JP7021029B2 - Gas sensor - Google Patents

Description

The present invention relates to a gas sensor provided with a sensor element that is exposed to a gas to be detected and detects a specific gas component therein.

Oxygen sensors and air-fuel ratio sensors that detect the oxygen concentration in exhaust gas are known as gas sensors that improve the fuel efficiency of internal combustion engines such as automobile engines and control combustion.
As such a gas sensor, there is a structure in which a sensor element having a detection unit for detecting the concentration of a specific gas on the tip side is held in the main metal fitting, and the detection part protruding from the tip of the main metal fitting is covered with a metal protector to protect the sensor element. It is known (see, for example, Patent Document 1).
However, such a gas sensor is exposed to high-temperature exhaust gas, and in particular, the welded joint between the protector on the tip side of the gas sensor and the main metal fitting becomes hot, and then repeated stress is generated by the cold heat cycle that is cooled, resulting in welded joint. The part may break. Therefore, in the gas sensor of Patent Document 1, the difference in the coefficient of thermal expansion between the protector and the main metal fitting is reduced, the repeated stress is reduced, and the breakage of the welded joint is suppressed.

Japanese Patent No. 3932881 (Fig. 1)

However, in recent years, the demand for increasing the heat-resistant temperature of automobile parts has become stricter, and it has become necessary to change the constituent materials of the parts to materials having high heat resistance. In particular, since the protector located at the outermost tip of the gas sensor is most required to have heat resistance, it is conceivable to change the protector to a heat resistant material (Inconel (registered trademark) alloy), but there is a problem that the cost is high.
For this reason, as an inexpensive material having excellent heat resistance, for example, austenitic stainless steel to which Nb is added may be used for the protector, but this material has a thermal expansion coefficient of the main metal fitting (for example, SUS430). It is higher than the thermal expansion rate, and the above-mentioned repeated stress causes the protector to expand more than the main metal fitting, which causes a problem of accelerating the breakage of the welded joint.

The present invention has been made in view of the present situation, and even when the thermal expansion rate of the constituent material of the protector is higher than the thermal expansion coefficient of the constituent material of the main metal fitting, the joint between the two is damaged or the like. It is an object of the present invention to provide a gas sensor that suppresses.

The gas sensor of the present invention includes a sensor element that extends in the axial direction and has a detection unit on the tip side, and a metal tubular main metal fitting that surrounds the sensor element while projecting the detection unit from its own tip. A gas sensor including a metal protector that houses the detection unit and is fixed to the main metal fitting, wherein the protector includes an inner protector arranged with a gap between the detection unit and the inner protector. A rear end joint portion in which the inner protector and the rear end portion of the outer protector overlap is joined to the outer surface of the main metal fitting to form a joint portion, including an outer protector arranged through a gap, to form a joint portion of the protector. The thermal expansion rate of the constituent material at 800 ° C. is higher than the thermal expansion rate of the constituent material of the main metal fitting at 800 ° C., and when viewed in a cross section including the joint portion, from the outer surface of the main metal fitting to the outer surface of the protector. The minimum distance t1 of the distance and the minimum distance t2 of the distance from the outer surface of the main metal fitting to the inner surface of the main metal fitting satisfy the relationship of 0.6 ≦ (t1 / t2) ≦ 2.0, and the main metal fitting satisfies the relationship. The first inner surface of the main metal fitting in the region where the joint portion on the outer surface of the main metal fitting is projected onto the inner surface of the main metal fitting is radially outward from the rear end side inner surface of the main metal fitting connected to the rear end side of the first inner surface. Spread, the first inner surface and the rear end side inner surface of the main metal fitting are not flush with each other .

