JPH07156329A - Composite pipe - Google Patents

Abstract

PURPOSE:To obtain an excellent thermal fatigue resistance characteristics by making an inner pipe of a double pipe of a ferrite stainless steel pipe and by making an outer pipe thereof of an austenite stainless steel pipe in regard to an heat-insulating composite pipe which has an intermediate layer constituted of an inorganic material in a gap between the inner and outer pipes. CONSTITUTION:An intermediate layer constituted of an inorganic material in a gap part between an inner pipe 3 and an outer pipe 2 of a double pipe is provided, a heat-insulating composite pipe 1 to be used for a piping of an exhaust manifold of an automobile, for instance, is obtained. In order to reduce a thermal stress caused by the terminal expansion of the inner pipe 3 and the outer pip 2 of a composite pipe 1, the inner pipe 3 exposed to high temperature is constructed of a material having small thermal expansion coefficient and the outer pipe 2 exposed to low temperature of a material having a large thermal expansion coefficient. Besides, the inner pipe 3 is made of a material being excellent in an oxidation resistance and a high-temperature strength, while the outer pipe 2 is made of a material being excellent in a corrosion resistance, in view of its direct contact with the air and so as to prevent the damage from salt at a high temperature in a cold district. Therefor a ferrite stainless steel pipe is used for the inner pipe 3 and an austenite stainless steel pipe for the outer pipe 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite pipe having an intermediate layer made of an inorganic material in a gap between an inner pipe and an outer pipe of a double pipe, and more particularly to a composite pipe used for an exhaust manifold of an automobile. .

[0002]

2. Description of the Related Art FIG. 2 is a diagram for explaining the structure of an automobile exhaust system. As shown in FIG. 2, exhaust gas from an engine 11 is collected by an exhaust manifold 12 and purified by an exhaust gas purification device 13. After that, the structure is such that the sound is silenced by the muffler 14 and released into the atmosphere through the end pipe 15.

The temperature of the exhaust gas in this exhaust system is
The closer the temperature to the engine 11 is, the higher the temperature is, and the temperature inside the exhaust manifold 12 usually reaches 800 to 900 ° C. Therefore, the conventional exhaust manifold material is made of stainless cast steel having excellent heat resistance and oxidation resistance.

In recent years, from the viewpoint of protecting the global environment, as the demand for improving fuel efficiency of automobiles has increased, it has been proposed to apply a thin-walled stainless steel pipe to an exhaust manifold, and an attempt has been made to achieve fuel efficiency improvement by reducing the thickness and weight. It is being done.

When the exhaust manifold is made thin and lightweight, the heat capacity of the exhaust manifold is reduced, so that the exhaust gas temperature in the exhaust gas purifying device is suppressed from decreasing.
The filter-type exhaust gas purification device with a built-in catalyst has an additional effect that sufficient gas can be purified even if the amount of the catalyst is reduced. However, a simple thin-walled steel pipe also causes a large amount of heat to be leaked to the outside, which may lead to thermal damage to electronic components that have been widely used in automobiles recently.

As a countermeasure against this, it is conceivable to provide a heat shield plate between the exhaust manifold and other parts, but this is not preferable because it increases the weight. Therefore, in order to impart heat insulating property to the steel pipe itself, as a structure in which a non-metallic material such as slag wool having heat insulating property is interposed between the inner pipe and the outer pipe of the double pipe, heat insulation, sound insulation, and vibration control are provided. A multi-layer tube aimed at the effect (see Japanese Patent Publication No. 60-54803) and a heat insulating composite metal tube which is composed of a woven fabric in which the intermediate layer is woven with inorganic fibers to prevent damage or breakage of the intermediate layer due to bending work 4-30
9410) is proposed.

