JP2007173116A - Spark plug - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spark plug equipped with a noble metal chip having excellent spark depletion resistance, improved in oxidation of its ground electrode, and excelling in high-temperature strength. <P>SOLUTION: A Ni-based alloy constituting a ground electrode 40 for jointing a chip 43 made of a noble metal thereto contains not smaller than 68 mass% of Ni, and further contains 15-23 mass% of Cr, 0.8-3.5 mass% of Mo, 0.01-0.05 mass% of C, not greater than 3 mass% of Fe and 0.003-0.01 mass% of B. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spark plug used by being attached to an internal combustion engine, and more particularly to a spark plug in which a tip having excellent wear resistance is joined to the tip of a ground electrode.

  A spark plug that is mounted on an internal combustion engine such as an automobile engine and used for ignition is generally joined to a metal shell with a cylindrical metal shell, an insulator to which a center electrode held by the metal shell is fixed, and And a center electrode and a ground electrode that forms a spark discharge gap. In addition, in consideration of wear resistance and ignitability of the center electrode and the ground electrode, there is one in which a noble metal having excellent spark wear resistance and oxidation wear resistance is disposed on the portion of each electrode facing the spark discharge gap.

  Ni-based materials such as Inconel 600 (registered trademark) are used as materials constituting these center electrode and ground electrode (see, for example, Patent Document 1). This is because this material has properties that are excellent in high-temperature strength and high-temperature oxidation resistance.

By the way, in recent years, internal combustion engines have been improved in combustion efficiency for higher output and fuel saving, and the combustion temperature tends to be higher. For this reason, the spark plug exposed to the combustion chamber is required to have higher temperature resistance. In particular, the ground electrode is generally protruded from the metal shell attached to the internal combustion engine toward the center of the combustion chamber, so that it tends to be hotter than the center electrode held by the insulator. . For this reason, there is concern about insufficient high-temperature strength, and adding W or Mo as a countermeasure has been studied (see, for example, Patent Document 2).
JP 2005-44627 A JP 2003-257583 A

  However, when W or Mo is contained, the high temperature strength is improved, but the high temperature oxidation resistance is lowered. Therefore, it is conceivable to increase the Cr content in order to compensate for the high-temperature oxidation resistance that decreases. However, when Cr is contained in a large amount, the thermal conductivity of the electrode is lowered, and the heat resistance of the tip bonded to the tip is deteriorated, so that the durability of the tip and consequently the durability of the spark plug is lowered. There's a problem.

  The present invention has been made in view of the above-mentioned problems, and even if the ground electrode has the same thermal conductivity and oxidation resistance performance as those of the prior art and has improved high-temperature strength, it is possible to drop the joined chip, etc. The object is to provide a spark plug that can be used without any problems.

Therefore, the present invention provides
A cylindrical metal shell, a cylindrical insulator whose outer periphery is held in the inner hole of the metal shell, a center electrode disposed in the inner hole at the tip of the insulator, and wear-resistant at one end A spark plug comprising: a chip excellent in bonding, forming a spark discharge gap between the chip and the tip of the central electrode, and a ground electrode bonded at the other end to the tip of the metal shell,
The ground electrode contains 68% by mass or more of Ni as a main component, 15% by mass to 23% by mass of Cr as subcomponents, 0.8% by mass to 3.5% by mass of Mo, and C. It is characterized by containing 0.01 mass% or more and 0.05 mass% or less, Fe 3 mass% or less, and further containing B 0.003% or more 0.01 mass% or less.

  As described above, in the present invention, Mo is contained in an amount of 0.8 mass% to 3.5 mass% with respect to the Ni alloy. As described above, Mo can be expected to have the effect of improving the high-temperature strength as with W. Therefore, 0.8% by mass or more is added in order to eliminate insufficient high temperature strength. However, if too much is added, the thermal conductivity is lowered, so there is a concern that the durability of the chip bonded to the tip of the ground electrode may be lowered. Therefore, the upper limit is set to 3.5% by mass.

