Metal treatment – Process of modifying or maintaining internal physical... – With measuring – testing – or sensing
Reexamination Certificate
2001-02-08
2002-07-16
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With measuring, testing, or sensing
C702S099000
Reexamination Certificate
active
06419767
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling distortion due to heat treatment during quenching of automotive parts or the like, and a device for measuring cooling power of liquid cooling medium for use in a quenching device.
2. Description of the Background Art
It is often the case that, once quenching of automotive parts is completed, the parts are put to use without being subjected to further processing. Thus, distortion due to the quenching should be limited in a small range. In particular, with wheel driving parts, such distortion will lead to noise at the time of engagement of toothed gears, or degradation in durability, and therefore, restriction of such distortion is a critical issue. A number of examinations and analyses have been made in an effort to restrict the distortion. However, such distortion due to heat treatment results from a variety of factors that affect to one another in a complicated manner. Further, the testing itself incorporates variation therein. Thus, detailed analyses have not been made successfully; a general tendency for each factor would be found at best.
In an effort to stabilize distortion, e.g., in over pin diameter (OPD) of an outer ring of constant velocity universal joint (CVJ), the inventors have focused on management of the following factors causing such distortion: (1) an output of high-frequency coil for heating; (2) concentration of coolant within liquid cooling medium; and (3) temperature of the cooling medium, and have succeeded in producing good results.
Due to increasingly stringent demands for suppressing distortion of automotive parts, however, it has become no longer possible to fulfill such demands with the conventional techniques. Thus, to quantitatively extract the effects leading to the distortion as described above, the inventors conducted a measurement of cooling power of liquid cooling medium in a quenching line in a strict manner allowing no variation to be incorporated therein, and examined a relation between the cooling power and the distortion. As a result, the inventors have succeeded in clarifying the effects of the cooling power of the cooling medium on the distortion due to heat treatment, which had been uncertain before conduction of such strict measurement.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a distortion control method that allows suppression of distortion due to heat treatment during quenching of a member, and a cooling power measuring device that enables strict measurement of cooling power of cooling medium that plays an important role in such distortion control.
According to a distortion control method of the present invention, when a member is being subjected to quenching using liquid cooling medium, cooling power of the cooling medium being used is maintained within a prescribed range so as to suppress variation in distortion suffered by the member.
In the course of examination of a relation between distortion due to the quenching and cooling power of the cooling medium, the inventors made a measurement of the cooling power with high precision excluding variation inherent to such cooling power measurement, and as a result, they succeeded in clarifying a significant influence of the cooling power on the distortion. The cooling power is measured as a cooling time that is required for cooling a sample member of a prescribed form by a prescribed temperature range. The correlation between this cooling time and the distortion is not clearly recognized if (a) the material of the sample member transforms in the prescribed temperature range; (b) an immersion depth of the sample member in the cooling medium varies in a range of ±1 mm; and (c) a thick oxide film is formed on the surface of the sample member when heating the sample member. However, by measuring the cooling power using, as a material of the sample member, Ni-based alloy such as Inconel that (a′) maintains an austenite phase from room temperature to high temperature and (b′) is excellent in oxidation resistance, thereby forming almost no oxide film, and (c′) by positioning the sample member in the cooling medium with accuracy within a range of ±0.03 mm, it has become possible to confirm that the cooling power of the cooling medium in the quenching device significantly affects the distortion. This phenomenon was made clear for the first time as the result of the high-precision measurement of the cooling power. Conventionally, the effect of the cooling power on the distortion was not recognized exactly, but was known vaguely as a kind of tendency. As a result of clarification of such phenomenon as described above, it has become clear that the distortion due to heat treatment can be controlled by maintaining the cooling power of the cooling medium in a fixed range. The quenching as mentioned above may be induction hardening, or the entire member may be heated in a heating furnace. Throughout the specification, the cooling power is expressed using a convenient measure. More specifically, a cooling time in which a sample member is cooled by a prescribed temperature range, or a concentration of coolant included in liquid cooling medium that is expressed as an equivalent new coolant concentration, as will be described later, is used as the measure to express the cooling power.
In the distortion control method of the preset invention, the cooling medium is cooling water including coolant, and a change of cooling power of the cooling water due to a running change of the coolant is maintained in the fixed range.
The coolant is a water-soluble liquid polymer, such as polyalkylene glycol (PAG). Normally, the coolant is dissolved into cooling water in concentration of 5-20% for prevention of quenching crack. Using the coolant, a uniform vapor film is formed on the surface of the member undergoing the heat treatment, which helps slow down the cooling, thereby preventing the quenching crack. Thus, by dissolving the coolant in the cooling medium and maintaining the cooling power in a fixed range, it becomes possible to suppress the distortion while preventing the quenching crack. Since the coolant consists of liquid polymer as described above, the polymerization degree of such high polymer is lowered as heat history is accumulated during the quenching. Therefore, as operating days pass from the first day of use of new liquid of coolant, the cooling power of the cooling medium increases in a constant manner. If the cooling power is expressed using the equivalent new coolant concentration as will be described below, the value decreases in a constant manner. During this, however, the concentration of coolant measured by a saccharimeter does not exhibit a significant change. This means that the cooling power cannot be estimated by only measuring the concentration using the saccharimeter. The cooling power ceases to increase after 30 days have passed since the day on which cooling medium in the quenching device was renewed and the use of new liquid of coolant was started. It is said that, when the polymerization degree of polymer is lowered to a certain extent, the polymer is stabilized against thermal shock. The stabilization of the cooling power after a lapse of 30 days as described above is considered because the polymer has reached such low polymerization degree. The distortion would not be controlled accurately if only the cooling power of newly applied cooling medium is considered without paying attention to such change in cooling medium over time.
The distortion control method of the present invention is a method of measuring cooling power of cooling medium employing quenching of a sample member in a prescribed form made of a material that does not transform in a temperature range to be measured. The measurement is conducted by immersing the sample member into the cooling medium and positioning the member at its quenching stop position with accuracy within a range of ±0.03 mm.
By performing such high-precision cooling power
Iihara Michio
Maruyama Takeshi
Yamaguchi Masami
Yoshitomi Jun
Ip Sikyin
McDermott & Will & Emery
NTN Corporation
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