JPS6356036B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] The present invention relates to Inconel-based alloys, and particularly to the composition of Inconel-based alloys that have excellent intergranular stress corrosion cracking properties. [Prior art] Inconel welding materials containing 13 to 20% by weight of chromium and 65 to 75% by weight of nickel are used in welds or weld overlays of various industrial equipment used in corrosive environments. It may be used in a corrosive environment in the welded state. Inconel alloys are said to have corrosion resistance, but the welded metal parts are actually tested using the sulfuric acid-ferric sulfate corrosion test method (Japanese Industrial Standard G0572), which is known as an accelerated test for intergranular corrosion or stress corrosion cracking. According to the evaluation, intergranular corrosion occurs in conventional products, and the corrosion resistance is not necessarily satisfactory. As a nickel-based alloy having excellent stress corrosion cracking resistance, an invention as described in, for example, Japanese Unexamined Patent Publication No. 51-52320 has been proposed. In the nickel-based alloy of this invention, titanium is added mainly to stabilize carbides, and its content is set at 0.5% by weight or less, considering both stress corrosion cracking resistance and intergranular corrosion resistance. The ratio Ti/C of carbon and titanium in the alloy is 4 to 7. In addition, Nb in the alloy is added to stabilize carbides, and if it is added in excess of 0.3% by weight, it will impair intergranular corrosion resistance and stress corrosion cracking resistance, so its content is regulated to 0.3% by weight or less. It is said that it is necessary to In this nickel-based alloy, the ratio of titanium and niobium contents (Ti+0.5Nb)/C, which corresponds to the carbon content in the alloy, is in the range of 6 to 9. Although this nickel-based alloy is effective under the conditions of use in high-temperature, high-pressure water, such as in light water reactors for nuclear power generation, the amount of coexistence of titanium and niobium relative to carbon in the alloy is inappropriate, and it is particularly susceptible to intergranular stress corrosion. Regarding the properties, the grain boundary erosion rate is high, and the performance is insufficient in that respect. [Objective] An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide an Inconel alloy that is particularly excellent in intergranular stress corrosion cracking properties among corrosive properties. [Structure] In order to achieve the above-mentioned object, the present invention provides an Inconel alloy in which the weight ratio Ti/C of the titanium content (Ti) to the carbon content (C) is 1.4 or less,
And the weight ratio of niobium content (Nb) and titanium content (Ti) to carbon content (C) is calculated using the following formula, and the value of the stabilization parameter is 7 to 30.
It is characterized by being regulated within the range of . = 0.13 (Nb + 2Ti) / C As a result of various studies on the intergranular stress corrosion cracking properties of Inconel alloys, the present inventors found that the precipitation and growth of chromium carbides at grain boundaries cause intergranular stress corrosion cracking. I found out what was causing it. As a countermeasure to this, in other words, as a method to suppress the precipitation of chromium carbides, we have determined from various experimental results that the amount of carbon in the alloy and the ratio of the amount of titanium and niobium added to stabilize the carbon without combining with chromium. It was discovered that an Inconel alloy with excellent intergranular stress corrosion cracking properties can be obtained by setting a specific ratio of . In other words, as mentioned above, titanium and niobium are added as stabilizing elements and to improve mechanical strength and weldability at high temperatures, but if the amount of titanium is large compared to carbon, The remaining unreacted titanium reacts with surrounding nickel to form Ni ₃ Ti. This formation facilitates the formation of a γ' phase, which is easily corroded in the sulfuric acid-ferric sulfate corrosive solution, and the intergranular corrosion resistance is reduced. Further, when the γ' phase precipitates, ductility decreases and cracks occur due to weld cracking or heat treatment after welding. On the other hand, niobium has the feature that even if there is residual niobium that does not react with carbon, it hardly reacts with nickel, and the γ' phase does not precipitate, so the above-mentioned disadvantages do not occur. Therefore, even if titanium is added, it is necessary to suppress the Ti/C ratio, and according to experiments by the present inventors, Ti/C is limited to 1.4 or less from the viewpoint of corrosion cracking resistance. Under these conditions where the ratio of titanium to carbon is kept low, if the stabilization parameter is 7 or more in the formula below, the Inconel alloy will virtually never experience intergranular stress corrosion cracking even in a corrosive environment. becomes. In addition, Nb in the formula,
Ti and C are the contents (% by weight) in the alloy. =0.13(Nb+2Ti)/C The following Table 1 shows the relationship between Inconel alloys of various compositions and stabilization parameters. In addition, the units of numerical values except for are weight%.
In addition to the components shown in the table, each alloy contains trace amounts of silicon, manganese, phosphorus, sulfur, etc., but their descriptions are omitted. Note that the numbers in parentheses in the table indicate the Nb content.

【table】

[Table] Figure 1 shows the results of measuring the grain boundary erosion rate based on the sulfuric acid-ferric sulfate corrosion test method described above after heat treating the various sample alloys shown in this table under the same conditions. As shown in this figure, the grain boundary erosion rates of each sample alloy are as shown in Table 2 below.

〔effect〕

As described above, the present invention provides that the amount of niobium and titanium for stabilizing the carbon in the Inconel alloy is such that the Ti/C (weight ratio) is 1.4 or less and satisfies the above formula. Products with a stabilization parameter of 7 or more when calculated based on the above have excellent intergranular stress corrosion cracking properties even under extremely severe corrosion conditions such as sulfuric acid-ferric sulfate corrosion tests. A highly reliable Inconel alloy can be provided.

[Brief explanation of drawings]

FIG. 1 is a characteristic diagram showing the relationship between stabilization parameters and grain boundary erosion rate. Figures 2 and 3 are micrographs of conventional Inconel alloy specimens and Inconel alloy specimens according to the present invention after corrosion tests;
5 and 6 are enlarged sectional views of essential parts showing examples of use of the Inconel alloy according to the present invention. 5, 7... Welded metal part, 13... Fillet weld part.

Claims (1)

[Claims] 1. 67 to 72% by weight of nickel, 14 to 15% by weight of chromium, 0.02 to 0.04% by weight of carbon, and 0.01 to 0.01% of titanium.
Nickel-based alloy containing 0.04% by weight, titanium content (Ti) relative to carbon content (C) in the alloy
The value of the weight ratio Ti/C is 1.4 or less, and the value of the stabilization parameter is 7 to 30 by calculating the weight ratio of the niobium content (Ti) to the carbon content (C) using the following formula. An Inconel alloy with excellent intergranular stress corrosion cracking properties that are regulated within a certain range. =0.13(Nb+2Ti)/C