*4.5. Analytical Methods*

#### 4.5.1. Metallographic Investigations and Hardness Measurements

For microstructural characterization of the interface zone, cross-sections were extracted from the bearing washers in a radial direction. Metallographic investigations were carried out after deposition welding, hot forming, and heat treatment in order to examine the microstructural evolution. The samples were prepared metallographically by grinding, polishing, and subsequently etched with the reagen<sup>t</sup> Beraha I or 2% nitric acid solution (CRIDA Chemie, Wenden, Germany). To investigate the hardness distribution across the joining zone, hardness measurements were carried out after each process step. Hardness testing on a ATM Q10A+ (ATM Qness GmbH, Mammelzen, Germany) according to Vickers HV 0.5 standard was used for this purpose [20]. Hardness profiles were measured from the AISI 52100 surface of the bearing disc in the AISI 1022M base material. To compare the surface hardness of the tailored forming bearing washer with an industrial bearing washer, hardness measurements were carried out after the bearing washer was manufactured. The Rockwell HRC hardness test according to DIN EN ISO 6508 was used for this purpose [21].

#### 4.5.2. Residual Stress Measurements

Due to different thermal expansion coefficients of the materials, delayed cooling of the subsurface and core, as well as local structural transformations limited to the surface layer; residual stresses were caused after forging and the heat treatment process, which were modified by subsequent machining of the hybrid component [22,23]. Residual stresses have a significant influence on the operating behavior and therefore the lifetime of a component. The fatigue life of a cyclically loaded component is reduced by high internal tensile residual stresses because they lead to failure of the component by crack formation and propagation. Compressive residual stresses, however, can increase the component's service life by reducing crack initiation and propagation [24–26]. For this reason, it was necessary to analyze the residual stress condition of a component before the tribological investigations.

The residual stresses were determined radiographically using the sin<sup>2</sup>ψ-method [27,28]. A Seifert XRD 3003TT two circle X-ray diffractometer (Röntgenwerk Rich. Seifert & Co., Ahrensburg, Germany), equipped with a Cr tube and a spatially resolved detector, was used for this purpose. The point focus measuring spot was delimited with a 2-mm point collimator. The ω-axis of the diffractometer served as tilting axis ψ. For measurement on the martensitic phase, the tilt range of ψ varied from −45◦ to +45◦ with a total of nine tilt positions. To determine the net plane spacing *d*211 of the α-iron, the intensity was recorded over 2θ in the range between 144.0◦ and 164.8◦ with a step size of 0.2◦. The measurement time per step was 36 s. The maximum information depth of the X-radiation was τ*max* = 5.5 μm. For measurements at a greater distance from the surface, material was successively removed by electrolytic polishing. For the X-ray elasticity constants *s*1, 12 *s*2, and the reference values for the unstressed material, the values of the pure α-iron grid were assumed [29].The evaluation of the residual stress measurements was carried out with the RayfleX software (Version 2.501, Röntgenwerk Rich. Seifert & Co., Ahrensburg, Germany) from General Electric. After data reduction, the position of the diffraction reflections was determined using the parabolic fit method. The data reduction was carried out according to the following procedure: smoothing according to the Savitzky and Golay algorithm, left-sided background correction, intensity corrections, and a parabola fit with a threshold value of 70% of the maximum intensity. The measuring accuracy of the X-ray diffractometer specified by the manufacturer for flat sample geometries was σ = ±10 MPa.

