GB1593373A

GB1593373A - Heat treatment of workpieces by induction heating

Info

Publication number: GB1593373A
Application number: GB29042/76A
Authority: GB
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1977-10-13
Filing date: 1977-10-13
Publication date: 1981-07-15

Description

(54) IMPROVEMENT IN OR RELATING TO HEAT TREATMENT OF WORKPIECES BY INDUCTION HEATING (71) We, THE ELECTRICITY COUNCIL, a British Body Corporate, of 30 Millbank, London, SW1P 4RD, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to heat treatment of workpieces by induction heating.

It is known to heat treat workpieces by passing them in abutting relationship through the treatment region of an elongate induction heating apparatus or furnace where the workpieces become heated by means of the eddy currents induced in them.

However, induced eddy currents themselves act to modify the original flux distribution in the treatment region of the apparatus, and the effect of the abutting workpieces within this treatment region is to modify the original flux distribution to such an extent that most of the induced currents are confined to the outer regions of the workpieces, thus giving rise to a very non-uniform heating pattern within the workpieces.

According to the present invention there is provided a method of heat treating a rotationally symmetrical workpiece by induction heating comprising mounting the workpiece on a refractory transport means and passing the transport means longitudinally through an elongate treatment region of an induction heating apparatus with the axis of symmetry of the workpiece parallel to the magnetic flux of the heating apparatus, the workpiece being accompanied on at least part of its passage through the treatment region by at least one flux modifying member which acts to modify the magnetic flux distribution by the effects of the eddy currents induced in that member and/or its magnetic permeability so that the resultant distribution across the workpiece transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece. As will be further explained later, this distribution of the axial flux leads to uniform heating of a rotationally symmetrical article.

The coil system may be axially symmetrical.

In some cases however it is preferred to use a long coil along which the workpiece is moved. The required flux distribution is provided across the workpiece. Thermal diffusion takes place to assist uniformity of temperature and, as described later, the workpiece may be rotated about its axis or moved to and fro with respect to the coil.

The invention furthermore includes within its scope a method of heat treating rotationally symmetrical workpieces by induction heating comprising mounting the workpieces spaced apart along a refractory (preferably ceramic) transport means and passing the transport means longitudinally through an elongate treatment region of an induction heating apparatus with the axis of symmetry of the workpieces parallel to the magnetic flux of the heating apparatus, there being a multiplicity of workpieces in the treatment region at any one time, each workpiece being accompanied on at least part of its passage through the treatment region by at least one flux modifying member which acts to modify the magnetic flux distribution by the effects of the eddy currents induced in that member and/or its magnetic permeability so that the resultant distribution across the workpiece transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece. By this method each workpiece is heat-treated without substantial interference from its neighbouring workpieces. In other words, as compared to passing the workpieces through the treatment region with the work pieces in abutting relationship, there is a reduction in the amount of heat transfer from neighbouring workpieces and a reduction in the disturbance of the flux through the workpiece, this flux disturbance being due to the eddy currents in neighbouring workpicces.

The present invention also provides an induction heating apparatus for the heat treatment of a rotationally symmetrical workpiece passed through an elongate treatment region thereof, comprising a refractory transport means movable longitudinally through the treatment region, said transport means being arranged to carry the workpiece with its axis of symmetry in a predetermined direction with respect to the transport means, means for producing an alternating magnetic flux in said treatment region with the flux parallel to the axis of symmetry of the workpiece, flux modifying members arranged to accompany the workpiece in predetermined positions with respect thereto on at least part of its passage through the treatment region, which modifying members act to modify the flux in the treatment region by the effects of the eddy currents induced in said members and/or their magnetic permeability so that the resultant distribution across the workpiece transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece.

A plurality of workpieces may be passed sequentially through an elongate treatment region of the apparatus.

The choice of the spacing of the workpieces will depend upon the degree of improvement required over the abovementioned known method and the requirement for a compact furnace which gives better atmosphere control, less heat loss and uses less floor space than a larger furnace, i.e. one requirement will determine a minimum spacing and the other a maximum spacing.

The workpieces may be spaced apart by the use of spacers which are nonconductive. Alternatively, the workpieces can be moved by a transport means arranged to carry the workpieces at predetermined spaced positions. One form of such a transport means comprises a multiplicity of non-conductive elements arranged to be fitted together in a line, and formed so as to provide respective cavities for receiving the workpieces. Such an arrangement is particularly useful in connection with vertically disposed treatment regions since the elements fit one on top of another as a tower and thus possess an inherent stability. A further advantage of separating workpieces in such vertical treatment regions is that the work pieces at the bottom of the pile would otherwise have to support the weight of those above them and the compressive force between adjacent workpieces can result in the workpieces becoming welded together.

