JPH0578712A - Production of sintered part - Google Patents

Abstract

PURPOSE:To increase the mechanical strength and to uniformize the internal hardness of a sintered part by adding C and a forming lubricant to a composite alloy steel powder of specified composition, sintering the admixture and then continuously cooling the sintered compact. CONSTITUTION:From 0.6 to 0.9wt.% C and a forming lubricant are added to a composite alloy steel powder consisting of 4.0-6.0% Ni, 1.0-2.0% Cu, 0.4-0.6% Mo and the balance Fe. The mixed powder is placed in a die and compacted to >=7.0g/cm<3> density, and the compact is sintered at 1250-1300 deg.C. The sintered compact is continuously cooled at a rate of about 5-20 deg.C/min to form a mixed structure of martensite and bainite. Consequently, heat treatment after sintering is dispensed with, and a high-hardness product excellent in tensile strength and impact value and having uniform internal hardness is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a sintered part by sintering a powder material obtained by adding an alloying element to iron powder.

[0002]

2. Description of the Related Art The powder metallurgy method has been conventionally used for manufacturing parts having complicated shapes which require high strength, high toughness, high wear resistance such as vane pump rotors, cam rings and gears.

In the powder metallurgy method, as shown in FIG.
C (carbon) is added to and mixed with the composite alloy steel powder material in which fine powders of alloy elements such as i are diffused and fixed by heat treatment, and the mixed powder is put into a mold and pressure-molded, and then 11
After heating at a temperature of 00 to 1250 ° C. for 20 to 60 minutes to sinter, sizing if necessary, heat treatment is performed to increase mechanical strength, and further subzero treatment for cooling to 0 ° C. or less is performed. Sintered parts are manufactured so as to prevent aging due to crystal structure transformation.
Here, the chemical composition of the above mixed powder is generally shown in Table 1 below.

[0004]

[Table 1]

[0005]

In the conventional method, heat treatment is carried out after sintering in order to enhance mechanical strength. In this heat treatment, the sintered body is heated at 850 to 900 ° C. for 1 to 3 hours, oil-quenched, and then heated at 150 to 180 ° C. for 1 to 1.
I try to temper it over 2 hours.

However, the heat treatment described above not only takes a long treatment time and causes a large loss of heat energy, but also because the sintered body is porous, the hardenability is poor, and the difference in hardness and strength between the inside and outside of the product is large. . Equipment for heat treatment,
Space and personnel may be required, and a grinding process may be required to remove strain caused by heat treatment.
Although a high Ni alloy steel showing sufficient hardenability is also known, sufficient mechanical strength cannot be obtained and the cost is high.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above problems and eliminates the heat treatment after sintering in the conventional process, and obtains a product having excellent mechanical strength and uniform internal hardness of the sintered parts. And the chemical composition is Ni; 4.0 to 6.0% by weight, C
u: 1.0 to 2.0% by weight, MO: 0.4 to 0.6% by weight, the balance being C in a composite alloy steel powder consisting essentially of iron;
A powder material added with 0.6 to 0.9% by weight and a molding lubricant was molded to a density of 7.0 g / cm ³ or more, and then this molded body was sintered at 1250 to 1300 ° C. It is characterized by having a mixed structure of martensite and bainite by continuous cooling.

Of the above compositions, Ni, Cu and M0 are the same as the conventional ones, for example, Ni; 4.0 to 4.8.
% By weight, Cu; 1.3 to 1.7% by weight, MO; 0.45
A commercially available product having 0.55% by weight can be used.

The higher the density of the green compact, the higher the mechanical strength. However, considering the die strength and wear,
It is preferably 7.0 g / cm ³ or more and 7.3 g / cm ³ or less.

Regarding the sintering temperature, Ni, Cu, M
Since O and C have a high diffusion temperature to Fe and a low diffusion rate, the solid solution of alloying elements becomes insufficient at the same temperature as in the past, so sintering is performed at 1250 to 1300 ° C. The amount of carbon added is 0.6 to 0.9% by weight. This is because the tensile strength and hardness are significantly reduced outside this range.

Further, upon cooling after sintering, the cooling rate in the CCT diagram (continuous cooling transformation curve) is 5 to 20 ° C. /
It is preferably set to min. This is because the required martensite structure does not appear uniformly when the temperature is less than 5 ° C / min, and a new cooling device is required for the equipment when the temperature exceeds 20 ° C / min.

[0012]

It is generally said that the strength decreases when 0.6% by weight or more of carbon is added. However, even if 0.6% by weight or more of carbon is added, the sintering temperature is increased and the continuous cooling rate is within a predetermined range. It has been found that the mechanical strength and hardness are improved by the above.