## 4.5.3. Non-Destructive Examination

The nanomechanical properties of the bearing washers were investigated before forming and after finishing. The aim of the tests was to determine the frictional and mechanical surface properties of the hybrid bearing washer. Nano-scratch tests were carried out with a Hysitron TriboIndenter TI950 (Bruker Corporation, Billerica, MA, USA). A cono-spherical diamond tip with a radius of 300 nm was used. The measured values were used to determine the elastic and plastic deformation behavior and the penetration-dependent friction coefficient μ during ploughing. The measuring tip traveled along the specimen surface with linearly increasing normal load and constant speed. It was moved over the sample with electrostatic force resulting in a tangential force. As the normal force increased during ploughing (Scratch test), the tangential force also increased. By the change in capacitance, the system detected the tangential force in dependence on the normal force and the depth displacement. According to these data, the coefficient of friction μ based on Coulomb friction was calculated. In the tests, a maximum load of the tip of 1 mN was used. The surface profile was analyzed before and after the scratch test using Scanning Probe Microscopy (SPM). The travel length of the tip was about 8 μm. The test was divided in three steps. First, the initial surface profile was scanned by the tip under a low load on the scratch path (prescan). Next, the scratch was performed on the same route with a load magnitude of 1 mN (scratch). To determine the resulting plastic deformation, the scratch route was scanned again with a low force magnitude (postscan). By analyzing the prescan, scratch, and postscan profile, it was possible to record the elastic and plastic deformation by calculating the different penetration depths. The plastic behavior was calculated based on five measurements and the standard deviation was determined.

Non-destructive testing using scanning acoustic microscopy (SAM) with a modified PVA TePla SAM 301 system (PVA TePla, Wettenberg, Germany) was used to examine the components for sub-surface damages after production and after fatigue testing. Here, the scanner-mounted transducer generates a pulse into the sample in the ultrasonic frequency range. The sound pulse is partially or completely scattered and reflected by inhomogeneities in the material, which are then measured (pulse-echo method). Distilled water served as a coupling medium between transducer and samples. Penetrating oil was used as a corrosion inhibitor. The surface roughness of the raceway was measured tactilely with a Mahr Perthometer PCV (Mahr-Gruppe, Göttingen, Germany). The measurement was carried out in radial and in circumferential direction of the disc at five points each.

#### 4.5.4. Bearing Fatigue Testing

In order to investigate the performance of the hybrid bearing washers, bearing fatigue tests were carried out on a self-designed FE8 test rig according to DIN 51819 [30]. The test rig was originally used for the dynamic-mechanical testing of automotive and industrial lubricants. In the FE8 test head, two cylindrical roller thrust bearings of type 81212 were mounted on a single shaft, see Figure 7. Each bearing consisted of a bearing washer that was fixed in the housing and a washer that rotated with the shaft, as well as the rolling elements and cage. Only one tailored forming housing washer was used for bearing fatigue testing, while the other washers were taken from conventional 81212 bearings. For these investigations, 19 rolling elements mounted in a fiberglass-reinforced polyamide cage were used. The conventional rolling elements and washers were made from martensitic through-hardened AISI 52100. The force was applied by a disc spring assembly in axial direction and measured by a HBM C2 load cell (Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany) during mounting. For an initial functional test, the test parameters from previous tests with other cladding materials were applied [13,14]. Here, the axial load was 40 kN. Due to very high run times, the load was increased to 60 kN for the subsequent fatigue tests. The test parameters are specified in chapter 0. A circulating lubrication system with a filtration quotient of β10 = 200 according to ISO 16889 supplied each bearing with lubricant at a rate of 0.1 L/min via the housing side [31]. The lubricant used was a commercial gear oil based on synthetic polyalphaolefins with a viscosity grade of ISO VG 68 (Fuchs Petrolub SE, Mannheim, Germany). The oil contained, among other additives, extreme-pressure and anti-wear additives. Failure by spalling on the washer's raceway was manifested as a sudden increase in vibration, which was used as a termination criterion. A threshold of 150% of the steady state signal had to be exceeded for the test to shut down. The tests were carried out using the sudden death method.

