JP2001003149A - METHOD FOR HEAT TREATING Co BASE ALLOY - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for heat treating a Co base alloy capable of suppressing deterioration in the intergranular corrosion resistance of a Co base alloy even in the case arc welding such as PTA welding and TIG welding is used. SOLUTION: This method includes a process in which a Co base alloy having chemical components of, by weight, 0.9 to 1.4% C, <=1.0% Mn, 0.4 to 2.0% Si, 26.0 to 32.0% Cr, 3.0 to 6.0% W, <=1.0% Mo, <=3.0% Ni, <=3.0% Fe, and the balance Co with <=0.5% other impurities is welded to a carbon steel and is heated to >=700 deg.C at the point of time in which the content of Fe reaches >=5.0%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for heat-treating a Co-based alloy.

[0002]

2. Description of the Related Art As a conventionally known Co-based alloy, its chemical component is expressed by weight percent and C: 0.9 to 0.9%.
1.4%, Mn: 1.0% or less, Si: 0.4 to 2.0
%, Cr: 26.0 to 32.0%, W: 3.0 to 6.0
%, Mo: 1.0% or less, Ni: 3.0% or less, Fe:
3.0% or less, other impurities are 0.5% or less,
There is an alloy whose balance is made of Co. This alloy is generally sold under the trade name “Stellite No. 6”, and is mainly used in valves of valve devices because of its excellent corrosion resistance, seizing wear resistance and cavitation resistance. Widely used for seats.

Further, as another Co-based alloy, the chemical composition is C: 0.2-0.3% by weight,
i: 0.9 to 1.5%, Mn: 1.0% or less, Ni:
1.75 to 3.25%, Fe: 2.0% or less, Cr: 2
5.5-29.0%, Mo: 5.0-6.0%, B:
There is an alloy containing 0.007% or less, with the balance being Co. This alloy is generally sold under the trade name “Stellite No. 21” and has excellent corrosion resistance and cavitation resistance. It is used for the valve seat of the throttle valve.

When using Stellite No. 6
Has been generally welded to the base metal by gas welding, but recently, from the viewpoint of reducing the number of gas welding engineers and improving welding efficiency, plasma transfer
There is a tendency that arc welding (hereinafter, referred to as “PTA welding”) and TIG welding (hereinafter, referred to as “TIG welding”) are employed. In addition, Stellite No. In most cases, 21 is conventionally welded to the base material by PTA welding or TIG welding. When welding is performed, post-heat treatment is performed. This heat treatment has effects of both softening of the heat-affected zone caused by welding and removal of residual stress, and is referred to as stress relief annealing. Have been.

Here, in general welding between carbon steels, a heat treatment condition of 595 ° C. or higher is set forth in “Ministerial Ordinance for Specifying Technical Standards for Welding Electric Workpieces” (Ordinance of the Ministry of International Trade and Industry No. 81 of 1970). It is specified that post-weld heat treatment should be performed in a temperature range of 700 ° C. or less.

Therefore, Stellite No. 6 and Stellite No. 6 Also, when a Co-based alloy such as No. 21 (hereinafter, also simply referred to as “stellite”) is welded to carbon steel, generally, the heat treatment conditions specified in the above Ministerial Order No. 81 are applied, and 650 ° C. ± 25. Post-heat treatment in the temperature range of ° C. has conventionally been customary.

[0007]

However, according to a test on intergranular corrosion by the present applicant, the above-mentioned PT
When welding stellite to carbon steel by electric arc welding such as A welding or TIG welding, the base metal inevitably penetrates, the Fe content of stellite increases, and due to the increase in Fe content, It has been confirmed that the intergranular corrosion resistance of stellite is significantly reduced.

[0008] In the post-heat treatment, it is known that the temperature range specified in the above ministerial ordinance No. 81 concerning welding of carbon steels is a temperature range in which stellite welding greatly promotes intergranular corrosion.

Accordingly, the present invention has been made to solve the above-mentioned conventional problems, and has been made in consideration of PTA welding and TI.
An object of the present invention is to provide a heat treatment method for a Co-based alloy that can suppress a decrease in intergranular corrosion resistance of a Co-based alloy even when using arc welding such as G welding.

[0010]

In order to achieve the above-mentioned object, according to the heat treatment method of the present invention, the chemical components are as follows: C: 0.9 to 1.4%; Mn: 1.0% or less; S
i: 0.4 to 2.0%, Cr: 26.0 to 32.0%,
W: 3.0-6.0%, Mo: 1.0% or less, Ni:
A Co-based alloy consisting of 3.0% or less, Fe: 3.0% or less, other impurities of 0.5% or less, and the balance being Co, was welded to carbon steel, and the Fe content was 5.0%. % Or more, heating to 700 ° C. or more.

