spark plug

Abstract

Spark plug (10) comprising a metal sheath (15, 40, 50, 60), wherein the spark plug contains: a cylindrical shaft section (16) extending along an axial line (O) from one side of the front end to one side of the rear end and having an external thread (17) formed on an outer circumferential surface thereof, and a seat section (18) which is adjacent to a rear end (16a) of the shaft section (16) and which projects radially outwards, wherein the shaft section (16) has: a first region (31, 41, 51, 61) which extends in the direction of the axial line (O) at least over a region (16b) which extends from a rear end (16a) of the shaft section (16) to a first screw thread section (17b) of the external thread (17), counted from a rear end (17a) of the external thread (17), and which extends partly in a circumferential direction of the shaft section (16), and a second area (32, 42, 52, 62) which is adjacent to the first area (31, 41, 51, 61) in the circumferential direction, and where the first area (31, 41, 51, 61) has a higher Vickers hardness than the second area (32, 42, 52, 62).

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a spark plug, particularly to a spark plug including a metal shell having an external thread formed on a portion of its outer peripheral surface. BACKGROUND OF THE INVENTION

[0002] A spark plug is mounted on an engine by screwing an external thread of a metal shell of the spark plug into a threaded hole of the engine. Japanese Patent Application Laid-Open (Kokai) No. JP 2007-280942 A discloses a technique for preventing breakage of the metal shell due to the application of excessive tightening torque. According to the disclosed technique, after forming an external thread on a tubular member to become the metal shell, the tubular member is carburized or quenched so that the screw portion of the metal shell is hardened over its entire circumference.

[0003] The conventional technique described above can increase the breaking load of the metal shell by hardening the bolt section. However, since the hardened bolt section has reduced toughness, the metal shell may break at a load smaller than the breaking load if the tightening speed is low, and at a high tightening (screwing) speed.

[0004] Further relevant prior art is described in DE 103 15 957 A1 and DE 10 2010 055 210 A1. SUMMARY OF THE INVENTION

[0005] The present invention was conceived to address the problem described above. One advantage of the invention is a spark plug that can prevent the breakage of its metal shell regardless of the tightening speed.

[0006] To solve the above-described problem, corresponding spark plugs with the features of claim 1 and claim 2 are specified. The spark plug according to claim 1 is characterized in particular in that the first region has a higher Vickers hardness than the second region. The spark plug according to claim 2 is characterized in particular in that the first region and the second region have different structures, and matensite is present in the first region. Further advantageous embodiments are defined in the dependent claims. Some examples of spark plugs are described below. The scope of protection, however, arises from the claims.

[0007] A spark plug is provided that includes a metal shell having a cylindrical shaft portion extending along an axial line from a front end side toward a rear end side and having an external thread formed on an outer peripheral surface thereof, and a seat portion located adjacent to a rear end of the shaft portion and projecting radially outward. The shaft portion has a first region and a second region. The first region extends in the axial line direction at least over a range extending from a rear end of the shaft portion to a first screw thread portion of the external thread, counted from a rear end of the external thread, and extending axially in a circumferential direction of the shaft portion. The second region is located adjacent to the first region in the circumferential direction.The first area has a higher Vickers hardness than the second area.

[0008] A spark plug is provided that includes a metal shell having a cylindrical shaft portion extending along an axial line from a front end side toward the rear end side, having an external thread formed on an outer peripheral surface thereof, and a seat portion located adjacent to a rear end of the shaft portion and projecting radially outward. The shaft portion has a first region and a second region. The first region extends in the axial line direction at least over a range extending from a rear end of the shaft portion to a first screw thread portion of the external thread, counted from a rear end of the external thread, and partially extends in a circumferential direction of the shaft portion. The second region is located adjacent to the first region in the circumferential direction.The first area and the second area have different structures and martensite is present in the first area.

