JPH04325624A - Austempering treatment for spheroidal graphite cast iron - Google Patents

Abstract

PURPOSE:To improve the machinability of high strength austempered ductile cast iron by subjecting spheroidal graphite case iron to heat treatment in a decarburizing atmosphere, further to austenitizing at specific temp., to holding at the isothermal transformation temp., and then to air cooling. CONSTITUTION:Spheroidal graphite case iron is heat-treated in a decarburizing atmosphere. It is preferable to regulate the decarburizing atmosphere and the decarburizing treatment time to 900-950 deg.C and 3-5hr, respectively. By the above procedure, the surface of the spheroidal graphite case iron is formed into decarburized state. Further, austenitizing is applied to the above at 850-950 deg.C. It is preferable to regulate the austenitizing time to 30-90min. The above decarburizing treatment can be done prior to or simultaneously with austenitizing treatment. By the above procedure, the surface of the spheroidal graphite case iron can be formed mainly into ferrite matrix. Subsequently, the cast iron is held at an isothermal transformation temp. of 300-400 deg.C for 30-90min and air-cooled. By this method, the ferrite in the decarburized layer is subjected to pearlite transformation and the matrix is transformed from austenite into bainite structure, by which the machinability of the austempered ductile cast iron can be improved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austempering process for improving the machinability of austempered ductile cast iron.

0002

2. Description of the Related Art Spheroidal graphite cast iron, which is an inexpensive high-strength cast iron, is often used as a material for parts that require strength, such as automobile parts and construction machine parts.

[0003] In order to improve the strength and toughness of this spheroidal graphite cast iron, an austenburizing process is performed to change the austenite structure by quenching into a bainite structure. This austempering treatment basically involves, for example, hardening spheroidal graphite cast iron at around 900°C (about 1 hour) and then heating it at 400 to 450°C.
, and kept at constant temperature for 1 hour to form a structure in which austenite remains in the bainite base.

The presence of retained austenite impairs machinability, making it difficult to expand the use of austempered ductile cast iron for parts that require machining. Therefore, it is considered to completely transform austenite into bainite in order to improve machinability, but the processing equipment and processing control for this are complex, and improvements in practicality are desired. .

[0005]

[Problems to be Solved by the Invention] The problems to be solved by the present invention are to eliminate the above-mentioned problems of the prior art, improve the machinability of austempered ductile cast iron, and further improve its use in parts requiring strength. It is about expanding.

[0006]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention basically uses means for creating a decarburized layer in spheroidal graphite cast iron prior to or simultaneously with the austenitizing treatment.

More specifically, the present invention provides spheroidal graphite cast iron by heat treating the spheroidal graphite cast iron in a decarburizing atmosphere, quenching it at 850 to 950°C, then keeping it at a constant temperature, and then cooling it in air. Provides an Austemper treatment method.

[0008] Preferably, the decarburization atmosphere is 900 to 950
℃, the decarburization treatment time is 3 to 5 hours, the austenitization time is 30 to 90 minutes, the isothermal transformation temperature time is 30 to 90 minutes, and the temperature is 300 to 400℃. Decarburization treatment and austenitization treatment can be performed simultaneously.

[0009] When decarburized before or at the same time as austenitization, the surface of spheroidal graphite cast iron becomes decarburized, and due to austenitization, this decarburized state becomes a low C, mainly ferritic, and does not become bainite, but becomes a pearlite. becomes. The pearlite layer is approximately 0.15mm deep,
In other cases, austenite metamorphoses into bainite structure. The pearlitized layer improves machinability, so approximately 1.
The present invention is extremely useful for parts that require a cutting amount of 5 mm.

0010

[Example 1] As spheroidal graphite cast iron, one that had been subjected to spheroidization treatment by adding Mg was used. Chemical composition: 3.75%C,
2.75%Si, 0.20%Mn, 0.03%P, 0.
008% S, 0.025% Cr, and 0.040% Mg.

[0011] Spheroidal graphite cast iron with the above components, outer diameter 30φ×
A round bar with a length of 200 mm was cast and used as a common sample.

[0012] Sample 1 was kept in a decarburizing atmosphere kept at 900 to 950°C for 4 hours, and about 0.15 mm from the outer surface was removed.
A decarburized layer (see Figure 1) was created up to the depth.

[0013] Decarburized sample 1 was heated to 900 to 950°C.
It was quenched (austenitized) at a temperature of 350° C. for 1 hour, and then kept at a constant temperature of 350° C. for 1 hour. A constant temperature condition was maintained using a salt bath. Then air cooled.

As is clear from FIG. 2, on the surface of specimen 1 maintained at a constant temperature transformation temperature, the ferrite in the decarburized layer is transformed to pearlite, and the matrix is transformed from austenite to bainite structure.

[0015]

[Example 2] A specimen with the same components and the same shape as the specimen shown in Example 1 was designated as specimen 2. Sample 2, 850-1
Decarburized austenitization treatment was carried out for 3 to 5 hours in a decarburized atmosphere maintained at 000°C, followed by 350°C decarburization using a salt bath.
A constant temperature state of 0.degree. C. was maintained for 1 hour. It was then air cooled.

The surface of the sample 2 has been transformed into pearlite, as shown in FIG. In addition, the base is transformed from austenite to bainite structure.

[0017]

[Comparative Example 1] A sample having the same components and the same shape as the sample shown in Example 1 was designated as sample 3. 900 to 950 samples
It was quenched (austenitized) at a temperature of 350° C. for 1 hour, and then subjected to constant temperature treatment at 350° C. for 1 hour using a salt bath to transform austenite into a bainite structure.

[0018]

[Experimental example] The outer surfaces of round bars of specimens 1 to 3 were cut under the following conditions. Bit Mitsubishi HT: 10 Depth of cut 1.0mm Feed 0.2mm/rotation speed
600rpm

Samples 1 and 2 had cutting times approximately 1.5 times faster than sample 3 using the same cutting tool.

[0020]

Effects of the Invention According to the present invention, the machinability of spheroidal graphite cast iron on austempered ductile cast iron subjected to austempering treatment is improved, and the durability of the cutting tool is improved, making it possible to extend the tool life by nearly 50%.

[Brief explanation of the drawing]

FIG. 1 is a metal structure showing a ferrite structure of a decarburized layer on the outer surface of a specimen.

FIG. 2 is a metal structure showing a pearlite structure on the surface of a sample.

Claims (3)

[Claims] 1. A method for austenburizing spheroidal graphite cast iron, which comprises heat-treating spheroidal graphite cast iron in a decarburizing atmosphere, austenitizing it at 850 to 950°C, then maintaining it at a constant temperature transformation temperature, and then air cooling. 2. Decarburization atmosphere: 900 to 950°C, decarburization treatment time: 3 to 5 hours, austenitization time: 30 to 9
0 minutes, isothermal transformation time 30-90 minutes and temperature 300-
The austempering method for spheroidal graphite cast iron according to claim 1, wherein the temperature is 400°C. 3. The austenburizing method for spheroidal graphite cast iron according to claim 1, wherein the decarburization treatment and the austenitization treatment are performed simultaneously in a decarburization atmosphere.