According to this gas sensor, the ratio (t1 / t2) = 1 even if the gas sensor uses a material whose thermal expansion rate at 800 ° C. is higher than the thermal expansion rate of the main metal fitting for the protector and a thermal cycle that repeats heating and cooling is added. The balance between the strength of the rear end of the protector and the main metal fitting at the joint can be appropriately balanced to reduce repeated stress and deformation of the main metal fitting, and damage to the joint can be suppressed.
When the ratio (t1 / t2) is less than 0.6, the thickness of the rear end portion becomes thinner than the thickness of the main metal fitting, the strength thereof decreases, and the repetitive stress increases. On the contrary, when the ratio (t1 / t2) exceeds 2.0, the thickness of the main metal fitting becomes thinner than the thickness of the rear end portion and the strength thereof decreases, and the main metal fitting is deformed and the joint is damaged. Becomes noticeable.

Further , according to this gas sensor, a sensor in which the first inner surface of the main metal fitting having a joint portion on the outer side is surrounded by the entire inner surface of the main metal fitting as compared with the case where the first inner surface of the main metal fitting is parallel to the inner surface on the rear end side in the axial direction (parallel). The space near the detection unit of the element increases, the gas to be detected easily enters and exits the detection unit, and the detection accuracy is improved. Further, the protector can have a larger diameter (away from the detection part) and the protector (outside) as compared with the case where the first inner surface of the main metal fitting having the joint portion on the outer side is parallel to the inner surface on the rear end side in the axial direction. Water coverage from the outer surface of the protector) to the sensor element can be further suppressed.

In the gas sensor of the present invention, the tensile strength specified in JIS-G0567 at 800 ° C. of the constituent material of the protector may be higher than the tensile strength of the constituent material of the main metal fitting.
According to this gas sensor, the heat resistance of the protector located on the outermost side of the gas sensor on the most advanced side is further improved, so that the heat resistance of the entire gas sensor can be further improved.

In the gas sensor of the present invention, the constituent material of the protector may be austenitic stainless steel, and the constituent material of the main metal fitting may be ferritic stainless steel.
According to this gas sensor, it is possible to suppress damage to the joint portion while improving the heat resistance of the entire gas sensor at low cost.

According to the present invention, even when the thermal expansion rate of the constituent material of the protector is higher than the thermal expansion coefficient of the constituent material of the main metal fitting, it is possible to obtain a gas sensor in which damage to the joint portion between the two is suppressed.

It is sectional drawing which follows the longitudinal direction of the gas sensor which concerns on embodiment of this invention. It is a partially enlarged view of FIG. 1 around the joint portion. It is a schematic diagram which shows the influence on various characteristics when the ratio (t1 / t2) of the distance between the rear end joint part and the main metal fitting in a joint part is changed. It is a figure which evaluated the breakage of the joint part when the ratio (t1 / t2) of the distance between the rear end joint part and the main metal fitting in an actual joint part was changed.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 is an overall cross-sectional view of the gas sensor (oxygen sensor) 200 according to the embodiment of the present invention along the longitudinal direction, and FIG. 2 is a partially enlarged view of FIG. 1 around the joint portion.
The gas sensor 200 is an oxygen sensor (whole area air-fuel ratio gas sensor) that detects the oxygen concentration in the exhaust gas of an automobile or various internal combustion engines.
The gas sensor 200 is an example in which the protector 140 described later forms a double protector of the inner protector 143 and the outer protector 142.

In FIG. 1, the gas sensor 200 has a tubular main metal fitting 138 in which a threaded portion 139 for being fixed to an exhaust pipe is formed on an outer surface, and an axis O direction (longitudinal direction of the gas sensor 200: vertical direction in the figure). An extending plate-shaped sensor element 10, a tubular ceramic sleeve 106 and a ceramic holder 151 arranged so as to surround the radial circumference on the rear end side of the sensor element 10, and an insertion hole 168 penetrating in the axial direction. A ceramic separator 166 arranged inside the tip side so as to surround the rear end of the sensor element 10, and five terminal fittings 21 arranged between the sensor element 10 and the separator 166 (FIG. In No. 1, only three are shown), and a double protector 140 fixed to the tip end side of the main metal fitting 138 is provided.
Further, the detection portion 10a at the tip of the sensor element 10 is covered with a porous protective layer 20 such as alumina.

