# The Influence of Grain Size on the Abrasive Wear Resistance of Hardox 500 Steel
## 1 Introduction  
Abrasive wear is a process in which the surface layers of interacting components degrade through the detachment of material particles, resulting in a loss of mass and thickness (dimensional reduction), as well as alterations to the structure and physical properties of the surface layer. For materials in these components, essential properties include high hardness, resistance to dynamic loads, and brittle fracture resistance, alongside compatibility with specific metal joining and processing techniques [1]. Among the materials with improved resistance to abrasive wear, high-strength martensitic boron steels are a leading group [2, 3]. These steels are widely applied in machine components exposed to abrasive wear. The use of boron in steel production, however, has a historical background, with initial studies on boron's effects on steel properties dating back to 1907. Early experiments added excessive amounts of boron (0.2-1.5% by weight) to the melt, which produced undesirable results due to the formation of coagulated intermetallic boron phases that lowered mechanical properties [4]. In 1921, researchers attempted boron additions at lower concentrations, leading to the development of self-hardening steels. Nevertheless, inconsistent results,attributed to the low metallurgical purity of available ores, hindered the widespread adoption of these promising materials. This situation persisted until 1935, when an American steel mill introduced an Al-Si-Zr-Ti ferroalloy during steel deoxidation. The addition of these elements mitigated boron compounds' adverse effects on hardenability, facilitating the production of bainitic steels with tensile strengths (\(R_{m}\)) up to 1200 MPa. In this case, boron was an incidental addition due to insufficient ore purification [5]. Limited by the research technology of the era, scientists were unable to fully explain boron's mechanism on hardenability. However, during World War II and later, the Korean War, boron microalloyed steels saw increasing use, as shortages of costly alloying elements like nickel, chromium, and molybdenum drove a shift towards less expensive alternatives. Today, economic pressures continue to motivate the use of boron in steel, with applications across industries like mining, agriculture, construction, and machinery, where components undergo intensive abrasive wear [4; 6]. The influence of boron on microstructural properties, specifically as defined by the grain size of the prior austenite, also warrants examination. Although martensitic boron steels are classified as fine-grained, with an initial grain size of approximately 20 \(\upmu\)m as delivered from the mill [7], these materials are subject to heat treatments that may lead to grain growth, such as during welding of components made from them [8]. It is essential to consider that martensite has a hierarchical structure, comprising packets, blocks, and laths within the prior austenite grain. Research suggests that boron positively affects grain refinement by reducing bainite lath thickness [9], facilitating high tensile strengths (\(R_{m}\)) exceeding 2000 MPa in mass-produced steels [10]. Nonetheless, boron may have adverse effects on the examined microstructural aspects if improper processing of these steels leads to the formation of boron nitrides (BN) [11; 12]. The most effective inhibitors of grain growth are primarily nitrogen compounds with titanium (TiN), which remain stable even at temperatures of 1150 \({}^{\circ}\)C [13], as well as nitrogen compounds with aluminum (AlN). The microstructural aspect of grain boundary strengthening is crucial in enhancing steel strength, as described by the Hall-Petch relationship. According to this theory, increasing the yield strength at room temperature is achievable by reducing grain size within the microstructure [14; 15; 16]. This mechanism relies on grain boundaries to hinder dislocation movement by introducing changes in the crystal lattice orientation and atomically mismatched regions.