INDUCTION HARDENING SYSTEM AND INDUCTION HARDENING METHOD

Abstract

An inductive hardening system for hardening a component includes a holding unit for holding the component, an induction coil configured to induce an electrical current in the component to heat the component, and a control unit configured to control the induction coil to produce a first amount of heat per unit area in the component until a predetermined temperature is reached and/or a predetermined time is elapsed and after the predetermined temperature is reached and/or the predetermined time is elapsed, to control the induction coil to produce a second amount of heat per unit area in the component, the second amount of heat being from 3% to 80% of the first amount of heat.

Claims

1. An inductive hardening system for hardening a component, the inductive hardening system comprising: a holding unit for holding the component; an induction coil configured to induce an electrical current in the component to heat the component, a control unit configured to control the induction coil to produce a first amount of heat per unit area in the component until a predetermined temperature is reached and/or a predetermined time is elapsed, and after the predetermined temperature is reached and/or the predetermined time is elapsed, to control the induction coil to produce a second amount of heat per unit area in the component, the second amount of heat being from 3% to 80% of the first amount of heat.

2. The inductive hardening system according to claim 1, wherein the induction coil is energizable by a generator with alternating current of predetermined magnitude, and wherein the control unit is configured to control the generator to adjust a current strength and/or a voltage and/or a frequency of the alternating current to produce the first amount of heat per unit area and the second amount of heat per unit area.

3. The inductive hardening system according to claim 2, including a holder configured to hold the induction coil at a coupling distance from the component, wherein the control unit is configured to control the holder to set a first coupling distance to produce the first amount of heat per unit area and to set a second coupling distance to produce the second amount of heat per unit area, the second coupling distance being less than the first coupling distance.

4. The inductive hardening system according to claim 1, wherein the induction coil is configured to partially cover the component, wherein the induction coil and the component are movable relative to each other, and wherein the control unit is configured to control a relative speed between the induction coil and the component.

5. The inductive hardening system according to claim 4, wherein the control unit is configured to set the relative speed of the induction coil and the component to a first speed until the predetermined temperature is reached and/or the predetermined time is elapsed and to set the relative speed of the induction coil and the component to a second speed greater than the first speed after the predetermined temperature is reached and/or the predetermined time is elapsed.

6. The inductive hardening system according to claim 5, wherein the first relative speed is selected to produce the first amount of heat per unit area, and the second relative speed is selected to produce the second amount of heat per unit area.

7. The inductive hardening system according to claim 1, wherein the predetermined temperature is an austenitization start temperature, an austenitization end temperature, or a temperature between the austenitization start temperature and the austenitization end temperature.

8. The inductive hardening system according to claim 1, wherein the control unit is configured to alternate during the hardening between a first heat input and a second, reduced heat input.

10. A method for inductively hardening a component, comprising: controlling an induction coil and/or a relative movement between the induction coil and the component to produce a first amount of heat per unit area in the component until a predetermined temperature is reached and/or a predetermined time is elapsed, and after the predetermined temperature is reached and/or the predetermined time is elapsed, controlling the induction coil and/or a relative movement between the induction coil and the component to produce a second amount of heat per unit area in the component, the second amount of heat being from 3% to 80% of the first amount of heat.

11. An inductive hardening system for hardening a component, the inductive hardening system comprising: a holding unit for holding the component; an induction coil configured to induce an electrical current in the component to heat the component, and a control unit configured to control the induction coil to produce a first amount of heat per unit area in the component or a second amount of heat per unit area in the component, the second amount of heat being less than the first amount of heat, and to alternate between controlling the induction coil to produce the first amount of heat and the second amount of heat until a predetermined temperature is reached and/or a predetermined time is elapsed, wherein the second amount of heat is from 3% to 80% of the first amount of heat.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a schematic representation of a preferred exemplary embodiment of an induction hardening system.

[0042] FIG. 2 is a schematic representation of heat input zones of the hardening system depicted in FIG. 1.

[0043] FIG. 3 is a schematic representation of a varying heat input.

[0044] FIG. 4 is a schematic representation of various preferred exemplary embodiments of the hardening method.

[0045] FIG. 5 is a schematic representation of a first preferred exemplary embodiment for varying a heat input.

[0046] FIG. 6 is a schematic representation of a second preferred exemplary embodiment for varying a heat input.