  Further, C is contained in an amount of 0.01% by mass to 0.05% by mass. This is because the inclusion of C has the effect of improving the high-temperature strength. However, if it is excessively contained, it forms a Cr component and a carbide, which will be described later, and reduces oxidation resistance.

  On the other hand, in order to prevent high-temperature oxidation of the ground electrode, it is effective to contain Cr that forms a protective oxide film for preventing oxygen from entering the surface of the ground electrode. However, Cr reacts with each other depending on the C content to form a carbide of Cr. Further, Cr, like Mo, lowers the thermal conductivity of the Ni alloy, so it is not advisable to add Cr. That is, it is necessary to consider the balance between the decrease in thermal conductivity due to the inclusion of Mo and the formation of Cr carbide due to the inclusion of C in the Cr content. % Or more and 23% by mass or less.

  The ground electrode having the above structure is excellent in high temperature strength, can form a protective oxide film imparting high temperature oxidation resistance, and can be said to suppress the decrease in thermal conductivity to a minimum. However, it is an important requirement that the characteristics desired as a ground electrode of a spark plug are not only resistant to such an environment but also can be manufactured more easily and reliably.

  Therefore, the ground electrode of the present invention contains Fe at 3 mass% or less. Since Fe has an effect of preventing cracking during hot forging, it can be said that it is an indispensable element for forming a ground electrode of a spark plug in terms of improving hot workability. However, as is well known, Fe has the effect of lowering the high temperature strength, so it is desirable to suppress its content as much as possible.

  Since the Fe content is suppressed, there is a concern about the occurrence of cracks during hot forging during formation of the ground electrode. Therefore, in the present invention, B is contained in an amount of 0.003% to 0.01% by mass. B has an effect of improving the grain boundary strength of the structure constituting the ground electrode, and can prevent cracking during hot forging. However, if it is excessively contained, the cold forgeability deteriorates, so it is desirable to keep it in the above range.

  Since the ground electrode of the spark plug of the present invention has the above composition, a long life can be achieved even if the spark plug including a noble metal tip is used in a harsh environment.

  In addition to the said structure, when this invention contains Si, Mn, Al, and Ti, it is desirable to make it each contain 0.3 mass% or less. These components have the effect of removing P, S, etc. that may be contained as impurities in the raw material of the ground electrode. In producing the Ni alloy as the ground electrode material of the spark plug of the present invention, Si, Mn, Al, and Ti are contained to remove the impurities, and the ground electrode material thus prepared has an adverse effect as a spark plug. It does not contain impurities such as P and S. If the added amount of Si, Mn, Al, and Ti is small, the impurities in the Ni alloy will not be completely removed. Paradoxically, the fact that Si, Mn, Al, and Ti are contained in the ground electrode material ensures that impurities that adversely affect the spark plug are not contained.

  By the way, for example, Si and Al are known to have an effect of improving high-temperature oxidation resistance and spark wear resistance, but can also contribute to promoting internal oxidation of the ground electrode. In the present invention, high temperature oxidation resistance and spark wear resistance are improved by arranging a noble metal tip on the ground electrode, so that it can be ensured that no impurities are contained as described above, Si, Mn, Al and Ti are each contained at 0.3 mass% or less. Moreover, it is more desirable that the total content of Si, Mn, Al, and Ti is 0.8% by weight or less.

  When the spark plug of the present invention is compared with an electrode material used for a ground electrode of a conventionally known spark plug, for example, the electrode material described in JP-A-2000-336446 contains Mo and B. Absent. Since Mo is not contained, the high-temperature strength is insufficient, and since B is not contained, a relatively large amount (over 3% by mass) of Fe must be contained in order to ensure workability. Even in the point of containing a large amount of Fe, the high temperature strength is still insufficient and the problem of the present invention cannot be solved.

