Glow plug and method for manufacturing the same

Abstract

Glow plug, including: a heater (30) which extends in an axial direction; a heater-holding sleeve (60) which holds the heater (30) from its radially outer side and which has a rearward-facing end surface (61); a cylindrical housing (10) which has a front end face (10f) and which is connected at their common boundary (VP) to a rear end of the heater-holding sleeve (60) via an annular welded area (12) which extends along the boundary (VP) between the front end surface (10f) of the housing (10) and the rearward end surface (61) of the sleeve, and a corrosion-resistant metal coating (11), which are located on an outer circumferential surface of the housing (10) and adjacent to the welding area (12), wherein radially inwards of the welding area (12) the front end surface (10f) is in direct contact with the rearward facing end surface (61).

Description

Technical area

[0001] The present invention relates to a glow plug (which may hereinafter simply be called a plug) which is used, for example, to preheat or assist in starting a diesel engine, in particular to a glow plug whose surface is rust-proof, and to a method for producing the same. Background technology

[0002] Traditionally, a glow plug has been mainly used to assist starting a diesel engine or to purify exhaust gas. Glow plugs are roughly classified into two types: one is a metal glow plug, in which a heating coil is housed within a metal tube to form a heater, and a casing holds the heater therein; and the other is a ceramic glow plug, in which a heating element made of an electrically conductive ceramic is attached to an electrically insulating ceramic substrate to form a ceramic heater, and a casing holds the ceramic heater therein. In some ceramic glow plugs, a casing holds a ceramic heater therein via an intermediate member called an outer metal tube (heater-holding sleeve) (see, for example, Patent Document 1, which is Japanese Patent Application Laid-Open (kokai) JP 2004-205148 A).In such a configuration that a heater holding sleeve is used to hold a ceramic heater in a housing, the ceramic heater is held in the heater holding sleeve by brazing or press fitting; the heater holding sleeve is made to abut against the housing; and laser welding is performed at the abutment area to ensure the connection. Then, a rust preventive is applied to the outer surface of the housing and to the surface of a welding area formed by welding, for example, by applying a coating material containing an imperceptible aluminum powder (hereinafter referred to as an antirust aluminum coating). In other words, in manufacturing such a glow plug, a rust preventive coating is performed in a final step after component elements are assembled into a glow plug.This is because, in terms of material cost and machinability, an iron-based alloy is used to form the housing.

[0003] To perform rust prevention treatment after the housing and the heater support sleeve are joined together, an additional step is required, such as applying a rust prevention coating to individually supported plugs. This may result in increased manufacturing costs. Due to the implementation of high output and performance of a diesel engine, to which plugs are attached, the working environment for the plugs is demanding. Therefore, an anti-rust aluminum coating may fail to provide sufficient corrosion resistance. This raises the need to improve corrosion resistance by metal plating (metal coating) on ​​the housing. In such a situation, it has been studied to coat the housing with, for example, zinc (Zn) before the housing and an outer metal tube are joined together. A Zn plating layer can be formed at a comparatively low cost.If further rust resistance is required, a nickel (Ni) plating or a Zn-Ni alloy plating is a treatment to consider.

[0004] The document US 2005 / 0284860 A1 discloses a glow plug in which a hollow cylindrical sleeve is welded to a housing for a heater. Disclosure of the inventionProblems solved by the invention

[0005] Although the above-mentioned overlay layers (coatings) can easily implement corrosion resistance, when metals with such an overlay layer present in an interlayer between the metals are welded together, the joint performance is deteriorated. In particular, when metal members with a Zn overlay layer present on an interfacial layer between the metal members are laser-welded together, a recess (also called a blowhole) is formed in a welding area or a sputtering trace is formed at the welding area due to spark generation (sputtering). This damages an external appearance and may deteriorate the joint condition as well as corrosion resistance. These disadvantages can occur regardless of the composition of the overlay layer, but are particularly noticeable when the overlay layer contains Zn.Since Zn has a low boiling point of 907 °C, it is conceivable that Zn evaporates during welding and is ejected in the form of a bubble from within a molten region.

[0006] An object of the present invention is to provide a glow plug which is free from inferior corrosion resistance and free from the above-mentioned disadvantages even when a housing coated with a Zn-containing material and a heater-holding sleeve are bonded together. Means to solve the problems

[0007] To achieve the above object, the glow plug of the present invention comprises a rod-like heater extending in an axial direction; a heater-supporting sleeve supporting the heater from its radially outer side and having a rearward-facing end face; a cylindrical housing having a front end face and connected to a rear end of the heater-supporting sleeve via a weld portion formed extending along a boundary between the front end face of the housing and the rearward-facing end face of the sleeve; and a corrosion-resistant overlay layer formed on an outer peripheral surface of the housing and adjacent to the weld portion. In the glow plug, the front end face is in direct contact with the rearward-facing end face within the weld joint.

