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

几乎所有用于汽车动力传动装置的齿轮都经过渗碳表面硬化处理。近来，对于降低动力传动齿轮的运行噪声和振动的严格要求在大多数情况下要求在渗碳之后进行额外的精加工操作，例如磨削和/或珩磨。齿轮的氮化可以在低于相变温度的约820 K的温度下进行，因此不需要淬火，从而导致与渗碳齿轮相比明显更小的热处理变形。氮化齿轮的表面硬度与渗碳齿轮的表面硬度相差无几，本文利用自行设计制造的冲击弯曲疲劳试验机，对氮化齿轮难以用于动力传动的原因进行了研究。采用了八种铁基材料，进行了大量的实验。结果发现，氮化降低了冲击弯曲疲劳寿命的齿轮，在几乎所有的情况下，当齿轮材料的种类是相同的。然而，我们发现由高张力钢制成的氮化齿轮比由常规渗碳钢制成的渗碳齿轮更坚固。高张力钢还没有被用作齿轮材料，它已被用于大型压力容器和具有焊接接头的大型桥梁的材料。高张力钢的碳含量与渗碳钢的碳含量几乎相同。因此，测试齿轮由高张力钢制成并渗碳，从而产生最强的表面硬化齿轮。虽然进行了许多实验，但这项研究成功的主要原因取决于发现了适合氮化的高张力钢。

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.