The bolsters and side frames of railway vehicles are the key parts of the running parts of the vehicles, and their quality directly affects the safety of railway transportation. With the acceleration of railway vehicles, the quality requirements for bolsters and side frames are also increasing. Due to the complex thin-walled structure of bolsters and side frames, defects such as deformation and cracks are easily generated in the casting process. However, a reasonable casting process often requires repeated cycles of “design, test pouring, calibration, and optimizationâ€. This often requires a lot of manpower and material resources and time. Computer numerical simulation can greatly simplify this process, which not only saves costs but also greatly shortens the time for process optimization. Therefore, it is imperative to use numerical simulation to arm the foundry industry, which is now relatively backward.
1 Overview of bolsters and side frames The structures of bolsters and side frames are complex and box-shaped sections are large thin-walled parts. In the casting process, due to the uneven wall thickness, it is impossible to simultaneously solidify during cooling, the local thermal stress is large, and the tendency to form hot cracks is large. Moreover, due to the large profile and complex structure of the castings, the castings are easily deformed and easily cracked. Most of the cracks are tortuous and irregular in shape. The surface is wide and the inside is narrow, extending from the surface to the inside. The section is oxidized and has no metallic luster. After the cracks are removed, there are no other casting defects such as sand inclusions, pores, and shrinkage holes. From these characteristics of the crack, it can be judged that the crack belongs to a cast hot crack. The bolster and side frame key detection locations are shown in Figure 1, Sections A and B.
Figure 1 Schematic diagram of the side frame focus detection
Figure 2 Schematic diagram of the key detection area of ​​the bolster
2 Status Quo of Numerical Simulation of Thermal Cracking At present, the models for thermal cracking prediction can be simply summarized as the following three models:
(1) Thermal crack simulation based on one-dimensional model of resistance;
(2) Thermal cracking simulation based on solidification conditions and feeding capacity;
(3) Thermal cracking simulation based on the high temperature stress and strain field of the casting.
2.1 One-Dimensional Impedance Model for Hot Cracking of Castings The one-dimensional model of hot cracking of castings is based on the idea of ​​concentrated deformation and restraint at both ends. Isobe, T. used thermal zone deformation and the critical deformation of the alloy to determine the formation of hot cracks as a function of temperature; Kubota, M. proposed the critical parameters determined by the high temperature mechanical properties of the alloy and the heat determined by the cooling conditions. The cracking parameters are compared to analyze the thermal cracking, or the ratio of the cooling rate at the center of the casting to the cooling rate at the outer edge is used as the thermal cracking criterion. The thermal cracking simulation of a one-dimensional model of resistance mainly takes into account the thermal cracking conditions at both ends of the test bar, which is a considerable distance from the actual situation.
2.2 Solidification and Filling Models for Hot Cracking of Castings The thermal cracking solidification feeding model is mainly used to predict the hot cracking of castings from the solidification or feeding conditions of the castings, which is based on the work of Clyne.TW and Feurer.U. Clyne.TW divides the solidification process into stress relaxation and cracking. The possibility of thermal cracking is thus determined from the ratio of the time intervals between the stress relaxation phase and the cracking phase. Feurer.U believes that the thermal cracking is due to the solidification shrinkage of the alloy can not be fully formed. He used Darcy's theory of flow in porous media to calculate the interdendritic fluid's retraction ability, and compared the obtained retraction ability with the alloy's solidification shrinkage, and proposed that if the retraction ability is greater than the solidification contraction, no thermal cracking occurs. On the contrary, there is thermal cracking.
2.3 Numerical Simulation of Thermal Cracking Based on Stress and Strain Field of Castings The thermal cracking model based on stress-strain field mostly uses finite element software developed or large-scale engineering finite element software to simulate high temperature stress-strain behavior.
3 Test contents and results Five slats ranging in length from 300mm to 900mm were designed on the test casting structure. Both ends of the slats pass through the runners and the block structure, so that their solidification shrinkage is hindered. The longer the slats, the greater the stress strain produced by the solidification shrinkage, and the easier it is to generate hot cracks at the hot junction where the slats and runners meet. According to the cracks at the hot section, the critical value of the hot cracking tendency of the ZG25MnNi casting at 1565°C is finally determined.
The test uses the same critical materials as the bolster and sideframe casting process, the casting mold, and the pouring temperature for the purpose of finding the critical value of the bolster and side frame hot tearing tendency. The specific test conditions are as follows:
Pouring temperature: 1565 °C;
Casting material: ZG25MnNi;
Casting material: organic ester water glass sand;
Cold iron material: A3 steel.
The test plan process map is shown in Figure 3. Figures 4, 5 and 6 are actual drawings of castings. Figure 4 shows the macro appearance of the two-box casting. The main part of the macro-deformation of the casting is indicated by a red line in the figure. Figures 5 and 6 are close-up views of the crack areas of test pieces #1 and #2, respectively. The red lines in the figure indicate the area where hot cracks can be observed with the naked eye.
Fig. 3 Test plan Process chart
Fig. 4 Macro outline drawing of two box castings
Fig. 5 Close-up view of the crack area of ​​test piece No. 1
Fig. 6 Close-up view of test piece No. 2 crack area
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