**Figure 7.** FE8 test head with type 81212 cylindrical roller thrust bearing.

#### **5. Results and Discussion**

#### *5.1. Microstructure Evolution during Processing*

The micrographs, showing the transition of the cladding layer into the base material, are illustrated in Figure 8. Figure 8a shows the microstructure after deposition welding. The base material AISI 1022M has a ferritic-pearlitic microstructure with large grains. Close to the joining zone, needle-shaped Widmanstätten ferrite could be observed. The microstructure of the cladding material was mainly pearlitic. Figure 8b shows the microstructure of the hot-formed and air-cooled bearing washers. Here, the base material also consisted of a ferritic-pearlitic microstructure and the cladded material of a mainly pearlitic microstructure. However, the microstructure was more homogeneous and fine-grained compared to the welded state due to hot forming-induced recrystallization. The micrographs taken after hardening, i.e., quenching and tempering, are shown in Figure 8c. The microstructure of the cladding material was completely transformed into martensite during hardening by quenching, featuring a needle-shaped morphology. Martensite was also formed close to the interface zone in the base material, though the remainder of the base material was ferritic-pearlitic. The results from the hardness measurements (Figure 9) were in accordance with the metallographic investigations. The hardness profiles of the bearing washers after deposition welding and after forming were very similar, since the components were cooled in air after the respective process step and thus have not been subjected to any specific heat treatment to increase the hardness. After the targeted heat treatment by quenching and tempering of the bearing washers, a hardness of 880 HV0.5 was determined in the cladding layer and of 250 HV0.5 in the base material. The base material remained ductile after hardening, while the cladding featured high hardness values as desired to withstand the rolling contact loads. The lower hardness in the base material resulted from the low carbon content of the steel AISI 1022M. The hardness was examined again on the cutting surface after the process steps of deposition welding, forming, and hardening (see Figure 9). The results show that the mean hardness after deposition welding and the forming process did not di ffer in the cladding material. It can be observed that there was a gradient from the cladding material to the base material. The hardness in the cladding material increased again after targeted heat treatment of the bearing washer. While an almost defect-free material transition between the base material and the cladding material was achieved by upsetting, the subsequent heat treatment allowed to achieve hardness values for the tailored forming bearings that are similar to those oftheindustrialbearings,whichwerealsomeasuredat60HRC.

**Figure 8.** Micrographs of the joining zone: (**a**) After deposition welding; (**b**) after forming; (**c**) after a quenching and tempering heat treatment; (**<sup>a</sup>**,**<sup>c</sup>**) etched with 2% nitric acid solution; (**b**) etched with Beraha I reagent.

**Figure 9.** Hardness profiles of the hybrid bearing washers.

The chemical composition measured by spark spectroscopy showed that the carbon content was slightly below the values specified for AISI 52100 (0.87% instead of at least 0.93%, see EN 10132-4), c.f. Table 1. The reduced carbon content is attributed to the welding process, as the elemental content was slightly reduced due to the material dilution between the base material and the cladding material.

#### *5.2. Residual Stress State*

In Figure 10, the residual stress depth profiles and the corresponding peak halfwidths (full width at half maximum, abbreviated as FWHM) depth profiles of a machined hybrid axial-bearing washer are presented. The residual stress depth curves show that in both directions (circumference ϕ = 0◦ of the bearing washers as well as radially ϕ = 90◦) maximum compressive residual stresses occured near the surface of the hybrid component. With increasing surface distance, a very abrupt shift in the direction of tensile residual stresses was noticeable. A significant di fference in the residual stress profile depending on the measuring direction was not visible in the surface-near area. Only from a surface depth of approx. 50 μm did deviations occur in the residual stress profiles between radial and circumferential direction.

Taking a closer look at the associated FWHM in connection with the residual stress measurement, it is clear that a relatively similar course of the profiles can be seen here as well. The di fferences between the two measuring directions are not significant. It is apparent that the FWHM at the surface was high, then decreased with further increasing surface distance and reached a minimum at approx. 4 μm depth. With further increasing surface distance, the FWHM value increased again and reached the value of the basic structure. The area that was influenced by the grinding process was only minimal and lies approximately in the range between 0 and 50 μm. The FWHM was an indication of the plastic deformation and thus of the hardening state of the surface as a result of increased or decreased dislocation density. There was a proportional relationship between plastic deformation and FWHM, which means that, with increasing plastic deformation, the FWHM would also increase. The grinding process caused a softening of the material in the surface-near area. The reason for this could be the heat transfer during grinding. In recent research projects, references were made to the high gradient in the residual stress depth curve caused by the high temperature development during grinding of hard materials [32–34]. This would also explain the abrupt drop of the high compressive residual stresses.

**Figure 10.** Residual stress and full width at half maximum (FWHM) depth profile after machining.
