# The Influence of Grain Size on the Abrasive Wear Resistance of Hardox 500 Steel
## 3 Results and Discussion
### Microstructural Analysis
The average grain size of prior austenite in the as-delivered state is 15 \(\upmu\)m, similar to that observed after austenitizing at 850 \({}^{\circ}\)C and 900 \({}^{\circ}\)C. In these states, the mean values closely align with the median size of 14 \(\upmu\)m. Increasing the austenitizing temperature to 1000 \({}^{\circ}\)C results in significant grain growth, with an average grain size of 41 \(\upmu\)m (an increase of 173% compared to the previous states) and a greater difference between the mean and median, which rises to 31 \(\upmu\)m (a 130% increase). These noted differences in parameters clearly indicate selective growth of certain grains, identifying 1000 \({}^{\circ}\)C as the threshold temperature for grain growth, especially as the maximum recorded grain size reaches 158 \(\upmu\)m.  
Raising the austenitizing temperature by an additional 100 \({}^{\circ}\)C further intensifies boundary migration processes, leading to an increase in the average grain size to 68 \(\upmu\)m and the median to 60 \(\upmu\)m (with observed grain sizes ranging from 17 to 156 \(\upmu\)m). Compared to the previous state, these sizes represent an increase of 66% and 93%, respectively. At the highest applied heat treatment temperature (\(T_{A}\) = 1200 \({}^{\circ}\)C), the average austenite grain size rises to 115 \(\upmu\)m, with a maximum size of 312 \(\upmu\)m and a minimum of 31 \(\upmu\)m. The relationship between grain size and temperature, including data from the as-delivered state, can be approximated by an exponential distribution, as shown in Figure 3. This visualization also reveals additional patterns: starting from the austenitizing temperature of 1000 \({}^{\circ}\)C,the difference between the smallest and largest grains increases markedly, ranging from 139 to 281 \(\upmu\)m, reaching its peak after austenitizing at 1200 \({}^{\circ}\)C. Similarly, the interquartile range, or the difference between the third and first quartiles, also expands. Overall, the increase in average grain size from 850 to 1200 \({}^{\circ}\)C is 667%, with median growth of 664% and a maximum value increase of 743%. Figure 3: Microstructure of Hardox 500 steel in its as-delivered state and after heat treatment at various austenitizing temperatures: (**a**) as-delivered state; (**b**) T\({}_{\rm A}\) = 850 \({}^{\circ}\)C; (**c**) T\({}_{\rm A}\) = 900 \({}^{\circ}\)C; (**d**) T\({}_{\rm A}\) = 1000 \({}^{\circ}\)C; (**e**) T\({}_{\rm A}\) = 1100 \({}^{\circ}\)C; and (**f**) T\({}_{\rm A}\) = 1200 \({}^{\circ}\)C. LM, etched with reagent No. 3 per ASTM E407. Differences in martensitic microstructure morphology prompted a comparative analysis of prior austenite grain growth under different austenitizing conditions. Based on micrographs depicting prior austenite grains (Figure 5a-f) and the grain size distributions in each heat treatment condition (Figure 6a-f), unique morphological characteristics of the obtained microstructures can be illustrated. In the as-delivered state, the small grains display uniform size (Figure 5a), with the distribution approximated by a log-normal model. A similar uniform and fine-grained microstructure is observed after austenitizing at 850 \({}^{\circ}\)C and 900 \({}^{\circ}\)C (Figure 5b,c), with grain size analysis indicating dominant grain sizes in the ranges of 10-17 \(\upmu\)m and 9-23 \(\upmu\)m, respectively (Figure 6b,c). Significant microstructural changes appear after heat treatment at 1000 \({}^{\circ}\)C (Figure 5d), where substantial grain growth occurs, with grain sizes ranging from 7 to 158 \(\upmu\)m (Figure 6d). This state is characterized by heterogeneous grain growth (abnormal growth), resulting in a marked division of the microstructure into larger grains surrounded by colonies of smaller grains. Moreover, the resulting distribution does not match any theoretical distribution model. After heat treatment at 1100 \({}^{\circ}\)C (Figure 5e), grain growth intensifies further, leading to a clearly coarse-grained microstructure.