When a composite pipe as shown in FIG. 1 described later is used as an exhaust manifold, the maximum temperature of the inner pipe 3 reaches 900 ° C. or higher because the exhaust gas temperature changes depending on the running state of the automobile. There are cases. On the other hand, between the inner pipe 3 and the outer pipe 2, the intermediate layer 4 which is a heat insulating material is provided.
Therefore, even if the temperature of the inner pipe 3 rises above 900 ° C, the temperature of the outer pipe 2 rises only to 300 to 450 ° C at the highest. When materials having the same coefficient of thermal expansion are used as the materials of the inner tube and the outer tube of the composite tube, the difference in the amount of thermal expansion due to the temperature difference between the inner tube and the outer tube becomes large.

FIG. 3 is a view showing a situation of pipe end processing when these composite pipes are applied to an exhaust manifold. As shown in the figure, after the outer tube 2 is subjected to the drawing process 6 and the heat insulating material is enclosed, the inner tube 3 is penetrated through the flange 5 in a state where the end of the outer tube 2 is not joined to the surface of the inner tube 3. Stick to. It is connected to the engine and the exhaust gas purifying device via the flange 5.

When the end portion of the outer pipe 2 is used as an exhaust manifold without being joined to the inner pipe as described above, if there is a temperature difference between the inner pipe and the outer pipe as described above, both of them are Due to the difference in thermal expansion, a gap is generated in the contact portion 7 between the inner pipe surface of the pipe end drawing portion 6 and the outer pipe inner surface, and the intermediate layer 4
However, there is a problem that the heat insulating material is scattered and deteriorates the heat insulating property and the vibration damping property.

FIG. 4 and FIG. 5 show a method of preventing this, which is a view showing a state in which the inner pipe and the outer pipe are joined by welding or the like. FIG. 4 shows a case where the end portion of the drawn outer pipe 2 is joined to the surface of the inner pipe 3, and FIG. 5 shows a case where the end portion of the outer pipe 2 is joined to the side surface of the flange 5. Although the generation of a gap in the contact portion 7 between the inner tube and the outer tube is suppressed, the thermal expansion of the inner tube 3 is restrained by the outer tube 2, and the inner tube 3 receives the compressive thermal stress and the outer tube does not. Tensile thermal stress is generated.

This thermal stress is repeatedly applied as the temperature of the exhaust gas fluctuates due to fluctuations in the running condition of the automobile, causing thermal fatigue cracks in the inner and outer tubes and the joint 8 between the inner and outer tubes. It may cause disbonding at. This also reduces the functions of the exhaust manifold, such as heat insulation and vibration damping, and there is a risk of damage.

[0012]

As the material of the inner pipe and the outer pipe of the conventional composite pipe, various combinations such as carbon steels, carbon steels and aluminized carbon steels, carbon steels and stainless steels, stainless steels are proposed. Has been done. These are basically composed of the same kind of material or a combination of materials having substantially the same thermal expansion coefficient, and there are still problems to be solved before being applied to the exhaust manifold.

An object of the present invention is to provide a composite pipe having excellent heat resistance and fatigue resistance even for a composite pipe having a joint portion between an inner pipe and an outer pipe and suitable for an exhaust manifold.

[0014]

In order to solve the above problems, the inventors of the present invention have variously studied the metallographical characteristics of a damaged exhaust manifold and the distribution state of thermal stress, and have made the following description on a composite pipe suitable for an exhaust manifold. I got the knowledge of.

In order to reduce the thermal stress due to the difference in thermal expansion between the inner tube and the outer tube of the composite tube, the inner tube side that is at a high temperature is made of a material having a small thermal expansion coefficient, and the outer tube that is at a low temperature is formed. It is preferable that the side is made of a material having a large coefficient of thermal expansion.

The inner pipe is in direct contact with the exhaust gas, and the maximum temperature is 900 ° C.
Since it is exposed to the above high temperatures, it should be a material having excellent oxidation resistance and high-temperature strength.

The outer pipe is in direct contact with the atmosphere, and in cold regions, it is necessary to use a material having excellent corrosion resistance in order to prevent high temperature salt damage due to sodium chloride to prevent freezing.

