# The Influence of Grain Size on the Abrasive Wear Resistance of Hardox 500 Steel
## 2 Materials and Methods  
### Material  
The study utilized 10 mm thick sheets of Hardox 500 steel, supplied by an authorized distributor, STAL-HUT. The chemical composition analysis was conducted using the spectral method with a Leco GDS500A glow discharge emission analyzer. The following parameters were applied to enable ionization of the inert gas: U = 1250 V; I = 45 mA; 99.999% argon. The obtained results represented the arithmetic mean of at least five measurements. According to the chemical composition, Hardox 500 steel is classified as medium-carbon steel (C = 0.29% by weight), with the key elements enhancing hardenability being manganese (0.74% by weight), chromium (0.61% by weight), and a micro-addition of boron (0.0009% by weight). The inclusion of aluminum and titanium prevents an undesirable reduction in hardenability by minimizing boron depletion at grain boundaries through the formation of BN and B\({}_{2}\)O\({}_{3}\) compounds. These elements bind oxygen and nitrogen into intermetallic phases, thereby preserving the required boron content within the interstitial sites of the crystal structure. Niobium can also positively influence hardenability by inhibiting the precipitation of the Fe\({}_{23}\)(C,B)\({}_{6}\) compound [36], binding carbon into niobium and titanium carbonitrides (Nb, Ti) (C, N) or molybdenum and titanium carbonitrides (Ti, Mo) (C, N). However, niobium is not present in Hardox 500 steel. Additionally, the low levels of harmful elements (P and S) help maintain high mechanical properties. The chemical composition of the analyzed material is presented in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**C** & **Mn** & **Si** & **P** & **S** & **Cr** & **Ni** & **Mo** \\ \hline
0.29 & 0.74 & 0.28 & 0.007 & 0.001 & 0.61 & 0.06 & 0.018 \\ \hline
**V** & **Cu** & **Al** & **Ti** & **Nb** & **Co** & **B** & **Zr** \\ \hline
0.012 & 0.010 & 0.054 & 0.003 & – & – & 0.0009 & – \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of Hardox 500 steel.  
Heat treatments were carried out in gas-tight chamber furnaces (FCF 12 SHM/R by [PERSON], Jastrzebie-Zdr\(\acute{\text{o}}\)j, Poland) with a protective atmosphere of 99.95% argon. Austenitizing was performed at five target temperatures: 850 \({}^{\circ}\)C, 900 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, 1100 \({}^{\circ}\)C, and 1200 \({}^{\circ}\)C. Samples were held at the specified temperature in 120 min, followed by water quenching at 30 \({}^{\circ}\)C. Temperature and holding time were controlled using automated furnace settings, with deviations monitored and maintained within \(\pm\)2 \({}^{\circ}\)C and thermocoupled.  
For prior austenite grain size analysis, additional tempering was applied after austenitizing. The samples were heated to 550 \({}^{\circ}\)C, held for 30 min in the furnace, and subsequently cooled in the furnace. This tempering step was essential for carbide precipitation along the prior austenite grain boundaries, which enabled their visualization during etching with reagent No. 8, as specified in ASTM E407 [37].  
In contrast, for hardness testing and abrasive wear resistance evaluation, samples were not tempered after quenching. This was to preserve the as-quenched martensitic microstructure, as tempering significantly alters both the microstructure and mechanical properties. The lack of tempering ensured that the tested samples reflected the direct effects of austenitizing conditions, enabling accurate comparisons of wear resistance and hardness across the different heat treatments. Detailed parameters of the heat treatment are presented in Table 2.