention has a weight ratio of Cr: 18 to 2.
5%, Mo=5-12%, Nb and/or Ta:2
~6%, Fe: 9% for nickel-based alloys containing 10% or less, with the remainder consisting of Ni and unavoidable impurities.
It is characterized by performing solution heat treatment at a temperature of 50 to 1150°C, and after cooling, performing plastic working of 15% or more.

The present inventor has discovered that stress corrosion cracking (S C
As a result of research on the mechanism of occurrence of C), the following findings were obtained. That is, it has been found that stress corrosion cracking is difficult to occur in the solution heat treatment state in which precipitates at the grain boundaries and within the grains are dissolved, but occurs due to the subsequent age hardening heat treatment.

Usually, the strength of the alloy is increased by age hardening heat treatment.
A phase (Ni3(Al, Ti)) is precipitated, and at the same time, Cr carbide is precipitated at the grain boundaries. It is presumed that when this Cr carbide precipitates, the Cr concentration near the grain boundaries decreases, and as a result, corrosion tends to occur selectively in the areas where the Cr concentration is decreased, resulting in stress corrosion cracking.

As a result of further research focusing on the above findings, the present inventors have found that the composition of the alloy, the solution heat treatment temperature, and the processing rate of subsequent plastic working can be improved without substantially performing the conventional age hardening heat treatment. It has been discovered that by strictly controlling , it is possible to harmoniously achieve improvements in both mechanical strength and SCC resistance.

The present invention will be specifically explained below.

First, in the Ni-based alloy used in the present invention, Cr is 18
-25%, more preferably 23-25%.

Cr is a component that contributes to corrosion resistance, and if it is less than 18%, its effect is poor, while if it is added in an amount exceeding 25%, it conversely reduces workability, which is not preferable.

Mo is a component that contributes to solid solution strengthening and improvement of corrosion resistance, and is added in an amount of 5 to 12%, more preferably 10 to 12%. Corrosion resistance improves as the amount added increases, but on the other hand it has a negative effect on workability, so it is desirable to limit it to the above range.

Nb and/or Ta are also components that contribute to solid solution strengthening and improvement of corrosion resistance, like the above Mo, and are present in an amount of 2 to 6%.
More preferably, it is added in an amount of 2 to 4%. Corrosion resistance improves as the amount added increases, but on the other hand it has a negative effect on workability, so it is desirable to limit it to the above range.

Fe is a component that contributes to improving ductility, but on the other hand, it reduces corrosion resistance, so it is desirable to limit it to 10% or less, more preferably 5% or less.

Although the above components are essential components, in the present invention, Ti or A1 may be added as a deoxidizing agent.

Also, since C increases the formation of Cr carbide, 0.1
% or less, more preferably 0.05% or less.

In the present invention, an ingot is manufactured by melting the alloy components in the above-mentioned composition range according to a conventional method, for example, and then forged, further hot rolled, and then subjected to solution heat treatment. This solution heat treatment is performed at a temperature range of 950 to 1150°C. If the temperature is lower than 950°C, the solid solution of the precipitates generated in the pre-processing will be incomplete and the effect of solid solution cannot be obtained;
Heating above 0°C is not preferred because the strength decreases. More preferably it is 1000-1100°C.

After the solution heat treatment, it is preferable to once rapidly cool the alloy. Such cooling is preferable in order to prevent the generation of Cr carbides.

Next, after cooling, plastic working is performed to form a predetermined member shape. It is important that this plastic working has a working rate of at least 10%. According to the findings of the present inventors, the development of SCC resistance is achieved by the above-mentioned solution heat treatment; however,
By the above-mentioned plastic working of 10% or more, even better mechanical properties can be obtained. However, when the processing rate exceeds 30%, the ductility decreases and when used as a structural material, 1
A preferable range is 0 to 30%.

(Example) Ingots were produced for each sample by melting alloy components having the compositions shown in Table 1 below.

Table 1 The resulting ingot is then forged, the resulting billet is hot rolled, and then rapidly cooled. Add this to 105
Solution heat treatment was performed at a temperature of 0°C.

Next, after the above treatment, cold working was performed at various processing rates to test the member properties.

First, the tensile test was conducted at 0.2% according to JIS Z2241.
The tensile strength was 2 kgf/-/see until the yield strength was determined, and the tensile rate was 50%/m1n after the yield strength until breakage.

In the corrosion test, a test piece with a size of 3 x 15 x 15 mm is washed, and then the weight and surface area of the test piece are measured. Next, this test piece is immersed in a sulfuric acid solution having a sulfuric acid concentration of 50% for 10 hours. The weight of the test piece after the test is measured, and the corrosion rate (g/g/h) is determined from the difference in weight before and after the test.

In the stress corrosion cracking (SCC) test, as shown in Figure 1, a test piece 1 with a size of 2 x 10 x 50 seconds is inserted between a jig 3 and 3' together with carbon graphite 2, and a constant stress is applied to the test piece 1. The specimens were kept in the same environment as inside the reactor (288°C, 70 atm) for 500 hours with the applied pressure applied to them, and after further holding, the cross section of the test pieces was polished and cracks caused by SCC were removed from the entire surface using a microscope. Observe for a long time,
This was done by measuring the maximum SCC depth (μm).

The measurement results of each test described above are shown in Table 2 below.

Table [Effects of the Invention] In the present invention, since the composition of the alloy, the solution heat treatment temperature, and the processing rate of the subsequent plastic working are controlled within a certain range, both mechanical properties and SCC resistance are improved. Provided is a nickel-based alloy in which the following properties are harmoniously achieved.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view schematically showing a test jig according to the CBB test method. 1... Test piece, 2... Carbon graphite, 3...
··jig.

Claims (1)

[Claims] 1. Weight ratio: Cr: 18-25%, Mo: 5-12%
, Nb and/or Ta: 2 to 6%, Fe: 10% or less, and the remainder is Ni and unavoidable impurities. Solution heat treatment is performed at a temperature of 950 to 1150 ° C., and after cooling. A method for producing a nickel-based alloy member, characterized by performing plastic working of 10% or more. 2. A nickel-based alloy member obtained by the method according to claim 1.