JPS6144137B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a Ni-based alloy with excellent intergranular corrosion resistance, stress corrosion cracking resistance, and mechanical strength.In particular, the present invention relates to a Ni-based alloy with high Cr content that improves intergranular stress corrosion cracking in high-temperature water. This relates to Ni-based alloys. In recent years, due to the development of the chemical and energy industries, equipment materials are being used in a wide variety of environments and are often used under harsh conditions.As demands for safety and reliability have increased, materials with stable corrosion resistance have been needed. requested. For these reasons, stainless steel and Ni-based alloys are widely used in harsh environmental conditions. Ni-based alloys containing Cr have excellent corrosion resistance in general and particularly against intragranular stress corrosion cracking in chloride environments, but carbides may precipitate at grain boundaries and become sensitive, resulting in intergranular corrosion and intergranular corrosion. Interfacial stress corrosion cracking may occur. Conventionally, measures have been taken to prevent these grain boundary sensitizations by adding stabilizing elements such as Ti or Nb and performing stabilization treatment to fix solid solute C in advance. In some areas, the above-mentioned stabilizing effect disappeared and the grain boundaries became sensitized during welding and subsequent heat treatment such as strain relief annealing. Among Ni-based alloys containing Cr, when the Cr content is relatively low, such as Inconel 600, a large amount of Nb is required to suppress grain boundary sensitization, and grain boundary sensitization is required, including in welded areas. In order to suppress this, an even larger amount of Nb is required. In order to improve the intergranular corrosion resistance and intergranular stress corrosion cracking resistance of Inconel 600, C should be reduced to 0.01% or less, as described in page 268 of the Japan Institute of Metals lecture abstracts held in April 1981. or
It is effective to add Nb, and N is 0.01
% or more, it is known that grain boundary sensitization is promoted. However, when the C and N content decreases, there is a drawback that the 0.2% yield strength among mechanical strength decreases. In addition, as stated in page 160 of the Abstracts of the Spring Academic Conference of the Corrosion Prevention Association held in May 1982, Nb and Ti are effective in improving the intergranular corrosion resistance of the deposited metal of Inconel 600 alloy. It is known. However, the above description relates to welded metal, and there is no description of mechanical strength, nor is there any description of the influence of N. Furthermore, according to the two lecture summaries mentioned above, since the Cr content of the test piece is low at 21% or less, it is necessary to increase the amount of Nb required to suppress grain boundary sensitization due to C. If the number increases, hot workability deteriorates, which not only impairs manufacturability but also increases costs. Also,
In the case of Inconel 600 base, even if a large amount of Nb is contained, it is extremely difficult to completely suppress grain boundary sensitization because the Cr content is relatively low. In order to meet the high safety requirements for various types of equipment in conventional chemical plants and the energy industry, there is a demand for alloys with extremely excellent intergranular corrosion resistance and stress corrosion cracking resistance. By the way, Inconel, which is a high Cr Ni-based alloy,
Regarding 690, it is known that grain boundary sensitization can be suppressed by lowering the C content to 0.02%, as described in Corrosion Prevention Technology Vol. 28 (1979), page 82. However, there is no description of the N content in the above alloy, and there is a drawback that mechanical strength decreases simply by lowering the C content. The object of the present invention is to provide an alloy that eliminates and improves the disadvantages of the above-mentioned conventionally known alloys, and the above object can be achieved by providing the alloy described in the claims. Next, the present invention will be explained in detail. The present inventors have investigated the effects of N to reduce mechanical strength caused by lowering C to suppress grain boundary sensitization.
It was newly discovered that N is very effective and that N does not promote grain boundary sensitization. Furthermore, it was found that Nb, like C and N, is effective in improving mechanical strength, and even if the sum of C and N is low, sufficient mechanical strength can be obtained by adding Nb. In other words, by achieving an appropriate balance of C, N, and C, N, and Nb, we have developed a Ni-based alloy that has excellent intergranular corrosion resistance, intergranular stress corrosion cracking resistance, and mechanical strength. Completed the invention. That is, the alloy of the present invention is an alloy superior to the conventional Inconel 690 in terms of intergranular corrosion resistance, intergranular stress corrosion cracking resistance, and mechanical strength. Next, the present invention will be explained using experimental data. Inventive alloys 1 to 6 and comparative alloys 7 to 8, whose compositions are shown in Table 1, were each melted in an atmospheric induction furnace and weighed 6 kg.
A steel ingot is forged to a thickness of 10 mm and a width of 70 mm.
As a material heat treatment, the material was heated at 1100°C for 1 hour and then cooled with water, and further heated at 870°C for 2 hours and then cooled with water. The thus obtained steel pieces were subjected to mechanical tests. On the other hand, for corrosion resistance testing, as shown in Fig. 1, the bevel was processed and multi-layer overlay was applied, heat treated at 600°C for 20 hours, then air cooled.
A steel billet was made by heat-treating at 500℃ for 40 hours and cooling it in air. Welding was performed by TIG welding, and the filler metal composition shown in Table 2 was used. For each specimen, the cross section of the welded part was cut out and wet-polished to a final #800. Table 3 shows the results of the 0.2% proof stress, intergranular corrosion test, and high temperature water stress corrosion cracking test. After the intergranular corrosion and high temperature water stress corrosion cracking tests, the cross section of the test piece was observed with an optical microscope, and in the case of intergranular corrosion, the maximum degree of corrosion d was measured. In the case of high-temperature water stress corrosion cracking, the presence or absence of cracking was investigated.