In another heat treatment method of the present invention for achieving the same object, the present invention provides a heat treatment method, wherein the chemical component is C: 0.2 to
0.3%, Si: 0.9 to 1.5%, Mn: 1.0% or less, Ni: 1.75 to 3.25%, Fe: 2.0% or less, Cr: 25.5 to 29 0.0%, Mo: 5.0-6.
0%, B: 0.007% or less, Co-based alloy consisting of Co with the balance being welded to carbon steel, the Fe content being 5.
The method is characterized by including a step of heating to 700 ° C. or more when the temperature becomes 0% or more.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, as an embodiment of the present invention, a stellite test material subjected to a heat treatment of the present invention will be described. First, in the welding of stellite, the amount of carbon steel, which is the base metal,
In the case of A welding, the welding conditions
Less than 15% and 30% even in TIG welding
The following can be managed.

Therefore, Stellite No. 6 or Stellite No. 6 A plurality of primary test materials having a Fe content in stellite in the range of about 0 to 40% were produced by mixing PTA powder No. 21 and Fe powder and casting using an arc furnace in an argon atmosphere. FIG. 1 shows the dimensions of the cast primary test material. That is, these primary test materials have an upper surface diameter of 50 mm, a lower surface diameter of 40 mm, and a height of 2 mm.
0 mm truncated cone. Also, in Table 1 below, stellite No. 7 types of primary test materials S0, S5, S10, S15, S20, S3 by mixing 6 powder and Fe powder
0 and S40 are shown in Table 2.
o. 21 shows the chemical components of six types of primary test materials T0, T5, T10, T15, T20, and T30 obtained by mixing powder No. 21 and Fe powder.

[0014]

[Table 1]

[0015]

[Table 2]

[0016] These primary test materials were added at 650 ° C and 750 ° C.
A heat treatment was performed at 4 ° C. and 900 ° C. for 4 hours. here,
Since the heat treatment time is generally one hour per 25 mm of the product thickness, the heat treatment time of four hours corresponds to the heat treatment time of a product having a thickness of 100 mm. Then, the cast primary test material after the heat treatment is processed to have a diameter of 16 as shown in FIG.
A disk-shaped secondary test material having a thickness of 3 mm and a thickness of 3 mm is manufactured.

After the heat treatment, these secondary test materials were subjected to JI
The corrosion test method for sulfuric acid and copper sulfate of stainless steel specified in S Standard G0575 (hereinafter referred to as “Strauss test”) was performed for 16 hours, and the Fe content of the secondary test material and the temperature of heat treatment showed intergranular corrosion. A test was conducted on the effect on the properties. 3 and 4 show Stellite No. 6 and Stellite No. 6 21 shows the test results of the test material No. 21.

As shown in FIG.
When the test material composed of No. 6 is heat-treated at 650 ° C., intergranular corrosion does not occur when the Fe content is about 1%, but intergranular corrosion starts to occur when the Fe content is 5%, and further F
As the e content increases, intergranular corrosion remarkably progresses.
That is, this indicates that the contained Fe significantly promotes intergranular corrosion.

When heat treatment was carried out at 750 ° C., intergranular corrosion did not occur up to an Fe content of 15%, but intergranular corrosion was observed at an Fe content of 20%.

Further, when heat-treated at 900 ° C.,
No intergranular corrosion occurred up to a content of 30%, and the occurrence of intergranular corrosion was recognized only when the Fe content was 40%.

As described above, Stellite No. In the test material composed of No. 6, the intergranular corrosion was remarkably promoted by the heat treatment at 650 ° C., but when the heat treatment temperature was high, such as 750 ° C. and 900 ° C., the intergranular corrosion susceptibility was low even when the Fe content was large. Got lower.

Similarly, based on FIG.
o. The test material 21 is described. First, 65
When heat treatment was performed at 0 ° C., intergranular corrosion did not occur when the Fe content was about 1%. However, when the Fe content became 5%, intergranular corrosion began to occur, and when the Fe content further increased, the grain boundary corrosion increased. Interfacial corrosion progressed significantly.

In the case of heat treatment at 750 ° C., intergranular corrosion did not occur up to an Fe content of 19%, but intergranular corrosion was observed at an Fe content of 33%.

Further, when heat-treated at 900 ° C.,
Although the intergranular corrosion with a content of 33% and a depth of 0.05 mm occurred, the intergranular corrosiveness almost disappeared.

The intergranular corrosion of stellite is not caused by carbides (hereinafter referred to as "primary carbides") that crystallize when the stellite solidifies, and similarly to the austenitic stainless steel 300 series stainless steel. This is caused by the formation of a Cr-deficient layer adjacent to a carbide (hereinafter referred to as “secondary carbide”) precipitated by the heat treatment.