The main metal fitting 138 is made of stainless steel, has a through hole 154 penetrating in the axial direction, and has a substantially cylindrical shape having a shelf portion 152 protruding inward in the radial direction of the through hole 154. In the through hole 154, the sensor element 10 is arranged so that the tip portion including the detection portion 10a of the sensor element 10 protrudes from its own tip. Further, the shelf portion 152 is formed as an inwardly tapered surface having an inclination with respect to a plane perpendicular to the axial direction.

Inside the through hole 154 of the main metal fitting 138, an annular ceramic holder 151 made of alumina, a powder packed bed 153 (hereinafter, also referred to as a talc ring 153), and a talc ring 153 in a state of surrounding the radial circumference of the sensor element 10 are provided. The above-mentioned ceramic sleeves 106 are laminated in this order from the front end side to the rear end side.
Further, a crimping packing 157 is arranged between the ceramic sleeve 106 and the rear end portion 158 of the main metal fitting 138. The rear end portion 158 of the main metal fitting 138 is crimped so as to press the ceramic sleeve 106 toward the tip end side via the crimp packing 157.

Further, the ceramic holder 151 is made of an insulating ceramic (for example, alumina), has a substantially short cylindrical shape, and has a tip facing surface 151a formed in a tapered shape that tapers toward the tip. Then, the ceramic holder 151 is pressed by the talc ring 153 from the rear end side while the portion near the outer periphery of the tip facing surface 151a is locked to the shelf portion 152 of the main metal fitting 138, so that the ceramic holder 151 is inside the main metal fitting 138. Is positioned and fitted in the gap.
Further, on the tip end side of the ceramic holder 151, a recess 151h that surrounds the insertion hole of the sensor element 10 and is recessed rearward is formed.

On the other hand, as shown in FIG. 1, the outer periphery of the tip portion 138s (lower side in FIG. 1) of the main fitting 138 covers the tip portion (including the detection portion 10a) of the sensor element 10 protruding from the main fitting 138, and also covers the outer periphery. A protector 140 forming a double metal tubular protector having a plurality of holes is attached by welding or the like.
This double protector 140 includes a bottomed cylindrical inner protector 143 arranged through a gap between the detection unit 10a and a bottomed tubular outer protector 142 arranged through a gap between the inner protector 143 and the inner protector 143. The rear end portion that is the opening of the inner protector 143 and the outer protector 142 overlaps to form the rear end joint portion 140b, and both are bonded to each other.
Further, the rear end joint portion 140b is joined to the outer surface of the tip portion 138s of the main metal fitting 138 by welding (in this example, all-around welding) to form a joint portion W.

An outer cylinder 144 is fixed to the outer periphery of the rear end side of the main metal fitting 138. Further, the opening on the rear end side (upper side in FIG. 1) of the outer cylinder 144 is electrically connected to each of the five terminal fittings 21 (only three are shown in FIG. 1) of the sensor element 10. A rubber grommet (seal member) 170 having a lead wire insertion hole 170h through which five lead wires 146 (only three are shown in FIG. 1) is inserted is arranged.

Further, a separator 166 is arranged on the rear end side (upper side in FIG. 1) of the sensor element 10 protruding from the rear end portion 158 of the main metal fitting 138, away from the main metal fitting 138. The separator 166 is arranged around a total of five electrode pads (not shown) formed on the main surface on the rear end side of the sensor element 10. The separator 166 is formed in a cylindrical shape having an insertion hole 168 penetrating in the axial direction, and is provided with a flange portion 167 protruding radially outward from the outer surface. The separator 166 is held inside the outer cylinder 144 by bringing the flange portion 167 into contact with the step portion of the outer cylinder 144 and crimping the outer cylinder 144 via the holding member 169.