[0008] The housing and the heater holding sleeve are joined together by welding at the boundary, with the housing holding the heater therein. A region which is melted by welding includes an outer periphery of the front end surface of the housing and an outer periphery of the rear end surface of the heater holding sleeve. Before welding is performed, the front end surface of the housing and the rear end surface of the heater holding sleeve are positioned. After welding is performed from a radially outer side as viewed on a longitudinal section including the axis of the glow plug, both the front end surface of the housing and the rear end surface of the heater holding sleeve, which are adjacent to each other, remain unmelted and are radially adjacent to the welding region.As mentioned previously, one reason for a gas bubble in the weld portion or a sputter mark at the weld portion is the presence of a plating layer containing Zn at the boundary layer when the housing and the heater-holding sleeve are in direct contact with each other without such a plating layer or the like being interposed at the boundary. Accordingly, in a glow plug thus formed, occurrence of a disadvantage such as inferior corrosion resistance or the like caused by a gas bubble or a sputter mark can be suppressed and can have a good external appearance. To obtain such a structural feature, a plating layer (coating) formed on a front end surface of the housing can be removed by polishing.Or, a front end surface of a housing may be masked to prevent the front end surface of the housing from being overlaid. In the case where masking is performed, the area to be masked is desirably limited to the area that will be included in the weld area after welding. This is because if an area that will not be included in the weld area is masked, corrosion resistance cannot be ensured for the area that will not be included in the weld area after welding and is exposed as a portion of the housing.

[0009] To ensure corrosion resistance for a portion of the heater-holding sleeve that is not included in the weld area after welding and is exposed as part of the heater-holding sleeve, the heater-holding sleeve may have a corrosion-resistant overlay layer formed thereon. In this case, the overlay layer is preferably not formed on a portion adjacent to the housing. Alternatively, the heater-holding sleeve itself may be formed of a corrosion-resistant material, thus omitting the step of providing the overlay layer.

[0010] In the glow plug of the present invention, the plating layer preferably comprises a Zn component. A preferred range of the Zn component is 80 wt% or more, which includes a plating layer containing 100 wt% Zn or a Zn-Ni alloy plating layer containing 80 wt% or more of a Zn component.

[0011] In the glow plug of the present invention, the thickness of the overlay layer is preferably 5 µm or greater.

[0012] The plating layer is expected to improve corrosion resistance. To adequately meet this requirement, the plating layer must have a sufficient thickness. On the other hand, the upper limit of the plating layer thickness can be set at 30 µm. If the plating layer thickness is greater than 30 µm, it will take excessive time to remove a portion of the plating layer formed on the front end surface of the housing after the plating layer is formed on the entire outer surface of the housing.

[0013] In the glow plug of the present invention, the heater holding sleeve has an engagement portion disposed radially inward of the rearward facing end surface and protruding rearward from the rearward facing end surface, the engagement portion being firmly engaged with an inner peripheral surface of the housing and including a spacing portion disposed radially inward of the front end surface of the housing, the spacing portion being spaced from the heater holding sleeve.

[0014] The engaging portion, which projects rearward and has substantially the same diameter as that of a recess of the housing, is formed at a radially inner end portion of the rearward end surface of the heater holding sleeve, which is adjacent to the front end surface of the housing. This facilitates positioning of the housing and the heater holding sleeve when the housing and the heater holding sleeve are joined together. However, adoption of these configurations may cause the following disadvantage. Since the overlay layer is formed on the surface of the housing before the housing is joined to the heater holding sleeve, the overlay layer may peel off when the housing is fitted to the engaging portion (by press fitting).If exfoliation of an overlay occurs between the front end face of the housing and the rearward end face of the heater-holding sleeve, it may become difficult to achieve abutment of the front end face of the housing against the rearward end face of the heater-holding sleeve. If the front end face of the housing and the rearward end face of the heater-holding sleeve are forced to abut each other by force, the concentricity between the housing and the heater-holding sleeve may be seriously deteriorated. According to the present invention, the rearward end face of the heater-holding sleeve and the spacing portion of the housing are separated from each other at a radially inner end portion of the boundary intermediate layer, thereby forming a "recess" space.Flaking of a coating is accommodated in this space, thereby preventing the occurrence of disadvantages associated with joining. As a result of forming the recess space, an abutment area between the housing and the heater-holding sleeve, and the periphery of the abutment area, from the radially outer side, includes the welding area, the boundary layer, the space (where the rearward-facing end surface and the spacing area are separated from each other), and the engagement area.