The gist of the present invention based on the above findings is as follows.
“A composite pipe as shown in FIG. 1, which has an intermediate layer (4) made of an inorganic material in the space between the inner pipe (3) and the outer pipe (2) of the double pipe. (3) is a ferritic stainless steel, and the outer pipe (2) is an austenitic stainless steel composite pipe ”.

[0019]

The reason why the material forming the composite tube of the present invention is specified as described above will be described below.

The inner pipe material is composed of ferritic stainless steel: Since the inner pipe is directly contacted with exhaust gas and exposed to high temperature of 900 ° C or higher, ferritic stainless steel excellent in oxidation resistance and high temperature strength is used. To be As ferritic stainless steel, JIS-indicated SUH 409, SUS 43
0J1L, SUS 410L, SUS 444, SUS 436L and their modified steels are used. These materials have a coefficient of thermal expansion
This is because it is as small as 12.8 × 10 ^{-6 to} 13.4 × 10 ^-6 , has excellent oxidation resistance, and has excellent thermal fatigue properties in the temperature range up to a maximum temperature of 900 ° C.

The outer pipe material is composed of austenitic stainless steel: the outer pipe is in direct contact with the atmosphere, and in cold regions, austenitic stainless steel excellent in corrosion resistance is used to prevent high temperature salt damage due to sodium chloride for freezing prevention. Used. As austenitic stainless steel, J
In terms of IS, SUS 304, SUS XM15J1, SUS 310S and their improved steels are used. This is because these materials have a large coefficient of thermal expansion of 19.0 × 10 ^{−6 to} 19.2 × 10 ⁻⁶ and are excellent in corrosion resistance.

The ratio of thermal expansion coefficients (thermal expansion coefficient of austenitic stainless steel / thermal expansion coefficient of ferritic stainless steel) obtained by the above combination is 1.42 to 1.5, and the temperature difference between the inner pipe and the outer pipe is 300 to At 500 ℃, the reduction rate of thermal stress is 25-83%. In order to reduce the occurrence of thermal stress and prevent the occurrence of fatigue cracks due to thermal stress, it is preferable that the difference in the coefficient of thermal expansion between the materials used for the inner tube and the outer tube is 1.4 times or more.

In the case of the inner pipe, the specific material may be selected according to the maximum temperature of the exhaust gas from the viewpoint of oxidation resistance and high temperature strength. For example, SUH 409 steel may be used in the low temperature range of the exhaust gas temperature up to 700 ° C, and SUS430J1L steel may be used in the case of over 700 ° C up to about 900 ° C. Further, when the temperature exceeds 900 ° C, ferritic stainless steel containing high Cr and Mo is proposed. The outer tube is determined from the level of corrosion resistance required by the use environment, and SUS304 steel, SUSXM15J1B steel, etc. are particularly suitable.

[0024]

EXAMPLES The effects of the present invention will be described more specifically below with reference to examples.

FIG. 1 is a diagram showing the structure of the composite pipe of the present invention. As shown in FIG. 1, the composite pipe used as a test material has an outer diameter.
42.7 mm, 1.2 mm thick outer tube and 32.3 mm outer diameter, 1.2 mm thick wall
4.0 mm thick ceramic fiber (Al ₂ O
It is a composite pipe with an outer diameter of 42.7 mm and an inner diameter of 29.9 mm in which a ₃ -SiO ₂ ) plain weave cloth material is inserted as an intermediate layer (heat insulating material) 4.

The materials shown in Table 1 were used as the materials for the outer tube and the inner tube, and a composite tube was produced by combining the materials shown in Table 2. Oh, material 3 in Table 1 is an improved material of ferritic stainless steel.

[0027]

[Table 1]

[0028]

[Table 2]

For the thermal fatigue test, the test body shown in FIG. 5 was used. This is because the composite pipe 1 cut into a length of 200 mm is subjected to pipe end processing 6 for narrowing both ends of the outer pipe 2 to the outer diameter of the inner pipe 3, and then the end portion of the inner pipe 3 and the drawn outer pipe 2 It is manufactured by welding the end portion (outer pipe / flange joint portion 10) to the flange 5.

