Improvement of Load-Carrying Capacity of Gears Case-Hardened by Plasma Nitriding

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Almost all gears used for power transmissions of automobiles have been case-hardened by carburizing. Recently, strict demand for reducing running noise and vibration from the power transmission gears requires, in most cases, an additional finishing operation such as grinding and/or honing after carburizing. Nitriding of the gears can be conducted at a temperature of about 820 K which is lower than the transformation temperature, and thus quenching is not required, resulting in an appreciably smaller heat-treatment distortions in comparison with those of carburized gears. The surface hardness of nitrided gears is almost the same as that of carburized gears.This investigation was conducted to clarify the reason why nitrided gears hardly used for power transmissions using an impact bending fatigue testing machine which was originally designed and made by the present investigators. Eight kinds of ferrous materials were used and many experiments were conducted. It was found that the nitriding reduced the impact bending fatigue lives of the gears, in almost all cases, when the kind of gear materials is the same. However, we found that the nitrided gears made from a high tension steel were stronger than carburized gears made from conventional carburizing steel. The high tension steel has not been used as for gear material and it has been used for a material of large pressure vessels and of large bridges with welded joints. The carbon content of the high tension steel is almost the same as that of the carburizing steel. Therefore, test gears were made from the high tension steel and carburized, resulting in the strongest case-hardened gears. Although many experiments were conducted, the main reason for the success of this investigation depended on a finding of the high tension steel suitable for nitriding.