【table】

【table】

【table】

[Table] According to Table 3, alloys 1 to 6 of the present invention are superior to comparative alloys 7 to 8 in terms of mechanical strength, that is, 0.2% proof stress, intergranular corrosion resistance, and intergranular stress corrosion cracking resistance. It has excellent properties and 0.2
The % proof stress exceeds the target value of 27Kg/ ^mm2 , and the maximum corrosion degree in the intergranular corrosion test is 40μ.
m/day, and no cracking occurred in any of the high temperature water stress corrosion cracking tests. Comparative Alloy 7, on the other hand, has a sufficient 0.2% yield strength, but in the intergranular corrosion test, the maximum corrosion rate was 630μ.
m/day, and cracking occurred in the high temperature water stress corrosion cracking test. Comparative alloy 8 has sufficient corrosion resistance in intergranular corrosion tests and high-temperature water stress corrosion cracking tests, but has poor mechanical strength.
The 0.2% proof stress is below 27Kg/ ^mm2 . Figure 2 shows the influence of C and N contents on mechanical strength, that is, 0.2% proof stress ^σ0.2 when Nb is not included. The sum of and N must be 0.040% or more. Figure 3 shows the effect of Nb on mechanical strength 0.2% proof stress,
This is a diagram showing the influence of C and N. Nb is effective for mechanical strength like C and N. When containing Nb, in order for the 0.2% yield strength to exceed 27 Kg/ ^mm2 , Nb, C, It is necessary to satisfy the relationship between N and the following formula, %Nb+50(%C+%N)2.0. Figure 4 is a diagram showing the influence of C and Nb contents on the maximum corrosion degree d in intergranular corrosion tests. In order to maintain sufficient intergranular corrosion resistance,
It is necessary to contain 50 (%C - 0.03)% or more of Nb. Also, if C is less than 0.03%,
It is not necessary to contain Nb. Next, the reason for limiting the composition of the alloy of the present invention will be explained. When C does not contain Nb, the amount of C exceeds 0.03%, or when it contains Nb, the amount of C exceeds 0.080%.
Since the corrosion resistance of the weld heat-affected zone deteriorates if it exceeds 100%, the amount of C needs to be 0.030% or less when it does not contain Nb, and it needs to be 0.080% or less when it contains Nb. If Si exceeds 1.0%, intergranular corrosion resistance deteriorates, so Si should be 1.0% or less. If Mn exceeds 1.0%, intergranular corrosion resistance deteriorates, so Mn must be kept at 1.0% or less. If P exceeds 0.030%, hot workability and weldability deteriorate, so P must be kept at 0.030% or less. If S exceeds 0.030%, hot workability deteriorates, so it must be kept at 0.030% or less. Cr is an essential element for exhibiting corrosion resistance. If Cr is less than 26%, corrosion resistance is insufficient, and if Cr exceeds 35%, hot working becomes extremely difficult. It is necessary to exceed 35% or less. If Fe exceeds 25%, the intragranular stress corrosion cracking resistance in a chloride environment, which is a characteristic of Ni-based alloys, will deteriorate, so it must be kept at 25% or less. N is effective in improving mechanical strength, but 0.2%
If the N content exceeds 0.2%, blowholes may occur in the steel ingot and productivity will deteriorate, so the N content must be 0.2% or less. Nb is an effective element for intergranular corrosion resistance and mechanical strength. C is 0.030 from the viewpoint of intergranular corrosion resistance.
% or less, it is not necessary to include Nb, and if C exceeds 0.030%, Nb must be included.
It is necessary to contain 50 (%C - 0.030)% or more. Furthermore, from the viewpoint of mechanical strength, when Nb is included, the following formula must be satisfied between the Nb, C, and N contents: %Nb + 50 (%C + %N) 2.0 . Ti, Zr, and Al are effective elements for improving hot workability as deoxidizers, and among them, Ti, Zr, and
is an element that has a large effect of suppressing the occurrence of blowholes and improves the corrosion resistance of the welded high-temperature heat-affected zone, but one or more selected from Ti, Zr, and Al must be present in a total of 1%. If the amount is more, the above-mentioned effects cannot be expected, so it is necessary to keep it at 1% or less. B and Mg improve hot workability, but if B and Mg exceed 0.005% and 0.05%, hot workability deteriorates, so B and Mg should be 0.005% or less and 0.05% or less, respectively. There is a need. As mentioned above, the 0.2% yield strength of the Ni-based alloy of the present invention is 27Kg/mm ²
As described above, it is an alloy with excellent mechanical strength, intergranular corrosion resistance, and stress corrosion cracking resistance, and has excellent properties in the chemical industry and energy industry, especially as equipment for nuclear power generation.