[0009] A spark plug as described above is provided, wherein in a portion of the stem portion of the metal shell on which a high axial tightening force acts when the spark plug is tightened; that is, in a region extending from the rear end of the stem portion to the first screw thread portion of the external thread, counted from the rear end of the external thread, the first region and the second region adjacent to the first region in the circumferential direction and having a lower Vickers hardness than the first region are provided. Since the second region, which has a lower Vickers hardness than the first region, is parallel along the direction in which the axial tightening force acts, the axial tightening force is distributed between the first region and the second region such that the amount of extension of the first region becomes equal to the amount of extension of the second region when the external thread is tightened.As a result, unlike the case where the second region is absent, if the tightening speed is low, the metal shell is prevented from breaking at a load less than the breaking load when tightening at high speed. Accordingly, the metal shell's breakage can be prevented regardless of the tightening speed.

[0010] A spark plug as described above is provided, wherein the stem portion of the metal shell has a first region extending from the rear end of the stem portion to the first screw thread portion of the external thread, counted from the rear end of the external thread, and a second region located adjacent to the first region in the circumferential direction. The first region and the second region have different structures, and no martensite having a high hardness is present in the second region. Since the second region, which is softer than the first region, extends parallel to the first region along the direction in which the axial tightening force acts, an effect similar to that described above with respect to the third aspect of the invention can be provided.

[0011] A spark plug as described above is provided, wherein the second region is arranged on each of the opposite sides of the first region in the circumferential direction such that the second region is adjacent to the first region. Thus, in addition to the effect of the third and fourth aspects of the invention, an effect of more reliably preventing breakage of the metal shell is provided regardless of the tightening speed.

[0012] A spark plug as described above is provided, in which martensite is present in the first region. Thus, in addition to the effect of the third aspect of the invention, an effect of enabling the production of the metal shell from carbon steel is provided.

[0013] A spark plug as described above is provided, wherein the first region extends in the axial direction from the rear end of the shaft portion to the front end of the external thread. Thus, in addition to the effect of any of the third to sixth aspects of the invention, an effect of increasing strength is provided in the region extending from the rear end of the shaft portion to the front end of the external thread and to which the axial tightening force acts. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a half-sectional view of a spark plug according to a first embodiment. Fig. 2A is a side view of a metal shell. Fig. Figure 2B is a sectional view of the metal shell along the line IIb-IIb of Fig. 2A. Fig. Figure 2C is a sectional view of the metal shell along the line IIc-IIc of Fig. 2A. Fig. Figure 3 is a graph showing the relationship between the axial tightening force and the elongation of an external thread of the metal shell. Fig. 4A is a side view of a metal shell of a spark plug according to a second embodiment. Fig. 4B is a side view of a metal shell of a spark plug according to a third embodiment. Fig. 4C is a side view of a metal shell of a spark plug according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferred embodiments of the present invention are described below with reference to the accompanying drawings. Fig. 1 is a half-sectional view of a spark plug 10 according to a first embodiment, taken along an axial line O. Fig. 1, the screw thread of an external thread 17 is shown in a simplified manner (this also applies Fig. 2A and Fig. 4A to 4C). The lower side of Fig. 1 is called the side of the front end of the spark plug 10 and the upper side of Fig. 1 is called the side of the rear end of the spark plug 10.

[0015] As in Fig. As shown in Figure 1, the spark plug 10 includes a metal shell 15 supporting an insulator 11. The insulator 11 is an approximately cylindrical member formed, for example, from aluminum oxide, which has excellent mechanical properties and excellent high-temperature insulating performance. The insulator 11 has an axial hole 12 extending therethrough along the axial line O.

[0016] A center electrode 13 is a rod-shaped electrode inserted into the axial hole 12 and held by the insulator 11 so as to extend along the axial line O. The center electrode 13 is arranged in the axial hole 12 so that the center electrode 13 protrudes from the front end of the insulator 11. The center electrode 13 is configured such that a core with excellent thermal conductivity is embedded in an electrode base member. The electrode base member is formed of a metal material made of Ni or an alloy containing Ni as a main component. The core is formed of copper or an alloy containing copper as a main component. The core may be omitted.

[0017] The metal terminal 14 is a rod-shaped member to which a high-voltage cable (not shown) is connected. A portion of the metal terminal 14 on the front end side is arranged in the insulator 11. The metal terminal 14 is electrically connected to the center electrode 13 within the axial hole 12. The metal shell 15 is attached to a portion of the outer periphery of the insulator 11 on the front end side.

