Effect of Microstructure and Compressive Residual Stress on the Fatigue Performance of AISI 4140 Steel with QPQ Salt-Bath Nitro-Carburizing
Abstract
:1. Introduction
2. Material and Experimental Procedures
2.1. Preparation of the Sample
2.2. Measurement of Hardness and Fatigue Testing
2.3. Microstructural and Fracture Feature Observation
2.4. Measurement of Residual Stress
3. Results
3.1. Surface Texture
3.2. Microstructures of the QPQ-Treated Sample
3.3. Hardness Profile of the QPQ-Treated Sample
3.4. EBSD Analysis
3.5. Residual Stress Measurements
3.6. Fatigue Tests
3.7. Fractured Surface Examinations
4. Discussion
5. Conclusions
- After QPQ treatment, O was more likely to concentrate in the outermost layer, forming a continuous oxide film, whereas N was enriched in the overall compound layer. In addition, O diffused inwards and assisted in forming a strike-like oxide film inter-dispersed in the compound layer. After salt-bath nitrocarburizing, Fe3N mixed with a small amount of Fe3O4 was found on the top surface of the treated sample. Moreover, Fe2N instead of Fe3N and an increased amount of Fe3O4 were found in the post-oxidized sample.
- A gradual drop in hardness from the external surface of HV 700 to the interior of HV 270 at a depth of about 160 μm was obtained for both QPQ-treated samples. Moreover, MSP only caused a slight increase in the hardened depth and compound layer thickness after QPQ treatment. It could be that the associated tempering effect during nitro-carburizing in a salt bath at 570 °C/2 hrs had caused a relaxation of RCS introduced by MSP. Therefore, the SP-QPQ and NP-QPQ samples had similar residual stress profiles.
- Cracking and spalling the superficial compound layer resulted in the fatigue crack initiation and propagation of all of the QPQ-treated samples fatigue-loaded at/above 875 MPa. The CRS field and hard case in the QPQ-treated specimen hindered dislocation motions around the surface, thereby increasing its fatigue resistance. Fatigue crack initiation was more likely to occur at the subsurface inclusions in the SP-QPQ sample fatigue-loading at 850 MPa or slightly above the fatigue limit (825 MPa). By contrast, multiple cracks were initiated at the external periphery and propagated inwards for the fatigue-fractured QT570 sample. The ease of slip on the external surface of the QT570 sample was responsible for its fatigue limit of about 400 MPa relative to that of about 825 MPa of the QPQ-treated samples.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
BC | Band contrast |
CRS | Compressive residual stress |
EBSD | Electron backscatter diffraction |
EDS | Energy-dispersive X-ray spectroscopy |
EPMA | Electron probe microanalysis |
FWHM | Full width at half maximum |
IPF | Inverse pole figure |
MSP | Micro-shot peening |
PM | Phase map |
QT200 | 4140 tempered at 200 °C |
QT570 | 4140 tempered at 570 °C |
SEM | Scanning electron microscope |
XRD | X-ray diffractometer |
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Sample | Surface Roughness (μm) | ||
---|---|---|---|
Sa | Sp | Sv | |
QT200 [28] | 0.31 | 2.02 | 1.75 |
QT200 + SP [28] | 0.50 | 2.46 | 2.35 |
NP-QPQ | 0.28 | 2.42 | 1.31 |
SP-QPQ | 0.60 | 4.08 | 2.48 |
Location | Distance from Surface (μm) | Chemical Composition (wt.%) | |||||||
---|---|---|---|---|---|---|---|---|---|
C | N | O | Mn | Cr | Mo | Si | Fe | ||
Compound layer | 2 | 0.45 | 5.21 | 8.15 | 0.41 | 0.91 | 0.08 | 0.25 | Bal. |
8 | 1.30 | 5.45 | 0.57 | 0.78 | 0.88 | 0.10 | 0.19 | Bal. | |
Diffusion zone | 20 | 0.72 | 0.55 | 0.08 | 1.01 | 1.06 | 0.13 | 0.18 | Bal. |
50 | 0.43 | 0.38 | 0.04 | 0.61 | 0.87 | 0.16 | 0.22 | Bal. | |
Base metal | 150 | 0.38 | 0.24 | 0.04 | 0.98 | 1.08 | 0.15 | 0.21 | Bal. |
250 | 0.44 | ― | ― | 0.82 | 1.00 | 0.14 | 0.21 | Bal. |
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Chen, H.; Chen, T.-C.; Hsu, H.-H.; Tsay, L.-W. Effect of Microstructure and Compressive Residual Stress on the Fatigue Performance of AISI 4140 Steel with QPQ Salt-Bath Nitro-Carburizing. Materials 2025, 18, 1995. https://doi.org/10.3390/ma18091995
Chen H, Chen T-C, Hsu H-H, Tsay L-W. Effect of Microstructure and Compressive Residual Stress on the Fatigue Performance of AISI 4140 Steel with QPQ Salt-Bath Nitro-Carburizing. Materials. 2025; 18(9):1995. https://doi.org/10.3390/ma18091995
Chicago/Turabian StyleChen, Hao, Tai-Cheng Chen, Hsiao-Hung Hsu, and Leu-Wen Tsay. 2025. "Effect of Microstructure and Compressive Residual Stress on the Fatigue Performance of AISI 4140 Steel with QPQ Salt-Bath Nitro-Carburizing" Materials 18, no. 9: 1995. https://doi.org/10.3390/ma18091995
APA StyleChen, H., Chen, T.-C., Hsu, H.-H., & Tsay, L.-W. (2025). Effect of Microstructure and Compressive Residual Stress on the Fatigue Performance of AISI 4140 Steel with QPQ Salt-Bath Nitro-Carburizing. Materials, 18(9), 1995. https://doi.org/10.3390/ma18091995