EP1616969B1 - Method for induction heat treatment of parts with rotational symmetry - Google Patents

Abstract

Heat treatment of a revolving component, e.g. a toothed wheel, comprises : (a) high frequency induction heating; (b) medium frequency induction heating; (c) quenching. An independent claim is also included for the device used for heat treatment.

Description

The present invention relates to a heat treatment process for improving the mechanical properties of a mechanical part of revolution such as a pinion, or gear wheel, gearbox.

There are different ways to improve the mechanical properties of a part and in particular its surface hardness and this, without resorting to expensive steel compositions with high concentration of carbon, nickel or chromium. The most commonly used method is carbonitriding. This method consists of superficially modifying the composition of the steels, in particular by increasing the carbon content. The treatment process involves bringing these steels to high temperatures in controlled atmospheres where the carbon concentration is high. The required ovens are very important and expensive, as for the treatment times they often exceed one or two hours.

To overcome the disadvantages of carbonitriding, alternative solutions have been proposed. One of these alternative solutions is to improve the properties of mechanical parts such as gears, by quenching and using inductive heating. The document US4855556 discloses an installation of this type. Long time the development of such a method, a priori simple and fast, stumbled on the complexity to achieve a reliable and repetitive industrial tool, not altering the teeth of the parts and the development of a range of appropriate treatment allowing to treat thousands of pieces / day.

The object of the present invention is therefore to propose a suitable method of treatment.

• chauffage inductif haute fréquence ;
• chauffage inductif moyenne fréquence ;
• trempe.
• chauffage inductif moyenne fréquence;
• chauffage inductif haute fréquence ;
• trempe.

According to the invention, the heat treatment method for a mechanical part of revolution, such as a toothed wheel, is characterized in that it comprises the following successive steps:

• inductive high frequency heating;
• inductive heating medium frequency;
• tempering.
• inductive heating medium frequency;
• inductive high frequency heating;
• tempering.

According to another characteristic of the treatment method according to the invention, during all or part of said steps, the mechanical part is rotated.

According to another characteristic of the treatment method according to the invention, during the inductive high frequency heating steps, the induction frequency is between 100 kHz and 300 kHz

According to another characteristic of the treatment method according to the invention, the duration of the inductive high frequency heating steps is between 0.3 s and 5 s.

According to another characteristic of the treatment method according to the invention, during the steps of inductive heating with a medium frequency, the induction frequency is between 0.5 kHz and 10 kHz

According to another characteristic of the treatment method according to the invention, the duration of the inductive high frequency heating steps is between 1 s and 10 s.

According to another characteristic of the treatment method according to the invention, during the inductive heating steps, the power supplied is between 100 kW and 1500 kW.

According to another characteristic of the treatment method according to the invention, the quenching bath is formed of a water-polymer mixture.

• la figure 1 est une vue schématique d'une machine-outil pour mettre en oeuvre l'invention;
• La figure 2 est un bloc diagramme détaillant un traitement selon l'invention.

The invention will be better understood, and other objects, characteristics, details and advantages thereof will appear more clearly in the explanatory description which will follow with reference to the accompanying drawings given solely by way of example illustrating an embodiment. of the invention and in which:

• Figure 1 is a schematic view of a machine tool for implementing the invention;
• Figure 2 is a block diagram detailing a treatment according to the invention.

The machine referenced 1 allows the heat treatment of mechanical parts of revolution such as the toothing 21 of a gear wheel 2 for gearboxes. In the present example, the toothed wheel 2 comprises a central bore 20, an outer peripheral wall and two lateral faces. The toothing 21, of the type for example helicoidal, is disposed on the outer peripheral wall of the toothed wheel 2

The machine comprises a frame supporting a non-figured control panel, induction heating means 4 and 5, a quench tank 6 and its supply and discharge circuit, handling means for moving the workpiece to be processed between the heating means 4, 5 and the quench tank 6 enclosing a quench bath such as a water-polymer mixture. The quenching bath, and in particular its composition, is adapted to ensure a reasonable cooling rate limiting the stresses in the metal.