[0018] The metal shell 15 is an approximately cylindrical member formed from an electrically conductive metal material (e.g., low-carbon steel). The metal shell 15 includes a cylindrical shank portion 16, a seat portion 18 located adjacent to the rear end of the shank portion 16 and projecting radially outward, and a rear end portion 19 located adjacent to the rear end of the seat portion 18. The rear end portion 19 includes a thin-walled portion 20 whose wall thickness is smaller than that of the seat portion 18, and a tool engagement portion 21 projecting radially outward from the thin-walled portion 20.

[0019] The shaft portion 16 is a portion surrounding a portion of the insulator 11 on the front end side and has an external thread 17 formed on the outer periphery thereof. The external thread 17 screw-engages with a threaded hole 25 of a prime mover 24, thereby fixing the metal shell 15 to the prime mover 24. The seat portion 18 limits the amount of screwing of the external thread 17 into the prime mover 24 and closes the gap between the external thread 17 and the wall surface of the threaded hole 25. In the present embodiment, a gasket 23 is attached to the front end of the seat portion 18. The gasket 23, interposed between the seat portion 18 and the prime mover 24, seals the gap between the external thread 17 and the wall surface of the threaded hole 25.

[0020] The thin-walled portion 20 is a portion that is plastically deformed and crimped for fastening when the metal shell 15 is installed on the insulator 11. The tool engagement portion 21 is a portion with which a tool such as a wrench engages when the external thread 17 is screwed into the threaded hole 25 of the engine 24.

[0021] A ground electrode 22 is a rod-shaped member made of a metal (e.g., a nickel alloy) and connected to the shaft portion 16 of the metal shell 15. A spark gap is formed between the ground electrode 22 and the center electrode 13. In the present embodiment, the ground electrode 22 is bent.

[0022] The spark plug 10 is manufactured, for example, by a method as described below. First, the center electrode 13 is inserted into the axial hole 12 of the insulator 11 and arranged such that the front end of the center electrode 13 protrudes from the front end of the insulator 11. Subsequently, the metal terminal 14 is inserted into the axial hole 12 of the insulator 11 to ensure electrical continuity between the metal terminal 14 and the center electrode 13. Subsequently, the insulator 11 is inserted into the metal shell 15 to which the ground electrode 22 has been connected in advance, and the rear end of the metal shell 15 is bent to fix the metal shell 15 to the insulator 11. The ground electrode 22 is bent, and the gasket 23 is attached, thus completing the spark plug 10.

[0023] The metal jacket 15 is Fig. 2A to 2C. Fig. 2A is a side view of the metal shell 15, Fig. Figure 2B is a sectional view of the metal shell 15 along the line Ilb-Ilb of Fig. 2A and Fig. Figure 2C is a sectional view of the metal shell 15 along the line IIc-IIc of Fig. 2A. Fig. Figure 2A shows the metal sheath 15 in a state before it is installed on the insulator 11. The lower side of Fig. 2A is called the side of the front end of the metal shell 15 and the upper side of Fig. 2A is called the side of the rear end of the metal shell 15 (this also applies Fig. 4A to 4C).

[0024] The metal shell 15 is manufactured, for example, by a method described below. First, a tubular workpiece (not shown) is formed by cold forging, cutting, or the like, from which the metal shell 15 is manufactured. An engaging portion 30 is provided in the workpiece adjacent to the front end of the seat portion 18. The engaging portion 30 is a portion with which the seal 23 (see Fig. 1) is in intervention.

[0025] After the ground electrode 22 (a straight electrode before being bent) is cut as shown in Fig. 2A, is connected to the shank portion 16 of the workpiece, the external thread 17 is formed on the shank portion 16 by means of roll forming, cutting, or the like. The engagement portion 30 is located adjacent to the rear end 17a of the external thread 17. As shown in Fig. As shown in Fig. 2B, the core diameter of the external thread 17 is approximately equal to the outer diameter of the engaging portion 30. After forming the external thread 17 on the shaft portion 16, a first region 31 and a second region 32 are formed on the shaft portion 16, thereby obtaining the metal shell 15. Plating or the like is performed for the metal shell 15 as needed.