[0047] FIG. 7 is schematic representations of the heat input with a hardening system according to FIG. 1.

DETAILED DESCRIPTION

[0048] In the following, identical or functionally equivalent elements are designated by the same reference numbers.

[0049] FIG. 1 schematically shows an inductive hardening system 100. With the depicted hardening system 100, a component 2, for example, a bearing ring as depicted here, is supported on a work table 4, and with the aid of drive devices 6 can be traversed along an induction coil 8, in particular can be repeatedly traversed (so-called pulse hardening). Alternatively of course, the induction coil 8 can also be moved along the component 2 with the aid of a drive mechanism 6.

[0050] In the depicted exemplary embodiment of the inductive hardening system 100, two coils 8-1 and 8-2 are present that are disposed opposite each other. It is of course also possible to use only one coil or more than two coils.

[0051] In the drive mechanism 6 a plurality of drive mechanisms can also be provided, e.g., the drive mechanisms (6-1, 6-2, 6-3); however, more or fewer drive mechanisms can also be present for moving the component 2 (or alternatively the induction coil(s)).

[0052] In the exemplary embodiment depicted, the induction coils 8-1; 8-2 are each held by an associated induction coil holder 12-1; 12-2 that ensures that the coil 8 is held at a certain coupling distance d with respect to the workpiece 2. The induction coil 8 itself is furthermore supplied with alternating current by a generator 16 wherein in the depicted exemplary embodiment both coils 8-1, 8-2 can be energized with the same generator 16, but a separate generator can be provided for each coil.

[0053] Furthermore, a control unit 10 is provided that controls both induction coils 8, in particular their coupling distance d or their energization (current strength, frequency, voltage) and the drive mechanism 6. Here it is advantageous in particular that the control unit is configured to control the drive mechanism 6 such that a rotational speed of the rotating component is adjustable or controlled. Furthermore, it is possible to use different control units 10 for the coils 8 or the drive mechanism 6 or to provide individual control units in the respective components, in particular in the induction coil 8 and in the drive mechanism 6, which control units act individually on coil 8 or drive mechanism 6. The induction coil holder 12 can also be controlled by the control unit 10 or a separate control unit in order to set the coupling distance d. The control unit 10 can also be configured to control the generator 16 in order to supply the coil 8 with a certain current of certain frequency, voltage, and strength, whereby the control unit can be provided separately or integrally.

[0054] It is to be noted that the induction hardening system 100 depicted in FIG. 1 represents only an exemplary embodiment, and other induction hardening systems can also be similarly controlled with the method steps described in the following in order to achieve a uniform-as-possible temperature input. The steps preferred for this purpose are described in the following for an exemplary embodiment in which a single control unit 10 is provided that can control not only two coils 8-1; 8-2, their associated holders 12-1; 12-2, the drive mechanism 6-1; 6-2; 6-3, and the generator 16, but also further parts, not shown here, of the induction assembly 100 in order to achieve a uniform-as-possible temperature distribution.

[0055] In order to achieve this, a heat input introduced by the coils 8 into the component 2 during the hardening process is variable, with a first heat input being introduced into the component 2 up to a predetermined time or up to a predetermined temperature reached, and the heat input is preferably maximized in the component 2 with the help of the first heat input, and from reaching the predetermined temperature or the predetermined time, the heat input is reduced. The first, preferably maximized, or second, reduced, heat input can be defined in advance in a manner depending on the properties of the to-be-hardened component 2, and in particular its material properties, the final hardness to be achieved, and/or the hardness penetration depth to be reached.

[0056] Furthermore in the hardening system 100 depicted in FIG. 1, due to the only partial covering of the component 2 with the induction coils 8-1, 8-2, the component is not heated everywhere simultaneously but rather always only in the region under the coil 8-1, 8-2. Thermal input zones and cooling-down zones thus arise over the component 2. These zones are schematically depicted in FIG. 2 in which a heating respectively takes place in the zones I-1, I-2, while no heating takes place in the zones II-1, II-2 since the component 2 is not covered by the coils 8.

[0057] A point P (see FIG. 1) on the component thus passes under the coils 8-1, 8-2 in the zones I-1, I-2 and heated, while outside the coils 8-1, 8-2 it cools down again-no heat input takes place herein the zones I-1, I-2. If the temperature is measured at the points A and B, i.e., directly after exiting from the coverage of the first coil 8-1 (point A) and shortly before entry into the coverage region of the second coil 8-2 (point B), then a temperature difference AT arises that should be as low as possible at the end of the hardening process.