[0015] In the glow plug of the present invention, the heater support sleeve is made of a material that has higher corrosion resistance than that of the housing. For example, the heater support sleeve is made of an Fe-Cr alloy containing 16 wt% or more Fe (e.g., a stainless steel such as SUS430, SUS630).

[0016] In the glow plug of the present invention, the heater holding sleeve is formed of an Fe-Cr alloy; a relationship of Wh < Wm < Wc is satisfied, where Wh is an average Cr content of the shell, Wc is a Cr content of the heater holding sleeve, and Wm is a Cr content of the weld portion; and Wm is 5 wt% or greater.

[0017] Since the heater support sleeve must have a hardness sufficient to support the ceramic heater, a material containing a large amount of Cr, such as stainless steel, is suitable for the heater support sleeve. As with the housing, carbon steel is used to form the housing due to its easy machining. However, carbon steel (Fe-based alloy) does not necessarily exhibit excellent corrosion resistance. As mentioned above, therefore, a plating layer containing Zn is formed on the outer peripheral surface of the housing.

[0018] Cr is a component for improving corrosion resistance and is known to significantly improve the corrosion resistance of Fe-based alloys when incorporated into Fe-based alloys. Thus, by performing welding such that the weld area has an average Cr content Wm of 5 wt% or greater, a glow plug with excellent corrosion resistance can be formed without the need for an additional step after joining to improve corrosion resistance, such as the conventional application of an anti-rust aluminum coating. Using the above Cr relationship makes it possible to coat the shell with a material that has better corrosion resistance than the conventional anti-rust aluminum coating.The thus prepared housing can be used in a glow plug manufacture, thereby eliminating the need to perform rust protection after the housing and the heater holding sleeve are joined together.

[0019] Furthermore, in the glow plug of the present invention, when the weld area is viewed in a cross section including a heater axis, a relationship of Sb < Sf is satisfied, where Sb is an area of ​​a shell-side portion of the weld area located behind the boundary, and Sf is an area of ​​a sleeve-side portion of the weld area located forward of the boundary. With this configuration, the corrosion-resistant component included in the heater-supporting sleeve can spread over the entire weld area.

[0020] To manufacture a glow plug of the present invention, the method comprises: forming a corrosion-resistant overlay layer on an outer peripheral surface of the housing but not on the front end surface of the housing; aligning the housing having the corrosion-resistant overlay layer with the heater supporting sleeve by abutting the front end surface, on which the corrosion-resistant overlay layer is not formed, against the rearward end surface of the heater supporting sleeve; and irradiating a laser beam so as to melt a portion of the housing and a portion of the heater supporting sleeve and form a weld portion adjacent to the corrosion-resistant overlay layer formed on the outer peripheral surface of the housing.Since the corrosion-resistant overlay layer is not formed on the front end surface of the housing, which is to be abutted against the rear end surface of the heater-holding sleeve, the welding region does not include a gas bubble caused by the overlay layer. To eliminate a gap between the corrosion-resistant overlay layer and the welding region, it is preferable to irradiate the laser beam so as to overlap the corrosion-resistant overlay layer to melt a front end of the corrosion-resistant overlay layer. To further enhance the corrosion resistance of the welding region, it is preferable to irradiate a laser beam in the melting step so that an innermost part of the welding region is located in front of the boundary. Short description of the drawings Fig. 1 is a view, in longitudinal section, showing the overall configuration of a glow plug 100 of the present invention, and is accompanied by an enlarged partially cutaway explanatory view. Fig. 2 is a series of views showing a process for manufacturing the glow plug 100 of the present invention. Fig. 3 is a sequence of views showing a process for joining a heater and axial element assembly 120 and a housing 10. Fig. 4 is a series of views showing, on an enlarged scale, a process of joining the heater-axial member assembly 120 and the housing 10 for explaining the effect of the present invention. Fig. 5 is a view explaining a welding portion 12 which joins a heater holding sleeve 60 and the housing 10 together. Fig. Figure 6 is a view of an EDS analysis of the welding area 12 and its periphery. Fig. 7 is a view of a line analysis of the weld area 12 with respect to a Cr content. Fig. 8 is a view for explaining the achievement of excellent corrosion resistance of the glow plug 100 of the present invention. Fig. 9 is a series of views for explaining modifications of the present invention. Fig. 10 is a view for explaining a modified embodiment of the present invention in which the deepest portion P of the welding portion 12 is located in front of the boundary plane VP. Fig. 11 is a view for explaining a modified embodiment of the present invention and showing the areas (Sb, Sf) of welding portions separated by the boundary plane VP. Fig. 12 is a series of views for explaining a modified embodiment of the present invention in which the boundary plane VP is oblique to the axis line AX. Best mode to carry out the invention

