| Popis: |
This research explores intrinsic properties of the carbon-rich subsurface zone (“case”) that low-temperature carburization generates in AISI-316 austenitic stainless steel. Foils of this steel were carburized to obtain concentrated interstitially dissolved carbon distributed uniformly throughout their thickness. Compared to the as-received AISI-316 foils, such “full” carburization increases the ultimate tensile strength to 3 times, the yield strength to 4 times, and Young’s modulus to 1.5 times, respectively. On the other hand, the strain to failure decreases to (9 ± 1) 10^(-3). For comparison, foils with larger thickness were carburized as well. Decreasing the ratio of “case” to foil thickness was found to decrease the ultimate tensile strength, yield strength, and Young’s modulus, while increasing the strain to failure. This research also investigates the impact of concentrated interstitial carbon on electrical conductivity, thermal conductivity, and conduction electron density and mobility. Foils with uniform carbon levels exhibit room-temperature electrical and thermal conductivity corresponding to only 0.8 and 0.7 times those measured in the as-received state, respectively. Hall-effect measurements revealed that concentrated interstitial carbon does not significantly reduce conduction electron mobility, but decreases the electron density to 0.7 of what we measured for as-received material. These observations suggest that the interstitial carbon atoms form covalent bonds with the metal atoms. With their unique combination of properties, free-standing uniform concentrated solid solutions of interstitial carbon in austenite can be regarded as a new material. Besides, this research also explores the thermal stability of the “case” at elevated temperature. Between 1300-1400 K, carbide transformation from M_{23}C_6 to M_7C_3 was observed. Finally, this research introduces a potential biomedical application of “case” on the Co–Cr–Mo alloy for surface wear improvement. A low-temperature encapsulation nitro-carburization method was developed. After heat-treating for 7.2 ks at 800 K, the resulting “case” features nitrogen and carbon fractions up to 0.075 and 0.035 respectively. This increases the surface hardness of the alloy to 16 GPa, twice as high as in the as-received state. Moreover, the treatment significantly improves the wear resistance of Co–Cr–Mo alloy in air as evaluated according to ASTM G133-05. |
