## 4 Discussion  
In the present investigation, specimens were heat-treated with comparable austenitization conditions but different quenching rates. A typical partitioning was omitted in order to isolate the quenching process, which is responsible for the formation of martensite. Hardness measurements were used to prove that a homogeneous microstructure was present. Accordingly, the specimens were chosen to be small enough to be able to neglect the influences of the temperature gradient in the specimen. In accordance with the current state of knowledge, a higher hardness was achieved with a more intensive quenching; see Figure 4.  
Since microstructure size effects can contribute to an increase in hardness, we investigated how the quenching intensity affects the microstructure scales. For the same material, an influence of the cooling rate on the resulting block size was determined in [22]. However, with a comprehensive evaluation of the sizes of the hierarchical microstructure, no significant systematic influence of the cooling rate could be identified. The difference could be due to the size of the measurement domain, which was chosen to be larger in the present investigation. Furthermore, in [22], the authors do not describe how the values which were considered representative for the block sizes were evaluated in detail. Figure 8: Packet cross-section area depending on the prior austenite grain (PAG) cross-section area: (a) measured data, (b) representation of measured data with fitted curves. Figure 9: Martensite block cross-section area depending on the prior austenite grain (PAG) cross-section area: (a) measured data, (b) representation of measured data with fitted curves. Due to the comparable austenitization conditions, the PAGs were expected to have the same appearance in size and shape. In order to verify this, the mean and median values of the areas were analyzed. It was shown that the median values were very similar and the mean values differed. This was due to the size of the measurement domain. However, in this study, our focus was on martensite; a possible influence of the austenite grain size was considered by the presentation of the data in relation to the austenite grain cross-section areas; see Figures 8 and 9. The following can be derived from these analyses. The packet size as well as the number of packets is dependent on the austenite grain size. This is in agreement with the concept of pre-existing nuclei of martensite, representing crystal lattice defects in the parent austenite phase [47]. Assuming that packets are generated from these kind of nucleation points, the probability of lattice defects increases with the size of the PAG. Considering fitting curves in order to evaluate the holistic trend of block sizes with regard to PAG sizes reveals a slight dependency, which decreases with increasing \(A_{\text{PAG}}^{\text{2D}}\). This is consistent with the Hall-Petch concept of grain boundary strengthening, where a term of type \(k\,d^{-0.5}\) is added to the initial yield strength. In this term, \(k\) represents a material specific constant and \(d\) is the diameter of the grain. Thus, the results in [48] can be confirmed, where the effect of the austenite grain size on the martensitic transformation in stainless steels is investigated by simulation. The observations of the size effect in this work are in contrast to the phenomenon observed in the majority of the literature, in which martensite sizes become smaller with increasing cooling rates [19; 20; 21; 22; 23; 24]. This motivates a detailed discussion of the differences. For this purpose, relative differences of the equilibrium temperature and martensite start temperature \((T_{0}-T_{\text{MS}})/T_{0}\) as well as the chemical driving forces \(\Delta G_{\text{chem}}^{\text{MS}}\) necessary to initiate the martensite transformation are calculated with Thermo-Calc TCFE 8 and compared. These values give an indication of the size of the energy barrier, which must be overcome.