For this reason it is known to feed cold workpieces in at the bottom of the treatment region so that as they move upwards and get hotter this force is progressively reduced. Such an arrangement places undesirable constraints on the flow of a controlled atmosphere through the treatment region however and the use of the elements obviates both problems simultaneously.

When the heating coil of the apparatus is of the flat-coil type and is elongate, the treatment region being that region between the spaced elongate portions of the coil, an alternative form of transport means may be used comprising a movable body of nonconductive material including mounting means arranged to mount workpieces at predetermined spaced apart positions. For workpieces which have a cavity or a through passage, the mounting means may be in the form of respective posts which may be fixed to the body or located within respective recesses in the body. The movable body may be in the form of an endless loop of material with its working run arranged such that the workpieces pass longitudinally through the treatment region. Another form of the movable body is a rotatable disc with the mounting means arranged circumferentially at or adjacent the periphery of the disc. In this latter case the heating coil is arcuate and provides an arcuate treatment region. The mounting means associated with the disc may also be in the form of respective posts.

Preferably the mounting posts are arranged to extend at right angles to the longitudinal direction of the treatment region thereby permitting rotation of the workpieces to obtain a more uniform heating of the workpieces. This rotation may be effected simply by contact between the workpieces and a stationary surface, i.e. the workpieces roll on the stationary surface, or the stationary surface may be replaced by a movable surface where a greater rotational rate is required.

In another alternative the mounting posts may be rotated about their longitudinal axes and cause rotation of the workpieces by means of frictional forces therebetween. For rotating the mounting posts, a rack and pinion arrangement may be used, the pinion being arranged at one end of the mounting posts.

Preferably, the workpieces are passed through the treatment region in such a manner that neither the workpieces nor the spacers (or the transport means) come into contact with a refractory lining of the induction heating apparatus. Such linings are used to reduce the heat loss from the workpieces, thereby improving the efficiency of the heat treatment and protecting the heating coil from radiation from the hot workpieces. By ensuring that there is no mechanical contact with the lining, it can be made thinner and/or of less strong or durable material and thus more cheaply as compared with known apparatuses wherein the lining has to be able to withstand knocks and scrapes from hot workpieces. In an apparatus having a transport means, the workpieces are constrained to travel at their respective predetermined positions relative to the transport means, and the abovementioned avoidance of contact with the refractory lining can be achieved by the use of guide means arranged to limit movement of the transport means in a transverse direction as it passes through the treatment region.

To improve the efficiency and uniformity of the heating of the workpieces, each workpiece is accompanied on at least a part of its passage through the treatment region by at least one flux modifying member which acts to modify the magnetic flux distribution by the effects of the eddy currents induced in that member and/or its magnetic permeability. The positioning of such a member will depend upon the particular workpiece to be heat treated, for instance where the workpiece has a bore a member may be disposed within the bore.

Such member can be arranged extending between adjacent workpieces, longitudinally of the treatment region.

For the purposes of the present invention, a flux modifying member can be made of a magnetic material having high resistivity, such as the material forming iron dust cores in I.F. transformers. It can also be made of an electrically conductive material, in which case it is not necessary for it to have any significant magnetic permeability, the flux modifying effect being obtained by the combined effects of eddying currents induced in the material, and of its magnetic permeability, if of significant value. It will thus be appreciated that a suitable material will possess significant magnetic permeability and/or electrical conductivity.

In the case where the mounting means of an above-mentioned transport means is in the form of posts, these posts can be made of magnetic material and thereby constitute flux modifying members for modifying the magnetic flux distribution.

It may be necessary to position thermally insulating material between the workpieces and magnetic flux modifying members to reduce or prevent heating of the magnetic material by the workpieces to a temperature above its Curie temperature.

The flux modifying members may be cooled by passing a coolant through and/or over the members.

A pressed powder technique may be used to form magnetic flux modifying members from say, iron dust, preferably mixed with magnetite to reduce the conductivity of the material. The relative permeability of such a pressed powder member is generally in the range 20-30. Alternatively, the flux modifying member can be made by the steps of forming a bundle of strands of a high permeability/low conductivity material and encapsulating the bundle i.e. filling the interstices with a substance to hold the strands in position. Preferably a nonconductive encapsulant is used, for example a ceramic cement. The material of the strands can be that known by the trade name "Kanthal", or that known by the trade name "Aluchrome", or a similar material, particularly the ferritic stainless steels commonly used in the manufacture of heating elements.

The bundle of strands can be formed into a desired shape prior to their encapsulation, or the final article can be cut or machined from a preformed block of the material.

Although reference has been made to rotating workpieces mounted on the mounting posts, more generally the invention preferably includes rotating the workpieces about respective axes at right angles to the plane of the heating coil during their passage through the treatment region. This technique of rotating the workpieces improves the heating pattern in the workpieces and is most conveniently used where the workpieces have a central bore and are disposed around respective mounting posts on a transport means, the workpieces thus being rotated generally about the axis of the bore.

