# Simulation of the Thermal Behavior of Cast Iron Brake Block during Braking Maneuvers
## 1 Introduction  
Most of the freight trains operating in the EU are equipped with braking systems that use brake blocks rather than disc brakes, which, on the other hand, are adopted for passenger transport [1]. Failures of the elements composing the running gear and the braking system represent the greatest amount of the total costs of the wagon. If the failure is such to cause the wagon to stop during operation, in addition to the maintenance costs of the component, other costs must also be sustained, e.g., the recovery of the wagon on the track, the recovery of the transported goods, and the delivery delays to the customer. On freight wagons, braking torque is often achieved by means of shoes (made of cast iron or synthetic materials) which directly act on the surface of the wheel tread. Due to wear phenomena, brake blocks are the elements that are most frequently replaced. Obviously, wear phenomena also affect the wheel tread profile, and for this reason the maintenance activity of re-profiling the wheels is necessary to ensure an equivalent conicity, which guarantees a proper dynamic behavior [2, 3, 4]. On the other hand, the wheel wear due to the braking action also has a beneficial effect because it machines the profile by removing some types of irregularities, such as wheel flats, out-of-roundness, burns, etc. During braking operations, brake blocks are pressed against the wheel rolling surface and the kinetic energy and potential energy of the train are converted into heat, which is generated at the block-wheel interface [5]. The friction heat generated during braking is distributed, with a different proportion, between the shoe and the wheel. Several research works [6] have been carried out to study the distribution of heat as a function of the material of the block and of the geometry of the whole system, also considering the contribution of cooling due to the wheel-rail contact (rail chill effect) and to convection with ambient air. During braking maneuvers, the blocks and the wheels are subjected to strong thermomechanical stresses [7]. In recent years, composite organic and sinter brake shoes havebeen introduced on freight wagons to replace the traditional cast iron blocks, which have low replacement costs but high noise levels during braking operations, as they are prone to the generation of hot spots due to thermoelastic instability (TEI) phenomena [8; 9; 10]. The substitution of traditional cast iron shoes with composite ones aims at a considerable reduction of the noise levels produced during braking operations. One of the greatest benefits of composite shoes is that their composition can be changed by the manufacturer to obtain different mechanical, frictional, and thermal properties [11; 12]. Composite brake blocks are classified into LL and K types, depending on their friction level. The first type can directly replace the cast iron shoes without requiring modifications to the brake system, while the second one, featuring a higher mean friction coefficient, requires changes in the brake cylinder bore or in the rigging ratio. The main advantage of K-type brake blocks is that the friction coefficient has a stable behavior at varying vehicle speed and braking force. However, the use of composite shoes, especially the K-type, affects the temperature gradients on the wheel surface due to the different thermal conductivity of synthetic material with respect to cast iron. Therefore, a larger portion of the total friction heat generated at the wheel-shoe interface enters the wheel, and this can lead to changes in the wheel steel microstructure [13] as well as to residual thermal stresses and strains [14], which can cause crack growth and propagation and wheel warping, respectively, and the latter can increase the derailment risk. [PERSON] et al. [15] experimentally determined the friction coefficients of different shoe materials, highlighting that cast iron blocks are the ones that generate the lowest thermal load on the wheels of the rolling stock, while [PERSON] et al.