Abstract
To understand the microstructure transformation characteristics of low alloy steel materials used in cast steel and forged steel brake discs for high-speed trains, the continuous cooling transformation (CCT) curves were measured using a DIL 805L dilatometer. The microstructure and microhardness of the brake disc materials were analyzed at various cooling rates. The results indicate that the austenite-to-ferrite-pearlite transition occurs at a cooling rate of 0.2 °C/s for the forged steel brake disc material, while the cast steel brake disc material attains complete martensitic transformation at a cooling rate exceeding 20.0 °C/s. In contrast, the critical cooling rate for complete martensitic transformation in the forged steel brake disc material is 50.0 °C/s. This study provides valuable insights into the microstructural stability and mechanical performance of brake discs during their service life.
1. Introduction
Brake discs play a crucial role in ensuring the safe and reliable operation of high-speed trains by absorbing and dissipating kinetic energy during braking. Cast steel and forged steel brake discs, fabricated from low alloy steels, are widely employed in various high-speed train models due to their exceptional mechanical properties and thermal stability. The microstructural evolution of these brake disc materials under complex thermal-mechanical loading conditions significantly impacts their service life and performance.
This study aims to determine the CCT curves of cast steel and forged steel brake disc materials used in high-speed trains. By analyzing the microstructure and microhardness at different cooling rates, we gain a comprehensive understanding of the phase transformation behavior and the critical cooling rates required for specific microstructural outcomes. This knowledge is essential for optimizing brake disc material selection and design to enhance their durability and performance.
2. Materials and Methods
2.1 Materials
The test materials were cast steel and forged steel brake disc samples sourced from high-speed trains. The chemical compositions of these materials are presented in Table 1.
Table 1. Chemical Composition of Cast Steel and Forged Steel Brake Disc Materials (wt.%)
| Element | Cast Steel | Forged Steel |
|---|---|---|
| C | 0.26 | 0.30 |
| Si | 0.58 | 0.40 |
| Mn | 1.04 | 0.75 |
| P | 0.014 | 0.009 |
| S | 0.011 | 0.007 |
| Cr | 0.90 | 1.20 |
| Ni | 1.00 | 0.40 |
| Mo | 0.55 | 0.65 |
| V | 0.10 | 0.30 |
2.2 CCT Curve Measurement
The CCT curves were measured using a DIL 805L dilatometer following the procedures outlined in standards YB/T 5127—2018 and YB/T 5128—2018. The samples were heated to 900 °C at a rate of 500 °C/h up to 500 °C, then at 180 °C/h until 900 °C, held for 10 minutes to ensure complete austenitization, and subsequently cooled to room temperature at various rates (0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0, and 50.0 °C/s) using a quenching device.
2.3 Microstructure and Microhardness Analysis
After CCT testing, the samples were mounted, polished, and etched with a 4% nitric acid-alcohol solution. The microstructures were examined using an optical microscope (ZEISS Axio Vert.A1) and a scanning electron microscope (ZEISS Gemini 500). Microhardness measurements were conducted using a VTD-512 microhardness tester.
3. Results and Discussion
3.1 Austenitization and Microstructure Transformation Temperatures
The austenitization temperature (Ac3) and martensitic start temperature (Ms) were determined from the dilatometer data. The cast steel brake disc material exhibited an Ac3 temperature of 847 °C and an Ms temperature of 347 °C, whereas the forged steel brake disc material had an Ac3 of 862 °C and an Ms of 368 °C.
3.2 Microstructural Evolution at Different Cooling Rates
3.2.1 Cast Steel Brake Disc Material
The microstructural evolution of the cast steel brake disc material at various cooling rates is summarized in Table 2 and illustrated in Figures 1 and 2.
Table 2. Microstructure Evolution of Cast Steel Brake Disc Material at Different Cooling Rates
| Cooling Rate (°C/s) | Microstructure |
|---|---|
| 0.2 | Granular bainite (GB) |
| 1.0 – 2.0 | Granular bainite + lath bainite (LB) |
| 5.0 – 20.0 | Lath bainite + martensite (M) |
| > 20.0 | Predominantly martensite |
Figure 1. Optical Micrographs of Cast Steel Brake Disc Material at Different Cooling Rates
(Insert figures here showing optical micrographs for various cooling rates)
Figure 2. SEM Micrographs of Cast Steel Brake Disc Material at Different Cooling Rates
(Insert SEM micrographs for selected cooling rates)
At low cooling rates (0.2 °C/s), the microstructure consisted primarily of granular bainite. As the cooling rate increased to 1.0–2.0 °C/s, a mixture of granular and lath bainite formed. At cooling rates of 5.0–20.0 °C/s, the microstructure transformed into a blend of lath bainite and martensite, with the martensite fraction increasing with cooling rate. At cooling rates exceeding 20.0 °C/s, the microstructure consisted predominantly of martensite.
