1. Field of the Invention
The present invention provides an oil composition for heat treatment of a gear and a gear treated by using the oil composition and, more particularly, an oil composition for heat treatment of a gear which is used in the quenching step of the gear so that a coating film is formed on the surface of the gear simultaneously with hardening of the gear and the gear can be provided with resistance to pitching, and a gear quenched by using the oil composition.
2. Description of the Related Arts
Gears for automobiles are, in general, produced from raw materials such as SCR415, SCR420 (AISI5120), SCR430 (AISI5130), SCM415, SCM420 and SCM430 (AISI4130) by cutting or deformation processing of the raw materials, followed by carburisation, quenching and tempering. For cooling in the quenching step, in general, so-called quenching oil such as marquenching oil and modified marquenching oil is used.
Gears for automobiles are under increasingly greater loads due to decreases in the size and the thickness accompanied with a decrease in the weight of automobiles and also due to an increase in the output power accompanied with improvements in the performance of automobiles. To satisfy these requirements, materials for gears and the method for quenching have been improved. As the result of the improvements, damages on gears take place more frequently on the surface of teeth than at the dedendum. For example, damages such as pitching, scoring and wear are more frequent than damages such as fracture of the dedendum of the gears which has heretofore been the major damage. In particular, the damage of the surface of teeth due to pitching is the major damage under the condition of the ordinary use.
To decrease the damage due to pitching, teeth are polished after quenching or compression stress is applied to the surface by shot peening.
However, these methods have a drawback in practical application in that the operation of the treatment is complicated and requires a great amount of time and cost of the treatment increases. Therefore, these methods are applied only to gears subjected to extraordinarily great loads and gears used for high grade automobiles.
Therefore, development of a method for improving resistance to pitching of a gear which can be practiced in a simple operation easily at a low cost and practically applied widely has been strongly desired.
The present invention has an object of providing an oil composition for heat treatment of a gear which can improve resistance to pitching of the gear effectively in a simple operation.
Extensive studies have been made by the present inventors to achieve the above object. In the quenching step of a gear for automobiles, it has heretofore been considered to be most important that heat treated gear having stable quality is obtained. Therefore, no active compounds reacting with steel are added to the quenching oil so that the properties do not change even when the quenching oil always has steel of about 850xc2x0 C. placed therein. Despite the above conventional practice, the present inventors paid attention to using in the quenching oil an additive which react with the surface of steel and can improve resistance to pitching of the gear. Thus, it was found that the resistance to pitching was improved by adding a phosphoric acid ester compound to a quenching oil. It was also found that, although stability to oxidation is adversely affected by the use of the above compound, the change in the property could be suppressed by suitably adding an alkenylsuccimide compound, an alkylsuccinimide compound or a salicylate, phenate or sulfonate of an alkaline earth metal and a heat treated gear exhibiting excellent resistance to pitching and stable quality could be obtained. The present invention has been completed based on the knowledge.
The present invention provides:
(1) An oil composition for heat treatment of a gear which comprises mineral oil having a kinematic viscosity of 5 to 40 mm2/second at 100xc2x0 C. as a base oil and, based on a total amount of the composition, 0.01 to 5% by weight of (a) a phosphoric acid ester compound (Composition I);
(2) An oil composition for heat treatment of a gear which comprises mineral oil having a kinematic viscosity of 5 to 40 mm2/second at 100xc2x0 C. as a base oil and, based on a total amount of the composition, 0.01 to 5% by weight of (a) a phosphoric acid ester compound and 0.5 to 10% by weight of (b) one compound selected from alkenylsuccinimide compounds, alkylsuccinimide compounds and addition products of boron with alkenylsuccinimide compounds or alkylsuccinimide compounds (Composition II);
(3) An oil composition for heat treatment of a gear which comprises mineral oil having a kinematic viscosity of 5 to 40 mm2/second at 100xc2x0 C. as a base oil and, based on a total amount of the composition, 0.01 to 5% by weight of (a) a phosphoric acid ester compound and 0.5 to 10% by weight of (c) at least one compound selected from salicylates, phenates and sulfonates of alkaline earth metals (Composition III);
(4) An oil composition for heat treatment of a gear which comprises mineral oil having a kinematic viscosity of 5 to 40 mm2/second at 100xc2x0 C. as a base oil and, based on a total amount of the composition, 0.01 to 5% by weight of (a) a phosphoric acid ester compound, 0.5 to 10% by weight of (b) one compound selected from alkenylsuccinimide compounds, alkylsuccinimide compounds and addition products of boron with alkenylsuccinimide compounds or alkylsuccinimide compounds and 0.5 to 10% by weight of (c) at least one compound selected from salicylates, phenates and sulfonates of alkaline earth metals (Composition IV); and
(5) A gear quenched by using any of Compositions I to IV.