Abstract:
A method for producing a rigid magnetic circuit component for an electromagnetically operable valve includes: a) providing a base element made of a magnetic or a magnetizable material, b) complete first heat treatment of the base element, c) a local second heat treatment of the base element so as to form a subregion having a microstructure of martensite and residual austenite in the otherwise martensitic base element, and d) installing the finished processed base element as the magnetic circuit component in a magnetic circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing a rigid magnetic circuit component for an electromagnetically operable value. 
     2. Description of Related Art 
       FIG. 1  shows a known fuel injector from the related art, which has a classical three-part construction of an inner metallic flow guidance part and a housing component at the same time. This inner valve pipe is formed by an intake nipple forming an inner pole, a nonmetallic intermediate part and a valve-seat support accommodating a valve seat, and is described in more detail in the associated description of  FIG. 1 . 
     A method is known from published German patent document DE 35 02 287 for producing a hollow cylindrical metallic housing having two magnetizable housing parts and a magnetic housing zone lying between them and separating the housing parts magnetically. This metallic housing is pre-worked, in this context, from a magnetizable blank in one piece, right down to an oversize in the outer diameter, an annular groove being cut into the inner wall of the housing to a width of the desired middle housing zone. With the housing rotating, a nonmagnetizable filler material is filled into the annular groove, while the annular groove region is heated, and the rotation of the housing is kept going until the filler material solidifies. The housing is subsequently over-rotated on the outside up to the end measure of the outer diameter, so that there is no longer any connection between the magnetizable housing parts. A valve housing produced in this manner may be used, for instance, in magnetic valves for antilock systems (ABS) of motor vehicles. 
     Methods are also known from published German patent document DE 42 37 405 for producing a rigid core for injection valves for internal combustion engines (see  FIG. 5  of the cited German patent document). The methods are distinguished in that they provide a one-piece, sleeve-shaped, magnetic martensitic workpiece, directly or via prior conversion processes, which experiences a local heat treatment in a middle section of the magnetic, martensitic workpiece for converting this middle section into a nonmagnetic, austenitic middle section. Alternatively, during the local heat treatment, using a laser, elements forming molten austenite or molten ferrite are added to the location of the heat treatment to form a nonmagnetic, austenitic middle section of the rigid core. 
     BRIEF SUMMARY OF THE INVENTION 
     The method, according to the present invention, for producing a rigid magnetic circuit component having the characterizing features of the main claim, has the advantage that, in a particularly simple and cost-effective method, housings are reliably producible that have magnetic separation and magnetic circuit components having locally adjusted magnetic properties especially in edge layers, using mass-production techniques. 
     In particular, because of the simplicity of the individual components, only a reduced expenditure on special tools is required, compared to known production methods. 
     It is also of advantage that great flexibility is made possible in the development of the geometry of the magnetic circuit component itself, such as length, outside diameter and gradations. 
     It is of special advantage that one is able to do without coating methods, such as carbonitriding, which are usually required to generate edge layers that are modified in their magnetic properties. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a fuel injector according to the related art, having a three-part inner metallic valve pipe as housing. 
         FIGS. 2 to 7  show schematic method steps of a method according to the present invention, for producing a rigid magnetic circuit component in the form of a pipe-shaped housing. 
         FIG. 8  shows a schematic cutout from an injector valve having a housing produced according to the present invention. 
         FIGS. 9 to 13  show schematic method steps of the method according to the present invention, for producing a rigid magnetic circuit component in the form of an armature bolt. 
         FIG. 14  shows a schematic cutout from a magnetic circuit in a plunger-type execution, having an armature bolt produced according to the present invention. 
         FIG. 15  shows a schematic cutout from a magnetic circuit in a flat-type armature execution, having a tie plate produced according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing the method steps according to the present invention, for producing a rigid magnetic circuit component, with the aid of  FIGS. 2 to 15 , we shall explain in greater detail a fuel injector of the related art, with the aid of  FIG. 1 , as a possible product for the insertion of a magnetic circuit component produced according to the present invention. 