[0026] The Vickers hardness of the first regions 31 is higher than the Vickers hardness of the second regions 32. In the present embodiment, the metal shell 15 is formed of low-carbon steel, and portions of the surface of the external thread 17 are irradiated with a laser beam for heating. As a result, heated portions of the external thread 17 are quenched, thereby forming the first regions 31. Regions (portions) that are not sufficiently heated become the second regions 32. If necessary, tempering is performed after quenching. Due to quenching, martensite is present in the structure of each first region 31, and austenite and / or pearlite is present in the structure of each second region 32. Since the metal shell 15 formed of carbon steel is used, the first regions 31 and the second regions 32, whose hardnesses differ, can be formed by heat treatment.

[0027] In particular, the method for forming the first regions 31 is not limited to laser quenching. Of course, the first regions 31 can be formed by electron beam quenching, discharge hardening, or shot peening. Since discharge hardening and shot peening do not harden the structure using a carbon solid solution, the material of the metal shell 15 is not limited to carbon steel. In this case, the material of the metal shell can be appropriately selected from various metal materials, taking into account thermal conductivity, thermal resistance, etc.

[0028] Furthermore, the method for forming the first and second regions 31 and 32 is not limited to the method for forming the first regions 31 by partially hardening the external thread 17. Of course, it is possible to form the second regions 32 by partially softening the external thread 17. In this case, the remaining portions become the first regions 31. For example, portions of the surface of the external thread 17 are irradiated with a laser beam or electron beam to heat them, so that portions of the external thread 17 are annealed after the metal shell 15 with the external thread 17 formed thereon is hardened by quenching and tempering, thereby forming the second regions 32. The regions (portions) that are not sufficiently annealed become the first regions 31.

[0029] As in Fig. 2C, the first regions 31 are portions of the shaft portion 16 that extend partially in the circumferential direction, and the second regions 32 are located circumferentially adjacent to the first regions 31. The first regions 31 and the second regions 32 are present in a region 16b of the shaft portion 16, in which a high axial tightening force acts on the shaft portion 16 when the external thread 17 of the spark plug 10 is screwed into the threaded hole 25 of the engine 24. The region 16b extends from the rear end 16a ( Fig. 2B) to a first screw thread portion 17b of the external thread 17, counting from the rear end 17a of the external thread 17. The first screw thread portion 17b can be a fully threaded portion or an incomplete threaded portion. In the present embodiment, the first regions 31 and the second regions 32 extend from the rear end 16a of the shaft portion 16 to the front end 17c of the external thread 17. The second regions 32 are arranged on opposite sides of each first region 31 in the circumferential direction, so that the two second regions 32 are adjacent to the first region 31. In the present embodiment, the number of first regions 31 and the number of second regions 32 is three.

[0030] The Vickers hardness of the first regions 31 and the second regions 32 is measured by a method described in JIS Z2244: 2009. The shank portion 16 is cut along a plane perpendicular to the axial line O, and the cut surface is mirror-polished, thereby obtaining a specimen for Vickers hardness measurement. In the Vickers hardness test, a notching tool is pressed against the specimen (the shank portion 16) at multiple measurement points spaced apart in the circumferential direction, forming indentations. The distances from the outer peripheral surface of the shank portion 16 to the respective measurement points are the same, and the same test force is applied to the notching tool at the respective measurement points for the same period of time.

[0031] In the present embodiment, the first regions 31 in the thickness direction (radial direction) of the shaft portion 16 are fully hardened as a result of laser quenching. Thus, the measurement points in the thickness direction are located at the center of the shaft portion 16. However, the Vickers hardnesses of the hardened (or softened) portions are measured if the first regions 31 and the second regions 32 are present only in a surface portion of the external thread 17 (e.g., if the first regions 31 are formed by shot peening or the like).

[0032] Since the Vickers hardness between the first regions 31 or between the second regions 32 differs by approximately ±50 HV, the first regions 31 and the second regions 32 are formed such that a Vickers hardness difference of 100 HV or higher exists between the first regions 31 and the second regions 32. For example, the Vickers hardness of the first regions 31 is 500 HV or higher, and the Vickers hardness of the second regions 32 is less than 400 HV.