  Further, for example, the electrode material described in Japanese Patent Application Laid-Open No. 2002-129268 describes an example containing Mo unlike the above-mentioned publication. However, it still contains a large amount of Fe, and there is a concern that the high temperature strength is insufficient.

  On the other hand, it is disclosed that the electrode material described in JP-A-2002-235137 contains B. However, it can be seen that Mo is not contained and the high-temperature strength is low, and also Fe is contained in a large amount as in the above two publications, and the high-temperature strength is not emphasized.

  Thus, conventionally known Ni alloy electrode materials for spark plugs have not solved the problems of the present invention. This is because none of the ones described in the above publication is supposed to be a spark plug having a noble metal tip, and the present invention has been made on the assumption that the noble metal tip is provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an overall view of a spark plug 100 of the present invention. The spark plug 100 is generally configured by combining an insulator 30 having a center electrode 10 and a terminal electrode 20 and a metal shell 50 having a ground electrode 40. Hereinafter, each member will be described.

  The center electrode 10 is formed in a substantially rod-like shape having a base 11 made of Ni-based alloy such as Inconel 600 and having a flange 11 at the rear end. A Cu alloy constitutes the core 12 for the purpose of improving thermal conductivity at the center of the base material made of Ni-based alloy. Further, a tip 13 made of a noble metal alloy containing Pt, Ir or the like and having excellent wear resistance is joined to the tip of the center electrode 10. On the other hand, the ground electrode 40 provided in the metal shell 50 is formed of a Ni alloy described later as a base material, is formed in a substantially rectangular shape, and is joined to the tip of the metal shell 50. The ground electrode 40 is bent at a substantially right angle so as to form a spark discharge gap G with a tip whose one side is joined to the center electrode 10. A tip 43 made of a noble metal alloy is joined to one side surface of the ground electrode 40 in the same manner as the center electrode 10 for the purpose of improving ignitability and wear resistance.

  The metal shell 50 is formed into a substantially cylindrical shape by subjecting an iron-based metal member such as S15C or S25C or a metal member such as stainless steel to plastic processing, and its rough shape is formed after finishing such as cutting. A threaded portion 51 for attaching the spark plug 100 to an internal combustion engine (not shown) is rolled on the distal end side of the outer peripheral surface of the metal shell 50. On the rear end side of the threaded portion 51, there is formed a flange portion 52 having a seating surface for maintaining airtightness by contacting the outer surface of the threaded portion 51 through the gasket 8 when attached to the internal combustion engine. A tool engaging portion 53 is formed on the rear end side of the portion 52 to engage with a tool such as a plug wrench when attached to the internal combustion engine. The tool engagement portion 53 and the flange portion 52 are formed thin so as to be buckled when the insulator 30 is assembled (FIG. 1 shows the shape after buckling). ).

  The rear end side of the tool engaging portion 53 is formed in a thin cylindrical shape so that a caulking lid 60 that becomes the rearmost end portion of the metal shell 50 is formed when the spark plug 100 is completed (in FIG. The shape after the caulking lid is formed is shown.). The inner hole 57 of the metal shell 50 is formed with a small-diameter hole 54 at the axial position where the threaded portion 51 is formed, and a shelf 55 protruding radially inward is formed at the distal end side of the small-diameter hole 54. . On the rear end side connected to the small-diameter hole 54, a large-diameter hole 56 is formed up to the rear end with the axis position where the flange portion 52 is formed as a boundary. The ground electrode 40 is joined to the tip of the metal shell 50 formed in this way. This joining is performed by resistance welding, and after removing the welding sag, the metal shell 50 is plated with zinc or the like together with the ground electrode 40.