FIG. 2 shows a partially enlarged view of FIG. 1 around the joint portion W. Here, the joint portion W is not only in contact with the outer surface of the main metal fitting 138 (tip portion 138s) and the rear end joint portion 140b, but also with the outer surface of the main metal fitting 138 (tip portion 138s) and the rear. A portion where the end joint portion 140b is integrated. Specifically, in the enlarged image of the cross section in the axis O direction of the portion where the outer surface of the main metal fitting 138 and the rear end joint portion 140b are in contact with each other, the boundary lines B1 and the inner protector between the inner protector 143 and the outer protector 142 are shown. It refers to the portion P where the boundary line B2 between the main metal fitting 138 (the tip portion 138s) of the main metal fitting 138 has disappeared. This is because the parts of the inner protector 143, the outer protector 142, and the main metal fitting 138 in which the materials are not integrated are separated from each other and thermally expand, so that repeated stress hardly occurs.

When viewed in a cross section including the joint portion W, the minimum distance from the outer surface of the main metal fitting 138 to the outer surface of the protector 140 is t1, and the minimum distance from the outer surface of the main metal fitting 138 to the inner surface of the main metal fitting 138 is t2.
Here, the outer surface of the protector 140 is, in the case of a double protector, the outermost surface of the entire protector 140, that is, the outer surface of the outer protector 142.
In the calculation of the distances t1 and t2, the cross section including the joint portion W is used as a reference, so that the outer surface and the inner surface of the tip portion 138s of the main metal fitting 138 in the joint portion W are the reference for the calculation of t2.
Further, in the joint portion W, since the rear end joint portion 140b and the main metal fitting 138 are integrated, the boundary between the two is a virtual straight line connecting the boundary lines B2 in contact with the front end side and the rear end side of the joint portion W. And. Therefore, the distance t2 is the distance between the inner surface of the main metal fitting 138 at the joint portion W and the virtual straight line. The distance t1 is the distance between the outer surface of the rear end joint portion 140b at the joint portion W and the virtual straight line.

Here, in the present invention, the thermal expansion rate of the constituent material of the protector 140 (inner protector 143 and outer protector 142) at 800 ° C. is larger than the thermal expansion coefficient of the constituent material of the main metal fitting 138 at 800 ° C. .. For example, in this example, the protector 140 is made of austenitic stainless steel to which Nb is added, and the main metal fitting 138 is made of SUS430LX (JIS standard) which is a ferritic stainless steel.
Therefore, when the gas sensor 200 is exposed to high-temperature exhaust gas or the like and then repeatedly cooled, the protector 140 is located at the outermost tip of the gas sensor 200, and the protector 140 is the main metal fitting. As described above, it expands more than 138 and promotes breakage of the welded joint (joint W).

Therefore, in the present invention, by controlling 0.6 ≦ (t1 / t2) ≦ 2.0, it is possible to suppress damage to the joint portion W between the protector 140 and the main metal fitting 138. Further, the heat resistance of the entire gas sensor 200 can be improved by using a material for the protector 140, which has excellent heat resistance and is inexpensive, but whose thermal expansion rate at 800 ° C. is higher than that of the main metal fitting at 800 ° C. Can be done.
The reason why damage to the joint portion W can be suppressed by controlling the ratio (t1 / t2) will be described with reference to FIG.

FIG. 3 is a schematic diagram showing the influence on various characteristics when the ratio (t1 / t2) of the distance between the rear end joint portion 140b and the tip portion 138s of the main metal fitting 138 in the joint portion W is changed.
As a premise, in the present invention, t2 is smaller than that of the conventional main metal fitting, and is about 1.7 times as large as t1 ((t1 / t2) = 0.6) at the maximum. Therefore, by changing the ratio of t2 and t1, the influence of repeated stress can be adjusted. On the other hand, in the conventional configuration in which t2 is significantly larger than t1 (for example, (t1 / t2) is 0.2 or less), the degree of deformation of the main metal fitting changes almost even if (t1 / t2) is changed. It is difficult to adjust the effect of repeated stress.

