JP2000218392A - Corrosion resistant cobalt group welding material, tig welding electrode composed of material, plasma transfer arc welding powder, covered electrode, and valve - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion resistant cobalt group welding material which can be used in a corrosive fluid without causing grain boundary corrosion. SOLUTION: This corrosion resistant cobalt group welding material is chemically composed of, 0.20-0.50 wt.% C, 25.0-30.0 wt.% Cr, 4.0 wt.% or less Ni, 5.0-6.0 wt.% Mo, 1.6-10.0 wt.% Nb, and the balance of substantially Co. Nb is easier than Cr to be combined with C in the material, and so C in the alloy is combined with Nb to generate NbC (niobium carbide). Then, since Cr is remained in a matrix, Cr carbide is not generated in the alloy. As a result, Cr shortage layers are not caused at an adjacent part of the Cr carbide, so as to prevent such a grain boundary corrosion that a local part around the Cr carbide is corroded.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material to be build-up welded on equipment used with a corrosive fluid, and more particularly to a corrosion-resistant cobalt-based welding material.

[0002]

2. Description of the Related Art Corrosion resistance and cavitation resistance of valves and other fluids that are used for corrosive fluids and that are of a type other than a type in which a valve seat slides, such as a gate valve, are known. Where necessary, referred to as Stellite No. 21,
Chemical components are% by weight, C: 0.20 to 0.30, Cr:
25.5-29.0, Ni: 1.75-3.25 or less,
Mo: 5.0 to 6.0, and the rest has been applied with build-up welding using a cobalt-based welding material composed of Co.

[0003]

However, when this welding material is applied to carbon steel or austenitic stainless steel, build-up welding is performed such that the penetration of the base material (mainly iron: Fe) is 5% or less. If not performed, carbides are formed at the crystal grain boundaries of the welded portion, and there is a tendency that intergranular corrosion occurs. It has been known that such a tendency is promoted as the penetration amount of the base material increases.

FIG. 1 shows a normal stellite No. 2
It is a schematic diagram which shows the distribution state of carbide | carbonized_material of 1 welding part. As shown in FIG. 1, the original metal structure of stellite No. 21 as a welding material is such that a carbide mainly composed of Cr is distributed in a matrix in a Co-Cr-Mo matrix.

[0005] However, as the penetration of the base material increases, the amount of carbides in the form of dots decreases, and as shown in the schematic diagram of FIG. 2, carbides are generated at the crystal grain boundaries. This phenomenon is caused by the fact that when Fe, which mainly constitutes the base material, dissolves, the amount of Cr in the alloy relatively decreases, so that carbide cannot be generated in crystal grains where solidification occurs first, and the solute is concentrated. This tends to increase the tendency of carbide formation at the crystal grain boundaries to be formed.

At this time, when carbide is generated at the crystal grain boundary, the amount of Cr in the vicinity of the grain boundary is extremely reduced because the carbide is a compound mainly composed of Cr, and this Cr-deficient layer is selectively formed. Corrosive conditions are formed. As a result, there arises a problem that intergranular corrosion occurs.

On the other hand, in welding using an electric arc, penetration of Fe is an unavoidable problem. Therefore, such a tendency of intergranular corrosion always occurs, and therefore, a material equivalent to stellite No. 21 which does not cause such intergranular corrosion even when the penetration of Fe occurs by performing arc welding is developed. It was desired.

Accordingly, the present invention has been made in order to solve the above-mentioned problems of the prior art, and provides a corrosion-resistant cobalt-based welding material which does not cause intergranular corrosion and can be used in a corrosive fluid. Its primary purpose is to provide.

[0009]

In order to achieve the above object, according to the present invention, there is provided a corrosion-resistant cobalt-based welding material comprising:
0, Cr: 25.0 to 30.0, Ni: 4.0 or less, M
o: 5.0 to 6.0, Nb: 1.6 to 10.0, and the balance substantially consists of Co.

[0010] The corrosion-resistant cobalt-based welding material includes a TIG welding rod, a powder for plasma transfer arc welding,
It can be applied to a covered arc welding rod, a valve and the like.

[0011]

[Function] Corrosion-resistant cobalt-based welding material is made of Stellite No.
By adding Nb to the material corresponding to No. 21 to make the above-mentioned chemical components, Nb is more likely to be present in the material than in Cr.
C in the alloy is bonded to Nb to form N
Since bC (niobium carbide) is generated and Cr remains in the matrix, the generation of Cr carbide in the alloy is eliminated. As a result, a Cr deficiency layer adjacent to the Cr carbide is not generated, thereby preventing intergranular corrosion in which a local portion around the Cr carbide is corroded.

