Mechanical Properties of Partially Solution-nitrided 316 L Austenitic Stainless Steel

摘要

Austenitic stainless steels (304 and 316 L type) are clinically applied for osteosynthesis plates because of the high strength, ductility and corrosion resistance. In order not to affect the soft tissue around the plate, it is necessary to reduce the thickness of the steel plate with improving its properties. Authors have tried to enhance the strength of austenitic stainless steel by solution nitriding. Solution-nitrided 316 L austenitic steel is expected to possess high tensile strength with sufficient ductility, as well as excellent corrosion resistance; therefore, it is expected as a new material for osteosynthesis plates, However, solution nitriding needs very long time to complete the nitrogen absorption, especially in the case of thick plates. In order to apply this method to commercial plates, it is important to understand partially solution-nitrided material and find the optimal treatment condition from the practical viewpoint. In this study, the kinetics of nitrogen absorption behavior during solution nitriding was discussed, and then mechanical properties were investigated for the partially solution-nitrided 316 stainless steel, as well as fully solution-nitrided one. Type 316 L austenitic stainless steel plate (AISI 316 L) were subjected to solution nitriding. The solution nitriding time was varied up to 36 ks to change the nitrogen concentration distribution in the steel plates, and then the material properties of these steels were investigated to demonstrate the effect of the unsaturated solution nitriding (partial solution nitriding). As a result, it was found that the hardness distribution is well consistent with the nitrogen concentration profile theoretically calculated according to the Fick' s law. The saturated nitrogen concentration was 0.46 mass% in the 316 L stainless steel, and the time required for full nitrogen absorption to the equilibrium state was around 4.8 ks in the case of 0.6 mm thick plate. Tensile testing for the solution-nitrided steels revealed that the yield and tensile strengths of 316 L austenitic steel increases with solution nitriding time. An industrially important tendency was found in the strengthening behavior during solution nitriding. That is, the yield strengm was almost identical irrespective of the solution nitriding time even in the partially solution-nitrided specimens, while the tensile strength was monotonously enlarged with increasing the solution nitriding time and then saturated when the nitrogen absorption was completed. This suggests that it is possible to obtain the requested mechanical properties even by short-time treatment if the maximum properties are not necessarily required.