*3.3. Heat Treatment*

The forming process was followed by a process-integrated surface hardening by quenching using an air-water spray cooling (Figure 5). The residual heat in the bevel gear after forging was employed for process-integrated heat treatment. By applying a short intensive cooling, a martensitic surface layer was formed; by interrupting the intensive cooling after a given time, a subsequent self-tempering of this surface layer was realized. This self-tempering of the martensitic surface layer occurs when residual heat from the core of the not fully quenched bevel gear flows to the surface.

**Figure 5.** Quenching of a rotating bevel gear in an air-water spray.

The spray cooling system consisted of eight air-water spray nozzles (Internal Mix Nozzles, SUJ12, Spraying Systems Co®, Wheaton, IL, USA) annularly arranged around a rotating mount for the bevel gear. The nozzles were aligned at a distance of 100 mm from the workpiece surface. By varying air and water inlet pressures, the cooling rate was adjusted. A martensitic surface layer in the toothing area of the bevel gears was produced by a short quenching phase with high cooling rates. Subsequently, the residual heat remaining in the core of the bevel gear was employed to temper the surface layer. By means of employing active cooling, the self-tempering temperature could be controlled. A pyrometer was used to monitor the set self-tempering temperature. The pyrometer recorded the temperature at the tooth tip during the self-tempering phase. Since air-water spray was employed during self-tempering, measurement was only possible between two air-water spray pulses. Hence, short pulses of air-water spray were employed automatically every time the surface

temperature exceeded the desired self-tempering temperature. To ensure a uniform heat treatment start temperature after transporting the forged bevel gears from the forging press to the spray cooling system, a second pyrometer measured the temperature on the top side of the bevel gear. By means of numerical simulations of process-integrated surface hardening and tempering, the spray parameters (inlet pressures), the duration of the quenching phase and the self-tempering temperature were determined; see Table 1. The heat transfer coefficients for the simulation were estimated by prior cooling tests on bevel gears. To compute the cooling curves by numerical simulations of the quenching process, boundary conditions were adapted as described in [21].


**Table 1.** Heat treatment parameters of the different hybrid bevel gears.

Figure 6 shows the time-temperature curves recorded during heat treatment of the bevel gears with a first phase of quenching followed by the self-tempering phase. The temperatures were measured at the tooth flanks with a pyrometer and used to control the self-tempering process. If the specified self-tempering temperature of 300 ◦C in the bevel gears with the material combination 41Cr4/C22.8 was exceeded, the air-water spray pulse was activated for a short period to reduce surface temperature. This resulted in temperature oscillations (green and orange curves) during the tempering phase starting at about 15 s. Due to the higher recommended self-tempering temperatures of up to 750 ◦C for the steel X45CrSi9-3, no air-water spray cooling was required in this heat treatment phase; hence, for the material combination X45CrSi9-3/C22.8, no such cooling related oscillations occurred. Instead, only oscillations with lower amplitude are visible caused by the rotation of the bevel gears during heat treatment. The system was manually stopped when an active control of the tempering temperature was no longer required, so that the total self-tempering times seen in Figure 6 vary. The sudden temperature drop at the end of each plot is caused by stopping the measurement. After self-tempering, the bevel gears were placed outside of the spray cooling arrangemen<sup>t</sup> and cooled down in still air.

**Figure 6.** Temperature curves measured with a pyrometer at the tooth flank of the bevel gear.

#### *3.4. Investigation of the Joining Zone*

To characterize the microstructural evolution after each process step, cross-sections were extracted from the hybrid workpieces and the bevel gears. Due to deviating forming temperature and strain distribution in the upper and in the lower part of the bevel gear, two sampling positions A and B were employed (Figure 7a). After metallographic preparation, the specimens were etched with 5% nitric acid solution. To reveal the martensitic microstructure of the X45CrSi9-3 steel, an etching with Beraha II reagen<sup>t</sup> was applied to the cladded and forged samples, and a V2A etchant was used on the heat-treated specimens. A detailed microstructural analysis of the combination X45CrSi9-3/C22.8 is given in [19]. Hardness measurements according to Vickers (HV0.5) were carried out for both the cladding layer and the substrate close to the interface [22]. In the bevel gears, the hardness was examined in the tooth tip area. The average values and the standard deviations given in Table 2 were calculated based on 10 indentations each in the cladding layer and in the substrate.

**Figure 7.** Sampling positions of the cross-sectional micrographs for metallographic examination and hardness measurements (**a**), extraction position of the sample plates (**b**), extraction position of the tensile specimens from the sample plates (**c**), geometry of the micro tensile specimen (**d**).



For a mechanical characterization of the interface, tensile test specimens with the geometry given in Figure 7d were cut from the longitudinal cross section of the cladded workpieces and the hybrid bevel gears using wire electrical discharge machining (EDM). The samples from the bevel gears were cut from the tooth tip area according to the sampling position depicted in Figure 7b. At first, three thin plates with a thickness of 1 mm were eroded from each section and etched with FeCl3 reagen<sup>t</sup> to reveal the material distribution. Thus, the tensile specimen from both positions A and B could be prepared precisely by means of wire EDM featuring the joining zone in the center of the specimen as shown in Figure 7c. Overall, six specimens were extracted from each bevel gear and each workpiece. The experiments were carried out using a tensile testing machine Zwick Retro (Line ZwickRoell, Ulm, Germany) with a maximum capacity of 10 kN. Prior to testing, the specimens were pre-stressed with a load of 5 N. The stress increase rate was set to 30 MPa/s according to standard EN ISO 6892-1 [23].
