In order to obtain a mechanical component of steel having a surface layer portion enhanced in strength, a treatment is sometimes performed to provide the surface layer portion with a layer having a higher nitrogen concentration than the remaining region, i.e., a nitrogen enriched layer. For example, a nitriding treatment is performed. A conventional nitriding method for steel is representatively a gas soft nitriding treatment in which steel is heated in an atmosphere containing ammonia or a similar gas serving as a source of nitrogen to cause the nitrogen to penetrate into a surface layer portion of the steel. However, when steel containing chromium in a large amount such as at least 3.75 mass %, for example, is used to produce a mechanical component, the mechanical component has a surface layer portion having chemically stable oxide film. As such, when the mechanical component formed of steel containing chromium in the large amount undergoes the gas soft nitriding treatment the surface layer portion is not penetrated by nitrogen and the nitrogen enriched layer is not formed.
To address this, a plasma nitriding process is proposed as follows: an object of steel to be treated is placed in a vacuumed furnace and a gas containing a gas serving as a source of nitrogen is introduced into the furnace, and between the object and a member, such as a wall of the furnace, disposed to face the object a difference in potential is caused to cause glow discharge to cause nitrogen to penetrate into a surface layer portion of the steel configuring the object (see Japanese Patent Laying-open No. 2-57675 (Patent Document 1) for example). The plasma nitriding process is controlled for example as based on a spectral analysis of glow discharge, a density of a current flowing in the object, or the like, as proposed in Japanese Patent Laying-Open No. 7-118826 (Patent Document 2) and Japanese Patent Laying-Open No. 9-3646 (Patent Document 3). This allows a mechanical component formed of steel containing at least 3.75 mass % of chromium to have a surface layer portion provided with a nitrogen enriched layer.
Following the recent improvement in performance and efficiency of a mechanical device employing a rolling bearing, high durability in a severe environment tends to be required to the rolling bearing. More specifically, a rolling bearing employed in a contaminated environment penetrated by hard foreign matter may be damaged in an early stage (in an operating time shorter than a calculated life of the bearing) due to gripping of the foreign matter. Furthermore, a rolling bearing rotating at high speed may have smearing even if the bearing is under relatively small load. Furthermore, a rolling bearing used in an insufficiently lubricated environment may cause seizure. When the rolling bearing is used in a high-temperature environment of a temperature exceeding 200° C., for example, hardness of components (bearing components) constituting the rolling bearing may be reduced, to reduce the durability of the rolling bearing.
When the bearing components are made of steel, strength at a high temperature can be improved for improving the durability of the rolling bearing in the high-temperature environment by adding at least 3.75 mass % of chromium to the steel thereby improving tempering softening resistance of the steel. In order to improve the durability in the contaminated environment, a treatment of forming nitrogen-enriched layers having higher nitrogen concentrations than the remaining regions on surface layer portions of the bearing components by performing nitriding, for example, can be employed.
In bearing components made of steel having a high chromium content of at least 3.75 mass %, for example, chemically stable oxide films are formed on the surface layer portions. When ordinary nitriding is performed on these components, therefore, nitrogen does not penetrate into the surface layer portions thereof, and no nitrogen-enriched layers are formed. In relation to this, there is proposed a countermeasure of forming nitrogen-enriched layers by performing a plasma nitriding process, as described above.
In order to improve seizure resistance, there is proposed a countermeasure of dipping balls serving as the bearing components in an organic phosphorus compound for forming reaction layers on the surfaces thereof (refer to Japanese Patent Laying-Open No. 9-133130 (Patent Document 4), for example).
Patent Document 1: Japanese Patent Laying-open No. 2-57675
Patent Document 2: Japanese Patent Laying-open No. 7-118826
Patent Document 3: Japanese Patent Laying-open No. 9-3646
Patent Document 4: Japanese Patent Laying-open No. 9-133130