Further, a TIG welding rod for welding a valve exposed to a corrosive fluid or a device exposed to the fluid, a powder for plasma transfer arc welding, a coated arc, When used for a welding rod or the like, a highly reliable device or the like can be obtained because of its excellent corrosion resistance.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the grounds for selecting the ratio of each chemical component of the corrosion-resistant cobalt-based welding material according to the present invention will be described.

C (carbon) is a conventional stellite No. 21
In the case of, bond with Cr to form carbides, increase the hardness of the alloy and contribute to maintaining the strength. On the other hand, the material according to the present invention combines with Nb to form Nb carbide (NbC) and also increases the hardness of the alloy, which is effective for maintaining a large strength. For this reason,
If the C content is small, the required hardness and strength cannot be obtained,
If the amount is too large, the amount of Nb for converting C into Nb carbide increases. Therefore, the range of the content of C is% by weight,
0.20 to 0.50%.

[0015] Cr (chromium) improves the oxidation resistance of the present alloy, and also improves the corrosion resistance to oxidizing acids such as nitric acid. For this reason, the range of the content of Cr is% by weight,
25.0 to 30.0%.

Ni (nickel) has similar properties to Co, but has higher oxidation resistance than Co. Also, Ni
Improves the corrosion resistance to reducing acids such as sulfuric acid,
When contained in a large amount, it promotes the Co alloy to be face-centered cubic and prevents the Co alloy from transforming into a hexagonal ε phase. The reason why the Co-based alloy exhibits excellent cavitation resistance is that the impact energy at the time of collapse of bubbles in the fluid is absorbed by transforming from face-centered cubic to hexagonal. Therefore, the range of the content of Ni is as follows:
The upper limit is set to 4.0%.

Mo (molybdenum) forms a solid solution mainly in a matrix of Co and is effective for improving high-temperature strength. Therefore, the content of Mo is 5.0 to 5.0% by weight.
6.0%.

Nb (niobium) combines with C to form Nb carbide and fixes C as Nb carbide so that Cr carbide is not formed. Therefore, in the present invention, it is the most important element. Further, the amount of Nb to fix C is more effective than C by 8 times. However, in the present invention, in order to make this effect more thorough,
Add up to 20 times C. Therefore, the content of Nb is set to 1.6 to 10.0% by weight.

From the above study, it was found that stellite N
The chemical composition of the corrosion-resistant cobalt-based welding material according to the present invention, which corresponds to O21, is represented by C: 0.20-0.
50, Cr: 25.0 to 30.0, Ni: 4.0 or less,
Mo: 5.0 to 6.0, Nb: 1.6 to 10.0, and the balance were selected to be substantially composed of Co. Next, the mechanical properties of the corrosion-resistant cobalt-based welding material according to the present invention will be discussed. In conducting the study, the present inventors conducted the following two types of comparative experiments.

[Comparative experiment 1] The present inventor mixed a metal powder, arc-melted in an inert gas atmosphere in argon, and obtained the results shown in Table 1 (attached to the end of the detailed description of the invention in the specification).
About 200 of Φ40 × 20t having the chemical composition shown in
g of alloy ingot was prepared and used as a test sample. As test samples, comparative samples simulating the case where carbon steel was dissolved in Stellite No. 21: F1, F2, and about 5% N
Five types of FN1, FN2 and FN3 to which b was added were prepared. Note that these test samples were subjected to stress relief annealing at 650 ° C. for 4 hours as in the case of the actual product.

In the test, a corrosion test specimen was processed from these test samples, and subjected to a 16-hour intergranular corrosion test using a boiling solution of sulfuric acid / copper sulfate specified in JIS G0575.
And 24 hours of intergranular corrosion test using a boiling solution of sulfuric acid and ferric sulfate specified in G0572.

Table 1 shows the test results in each test. As can be seen from Table 1, Samples F1 and F2 to which Nb was not added caused significant intergranular corrosion in both tests.

However, the sample FN to which Nb was added
1, FN2 and FN3 did not cause intergranular corrosion in a test using a sulfuric acid / copper sulfate solution. In the test using a sulfuric acid / ferric sulfate solution, the sample FN1 was 0.04 m
m, intergranular corrosion of 0.05 mm occurred in sample FN2,
Compared to the intergranular corrosion amounts of 0.48 mm and 0.69 mm of the samples F1 and F2 to which Nb was not added, the corrosion depth was about 1/10 and much smaller.