This machine and more particularly the induction heating means are connected to appropriate medium and high voltage electrical generators.

The handling means comprise a work holding plate 7 integral with a shaft 70 extending vertically. The ring 2 is centered on the plate 7 by means of the end of the shaft 70 projecting above the plate and which penetrates into the central bore of the toothed wheel 2. The shaft 70 is rotated by first controlled means 71 and in vertical sliding by second controlled means 72. Clamping means, such as a controlled piston 73 immobilizes the wheel 2 on the door piece 7. This means 73 can also be used to reduce deformation by keeping the piece in press.

The heating means comprise two separate devices substantially identical in design and arranged one below the other. The device 4 is connected to a medium frequency generator as for the device 5 it is connected to a high frequency generator. Given their similarity of design, only the device 4 will be detailed further.

The heating device 4 is composed of three turns coaxial with the shaft 70, respectively 41, 42 and 43. These turns are in the form of an unsealed ring of the omega-type, and consist of annular hollow tubes. These metal turns are arranged so that when the wheel 2 is positioned so that its toothing 3 is vis-à-vis the intermediate coil, the median plane of the wheel forming a plane of symmetry of the intermediate coil 42, the Extreme turns 41 and 43 are then on either side of the wheel 2 without being vis-à-vis the toothing 3.

The three turns 41, 42 and 43 are fixed at one end to a first metal interconnection strip 44. The second ends of the three turns are fixed to a second interconnection bar not shown. Both connectors are connected to terminals of a medium frequency electric generator. An insulating wall separates the two bars.

Suitable control means make it possible to control for each inductive heating device both the duration, the frequency and the power of the electric current flowing through the turns.

A cooling fluid circulates inside each turn 41, 42, 43. The supply means, evacuation and circulation of the cooling fluid for each of these turns are not detailed further as well as the means for fixing the turns to the frame of the machine 1.

The respective dimensions of each of the turns 41, 42 and 43, namely inside diameter, height and section and their distance from each other are adjusted to the dimensions of the toothed wheel 2. In particular the height of the central turn is less than the height of the toothing.

The handling means are adapted to allow the vertical displacement of the toothed wheel 2 between 4 positions: a position I of setting up the crown, a position II of heating by the medium frequency device 4, a position III of heating by the high frequency device 5 and an IV position of immersion in the quench bath 61.

This machine 1 thus described with reference to Figure 1 allows the implementation of the method detailed in Figure 2.

The toothed wheel 2 first brought from the initial position I to the position III, inside the high frequency heating device 5. Once the wheel is positioned in front of the central turn of the device 5, a frequency of about 150 kHz is applied for 2 seconds. A power of the order of 400 kW is then provided. The teeth 21 of the wheel 2 are then brought to about 1100 ° C. All the time of this high frequency heating, the wheel 2 is rotated by means of the shaft 70 at about 150 rpm in order to standardize the heating over the entire periphery of the toothing 21.

This first stage of treatment completed, the wheel is then brought inside the medium frequency heating device 4, position II. Once the wheel is positioned in front of the central turn of the device 4, a frequency of about 2 kHz is also applied for 2 seconds. Again, a power of the order of 400kW is provided. The bottom of the teeth is in turn raised to about 1000 ° C - 1100 ° C. All the time of this medium frequency heating, the wheel is rotated at about 150 rpm and in order to standardize the heating over the entire periphery of the wheel.

After immobilizing the rotating wheel, the latter is immersed, in position IV, in the quenching bath 61 for 15 to 30 seconds. The wheel 2, initially immobile, is progressively rotated to reach a speed of rotation between 10 and 20 rpm and in a direction adapted to the direction of the toothing so as to force the quenching liquid to circulate from bottom up between the teeth. So if the toothing is on the right, the direction of rotation is clockwise. This circulation of the liquid between the teeth avoids the phenomena of calefaction and allows a homogenization of the treatment on the set of teeth.