3.2.2 Forged Steel Brake Disc Material
The microstructural evolution of the forged steel brake disc material at various cooling rates is summarized in Table 3 and illustrated in Figures 3 and 4.
Table 3. Microstructure Evolution of Forged Steel Brake Disc Material at Different Cooling Rates
| Cooling Rate (°C/s) | Microstructure |
|---|---|
| 0.2 | Ferrite + pearlite + trace bainite |
| 1.0 – 20.0 | Bainite + lath martensite |
| > 50.0 | Predominantly martensite |
Figure 3. Optical Micrographs of Forged Steel Brake Disc Material at Different Cooling Rates
(Insert figures here showing optical micrographs for various cooling rates)
Figure 4. SEM Micrographs of Forged Steel Brake Disc Material at Different Cooling Rates
(Insert SEM micrographs for selected cooling rates)
At the slowest cooling rate (0.2 °C/s), the microstructure comprised ferrite, pearlite, and trace amounts of bainite. As the cooling rate increased to 1.0–20.0 °C/s, a mixture of bainite and lath martensite formed. Complete martensitic transformation occurred only at cooling rates exceeding 50.0 °C/s.
3.3 CCT Curves and Microhardness
The CCT curves for the cast steel and forged steel brake disc materials are presented in Figures 5 and 6, respectively. The microhardness values measured at different cooling rates are also plotted on these graphs.
Figure 5. CCT Curve of Cast Steel Brake Disc Material
(Insert CCT curve for cast steel brake disc material)
Figure 6. CCT Curve of Forged Steel Brake Disc Material
(Insert CCT curve for forged steel brake disc material)
For the cast steel brake disc material, the microhardness increased significantly from 383 HV at 0.2 °C/s to 490 HV at 1.0 °C/s due to the formation of harder bainitic microstructures. Between 1.0 and 20.0 °C/s, the microhardness increased moderately from 490 to 520 HV as the martensitic fraction increased. Above 20.0 °C/s, the microhardness remained relatively constant due to the stable martensitic microstructure.
In contrast, the forged steel brake disc material exhibited a more gradual increase in microhardness with increasing cooling rate. The initial increase from ferrite-pearlite to bainitic microstructures was followed by a more pronounced rise as martensite formed above 50.0 °C/s.
4. Discussion
The differences in Ac3 and Ms temperatures between the cast steel and forged steel brake disc materials can be attributed to their distinct chemical compositions. The higher Mo and V contents in the forged steel brake disc material contribute to a higher Ac3 temperature, while the Ni content in the cast steel brake disc material favors a lower Ac3 temperature. The higher Ms temperature in the forged steel material reflects its increased hardenability.
The distinct microstructural evolution at various cooling rates highlights the importance of material selection and process optimization in brake disc design. The cast steel brake disc material readily transforms to martensite at moderate cooling rates, imparting higher hardness and strength. In contrast, the forged steel brake disc material requires significantly higher cooling rates to achieve complete martensitic transformation.
During brake disc service, localized overheating on the friction surface can exceed the Ac3 temperature, leading to austenitization followed by rapid cooling. The CCT curves provide valuable insights into the expected microstructural changes and associated changes in mechanical properties under these conditions. The cast steel brake disc material’s lower critical cooling rate for martensitic transformation suggests improved microstructural stability during repeated thermal cycling.
5. Conclusion
This study successfully determined the CCT curves of cast steel and forged steel brake disc materials used in high-speed trains. The cast steel brake disc material exhibited an Ac3 temperature of 847 °C and an Ms temperature of 347 °C, while the forged steel brake disc material had an Ac3 of 862 °C and an Ms of 368 °C. Microstructural analysis revealed that the cast steel material readily transforms to martensite at cooling rates exceeding 20.0 °C/s, while the forged steel material requires cooling rates above 50.0 °C/s for complete martensitic transformation.
The CCT curves and microhardness data provide critical insights into the microstructural stability and mechanical properties of brake disc materials during service. The cast steel brake disc material’s lower critical cooling rate for martensitic transformation indicates potential advantages in terms of improved microstructural stability under repeated thermal cycling conditions encountered during braking operations. This study underscores the importance of material selection and process optimization in ensuring the durability and reliability of high-speed train brake discs.