     The valve that is operable electromagnetically, shown in exemplary fashion in  FIG. 1  in the form of an injector for fuel injection systems of mixture-compressing, externally ignited internal combustion engines, has a core  2 , surrounded by a magnetic coil  1 , used as fuel intake neck and inner pole, which has, for example, a constant outer diameter over its entire length. A coil shell  3  graded in the radial direction accommodates a winding of magnetic coil  1  and, in conjunction with core  2 , enables the fuel injector to have a compact design in the region of magnetic coil  1 . 
     A tubular, metal, nonmagnetic intermediate part  12  is connected to a lower core end  9  of core  2  by welding, concentrically to a longitudinal valve axis  10 , and partially surrounds core end  9  in an axial manner. A tubular valve-seat support  16 , which is rigidly connected to intermediate part  12 , extends downstream from coil shell  3  and intermediate part  12 . An axially movable valve needle  18  is situated in valve seat support  16 . At downstream end  23  of valve needle  18  a spherical valve closure member  24  is provided, at whose circumference, for example, five flattened regions  25  are provided for the fuel to flow past. 
     The fuel injector is actuated electromagnetically, in the known manner. For the axial displacement of valve needle  18 , and thus for the opening counter to the spring force of a restoring spring  26 , or for the closing of the fuel injector, the electromagnetic circuit having magnetic coil  1 , core  2  and an armature  27  is utilized. Pipe-shaped armature  27  is rigidly connected to the end of valve needle  18  facing away from valve-closure member  24 , by a welded seam, and is aligned with core  2 . A cylindrical valve-seat member  29  having a fixed valve seat  30  is mounted in the downstream end of valve-seat support  16  facing away from core  2 , using welding, so as to form a seal. 
     Spherical valve-closure member  24  of valve needle  18  interacts with the valve seat  30  of valve-seat member  29 , which is frustoconically tapered in the direction of flow. At its lower end face, valve seat member  29  is connected to a pot-shaped spray orifice disk  34 , for example, rigidly and sealingly by a welded seam that is developed, for example, using a laser. In spray orifice disk  34 , at least one, but, for example, four, spray-discharge orifices  39  are provided that are formed by eroding or stamping, for example. 
     In order to conduct the magnetic flux for the optimal activation of armature  27 , when magnetic coil  1  is supplied with current, and with that to the secure and accurate opening and closing of the valve, magnetic coil  1  is surrounded by at least one conductive element  45 , developed, for instance, as a bracket and used as a ferromagnetic element, which surrounds magnetic coil  1  at least partially in the circumferential direction, and which lies with its one end against core  2  and with its other end against valve seat support  16 , and is able to be connected to the latter, for instance, by welding, soldering or adhesion. Core  2 , nonmagnetic intermediate part  12  and valve seat support  16  form an inner metallic valve pipe as skeleton and, with that also the housing of the fuel injector, and they are firmly connected to one another and altogether extend over the entire length of the fuel injector. All additional functional groups of the valve are ordered within or round about the valve pipe. This arrangement of the valve pipe involves the classical three-part design of a housing for an electromagnetically operable aggregate, such as a valve, having two ferromagnetic or magnetizable housing regions which, for the effective conduction of the magnetic circuit lines of force in the region of armature  27 , are magnetically separated from each other or at least connected to each other via a magnetic throttling point, using a nonmagnetic intermediate part  12 . 
     The fuel injector is largely surrounded by a plastic extrusion coat  51 , which extends in the axial direction from core  2 , over magnetic coil  1  and the at least one conductive element  45 , to valve-seat support  16 , the at least one conductive element  45  being completely covered in the axial and circumferential directions. Part of this plastic extrusion coating  51  is a likewise extruded electrical connection plug  52 , for instance. 
     Using the method steps of the method according to the present invention that are schematically indicated in  FIGS. 2 to 7 , for producing a rigid magnetic circuit component, it is advantageously possible to produce, especially simply and cost-effectively, thin-walled housings  66  for various utilization purposes, among other things, preferably electromagnetically operable valves which are able to replace a three-part valve pipe described above. 