[0021] A glow plug of the present invention will next be described with reference to the drawings. In the following description, the vertical direction of the drawings is referred to as the direction of an axis AX; the lower end of the drawings is referred to as a front end; and the upper end of the drawings is referred to as a rear end.

[0022] Fig. 1 is a view, substantially in longitudinal section, showing the overall configuration of a glow plug 100 according to an embodiment of the present invention, accompanied by an enlarged partial view for explanation. The glow plug 100 includes a housing 10, an axial member 20, a ceramic heater 30, a ring member 40, a terminal pin 50, and a heater-supporting sleeve 60.

[0023] The housing 10 is made of carbon steel equivalent to S45C; assumes a cylindrical shape having a recess 10h extending therethrough along the axis AX; and has an external threaded portion 10n formed by rolling or cutting on an axially intermediate portion of its outer peripheral surface, and is mounted on a diesel engine. A rear end portion 10k, located rearward of the external threaded portion 10n, has a tool engagement portion 10e with which a tool, such as a hexagonal wrench or a spark plug wrench, is engaged for mounting on the diesel engine. The inner wall of the inner end portion 10k has a tapered portion 10t for expanding the diameter of the recess 10h toward the rear end, and a large-diameter opening 10d adjacent to the rear end of the tapered portion 10t. As shown in Fig. As shown in Fig. 3, a front end portion 10s is chamfered (a portion of 0.3 mm or more is removed from the corner) so as to remove an edge defined by the inner peripheral surface (recess 10h) and a front end surface 10f, and a radially inner end portion of the front end surface 10f is formed into a chamfer portion (spacing portion) 10m. The chamfer 10m corresponds to a structure in which "the rearward end surface of the heater-holding sleeve and the front end surface of the housing are separated from each other" and forms a so-called "recess." The front end surface 10f includes the chamfered portion.

[0024] The heater holding sleeve 60 is formed of SUS430; assumes a generally cylindrical shape having a flange portion (large cylindrical portion 60d) and a recess 60h extending therethrough; and holds a ceramic heater 30 therein by press-fitting the ceramic heater 30 into the recess 60h.The heater-holding sleeve 60 includes a small cylindrical portion 60s located on one side toward the front end and having a relatively thin wall; a taper 60t adjacent to the rear end of the small cylindrical portion 60s and increasing in diameter toward the rear end; a large cylindrical portion 60d adjacent to the rear end of the taper 60t and having an outer diameter larger than that of the small cylindrical portion 60s and substantially equal to that of the front end portion 10s of the housing 10; and an engaging portion 60k located rearward of the large cylindrical portion 60d via a rearward-facing end surface 61 and having an outer diameter smaller than that of the large cylindrical portion 60d.When the glow plug 100 is mounted on a non-illustrated diesel engine, the taper 60t serves as a seal to ensure gas tightness of an engine chamber.

[0025] The axial member 20 is formed from S430 and has a rod-like shape. A front end portion of the axial member 20 has a small-diameter portion 20s at the front end, which is formed via an engaging end surface 20t so as to be fitted into the ring member 40. A rear end portion 20k has a diameter slightly smaller than that of the other portion to allow attachment of the terminal pin 50.

[0026] The ring member 40 is made of SUS630 and has a cylindrical shape. The inner diameter of the ring member 40 is substantially equal to the outer diameter of the inner end portion 30k of the ceramic heater 30 and the outer diameter of the small-diameter portion 20s at the front end of the axial member 20. The outer diameter of the ring member 40 is substantially equal to the outer diameter of a portion of the axial member 20 located behind the engaging end surface 20t.