[0058] In the case of a pulse hardening, i.e., with a repeated passing over of a point P with the inductor during the hardening, the heating of the component 2, measured at a point P at the locations A and B thus follows a heating curve 20 depicted graphically in FIG. 3 in which the time t is plotted on the x-axis, and the temperature T is plotted on the y-axis.

[0059] As can be seen from the graph 20, the observed location P is strongly heated with each pass of the coil 8-1, 8-2 so that for example, during the second pass at the location A (see FIG. 2), i.e. shortly before the exit from the influence region of the coil 8-1, the temperature at location P has a maximum value T.sub.A1. If the point P is moved out from the influence of the coil, the temperature decreases until shortly before entry into the (next) coil 8-2 (see location B) it has a minimum value T.sub.B1. The temperature difference ΔT.sub.1 is comparatively large.

[0060] However, despite the cooling down between the coils, as can be seen from FIG. 3, the temperature of the component increases overall, and at time t.sub.X finally reaches or exceeds the predetermined temperature T.sub.X. This can be, for example, the austenitization start temperature, the austenitization end temperature, or a temperature in the range between austenitization start temperature and austenitization end temperature. In the diagram of FIG. 3, the predetermined temperature T.sub.X is reached at approximately half of the entire hardening time t.sub.final. This reduced heat input is characterized in the curve 20 by an overall flattened temperature increase as well as a reduced temperature fluctuation range ΔT.sub.2 between the maximum temperature T.sub.A2 and the minimum temperature T.sub.B2.

[0061] Furthermore, FIG. 3 shows that with when a final temperature T.sub.final is reached or at the end of the heating time t.sub.final, the component 2 is quickly quenched as usual to a temperature below the martensite start temperature, and the inductive hardening process is thus completed.

[0062] In addition to the control depicted in FIG. 3, in which starting from a certain temperature value T.sub.X a reduction of the heat input occurs, other variable heat inputs are also possible. Thus, FIG. 4 shows a plurality of hardening method options that work with a variable heat input in order to reach the final temperature T.sub.final at the time t.sub.final, at which the heating process is completed and the component 2 is quickly cooled down to a temperature below the martensite start temperature (T.sub.Ms). Here, in an analogous manner to the method discussed in FIG. 3, in which methods designated as A and C are used to reach a certain temperature T.sub.X (see method C) or when reaching a certain time t.sub.X (see method A), the heat output is reduced so that the temperature increase overall runs flatter. After the reaching of the time t.sub.final, the heat input is stopped and a usual quenching process is initiated after the induction hardening.

[0063] Here T.sub.X can again be the austenitization start temperature, the austenitization end temperature, or a temperature between the two.

[0064] In the method designated as E, a heat input reduced in comparison to the methods A-D is effected over the entire time t.sub.0 to t.sub.final, which is also further continued after the reaching of the time t.sub.final, in which the heat input is reduced even further. In the method according to E, the heating process is thus extended beyond reaching the heating time t.sub.final, in which the strongly reduced further heat input after the reaching of the time t.sub.final ensures a further temperature harmonization of the component in the circumferential direction. Even when in this method the maximum possible heat input for the system is not used (which can be seen from the less-steeply extending curve), then for the method E itself, up to reaching the time t.sub.final, the heat input is nevertheless maximized in the context of the parameters used for the method.

[0065] In addition to a section with reduced heat input B.sub.1; D.sub.1, the methods designated as B and D include at least one further section B.sub.2; D.sub.2 with maximized heat input. Thus, for example, in the method designated with B, starting from reaching the predetermined temperature T.sub.X=T.sub.1, up to reaching a second temperature T.sub.2, a reduced heat input is provided, wherein starting from the reaching of the temperature T.sub.2 the heat input is again maximized until the temperature T.sub.final is reached at the time t.sub.final.

[0066] Here T.sub.1 can be the austenitization start temperature and T.sub.2 the austenitization end temperature. However, T.sub.1 or T.sub.2 can also lie between the austenitization start temperature and the austenitization end temperature.

