## 2 Experimental Setup and Methodology  
For the experimental investigations in this work, referring to previous studies, the material 50 CrMo4 (SAE 4150) is used [1; 22; 40; 41]. For the sake of completeness, mean values of the chemical composition are documented in Table 1. These values were determined by spark emission spectroscopy analysis [1]. The initial material condition is pre-hardened. Different types of cylindrical specimens are heat-treated in order to form a homogeneous and purely martensitic microstructure. In Figure 1a, the three types are depicted with their nominal dimensions. Types A and B are analyzed with regard to the morphology of their martensite structure. The reason for the hole in type B is that, due to the reduced mass, a higher quenching rate is achieved with the same quenching medium. Specim type C only serves as an accompanying specimen to measure the temperature inside. The temperature is measured with a thermocouple (type K), which is inserted and fixed in the blind hole of specimen type C.  
In order to achieve different quenching rates, two different heat treatment processes were used. The first was performed with the dilatometer DIL 805A from TA Instruments. Specimens of type A and B were heated by induction at a rate of 20.0 Ks\({}^{-1}\) to a temperature of 860 \({}^{\circ}\)C. A vacuum in the chamber containing the specimen prevented the decarburization of the surface layer of the specimen. For supercooling, the chamber was flooded with nitrogen, where a linear temperature curve was specified. The temperature was measured with a type S thermocouple. With this experimental setup, the critical quenching rate necessary to obtain a fully martensitic microstructure was determined step by step with specimen type A. Furthermore, the highest cooling rate possible with this test configuration was tested with specimen types A and B. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline
**Material** & **C** & **Si** & **Mn** & **P** & **S** & **Cr** & **Mo** \\ \hline
50 CrMo4 [1] & 0.52 & 0.26 & 0.74 & 0.014 & 0.008 & 1.31 & 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the investigated material 50 CrMo4 (SAE 4150) in wt.%; the remainder is Fe. Figure 1: Specim geometry and measurement methodology, (**a**) Specim types with nominal dimensions of material 50 CrMo4 (SAE 4150); specimen type C is only used for temperature measurement. (**b**) Specim handling after heat treatment. EBSD: electron backscatter diffraction. As the quenching rate of the dilatometer was limited, a second experimental setup was used. With this setup, specimen types A and C were heated in a furnace and then quenched in a water bath. The temperature inside (1) and outside (2) the specimen was measured; see Figure 1a. With both experimental setups, a holding time of 30 min at high temperature was employed to guarantee full austenitization. After heat treatment, the specimens were divided crosswise as depicted in Figure 1b. In order to check the homogeneity of the microstructure, hardness measurements were taken on the new surface highlighted in gray with the low-load Vickers hardness tester KB30 BVZ. Two runs of measurements were performed in each case. Before each run, the specimens were ground and polished (final polishing step: 3.0 \(\upmu\)m diamond suspension). A second cut lengthwise exposed the surface on which the microstructure was characterized; see Figure 1b. For the EBSD analyses, the surface was ground with SiC sand paper with grades up to 4000 followed by diamond polishing with suspension sizes of 3.0 \(\upmu\)m and 1.0 \(\upmu\)m. The final polish was performed with a 0.06 \(\upmu\)m aluminum suspension (Eposal). The EBSD measurements were performed with the scanning electron microscope Zeiss Supra 55 VP. Areas of 114 \(\upmu\)m \(\times\) 85 \(\upmu\)m with a step size of 0.15 \(\upmu\)m were measured. Only phases that could be assigned to martensite or retained austenite were indexed.