Method for treating a component such as a gearwheel

Abstract

A method of manufacturing a mechanical component made of steel including a surface-hardening phase. The method includes creating a rough form of the component with regions to be hardened, then successive case hardening with cooling without quenching, heating to an austenitizing temperature of the steel by induction heating the zones, and quenching.

Claims

1. A method of manufacturing a steel toothed gearwheel with a phase of hardening surface zones, comprising: creating a blank of the gearwheel, the gearwheel including a tooth set that is to be hardened; and then carburizing a surface layer of the tooth set, the carburizing of the surface layer being followed by a cooling without quenching; localized induction heating using at least one medium-frequency or low-frequency current generator, which is applied to an entire depth of the tooth set of the gearwheel from a tip to a root of each tooth of the tooth set and to a gullet between adjacent teeth of the tooth set, to an austenitizing temperature of the steel; and quenching.

2. The method as claimed in claim 1, wherein the localized induction heating is completed using only the at least one medium-frequency or low-frequency current generator such that a high-frequency generator is not used.

3. The method as claimed in claim 1, wherein, after the quenching, the gearwheel has a surface hardness of at least 680 HV.

4. The method as claimed in claim 1, wherein, after the quenching, a hardness of a portion of the tooth set beneath the carburized surface layer is at least 450 HB.

5. The method as claimed in claim 1, wherein, prior to the quenching, a hardness of a core of the gearwheel is between 100 and 400 HB.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention is now described in greater detail with reference to the attached drawings in which:

(2) FIG. 1 is the depiction of a gearwheel with its various zones that need to be hardened;

(3) FIG. 2 is a schematic cross section of the structure of the tooth set of a gearwheel after the entire component has been carburized and quenched;

(4) FIG. 3 is a schematic cross section of the structure of the tooth set of a gearwheel after what is known as contour hardening;

(5) FIG. 4 schematically shows the structure of the tooth set after the treatment of the invention.

DESCRIPTION OF THE INVENTION

(6) According to the invention, a steel bar is machined, forged or converted in a suitable way to obtain a blank of the component that is to be manufactured. The steel used has a low carbon content. This is, for example, of the order of 0.3%. In the case of a gearwheel, the peripheral tooth set is machined. So are the other zones that are to be hardened. In the case of a gearwheel as depicted in FIG. 1, the cylindrical part that forms the support for the rolling bearing and the splines for transmitting driving load rotationally between a shaft and the gearwheel are advantageously machined in the blank prior to the hardening heat treatment or thermochemical treatment that will follow.

(7) Once the blank has been prepared, it is subjected to a diffusion treatment aimed at increasing, notably up to around 0.8%, the carbon content in the surface layers over a desired depth. The aim here is to increase the carbon content in the zones that are to be hardened. The treatment may be a conventional one, for example carburizing in a furnace under a partial pressure in the case of the known technique of low pressure carburizing LPC, or at atmospheric pressure in the case of the known technique of gas carburizing in a controlled flow of a reactive gas mixture. For example, in the case of LPC, the carburizing process involves the following steps of evacuating the chamber, heating under vacuum, by radiation or other equivalent means, gradually in stages with homogenization soaks up to the carburizing temperature, injecting the carburizing gaseous mixture under controlled partial pressure in the case of LPC, and at a controlled mass flow rate, a succession of diffusion and carburizing sequences, according to the desired depth and profile.

(8) The invention is not restricted to this thermochemical treatment mode. Any mode of treatment that achieves this result will suit. Thus, in the case of gas carburizing, the heating is the conventional one.

(9) The component is then gradually cooled down to ambient temperature. It should be noted that the cooling rate is chosen according to the amount of through-hardening that it is desired the component should have. For preference, the cooling rate is sufficiently removed from the quench rate that a non-through-hardened state is achieved.

(10) The next step is to heat the component locally in the desired zones by magnetic induction up to the austenitizing temperature of the steel, then to quench.

(11) Electromagnetic induction hardening is a method known per se making it possible to obtain uniform rapid heating over a controlled and reproducible depth from 0.5 mm to several centimeters. The ferromagnetic material component is placed inside a solenoid through which a high-frequency, medium-frequency or low-frequency AC current passes. This solenoid, with the component, behaves like a transformer and develops an induced current within it. The heating effect at the periphery of the component is very rapid.

(12) Induction contour hardening uses high-frequency currents the associated frequencies of which range between 20 and 600 kHz, and requires a generator with a power in excess of 100 kW.

(13) Medium-frequency induction surface hardening uses a medium-frequency current generator with a power of the order of 50 kW and the associated current frequencies of which are of the order of 10 kHz.

(14) Low-frequency induction surface hardening uses a low-frequency current generator with a power below 1 kW and the associated current frequencies of which are below 1 kHz. The choice of generator is generally dependent on the chosen depth of treatment.

(15) After heating, the component is immersed in a quenching fluid, generally water incorporating one or more suitable additives.

(16) Within the context of the invention, this differs from the electromagnetic induction hardening technique referred to as contour hardening.

(17) It will be recalled that the contour heating of a gearwheel involves placing this gearwheel within the alternating magnetic field of a single-turn or multi-turn inductor coaxially surrounding the toothed periphery of this gearwheel in order to create an axial field. The alternating field with which the inductor is supplied for a contour hardening of tooth sets is generally a high-frequency field, with a frequency ranging from 20 to 600 kHz, with a current generator of a power in excess of 100 kW.

(18) What the invention is proposing is low-frequency or medium-frequency hardening because the invention proposes to achieve surface hardening that does not, however, exactly follow the contour of the tooth.

(19) Thus, according to the invention, what is referred to as full tooth induction heating is performed whereby surface hardening is achieved that does not follow the exact contour of the tooth set. The heating is not as accurate but can be carried out by a low-frequency or medium-frequency current generator. The heating is easier to optimize because the currents do not have to follow the contour of the tooth.

(20) Following heating, the component is subjected to a quench, for example being quenched in water.

(21) FIGS. 2, 3 and 4 depict the effects of the various treatment modes, the first two corresponding to the prior art and the third to the invention.

(22) The tooth set 5 of FIG. 2 has a carburized state in the surface layer 51. The component 3 is through-hardened. Use is made of an alloy steel with a low carbon content of 0.15%. Through the carburizing process the carbon content in the surface layer reaches 0.8%.

(23) The tooth set 5 in FIG. 3 has a surface layer 51 which is hardened by induction contour hardening. This surface layer may be relatively thick. In order to maintain a certain level of toughness within the core, the steel has a higher carbon content than before, namely 0.6%. The content is the same throughout the component.

(24) The tooth set 5 in FIG. 4 has a carburized surface layer 51 the surface carbon content of which is high at 0.8%. Because of the induction hardening applied to the entire tooth set, the whole of this part 52 formed by the whole of the teeth from the tips to the root and the gullet between two adjacent teeth has a hardness greater than that of the underlying part of the disk. The material is not through-hardened.