# The Influence of Grain Size on the Abrasive Wear Resistance of Hardox 500 Steel
## 3 Results and Discussion  
### Microstructural Analysis  
Figure 3a-f illustrates the microstructures of Hardox 500 steel in its as-delivered state and after quenching from temperatures ranging from 850 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C. In its as-delivered state, Hardox 500 steel exhibits a lath martensite microstructure characterized by a three-tier hierarchical structure of laths, blocks, and packets. The martensite laths forming a block share the same crystallographic orientation, indicating that they represent the same martensite microstructure variant. In contrast, packets consist of blocks with a uniform habit  
Figure 2: Shapiro-Wilk normality plot with statistics: (**a**) abrasive wear resistance, (**b**) hardness. DS, as-delivered state. plane, corresponding to the [111] plane of the primary austenite. Additionally, regions of coalesced quenched martensite are evident, which is attributed to the steel's low tendency for spontaneous tempering. This occurs because martensite blocks with the same habit plane and similar crystallographic orientation relative to austenite tend to overlap without intermediary phases, forming thicker structures. However, regions with stronger etching effects can be distinguished, potentially indicating fine-dispersed phases suggestive of lower bainite or tempered martensite. Furthermore, the fine-lath microstructure displays lighter bands resulting from thermomechanical treatment applied by the manufacturer (Figure 3a). The microstructure of the steel quenched from 850 \({}^{\circ}\)C (Figure 3b) exhibits similar morphological characteristics to the as-delivered state. However, this sample shows more regions of brighter martensite laths undergoing coalescence, with wider laths. In addition, laths subjected to more intense etching are interspersed, which may suggest chemical microsegregation resulting in carbon-rich micro-areas or fine-dispersed carbide phases in these regions, potentially indicating the presence of tempered martensite or lower bainite. As the austenitizing temperature increases, a gradual thickening of coalesced martensitic regions is observed, accompanied by a reduction in the presence of laths undergoing intense etching (Figure 3c). Notably, the microstructure remains fine-grained without the ability to distinguish prior austenite grain boundaries. In contrast, the grain boundaries of the former austenite become more distinct in the micrographs of the sample quenched from 1000 \({}^{\circ}\)C (Figure 3d). The morphological characteristics of this microstructure also indicate selective growth of martensite laths, packets, and blocks, marking this temperature as the onset of growth in these microstructural elements. Areas delineated by prior austenite grain boundaries exhibit needles that undergo intensive etching and contrast against a bright background, confirming the presence of more pronounced microsegregation. Moreover, the structure of the darker, carbon-rich regions shifts from a lath to a needle-like form, typical of steels with higher carbon content, further suggesting potential microsegregation of this element. The microstructures obtained after austenitizing at the two highest temperatures (Figure 3e,f) display large laths and needles, along with clear prior austenite grain boundaries. The contrast between the dark needles and laths and the bright background becomes even more pronounced. The described morphological characteristics of the martensitic microstructure, based on the obtained micrographs, are reflected in the quantitative assessment of prior austenite grain size and the qualitative evaluation based on the revealed grain boundaries, which will be discussed below. Based on the grain growth analysis of prior austenite (Figure 4), it can be concluded that the proposed heat treatment in the temperature range of 850-900 \({}^{\circ}\)C is safe for maintaining a fine-grained microstructure.