Heat Treatment System

Abstract

A nitrogen enriched layer is formed in a primary heat treatment, and requenching is conducted in a secondary heat treatment, while the heat treatment efficiency is improved across the entire system. In a primary heat treatment device 1, a bearing component is heated in a heater 11 at a temperature exceeding the A1 transformation point, and is then cooled in a cooler 12 to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component. The bearing component that has undergone primary heat treatment is then subjected to high frequency heating in a heater 21 of a secondary heat treatment device 2, at a temperature exceeding the A1 transformation point, and is then cooled in a cooler 22 to a temperature less than the A1 transformation point. Following cooling by the cooler 22, the component is tempered using high frequency heating.

Description

TECHNICAL FIELD [0001] The present invention relates to a heat treatment system for conducting a two-stage heat treatment on a steel component. BACKGROUND ART [0002] An example of a heat treatment method that has been adapted for steel machine components that require a long service life to rolling fatigue, such as the bearing portions of a rolling bearing, is disclosed in Japanese Patent Laid-Open Publication No. 2003-226918. This method comprises the steps of subjecting the steel for the bearing components to carbonitriding treatment at a carbonitriding treatment temperature that exceeds the A1 transformation point, subsequently cooling the steel to a temperature less than the A1 transformation point, and then reheating and quenching the steel within a quenching temperature region (790 to 830° C.) exceeding the A1 transformation point but less than the carbonitriding treatment temperature. [0003] According to this method, the presence of a carbonitrided layer at the surface of the ...

AI Technical Summary

Benefits of technology

[0011] According to this heat treatment system, the heat treatment by the primary heat treatment device forms a nitrogen enriched layer in which nitrogen is diffused throughout the component surface, thus increasing the surface hardness of the steel component. On the other hand, the austenite grains within the steel structure following the primary heat treatment are considerably large, but because the secondary heat treatment is subsequently conducted while controlling a heating temperature and a heating time by the subsequent induction heating, the austenite grains can be reduced in size to approximately one half of the size observed in conventional components, enabling a fine grain size with an austenite grain size number exceeding 10 to be achieved. As a result of these characteristics, abrasion resistance and cracking resistance can be improved in comparison with conventional components, enabling a significant increase in the service life relative to rolling fatigue.

[0031] According to this aspect of the present invention, a nitrogen enriched layer is formed in the primary heat treatment, and when requenching is then conducted within the secondary heat treatment, the heating efficiency within the secondary heat treatment can be improved dramatically. Accordingly, the heat treatment efficiency within the primary heat treatment and the secondary heat treatment can be balanced, enabling an improvement in the heating efficiency across the entire system.

Problems solved by technology

For example, the bearings used in reduction gears, drive pinions, and transmissions and the like operate under severe conditions, including high loads and contamination of the lubricant with foreign matter (such as abrasion dust from the gears), and with recent demands for even higher speeds and greater miniaturization, the severity of these operating conditions continues to increase.

If this secondary heating is conducted in an atmospheric furnace, in a similar manner to a conventional quenching process, and the atmospheric temperature inside the furnace is measured, then differences can develop between the measured temperature and the actual temperature of the bearing component, making strict temperature control difficult.

In the case of a tapered roller bearing, since the thickness of the outer ring and the inner ring is also non-uniform, there is a real danger of quenching distortion.

Furthermore, in the invention disclosed in the above publication, a total of two heat treatments, namely a primary treatment and a secondary treatment, are performed, but because the heating time required for the primary heat treatment is different from that required for the secondary heat treatment, the heating times for the primary heat treatment and the secondary heat treatment cannot be easily balanced, and there is a danger that the heat treated product must be halted within the production line, causing longer down times for the machinery used to perform the heat treatments, and resulting in reduced heat treatment efficiency.

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