The thermal fatigue test is a thermal cycle test in which a high temperature gas is intermittently blown into the pipe of the test body (high temperature gas: 15 minutes, cold air: 10 minutes), and after 500 cycles, a crack is generated in the inner and outer pipes. And the maximum temperature of the inner and outer tubes during the test was measured. The results are shown in Table 2.

From these results, the test specimens A to A of the present invention are shown.
In C, the outer tube is made of a material having a large coefficient of thermal expansion (the ratio of the coefficient of thermal expansion of the outer tube and the inner tube (outer tube / inner tube) is 1.42 or more). The difference in the maximum temperature of the inner pipe
Although the temperature was 350 to 500 ° C, no crack was observed.

On the other hand, the test sample D of the comparative example has an austenitic stainless steel (SUS 304
Since the ratio of the coefficient of thermal expansion of the outer tube to the inner tube was 1), the flange joint was broken at 400 cycles.

Both the outer and inner pipes of the test bodies E and F were made of ferritic stainless steel (SUH409, SUS 430J1L).
As a result, the cracks were found in the inner pipe.

In the test body G, the outer tube was made of austenitic stainless steel (SUS 304) and the inner tube was made of carbon steel (SPCC).
The ratio of the coefficient of thermal expansion of the outer tube to the inner tube (outer tube / inner tube) was 1.41, but abnormal oxidation occurred in the inner tube (carbon steel), and more remarkable cracking was observed than in other comparative examples.

In the test piece H, the outer tube was made of ferritic stainless steel (SUH 409) and the inner tube was made of austenitic stainless steel (SUS 304). The ratio of the coefficient of thermal expansion of the outer tube to the inner tube (outer tube (/ Inner pipe) was 0.71, so the flange joint fractured after 300 cycles.

[0036]

In the composite pipe of the present invention, the inner pipe is made of ferritic stainless steel, so it is excellent in high-temperature oxidation resistance, and the outer pipe is made of austenitic stainless steel, so that it is excellent in corrosion resistance. Moreover, since the structure takes into account the thermal expansion coefficient of the inner and outer pipe materials, there is no occurrence of thermal cracks at the joint between the inner pipe and the outer pipe, and it can be used for piping of an exhaust manifold of an automobile. When used in an exhaust manifold, it also has the effect of increasing the exhaust gas passage temperature and improving exhaust gas purification performance due to its excellent heat insulation.

This composite pipe can be used in fields where the above excellent characteristics can be utilized.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view showing a structure of a composite pipe of the present invention.

FIG. 2 is a diagram showing an outline of an exhaust system of an automobile.

FIG. 3 is a cross-sectional view showing a manner of pipe end processing when a composite pipe is applied to an exhaust manifold, and is a diagram showing a case where an outer pipe is not joined to an inner pipe.

FIG. 4 is a cross-sectional view showing a manner of pipe end processing when a composite pipe is applied to an exhaust manifold, and is a diagram showing a case where an outer pipe is joined to an inner pipe.

FIG. 5 is a cross-sectional view showing a manner of pipe end processing when a composite pipe is applied to an exhaust manifold, and is a diagram showing a case where an outer pipe is joined to a flange.

[Explanation of symbols]

1: Composite pipe 2: Outer pipe 3: Inner pipe 4: Middle layer 5: Flange 6: Pipe end drawing part 7: Contact part 8: Inner pipe / outer pipe joint 9: Inner pipe and flange joint 10 : Joint between outer pipe and flange 11: Engine 12: Exhaust manifold 13: Exhaust gas purifier 14: Muffler 15: End pipe

Claims (1)

[Claims] 1. A heat-insulating composite pipe having an intermediate layer made of an inorganic material in a space between a double pipe inner pipe and an outer pipe, wherein the inner pipe is a ferritic stainless steel pipe and the outer pipe is an austenitic stainless steel pipe. A composite pipe characterized by being present.