## 3 Results
### Microstructures  
The hot-rolled sample (HR) showed a banded ferritic-pearlitic microstructure with no martensite present (Figure 2a) and with a hardness below 200 HV10. The absence of martensite and bainite was due to the low level of alloying combined with the fact that the hot-rolled material was coiled at around 700 \({}^{\circ}\)C, after which it cooled very slowly, which can take up to three days to cool down to room temperature. Figure 3a shows the resulting ferritic-pearlitic microstructure. This is a typical microstructure for plain carbon steels when quenching has not been applied. Figure 2: Optical micrographs of the hot-rolled (HR) microstructure and Gleeble samples with different cooling rates: (**a**) hot-rolled, (**b**) 3 \({}^{\circ}\)C/s, (**c**) 12 \({}^{\circ}\)C/s, (**d**) 96 \({}^{\circ}\)C/s. Microstructural evolution during tempering is seen in the quenched and tempered samples in the FESEM images in Figure 3b-f. Starting from the water quenched sample, the microstructure consisted of lath-like martensite comprising packets, blocks, and sub-blocks. The darkest auto-tempered martensite was the most dominant phase in the water quenched microstructure (Figure 3b), as is typical of other commercial medium carbon steels, such as some wear-resistant steels [19; 20; 21; 22; 23]. Auto-tempering is desirable for this type of steel in order to improve the impact toughness in the as-quenched state. Despite being subjected to rapid water cooling, the samples did not display any signs of immediate or delayed quench-cracking indicating that milder quenching media are not required for the 10 mm plate thickness concerned, at least. The effects of tempering begin to appear more distinctly at 150 \({}^{\circ}\)C. The fraction of precipitate-rich, light areas increased, but the precipitate size was generally very small. The sample tempered at 200 \({}^{\circ}\)C showed significantly less untempered martensite, i.e., the darkest areas without visible traces of precipitates, than the water quenched (WQ) and 150 \({}^{\circ}\)C samples. This is consistent with the first stages of tempering, i.e., precipitation of fine, presumably transition, carbides [12; 13]. Both the 150 \({}^{\circ}\)C and  
Figure 3: Field emission scanning electron microscope (FESEM) micrographs of the tested steels. Images taken from \(\frac{1}{4}\) depth from the top surface in normal-to-rolling direction (longitudinal). Samples: (**a**) hot-rolled, (**b**) water quenched, (**c**) tempered at 150 \({}^{\circ}\)C, (**d**) tempered at 200 \({}^{\circ}\)C, (**e**) tempered at 300 \({}^{\circ}\)C, (**f**) tempered at 400 \({}^{\circ}\)C.  
200 \({}^{\circ}\)C samples contained a high number density of fine transition carbides, but in the sample tempered at 200 \({}^{\circ}\)C the precipitates were notably larger, and their needle-like shape could be distinguished more easily. These differences result in the microstructure of the 200 \({}^{\circ}\)C sample showing more light contrast in the low magnification FESEM images. The water quenched (WQ) sample had more carbide-free areas while the auto-tempering had introduced some carbide formation (Figure 4a). In the FESEM images, this was also seen as the different depth of the areas as the regions could be assumed to have different carbon concentrations in the solid solution [24]. This supports the concept of martensite formation in rapid sequences rather than as a single massive transformation throughout the whole sample at once. The darker areas appeared as there would have been nearly a total absence of precipitates. Therefore, this martensite may be assumed to have had formed in the later stages of the phase transformation, having less time for the interstitial carbon to diffuse into precipitates.