[0027] The ceramic heater 30 is configured such that a resistance heating element 32 formed of an electrically conductive ceramic and a pair of conductor members 33 are embedded in a rod-like, electrically insulating ceramic substrate 31. The resistance heating element 32 is disposed within a front end portion of the ceramic heater 30 and is formed of an electrically conductive ceramic in a U-shape. The two conductor members 33 extend rearward from the respective ends of the resistance heating element 32 and are formed of an electrically conductive ceramic. One conductor member 33 has an electrode lead-out portion 34 exposed on the surface of the ceramic substrate 31 in a radial direction perpendicular to the axis AX and can electrically communicate with the ring member 40 at the rear end portion 30k of the heater 30.The other conductor part 33 has an electrode lead-out portion 35 which is arranged in front of the electrode lead-out portion 34 and is similarly exposed in a radial direction and which can electrically communicate with the heater-holding sleeve 60.

[0028] Next, the process of installing elements into the glow plug 100 is described with reference to the Fig. 2 to 5 are described.

[0029] The description of conventionally known structural areas is simplified or omitted.

[0030] The ring member 40 is formed by cutting a SUS630 pipe material, forming the cutting piece into a predetermined cylindrical shape, forming a Ni strike plating layer, and then forming an Au plating layer. Similarly, the heater-holding sleeve 60 is formed by cutting a SUS430 pipe material, forming the cutting piece into a predetermined shape by cutting or the like, and forming a plating layer on its inner peripheral surface in a manner similar to the case of the ring member 40. This plating layer has a thickness of 0.8 µm.

[0031] After the overlay layer is formed, the ceramic heater 30 is inserted into the ring element 40 (by press fitting) to thereby form a subassembly. The ceramic heater 30, which is assembled as a subassembly with the ring element 40, is inserted into the heater-holding sleeve 60 (by press fitting), thereby forming a subassembly 110, as shown in Fig. 2(a). An axial element is press-fitted into the ring element 40 of the sub-assembly 110, and laser welding is applied to form a heater-axial element assembly 120 (see Fig. 2(b)).

[0032] The housing 10 is formed by cutting a pipe material formed by drawing a carbon steel bar of S45C into a predetermined length, and forming the tool engagement portion 10e and the large-diameter recess 10d of the rear end portion 10k using a machining center, a reamer, and the like. Also, the external thread portion 10n is formed using a thread rolling die. A method for forming a general shape is not limited to the above method, but may be as follows: a housing member having an almost completed shape is formed, and subsequently, the housing member is immersed in a Zn-Ni alloy plating bath, thereby forming a Zn-Ni alloy plating layer 11 on its surface. Furthermore, a chromate treatment may be performed on the plating layer 11.After the plating layer 11 is formed, the front end surface 10f of the housing 10 is polished to remove the plating layer 11 from a portion of the front end surface 10f adjacent to the heater-supporting sleeve 60. According to the present embodiment, chamfering is performed at this stage to form the chamfer 10m. To keep the drawings clear, the plating layer 11 is shown only in FIGS. Fig. 4 and Fig. 5. The overlay layer 11 has a thickness of 5 µm. It is conceivable that the overlay layer 11 essentially has a thickness of approximately 20 µm. A film of trivalent chromate can be formed on the overlay layer 11.

[0033] In the case where an overlay layer is formed on a relatively long cylindrical member, it is difficult to uniformly form the overlay layer on the inner cylindrical surface. However, an overlay layer can be formed on the inner surface of an opening portion of the cylindrical member in a manner similar to that of the outer peripheral surface. Accordingly, in the completed glow plug 100, the overlay layer 11 is formed on a portion of the recess 10h of the housing 10 that comes into contact with a rear end portion (engagement portion 60k) of the heater-holding sleeve 60.

[0034] The component 120 consisting of heater and axial element and the housing 10 are connected together (see Fig. 2(c)). In this joining, first, a front end portion of the recess 10h of the housing 10 is fitted to the engagement portion 60k (by press fitting) such that the front end surface 10f of the housing 10 abuts the rearward end surface 61 of the heater-holding sleeve 60, whereby both elements are positioned for joining (see Fig. 3(a) and Fig. 3(b)). Then, a laser beam is radiated radially onto an abutment area between the front end face 10f of the housing 10 and the rearward end face 61 of the heater-holding sleeve 60, thereby laser-welding both elements together (see Fig. 3(b) and Fig. 3(c)).

[0035] In this positioning, the chamfer 10m provided on the front end surface 10f of the housing 10 provides the effect of facilitating press fitting and the effect of ensuring abutment between the front end surface 10f of the housing 10 and the rearward end surface 61 of the heater-holding sleeve 60. These effects will be described in detail with reference to Fig. 4, which describes the process of Fig. 3(a) and Fig. 3(b) shows in enlarged scale.

