JP2000046182A - 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 double tube bellows having improved stress corrosion cracking resistance as well as heat retaining property comprises an outer tube 2 and an inner tube 1 both of a ferritic stainless steel, where the outer tube stainless steel is larger in the yield stress value by at least 20 MPa than the inner tube stainless steel. The outer and inner tubes 2 and 1 preferably include an appropriate amount of C, Cr, Mo, Ni, Cu, Nb and Ti.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Bellows used for automobile exhaust systems are used to absorb vibration from an engine.
The inside is exposed to a high-temperature oxidizing environment caused by high-temperature exhaust gas, and the outside is exposed to a high-temperature salt-corrosion environment caused by snow-melting salt or sea salt particles. 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 that reason,
As disclosed in JP-A-5-339682,
Attempts have been made to use materials to which elements that suppress stress corrosion cracking are added.

[0003] On the other hand, in order to make the exhaust gas harmless, attempts have been made to introduce higher-temperature exhaust gas into a catalytic converter. For this reason, the bellows installed before the catalytic converter in the exhaust system is required to have a higher exhaust gas heat insulating property than before. However, these measures have not been taken yet.

[0004]

First, it is effective to change the material in order to prevent stress corrosion cracking. That is, it is effective to apply a material to which an element effective for stress corrosion cracking resistance is added to the bellows. However, even if an attempt is made to add such an element, 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 may occur due to stress corrosion cracking. Once a micro crack is generated, the vibration applied to the bellows causes fatigue fracture starting from the crack. On the other hand, in order to keep the exhaust gas warm, it is not enough to simply change the material, and it is considered necessary to change the structure of the bellows itself.

[0005] It is an object of the present invention to reduce the stress corrosion cracking property of an automobile bellows as much as possible and to improve the heat retention of exhaust gas passing therethrough.

[0006]

According to the present invention, a ferritic stainless steel is used as a material, so that stress corrosion cracking does not occur, and a gap between the inner and outer tubes is reliably formed by defining a yield stress difference between the inner and outer tubes. To provide a double-tube bellows with excellent heat retention, ensuring heat insulation with the outside.
The summary is as follows.

(1) In a bellows having a double pipe structure having an outer pipe and an inner pipe, the outer pipe and the inner pipe are made of ferritic stainless steel, and the yield stress value of the ferrite stainless steel of the outer pipe is lower than the inner pipe. A double-pipe bellows excellent in stress corrosion cracking resistance and heat retention characterized in that the yield stress value of the ferritic stainless steel is 20 MPa or more. (2) Outer ferrite stainless steel
And containing C: 0.07% or less, Cr: 10 to 23%, the balance being Fe and unavoidable impurities, and the ferrite-based stainless steel of the inner tube is C: 0.03% or less by weight, Cr: : The double-pipe bellows according to the above (1), which contains 10 to 23% and has a balance of Fe and inevitable impurities, which is excellent in stress corrosion cracking resistance and heat retention.

(3) The ferrite stainless steel of the outer tube is Mo: 0.2-2%, Ni: 0.2-1% by weight.
%, Cu: 0.1 to 1%, and one or more of them. The double-tube bellows excellent in stress corrosion cracking resistance and heat retention described in (2) above, further comprising: (4) The ferrite stainless steel of the inner tube is
And Mo: 0.2 to 1.5%, Ni: 0.2 to 0.8
%, Cu: 0.1 to 0.7%, and further comprises one or more of 0.1 to 0.7%, and is excellent in stress corrosion cracking resistance and heat retention as described in (2) or (3) above. Double bellows.

(5) The ferrite stainless steel of the outer tube is at least 7 times and not more than 0.6% of the sum of the Nb: C content and the N content by weight, and the Ti: C content and the N content are not more than 0.6%. (2) to (2) to (4), which further contains one or more kinds of not less than 4 times and not more than 0.6% of the sum of the amounts.
The double pipe bellows according to any one of (4) and having excellent resistance to stress corrosion cracking and heat retention. (6) The ferrite stainless steel of the inner tube is
Nb: one kind of not less than 7 times and not more than 0.6% of the sum of the C content and the N content, and not less than 4 times and not more than 0.6% of the sum of the Ti: C content and the N content. Alternatively, 2 which is excellent in stress corrosion cracking resistance and heat retention described in any one of the above (2) to (5), further comprising two or more kinds.
Heavy pipe bellows.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors first studied a method for suppressing stress corrosion cracking by reducing the applied stress by changing the component structure in order to reduce stress corrosion cracking. The portion where the applied stress is large is the peak portion (convex portion) of the bellows. Therefore, several types of bellows with different peak heights, material thicknesses, intervals between peaks, etc. were prototyped, and their shapes were changed to reduce the stress applied to the peaks.