     In a first method step ( FIG. 2 ) a base element  55 , that is cylindrical, for example, is provided from which housing  66  is to be manufactured, and which is made of a magnetic or magnetizable material and is ferromagnetic or ferritic, for example, or has a martensitic microstructure. Base element  55  may be solidly developed, for the moment, and may be made from long rod material, for example, for an especially effective production of a plurality of housings  66 . The material of base element  55  is steel in each case, which forms residual austenite and martensite based on its alloy composition. The alloying elements in the material are the elements C, N, Ni and Mn, which stabilize austenite. 
     In order to achieve the different desired magnetic properties of the magnetic circuit component, base element  55  is submitted completely to a heat treatment, which is able to be performed, for instance, using hardening, deep cooling in deep-cooling refrigerators and/or by one-time or multiple reheating in ovens  56  ( FIG. 3 ). After hardening, the microstructure may still also be made up of residual austenite proportions which are transformed into martensite by the subsequent heat treatment steps. Alternatively to this, the microstructure may also be made up of ferrite, having intercalated particles such as carbides, nitrides or intermetallic compounds. The heat treatment takes place in such a way that a completely magnetic martensitic material microstructure forms in base element  55  ( FIG. 4 ). 
     An additional heat treatment is subsequently undertaken which, however, is only carried out in a locally limited fashion. A subregion of base element  55  is exposed, for this purpose, to short-term heat treatment using laser heating or induction heating  57  ( FIG. 5 ). In this way, the material of base element  55  is locally austenitized and homogenized in the subregion of the second heat treatment and, after cooling of base element  55  or self-quenching by the surrounding material, it is made up of martensitic regions  58  and subregion  59  having martensite and residual austenite ( FIG. 6 ). Base element  55  is now made up of zones having various microstructures and magnetic properties. 
     Base element  55  is then finally treated in such a way that there exists a rigid housing  66  as magnetic circuit component in a desired geometry. In the case of the use of a housing  66  produced according to the present invention, in a fuel injector, it may be advantageous specifically to form housing  66  into shape by measures of production technology, such as ironing, tumbling, round-kneading, flanging and/or flaring. Housing  66  then represents a component that is able completely to take over the sum of the functions of the valve pipe, consisting of core  2 , intermediate part  12  and valve seat support  16  in a known fuel injector according to  FIG. 1 , and consequently it extends, for example, over the entire axial length of a fuel injector. 
     Solid base element  55  is brought, for example, to form a pipe-shaped sleeve form, by production technology measures. Solid base element  55  may be provided, in this context, with an inner longitudinal opening  60  to form pipe-shaped housing  66  ( FIG. 7 ), either before or only after the local heat treatment. 
       FIG. 8  shows a schematic cutout of a fuel injector having a housing  66  produced according to the present invention, which is installed in the valve as a thin-walled sleeve, and thus surrounds core  2  and armature  27  radially and in the circumferential direction, and is itself, in this context, surrounded by magnetic coil  1 . It becomes clear that subregion  59  of housing  66 , that has been changed in its magnetic properties and is martensitic and residually austenitic, lies in the axial extension region of a working air gap  70  between core  2  and armature  27 , in order to conduct the magnetic circuit lines of force optimally and effectively in the magnetic circuit. Instead of bracket-shaped conducting element  45  shown in  FIG. 1 , the outer magnetic circuit component is executed, for instance, as a magnetic pot  46 , the magnetic circuit being closed between magnetic pot  46  and housing  66  via a cover element  47 . The method according to the present invention also makes it possible locally to change housing  66  in its magnetic properties, using greater wall thicknesses, so that a higher internal pressure stability is ensured in favor of the magnetic force, in spite of the minimized magnetically active region. 