[Brief explanation of the drawing]

Figure 1 is a perspective view of a welded specimen subjected to a corrosion test, Figure 2 is a diagram showing the influence of C content and N content on mechanical strength, and Figure 3 is a diagram showing the effect of (C+N) on mechanical strength. FIG. 4 is a diagram showing the influence of C content and Nb content on the maximum corrosion degree d in intergranular corrosion tests.

Claims (1)

[Claims] 1 C0.030% or less, Si1.0% or less, Mn1.0% or less, Cr more than 26% and 35% or less, Fe25% or less, P0.030
% or less, S0.030% or less, N0.2% or less Sum of C and N
A Ni-based alloy containing 0.040% or more, B0.005% or less, and Mg 0.05% or less, with the remainder being essentially Ni, which has excellent intergranular corrosion resistance, stress corrosion cracking resistance, and mechanical strength. 2 C0.030% or less, Si1.0% or less, Mn1.0% or less, Cr26% or more and 35% or less, Fe25% or less, P0.030
% or less, S 0.030% or less, N 0.2% or less, sum of C and N 0.040% or more, B 0.005% or less, Mg 0.05% or less,
Contains 1% or less of any one or more selected from Ti, Zr, and Al in total, and the remainder is substantially
A Ni-based alloy made of Ni that has excellent intergranular corrosion resistance, stress corrosion cracking resistance, and mechanical strength. 3 C0.080% or less, Si1.0% or less, Mn1.0% or less, Cr26% or more and 35% or less, Fe25% or less, P0.030
% or less, S0.030% or less, N0.2% or less, Nb4% or less and 50(%C-0.030)% or more, and [2.0-
50(%C+%N)〕% or more, B0.005% or less,
A Ni-based alloy that contains 0.05% or less of Mg and the remainder is essentially Ni, and has excellent intergranular corrosion resistance, stress corrosion cracking resistance, and mechanical strength. 4 C0.080% or less, Si1.0% or less, Mn1.0 or less,
Cr26% or more and 35% or less, Fe25% or less, P0.030% or less, S0.030% or less, N0.2% or less, Nb4% or less, and 50(%C-0.030)% or more, and [2.0-50
(%C+%N)〕% or more, B0.005% or less, Mg0.05
% or less, containing any one or two or more selected from Ti, Zr, and Al in total of 1% or less, with the remainder consisting essentially of Ni. Intergranular corrosion resistance, stress corrosion cracking resistance, and mechanical properties. Ni-based alloy with excellent strength.