The rotation of the workpieces can be continuous or it can be intermittent. A workpiece can be mounted on a rotatable base forming a mounting means and itself having projections by means of which it can be rotated. The abutments can be in the form of pins, and can be stationary or moving relative to the induction heating coil. In an alternative, gripping and turning means may be provided for producing the required rotation. Such means may be arranged to rotate all the workpieces as they pass through the treatment region or may be arranged to rotate a selected one or more of the workpieces.

An apparatus of this invention preferably includes means for controllably altering the spacing of the elongate coil portions. Such means can be powered in any suitable manner, for example hydraulically, pneumatically, or by an electric motor, and if desired the means may include a servo loop.

Where it is desired to heat treat work pieces of differing shapes and/or sizes in a single treatment region, or where the workpieces are of non-uniform shape, to reduce temperature diffcrentials in the workpieces there may be included the step of altering the position of the workpiece at right angles to the plane of the flat coil. As an example.

a workpicce with a thick end and a thin end may travel a major part of the treatment region with its thick end between the coils, and then be moved or displaced so that the thin end is brought between the coils. It will be appreciated that the amount and manner of the displacemcnt will be selected in accordance with the particular workpiece to be heat treated.

In the following description, reference will be made to the accompanying drawings in which: Figure I is a schematic sectional diagram showing spaced workpieces travelling vertically through a solenoid induction heating coil; Figures 2A and 2B show schematically the incorporation of magnetic cores to modify the flux distribution within the heating coil; Figure 3 shows spaced workpieces travelling horizontally through the treatment region of an elongate flat heating coil; Figure 4 shows schematically the heating coil of Figure 3 disposed vertically; Figure 5 shows schematically the heating coil of Figure 3 inclined; Figure 6 is a sectional view of a workpiece mounted on a transport means; Figure 7 is a sectional view of a workpiece mounted on an alternative transport means.

Figure 8 shows schematically a particular shaping of magnetic cores to obtain the desired flux distribution in a flat workpiece in accordance with the present invention Figitres 9 and JO are transverse sectional views through a magnetic core and a refractory sleeve showing respective arrange mcnts of ducts for the flow of coolant; and Figures 11 and 12 are schematic plan views of means for turning the workpicces.

In Figure 1 there is shown schematicallv a vertically disposed solenoid heating coil 10 of an induction heating apparatus, the remainder of which apparatus is not shown. A column of spacing elements ii, each of which carrics a workpiece 12 within a cavity 13 thereof, travels longitudinally within the elongate treatment region 14 inside an optional refractory lining 15 adjacent the heating coil 10. The spacing elements 11 have apertures (not shown) to permit access to the cavities 13 of a controlled atmosphere in the induction heating apparatus.

The spacing elements are formed of a non-conductive material, for example a ceramic material, in order to avoid direct eddy current heating of the elements which otherwise would heat the workpiece by radiation and conduction and could cause an undesirable distribution of heat in the workpiece in addition to reducing the overall efficiency of heating.

The workpieces 12 are spaced apart in the longitudinal direction of the heating coil 10 by approximately twice their greatest dimension, and under these conditions there is a worthwhile improvement in flux distribution at a workpiece compared with when the workpieces 12 are stacked one on top of another. The greater the spacing of the workpieces 12, the less the effect a workpiece has on its neighbouring workpieces, but the lower is the efficiency with poorer utilization of the inducing magnetic field.

The upper and lower portions of the spacing elements 11 have complementary formations, the upper portion having a rim and the lower portion having a peripheral recess for receiving such a rim, thus enabling the spacing elements to be securely fitted together. The column thus formed is very stable and is readily driven through the treatment region without coming into contact with the refractory lining 15.

It will be noticed that each workpiece 12 rests on the bottom of a respective recess 13 but that there is a clearance between the top of the workpiece and the bottom of the immediately above spacing element 11.

There is thus no compressive force on the workpieces. For a larger workpiece the recess 13 can be made deeper and/or a similar recess can be provided in the lower surface of the spacing element.

In the present invention, to improve the efficiency and uniformity of the heat treatment, flux modifying members are used to modify the flux distribution in a desired manner to be described more particularly with reference to Figure 8. Figures 2A and 2B show examples of flux modifying members.

In Figure 2A, workpieces 12-1 in the shape of a cylinder, whose axial length is of the same order as its radius, are carried in the cavities 13 of spacing elements 11-1.

Each magnetic core 16 is mounted in a ceramic spacer 17 to avoid direct contact between the core and the workpiece. Also mounted in the spacer 17 are spaced concentric hollow cylinders 16-1, and 16-2 which act as cores. The axial length of the magnetic cores 16 is selected such that the top of the magnetic core 16 is just below the underside of the adjacent spacing element.

