Force condition of the casting for the clamp
As shown in the figure, the hydraulic brake caliper is an executive component of the brake system, which is subjected to the dynamic load and impact force of the brake disc during braking. The main frame structure of the caliper, the bracket, is a cast part, which has been optimized in structure and reduced by 32% in mass. The force-bearing condition is shown in the figure. On the one hand, the bracket bears the axial clamping force exerted by the lever on the brake disc; on the other hand, it is subjected to the tangential friction force of the brake disc. Based on the force-bearing condition of the caliper, firstly, a simulation calculation is carried out for the caliper using the pre-processing software HyperMesh, with the solver using OptiStruct, and using second-order tetrahedral elements for simulation. The loading and constraint methods are shown. The clamping force is applied vertically to the contact surface of the lever and bracket, and the friction force is applied to the mounting hole of the bracket. The displacement constraint is at the mounting interface between the support frame and the vehicle.
Evaluation of the influence of steel casting defects on strength
Unlike ductile iron, cast steel materials have their own characteristics such as large shrinkage and poor fluidity during casting, which can easily lead to defects such as shrinkage cavity, sand hole, and slag inclusion in complex structural components during the casting process. Although the structural characteristics of the bracket are relatively complex, some defects can be avoided by optimizing the pouring and overflow system and placing internal and external chillers reasonably. However, some areas still have casting defects. There are several slag inclusion defects on the four threaded interface planes of the bracket, as shown in Figure 4. The simulation stress value in this area is 25.3 MPa, which belongs to the low stress risk area and therefore does not require welding repair. During the roughcast stage, it was found that there was a shrinkage defect near the bottom corner of the lever support (shown in the figure before the defect was treated and welded). From the force exerted on its characteristic points, it can be seen that during the clamping brake process, the lever joint acts as a fulcrum for the lever to withstand large tensile stresses. If there are casting defects, they may extend inward during repeated stressing over a long period of time, which may lead to fatigue failure risks. The simulation stress value in this area is 163.2 MPa, which belongs to the medium stress risk area. As the junction of three surfaces, it is difficult to solve the defect problem by casting process, so it is necessary to consider using welding repair process to compensate for the defect problem. However, E-grade steel is a high-strength steel with high carbon content, which has poor weldability. The presence of weld defects can lead to stress concentration and crack source expansion, resulting in structural damage. Whether the bracket can withstand the strength and impact during clamping use after welding repair requires further experimental verification.
Test for welding defects
According to the requirements of ISO 148-1:2016 “Metallic Materials – Charpy Pendulum Impact Test – Part 1: Test Method (E)”, several impact test blocks were processed, with 50% each of the weld and base metal thickness. The impact test results are shown in Table 3. It can be seen that the average impact energy of the test block is around 39.5 J, which meets the requirement of not less than 27 J for the base metal. Therefore, the impact performance of the test block after welding repair did not decrease.
The impact of welding repair on the bracket under impact conditions was tested by conducting an impact vibration test using the assembled complete set of clamps as the test specimen. According to the requirements of the industry standard IEC 61373-2010 “Railway Equipment – Rolling Stock Equipment – Impact and Vibration Testing”, the clamps mounted on the bogie frame were subjected to a Class II impact vibration test, including vertical, lateral, and longitudinal simulation long-life tests, impact tests, and functional random tests at enhanced random vibration levels.
In order to verify the fatigue strength of the bracket after welding, the assembled complete set of clamps is used as the test piece, and a fatigue strength test bench is built. The clamp fatigue test bench is shown in the figure, which consists of an electrical control cabinet, a hydraulic pump station, a test piece installation platform, and connecting pipelines. The electrical control cabinet is used to output control signals and control the pressure of the hydraulic pump station to output the required target oil pressure. The clamp installation interface position applied to the installation platform is the same as that on the vehicle, and the simulated brake disc is a lever structure. The force applied to the lever through the oil cylinder is converted into torque to the clamp, simulating the torque of the brake disc during braking conditions. Therefore, this fatigue test bench can achieve a strength test of the composite clamping force and radial friction force of the bracket by using the clamping force of the test piece during braking and the torque of the lever.
Conclusion
Based on the fact that complex castings made of E-grade steel materials for locomotives and vehicles are prone to casting defects such as shrinkage cavities and slag inclusions, the stress zone classification of the castings is carried out through theoretical simulation methods to evaluate the required welding areas. For areas with high stress and prone to defects, welding repair treatment is required. The welding repair bracket is tested and verified from three aspects: static strength sample tensile test, sample and bracket impact test, and complete machine clamp fatigue test. The results show that the tensile fracture of the E-grade steel part after welding repair does not occur at the welding repair site; the impact energy of the test block still meets the material standard requirements, and no deformation, cracks, or other defects occur at the welding repair site after impact vibration of the test piece; the fatigue strength test results simulating real vehicle operating conditions show that no crack propagation occurs at the weld after long-term durability testing. The theoretical experimental research in this article provides a theoretical basis for the application of high-strength steels such as E-grade steel in casting defects.The evaluation of the issues in the research provides a certain reference basis; it achieves weight reduction for the structural topology optimization of the casting parts, and provides feasibility for solving the strength problem with high-strength steel.