First, as shown by the solid line in FIG. 3, in the region sandwiching the ratio (t1 / t2) = 1, the larger the ratio value, the thicker the distance t1, that is, the thickness of the rear end coupling portion 140b, and the higher the strength thereof. It approaches the strength of the tip portion 138s of the main metal fitting 138. Therefore, the repetitive stress of the joint portion W due to the thermal cycle is reduced, and damage to the joint portion W is suppressed. Therefore, as long as attention is paid to the rear end coupling portion 140b, the larger the ratio value, the better.
On the other hand, as the value of the ratio becomes larger, the distance t2, that is, the thickness of the tip portion 138s of the main metal fitting 138 becomes thinner, and the strength of the main metal fitting 138 (tip portion 138s) decreases as shown by the broken line in FIG. Deformation of the main metal fitting 138 due to stress, and eventually damage to the joint portion W, etc. increase. Therefore, as long as the main metal fitting 138 is focused on, the smaller the ratio value, the better.
From these facts, the ratio (t1 / t2) has an appropriate range with 1 in between, and a specific range of the ratio was obtained by an experiment described later.

When the ratio (t1 / t2) is less than 0.6, the thickness of the rear end joint portion 140b becomes thinner than the thickness of the main metal fitting 138 at the joint portion W, the strength thereof decreases, and the repetitive stress increases. The joint portion W is significantly damaged.
On the contrary, when the ratio (t1 / t2) exceeds 2.0, the thickness of the main metal fitting 138 becomes thinner than the thickness of the rear end joint portion 140b at the joint portion W, and the strength thereof decreases, so that the main metal fitting 138 becomes thinner. Deformation, and eventually damage to the joint portion W, etc. become remarkable.
The ratio (t1 / t2) is preferably 0.6 to 1.6, more preferably 0.8 to 1.6. Further, the ratio (t1 / t2) is preferably 0.6 to 1.5, more preferably 0.8 to 1.5. Further, the ratio (t1 / t2) is preferably 0.6 to 1.3, more preferably 0.8 to 1.3.

In the present invention, as shown in FIG. 2, a region in which the joint portion W on the outer surface of the main fitting is projected radially inward onto the inner surface of the main fitting 138 is defined as R, and the main fitting 138 (tip portion 138s) in the region R is defined as R. ) Is the first inner surface 138i1. At this time, the first inner surface 138i1 extends radially outward from the rear end side inner surface 138i2 of the main metal fitting 138 connected to the rear end side of the first inner surface 138i1.
By doing so, the space in the vicinity of the detection unit 10a of the sensor element 10 surrounded by the inner surface of the main metal fitting 138 increases as compared with the case where the inner surface 138i1 and the rear end side inner surface 138i2 are parallel (parallel) in the axis O direction. The gas to be detected easily enters and exits the detection unit 10a, and the detection accuracy is improved. Further, the protector 140 can have a larger diameter (away from the detection unit 10a) and the protector 140 (outer protector 142) as compared with the case where the inner surface 138i1 and the rear end side inner surface 138i2 are parallel to the axis O direction (plane one). ) Can be more suppressed from being exposed to water from the outer surface of the sensor element 10.
The inner surface of the main metal fitting 138 in the region R is an inner surface including the region R in the axis O direction.

Further, in the present invention, if the tensile strength of the constituent material of the protector 140 at 800 ° C. specified in JIS-G0567 is higher than the tensile strength of the constituent material of the main metal fitting 138, the outermost side of the gas sensor 200 is the outer side. Since the heat resistance of the protector 140 located in is further improved, the heat resistance of the entire gas sensor 200 can be further improved.

It goes without saying that the present invention is not limited to the above embodiments and extends to various modifications and equivalents included in the idea and scope of the present invention.
The shape of the protector and the main metal fitting to which the protector is joined is not limited to the above. For example, in the above embodiment, the protector is a double protector of an inner protector and an outer protector, but it may be a single (single) protector.
The joint is not limited to welding, but welding is preferred.
Examples of the gas sensor include an oxygen sensor, an all-region gas sensor, and a NOx sensor. It may be a tubular sensor element.
It is preferable that the constituent material of the main metal fitting has higher strength than the constituent material of the protector.