Evaluations were made by measuring the spring constant and performing fatigue tests. It was estimated that the stress applied to the projections was still within the range that would cause stress corrosion cracking even when the shape was changed. Therefore, the present inventors considered that there was no method for reducing stress corrosion cracking other than changing the material, and examined the application of ferritic stainless steel to automotive exhaust materials as a material with less concern for stress corrosion cracking.

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, the material is required to have high-temperature salt damage resistance and high-temperature oxidation resistance. In order to improve such characteristics, addition of various elements is necessary, and such various elements must be added within a range where bellows processing is possible.

The present inventors have further studied the heat retention. BACKGROUND ART Conventionally, in various products, a method of loading a heat insulating material between inner and outer walls has been applied in order to improve heat retention. As the heat insulating material, a substance mainly containing many fine air bubbles is frequently used. By the way, the bellows has a double structure formed from a double tube, and the present inventors have studied loading a heat insulating substance between the double tubes. However, no heat insulating material that can withstand the high temperature of the exhaust gas was found.

Therefore, the present inventors have conceived of forming an air layer between the double tubes. Even if the temperature of the outer tube of the double tube is reduced, the amount of heat removed by the exhaust gas passing through the inside is reduced due to the heat insulating effect of the air layer. That is, by using a ferritic stainless steel and introducing an air layer between the double pipes, an attempt was made to produce a bellows with less concern for stress corrosion cracking and excellent heat retention of exhaust gas.

As a result of the inventors' determination of the structure of the double pipe having a gap and a study of the formation method, as shown in FIG. 1, the gap 3 was introduced into the concave portion 5 of the bellows having a large area in contact with air. It was decided to. Next, a method for introducing such voids was examined. Such a gap is formed due to the difference in the amount of springback of the uneven portion after bellows processing.
In other words, if a material having a large springback amount is used for the outer tube, the curvature in the uneven portion becomes large, and a gap as shown in FIG. 1 is generated. This springback amount depends on the yield stress. Therefore, a material having a large yield stress value is
If the small material is applied to the inner tube 1, the above-mentioned gap is formed at the time of bellows processing.

Such a difference in yield stress between the inner tube material and the outer tube material can be obtained by controlling the amount and type of the component added, controlling the crystal grain shape by heat treatment during the production of the material, and the like. It was estimated that if the bellows processing was performed with materials having different yield stress values, and the difference in the yield stress values of the materials in the tensile test was 20 MPa or more, voids sufficient to have the above-described effects could be obtained.

Next, the components of the ferritic stainless steel that can be bellowed were examined. Bellows are usually manufactured by hydraulic overhang from the inside of a double pipe and compression of the pipe itself. The severest part of the bellows processing is the protruding part that protrudes outward. As a result of various studies, it was considered necessary to improve the elongation of the material in order to enable processing in this part. For that purpose, a material in which the amount of elements other than Fe and Cr is reduced as much as possible is desirable. In particular, reduction of the content of C is effective for improving workability, and materials having a low C content were used for the inner pipe and the outer pipe. High-temperature salt damage resistance and high-temperature oxidation resistance can be obtained by limiting the amount of added Cr within a range that does not deteriorate workability.

Hereinafter, the reasons for limiting the components of the outer tube and the inner tube will be described in more detail. First, the component of the outer tube is that C is 0.0
When it is 7% by weight (hereinafter abbreviated as%) or less, sufficient workability for bellows processing can be obtained. Cr is required for corrosion resistance.
Addition of 10% or more is necessary for both the inner and outer tubes, but if it exceeds 23%, the workability is greatly deteriorated and the bellows cannot be machined. Therefore, the lower limit is 10% and the upper limit is 23%.

In a severe high-temperature salt corrosion environment, for example, when traveling in a beach area or an area where snow melting salt is frequently sprayed, the bellows using the above-described steel of the present invention is not sufficient for the outer pipe to withstand high-temperature salt damage. . 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 obtain this, Mo, Ni, Cu
The addition 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 were also limited for the same reason as for Mo. Ni defined the lower limit as 0.2% and the upper limit as 1%. Further, for Cu, the lower limit was defined as 0.1% and the upper limit was defined as 1%.

On the other hand, the bellows may be used in an environment where a very large vibration is applied. For example, there are cases where the vehicle frequently runs on a road surface with severe unevenness, and cases 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. For that purpose, it is effective to reduce the amount of solid solution C and the amount of solid solution N for hardening the steel. 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
In the case of Nb, it is seven times the sum of the C content and the N content, and in the case of Ti, it is four times that. With more addition, sufficient C
And N can be fixed by these elements. However, excessive addition degrades the workability required for bellows processing, so that it is necessary to set the upper limit of the addition amount, and it is desired that both Nb and Ti do not exceed 0.6%.

