**4. Results**

The results of the microstructural examination are summarized in Figure 8. In both types of PTAW-cladded workpieces made of 41Cr4/C22.8 (Figure 8a,b), the clad material (marked as CM) shows a pearlitic microstructure with ferrite along the former austenitic grain boundaries. The base material (marked as BM) has a ferritic-pearlitic microstructure with a prevailing ferrite fraction typical for low eutectoid steels. Close to the joining zone, a needle-shaped Widmanstätten structure is visible, which is induced by the high cooling rates after the cladding process. The forging process has a positive influence by completely transforming the coarse-grained weld microstructures. A smooth transition between cladding and substrates is visible. Grain refinement by recrystallization due to thermomechanical treatment took place at both investigated positions A and B (Figure 8d–h).

In the LHC workpieces combining X45CrSi9-3 and C22.8, the cladding material X45CrSi9-3 mainly consists of martensite and nodular-shaped pearlite. The substrate contains a mixture of ferrite and pearlite (Figure 8c). However, the Widmanstätten structure, which appears in the substrate close to the joining zone after cladding, is less pronounced compared to the 41Cr4/C22.8 workpieces. This can be explained by lower heat input during LHC in comparison to the PTAW, resulting in a lower thickness of the heat-a ffected zone (HAZ). Probably, there was also some di fference in the cooling rates as well as in the start temperature of the transformation. In the transition zone, a pearlite interlayer with a thickness up to 50 μm can be observed, which is retained after forging.

Contrary to the 41Cr4/C22.8 workpieces, the micrographs of the combination X45CrSi9-3-C22.8 show some microstructural di fferences at positions A and B (Figure 8f–i). While the substrate shows grain refinement in both cases, the former austenitic grains of the cladding material have a coarser structure in position A than in position B. This means that the cladding of X45CrSi-9 is more sensitive to the temperature gradient between lower and upper part of the bevel gear (Figure 4) and the strain di fferences than the cladding of 41Cr4. The higher forming temperature at position A can result in intensive grain growth, which cannot be compensated for by recrystallization due to the lower strain at this position.

The heat treatment showed the desired impact on the microstructure in all cases. Due to the high cooling rates during spray quenching, a fully martensitic microstructure free of pearlite was achieved in both claddings. The subsequent self-tempering, applied to reduce brittleness and residual stresses in the hardened microstructure, tempers the martensite in the cladding layer as depicted in Figure 8j–o. A similar microstructure is also present in the partially heat-a ffected substrate close to the joining zone, providing a smooth transition between cladding and substrate. Moreover, the pearlitic interlayer observed in the material combination X45CrSi9-3/C22.8 was fully suppressed by the heat treatment.

The average hardness values given in Table 2 are in line with the microstructural features described above. The hardness values of X45CrSi9-3 were in all cases higher than those of 41Cr4, since at least some fraction of martensite forms even for the low cooling rates when cooling in still air. For both investigated material combinations, the hardness of the substrate and of the cladding material remained at the same level after forging as after cladding. However, the heat treatment resulted in a substantial increase in the material strength, and the tempered martensite in the cladding layer showed hardness values above 500 HV 0.5. For X45CrSi9-3, a maximum hardness of approx. 730 HV 0.5 was achieved at position B and of approx. 590 HV 0.5 at position A. This variance can be attributed to the microstructural di fferences observed between positions A and B.

**Figure 8.** Microstructural evolution in the material combinations 41Cr4/C22.8 and X45CrSi9-3/C22.8 after cladding (**<sup>a</sup>**–**<sup>c</sup>**), forming (**d**–**i**) and heat-treatment (**j**–**<sup>o</sup>**); etched with 5% nitric acid solution excluding (**<sup>c</sup>**,**f**,**i**)—etched with Beraha II reagent—and (**l**,**<sup>o</sup>**)—Etched with V2A etchant.

The average tensile strength values for all investigated conditions are summarized in Figure 9. In all specimens, fracture in the hybrid samples occurred within the gauge section on the side of the base material with the lower strength. Thus, the measured values are not labeled as bond strength but as tensile strength. In analogy to the hardness values, the strength values for the conditions after cladding and after forming show no substantial differences (Figure 9a–c), and the strength values at positions A and B are similar. The values for mono-material specimens made of 41Cr4 (marked grey) are considerably higher after forming than those of the hybrid specimens. This can be attributed to the elevated mechanical properties of the steel 41Cr4 in comparison to the C22.8 substrate, in which all hybrid specimens fractured.

**Figure 9.** Tensile strength values of cladded workpieces (**a**), forged bevel gears at position A (**b**) and B (**c**), heat-treated bevel gears at position A (**d**) and B (**e**).

The integrated heat treatment increased the material strength as depicted in Figure 9d,e. In comparison to the condition after forging, the strength values of the samples of 41Cr4/C22.8 (marked green and orange) doubled and were close to those of the steel 41Cr4. The samples of X45CrSi9-3/C22.8 show a lower strength. This can be attributed to higher self-tempering temperatures, and thus a lower hardness of the substrate and an increased hardness difference between cladding and substrate compared to 41Cr4/C22.8. Despite the microstructural differences at position A and B, the corresponding strength values do not differ substantially from each other.
