Strength and Fracture Mechanism of an Ultrafine-Grained Austenitic Steel for Medical Applications

In this paper, we study the corrosion-resistant austenitic steel Fe-0.02C-18Cr-8Ni for medical applications. The microstructure and mechanical properties (tensile mechanical properties, torsional strength, impact toughness, and static and cyclic crack resistance) under different types of loading of the steel are investigated. The results are compared for the two states of the steel: the initial (coarse-grained) state and the ultrafine-grained state produced by severe plastic deformation processing via equal-channel angular pressing. It is demonstrated that the ultrafine-grained steel 0.08C-18Cr-9Ni has essentially better properties and is very promising for the manufacture of medical products for various applications that experience various static and cyclic loads during operation.

Influence of Severe Plastic Deformation on Static Recrystallization Microstructure of Pure Iron

In recent years, ultrafine-grained steel has been gaining increasing attention as a high-performance material. Accordingly, it is necessary to develop an efficient production method for ultrafine-grained steel. Severe plastic deformation is a critical factor that causes grain subdivision into ultrafine grains less than 1 µm in diameter. In this study, the effects of plastic deformation on the microstructure and static recrystallization of pure iron were studied by comparing orthogonal cutting and rolling. Orthogonal cutting yielded ultrafine grains with a diameter of 0.2 µm. It was found that a high strain rate in the thin shear plane generated during the cutting process caused a uniform subdivision of grains, and this uniform plastic deformation resulted in the uniform recrystallization of grains. In addition, a theoretical model was developed, and it was revealed that the number of recrystallized grains depended on the fraction of a large-misorientation area constructed with geometrically necessary boundaries (GNBs). It was suggested that the cutting process was more advantageous than rolling in producing ultrafine recrystallized grains because cutting could apply severe plastic strain uniformly on a work material, effectively generating GNBs.

Development of Fine-Grained High-Mn Steelby Cold Rolling and Annealing

The regularities of static recrystallization in an Fe-0.3C-17Mn-1.5Al TWIP steel subjected to cold rolling and annealing were studied. The cold rolling led to noticeable increase in the dislocation density, extensive mechanical twinning and shear banding. The subsequent annealing resulted in the development of recovered or recrystallized microstructure depending on the rolling reduction and the annealing temperature. An increase in the rolling reduction promoted the recrystallization development, which led to ultrafine-grained microstructure with a grain size below 10 μm. The developed ultrafine-grained steel samples are characterized by beneficial mechanical properties.

High Strength Microscrew with Ultrafine Grained Structure

While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. It was found that the reduction in area - tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensitic and bainitic steels. Formability of ultrafine-grained steel is examined by applying to form a M1.7 micro screw using these ultrafine-grained steels. Screws are formed through the process of cold heading and rolling. Relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. Reduction in area is a measure to determine formability on cold heading. Ultrafine grained steel wire with length of several hundred meter were developed with the technology of warm continuous multi-directional rolling. This wire also have a good formability which can form microscrews. High strength microscrew with ultrafine grained structure was obtained.