Preparation of high nitrogen hard duplex steel with nanometer grain size and its thermal stability

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A new technique for surface treatment of steel is studied, where soft high nitrogen austenite layer is initially formed and is decomposed into a mixture of equilibrium phases, i. e. α-Fe and γ'-Fe_4N, by low temperature aging. Because of the small grain size of several dozens of nanometers, the surface layer becomes as hard as martensite steel. It was found that nitrogen atoms taken up from nitriding atmosphere recombine at internal grain boundaries and turn out to be nitrogen gas bubbles, which embrittle the surface layer. With proper nitriding condition and improvement of quenching technique, the surface layer of 130 μm in thickness and as hard as 900 Hv was formed on a mild steel bar with a size of φ10mm x 100mm. Four patent proposals were submitted.