This first treatment range allows a quenching of all the teeth 21 and also of the wheel 2 to a depth of about 3 mm at the bottom of the teeth

After an optional spinning of the wheel of rotating the wheel at high speed. A second quenching is performed to create a zone of income under the teeth 21 conferring a certain ductility and this, while maintaining a quenching of the teeth. This makes it possible to combine with hardness and flexibility for the toothing 21. This second quench therefore begins with a new step where the toothed wheel 2 is first brought to the position II inside the medium-frequency heating device. 4. Once the wheel is positioned in front of the central turn of the device 4, a frequency of about 1 kHz is applied for 2 seconds as well. Again, a power of the order of 300kW is provided. The teeth are thus brought to about 300 ° C - 400 ° C. All the time of this medium frequency heating, the wheel is rotated at about 150 rpm and in order to standardize the heating over the entire periphery of the wheel.

After this preheating, the wheel is brought to position III, inside the high frequency heating device 5. Once the wheel is positioned in front of the central turn of the device 5, a frequency of about 150 kHz is applied during 0, 4 seconds. A power of the order of 1000 kW is then provided. The teeth 21 of the wheel 2 are then brought to about 1200 ° C. All the time of this high frequency heating, the wheel 2 is rotated by means of the shaft 70 at about 250 rpm, in order to standardize the heating over the entire periphery of the toothing 21.

As soon as this high frequency heating step is performed, the wheel is immediately immersed in the bath 61 and this, for a period of time sufficient for the cooling of the wheel 2 to then allow the transfer of the latter out of the machine 1. During this phase immersion, the wheel 2, initially immobile, is gradually rotated to reach a rotation speed between 10 and 20 rpm and in a direction adapted to the direction of the teeth so as to force the liquid of soaks to flow from bottom to top between the teeth. So if the toothing is on the right, the direction of rotation is clockwise. This circulation of the liquid between the teeth avoids the phenomena of calefaction.

Of course, the present invention is susceptible of various variations and modifications which will be apparent to those skilled in the art. In particular, it is possible to carry out induction heating of any toothed wheels that are herringbone, worm type tangents or globes. The present invention also applies to conical gears, and gears for which the toothing is located on a face substantially perpendicular to the axis of the wheel, the geometric parameters defining the device according to the invention being determined according to the teeth. treated.

Therefore, depending on the type of part processed, the various parameters of the heat treatment process detailed above can be adjusted.

Claims (8)

1. Method of heat treatment for a revolving mechanical component, such as a toothed wheel, characterised in that it includes the following successive steps:

   - high-frequency induction heating;

   - medium-frequency induction heating;

   - hardening;

   - medium-frequency induction heating;

   - high-frequency induction heating;

   - hardening.
2. Method according to Claim 1, characterised in that during all or some of said steps the mechanical component is driven in rotation.
3. Method according to any one of Claims 1 to 2, characterised in that during the steps of high-frequency induction heating, said frequency is comprised between 100 kHz and 300 kHz
4. Method according to any one of Claims 1 to 2, characterised in that the duration of the steps of high-frequency induction heating is comprised between 0.3 s and 5 s.
5. Method according to any one of Claims 1 to 2, characterised in that during the steps of medium-frequency induction heating, said frequency is comprised between 0.5 kHz and 10 kHz
6. Method according to any one of Claims 1 to 2, characterised in that the duration of the steps of medium-frequency induction heating is comprised between 1 s and 10 s.
7. Method according to any one of Claims 1 to 2, characterised in that during the steps of induction heating, the power supplied is comprised between 100 kW and 1500 kW.
8. Method according to any one of Claims 1 to 2, characterised in that the hardening bath is made of a water-polymer mixture.

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