The magnetic cores 16 are arranged approximately symmetrical with respect to the median transverse planes of the workpieces.

Figure 2B shows an arrangement corresponding to that of Figure 2A but for use with workpieces 12-2 having a bore 20. In this case the magnetic cores 16 extend longitudinally through the bores 20 with their lower ends located in respective recesses 21 in the spacing elements 11-2. The cores are protected from direct contact with the workpieees by a ceramic sleeve 22. The dimensions of the spacing elements and of the cores are selected such that the cores are positioned symmetrically in relation to the median planes of the workpieces, and preferably such that there is effectively a continuous magnetic core extending through the treatment region.

Figure 3 shows an alternative form of heating coil 25 which is wound as a flat coil having two long spaced parallel portions 26 and two short end portions 27 (see Figures 4 and 5). Such a coil is known as a hairpin coil. The region between coil portions 26 is the treatment region 13 through which workpieces 12-3 are moved. These workpieces 12-3 have a through bore 20 and are mounted on respective posts 28 spaced along and fixed to an endless transport mechanism 29.

Again, to improve the efficiency and uniformity of the heat treatment, the flux distribution is modified by forming the posts 28 of a magnetic material. In which case the posts constitute magnetic cores and are preferably thermally isolated from the workpieces as discussed later. It will be appreciated that the posts 28 prevent the workpieces from wandering into contact with the refractory lining (not shown) for the coil portions 26.

The flat coil 25 is not limited to a horizontal configuration and can be arranged vertically as shown in Figure 4, or sloping as in Figure 5. In Figures 4 and 5 the end portions 27 are shown displaced from the plane of the long coil portions 26 so as to provide open ends for the treatment region and facilitate the entry and exit of the workpieces. It will be observed that the axes of those posts 28 in the treatment region are at right angles to the plane of coil portions 26 and to the longitudinal direction of the treatment chamber. Thus if required the workpieces 12-3 can be rotated about the posts 28 to render more uniform the heating of the workpieces. A simple method of rotating the workpieces 12-3 is to press the workpieees against a stationary surface having a high coefficient of friction relative to the material of the workpieces whereby the workpieces roll along this stationary surface. The upper surface of ramp 30 shown in Figure 5 can constitute this stationary surface. If a greater rate of rotation is required this surface can be moved in the opposite direction to the movement of the transport mechanism 29. Alternatively the post can be rotated by contact with ramp 30, the workpiece being rotated by frictional engagement with the post.

Normally the posts 28 (and the cores 16) will be formed of a material of high permeability and low conductance in order to obtain modification of the flux distribution without this material itself becoming heated by eddy currents induced therein. If a material is used for the posts 28 which has substantial conductance, it may be necessary to prevent direct heating of the workpiece by the post, and this can be achieved by the use of thermal insulation suitably positioned. One form of such insulation is shown in Figure 6 which is a transverse section through the transport mechanism 29 of Figure 3 but incorporating a slight modification. A post 28 is shown with its base 31 fixed to the bottom of a recess 32 in the transport mechanism 29. At the top of the recess 32 is an annular rebate 33 in which is located an annular flange 34 at the lower end of a thermally insulating sleeve 35 fitting over the post 28. The workpiece 12-3 is disposed around the sleeve 35.

Figure 7 is a similar section through an alternative arrangement in which instead of a post 28 fixed to the transport mechanism 29, there is a separate magnetic core 36 fitted within a thermally insulating sleeve 37 whose lower end is located in a recess 38 in the transport mechanism. In use the core 36 and sleeve 37 are simply inserted through the bore in the workpiece to rest in the recess 38.

If required a magnetic core can be formed of a first part fixed in the manner of post 28, and a second part separate from the first part and arranged to provide an air gap between the parts. The relative positions of the parts can be altered to obtain control of the air gap.

Figure 8 illustrates the shaping of magnetic cores 16 necessary in one embodiment of the invention for obtaining near uniform heat treatment of a thin flat disc 12-4. It can be shown that uniform heating of such a disc (or indeed any rotationally symmetrical body) is obtained when the axial flux density varies inversely with the radius of the disc, and the pointed ends 39 of the magnetic cores provide a distribution which is a close approximation to the ideal desired distribution. In many cases the components to be heated are designed to fit onto shafts and hence have a hole through their centre. It does not matter how the flux is distributed within this hole providing it is related to the distribution outside it. It can be shown that this requirement is satisfied by a theoretical constant flux density across the bore hole equal to twice the value required at the edge of the hole. It is therefore certainly not necessary to produce an infinite flux density at r=o in order to realise the uniform heating condition. A suitably sized magnetic core is an ideal way of achieving this.

