JP2000046181A - Stainless steel double tube bellows with improved stress corrosion cracking resistance and heat retaining property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide bellows that is free from stress corrosion cracking and is excellent in heat retaining property. SOLUTION: A stainless steel double tube bellows having improved stress corrosion cracking resistance as well as heat retaining property comprises an outer tube 2 of a ferritic stainless steel and an inner tube 1 of an austenitic stainless steel. Preferably, the outer tube 2 includes an appropriate amount of C, Cr, Mo, Ni, Cu, Nb and Ti, and the inner tube 1 includes an appropriate amount of Cr and Ni.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a double pipe bellows made of stainless steel for use in an automobile exhaust system.

[0002]

2. Description of the Related Art Bellows used in automobile exhaust systems are intended to absorb vibrations from an engine. The interior of the bellows is in a high-temperature oxidizing environment due to high-temperature exhaust gas, and the outside is subjected to high-temperature salt corrosion caused by snow-melting salt or sea salt particles. Exposed to the environment.
As the bellows, stainless steel is used as a material having excellent high-temperature resistance characteristics, and in particular, austenitic stainless steel, which is easily bellowed, is used. However, in an external high-temperature salt-corrosion environment, the use of austenitic stainless steel may cause stress corrosion cracking. For this reason,
As disclosed in JP-A-339682, an attempt has been made to use a material to which an element for suppressing stress corrosion cracking is added.

[0003] However, even if an attempt is made to add various elements, the concern of stress corrosion cracking still remains. Even if the susceptibility of the material to stress corrosion cracking is suppressed as much as possible by the addition of these elements, there is a possibility that minute cracks will occur due to stress corrosion cracking. Once a minute crack occurs, it will be reduced by the vibration applied to the bellows. Fatigue fracture starts from the starting point.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, reduces the stress corrosion cracking properties as much as possible, and provides a heat insulating property of an exhaust gas by using a double pipe to provide an automobile having excellent heat insulating properties. An object of the present invention is to provide a bellows for an exhaust system.

[0005]

SUMMARY OF THE INVENTION The present invention is to reduce the stress corrosion cracking by thoroughly examining the material of the inner and outer tubes, and to reduce the yield stress difference between ferrite and austenitic materials between double tubes. The present invention has been completed in which a gap is reliably provided to thereby ensure heat retention, and the gist of the invention is as follows. (1) In a bellows having a double pipe structure including an outer pipe and an inner pipe, the outer pipe is made of ferritic stainless steel, and
A double pipe bellows having excellent stress corrosion cracking resistance and heat retention, characterized in that the inner pipe is made of austenitic stainless steel.

More specifically, (2) the ferrite-based stainless steel of the outer tube has a C content of 0.07% or less by weight,
r: 10 to 23%, the balance being Fe and unavoidable impurities, and the austenitic stainless steel of the inner tube is:
Excellent in stress corrosion cracking resistance and heat retention as described in (1) above, characterized by containing 16 to 25% by weight of Cr and 6 to 15% by weight of Ni, with the balance being Fe and unavoidable impurities. (3) Ferrite stainless steel of the outer tube is Mo: 0.2 to 2% by weight, N
i: 0.2 to 1%, and Cu: 0.1 to 1%, and one or more kinds thereof are further contained in the stress corrosion cracking resistance and the heat retention described in (2) above. It is an excellent double pipe bellows. (4) The ferrite stainless steel of the outer pipe contains 7% by weight of the sum of Nb: C content and N content.
It is characterized in that it further contains one or more kinds of not less than twice and not more than 0.6%, not less than four times the sum of the Ti: C content and N content and not more than 0.6%. The double pipe bellows according to the above (2) or (3), which is excellent in stress corrosion cracking resistance and heat retention.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied a method of suppressing stress corrosion cracking by first reducing the applied stress by changing the component structure. The portion where the applied stress is large is the peak portion (convex portion) of the bellows. So, the height of the mountain,
Several types of bellows with different material thickness, peak-to-peak spacing, etc. were prototyped, and their shapes were changed to reduce the load stress on the peaks. A spring constant measurement and a fatigue test were performed, but it was estimated that even when the shape was changed, the stress value applied to the convex portion was still within a range that would cause stress corrosion cracking.

[0008] The present inventors have considered that there is no method for reducing stress corrosion cracking other than changing the material. Ferritic stainless steel is a material that is less likely to cause stress corrosion cracking. Therefore, the present inventors have studied the application of ferritic stainless steel to automobile exhaust system materials.

First, the property required of the material is workability. Ferritic stainless steel is inferior in workability to austenitic stainless steel. In order to improve the workability of ferritic stainless steel, it is necessary to reduce elements other than Fe and Cr as much as possible.