  The insulator 30 is manufactured by mixing an insulating ceramic powder such as alumina or aluminum nitride with a binder, etc., forming a rough shape by a press, grinding and shaping with a grindstone, and firing. The insulator 30 has a substantially cylindrical shape, and has a shaft hole 31 formed therein, and a middle body portion 32 protruding outward in the radial direction is formed substantially at the center of the outer surface in the axis O direction. A front end side body portion 34 having a stepped portion 33 facing the front end is formed on the front end side of the middle body portion 32, while a rear end side slightly larger in diameter than the front end side body portion 34 is formed on the rear end side of the intermediate body portion 32. An end-side body portion 35 is formed. Note that the tip end side of the stepped portion 33 formed on the tip end side body portion 34 constitutes a leg length portion 36 that is exposed to the combustion gas when the spark plug 100 is completed. A support step portion 37 that supports the center electrode 10 is formed on the rear end side of the leg long portion 36 in the shaft hole 31, and the inner diameter of the shaft hole 31 is narrower at the front end side than the support step portion 37 than at the rear end side. It is formed.

  As for the assembly of the insulator 30, the center electrode 10 and the terminal electrode 20, as is well known in the art, a metal shell 50 in which the insulator assembly made of glass seal material 5 and resistor 6 is integrated and the ground electrode 40 is joined. Assembled. This assembly is also well known in the art, and the packing 7 is interposed between the stepped portion 33 of the insulator 10 on the tip side of the middle barrel portion 32 and the shelf portion 55 of the metal shell 50. Further, the insulator 10 is fixed to the metal shell 50 by the caulking lid 60 through the packings 3 and 3 and the talc 4 on the rear end side from the middle body portion 32.

  As described above, the spark plug 100 is completed by assembling the insulator 30 to which the center electrode 10 and the terminal electrode 20 are fixed and the metal shell 50 to which the ground electrode 40 is joined.

  The composition of the ground electrode 40 of the spark plug 100 of the present invention will be described. The ground electrode 40 has a Cr component of 16% by weight, a Mo component of 3.0% by weight, an Fe component of 2.0% by weight, a C component of 0.04% by weight, a B component of 0.004% by weight, and a Mn component. Is 0.2% by weight, Al component is 0.2% by weight, Si component is 0.2% by weight, Ti component is 0.1% by weight, and the remainder is Ni component (about 78.2% by weight). . In addition, Table 1 shows the compositions (Comparative Examples 1 to 12) of the ground electrode 40 prepared as a comparative example.

  The ground electrode 40 having the above composition is manufactured as follows. First, using a known vacuum melting furnace, a molten alloy having each component composition is prepared, and a 30 cm square bar-shaped ingot is formed by vacuum casting. This ingot is formed into a round bar having a diameter of 60 mm by hot forging, and this round bar is subjected to a drawing process to obtain a wire having a cross-sectional dimension of 1.5 mm × 2.8 mm. This wire is welded to the metal shell 50 to form the ground electrode 40.

  Next, the performance evaluation test of the spark plug 100 of the present invention will be described. The composition of the ground electrode is as described in the above Examples and Comparative Examples 1 to 11, and the performance evaluation test to be performed is as follows. In addition, the spark plug 100 was produced by using the same manufacturing method for both the example and the comparative example.

Oxide film thickness increase evaluation The oxide film thickness increase evaluation test is performed in the completed form of the spark plug 100 described above, and the tip 43 bonded to the ground electrode 40 has a cylindrical shape of φ1.0 mm and a length of 0.5 mm. (Composition: 80Pt-20Ir). The spark plug 100 is attached to a 4-cylinder 2 liter gasoline engine, the engine speed is 5000 rpm and the idle state (700 rpm) is repeated for 1 minute, and the oxide film thickness after operating the engine for 250 hours is determined for each ground electrode 40. Measure for composition. In addition, the measurement location took the cross section of the longitudinal direction of the ground electrode 40 which passes along the back surface of the site | part to which the chip | tip 43 was joined, observed with SEM, and measured on the enlarged image. As the judgment criteria, those in which the measured value of the oxide film thickness was less than 120 μm were evaluated as “◯”, those in the range from 120 μm to less than 180 μm were evaluated as Δ, and those in which the measured value was 180 μm or more were determined as “X”. In this evaluation test, a chip that has been dropped is also determined as x.