It is important to obtain uniform heating of workpieces made of pressed metal powder sincc, if the comparatively weak pressed powders are not heated uniformly, there is considerable dngcr of thermal distortion in the preform during the sintering I,ioccss leading to poorer dimensional oleralllces and possibly weakness in the Cill;ll product.

As has been mentioned, the magnetic cores may themselves become heated by induced eddy currents, and thermally in sulalting sleeves are used to prevent radiation from the cores to the workpieces.

However, more commonly the material of the magnetic cores will have a low conduct ancc and there will be no significant eddy current heating of the cores, but it will still be desirable to use the insulating sleeves to reduce heating of the cores by the hot workpieces so that the magnetic material does not exceed its Curie temperature.

In the situation where the workpiece is raised to a high temperature and the material of the magnetic core is one which becomes heated by eddy currents induced therein, the magnetic core is continuously cooled by passing a coolant gas through longitudinal ducts 40. As shown in Figure 9 the magnetic core has a first duct 4() in the form of an axial bore, 'ind has six longitu- dinal channels regularly spaced around its cvlindrical surface. which channels in conjunction with the inner surface of the insulating sleeve 35 constitute further ducts 40.

Figure 10 shows an alternative arrangement in which longitudinal channels are formed in the inner surface of the sleeve 35 and in conjunction with the cylindrical surface of the magnetic core constitute ducts 40. If required the ducts may be formed by channels in both the magnetic core and the sleeve, or by longitudinal bores in the magnetic core similar to the axial bore shown in Figure 9.

Ducts 40 may be incorporated into the arrangement shown in Figure 6, in which case coolant gas can be introduced into recess 32 from a passage (not shown) in the transport mechanism 29 for flow upwardly through the ducts.

Workpicces in the form of powdered metal compacts can be sintered by the abovedescribed methods of heat treatment, and it is particularly advantageous in such intering to control the heating levels in the early stages to allow heat to diffuse away from the hot spot arcas at the particle interfaces. This can be achieved by a variation in the turn density on the induction coil.

In a simple form of such a variation the induction coil can be formed as a series of short spaced apart sections at and adjacent the entrance of the treatment region. It is also particularly advantageous in sintering of powdered metal compacts to vary the current in the induction coil, or in the spaced apart sections when present, as a function of time in order to minimize the hot spots.

It is desirable to be able to vary the spacing of the elongate coil portions of a flat coil type induction heating coil in order to adapt to different sizes, of workpieces, and this can be achieved by arranging one or both of the elongate coil portions 26 to be movable in the X and/or Y directions of the heating coil under the control of a hydraulic or mechanical drive means, taking the longitudinal direction as Z. A sensor can be used to provide a signal relating to the position of the movable coil portion (or the spacing of the coil portions), such signal constituting an actual value signal in a servo-control system for controlling the drive means.

Different sizes or types of workpieces can require different currents in the heating coil and thus to provide for flexibility of operation, the above-described heating apparatus may include means for controlling the current. Conveniently, this control means is in the form of a variable output transformer coupled between the power supply and the heating coil.

In one alternative construction of m

The rails 41 and 42 are driven by means (not shown) reciprocatingly both longitudinally of the rails and transversely thereto such that each rail has a movement which is somewhat of a mirror image movement of that of the other as indicated in Figure 11 where tail 41 has the movement pattern referenced A and rail 42 has the pattern referenced B. Thus as rail 41 moves forwards, rail 42 moves backwards, and when rail 41 moves outwardly so does rail 42, thereby causing an intermittent turning movement of the workpieces as the refractory pads grip and exert a turning couple on the workpieces during longitudinal movement of the rails in their respective inner positions, the pads being clear of the workpieces when the rails are in their respective outer positions.

A modification of the arrangement of Figure 11 is shown in Figure 12 wherein a pair of rails 45, 46 extend transversely of the treatment region and have a spring-loaded refractory pad 43 mounted at their respective operative free ends for turning a single workpiece 12-3 through a predetermined angle in a similar manner to the arrangement of Figure 11. Several pairs of the rails 45, 46 can be provided spaced along the length of a treatment region with means for selecting operation of one or more of the pairs, as desired.

Whereas, as described above, the cores 16 and the elements for improving the flux distribution in workpieces are formed of magnetic material i.e. material having a significant value of magnetic permeability, they can alternatively be formed of material -having a low or insignificant value of permeability provided that it is electrically conductive and can effect modification of the flux distribution by means of the eddy current induced therein.

In the abovementioned embodiments the workpieees are constrained to travel at predetermined positions along a transport means, for example on posts 28. One form of such a transport means in an endless belt conveyor system made of a stainless steel mesh. The various links of such a mesh belt are in practice electrically insulated from each other by a film of chromium oxide which forms on the surface of the stainless steel. Where it is desired to use a mesh belt for conveying the workpieces at predetermined positions, there may be provided posts of the kind shown in Figure 3; these posts act as the abovementioned flux modifying members and they can be attached to the mesh belt via a ceramic mounting device. As an alternative to the abovementioned stainless steel mesh belt, an endless conveyor can be formed of ceramic elements linked together by for example stainless steel pins.