In a test of a sample FN3 having a high iron content in a sulfuric acid / ferric sulfate solution, general corrosion occurred, and no intergranular corrosion was observed.

In general, the intergranular corrosion resistance of a metal material is, for example, in a corrosive environment of about seawater, if the intergranular corrosion does not occur in a test using sulfuric acid / copper sulfate, a sufficient intergranular corrosion resistance is practically sufficient. It may be evaluated that there is. Also, the amount of penetration of the base material (mainly Fe) by welding is often about 15 to 30%. Accordingly, it can be determined from the test results that the welded portion welded using the material according to the present invention has sufficient corrosion resistance when used in seawater, steam, and high-temperature and high-pressure water.

The test using a sulfuric acid / ferric sulfate solution is as follows.
This is a test for more severe intergranular corrosion conditions. But,
Even under this test condition, the welded portion using the material according to the present invention has a higher weldability than the welded portion using stellite No. 21 even when iron is penetrated by about 35%.
It showed much better intergranular corrosion resistance.

FIG. 3 shows the Vickers hardness H of the welded portion using Stellite No. 21 and the material according to the present invention.
v is shown in relation to the Fe content. Vickers hardness is an index indicating the strength of the welded portion and the cavitation erosion resistance. As shown in FIG. 3, for the same Fe content, the weld using the material according to the present invention is harder than the weld using Stellite No. 21, and the mechanical properties of the weld are higher. It was presumed that the material according to the present invention was excellent.

[Comparative Test 2] Next, the present inventors mixed the metal powder, arc-melted it in an argon inert gas atmosphere, and attached Table 2 (attached to the end of the detailed description of the invention in the specification). ) Having a chemical composition shown in FIG.
An alloy lump of 00 g was manufactured and used as a test sample. As test samples, Stellite No. 21 itself and Stellite N
Comparative samples simulating the case where stainless steel (SUS316L) was dissolved in o21: S0 to S4, and SN0 to SN4 to which Nb was added to 1.3 to 2.5%, and SN5 to about 5% of Nb were added. Fourteen types of SN8 were prepared. These test samples were subjected to a solution treatment of air cooling at 1070 ° C. × 4 hours as in the actual product.

In the test, a corrosion test piece was processed from these test samples, and was subjected to JIS G05 as in [Comparative Test 1].
Two types of tests were performed: a 16-hour intergranular corrosion test using a boiling solution of sulfuric acid and copper sulfate specified in 75, and a 24-hour intergranular corrosion test using a boiling solution of sulfuric acid and ferric sulfate specified in JIS G0572. did.

Table 2 shows the test results in each test. As can be seen from Table 2, in the corrosion test using a boiling solution of sulfuric acid / copper sulfate, the intergranular corrosion was observed regardless of whether Nb was added or not, regardless of whether or not Nb was added, both in the case where stellite No21 itself and stainless steel were dissolved in stellite No21. Did not occur.

On the other hand, in a corrosion test of a sulfuric acid / ferric sulfate in a boiling solution, when iron did not dissolve into Stellite No. 21, the effect of Nb addition was not recognized. However, when the penetration of iron was 15 to 27%, Nb was 2.1 to 4.8 as compared with the samples without Nb (S1 and S2).
% Added samples (SN1 and SN2, SN6 and SN
In (7), the intergranular corrosion depth is about 1 compared to the case of no additive.
/ 2. The penetration of stainless steel is 35%
In the above description, the grain boundary corrosion property of the sample without Nb (S3 and S4) was better than that of the sample with Nb additive (SN3 and SN4).

As described above, regarding the intergranular corrosion resistance in seawater, steam and high-temperature and high-pressure water environments, if the intergranular corrosion does not occur in a test of a sulfuric acid / sulfuric acid solution, a practically sufficient grain boundary corrosion resistance is obtained. It can be evaluated as corrosive. Therefore, since no intergranular corrosion occurred in all test samples, it is not necessary to use the material according to the present invention as a welding material for stainless steel used in such a use environment.

However, in a more severe environment where the intergranular corrosion environment is typified by sulfuric acid / ferric sulfate boiling solution, the material according to the present invention is used for welding a welded portion using this material. This is particularly effective when the steel is dissolved up to an Fe content of 15 to 30%. This penetration amount corresponds to the amount that is often observed in the penetration amount of the base material in ordinary welding.