Next, materials are required to have high-temperature salt damage resistance and high-temperature oxidation resistance. In order to improve such properties, it is necessary to add various elements. However, since the addition of elements deteriorates the workability of the material, it is necessary to add various elements to the material as long as the workability into the bellows is possible. However, the range of components of a material that satisfies all these characteristics is limited, and their production is difficult.

The bellows is formed from a double pipe. Vibration of gas passing through the inside and reduction of exhaust noise from the engine are the reasons for using the double pipe. Therefore, the present inventors have tried to invent a bellows with as little fear of stress corrosion cracking as possible by using different materials for the outer bellows (outer pipe) and the inner bellows (inner pipe) of the double pipe structure. .

[0012] Stress corrosion cracking is promoted by elevated temperatures and corrosive environments. Therefore, the present inventors have studied a method for lowering the temperature of the outer tube. That is, when the gap 3 as shown in FIG. 1 is between the outer pipe 2 and the inner pipe 1, the temperature of the outer pipe 2 is reduced by the heat insulation of the air in the gap 3 as compared with the case where there is no gap. In addition, the concave portion 5 of the concave and convex portion is likely to accumulate salt and is considered to be a region where corrosion is severe. Therefore, it is considered that a temperature decrease in this portion suppresses stress corrosion cracking.

Further, the fact that the curvature of the concave portion 5 is larger than that of the inner tube makes it easy for the salt accumulated here to drop due to the vibration of the bellows during running. In an exhaust system structure in which the catalytic converter is installed behind the bellows, such a gap keeps the exhaust gas passing through the inside of the bellows warm and promotes the detoxification of the exhaust gas.

Such a gap is formed due to a difference in the amount of springback of the uneven portion after the bellows processing. That is, if a material having a large springback amount is used for the outer tube, the curvature in the uneven portion becomes large, and the gap 3 as shown in FIG. 1 is generated. This springback amount depends on the yield stress. Therefore, if a material having a large yield stress value is applied to the outer tube and a material having a small yield stress is applied to the inner tube, the above-described gap is formed during bellows processing.

The present inventors have noticed that the yield stress of austenitic stainless steel is generally lower than that of ferritic stainless steel, and used austenitic stainless steel for the inner tube and ferritic stainless steel for the outer tube. Bellows using steel was invented. That is, the present inventors have invented a bellows in which the yield stress value of the outer pipe material is made larger than that of the inner pipe material, thereby forming a gap between the inner pipe and the outer pipe to reduce stress corrosion cracking. did.

First, a material having excellent workability must be applied to the ferrite stainless steel for the outer tube.
For that purpose, a material of a component in which the amounts of elements other than Fe and Cr are reduced as much as possible is required. In particular, reduction of the content of C is effective for improving workability. On the other hand, the resistance to high-temperature salt damage and the resistance to high-temperature oxidation are as follows:
It can be obtained by limiting the amount of r. For the inner tube, workability and high-temperature oxidation resistance are required. Austenitic stainless steel suffices for workability, but high-temperature oxidation resistance can be obtained by limiting the amounts of added Cr and Ni.

The components of the raw material in the double pipe bellows of the present invention will be further described. First, workability is required for ferritic stainless steel used as an outer tube. If C is 0.07% or less, workability sufficient for bellows processing can be obtained. Cr is required to be 10
% Or more is required, but if it exceeds 23%, the workability is greatly deteriorated and the bellows cannot be processed. Therefore, the lower limit is 10% and the upper limit is 23%.

When traveling in a severe high-temperature salt corrosion environment, for example, in a beach area or an area where snow melting salt is frequently sprayed, the high-temperature salt damage resistance of the outer pipe of the bellows using the steel containing the above-described components is not sufficient. It is. When the bellows is used in such an environment, the material for the outer pipe is required to have excellent high-temperature salt damage resistance. To get this
The addition of o, Ni, and Cu is effective.

In order to obtain sufficient high-temperature salt damage resistance, M
Although the addition amount of o is required to be 0.2% or more, ductility is deteriorated as the addition amount increases, and bellows processing becomes difficult with the addition exceeding 2%. From the above, the upper limit of Mo addition is 2%,
The lower limit was set to 0.2%. The addition amounts of Ni and Cu are also limited for the same reason as for Mo. Ni is 0.2 to 1%, and Cu is 0.1%.
１1%.

On the other hand, the bellows may be used in an environment where an extremely large vibration is applied. For example, there are cases where the vehicle is frequently run on a road surface with severe unevenness, or where the vibration of the engine is extremely large. In such a case, the stress applied to the bellows during running increases, and there is a concern that fatigue fracture may occur in addition to stress corrosion cracking. In order to suppress this, it is necessary to reduce the stress applied to the uneven portion of the bellows as much as possible. It was considered that reducing the amount of solid solution C and the amount of solid solution N was effective for reducing the applied stress.