WHAT WE CLAIM IS: 1. A method of heat treating a rotationally symmetrical workpiece by induction heating comprising mounting the workpiece on a refractory transport means and passing the transport means longitudinally through ail elongate treatment region of an induction heating apparatus with the axis of symmetry of the workpiece parallel to the magnetic flux of the induction heating apparatus, the workpiece being accompanied on at least part of its passage through the treatment region by at least one flux modifying member which acts to modify the magnetic flux distribution by the effects of the eddy currents induced in that member and/or its magnetic permeability so that the resultant distribution across the workpiece transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece.

2. A method of heat treating rotationally symmetrical workpieces by induction heating comprising mounting the workpieces spaced apart along a refractory transport means and passing the transport means longitudinally through an elongate treatment region of an induction heating apparatus with the axis of symmetry of the workpieces parallel to the magnetic flux of the induction heating apparatus, there being a multiplicity of workpieces in the treatment region at any one time, each workpiece being accompanied on at least part of its passage through the treatment region by at least one flux modifying member which acts to modify the magnetic flux distribution by the effects of the eddy currents induced in that member and/or its magnetic permeability so that the resultant distribution across the workpiece transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece.

3. A method as claimed in claim 2 wherein the workpieces are spaced apart by the use of spacers.

4. A method as claimed in claim 3 wherein the spacers are non-conductive.

5. A method as claimed in claim 2 wherein the transport means is arranged to carry the workpieces at predetermined spaced positions.

6. A method as claimed in claim 5 wherein the transport means comprises a multiplicity of non-conductive elements fitting together in a line, and formed so as to provide respective cavities in which the workpieces are disposed.

7. A method as claimed in claim 6 wherein the treatment region is disposed vertically.

8. A method as claimed in claim 5

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The rails 41 and 42 are driven by means (not shown) reciprocatingly both longitudinally of the rails and transversely thereto such that each rail has a movement which is somewhat of a mirror image movement of that of the other as indicated in Figure 11 where tail 41 has the movement pattern referenced A and rail 42 has the pattern referenced B. Thus as rail 41 moves forwards, rail 42 moves backwards, and when rail 41 moves outwardly so does rail 42, thereby causing an intermittent turning movement of the workpieces as the refractory pads grip and exert a turning couple on the workpieces during longitudinal movement of the rails in their respective inner positions, the pads being clear of the workpieces when the rails are in their respective outer positions. A modification of the arrangement of Figure 11 is shown in Figure 12 wherein a pair of rails 45, 46 extend transversely of the treatment region and have a spring-loaded refractory pad 43 mounted at their respective operative free ends for turning a single workpiece 12-3 through a predetermined angle in a similar manner to the arrangement of Figure 11. Several pairs of the rails 45, 46 can be provided spaced along the length of a treatment region with means for selecting operation of one or more of the pairs, as desired. Whereas, as described above, the cores 16 and the elements for improving the flux distribution in workpieces are formed of magnetic material i.e. material having a significant value of magnetic permeability, they can alternatively be formed of material -having a low or insignificant value of permeability provided that it is electrically conductive and can effect modification of the flux distribution by means of the eddy current induced therein. In the abovementioned embodiments the workpieees are constrained to travel at predetermined positions along a transport means, for example on posts 28. One form of such a transport means in an endless belt conveyor system made of a stainless steel mesh. The various links of such a mesh belt are in practice electrically insulated from each other by a film of chromium oxide which forms on the surface of the stainless steel. Where it is desired to use a mesh belt for conveying the workpieces at predetermined positions, there may be provided posts of the kind shown in Figure 3; these posts act as the abovementioned flux modifying members and they can be attached to the mesh belt via a ceramic mounting device. As an alternative to the abovementioned stainless steel mesh belt, an endless conveyor can be formed of ceramic elements linked together by for example stainless steel pins. WHAT WE CLAIM IS:

1. A method of heat treating a rotationally symmetrical workpiece by induction heating comprising mounting the workpiece on a refractory transport means and passing the transport means longitudinally through ail elongate treatment region of an induction heating apparatus with the axis of symmetry of the workpiece parallel to the magnetic flux of the induction heating apparatus, the workpiece being accompanied on at least part of its passage through the treatment region by at least one flux modifying member which acts to modify the magnetic flux distribution by the effects of the eddy currents induced in that member and/or its magnetic permeability so that the resultant distribution across the workpiece transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece.