       FIGS. 9 to 13  show schematic method steps of the method according to the present invention, for producing a rigid magnetic circuit component in the form of an armature bolt  66 ′. The production of armature bolt  66 ′ takes place in a comparable manner to the previously described production of housing  66  according to  FIG. 7 . In a first method step ( FIG. 9 ), a thin cylindrical base element  55 ′ is provided, for instance, from which armature bolt  66 ′ is to be produced, and which is made of a magnetic or a magnetizable material, and is ferromagnetic or ferritic, for example, or which has a martensitic material microstructure. Base element  55 ′ may, for instance, be made of long rod material for an especially effective production of many armature bolts  66 ′. The material of base element  55 ′ is a steel in each case, which forms residual austenite and martensite based on its alloy composition. The alloying elements in the material are the elements C, N, Ni and Mn, which stabilize austenite. 
     In order to achieve the different desired magnetic properties of the magnetic circuit component, base element  55 ′ is submitted completely to a heat treatment, which is able to be performed, for instance, using hardening, deep cooling in deep-cooling refrigerators and/or by one-time or multiple reheating in ovens  56  ( FIG. 10 ). After hardening, the microstructure may still also be made up of residual austenite proportions, which are transformed into martensite by the subsequent heat treatment steps. Alternatively to this, the microstructure may also be made up of ferrite, having intercalated particles such as carbides, nitrides or intermetallic compounds. The heat treatment takes place in such a way that a completely magnetic martensitic material microstructure forms in base element  55 ′ ( FIG. 11 ). 
     Thereafter, additional heat treatment is performed, which is supposed to lead to a change in the magnetic properties, exclusively at the surface in the edge regions of base element  55 ′. A surface of base element  55 ′ is exposed, for this purpose, to short-term heat treatment using laser heating or induction heating  57  ( FIG. 12 ). In this way, the material of base element  55 ′ is locally austenitized and homogenized at the surface and, after the cooling of base element  55 ′ or self-quenching by the surrounding material, it is made up of an inner martensitic regions  58 ′ and an outer edge region  59 ′ having martensite and residual austenite ( FIG. 13 ). Base element  55 ′ or armature bolt  66 ′ is now made up of zones having various microstructures and magnetic properties. 
     If necessary, base element  55 ′ is then finally treated in such a way that there exists a rigid armature bolt  66 ′ as magnetic circuit component, in a desired geometry.  FIG. 14  shows a schematic cutout of a magnetic circuit in plunger-type execution, having an armature bolt  66 ′ according to the present invention, which plunges through a magnetic pot  46  and is displaceable there in a movable manner. In the case of plunger-type magnetic circuits, the dynamics and the magnetic force of the magnetic valve are able to be improved, using an armature bolt  66 ′, in which outer edge region  59 ′ has residual austenite proportions. Coating methods, such as carbonitriding, may be omitted. 
       FIG. 15 , a schematic cutout from a magnetic circuit in a flat-type armature execution is shown, having a tie plate  66 ″, produced according to the present invention. The production principle is again comparable to the previously described method steps for producing housing  66  or armature bolt  66 ′. The local second heat treatment takes place in such a way that a short-term heat treatment is performed, using laser heating or induction heating, at one side of the flat, plate-shaped base element. In this way, the material of the base element is locally austenitized and homogenized on this side and, after the cooling of the base element or the self-quenching by the surrounding material, it is made up of a martensitic region  58 ″ and an edge region  59 ″ facing magnetic coil  1 , having martensite and residual austenite. Tie plate  66 ″ is now made up of zones having various microstructures and magnetic properties. 
     Using such a tie plate  66 ″, an additional air gap is able to be generated in flat-type armature magnetic circuits. This additional air gap in edge region  59 ″ may be used so as to prevent the adhesion of tie plate  66 ″ to magnet pot  46 , so as to set a specified residual air gap in the magnetic circuit or so as to have it used as an air gap having wear protection. 
     The present invention is by no means restricted to use in fuel injectors or magnetic valves for antilock systems, but relates to all electromagnetically operable valves in different fields g 1  of application, and generally to all rigid housings in assemblies in which the zones of different magnetism are required successively.