The protector 140 was manufactured from austenitic stainless steel to which Nb was added, the main metal fitting 138 was manufactured from SUS430LX (JIS standard), and the gas sensor 200 shown in FIG. 1 was assembled by changing the ratio (t1 / t2). The protector 140 was fixed to the outer surface of the tip portion 138s of the main metal fitting 138 by welding all around.
This gas sensor 200 was tested under a cold / heat cycle condition of 500 cycles with 900 ° C. × 20 minutes −200 ° C. × 20 minutes as one cycle, and the presence or absence of damage or deformation of the joint portion W after the test was visually evaluated.
Evaluation ◯: No peeling of the rear end joint 140b of the protector 140 from the joint W or deformation of the main metal fitting 138 Evaluation ×: There was peeling of the rear end joint 140b or deformation of the main metal fitting 138. thing

The obtained results are shown in FIG.
As shown in FIG. 4, when 0.6 ≦ (t1 / t2) ≦ 2.0, there is no peeling of the rear end joint portion 140b of the protector 140 from the joint portion W and no deformation of the main metal fitting 138. It was possible to suppress damage to the joint portion W.
On the other hand, when (t1 / t2) <0.6, the rear end joint portion 140b of the protector 140 was peeled off from the joint portion W. Further, when (t1 / t2)> 2.0, the main metal fitting 138 was deformed.

10 Sensor element 10a Detection part 138 Main metal fittings 138i1 First inner surface of main metal fittings 138i2 Rear end side inner surface of main metal fittings 140 Protector 140b Rear end part (rear end joint part)
142 Outer protector 143 Inner protector 200 Gas sensor O-axis W joint t1 Minimum distance from the outer surface of the main metal fitting to the outer surface of the protector t2 Minimum distance from the outer surface of the main metal fitting to the inner surface of the main metal fitting R Project the joint on the inner surface of the main metal fitting Area

Claims (3)

A sensor element that extends in the axial direction and has a detection unit on the tip side,
A metal tubular main metal fitting that surrounds the sensor element while projecting the detection unit from its tip.
A metal protector that houses the detection unit and is fixed to the main metal fitting,
It is a gas sensor equipped with
The protector includes an inner protector arranged through a gap between the detection unit and the outer protector and an outer protector arranged through the gap between the inner protector and the detector.
The rear end joint portion where the inner protector and the rear end portion of the outer protector overlap is joined to the outer surface of the main metal fitting to form a joint portion.
The thermal expansion rate of the constituent material of the protector at 800 ° C. is higher than the thermal expansion rate of the constituent material of the main metal fitting at 800 ° C.
When viewed in a cross section including the joint portion, the minimum distance t1 of the distance from the outer surface of the main metal fitting to the outer surface of the protector, and the minimum distance t2 of the distance from the outer surface of the main metal fitting to the inner surface of the main metal fitting are. Satisfying the relationship of 0.6 ≤ (t1 / t2) ≤ 2.0 ,
The first inner surface of the main metal fitting in the region where the joint portion on the outer surface of the main metal fitting is projected onto the inner surface of the main metal fitting has a diameter larger than the inner surface on the rear end side of the main metal fitting connected to the rear end side of the first inner surface. Spreads outward in the direction,
A gas sensor in which the first inner surface and the inner surface on the rear end side of the main metal fitting are not flush with each other . The gas sensor according to claim 1 , wherein the tensile strength of the constituent material of the protector at 800 ° C. is higher than the tensile strength of the constituent material of the main metal fitting . The gas sensor according to claim 1 or 2 , wherein the constituent material of the protector is austenitic stainless steel, and the constituent material of the main metal fitting is ferritic stainless steel .

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