Ultrahigh-strength AISI-316 austenitic stainless steel foils through concentrated interstitial carbon

This research explores intrinsic properties of the carbon-rich subsurface zone ("case") that low-temperature carburization generates in an austenitic stainless steel (AISI-316). Foils of this steel were carburized to obtain concentrated interstitially dissolved carbon distributed uniformly throughout their thickness. Compared to the as-received foils, such "through" 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. Those foils retained a core of low carbon level. Decreasing the ratio of mean carbon depth to foil thickness was found to decrease the ultimate tensile strength, the yield strength, and Young's modulus, while increasing the strain to failure. Tensile testing partially carburized foils to failure confirmed ductility in the core, but revealed reduced ductility in the carbon-rich zone near the surface. On the surface, intergranular cracks were observed to propagate perpendicular to the tensile direction, indicating brittle fracture initiated by crack nucleation at grain boundaries. The isolated concentrated solid solution of interstitial carbon in austenite can be regarded as a new material with exceptional properties, particularly ultrahigh tensile strength, high yield strength, and high Young's modulus.