[0033] The first regions 31 and the second regions 32 of the shaft portion 16, which are adjacent in the circumferential direction, can be determined by comparing the Vickers hardnesses measured at the plurality of measurement points separated from each other in the circumferential direction. Furthermore, by measuring the Vickers hardnesses of a plurality of test pieces that differ from each other in the distance between the rear end 16a of the shaft portion 16 and the cut surface, the range in which the first regions 31 and the second regions 32 extend from the rear end 16a of the shaft portion 16 toward the front end side can be determined. By combining these measurements, the extents of the first regions 31 and the second regions 32 in the circumferential direction and in the axial direction in the shaft portion 16 can be determined.

[0034] Furthermore, the presence of martensite in the first region 31 can be checked as follows. The shaft portion 16 is cut along a plane 16 perpendicular to the axial line O, and the shaft portion 16 is polished so that a flat cross section appears. By structural observation of a composite image obtained by a metallurgical microscope or an SEM, the presence of martensite can be checked. If necessary, electrolytic etching, non-electrolytic etching, or the like is performed using an etching solution before the structural observation. Furthermore, by structural observation of a plurality of sections that differ from each other in terms of distance from the rear end 16a of the shaft portion 16, the range in which the first regions 31 and the second regions 32 extend from the rear end 16a of the shaft portion 16 toward the front end side can be determined.By combining these observations, the extents of the first regions 31 and the second regions 32 in the circumferential direction and in the axial direction in the shaft portion 16 can be determined.

[0035] Fig. 3 is a graph showing the relationship between the axial tightening force and the elongation of the external thread 17 of the metal shell 15. Fig. 3 shows not only a curve for the metal shell 15 of the present embodiment, but also a curve for a metal shell of a comparative example in which the first region 31 is formed over the entire circumference of the external thread 17 (hereinafter referred to as the “metal shell 31”), and a curve for a metal shell of a comparative example in which the second region 32 is formed over the entire circumference of the external thread 17 (hereinafter referred to as the “metal shell 32”).

[0036] When an excessively high tightening torque is applied to the external thread 17 of a metal shell, the stress of the metal shell exceeds a yield point, and the metal shell breaks. The metal shell 32 has a higher elongation than the metal shell 31 but a smaller ultimate load than the metal shell 31. Meanwhile, due to the hardening of the external thread 17, the metal shell 31 has a higher ultimate load than the metal shell 32. However, the amount of elongation of the metal shell 31 before breakage is small due to the reduced toughness of the external thread 17. Thus, when the tightening (screwing) speed is high, the metal shell 31 may break due to a tightening impact at a load smaller than the ultimate load when the tightening speed is low.

[0037] In contrast, in the metal shell 15, in the region 16b extending from the rear end 16a of the shaft portion 16 to the first screw thread portion 17b of the external thread 17, counted from the rear end 17a of the external thread 17, there are the first regions 31 and the second regions 32, each adjacent to the corresponding one of the first regions 31 in the circumferential direction and having a lower Vickers hardness than the first regions 31. Stress concentration is likely to occur in the region 16b at the time of tightening. Since the second regions 32 are parallel to the first regions 31 along the direction (axial direction) in which the axial tightening force acts, the axial tightening force is distributed between the first regions 31 and the second regions 32 in such a way that the amount of elongation of the first regions 31 becomes equal to the amount of elongation of the second regions 32 when the external thread 17 is tightened.As a result, unlike the case where the second regions 32 are not present (the case of the metal shell 31), the metal shell 15 is prevented from breaking at a load smaller than the breaking load when tightened at a high speed if the tightening speed is low. Accordingly, the breakage of the metal shell can be prevented regardless of the tightening speed.

[0038] Furthermore, in the metal shell 15, the first regions 31 are hardened and contain martensite. The first regions 31 differ in structure from the second regions 32, and no martensite is present in the second regions 32. Since, as described above, the second regions 32, which are softer than the first regions 31, run parallel to the first region 31 along the direction in which the axial tightening force acts, fracture of the metal shell can be prevented regardless of the tightening speed.

