# The Influence of Grain Size on the Abrasive Wear Resistance of Hardox 500 Steel
## 3 Results and Discussion
### Abrasive Wear Resistance
Table 7 also includes calculated \(\frac{Ra}{Rz}\) values, all of which approximate 2.0, further supporting these conclusions regarding surface topography characteristics. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{ \begin{tabular}{c} **Shape** \\ **Parameter** \\ **Ratios [-]** \\ \end{tabular}  }} & \multicolumn{5}{c}{**Heat Treatment–Sample Designation**} \\ \cline{2-7}  & **DS** & **T\({}_{\textbf{A}}\) = 850 \({}^{\circ}\)C** & **T\({}_{\textbf{A}}\) = 900 \({}^{\circ}\)C** & **T\({}_{\textbf{A}}\) = 1000 \({}^{\circ}\)C** & **T\({}_{\textbf{A}}\) = 1100 \({}^{\circ}\)C** & **T\({  
The analysis of roughness parameters did not establish a strict correlation between changes in surface layer characteristics and the steel microstructure, defined by prior austenite grain size. However, a weak correlation with surface hardness was observed for several indices (Figure 15a-c). The strongest correlation with hardness was noted for the parameter describing the depth of the deepest indentation (\(Rv\)). An initial increase in the hardness of Hardox 500 steel led to a rise in the \(Rv\) parameter, followed by a decrease. This relationship was approximated by a quadratic function with an \(R^{2}\) value of 0.51 (Figure 15b). The quadratic function, though with a lower determination coefficient \(R^{2}\) = 0.43, also approximates the change in mean groove width (\(RSm\)) with hardness (Figure 15c). The weakest correlation with hardness was observed for the peak height (\(Rp\)) (Figure 15b) and the arithmetic mean of the absolute heights of the five highest peaks and five lowest valleys (\(Rz\)) (Figure 15b). All parameters, except \(RSm\), belong to the group of vertical (amplitude) indicators. According to [PERSON] et al. (2019), the \(Rq\) parameter is more sensitive to deep valleys and high formations on the surface than \(Ra\). An important observation from their study, which subjected steels with diverse microstructures and hardness ranging from 186 HV to 712 HV to impact-abrasive wear tests, was that softer materials exhibited significantly higher surface roughness than harder materials. Similarly, prolonged test durations yielded slightly higher \(Rq\) values, though these differences were less pronounced. These observations were only partially confirmed for Hardox 500, as the \(Rq\) parameter was lower for samples austenitized at 1100 \({}^{\circ}\)C and 1200 \({}^{\circ}\)C than in the as-delivered state, despite the higher hardness achieved in this state compared to the reference (Figure 15a). An increase in austenitizing temperature plays a pivotal role in altering the microstructural characteristics of Hardox 500 steel. At elevated temperatures, the dissolution of carbides such as MoCrC or Cr\({}_{7}\)C\({}_{3}\) that occur in Hardox 500 steel (Hardox 5000) takes place, releasing carbon into the austenite matrix. This process increases the carbon solubility in austenite, which enhances grain boundary mobility due to reduced pinning effects exerted by carbides. This phenomenon has been extensively documented in martensitic and low-alloy steels, where the dissolution of fine carbides directly correlates with abnormal grain growth ([PERSON] et al., 2018; [PERSON] et al., 2018). The redistribution of carbon within the matrix at high austenitizing temperatures also leads to microsegregation, forming carbon-rich regions observable as darker etched areas in micrographs. These regions may act as sites for carbide precipitation during slower cooling or tempering processes. Such carbides can nucleate at grain boundaries or within grains, depending on the cooling rate and chemical composition of the steel. The precipitation of secondary phases at grain boundaries may reduce the cohesive strength of the boundaries, increasing the material's susceptibility to wear and crack propagation. Grain boundary migration at elevated temperatures is a key driver of grain coarsening. The mobility of grain boundaries increases exponentially with temperature due to enhanced atomic diffusion.