With the aim of extending the lifetime of molds used for continuous steel casting, we developed a composition-modulated Co-Ni alloy plating with alternately laminated layers of different compositions. Alloy films with laminated structures comprising few-micrometer-thick layers were formed using a single plating bath by periodically altering the agitation intensity in the bath during plating. The hardness of the as-plated composition-modulated Co-Ni alloy film was 350 HV, which was higher than that of a conventional non-laminated Co-Ni alloy plating film. Heat treatment slightly decreased the hardness. Nevertheless, the composition-modulated film exhibited higher tensile strength and higher hardness than the conventional alloy film after heat treatment at temperatures of 200-700 ℃. The breaking elongation of the composition-modulated film increased to 30% after heat treatment at 700 ℃. Finally, the composition-modulated Co-Ni alloy plating was applied to a mold surface, which was then used for continuous steel casting. It exhibited excellent resistance against crack formation by thermal shock resulting from contact with molten steel during continuous casting.
Industrial processes for plasma nitriding, such as ion nitriding and radical nitriding, are used extensively, but their daily production is severely limited by the duration of their associated heating and cooling times. Considering ecologically friendly processes and processing time reduction, near-infrared radiant（NIR）heating is a better heating method. Because NIR wavelengths are shorter than far-infrared radiation wavelengths, NIR gives higher energy input: it is better suited for heating metal. This paper presents a novel approach to reduce radical nitriding processing times. We prepared a new apparatus: an NIR heater mounted on a radical nitriding apparatus（IR-RN）.
Results demonstrated that IR-RN can reduce overall processing times from 900 to 380 min because IR-RN can increase the surface temperature immediately and cool it much faster than radical nitriding（RN）. Moreover, a sample treated using IR-RN showed the same surface hardness and roughness as the radical nitrided sample. No compound layer was observed in a surface layer with the sample treated using IR-RN.
The hardening depth of a layer treated using IR-RN for 60 min was half that of a layer treated using RN because of the temperature gradient, whereas a sample treated using IR-RN（513 K bottom temperature）for 300 min and IR-RN（573 K bottom temperature）for 180 min and 300 min were the same depth as the RN sample layer.