[0067] In the method designated with D, the heat input is preferably alternately reduced or maximized at regular intervals from the reaching of a temperature T.sub.D1 or a certain time t.sub.D1. In the depicted exemplary embodiment, three sections with maximum heat input alternate with three sections with reduced heat input. The temperature T.sub.D1 can also lie below the austenitization start temperature.

[0068] However, in all presented variants, during the hardening method the component is heated far above the austenitization end temperature up to the temperature T.sub.final, in order to dissolve to the greatest degree possible the alloy elements necessary for the to-be-achieved structure in the austenite.

[0069] As mentioned above, a reduction of the heat input is preferably effected during the first time reaching the austenitization start temperature (at approximately 700° C. to 1100° C., depending on the steel, microstructure state, and heating speed) in the component 2. Alternatively an adjusting can also be effected at the end of the heating time t.sub.final or after reaching a desired austenitization temperature or both in combination. The reduction of the heat input can optionally also be provided only at the time at which the full quenching effect of the quenching device is achieved on the component surface, that is, up to the time at which the quenching medium is brought with full power or full flow onto the component. A further rest time is then not provided between the end of the heating (also with reduced heat input) and quenching).

[0070] The variability of the heat input can be set, as mentioned above, with the aid of the control unit 10, in which in particular a current strength, a current voltage, a current frequency, (which in particular define the heating power of the coil), a speed of the relative movement, and/or a coupling distance can be varied.

[0071] In addition to the reduction of the heating power, the coupling distance can thus also be increased for a reduction of the heat input with the heating power remaining the same, or both in combination. In addition, the relative speed between component and tool (induction coil) can be increased, which is also conducive to the temperature uniformity in the ring.

[0072] FIG. 5 and FIG. 6 illustrate such a variable heat input in which the heat input can here can be achieved, for example, via the heating power W (see FIG. 5) or the coupling distance d (see FIG. 6). Here in FIGS. 5 and 6, the situation for the method A with heat input only reduced at the end, and the method D with alternating first, and second reduced, heat input is respectively illustrated.

[0073] In FIG. 5, with regard to method A, the heat input is reduced by a first high heating power W.sub.1 at time t.sub.X to the second heating power W.sub.2, or alternated between two heating powers W.sub.1, W.sub.2 (method D). As can furthermore be seen from FIG. 5, with alternating methods a third heating power W.sub.3 higher than W.sub.2 can also be used.

[0074] In FIG. 6, the reduced heat input is provided over a greater coupling distance d.sub.2. As can also be seen from FIG. 6, in the comparison of the different methods A, D, different coupling distances can also be used. Thus, for example, with the alternating method D, the narrowest coupling distance d.sub.0 is smaller than the smallest coupling distance d.sub.1 at the start of the method or in comparison with method A.

[0075] The maximum temperature difference ΔT to be expected in the circumferential direction of the component after the inventive hardening method is reduced by the above-mentioned measures to at most 40° C., preferably no more than 30° C., most preferably no more than 20° C. This preferably applies both for the austenitization temperature range between austenitization start temperature between, e.g., 700° C.-1100° C., and the austenitization end temperature between, e.g., 750° C. and 1150° C., depending on the steel, microstructure state, and heating speed, as well as for the point in time of the quenching used.

[0076] Due to the variable heat input, undesired microstructure components are preferably reduced or largely avoided after/during the quenching (bainite, perlite, ferrite). In addition, a premature lowering of the temperature (losses due to radiation, heat conduction, convection) between the times “end heating time” and “start quenching” can be prevented by the further heating with reduced heat input, whereby undesirable microstructure components can be avoided/controlled. If active heat input does not occur during the quenching delay/temperature equalization, but rather a rest time without any heat input, a temperature homogenization specifically also occurs, but the comparatively rapid cooling down requires a higher hardening temperature overall in order to compensate for the rapid temperature drop due to convection/conduction/radiation. In contrast, if further heat is actively introduced, a higher hardening temperature and a higher hardenability can be achieved overall.

[0077] FIG. 7 shows, for the method E, a comparison of the temperature development with (solid line) and without (dashed line) active post-heating with reduced heating power after reaching the heating time t.sub.final, in which the graph 22 represents the temperature development at point A, and the graph 24 represents the temperature measurement at point B. Here upon the reaching of the time t.sub.final, the heat input is reduced to 10% of the first heat input.