[0036] As in the Fig. As shown in Figure 4(a), the chamfer 10m functions as a conical inlet for fitting the housing 10 to the engagement portion 60k of the heater-holding sleeve 60, thereby providing the function of facilitating press-fitting. In this way, the elements are positioned by press-fitting prior to welding. At this time, the overlay layer 11 formed on the surface of the housing 10 may peel off (see Fig. 4(b)). Since the chamfer 10m of the front end surface 10f of the housing 10 is separated from the rearward-facing end surface 61 of the heater-holding sleeve 60, a space is formed between the chamfer 10m and the rearward-facing end surface 61. Flaking of a coating is absorbed in the space (see Fig. 4(c)). This can avoid a situation in which the front end surface 10f and the rearward end surface 61 fail to contact each other, as in the case where the chamfer 10m is not formed, as in Fig. 4(d).

[0037] As mentioned above, the housing 10 and the heater holding sleeve 60 are positioned (alignment step) and are then welded together by a laser (melting step) (see Fig. 2(c), Fig. 3(b) and Fig. 3(c)). Laser welding is desirably carried out such that a joint of a suitable size is formed by adjusting a laser output power. As in Fig. 5, laser welding is particularly preferably performed to form a welding portion 12 such that a portion of the interfacial layer remains between the front end face 10f of the housing 10 and the rear end face 61 of the heater-holding sleeve 60. As mentioned previously, deposit exfoliation may possibly be received in the space partially formed by the chamfer 10m. Thus, when laser irradiation is such that the entire interfacial layer is melted, the received deposit exfoliation evaporates, possibly resulting in a gas bubble or sputtering trace being formed in the welding portion 12. A YAG laser is used in this embodiment, but another type of laser, such as a CO2 laser, may be used.

[0038] As described above, the housing 10, the axial member 20, the ceramic heater 30, the heater holding sleeve 60, etc. are assembled. Subsequently, as in Fig. As shown in Fig. 2(d), an O-ring 101 and an insulation sleeve 102 are fitted to the rear end portion 20k of the axial member 20, which protrudes from the rear end portion 10k of the housing 10. Then, the previously formed terminal pin 50 is tightly crimped to the rear end portion 20k, thereby completing the glow plug 100.

[0039] The finished glow plug 100 was sectioned along a plane encompassing the AX axis. On the sectioned surface, the weld area 12 and its periphery were measured for Cr distribution using component analysis (EDS, EPMA, etc.). Fig. Figure 6 shows the results of this measurement. The EDS analysis uses an analytical scanning electron microscope (JSM-6380A, a product of JEOL) and is performed at an accelerating voltage of 20 kV. To investigate the distribution of Cr in the weld area 12, a line analysis was performed on the outer surface of the weld area 12 along the axial direction over a range from the housing 10 to the heater-supporting sleeve 60. Fig. 7 shows the results of this measurement. In Fig. 7, a position located 0.14 mm axially from the origin serves as an analysis starting point, which corresponds to the position of the boundary layer between the front end surface 10f of the housing 10 and the rear end surface 61 of the heater-holding sleeve 60. The Cr content of the weld portion 12 was obtained from the results of the line analysis. As a result, it was confirmed that the weld portion 12 contained Cr in an average amount of 5 wt% or more. The Cr content Wh of the housing 10 was 0 wt%, and the Cr content Wc of the heater-holding sleeve 60 was 16 wt%. Thus, it was confirmed that the relationship Wh < Wm < Wc was satisfied.

[0040] Next, the corrosion resistance of the glow plug 100 of the present invention was examined; particularly, the corrosion resistance of the welded portion 12, which is a connection between the housing 10 and the heater-holding sleeve 60, was examined. A salt water spray test was conducted in accordance with JIS H8502. Even after 900 hours, no red rust was observed, indicating excellent corrosion resistance. According to the results of the line analysis in Fig. 7, a small portion of the weld area 12, which is close to the housing 10, indicates that the Cr content is less than 5 wt%. However, the above-mentioned corrosion resistance verification test shows favorable results; thus, even if red rust develops, it has no fatal effects on the glow plug 100 of the present invention. This phenomenon will be discussed with reference to Fig. 8, which schematically shows on an enlarged scale the intermediate layer between the welding area 12 and the housing 10 and the periphery of the intermediate layer. For clarity, Fig. 8 the welding area 12 is not hatched.