Intergranular corrosion susceptibility is observed when heated to 420-650 ° C. At 650 ° C., the Cr concentration in the secondary carbide is high, as conceptually shown in FIG.
Therefore, the Cr concentration in the Cr-deficient layer adjacent to the secondary carbide is low, and intergranular corrosion is promoted. However, at 750 ° C. and 90
Since the Cr concentration of the secondary carbide precipitated at 0 ° C. is low, the Cr concentration of the Cr-deficient layer adjacent to the secondary carbide is high, and the high heat treatment temperature causes the diffusion of Cr to eliminate the Cr-deficient layer.

Here, Fe promotes intergranular corrosion. First, Fe dissolves in stellite and forms a solid solution with Co, the main element of stellite, to reduce the Cr concentration in the entire stellite. It may reduce the corrosion resistance of the matrix (base) of stellite. Second,
Secondary carbide having a high Cr concentration precipitates out of the matrix having a reduced corrosion resistance, depleting Cr in a portion adjacent to the secondary carbide, and further reducing the corrosion resistance of this portion. In this case, at 650 ° C., the Cr concentration of the secondary carbide is higher than in the case of the heat treatment at a higher temperature, so that the lowering of the Cr concentration in the Cr-deficient layer is further promoted. Third, Fe
If contained, the diffusion of Cr may be delayed. Further, since the diffusion of Cr becomes smaller at low temperatures and becomes larger at high temperatures, it takes more time at 650 ° C. than at a higher heat treatment temperature.

Thus, the Fe content has the effect of increasing the susceptibility of stellite to intergranular corrosion. And, as described above, even when Fe is contained,
Heat treatment at a temperature higher than 0 ° C. reduces the susceptibility of stellite to intergranular corrosion, but the lower limit of the heat treatment temperature at which this effect is substantially recognized is 700 ° C.

These heat treatment conditions are also effective in reducing the residual stress in the stellite welded portion as compared with the heat treatment at 650 ° C., and the propagation of stellite into cracks caused by the occurrence of intergranular corrosion occurs. Can be prevented.

[0030]

As described above, according to the heat treatment method of the present invention, the intergranular corrosion susceptibility of a Co-based alloy such as stellite can be reduced, and the arc welding such as PTA welding or TIG welding can be used. However, a decrease in intergranular corrosion resistance of the Co-based alloy can be suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram showing the shape and dimensions of a primary test material.

FIG. 2 is a diagram showing the shape and dimensions of a secondary test material.

FIG. 6 is a diagram showing the results of an intergranular corrosion test of a test material No. 6; FIG.

FIG. FIG. 9 is a diagram showing the results of a grain boundary corrosion test of a test material consisting of No. 21;

FIG. 5 is a schematic diagram showing a cause of occurrence of intergranular corrosion.

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[Procedure amendment]

[Submission date] June 16, 2000 (2006.1.
6)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0028

[Correction method] Change

[Correction contents]

Thus, the Fe content has the effect of increasing the susceptibility of stellite to intergranular corrosion. And, as described above, even when Fe is contained,
Heat treatment at a temperature higher than 0 ° C. reduces the susceptibility of stellite to intergranular corrosion, but the lower limit of the heat treatment temperature at which this effect is substantially recognized is 700 ° C. Therefore, in the present invention,
700 ° C or less when the Fe content becomes 5.0% or more
Heat on.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) C22F 1/00 650 C22F 1/00 650C 691 691A 694 694B

Claims (2)

[Claims] 1. The method according to claim 1, wherein the chemical components are in weight percent and C: 0.1.
9 to 1.4%, Mn: 1.0% or less, Si: 0.4 to
2.0%, Cr: 26.0-32.0%, W: 3.0-2.0%
6.0%, Mo: 1.0% or less, Ni: 3.0% or less,
When a Co-based alloy containing 3.0% or less of Fe and 0.5% or less of other impurities and the balance of Co is welded to carbon steel, and the Fe content becomes 5.0% or more. To 700
A heat treatment method including a step of heating to at least C. 2. The chemical composition, in weight percent, C: 0.
2 to 0.3%, Si: 0.9 to 1.5%, Mn: 1.0
% Or less, Ni: 1.75 to 3.25%, Fe: 2.0%
Hereinafter, Cr: 25.5 to 29.0%, Mo: 5.0 to 5.0
6.0%, B: 0.007% or less, the balance is Co-based alloy consisting of Co is welded to carbon steel, and when the Fe content is 5.0% or more, the temperature is increased to 700 ° C or more. A heat treatment method including a heating step.