# Mechanism and Effect Factor of Toughening of High-Speed Train Wheels
## 1 Introduction  
When a train's speed exceeds 200 km/h, the dynamic conditions of the train change significantly. The increased speed puts greater stress on the high-speed wheels, leading to notable changes in their operating conditions. Consequently, the main failure mode for these wheels after the speed increase is fatigue stripping of the tread. To ensure the safe and stable operation of the wheels, they must possess high strength, resistance to stripping, and overall fatigue resistance. Therefore, high-speed train wheels are characterized by their high technological content, added value, and associated risks. They impose higher demands on toughness and plasticity than conventional wheels (below 160 km/h). Research has shown that enhancing wheel toughness, especially fracture toughness, is the key to the production of high-speed train wheels [1; 2; 3].  
High-speed train wheels are usually manufactured using high-quality carbon steel with a carbon content ranging from 0.48 to 0.60%. These wheels are formed through a combination of forging and rolling processes [4]. Due to the wear and fatigue resistance requirements, high speed trains often need to exhibit higher strength and hardness. As the train speed increases, especially when it surpasses 200 km/h, there is a significant rise in traction power, dynamic forces between the wheel and the rail, and braking power, which puts higher demands on the toughness of the high speed train wheel [5; 6; 7].  
According to research studies [8; 9; 10], when the carbon content of the wheel steel grade is the same, high-speed train wheels require higher hardness than ordinary wheels. At the wheel wear limit, the hardness is about 10 HB higher. However, the toughness requirements remain the same. Currently, an effective method is to adopt a micro-alloyed compositiondesign to achieve high strength and hardness without compromising the toughness level of the wheel [11; 12; 13; 14; 15]. This study mainly focuses on investigating the fluctuation of fracture toughness in the case of micro-alloyed material.  
Ensuring the safety of high-speed wheels is an increasing concern. The braking process generates energy concentration, which, mixed with mechanical forces and heat, can lead to fatigue damage in the wheel tread [16]. Fracture mechanics research indicates that the most fundamental approach to preventing brittle fracture in wheels is to increase the allowable crack length and allowable stress of the wheel material, thereby improving its toughness.  
\[\mathrm{a_{c}}=(\mathrm{YK_{IC}}/\sigma_{\mathrm{c}})^{2}\]  
In the formula, \(\mathrm{a_{c}}\) represents the allowable crack length in the wheel, \(\sigma_{\mathrm{c}}\) denotes the allowable stress, \(\mathrm{K_{IC}}\) represents the fracture toughness, and \(\mathrm{Y}\) is the crack shape coefficient, typically ranging from 0.5 to 1.1. It is worth noting that improving the toughness of wheel materials is a shared goal pursued by various countries, considering the development trend of wheel materials. Therefore, it is necessary to analyze the factors influencing wheel toughness, such as smelting and physical metallurgy of wheel materials, microstructure control, etc. These factors can help identify methods for enhancing wheel toughness.  
Cracks in the wheel rim can lead to serious consequences [17]. Research shows that rim cracks generally originate from a depth of 10 to 20 mm beneath the wheel tread [18] and propagate around the rim under the action of contact fatigue loads. These cracks are difficult to detect before expanding out of the rim surface. Hence, the rate at which fatigue cracks propagate is an important factor affecting wheel safety. Fracture toughness testing is conducted to evaluate the ability of the wheel to resist crack propagation and assess the rate under fatigue loads. Given the non-uniformity of fracture toughness in high-speed wheels, this study focuses on understanding the reasons behind the very low fracture toughness observed in wheels and exploring methods to improve fracture toughness.