Next, as for the components of the inner tube material, the effects of the respective elements are the same as those of the outer tube, but in order to maintain the yield stress difference with the outer tube, the upper limit values of C, Mo, Ni and Cu are set. Care must be taken in the regulations. Cr content is necessary for high-temperature salt damage resistance,
It must be added in the same amount as the outer tube material. The amount of C is limited to 0.0 to maintain the yield stress difference with the outer tube.
It is necessary that the content be 3% or less.

When the exhaust gas of the engine is at a high temperature, for example, in the case of an automobile equipped with an engine with a large displacement or an automobile that frequently runs at high speed, the material for the inner pipe is required to have excellent high-temperature oxidation resistance. In order to obtain this property, it is effective to add Mo, Ni, and Cu, as in the case where the high-temperature salt damage resistance property is obtained. The addition amount is the same as the outer tube,
Mo: 0.2% or more, Ni: 0.2% or more, Cu: 0.
The above effect can be obtained by adding 1% or more, but the upper limit of the added amount must be set in consideration of the fact that the bellows processing is possible and the yield stress difference with the outer tube is obtained. , Mo: 1.5% or less, Ni: 0.8% or less, C
u: It is added in a range of 0.7% or less.

[0025]

EXAMPLES Table 1 shows the components of the materials subjected to the test. A ferritic stainless steel having a thickness of 0.2 mm was used. In the table numbers, plates of components A1 to A5 and R1 were used as materials for outer tubes, and plates of components B1 to B7 were used as materials for inner tubes. The difference between the yield stress values of these plates is 20 MPa or more. These plates were formed into pipes having outer diameters of 52.9 mm and 51.9 mm, respectively, of the outer pipe and the inner pipe by TIG welding, and bellows having the combinations shown in Table 2 were prototyped. 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. A similar test was performed using a SUS304 bellows that is currently used, and micro cracks generated on the bellows surface after 10 cycles were observed, and the crack state 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. No. 1
The reason why the evaluation of the prototype bellows of No. 6 was 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, the concern of stress corrosion cracking of the stainless steel bellows is reduced.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

According to the double pipe bellows of the present invention, the fear of stress corrosion cracking is extremely reduced as compared with the conventional pipe made of austenitic stainless steel, and by defining the yield stress difference between the inner and outer pipes, By reliably forming a gap between the heavy pipes, 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, so that an ideal bellows can be provided, especially in an automobile exhaust system. Therefore,
The present invention can be said to be an invention having extremely high industrial value.

[Brief description of the drawings]

FIG. 1 is a sectional view of a double pipe bellows according to the present invention.

[Explanation of symbols]

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

Claims (6)

[Claims] In a bellows having a double pipe structure comprising an outer pipe and an inner pipe, the outer pipe and the inner pipe are made of ferritic stainless steel, and the yield stress value of the ferrite stainless steel of the outer pipe is lower than that of the inner pipe. A stainless steel double pipe bellows excellent in stress corrosion cracking resistance and heat retention characterized by being 20 MPa or more larger than the yield stress value of ferritic stainless steel. 2. The ferritic stainless steel of the inner tube, wherein the ferrite-based stainless steel of the outer tube contains C: 0.07% or less and Cr: 10 to 23% by weight, the balance being Fe and unavoidable impurities. 2. The stress corrosion cracking resistance and heat retention according to claim 1, wherein the steel contains C: 0.03% or less and Cr: 10 to 23% by weight, and the balance consists of Fe and unavoidable impurities. Double pipe bellows made of stainless steel with excellent properties. 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-based stainless steel of the inner tube is, by weight%, Mo: 0.2 to 1.5%, Ni: 0.2 to 0.8%, Cu: 0.1 to 0.7%. The stainless steel double pipe bellows having excellent stress corrosion cracking resistance and heat retention according to claim 2 or 3, further comprising one or more of the following. 5. The ferrite-based stainless steel of the outer tube contains at least 7 times the sum of the Nb: C content and the N content and 0.6% by weight.
%: Ti: 4 times or more of the sum of C content and N content, and 0.6
% Or less, and the stainless steel double pipe bellows excellent in stress corrosion cracking resistance and heat retention according to any one of claims 2 to 4, further comprising: . 6. The ferrite-based stainless steel of the inner tube contains at least 7 times the sum of Nb: C content and N content, and 0.6% by weight.
%: Ti: 4 times or more of the sum of C content and N content, and 0.6
The stainless steel double pipe bellows excellent in stress corrosion cracking resistance and heat retention according to any one of claims 2 to 5, further comprising one or more of the following: .