4. A method as claimed in claim 3 wherein the spacers are non-conductive.

8. A method as claimed in claim 5

wherein the heating coil of the apparatus is of the flat-coil type and is elongate, the treatment region being that region between the spaced elongate portions of the coil and wherein the transport means comprises a movable body of non-conductive material including mounting means arranged to mount workpieces at predetermined spaced apart positions.

9. A method as claimed in claim 8 wherein the workpieces have a cavity or a through passage. and the mounting means comprises respective posts which may be fixed to the body or located within respective recesses in the body.

10. A method as claimed in claim 9 wherein the mounting posts are arranged to extend at right angles to the longitudinal direction of the treatment region and the workpieces are rotated about the mounting posts to obtain a more uniform heating of the workpieces.

11. A method as claimed in claim 10 wherein the rotation is cffccted by contact between the workpieces and a surface which is stationary or moving according to the rate of rotation required.

12. A method as claimed in claim 9 wherein the mounting posts are rotated about their longitudinal axes and cause rotation of the work pieces by means of frictional forces therebetween.

13. A method as claimed in claim 12 wherein a rack and pinion arrangement is used for rotating the mounting posts, the pinion being arranged at one end of the mounting posts.

14. A method as claimed in any one of claims 8 to 13 wherein the movable body is in the form of an endless loop of material with its working run arranged such that the workpieces pass longitudinally through the treatment region.

15. A method as claimed in any one of claims 8 to 13 wherein the movable body is a rotatable disc with the mounting means arranged circumferentially at or adjacent the periphery of the disc, and the elongate heating coil is arcuate and provides an arcuate treatment region.

16. A method as claimed in any one of claims 3 to 15 wherein the workpieces are passed through the treatment region in such a manner that neither the workpieces nor the spacers (or the transport means. as the case may bc) come into contact with a refractory lining of the induction heating apparatus.

17. A method as claimed in claim 8 and including the step of rotating the workpieces about respective axes at right angles to the plane of the heating coil during their pas sagc through the treatment region.

18. A method as claimed in claim 17 wherein the workpieces are rotated by gripping and turning means.

19. A method as claimed in any of claims 2 to 18 wherein the flux modifying members accompany the workpieces for only a part of their passage through the treatment region, and are guided for movement relative to the workpieces.

20. A method as claimed in claim 19 wherein the members are brought into their respective working positions in or adjacent the workpieces after the workpieces have entered the treatment region, or are withdrawn from their working positions before the workpieces reach the end of the treatment region. or undergo a combination of these movements whereby they are only in working position over an inner part of the treatment region.

21. A method as claimed in any of the preceding claims wherein the flux modifying members are made of a magnetic material having high resistivity.

22. A method as claimed in any of claims 1 to 20 wherein the flux modifying members are made of an electrically conductive material.

23. A method as claimed in any one of claims 9 to 13, wherein the posts are made of magnetic material and thereby constitute the flux modifying members for modifying the magnetic flux distribution.

24. A method as claimed in any one of claims 21 to 23 wherein the flux modifying members are formed using a pressed powder technique.

25. A method as claimed in claim 24 wherein the flux modifying members are formed of iron dust.

26. A method as claimed in claim 21 wherein the flux modifying members have been made by the steps of forming a bundle of strands of a high permeability/low conductance material and filling the interstices of the bundle with a substance to hold the strands in position.

27. A method as claimed in claim 26 wherein said substance is non-conductive.

28. A method as claimed in any of the preceding claims including providing thermally insulating material between the or each workpiece and the magnetic flux modifying members.

29. A method as claimed in any of the preceding claims including cooling the flux modifying members by passing a coolant through and/or over the members.

30. A method as claimed in any of the preceding claims wherein the or each workpiece has a central bore and the total quantity of flux in the bore is equivalent to a constant flux density across the bore equal to twice the value of flux density at the wall of the bore.

31. A method of heat treating a workpiece or workpieces by induction heating as claimed in either claim 1 or claim 2 and substantially as hereinbefore described with reference to the accompanying drawings.

32. An induction heating apparatus for the heat treatment of a rotationally symmetrical workpiece passed through an elongate treatment region thereof, comprising a refractory transport means movable longitudinally through the treatment region, said transport means being arranged to carry the workpiece with its axis of symmetry in a predetermined direction with respect to the transport means, means for producing an altcrnating magnetic flux in said treatment region with the flux parallel to the axis of symmetry of the workpiece, flux modifying members arranged to accompany the workpiece in predetermined positions with respect thereto on at least part of its passage through the treatment region. which mod ifving members act to modify the flux in the treatment region by the effects of the eddy currents induced in said members and/or their magnetic permeability so that the resultant distribution across the workpieces transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece.

