JPH05112817A - Method for heat treating adi parts - Google Patents

Abstract

PURPOSE:To provide the method capable of improving the mass effect of heat treatment only by the selection of heat teating conditions without the addition of expensive alloy elements. CONSTITUTION:In the austempering treatment of ADI (austempered spheroidal graphite cast iron) parts, at the time of executing rapid cooling from the austenitization temp. to the bainitization temp., this parts are subjected to primary cooling to a temp. at which the difference with the austenitization temp. is >=600 deg.C in the temp. range of 225 to <300 deg.C, are thereafter held to the bainitization temp. and are subjected to austermpering.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention adds a first stage cooling step of a specific temperature difference when cooling from an austenitizing temperature to a bainizing temperature and reduces the mass effect of heat treatment by increasing the cooling rate. The present invention relates to a heat treatment method for austempered spheroidal graphite cast iron (hereinafter referred to as ADI) parts.

[0002]

2. Description of the Related Art Generally, ADI parts can be remarkably toughened with a tensile strength of about 1000 N / mm ² and an elongation of about 10% by austempering spheroidal graphite cast iron (FCD), so that mechanical parts can be made compact. In order to meet the demand for lighter weight, adoption in automobiles and construction machinery is expanding. The reason for toughening is that austenite causes a large amount of high carbon bainite and retained austenite to appear, and it is said that the retained austenite transforms to martensite and becomes hard when external stress is applied, and it is said that it exhibits remarkable deformation resistance. ing.

Usually, the austempering method shown in FIG. 1 is used for the heat treatment for manufacturing ADI parts. Figure 1
The austemper according to the conventional method is austenitic heating of spheroidal graphite cast iron parts at 800 to 950 ° C., and heating and holding at that temperature for 0.5 to 3 hours to austenite the base structure, and then bainite at 300 to 400 ° C. It is cooled by quenching to the oxidization temperature and kept as it is for 0.5 to 4 hours, and isothermically transformed into bainite and retained austenite, and then air-cooled. The ADI parts manufactured in this way are manufactured according to JIS FCD4 as spheroidal graphite cast iron.
When 50 is used, the tensile strength is 1000 N / mm ² , and the elongation is about 10%, and the material is significantly toughened. This value is FCD4
This is an epoch-making product in which the tensile strength of 50 is about 500 N / mm ² and the elongation is 10%, while the tensile strength is doubled while maintaining the same elongation value.

However, as is well known, the heat treatment has a mass effect, and the mechanical properties obtained decrease as the size of the heat-treated product increases. This is because the cooling rate at the time of cooling from the austenitizing temperature to the isothermal holding temperature becomes smaller as the mass of the heat-treated product becomes larger, so that pearlite appears in the matrix structure, and the tensile strength and especially the elongation become small. Because.

In order to reduce this mass effect, in the isothermal transformation curve of FIG. 2, (1) move the pearlite transformation point to the long side, and (2) move the cooling curve of the heat-treated product to the left. There are two possible ways. Conventionally,
For example, JP-B-55-3422 and JP-B-59-10.
No. 988, etc., a large amount of ADI can be obtained by adding an appropriate amount of alloying elements such as Mo, Cu and Ni to FCD and moving the isothermal transformation curve to the right side as shown in FIG.
It is known to change. This prior art is based on the method (1) of the above two methods.

However, the addition of expensive alloying elements directly increases the cost of the product. Also,
In general, the non-alloy cast iron casting line and the alloy cast iron casting line coexist or coexist, which causes a problem that the management cost of the return material after the casting finish increases.

It is an object of the present invention to provide a method capable of improving the above-mentioned mass effect by means of (2), that is, without adding an expensive alloying element, and only by selecting a heat treatment condition.

[0008]

According to the present invention, in the austempering of ADI parts, rapid cooling from an austenitizing temperature to a bainizing temperature is performed.
In the temperature range of 5 ° C. or more and less than 300 ° C., the first stage cooling is performed to a temperature having a difference of 600 ° C. or more from the austenitizing temperature, and then the temperature is maintained at the bainitizing temperature and austempered, whereby It is a solution to the problem.

The present invention will be described below mainly with reference to the accompanying drawings. Fig. 3 shows FC of 25mmφ x 220mmL
D450 specimens were austenitized at temperatures 850 and 9
It is the result of having measured the cooling curve at the time of cooling to the bainitizing temperature of 00 degreeC to 375 degreeC about the surface part and the center part, respectively. Comparing the cooling curves at the respective austenitizing temperatures, the higher the austenitizing temperature of 900 ° C. is, the higher the cooling rate is obviously in both the surface portion and the central portion. Especially, the difference is the largest in the range of 500 to 700 ° C. Generally, the higher the austenitizing temperature, the larger the amount of heat held by the test piece, so it was thought that the cooling rate when cooled to the same temperature would be lower. However, as shown in FIG. 3, under the ADI heat treatment conditions, the larger the difference between the austenitizing temperature and the bainitizing temperature, the larger the cooling rate.

FIG. 4 shows the microscopic structure of the center part of these test pieces. The center part of the test piece cooled from 900 ° C. shows a 95% bainite structure, while 850
Those cooled from 0 ° C deposit about 30% of pearlite.