[0041] After mutual positioning, the housing 10 and the heater-holding sleeve 60 are welded together by a laser. The welding region 12 is formed by fusion of two elements, and a portion of the welding region 12 may be formed from the Zn-Ni alloy plating layer 11 formed on the outer peripheral surface of the housing 10 (the region of the laser region 12 is designated by reference numeral 12a and is delimited along a boundary β by the rest of the welding region 12). In the region 12a, the material of the housing 10 is predominantly molten; thus, the Cr content of the region 12a is low. As a result, the region 12a is possibly oxidized. However, the Zn-Ni alloy plating layer 11 is on the radially inner side (on the lower side in Fig. 8) of the area 12a, and the welding area 12, which has a higher Cr content, is on the front side (on the left side in Fig. 8) of the region 12a. Therefore, even if rust starts from the region 12a, rust is not progressive, so that a glow plug with excellent corrosion resistance can be obtained. In other words, the plating layer 11 on the outer peripheral surface of the housing is formed in such a manner as to be located under a rear end portion of the welding portion 12 that joins the heater-holding sleeve 60 and the housing 10 together, thereby further ensuring corrosion resistance.

[0042] In the thus-manufactured glow plug 100, the Zn-containing plating layer 11 is formed on the surface of the housing 10, and the weld portion 12 on which the plating layer 11 is not formed contains Cr in an average amount of 5 wt% or more as a result of supplying Cr from the material of the heater-holding sleeve 60 in association with welding the heater-holding sleeve 60 to the housing 10. Thus, a glow plug with excellent corrosion resistance can be obtained. In this welding, the Zn-containing plating layer does not come between members to be joined together (the housing 10 and the heater-holding sleeve 60). Even if the plating layer comes between them, the intervening plating layer is present away from the weld portion 12. Thus, a glow plug free from the formation of a gas pocket or a sputter mark can be obtained.

[0043] While the present invention has been described with reference to the above embodiment and the verification test, the present invention is not limited to the embodiment, but may be modified as appropriate without departing from the spirit or scope of the invention. For example, in the above embodiment, a part of the front end surface 10f of the housing is chamfered. Such treatment of an edge is not limited to chamfering, but may also include rounding. Also, the angle of chamfering is not limited to 45°. In the above embodiment, chamfering is performed before the overlay layer 11 is formed on the housing 10, and the overlay layer 11 is formed on the housing 10, which includes the chamfer 10m. However, chamfering to form the chamfer 10m may be performed after the overlay layer 11 is formed.This chamfering is performed to form a space for receiving exfoliations of the overlay layer 11. However, this space may be as shown in . Fig. 9(a); specifically, the rearward-facing end surface 61 of the heater-holding sleeve 60 is notched only around the engaging portion 60k. In this case, too, the rearward-facing end surface includes the notched portion.

[0044] This can also be Fig. 9(b) may be employed to increase the Cr content of the welding portion 12 to be formed; specifically, a radially outer end portion of the front end surface 10f of the housing 11 is chamfered; a corresponding portion of the rearward end surface 61 of the heater-holding sleeve 60 is formed to engage with the chamfered portion; and welding is performed with these portions engaged together. A means for increasing the Cr content of the welding portion 12 may be as shown in Fig. 9(c); specifically, a radially outer end portion of the front end surface 10f of the housing 10 and a radially outer end portion of the rearward end surface 61 of the heater holding sleeve are both chamfered, and welding is performed while adding an er-containing metal, thereby forming the welding portion 12.

[0045] By implementing the present invention, corrosion resistance can be ensured for the weld area and its periphery. Conventional anti-rust aluminum plating can be additionally applied. An organic protective layer for improving corrosion resistance can be formed. For example, a colored anode oxide film can be formed to improve the external appearance. In the above embodiment, the deepest portion of the weld area 12 is located around the boundary between the housing 10 and the heater-holding sleeve 60 in a cross section including the axis line AX. As shown in Fig. 10 and Fig. 11, on the other hand, the deepest area P of the welding area 12 is in front of the boundary plane VP between the housing 10 and the heater-holding sleeve 61. Furthermore, when the welding area 12 is separated by the boundary plane VP into a housing-side part and a sleeve-side part, an area Sf of the sleeve-side part can be set larger than an area Sb of the housing-side part. In addition, the front end surface 10 and the rear-facing end surface 61 need not be vertical to the axis line AX, but may be oblique thereto, as shown in FIGS. Fig. 12(a) and Fig. 12(b) illustrates.

[0046] The above embodiment was described while mentioning a ceramic glow plug. However, the present invention can be applied to a metal glow plug. In the case of a metal glow plug, for example, a heater-supporting sleeve and a heater tube can be integrally formed into a single element.

