Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2002-02-07
2004-04-20
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S660000
Reexamination Certificate
active
06723188
ABSTRACT:
This nonprovisional application claims priority under 35 U.S.C. § 119(
a
) on Patent Application No. 2001-32244 filed in Japan on Feb. 8, 2001, which is herein incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a steel workpiece oil quenching method and, in particular, to a method for processing a steel workpiece in a marquenching manner.
In general, when a steel workpiece maintained at a quenching temperature is immersed in quenching oil, the steel workpiece is cooled through the three stages of a vapor film stage (high-temperature region), a boiling stage (intermediate-temperature region) and a convection stage (low-temperature region). It is known that a cooling speed in the vapor film stage is slow and a cooling speed in the boiling stage is three to ten times faster than the above-mentioned speed. A high-temperature quenching oil (hot quenching oil), of which the cooling speed in the intermediate-temperature and low-temperature regions is slower than that of a low-temperature quenching oil (cold quenching oil), is therefore able to reduce the distortion attributed to a quenching transformation. However, it is also known that a thermal distortion attributed to a temperature difference in the high-temperature region tends to easily occur since the time of the vapor film stage is short and the end temperature in the vapor film stage is high. If the quenching oil is put in a reduced pressure state, as shown in
FIG. 2
, the time of the vapor film stage is prolonged by the reduction of the boiling point, and the end temperature in the vapor film stage is lowered. Accordingly, as a method for reducing the deformation attributed to the quenching taking advantage of the above-mentioned phenomenon, there has been put in practice a method for performing quenching by immersing a steel workpiece maintained at a quenching temperature in a high-temperature quenching oil or a method for performing quenching by immersing the steel workpiece in a quenching oil under a reduced pressure.
On the other hand, as an oil quenching method of a steel workpiece such as a gear, there is a method (marquenching method) for rapidly cooling a steel workpiece maintained at a specified quenching temperature to a temperature slightly higher than the martensite transformation start point (Ms point) by immersing the steel workpiece in a high-temperature coolant at a temperature slightly higher than the martensite transformation start point (Ms point), thereafter cooling the steel workpiece in the atmospheric air by taking out the steel workpiece out of the high-temperature coolant at a point of time when the entire steel workpiece comes to have roughly same temperature, and thereby effecting the martensite transformation. This method, which can reduce the quenching distortion and the quenching variation, has the problem that the cooling speed causes a temperature difference between the placement positions of workpieces in a tray and between the portions of a workpiece due to the cooling in the atmospheric air, and consequently the quenching distortion and the quenching variation attributed to the temperature difference cannot be avoided.
As a method for solving this problem, there has been proposed a method for rapidly cooling a steel workpiece maintained at a specified temperature by immersing the steel workpiece in a high-temperature coolant at a temperature higher than the martensite transformation start temperature and thereafter immersing the steel workpiece in a low-temperature coolant at a temperature lower than the martensite transformation start temperature at the point of time when the entire steel workpiece comes to have roughly same temperature (as described in Japanese Patent Laid-Open Publication No. 2-101113), a method for providing a circulation system for circulating a quenching oil, using a quenching bath that has a hood for surrounding the workpiece, immersing the workpiece inside the hood in a state in which the circulation system is stopped, raising the temperature of the quenching oil inside the hood close to the martensite transformation start temperature (Ms point) by the heat of the workpiece, and then rapidly cooling the workpiece to a temperature lower than the martensite transformation start temperature by circulating the quenching oil in the circulation system at the point of time when the entire workpiece has roughly same temperature (as described in Japanese Patent Laid-Open Publication No. 6-279838) or the like.
However, from the viewpoint of the construction and structure of the quenching apparatus, the former method, which needs not only the high-temperature coolant and the low-temperature coolant but also the quenching bath for the high-temperature coolant and the quenching bath for the low-temperature coolant, has the problem that the quenching apparatus becomes inevitably increased in size and complicated and has poor maintenance. The latter method, which solves the problem of the former method, has the problem that the quenching bath itself is complicated. Moreover, from the viewpoint of quenching distortion and variation, both are the systems for putting the entire steel workpiece into roughly same temperature by immersing the steel workpiece in the coolant, and therefore, it is difficult to bring the coolant that serves as a thermal medium in contact with all workpieces on the tray or the entire portions of the workpiece uniformly and sufficiently, and consequently a temperature difference occurs between the steel workpieces or the portions of the steel workpiece in the soaking stage. Accordingly, although those methods have the effect of reducing the quenching distortion and variation, the effect are not satisfactory.
As a result of detailed researches for solving the aforementioned problems, it was discovered that these quenching distortion and variation were attributed to the temperature difference between the steel workpieces or the portions of the steel workpiece during the martensite transformation and to the fact that the cooling speed in the high-temperature region (not lower than about 550° C.) was too fast.
SUMMARY OF THE INVENTION
Accordingly, the present invention has the object of reducing the temperature difference between steel workpieces or the portions of a steel workpiece in the martensite transformation stage and making the cooling speed in a high-temperature region (not lower than about 550° C.) a slow speed sufficient for restraining the thermal distortion, thereby reducing the quenching distortion and quenching variation.
In order to achieve the aforementioned object, a steel workpiece oil quenching method of the present invention comprises the steps of: rapidly cooling a steel workpiece maintained at a specified quenching temperature by immersing the steel workpiece in a high-temperature quenching oil until a temperature of a specified portion of the steel workpiece reaches a temperature just above a martensite transformation start point (Ms point); thereafter taking the steel workpiece out of the high-temperature quenching oil to soak the steel workpiece by heat possessed by the steel workpiece; and cooling the steel workpiece by subsequently immersing the steel workpiece in the high-temperature quenching oil.
According to one embodiment of the present invention, the steel workpiece is rapidly cooled in a rapid cooling process until a temperature of its portion producing the largest deformation amount (in the concrete, a portion having a small internal volume with respect to a unit area of the steel workpiece (for example, a gear tooth, a corner portion of a prismatic workpiece, or the like, hereinafter referred to as “a sharp portion”)) reaches a temperature just above the Ms point of the portion.
The present invention is based on the following knowledge. That is, according to the method of the present invention, the steel workpiece is first cooled to a temperature just above the martensite transformation start point (Ms point) by being immersed in the high-temperature quenching oil. In this case, it is pro
Miyamoto Kiyohiro
Ogawa Saburo
Shimosato Yoshikazu
Birch & Stewart Kolasch & Birch, LLP
Chugai Ro Co., Ltd
Yee Deborah
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