[0078] It can be seen in FIG. 7 that starting from the time t.sub.final, with the application of a reduced heat input a smoother subsiding or a smoother cooling-down of the component occurs than when at time t.sub.final the power supply would be completely stopped (see dashed line contour). This additional, even if slight, heat input can lead to a particularly good temperature compensation, since the component 2 itself is not solely responsible for the heat input into the colder zones, but is supported by the additional, even if reduced, heat input of the coils. This leads to a particularly homogeneous temperature distribution and also an increased hardening depth before the induction coils are completely switched off and the quenching process begins.

[0079] This can also be read directly from FIG. 7, since at a time t.sub.y after the heating time t.sub.final, ΔT, i.e., the temperature difference at points A and B is much larger with complete shutdown than ΔT.sub.E with active post-heating.

[0080] Furthermore, preferably at a depth of up to 50% of the nominal minimum hardening depth, the proportion of the non-martensitic components (bainite/perlite/ferrite) in the microstructure is usually at most 0.5%, preferably at most 0.4%, most preferably 0%, and at a depth of up to the nominal minimum hardening depth, the proportion of the non-martensitic components (bainite/perlite/ferrite) in the microstructure is at most 4.0%, preferably at most 3.5%, most preferably 0%.

[0081] Due to the active, although reduced, heat input after reaching the austenitization temperature, especially at depth, a more uniform microstructure transformation is achieved.

[0082] Furthermore, due to the variable heat input, an optimized residual stress distribution (in the circumferential direction and radial direction) is achieved: The additionally introduced heat, e.g., in the case of an extended heating time with reduced heat input before the quenching (see method E in FIG. 4 or FIG. 7) leads to an additional heating of the component core. In this way, during the subsequent quenching/cooling a contraction of the core leads to additional residual compressive stresses in the previously transformed, already martensitic hardened layer. The increase of the residual compressive stresses starting from a depth of 100 μm up to the lower nominal hardening depth (SHD) can preferably be at least 200 MPa, preferably 300 MPa, most preferably 400 MPa or more. However, residual compressive stresses of over 1,200 MPa should be avoided.

[0083] The improved stress distribution during the quenching due to avoiding a premature temperature loss in the near-surface regions of the component and the mechanical stresses and stress gradients associated therewith lead to a reduced risk of cracking during quenching.

[0084] As mentioned above, the temperature uniformity can also be improved over the circumference of the workpiece. This leads to a homogenization of the solution state in the microstructure, or a homogenization of the temperature associated with the solution state from which the martensite formation starts during the quenching. This in turn leads to a temporal equalization of the incipient martensite formation, whereby a stress reduction and increase in and between adjacent volumes is avoided by the accompanying change of the specific thickness/volume change.

[0085] This equalization in turn leads to a more uniform hardness distribution and consequently to a more uniform load-bearing capacity of the component overall.

[0086] The equalization of the temperature distribution and the residual stresses in the circumferential direction also lead to a reduced warpage after the hardening as well as subsequent manufacturing processes. An equalization of the temperature also leads to more uniform thermal expansions and thus to less warpage or plasticity during the heating.

[0087] In addition, the additional energy introduced prior to quenching can lead to higher temperatures inside the component, whereby a deeper hardness penetration can be set.

[0088] Overall, with the control discussed above or the method discussed above, it can be achieved that, before quenching, the component as a whole achieves a temperature distribution that is as uniform as possible in its hardness range. As a result, stresses in the component can be balanced and a particularly good and even hardness can be achieved. In addition, the hardening depth can also be increased as a result, since heat does not have to be extracted from the component from the inside to the outside to equalize the temperature before quenching.

[0089] Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above may be utilized separately or in conjunction with other features and teachings to provide improved induction hardening methods and systems.

[0090] Moreover, combinations of features and steps disclosed in the above detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

[0091] All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter.

REFERENCE NUMBER LIST

[0092] 2 Workpiece [0093] 4 Work table [0094] 6 Drive mechanism [0095] 8 Induction coil [0096] 10 Control unit [0097] 12 Induction coil holder [0098] d Coupling distance [0099] 16 Generator [0100] 100 Hardening system [0101] T Temperature [0102] t Time [0103] W Heating power [0104] T.sub.X Predetermined temperature [0105] t.sub.X Predetermined time [0106] t.sub.final Final heating time [0107] T.sub.final Final heating temperature