[0047] In summary, a glow plug is provided which has a housing with a Zn-containing plating layer formed on its surface, and in which the housing and a heater-holding sleeve for holding a heater are advantageously joined together, while a connection between the housing and the heater-holding sleeve and the periphery of the connection are improved in corrosion resistance. A housing having a Zn-containing plating layer on its outer peripheral surface is provided, and a front end surface of the housing and a rear end surface of a heater-holding sleeve are in direct contact with each other. The housing and the heater-holding sleeve are joined together by welding, whereby a radially outer end portion of a portion associated with this contact is melted. Description of reference symbols 10 housings 10f whose front end face 11 Overlay layer 12 Welding area 20 axial element 30 ceramic heater 40 ring element 50 pin connector 60 Heating holding sleeve 61 whose rearward facing end face 100 glow plugs VP border

Claims

[1] Glow plug, comprising: a heater (30) extending in an axial direction; a heater-holding sleeve (60) which holds the heater (30) from its radially outer side and which has a rearward-facing end surface (61); a cylindrical housing (10) having a front end surface (10f) and which is connected at their common boundary (VP) to a rear end of the heater-holding sleeve (60) via an annular welding portion (12) formed extending along the boundary (VP) between the front end face (10f) of the housing (10) and the rearward-facing end face (61) of the sleeve, and a corrosion resistance-implementing metal coating (11), which is formed on an outer peripheral surface of the housing (10) and formed adjacent to the welding area (12), wherein radially inward of the welding region (12) the front end face (10f) is in direct contact with the rearward facing end face (61). [2] Glow plug according to claim 1, wherein the overlay layer (11) comprises a Zn component. [3] Glow plug according to claim 1 or 2, wherein the coating layer (11) has a thickness of 5 µm or more. [4] Glow plug according to one of claims 1 to 3, wherein the heater holding sleeve (60) has an engagement portion (60k) which is arranged radially inward of the rearward facing end surface (61) and which projects rearward from the rearward facing end surface (61), the engagement portion (60k) being firmly engaged with an inner peripheral surface of the housing (10), and the housing (10) includes a shoulder portion (10m) which is arranged radially inward of the front end surface (10f) of the housing (10), the shoulder portion (10m) being spaced from the heater holding sleeve (60). [5] Glow plug according to one of claims 1 to 4, wherein the heater holding sleeve (60) is made of a material which has a higher corrosion resistance than that of the housing (10). [6] The glow plug according to any one of claims 1 to 5, wherein the heater holding sleeve (60) is formed of an Fe-Cr alloy; a relationship of Wh < Wm < Wc is satisfied, where Wh is an average Cr content of the housing (10), Wc is a Cr content of the heater holding sleeve (60), and Wm is a Cr content of the weld portion (12); and Wm is 5 wt% or greater. [7] Glow plug according to claim 5 or 6, wherein when the welding portion (12) is viewed in a cross section including an axis of the heater, a relationship Sb < Sf is satisfied, where Sb is an area of ​​a shell-side part of the welding portion (12) which is located behind a boundary (VP), and Sf is an area of ​​a sleeve-side part of the welding portion (12) which is located in front of the boundary (VP). [8] A method of manufacturing a glow plug comprising: a heater (30) extending in an axial direction; a heater-holding sleeve (60) having a rearward-facing end face (61), the heater (30) being held in the sleeve (60); a cylindrical housing (10) having a front end face (10f) and connected to a rear end (61) of the heater-holding sleeve (60) via a welding portion (12) formed extending along the boundary (VP) between the front end face (10f) of the housing (10) and the rearward-facing end face (61) of the sleeve (60); and a corrosion resistance-implementing metal coating (11) formed on an outer peripheral surface of the housing (10) and adjacent to the welding portion (12), the method comprising: Forming the corrosion resistance-implementing metal coating (11) on the outer peripheral surface of the housing (10) but not on the front end surface (10f) of the housing (10); Aligning the housing (10) having the anti-corrosive coating layer (11) with the heater-holding sleeve (60) by abutting the front end surface (10f) on which the anti-corrosive coating layer (11) is not formed to the rearward-facing end surface (61), and Melting a radially outer part of the housing (10) and a radially outer part of the heater-holding sleeve (60) by laser welding so as to form the welding region (12) adjacent to the anti-corrosive coating layer (11) formed on the outer peripheral surface of the housing (10). [9] A method of manufacturing a glow plug according to claim 8, wherein a front end of the corrosion-resistant plating layer (11) is melted by laser welding. [10] A method for manufacturing a glow plug according to claim 8 or 9, wherein a laser beam is irradiated in the melting step so that an innermost portion of the welding portion (12) is located in front of the boundary (VP).

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