[0039] Since two regions 32 are provided on opposite sides of each first region 31 in the circumferential direction, the second regions 32, which are softer than the first regions 31, can be distributed in the circumferential direction. As a result, breakage of the metal shell 15 can be more reliably prevented regardless of the tightening speed.

[0040] In addition, since the first regions 31 extend in the axial direction over a range from the rear end 16a of the shaft portion 16 to the front end 17c of the external thread 17, the strength of the shaft portion 16 can be increased not only in the range 16b extending from the rear end 16a to the first screw thread portion 17b and in which the stress concentration is likely to occur, but also in the range from the rear end 16a of the shaft portion 16 to the front end 17c of the external thread 17 and to which the axial tightening force is applied.

[0041] Based on Fig. 4A, Fig. 4B and Fig. 4C, the second to fourth embodiments are described. Fig. 4A is a side view of a metal shell 40 of a spark plug according to a second embodiment, Fig. 4B is a side view of a metal shell 50 of a spark plug according to a third embodiment and Fig. 4C is a side view of a metal shell 60 of a spark plug according to a fourth embodiment. The metal shells 40, 50, and 60 support the insulator 11 instead of the metal shell 15 of the spark plug 10 of the first embodiment. The same portions as those of the metal shell of the first embodiment are denoted by the same reference numerals and will not be described redundantly.

[0042] As in Fig. As shown in Figure 4A, two first regions 41 and one second region 42 are alternately formed in the metal shell 40 in the circumferential direction of the shaft portion 16. The first regions 41 have a higher Vickers hardness than the second region 42, and martensite is present in the structure of each first region 41.

[0043] The first regions 41 are present in a region 16b extending from the rear end 16a of the shaft portion 16 to the first screw thread portion 17b (see Fig. 2B). The second region 42 is in the axial direction (the vertical direction in Fig. 4A) over a region extending from the front ends of the first portions 41 to the front end 17c of the external thread 17. In the present embodiment, two first portions 41 are present in the region 16b, and two upper end portions of the second region 42 are present in the region 16b. Since the metal shell 40 has the first regions 41 and the second region 42 present in the region 16b where stress concentration is likely to occur, breakage of the metal shell 40 can be prevented as described for the first embodiment regardless of the tightening speed.

[0044] As in Fig. As shown in Fig. 4B, first regions 51 and second regions 52 are alternately formed in the metal shell 50 in the circumferential direction of the shaft portion 16. The first regions 51 have a higher Vickers hardness than the second regions 52, and martensite is present in the structure of each first region 51.

[0045] The first regions 51 are present in a region extending from the rear end 16a of the shaft portion 16 to the front end 17c of the external thread 17. In the present embodiment, the number of first regions 51 is six, and the number of second regions 52 is six. Since the widths of the first regions 51 and the second regions 52 in the circumferential direction can be made smaller compared to the first embodiment, the processing area of ​​the external thread 17 for forming the first regions 51 and the second regions 52 can be reduced. As a result, the metal shell 50 provides not only the function and effect described in the first embodiment, but also an effect of simplifying the process for forming the first regions 51 and the second regions 52.

[0046] As in Fig. As shown in Fig. 4C, first regions 61 and second regions 62 are alternately formed in the metal shell 60 in the circumferential direction of the shaft portion 16. The first regions 61 have a higher Vickers hardness than the second regions 62, and martensite is present in the structure of each first region 61.

[0047] The first regions 61 and the second regions 62 are formed in such a way that they extend spirally from the rear end 16a of the shaft portion 16. As a result, the first regions 61 and the second regions 62 can be formed in a manner that is smaller than in the case where the first regions and the second regions extend parallel to the axial line O (see Fig.2A), have longer lengths. Since the amounts of elongation of the first regions 61 and the second regions 62 can be increased by the amounts by which the lengths of the first regions 61 and the second regions 62 are increased, respectively, the metal shell 60 provides not only the function and effect described in the first embodiment but also an effect of expanding an additional range of tightening torque.

