Date

2022-2-11

Author

Yıldırım, Zafer

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Carburizing is a surface treatment process applied for improving the surface hardness, wear resistance, and also fatigue life of some critical aerospace and automotive components, such as gears, bearings, and bushings. It consists of carbon diffusion into low-C steels in the austenitized condition which results in a carbon gradient along with the depth, then followed by quenching, sub-zero treatment, and tempering. In a carburized component, a strong and wear-resistant surface layer (the case) with compressive residual stress, as well as hardness and residual stress gradients from the case to the core is expected. Residual stresses, which are critical for the performance and stability of the engineering components, are defined as the self-equilibrating elastic stresses in a component in the absence of external stresses, created during manufacturing. In carburized components, the C-gradient affects the phase transformations on cooling and, thus, the final patterns of the microstructure and the residual stress. Besides, depending on the carbon content, carburized steels may contain retained austenite up to 10%, which affects the magnitude and the depth profile of the residual stress, and also, the dimensional stability of the part. Controlling the success of the carburizing process is vital to satisfy the design requirements and to guarantee safety with expected performance and lifetime. This necessitates practical, reliable, and time- and cost-effective non-destructive evaluation methods by the manufacturers. The aim of this thesis is to determine non-destructively the effects of the parameters of the carburizing process chain on microstructure, hardness, and residual stress state by using the Magnetic Barkhausen Noise (MBN) method. In the experimental part, a series of SAE 9310 steel specimens having different microstructure and residual stress states were prepared by altering the cooling rate (air-cooling, quenching), the carburizing time (3h, 6h), the cryogenic treatment temperature (-25°C, -75°C, -100°C), and the tempering temperature (150°C, 300°C). The specimens were systematically characterized through metallographic investigations, hardness measurements, measurements of retained-austenite and residual stress by the XRD method, and MBN measurements. The results show that a good correlation exists between the results obtained by conventional methods and the parameters derived from the MBN signals.

Subject Keywords

Magnetic Barkhausen noise, Residual stress, X-Ray diffraction, Carburizing of steel, Microstructure

URI

https://hdl.handle.net/11511/96291

Collections

Graduate School of Natural and Applied Sciences, Thesis