Characteristics and Kinetics of Bainite Transformation Behaviour in a High-Silicon Medium-Carbon Steel above and below the Ms Temperature
Abstract
:1. Introduction
2. Materials and Methods
2.1. Experimental Material
2.2. Heat-Treatment Cycles
2.3. Microstructural Observation and Hardness Measurement
2.4. Microphase Analysis
2.5. Kinetics Data Collection
3. Results
3.1. Progress of Bainite Formation
3.2. Kinetic Data of Bainite Phase Transformation
3.3. Rate of Bainite Phase Transformation
3.4. Microstructural Features and Hardness Measurements
3.5. EBSD Imaging and XRD Confirmations
4. Conclusions
- 1.
- Isothermal holding above the Ms temperature (275 °C) facilitated bainitic transformation, which was completed after 1 h isothermal holding regardless of fractions transformed. In addition, a further decrease in holding temperature below the Ms resulted in the formation of initial athermal martensite, which provided additional nucleation sites for the accelerated formation of bainitic sheaves and enabled carbon partitioning to the adjacent austenitic areas, thereby enhancing the bainite transformation rate.
- 2.
- Avrami-type functions suitably predicted the progress of bainite transformation at different isothermal holding temperatures as a function of elapsed times, above and below the Ms (275 °C). The rate coefficients k corresponding to the bainite reactions in Q&P samples (0.03–0.04 s−1) were more than 10 times greater than those for Q&B samples (0.002–0.004 s−1), which showed a weak dependence in the bainitic regime. The Avrami exponent n, on the other hand, varied in the range 1.0 and 1.8 for the isothermal Q&B treatments (above Ms), increasing with the increasing bainitizing temperature, but was significantly lower (<0.4) for Q&P treatments. The decrease in the Avrami exponent (n) with holding below the Ms (250 and 200 °C) indicated that the bainite formation became site-saturated because of the presence of numerous nucleation sites at prior austenite–martensite interfaces.
- 3.
- Following Q&P processing (below Ms at 250 and 200 °C), a maximum rate of transformation occurred at the start of isothermal holding owing to the occurrence of such mechanisms as carbon partitioning and possible ledged growth of isothermal martensite in region I. A second peak seen in region II subsequently marked the decomposition of austenite to ultrafine bainite during longer holding.
- 4.
- Microstructural investigation confirmed the dilatation results and showed the extensive formation of bainitic microstructures after isothermal holding at 450 °C, whereas multiphase microstructures comprising complex martensite–bainite–retained austenite phase constituents were realized by holding at temperatures below the Ms temperature (200 and 250 °C). TEM images confirmed the presence of fine films of interlath RA and carbides formed during different isothermal holding treatments for 1 h both above and below the Ms temperature.
- 5.
- Hardness tests performed on the Q&B and Q&P heat-treated specimens corroborated the results of the dilatation curves and phase transformation evolution. Vickers hardness decreased continuously with increasing isothermal temperature between 200 and 450 °C due to the microstructural evolution from a multiphase microstructure comprising martensite–bainite–retained austenite (isothermal holding at 200 °C) to relatively complete bainitic structures at the isothermal holding temperature of 450 °C.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Isothermal Holding Temperature (°C) | Linear Regression Equations | |
---|---|---|
450 °C | y = 1.7855x − 6.2659 | R2 = 0.9433 |
400 °C | y = 1.4251x − 6.205 | R2 = 0.9634 |
350 °C | y = 1.2862x − 5.9329 | R2 = 0.9701 |
300 °C | y = 1.0029x − 5.6968 | R2 = 0.9157 |
250 °C | y = 0.3665x − 3.2404 | R2 = 0.9611 |
200 °C | y = 0.1622x − 3.5651 | R2 = 0.9188 |
Isothermal Holding Temperature (°C) | n | k (1/s) |
---|---|---|
450 °C | 1.7855 | 0.0019 |
400 °C | 1.4251 | 0.0021 |
350 °C | 1.2862 | 0.0027 |
300 °C | 1.0029 | 0.0040 |
250 °C | 0.3665 | 0.0392 |
200 °C | 0.1622 | 0.0283 |
Isothermal Holding Temperature (°C) | Macrohardness (HV30) |
---|---|
450 °C | 350 |
400 °C | 430 |
350 °C | 530 |
300 °C | 584 |
250 °C | 600 |
200 °C | 655 |
Isothermal Holding Temperature (°C) | RA (vol.%) | Carbon Content of RA (wt.%) |
---|---|---|
450 | ˂3 | - |
400 | ˂3 | - |
350 | 6.9 | 1.39 |
300 | 6 | 1.45 |
250 | 7.8 | 1.21 |
200 | 17.2 | 1.20 |
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Pashangeh, S.; Ghasemi Banadkouki, S.S.; Somani, M.; Kömi, J. Characteristics and Kinetics of Bainite Transformation Behaviour in a High-Silicon Medium-Carbon Steel above and below the Ms Temperature. Materials 2022, 15, 539. https://doi.org/10.3390/ma15020539
Pashangeh S, Ghasemi Banadkouki SS, Somani M, Kömi J. Characteristics and Kinetics of Bainite Transformation Behaviour in a High-Silicon Medium-Carbon Steel above and below the Ms Temperature. Materials. 2022; 15(2):539. https://doi.org/10.3390/ma15020539
Chicago/Turabian StylePashangeh, Shima, Seyed Sadegh Ghasemi Banadkouki, Mahesh Somani, and Jukka Kömi. 2022. "Characteristics and Kinetics of Bainite Transformation Behaviour in a High-Silicon Medium-Carbon Steel above and below the Ms Temperature" Materials 15, no. 2: 539. https://doi.org/10.3390/ma15020539