[0013]

EXAMPLE As shown in Table 2, Ni was 4.3% by weight and Cu
1.6% by weight, MO 0.5% by weight, Si 0.01%
%, Mn is 0.05% by weight, and a commercially available composite alloy steel base powder having the balance substantially iron is prepared. C (carbon), in Experimental Example 2, 0.5% by weight of C,
In Experimental Example 3, 0.6% by weight of C was added, in Experimental Example 4, 0.8% by weight of C was added, and in Experimental Example 5, 1.0% by weight was added.
Wt% C was added, and 0.8 wt% zinc stearate as a molding lubricant was further added to each experimental example.
Then, the above material powder was put into a mold and the density was 7.05 g / c.
m ³ molded body, and this molded body at 1300 ° C. for 60 minutes,
The atmosphere in the furnace was sintered with 90% N ₂ + 10% H ₂ , and the continuous cooling rate after sintering was set to 7 ° C./min to obtain a test piece.
For comparison of the sintering temperatures, as Experimental Example 6, a test piece having the same components as in Experimental Example 3 was sintered at 1150 ° C. × 40 min.

[0014]

[Table 2]

Table 3 and FIGS. 2 to 4 show the relationship between the tensile strength, hardness and impact value of each test piece obtained above and the amount of carbon added. As is clear from Table 3 and FIGS. 2 to 4, when the C content is less than 0.6% by weight, the tensile strength and hardness cannot reach the target values, and the C content is 1.
When the content is 0% by weight or more, except for a slight increase in hardness, the composition is excessively hardened, causing brittle fracture, and both tensile strength and impact value are significantly reduced. In the present invention, C: 0.6 to 0.9% by weight all satisfy the target value. For comparison, a test piece having the same composition as in Experimental Example 3 was used and the sintering temperature was 11
Sintering at 50 ℃ × 40min and heat treatment (850 ℃ × 1H)
FIG. 5 shows a comparison of the hardness from the surface of the test piece obtained by the conventional method and the test piece obtained by Experimental Example 4 of the present invention. As is clear from FIG. 5, the present invention has less variation.

[0016]

[Table 3]

FIG. 6 is a photomicrograph (400 times) showing the metal structure of the test piece obtained by the present invention, and FIG. 7 is a photomicrograph (400 time) showing the metal structure of the test piece obtained by the conventional method. From these photographs, it can be seen that in the case of the method of the present invention, the metal structure is a substantially uniform mixed structure of martensite and bainite, and there is no retained austenite structure scattered in the martensite structure as compared with the case of the conventional method.

[0018]

[Effect of the invention]
As is clear from Table 4 and FIGS. 2 to 5, according to the sintering method of the present invention, the heat treatment after sintering is unnecessary, and the tensile strength and the impact are higher than those of the products subjected to the heat treatment after sintering in the conventional method. It is possible to obtain a highly reliable product having excellent mechanical strength such as value, high hardness, and uniform internal hardness.

Further, since the heat treatment and the sub-zero treatment to be performed thereafter are unnecessary, it is possible to reduce the loss of thermal energy, greatly simplify the equipment, and reduce the cost.

[Brief description of drawings]

FIG. 1 is a schematic process diagram of a conventional powder metallurgy method.

FIG. 2 is a graph showing the relationship between the amount of carbon added and the tensile strength.

FIG. 3 is a graph showing the relationship between the amount of carbon added and the hardness (HRA).

FIG. 4 is a graph showing the relationship between the amount of carbon added and the impact value.

FIG. 5 is a graph showing the relationship between the depth from the surface and the hardness (HRA).

FIG. 6 is a micrograph showing the metal structure of a test piece obtained according to the present invention.

FIG. 7 is a micrograph showing the metal structure of a test piece obtained by a conventional method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Murakami 1-14-1 Fujiwara-cho, Gyoda-shi, Saitama Stock Company Showa Manufacturing Co., Ltd. Saitama Head Office Factory (72) Inventor Toshiyuki Minegishi Kawasaki-cho, Chiba-shi Chiba Prefecture Kawasaki Chiba Steel Works, Ltd. (72) Inventor Akio Sonobe 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Company, Chiba Works

Claims (1)

[Claims] 1. A chemical composition of Ni; 4.0 to 6.0% by weight,
Cu: 1.0 to 2.0% by weight, MO: 0.4 to 0.6% by weight, and C in the composite alloy steel powder consisting essentially of iron as the balance.
A powder material added with 0.6 to 0.9% by weight and a molding lubricant is molded to have a density of 7.0 g / cm3 or more, and then this molded body is sintered at 1250 to 1300 ° C. A method for manufacturing a sintered part, characterized in that it is cooled to form a mixed structure of martensite and bainite.