*4.2. Upsetting*

After deposition welding, the hybrid bearing washers were formed in a single-stage upsetting process. The application of forming allowed to improve the welded microstructure by thermomechanical treatment and to close the pores in the cladding layer [19]. The upsetting was performed on a hydraulic press (SPS Schirmer-Plate Siempelkamp GmbH, Krefeld, Germany) with a maximum capacity of 12,500 kN, as depicted in Figure 4a. The required forging temperature of 1050 ◦C was achieved by heating in a chamber furnace (Nabertherm, Lilienthal, Germany) in an inert gas atmosphere (Figure 4b) in order to prevent the surface decarburization and oxidation of the weld material. After the individual workpieces were put in the shielding gas container, the air was displaced by argon within a timeframe of 15 min. Then, the container was placed in the furnace until the workpieces were heated up within a timeframe of 20 min. Subsequently, the hot preforms for bearing washers were manually transferred to the forming tool and formed by upsetting from 15 mm to 9 mm. After forging, the bearing washers were cooled in air. A bearing washer after forming is shown in Figure 4c. The main parameters of the forging are summarized in Table 3.

**Table 3.** Parameters of upsetting process.


**Figure 4.** Forming setup: (**a**) hydraulic press; (**b**) removal of the shielding gas container from the furnace after heating of the workpieces; (**c**) forged bearing washer.

The macrographs depicted in Figure 5 show the effect of the die forging on the material distribution for the example of a bearing washer preform that is welded with a single seam. Both samples were prepared from the same workpiece, which was separated in two parts. One of them was subsequently formed; the second one remained without any further treatment. The mentioned parts were metallographically investigated at several positions. The representative results of this examination are illustratively demonstrated in Figure 5. After upsetting, the weld material was pressed into the substrate. A flattening of the cladding surface took place as a result of the forming process. After the deposition welding, some pores marked with red circles in Figure 5a were macroscopically observed close to the joining zone area. After forging, no pores at the investigated positions could be seen at the macroscopic scale (Figure 5b). A quantitative comparison of the porosity before and after forging was, however, not possible because the cross-sections of cladded and forged parts were not extracted from the same position. Forming is not only able to close pores; non-metallic inclusions are also pressed further into the depth of the surface, and thus out of the intended loaded functional area. This is also a desirable effect, as pores or defects directly below the surface are the most critical ones [6].

**Figure 5.** (**a**) Section view of the cladded workpiece welded with a single seam and (**b**) of a near-net-shaped bearing washers after forging (etched with nitric acid solution).

## *4.3. Heat Treatment*

To adjust the required strength and hardness of the bearing washers, a heat treatment in terms of quenching and tempering was carried out after the machining. For this purpose, the bearing washers

were placed in a self-designed hardening box with neutral annealing coal. The box was equipped with a thermocouple and preheated to 850 ◦C in an electrically heated chamber furnace. The bearing washers were austenitized at this temperature for 45 min (heating + holding time). In addition to the neutral annealing coal, argon was used to further prevent decarburization of the parts. To avoid stress cracks, quenching was carried out in an oil bath at room temperature. Subsequently, the bearing washers were tempered at 150 ◦C for 1 h to reduce brittleness and internal stresses, and achieve a target hardness of 60 according to Rockwell C (HRC) (measured on industrial bearing raceway).