[0048] The present invention has been described based on embodiments. However, it should be understood that the present invention is not limited to the above-described embodiments and can be improved and modified without departing from the spirit of the present invention. For example, the number of first regions 31 (41, 51, 61) and the number of second regions 32 (42, 52, 62) can be appropriately set as long as at least one first region and at least one second region are present.

[0049] In the embodiments, the spark plug 10 includes the gasket 23 disposed at the front end of the seat portion 18 of the metal shell 15 (40, 50, 60). However, the present invention is not limited to the spark plug 10 having the gasket 23. Of course, the present invention can be applied to a conical seat type spark plug (metal shell) that does not include a gasket, in which the front end surface of the seat portion 18 is inclined, and the seat portion 18 is brought into contact with the engine 24, thereby sealing the combustion gas.

[0050] In the embodiments, the first regions 31 (41, 51, 61) are formed by quenching, discharge hardening, or shot peening, and the second regions 32 (42, 52, 62) are formed by annealing. However, the method for forming the first regions and the second regions is not limited thereto. Of course, it is possible to manufacture a metal shell including the first regions and the second regions by preparing two materials whose Vickers hardnesses differ from each other by 100 HV or more, forming the first regions and the second regions using the two materials, and joining them together by welding or the like. DESCRIPTION OF REFERENCE SYMBOLS 10 Spark plug 15, 40, 50, 60 metal jacket 16 Shaft section 16a rear end of the shaft section 16b area 17 external threads 17a rear end of the external thread 17b screw thread 17c front end of the external thread 18 Seating section 31, 41, 51, 61 first area 32, 42, 52, 62 second area O Axial line

Claims

[1] Spark plug (10) comprising a metal shell (15, 40, 50, 60), the spark plug containing: a cylindrical shaft portion (16) extending along an axial line (O) from a front end side to a rear end side and having an external thread (17) formed on an outer peripheral surface thereof, and a seat portion (18) located adjacent to a rear end (16a) of the shaft portion (16) and projecting radially outward, wherein the shaft portion (16) comprises: a first region (31, 41, 51, 61) extending in the direction of the axial line (O) at least over a region (16b) extending from a rear end (16a) of the shaft portion (16) to a first screw thread portion (17b) of the external thread (17), counted from a rear end (17a) of the external thread (17), and extending partly in a circumferential direction of the shaft portion (16), and a second region (32, 42, 52, 62) located circumferentially adjacent to the first region (31, 41, 51, 61), and wherein the first region (31, 41, 51, 61) has a higher Vickers hardness than the second region (32, 42, 52, 62). [2] Spark plug (10) comprising a metal shell (15, 40, 50, 60), the spark plug containing: a cylindrical shaft portion (16) extending along an axial line (O) from a front end side to a rear end side and having an external thread (17) formed on an outer peripheral surface thereof, and a seat portion (18) located adjacent to a rear end (16a) of the shaft portion (16) and projecting radially outward, wherein the shaft portion (16) comprises: a first region (31, 41, 51, 61) extending in the direction of the axial line (O) at least over a region (16b) extending from a rear end (16a) of the shaft portion (16) to a first screw thread portion (17b) of the external thread (17), counted from a rear end (17a) of the external thread (17), and extending partly in a circumferential direction of the shaft portion (16), and a second region (32, 42, 52, 62) located circumferentially adjacent to the first region (31, 41, 51, 61), and wherein the first region (31, 41, 51, 61) and the second region (32, 42, 52, 62) have different structures and wherein martensite is present in the first region (31, 41, 51, 61). [3] Spark plug (10) according to claim 1 or 2, wherein the second region (32, 42, 52, 62) is arranged in the circumferential direction on each of the opposite sides of the first region (31, 41, 51, 61) so that the second region (32, 42, 52, 62) is adjacent to the first region (31, 41, 51, 61). [4] Spark plug (10) according to claim 1, wherein martensite is present in the first region (31, 41, 51, 61). [5] Spark plug (10) according to one of claims 1 to 4, wherein the first region (31, 51, 61) extends in the direction of the axial line (O) from the rear end (16a) of the shaft portion 16 to a front end (17c) of the external thread (17).

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