-High temperature strength evaluation In the high temperature strength evaluation test, the ground electrode 40 (but the chip 43 is not joined) before completion of the spark plug 100 is put into an electric furnace with one end and the other end supported in the longitudinal direction. Then, the strength when pulled in the longitudinal direction in a state heated to 700 ° C. is measured. The criterion was determined to be ○ when the strength was 350 N / mm ² or more, and × when the strength was less than that.

-Workability evaluation In the workability evaluation test, it was determined that the case where no defects such as cracks occurred in the wire processing step and the ground electrode formation step was ○, and the case where the failure occurred was ×. These steps are not special steps and are the same as conventionally known steps. In addition, the defect by the former process is set to x (hot), and the defect by the latter process is written x (cold).

-Tip durability evaluation The spark plug of the evaluation object similar to the above is attached to a 4-cylinder 2-liter engine, and the state of the noble metal tip is observed after operation at 5000 rpm for 400 hours. The judgment was made by comparing the area of the projections of the noble metal tip from the direction perpendicular to the axis O direction of the spark plug before and after the test, and the remaining amount of the noble metal tip was 90% or more without dropping the noble metal tip from the ground electrode. Were evaluated as ◯, less than 90% as 80% or more, Δ, and less than 80% as ×.

The following is confirmed from the evaluation test.
From the comparison between the Examples and Comparative Examples 1 and 2, the Cr component is preferably contained in an amount of 15 wt% to 23 wt%. Cr is oxidized to form a Cr ₂ O ₃ film. Since this Cr ₂ O ₃ film has a dense structure, oxygen around the ground electrode 40 is prevented from reacting with the tissue inside the ground electrode 40, and a protective oxide film that acts to reduce the oxidative consumption of the ground electrode 40 and Become. In Comparative Example 1, since Cr is less than 15% by weight, this effect cannot be expected, and the structure inside the ground electrode 40 reacts with oxygen to increase the oxide film thickness. Further, as a result, the bonding force between the chip 43 and the ground electrode 40 is reduced, resulting in a problem that the chip 43 falls off.

  On the other hand, since the comparative example 2 contains Cr excessively, thermal conductivity falls and the thermal radiation performance of the ground electrode 40 falls. As a result, the tip side of the ground electrode 40 of Comparative Example 1 is at a higher temperature than that of the example. When the tip of the ground electrode 40 reaches a high temperature, the amount of expansion at the joint surface between the ground electrode 40 and the chip 43 differs due to the difference in thermal expansion coefficient between the ground electrode 40 and the chip 43. Since the tip portion is in a high temperature state, the stress generated on the joint surface due to the difference in expansion amount is released, while the tip 43 falls off the ground electrode 40 due to the stress generated when the temperature is low. Conceivable.

  From the comparison between Examples and Comparative Examples 3 and 4, Mo is preferably contained in an amount of 0.8 wt% to 3.5 wt%. In Comparative Example 3, the high temperature strength is not sufficiently improved due to insufficient Mo content. On the other hand, in Comparative Example 4, since Mo was excessively contained, it was determined to be “x” in the evaluation of the oxide film thickness increase. Mo improves the high-temperature strength, but has an effect of inhibiting the formation of a protective oxide film. Therefore, if it is excessively contained, it causes an increase in the oxide film thickness.

  It can be confirmed that it is preferable not to contain Fe excessively as compared with Examples and Comparative Example 5. Fe is preferably not contained as much as possible because it lowers the oxidation resistance and high-temperature strength. However, in order to produce the ground electrode 40, it should have a certain degree of hot forging performance, and the allowable upper limit is In the configuration of the invention, it is 3% by weight.

