We have demonstrated the possibility of nitro-quenching by using an atmospheric-pressure plasma jet. Here we heat up a steel sample of JIS SPCC and supply nitrogen atoms to the surface by spraying the plasma-jet plume produced with nitrogen／hydrogen gas mixture, and water-quench the nitrided sample. The iron-nitrogen martensite phase is found to form non-uniformly in area. However, we achieved in invoking martensite transformation only at the jet-spraying center by optimizing the gap between the jet nozzle and the sample surface.

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The wear resistance without lubrication were studied experimentally for iron nitrides compound layer and high nitrogen martensite phase formed in the material surface by using industrial pure iron treated by nitriding and quenching followed by aging process. As a result both the compound layer where the generation of pores was suppressed by lowering the nitriding temperature and the high nitrogen martensite phase showed the same trend of high wear resistance. However, the pores in the compound layer decreased the wear resistance due to the acceleration of crack propagation.

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Quenching and partitioning (Q&P) treatment was applied to low-carbon steels containing a different amount of Cr ranging from 3 to 12 mass%, and then the effect of Cr content on the decomposition and stabilization behavior of untransformed austenite. Under the optimum Q&P treatment condition, the bainitic transformation and carbide precipitation could be effectively retarded by increasing Cr content. As a result, a larger amount of austenite was retained in a steel with higher content of Cr. Consequently, it was suggested that martensitic stainless steels intrinsically possess a favorable character for improving the mechanical property by Q&P treatment.

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The springbacks of the diaphragm in hydraulic bulging, were analyzed by the finite element method (FEM) with the parameters of bulge height and flange angle. Furthermore, FEM results were compared experimentally. The FEM result by using Yoshida-Uemori (Y-U) model showed a good agreement with the experimental result.
Next step, the effects of yield and tensile strengths of steel sheet, the friction between steel sheet and die, radius of die shoulder and hydraulic pressure on springback were examined by using FEM. It was revealed that the bulge height and the flange angle were linearly related to the yield and tensile strengths of steel sheet, as well as hydraulic pressure. Similarly, the friction between the steel sheet and the die has significant influence on the bulge height and the flange angle. However, the radius of die shoulder has little influence on these parameters.

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We have studied the corrosion resistance and wear behaviour of low-carbon steels by investigating the effect of microstructures formed at the surface by salt-bath nitrocarburising and gas nitriding. The microstructures included a lithium-containing ferroxide layer, a porous layer containing ε-Fe_2-3N, and a compound layer composed of ε-Fe_2-3N and γ′-Fe₄N phases. Corrosion resistances were evaluated by 5%-NaCl aq. splay testing, and corrosion potentials and anode polarization curves were evaluated by electrochemical testing using SPCC steel. Wear behaviours were studied by ball-on-disc sliding tests done under a Hertz’s pressure of 1.73 to 2.05 GPa without lubricant using S15C steel. Results showed that the surface oxide remarkably improved the corrosion resistance, that the porous layer decreased the resistance, and that the ε phase with a high amount of nitrogen had a higher resistance to corrosion compared with the γ′ phase. Furthermore, the oxide had an excellent adhesive resistance at a high contact pressure, the porous layer increased wear resistance because of the relaxation of contact stress during the sliding test, and the γ′ phase with high toughness resulted in better resistance to abrasion wear because the degree of delamination of the γ′ phase by brittle-crack generation was less than that of the ε phase.

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