Induction contour hardening
Induction contour hardening: Induction contour hardening of gear wheels belongs to effective heat treatment technologies especially recommended for high-tech applications in machinery, automotive, and aerospace industries.
In comparison with long-term, energy-consuming conventional heat treatment (carburizing and consequent quenching), its main positive features are characterized by high total efficiency, short duration, and relatively low energy consumption.
modeling of rapid induction heating
However, modeling the process is relatively complicated. The numerical model should contain both the multi-physic and non-linear formulation of the problem. The paper concentrates on the modeling of rapid induction heating being the first stage of the contour induction hardening process which is the time-consuming part of the computations.
It is taken into consideration that critical temperatures and consequently the hardening temperature is dependent on the velocity of the induction heating.
Numerical modeling of coupled non-linear electromagnetic and temperature fields is shortly presented. Investigations are provided for gear
wheels made of a special quality steel AISI 300M.
In order to evaluate the accuracy of the proposed approach, exemplary computations of the full induction contour hardening process are provided. The exemplary results are compared with the measurements and satisfactory accordance between them is achieved.
Induction Contour Hardening (ICH)
Induction Contour Hardening (ICH) of gear wheels is an innovative technological process making it possible to obtain a thin hardened zone along the whole working surface of teeth. The result is a hard microstructure in the contour zone and a less hard microstructure in the transition zone. The microstructure of the internal part of the gear wheel remains practically unchanged.
Such an environment-friendly heat treatment process is very effective. It is characterized by large productivity and low-specific energy consumption. It is more effective than a long-term, energy-consuming conventional heat treatment process of gear wheels consisting of carburizing and consequent quenching.
In many high-tech, industrial applications the crucial quality condition is the full uniform shape of the hardened contour zone. For the ICH of big gear wheels, such an expected result could be easily achieved by means of the Tooth-by-Tooth Induction Hardening (TTIH) method.
small gear wheels
However, for small gear wheels, such a method cannot be applied, and it is not so easy to obtain the uniform thickness of the hardened contour zone along with the whole tooth by ICH methods. The paper describes the full hardening process.
The induction heating stage of the ICH process can be realized mostly in one or two cycles. The one-cycle heating process consists of Single Frequency Induction Hardening (SFIH) or, more often, Simultaneous Dual Frequency Induction Hardening (SDFIH) in the medium frequency (MF) and high frequency (HF) electromagnetic fields.
MF induction heating
The two cycles process consists of MF induction heating first, followed by immediate shifting of the body between inductors, and then finally HF induction heating. The paper deals with this second kind of approach, which will be later defined as Consecutive Dual Frequency Induction Hardening (CDFIH).
In general, the CDFIH process consists of five consecutive stages: MF induction heating, immediate shifting the workpiece between MF and HF inductors, rapid HF induction heating, immediate shifting the gear wheel to the sprayer area, and finally the intensive cooling. Due to a short duration of induction heating, the austenitization process is completely different in comparison with any classical kind of hardening.
Critical temperatures characterizing transformation of any prior microstructure to uniform austenite microstructure are dependent on the velocity of induction heating.
In order to determine dependencies between critical temperatures and velocity of induction heating the Time-Temperature-Austenitization (TTA) diagram for investigated steel is measured.Austenitization begins at the lower critical temperature Ac1. When the temperature of the material reaches the upper critical temperature Ac3 we obtain the austenite microstructure, but it is non-uniform.
A fully uniform austenite microstructure is finally obtained only at the modified upper critical temperature Acm. The paper is aimed at modeling the ICH process with a special emphasis put on the induction heating stage. The main goal is formulated as an elaboration of the computation method making it possible to obtain an accurate prediction of hardness and microstructure distribution in a hardened element and determination of the shape of the contour zone. However, the modeling of the process is complicated because of the necessity to analyze
multi-coupled, non-linear physical fields. In order to reduce the duration of computations, magnetic field quantities are considered in a simplified way as harmonic.
However, in order to achieve the expected accuracy of hardness prediction, it is crucial to take into consideration:
The dependence of the critical temperatures for the investigated steel on the velocity of induction heating strongly influences upon accurate determination of the hardening temperature;
The material properties of the investigated steel as non-linear dependences on the temperature;
The dependence of hardness on the velocity of cooling should be elaborated based upon a set of continuous-cooling-transformation diagrams for the investigated steel starting at different hardening temperatures.
Figure 1. Exemplary dependence of critical temperatures on the velocity of induction heating for steel AISI 300M
In order to obtain uniform austenite microstructure, the average hardening temperature Th in the contour zone should be slightly higher than the modified upper critical temperature Acm.
Configuration of inductors–sprayer–gear wheel systems is shown in Figure2.
Figure 2. Configuration of the CDFIH system.
Both inductors have a ring shape. They are water-cooled. The HF inductor is equipped with a magnetic flux concentrator made of Fluxtrol 50.
The CDFIH process described in the paper is successfully applied for the energy-efficient contour hardening of gear wheels. The proposed mathematical model could be an effective tool for the design of such processes. It makes it possible to predict hardness and microstructure distributions with reasonable accuracy of about 20 HV. In order to minimize the computation time, magnetic field quantities are considered in a simplified way as harmonic. The model should contain additional numerical procedures based upon specialized measurements for the investigated steel (TTA and CCT diagrams), making it possible to determine hardening temperature as a function of the velocity of induction heating and hardness as a function of the velocity of cooling and hardening temperature. The next paper in this area will be aimed at the improvement of the described process in order to obtain the uniform shape of the contour zone.