  From the comparison between Examples and Comparative Examples 6 and 7, it can be said that C is preferably contained in an amount of 0.01 wt% to 0.05 wt%. In Comparative Example 6, since the content of C is small, the oxidation resistance is not deteriorated, but sufficient high-temperature strength cannot be obtained, and the risk of dropping of the chip 43 is caused by the same mechanism as in Comparative Example 1. . On the other hand, in Comparative Example 7, since it was excessively contained, the oxidation resistance performance was insufficient.

  The effect of B can be confirmed from the comparison between Example and Comparative Examples 8 and 9. Since Comparative Example 8 was less than 0.003% by weight, cracks occurred in the process of manufacturing the ground electrode 40. As a result, the workability is determined as x. In addition, since such a problem has occurred, an oxide film thickness increase evaluation test, a high temperature strength evaluation test, and a chip durability evaluation test are not performed. On the other hand, since Comparative Example 9 contains more than 0.01% by weight, the cold workability deteriorates, and the wire is drawn from the strand to form a ground electrode for testing. Samples that crack when the shape of the ground electrode 40 was formed frequently.

From the comparison between Example and Comparative Example 10, it can be said that it is preferable that the contents of Mn, Si, Al, and Ti additionally contained in the ground electrode 40 should be 0.3% by weight or less, respectively. . These components have higher affinity for oxygen than Cr and oxygen, and form an oxide at the interface between the Cr ₂ O ₃ film formed on the surface layer of the ground electrode 40 and the ground electrode 40. The formed oxide may destroy the Cr ₂ O ₃ film and reduce the oxidation resistance of the ground electrode 40 due to the protective oxide film. More desirably, these components may be 0.8% by weight or less in total. On the other hand, in order to ensure that impurities such as P and S are not contained, at least one of Mn, Si, Al, and Ti may be contained, for example, 0.1% by weight or more.

  As Comparative Example 11, a similar test was performed for NFC600, which is a conventional equivalent. Although the Cr content was sufficient, Fe was contained in a large amount of 7% by weight, so that the oxidation was promoted and a sufficient oxidation suppression effect was not obtained. In addition, the high temperature strength may be reduced. Moreover, since Mo is not contained, the high-temperature strength is not sufficient, and in this example, which is a severe evaluation test, the object of the present invention has not been solved.

  From the above evaluation tests, the spark plug of the present invention achieves spark wear resistance by providing a noble metal tip, and is sufficiently long even when the ground electrode to which the tip is joined is used under severe conditions. A lifetime can be achieved.

  Note that the spark plug of the present invention may be configured such that the ground electrode has a core material having good thermal conductivity such as Cu, and the present invention does not exclude the configuration. However, since the present invention has an effect of preventing the noble metal tip from falling off without using the core material, it goes without saying that the present invention is excellent in terms of manufacturing cost.

1 is an overall view of a spark plug 100 of the present invention, and is a partial cross-sectional view thereof.

Explanation of symbols

10 center electrode 20 terminal electrode 30 insulator 40 ground electrode 43 (noble metal) chip 50 metal shell 100 spark plug

Claims (3)

A cylindrical metal shell,
A cylindrical insulator whose outer periphery is held in the inner hole of the metal shell,
A center electrode disposed in the inner hole of the tip of the insulator;
A tip having excellent wear resistance is joined to one end, a spark discharge gap is formed between the tip and the tip of the central electrode, and a ground electrode having the other end joined to the tip of the metal shell,
A spark plug comprising:
The ground electrode is
Containing 68% by mass or more of Ni as a main component,
As subcomponents, Cr is 15% by mass to 23% by mass, Mo is 0.8% by mass to 3.5% by mass, C is 0.01% by mass to 0.05% by mass, and Fe is 3% by mass or less. A spark plug comprising: 0.003% by mass to 0.01% by mass of B. The ground electrode is
The spark plug according to claim 1, further comprising 0.3% by mass or less of Si, Mn, Al, and Ti, respectively. The spark plug according to claim 2, wherein the total content of the Si, Mn, Al, and Ti components is 0.8 wt% or less.

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