In order to fix these elements and reduce the amount of solid solution, it is effective to add Ti or Nb. The amount required to fix C and N is seven times the sum of the C content and the N content in the case of Nb, and four times that in the case of Ti. By further addition, C and N can be sufficiently fixed by these elements. However, excessive addition degrades the workability required for bellows processing, so it is necessary to set the upper limit of the addition amount, and both Nb and Ti are added within a range not exceeding 0.6%.

In the austenitic stainless steel used as the inner tube, the amount of Cr is necessary for obtaining high-temperature oxidation resistance. The lower limit of the amount is 16%, and the upper limit is 25% from the viewpoint of deterioration in workability. did.

The addition of Ni stabilizes the austenite phase and is necessary for improving the corrosion resistance. However, large additions produce carbides at the grain boundaries that cause intergranular corrosion by reducing the solubility of the C content. Therefore, it must be added in a range in which such carbides are not generated in a normal C amount. For the above reasons, the lower limit of the addition amount of Ni is set to 6% and the upper limit is set to 15%.

The method of manufacturing the double-pipe bellows of the present invention is not particularly limited.
It can be manufactured by compression of the pipe itself.

[0025]

EXAMPLES Table 1 shows the components of the material materials subjected to the test. Each material has a thickness of 0.2 mm, and No. A in the table
The plates of components 1 to A5 are ferritic stainless steels used as outer tube materials, and the plates of Nos. B1 to B7 are austenitic stainless steels used as inner tube materials. These outer tubing materials have a difference in yield stress value of 20 MPa or more than the inner tubing material. The outer diameters of the outer and inner tubes were 52.9 by TIG welding of these plates, respectively.
The bellows having the combinations shown in Table 2 were manufactured by trial production. As expected, voids necessary for heat retention were formed in the prototype bellows.

The bellows thus produced was 3%
The samples were subjected to a salt damage corrosion cycle test in which three steps of immersion in a saline solution for 5 minutes, holding in an air furnace at 700 ° C. for 2 hours, and slowly cooling to room temperature in the air were one cycle. However, only at the time of immersion in the saline solution, both ends of the bellows were sealed so that the saline solution did not enter the inside of the bellows. The test was performed up to 10 cycles. After 10 cycles, minute cracks generated on the bellows surface were observed, and the state of the cracks was set as an evaluation standard.

When it was judged that the cracks were smaller or no cracks than that state, the evaluation was "O", and when it was judged to be larger, the evaluation was "X". Table 2 shows the evaluation results. The prototype bellows of No. 16 used a combination of R1 component materials (SUS304) for the inner and outer tubes, and the evaluation of x was presumed to be due to the small amount of Cr in the outer tube material. As described above, by using ferritic stainless steels of different materials for the outer tube and the inner tube, concerns about stress corrosion cracking of the stainless steel bellows are reduced.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

According to the present invention, by providing a double pipe bellows using a ferritic stainless steel for the outer pipe, there is much concern about stress corrosion cracking as compared with the case where only the conventional austenitic stainless steel is used. Less. In addition, by using different materials for the inner and outer pipes, a gap is reliably provided between the inner and outer pipes, and the heat retaining effect of the exhaust gas is enhanced, which is very advantageous for detoxification of the exhaust gas by a catalytic reaction. Further, by adding an appropriate element, various characteristics required for an automobile member, such as corrosion resistance and fatigue characteristics, can be added. Therefore, an ideal bellows can be provided particularly for automobile exhaust system applications. Therefore, it can be said that the present invention is an invention having extremely industrial value.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a double pipe bellows belonging to the present invention.

[Explanation of symbols]

 1: inner tube 2: outer tube 3: void 4: convex portion 5: concave portion

Claims (4)

[Claims] 1. A bellows having a double pipe structure including an outer pipe and an inner pipe, wherein the outer pipe is made of ferritic stainless steel and the inner pipe is made of austenitic stainless steel. Stainless steel double pipe bellows with excellent cracking and heat retention. 2. The austenitic stainless steel of the inner tube, wherein the ferrite stainless steel of the outer tube contains, by weight%, C: 0.07% or less and Cr: 10 to 23%, the balance being Fe and unavoidable impurities. 2. The stress corrosion cracking resistance and heat retention according to claim 1, wherein the steel contains 16 to 25% of Cr and 6 to 15% of Ni by weight% and the balance is Fe and inevitable impurities. Excellent stainless steel double pipe bellows. 3. The ferrite stainless steel of the outer tube is one or two types by weight: Mo: 0.2 to 2%, Ni: 0.2 to 1%, Cu: 0.1 to 1%. The stainless steel double pipe bellows excellent in stress corrosion cracking resistance and heat retention according to claim 2, further comprising the above. 4. The ferrite stainless steel of the outer tube contains at least 7 times the sum of the Nb: C content and the N content in weight%,
Ti: 6% or less, 4 times or more of the sum of the Ti: C content and the N content;
The stainless steel double pipe bellows excellent in stress corrosion cracking resistance and heat retention according to claim 2 or 3, further comprising one or more kinds of 6% or less.