JPS621462B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a Ni-based alloy containing Cr, particularly a Ni--Cr alloy with improved stress corrosion cracking resistance. Ni-based alloys containing Cr are originally materials with excellent stress corrosion cracking resistance. Therefore, Alloy600 (75% Ni, 15%
High Cr, Ni-based alloys such as Cr, 8% Fe) are used. However, even with Alloy 600, stress corrosion cracking may occur under certain usage conditions when used in steam generator tubes. Stress corrosion cracking occurs when three factors are present: the presence of tensile stress, factors originating from the environmental conditions in which it is used, and factors related to the material itself. If even one of these factors is completely removed, This cracking can be prevented. However, when used in the above-mentioned steam generator, for example, residual stress due to surface polishing and bending, and tensile stress due to thermal stress during nuclear reactor operation, etc., are unavoidable. Furthermore, regarding factors caused by the usage environment, although extremely strict water quality control is carried out on the water used, it is difficult to say that such factors can be completely eliminated. Therefore, the best way to prevent stress corrosion cracking is to improve the material properties themselves and reduce the susceptibility to stress corrosion cracking. However, in high-Ni alloys containing Cr, the amount of solid solution of C is small, so even if the C content is reduced as much as possible during the refining process, it will not be possible in processes such as welding.
Since Cr carbides mainly precipitate at grain boundaries,
A Cr-deficient layer is formed at the grain boundaries, and the corrosion resistance of that area deteriorates. The present inventors conducted various studies with the aim of improving the stress corrosion cracking resistance of 30Cr-60Ni alloys, and found that B, which is normally contained as an impurity of 20 ppm or more in this type of Ni-based alloy, (Boron) precipitates and segregates at grain boundaries during final annealing, which is usually performed at 900 to 1050°C, or during subsequent cooling, and the sensitization of the alloy is accelerated by B precipitated and segregated at these grain boundaries. After obtaining the knowledge that stress corrosion cracking resistance decreases, and conducting a series of experiments, it was discovered that such sensitization can be prevented by limiting B as an impurity to 10 ppm or less, and the present invention has been developed. completed. Therefore, the gist of the present invention is as follows: C: 0.15% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 25-35%, Ti : 0.01~1.0%, Fe: 6.0~10.0%, Al: 0.01~1.0%, Zr: 0.01~1.5%, the balance is Ni, and the stress resistance is characterized by limiting B as an alloy impurity to 10 ppm or less. Ni-Cr with excellent corrosion cracking resistance
It is an alloy. The reason why the composition range of the alloy is limited as described above in the present invention is as follows. C: Since C is an element harmful to stress corrosion cracking resistance, in the present invention, the amount of C is set to 0.15% or less. Si, Mn: These are both deoxidizing elements, and each needs to be added in an amount of 1.0% or less. If it exceeds 1.0%, it will reduce the cleanliness of the alloy. Al: Al is a deoxidizing element, and in the present invention 0.01%
It is necessary to add more than 1%, but if it is added in excess of 1%, the cleanliness of the alloy will decrease. In other words, Al
By adding 0.01% or more, preferably 0.05% or more, the oxygen content in the steel can be reduced to 0.005% or less, improving workability. Further, even if it is added in an amount exceeding 1%, no further improvement in processability is observed. Cr: Cr is an essential element for improving corrosion resistance and has a content of 25%
If it is less than that, the corrosion resistance required in the present invention cannot be ensured. On the other hand, when it exceeds 35%, hot workability deteriorates significantly. Therefore, in the present invention, the Cr content is limited to 25 to 35%. P, S: These elements generally impair hot workability, but if they are 0.030% or less, they will not have any substantial effect on hot working, so in the present invention they are each limited to 0.030% or less. Fe: Fe needs to be added in an amount of 6.0% or more to ensure hot workability, but if it exceeds 10%, corrosion resistance decreases. In the present invention, Fe is limited to 6.0 to 10%. Ti: Ti improves hot workability by adding 0.01% or more, but on the other hand, the effect is saturated even if it is added in excess of 1.0%.
It shall be 1.0%. Zr: Zr is 0.01% when added in the present invention
When more than 1.5% is added, hot workability is improved, but when more than 1.5% is added, the cleanliness of the alloy decreases. B: B is an element that accompanies Ni as an impurity, and is generally contained in a Ni-based alloy such as the one targeted by the present invention in an amount of 0.002% or more, usually about 0.004%. However, as already mentioned, in the present invention, it has become clear that B is an element harmful to stress corrosion cracking resistance.
Limit to 0.001% or less. Ni: Ni is an effective element for improving corrosion resistance, especially
Ni to improve stress corrosion cracking resistance in high-temperature, high-pressure underwater alkaline environments containing NaOH.
≧50% is required. Next, the present invention will be explained in more detail with reference to Examples. It should be noted that it will be clear to those skilled in the art that the following examples are shown merely to explain the present invention, and the present invention is not limited thereto. Examples Various test materials whose alloy compositions are shown in Table 1 were melted in a 17Kg vacuum furnace, and the resulting ingots were forged, hot rolled, and heat treated according to conventional methods, followed by 30% cold rolling. Processing was performed and final annealing was performed at 950-1150°C. 2mm (thickness) x 10mm from each sample material obtained
(width) x 75 mm (length) stress corrosion crack test specimens were taken. These stress corrosion cracking specimens are made of emery paper 320
After polishing, the specimens were bent into a U-shape to obtain U-bent test specimens, which were then subjected to a 1000-hour immersion test in a 50% NaOH (caustic soda) solution at 330°C using an autoclave (high-temperature, high-pressure container). did. After the test, the depth of stress corrosion cracking was measured using a microscope.

【table】

【table】

[Table] The experimental results thus obtained are summarized in Table 2. It can be seen that the maximum crack depth of the material of the present invention is extremely small. When these results are further summarized regarding the B content, the graph shown in the attached drawing is obtained. It can be seen that by limiting the B content to 10 ppm or less, the maximum crack depth of stress corrosion cracking is significantly reduced. In the figure, each number indicates the sample material number. As described above, according to the present invention, even in stress corrosion cracking tests under conditions more severe than the actual steam generator environment in which nuclear reactor steam generator tube materials have been conventionally used, B as an alloy impurity can be suppressed. content
The significance of the present invention is significant because stress corrosion cracking can be significantly reduced simply by limiting the content to 10 ppm or less.

[Brief explanation of the drawing]

The attached drawing is a graph showing the relationship between B content and maximum stress corrosion crack depth.

Claims (1)

[Claims] 1 C: 0.15% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 25 to 35%, Ti: 0.01 to 1.0%. , Fe: 6.0~10.0%, Al: 0.01~1.0%, the balance is Ni, with B as an alloy impurity.
A Ni-Cr alloy with excellent stress corrosion cracking resistance, limited to 10ppm or less. 2 C: 0.15% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 25-35%, Ti: 0.01-1.0%, Zr: 0.01-1.5 %, Fe: 6.0 to 10.0%, Al: 0.01 to 1.0%, balance Ni, with B as an alloy impurity.
A Ni-Cr alloy with excellent stress corrosion cracking resistance, limited to 10ppm or less.