Next, a comparative study based on the difference in the amount of Nb added will be described. The amount of Nb added was determined according to Samples SN3 and SN in Table 2.
As can be seen from comparison with Sample No. 8, the sample with the Nb content of 4.4% exhibited better corrosion resistance in the range where the amount of Fe penetration was larger than the sample with the Nb content of 1.6%. .

However, samples SN0-SN3 and SN5
Examining the ratio of Nb / C of ~ SN8 and the depth of intergranular corrosion, there is no significant difference in the intergranular corrosion depth for Nb / C: 14 to 20 times and 29 to 49 times. Therefore, the Nb / C ratio of the material according to the present invention is selected up to 20 times C as described above.

FIG. 4 shows the Vickers hardness H of the welded portion using Stellite No. 21 and the material according to the present invention.
v is shown in relation to the Fe content. As described above, the Vickers hardness is an index indicating the strength of the welded portion and the cavitation / erosion resistance. As shown in FIG. 4, the Vickers hardness of the welded portion using the material to which Nb according to the present invention is added is higher than that of the welded portion using Stellite No. 21 to which Nb is not added. As for the amount of Nb added, the amount of Nb added is 1.3 to 2.2.
The sample with 4.2-4.8% addition became harder than the sample with 5%. Therefore, it was presumed that the mechanical properties of the welded portion welded using the material according to the present invention were superior to those welded using Stellite No. 21.

From the above two types of test results, when welding Stellite No. 21 to stainless steel and using it in a corrosive environment such as steam, high-temperature water, or seawater, N
No addition of b is necessary. However, it has been found that when stainless steel is used in a more severe corrosive environment, it is effective to use the cobalt-based welding material according to the present invention to which Nb is added. As described above, the case where the corrosion-resistant cobalt-based welding material according to the present invention is applied to a welding rod has been described, but the present invention is not limited to this example.
It can be suitably applied to valves used in severe corrosive environments.

[0038]

According to the present invention, the corrosion-resistant cobalt-based welding material according to the present invention is obtained by adding Nb to a material corresponding to stellite No. 21 to obtain a chemical component as described in claim 1.
Since Nb is more easily bonded to C in the material than Cr, C in the alloy combines with Nb to form NbC (niobium carbide), and Cr remains in the matrix. The generation of Cr carbide is eliminated. Therefore, Cr
The Cr deficiency layer adjacent to the carbide is not generated, thereby preventing intergranular corrosion in which a local portion around the Cr carbide is corroded.

Further, a TIG welding rod for welding a valve exposed to a corrosive fluid or a device exposed to the fluid, a powder for plasma transfer arc welding, a coated arc, When used for a welding rod or the like, a highly reliable device or the like can be obtained because of its excellent corrosion resistance.

[Table 1]

[Table 2]

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a distribution state of carbides in a normal stellite No. 21 weld.

FIG. 2 is a schematic diagram showing a distribution state of carbides of Stellite No. 21 in which about 30% of iron is dissolved.

FIG. 3 shows the relationship between Fe penetration and hardness when Stellite No. 21 and the material of the present invention are applied to carbon steel.

FIG. 4 shows the relationship between Fe penetration and hardness when Stellite No. 21 and the material of the present invention are applied to stainless steel.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Katsumi Hirano 1-14 Nakamachi, Moji-ku, Kitakyushu City, Fukuoka Prefecture Inside of Okano Valve Manufacturing Co., Ltd. (72) Tatsumi Watanabe 1-14 Nakamachi, Moji-ku, Kitakyushu City, Fukuoka Okano Valve Manufacturing Co., Ltd.

Claims (5)

[Claims] C. 0.20 to 0.1% by weight of a chemical component.
0.50, Cr: 25.0 to 30.0, Ni: 4.0 or less, Mo: 5.0 to 6.0, Nb: 1.6 to 10.0,
Corrosion-resistant cobalt-based welding material whose balance substantially consists of Co. 2. A TIG welding rod made of the corrosion-resistant cobalt-based welding material according to claim 1. 3. A powder for plasma transfer arc welding comprising the corrosion-resistant cobalt-based welding material according to claim 1. 4. A coated arc welding rod made of the corrosion-resistant cobalt-based welding material according to claim 1. 5. A valve to which the corrosion-resistant cobalt-based welding material according to claim 1 is applied.