A sensor of the quench test heated to an austenitizing temperature of 850 ° C. was put into a bainizing temperature of 200,
FIG. 5 shows the results of measuring the cooling curve and the cooling rate without adding the salt bath, which was maintained at the temperatures of 225, 250 and 375 ° C., without stirring. As is clear from FIG. 5, the cooling to 375 ° C., which is a general bainizing temperature, is 200 to 200 ° C.
It shows a remarkably small cooling rate as compared with cooling to a temperature too low as a bainizing temperature such as 250 ° C.

Generally, the most frequently used bainizing temperature is 350 to 400 ° C. In this case, if the austenitizing temperature is set to 950 to 1000 ° C., a temperature difference of more than 600 ° C. can be maintained, but such a high temperature is required. At the austenitizing temperature, brittleness due to crystal grain growth or the like is remarkable, and the target toughness of the austemper cannot be achieved. Therefore, increasing the austenitizing temperature to set the temperature difference from the normal bainizing temperature to 600 ° C. or more results in contrary to the original purpose of the heat treatment.

Therefore, in the present invention, the ADI product is
After austenitizing to 950 ° C., before cooling to the bainizing temperature, once in a temperature range of 225 to less than 300 ° C., first stage cooling is performed to a temperature at which the difference from the austenitizing temperature is 600 ° C. or more, After that, a predetermined bainizing temperature is maintained and austempering is performed to increase the cooling rate near the pearlite transformation temperature of 500 to 700 ° C. to suppress the precipitation of pearlite to a minimum. This heat treatment diagram of the present invention is shown in FIG.
It shows in (b).

FIG. 6B shows the temperature conditions of the method of the present invention and the conventional method with the austenitizing temperature γ on the vertical axis and the bainizing temperature B on the horizontal axis.
In FIG. 6 (b), the quadrangle PQTS indicates the temperature condition of the first stage cooling of the present invention, the rectangle QRVU indicates the temperature condition of the conventional method, and the upper part of the straight line ST indicates the austenitizing temperature and the bainizing temperature. Indicates a temperature range in which the difference of 600 ° C. or more.

The lower limit temperature of the first cooling in FIGS. 6A and 6B is 225 ° C. This is a general non-alloy FC
The martensite transformation point of D is in the range of 200 to 220 ° C,
The occurrence of martensitic transformation by this cooling depends on another toughening mechanism, and this has already been described in other patents, for example, JP-A-61-76612, Japanese Patent Publication No.
-42326 and the like.

The upper limit temperature 299 ° C. of the first cooling is the temperature closest to the lower limit 300 ° C. of the normal bainizing temperature. Further, the austenitizing temperature and the bainizing temperature are determined by the material, shape and size of ADI, required mechanical properties, etc., and are not restricted by the present invention.

Examples will be shown below.

EXAMPLE FIG. 7 shows a quench test at 850 ° C.
→ 225 ° C → 375 ° C and 850 ° C → 250 ° C → 375
The results of measuring the cooling curve and cooling rate when performing the austempering according to the present invention at ℃ are shown. Comparing this with the results of FIG. 5, the cooling in the pearlite precipitation range of 500 ° C. to 700 ° C. becomes as fast as the cooling to 225 to 250 ° C., and after cooling, it reaches 375 ° C. in 20 seconds and the bainite temperature is maintained. Will be done. With cooling to 375 ° C. (FIG. 5), the bainite temperature is finally maintained after 60 seconds.

Austenite at 900 ° C., 375 ° C.
25mmφ and 50mmφ × 220 bainized
The following table shows the results obtained by processing the JIS No. 4 test piece from the mmL ADI test piece and performing the tensile test for the conventional method and the method of the present invention in comparison. The initial cooling temperature in the case of the present invention is 225 ° C.

[0019]

From the above table, according to the ADI heat treatment method of the present invention, the improvement of the tensile strength at 25 mmφ is 50 m.
It is clear that the improvement in elongation is significant for mφ.

[0021]

As described above, the improvement in tensile strength according to the present invention is 15% at 25 mmφ and the improvement in elongation is 44% at 50 mmφ. Such an effect promotes the adoption of a tough ADI, leads to a reduction in fuel consumption due to the weight reduction of automobiles and construction machinery, and has a great effect.

[Brief description of drawings]

FIG. 1 is an austempering heat treatment diagram of ADI according to a conventional method.

FIG. 2 is a TTT curve diagram of FCD for reducing the mass effect.

FIG. 3 is an explanatory diagram showing a difference in cooling curves due to a difference in austenitizing temperature.

FIG. 4 is an explanatory diagram showing a difference in micrographs of a metal structure due to a difference in austenitizing temperature.

FIG. 5 is an explanatory diagram showing a relationship between various bainizing temperatures, a cooling curve, and a cooling rate.

6 (a) is an austemper heat treatment diagram according to the present invention, and FIG. 6 (b) is an explanatory diagram showing temperature ranges of the method of the present invention and the conventional method.

FIG. 7 is an explanatory diagram of a cooling curve and a cooling rate according to the present invention.

Claims (1)

[Claims] 1. ADI (Austempered spheroidal graphite cast iron)
When performing rapid cooling from austenitizing temperature to bainizing temperature in austempering of parts,
In the temperature range of 225 ° C. or higher and lower than 300 ° C., the first stage cooling is performed to a temperature having a difference of 600 ° C. or higher from the austenitizing temperature, and then the temperature is maintained at the bainizing temperature for austempering. Heat treatment method for ADI parts.