33. An induction heating apparatus for the heat treatment of rotationally symmetrical workpieces passed sequentially through an elongate treatment region thereof. comprising a refractory transport means movable longitudinally through the treatment region and along which the workpieces can be spaced, said transport means being arranged to carry each workpiece with its axis of symmetry in a predetermined direction with respect to the transport means, means for producing an alternating flux in said treatment region with the flux parallel to the axis of symmetry of the workpiece, flux modifying members arranged to accompany workpieces in predetermined positions with respect thereto on at least part of their passage through the treatment region, which modifying members act to modify the flux in the treatment region by the effects of the eddy currents induced in said members and/or their magnetic permeability so that for each workpiece the resultant distribution across the workpiece transversely to the axis thereof of the flux in the axial direction is substantially inversely related to the distance from the axis of rotational symmetry of the workpiece.

34. An apparatus as claimed in claim 33 wherein said transport means comprises spacers.

35. An apparatus as claimed in claim 34 wherein said spacers are formed of a nonconductive material.

36. An apparatus as claimed in claim 33 wherein the transport means is arranged to carry the workpieces at predetermined positions.

37. An apparatus as claimed in claim 36 wherein the transport means comprises a multiplicity of non-conductive elements arranged to be fitted together in a line, and formed so as to provide respective cavities for receiving the workpieces.

38. An apparatus as claimed in claim 37 wherein the treatment region is disposed so as to be vertical in use.

39. An apparatus as claimed in claim 36 wherein the heating coil of the apparatus to produce said flux is of the flat-coil type and is elongate, the treatment region being that region between the spaced elongate portions of the coil, and wherein the transport means comprises a movable body of nonconductive material including mounting means arranged to mount workpieces at the predetermined spaced apart positions.

40. An apparatus as claimed in claim 39 wherein the mounting means comprises respective posts which may be fixed to the body or located within respective recesses in the body.

41. An apparatus as claimed in claim 40 wherein the mounting posts are arranged to extend at right angles to the longitudinal direction of the treatment region thereby permitting rotation of workpieces thereon to obtain a more uniform heating of the workpieces.

42. An apparatus as claimed in claim 41 comprising a surface extending along the treatment zone adjacent the transport means for contact with workpieces mounted on the posts such as to rotate the workpieces by frictional force.

43. An apparatus as claimed in claim 42 including means for moving said surface in the direction to increase the rotation of the workpieces.

44. An apparatus as claimed in claim 40 wherein the mounting posts are arranged to be rotated about their longitudinal axes so as to cause rotation of the workpieces by means of frictional forces therebetween.

45. An apparatus as claimed in claim 44 including a rack and pinion arrangement for rotating the mounting posts, the pinion being arranged at one end of the mounting posts.

46. An apparatus as claimed in any one of claims 39 to 45 wherein the movable body is in the form of an endless loop of material with its working run arranged such that the workpieces pass longitudinally through the treatment region.

47. An apparatus as claimed in any one of claims 39 to 45 wherein the movable body is a rotatable disc with the mounting means arranged circumferentially at or adjacent the periphery of the disc, and the elongate heating coil is arcuate and provides an arcuate treatment region.

48. An apparatus as claimed in any one of claims 39 to 47 including means for controllably altering the spacing of the elongate coil portions.

49. An apparatus as claimed in any one of claims 39 to 48 including means arranged to altcr the position of the workpicce at right angles to the plane of the coil.

5(). An apparatus as claimed in claim 39 and including means for rotating the workpieces about respective axes at right angles to the plane of the heating coil during their passage through the treatment region.

51. An apparatus as claimed in claim 50 comprising for each workpiece a respective rotatable base forming a mounting means and itself having projections by means of which it can be rotated.

52. An apparatus as claimed in claim 50 including gripping and turning means for rotating the workpieces.

53. An apparatus as claimed in any one of claims 32 to 52 wherein the flux modifying members are made of a magnetic material having high resistivity.

54. An apparatus as claimed in any one of claims 32 to 53 wherein the flux modifying members are made of an electrically conductive material.

55. An apparatus as claimed in any one of claims 40 to 45, wherein the posts are made of magnetic material and thereby constitute the flux modifying members for modifying the magnetic flux distribution.

56. An apparatus as claimed in any of claims 32 to 53 wherein the flux modifying members have been formed using a pressed powder technique.

57. An apparatus as claimed in claim 56 wherein the flux modifying members are formed of iron dust mixed with magnetite.

58. An apparatus as claimed in claim 53 wherein the flux modifying members are in the form of a multiplicity of strands of a high permeability/low conductance material securely held together in a bundle by an encapsulant.

59. An apparatus as claimed in claim 58 wherein the encapsulant is non-conductive.

60. An induction heating apparatus for the treatment of workpieces passes sequentially through an elongate treatment region thereof, substantially as hereinbefore described with reference to and as shown in Figure 8.

61. An apparatus as claimed in any one of claims 32 to 60 comprising means for passing a coolant through and/or over the flux modifying members.