JPH032926B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat treatment method for cast iron parts, and more particularly to a heat treatment method for improving the toughness of a surface portion of a cast iron part.

Traditionally, cast iron materials have been heated to a solid solution formation temperature (900℃).
When the material is heated and held for a certain period of time (before and after) to austenite, then rapidly cooled and held at a constant temperature of 220°C to 420°C for a certain period of time, so-called austempering treatment, a bainitic structure is generated and its toughness is significantly improved. This austempering treatment is widely used to increase the toughness of various cast iron parts.

In the above austempering process, heating and holding at the solid solution formation temperature (around 600℃) is usually 1
It takes about an hour. The reason why the heating is maintained at a temperature of around 900°C for about one hour is to uniformly heat the cast iron material to around 900°C from the surface to the center.

However, there are many cast iron parts that require such austempering treatment, such as gears, for which it is sufficient to improve the toughness of only the surface portion. Therefore, even the center of such cast iron parts is unnecessarily heated to 900℃.
If the heating is maintained for a long period of about one hour as described above in order to heat back and forth, an extremely large amount of energy will be wasted. If the heat is maintained for as long as one hour, the time required for heat treatment will naturally increase, and furthermore, it will take time to rapidly cool down to a temperature of 220°C to 420°C, which will adversely affect the formation of bainite structure. The above problem also occurs.

An object of the present invention is to provide a method for heat treating cast iron parts that eliminates the above-mentioned problems.

The heat treatment method for cast iron parts of the present invention uses high-frequency heating to heat the surface part to the solid solution formation temperature (approximately 900°C) when performing the above-mentioned austempering treatment on cast iron parts that require improvement in toughness only in the surface part. The basic concept is to adopt the following.

However, when the above-mentioned high-frequency heating is started with the cast iron parts kept at room temperature, as shown in Figure 1,
Even during the cooling period when high-frequency heating is stopped, the temperature of the center of a cast iron component continues to rise as it absorbs heat from the high-temperature surface. As a result, the surface area becomes
Before the constant temperature holding process from 220℃ to 420℃, the temperature drops to below the Ms point (usually 210℃, the temperature at which martensitic transformation begins), and a martensitic structure is generated on the surface, achieving the desired toughness. This causes the problem that the characteristics cannot be obtained. It is not impossible to prevent the temperature of the surface from dropping below the Ms point if various conditions are set, but the heat transfer characteristics between the center and the surface will depend on the shape of the cast part, the ambient temperature, etc. Complexly related, uniform surface area at 220℃
Cooling down to ~420°C is extremely difficult at a practical level.

Therefore, the method of the present invention includes a step of holding the cast iron component at a temperature of 220° C. to 400° C. for a predetermined period of time before the high-frequency heating. By performing such preheating, the center part will not rapidly absorb heat from the surface part during the cooling period after the high-frequency heating is stopped. Therefore, the temperature of the surface portion does not drop suddenly below the Ms point.

In other words, in summary, the method for heat treatment of cast iron parts of the present invention involves holding the cast iron part at a temperature of 220°C to 420°C for a predetermined period of time, and then heating the cast iron part with high frequency to bring only the surface portion to a temperature of around 900°C. Rapidly heat the cast iron parts to 220℃~
After rapid cooling to a temperature of 420℃, this 220℃～
It is kept constant at a temperature of 420°C for a predetermined period of time.

Examples of the present invention will be described in detail below.

FIG. 1 shows temperature changes in a cast iron gear whose tooth surfaces are required to have toughness, when the gear is heat treated by the method of the present invention. The workpiece, a cast iron gear, is first placed in a preheating chamber consisting of a known furnace, etc., where both the center and surface are heated to 265℃.
The sample is heated until the temperature reaches 100.degree. C. and held at this temperature for a predetermined time _T1 .

Next, this preheated workpiece is placed on an induction hardening station, and only the tooth flanks that need to be toughened are heated to 910°C by high frequency. This high frequency heating time _T2 is the time required for austenitization,
Approximately 10 seconds is sufficient for perlite. As shown in the figure, according to this high-frequency heating, the tooth flank portion of the workpiece is rapidly heated to the above-mentioned 910°C.

Next, the workpiece is placed in a soluble, salt furnace, etc. and rapidly cooled to 265℃. Here, since only the tooth surface portion of the workpiece was heated to 910° C. as described above, this cooling was performed extremely rapidly, which is advantageous for the formation of a bainite structure.

Thereafter, the rapidly cooled work is kept at a constant temperature of 265° C. for a predetermined time T ₃ (for example, 5 minutes or more) in an electric furnace, fluidized bed furnace, etc., and then air cooled. This constant temperature maintenance transforms the base structure of the tooth surface portion of the workpiece into a bainite structure, forming a cast iron gear with a strong tooth surface.

In the above method, since only the portion (tooth surface portion) that requires heat treatment is subjected to high frequency heating as described above, the time T ₂ for heating this portion is only about 10 seconds. Since the preheating performed before this high-frequency heating is also performed at a temperature of about 220°C to 420°C, the energy consumption in the heat treatment method of the present invention is lower than that of conventional austempering in which the entire workpiece is held at around 900°C for about 1 hour. The energy consumption is much lower than that of processing.

As explained in detail above, the method of heat treating cast iron parts of the present invention consumes little energy and can be carried out within a short time, so it can be carried out at extremely low cost. Moreover, since the cooling after austenitization is performed at a very high speed, the quality of the heat-treated product is also improved.

[Brief explanation of the drawing]

Figure 1 is a graph showing the temperature change of the workpiece during austempering treatment using high-frequency heating;
The figure is a graph showing changes in temperature of a workpiece in a heat treatment method according to an embodiment of the present invention.

Claims (1)

[Claims] 1. Hold the cast iron part at a temperature of 220°C to 420°C for a predetermined period of time, then rapidly heat the cast iron part by high-frequency heating until only the surface reaches a temperature of around 900°C, and then heat the cast iron part to a temperature of 220°C to 420°C. A method for heat treatment of cast iron parts, which is characterized by rapidly cooling to a temperature of 420°C and then maintaining the temperature at a constant temperature of 220°C to 420°C for a predetermined period of time.