JPS5980714A - Manufacture of cast iron parts with high toughness and wear resistance - Google Patents

Abstract

PURPOSE:To manufacture the titled parts without carrying out isothermal transformation by heat-treating an alloy cast iron material having the composition of acicular cast iron to convert the matrix structure into granular pearlite, mechanically working the material, and heat-treating it again to convert the granular pealite into uniform bainite. CONSTITUTION:A material consiting of, by weight, 0.5-2.0% Cu, 1.0-2.0% Mo, 1.0-3.0% Ni and the balance cast iron is heated, held at 800-1,000 deg.C for 0.5- 6.0hr, and allowed to cool in the air. It is heated again, held at 670-760 deg.C for 0.5-6.0hr, and allowed to cool in the air or furnace-cooled. The material heat- treated as mentioned above so as to condition the structure is mechanically worked to the shape of a product. The worked material is heated to 800- 1,000 deg.C to austenitize the matrix structure, and it is allowed to cool in the air to convert the austenite into bainite. Thus, a cast iron product with both high workability (toughness) and wear resistance is manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing cast iron parts having high toughness and high wear resistance.

Conventionally, a manufacturing method using acicular cast iron has been well known as a manufacturing method for this kind of cast iron parts. This one has a composition of N
i: 1.0-8.0% by weight, Mo: 0.5-1.
By casting an alloy cast iron consisting of 0% by weight and the balance being cast iron, the matrix structure is changed to bainite by casting, thereby producing high-hardness cast iron parts without the need for constant temperature transformation treatment, which requires a lot of man-hours. It is. However, in the above-mentioned conventional manufacturing method, the hardness after as-casting is HRC28-44, which is too hard to perform machining, and there is a problem that workability and toughness are poor. On the other hand, if wear resistance is required, the hardness is insufficient, so the normal composition requires quenching.
As a result, there is a drawback that a refrigerant such as water, oil, salt, etc. is required, making the equipment complicated. Furthermore, when casting parts with inconsistent wall thickness, a uniform bainite base could not be obtained due to variations in the cooling rate, resulting in the problem of uneven hardness.

The present invention has been made in view of these points, and uses alloy cast iron having a composition in the above-mentioned acicular cast iron region as a material, and heat-treats the material to once transform the base structure into granular pearlite, thereby improving workability and toughness. After ensuring air hardenability, machining is performed, and then heat treatment is performed again to uniformly transform the base structure into bainite, improving wear resistance. This material has succeeded in achieving both toughness and wear resistance.

To achieve this objective, the method for manufacturing cast iron parts of the present invention has a composition of Cu: 0.5 to 2.0% by weight and Mo: 1.
．． 0 to 2.0% by weight, and Ni: 1. 800 to 1000 for a material consisting of O to 3.0% by weight and the balance being cast iron.
After heating and holding at 670 to 760°C for 0.5 to 6 hours, cooling in the air, then heating and holding at 670 to 760°C for 0.5 to 6 hours, followed by cooling in the air or in a furnace. The material is then machined into a product shape, and then
After heating to 00°C to austenite the matrix structure, the base structure is left to cool in the atmosphere to become bainite.

The present invention will be explained in detail below.

The composition of the material used in the present invention is that Cu is 0.5 to 2.
．． 0% by weight, MO 1.0 to 2.0% by weight, Ni 1.0' to 3.0% by weight, and the remainder C, Si, and Mn.

It consists of flake graphite cast iron containing P and S within the usual composition range. - Here, to explain the reason for limiting the blending ratio of each of the above elements, if each element of Cu, Mo, and Ni is kept below the lower limit value, the effect of the combination of the above three elements will be reduced during air cooling in the tertiary heat treatment described later. Since bainite formation cannot be achieved, a hard bainite base with excellent wear resistance cannot be obtained, and especially when Cu is reduced to 0.5% by weight or less, when obtaining granular pearlite in the primary and secondary heat treatments described below,
This is because pearlite decomposition resistance decreases. On the other hand, if the content of each element exceeds the upper limit, the above-mentioned combined effect will be saturated, and the amount of the blended elements will be wasted, resulting in an increase in cost.In particular, if Cu is made more than 2.0% by weight, the grain boundary This is because a large amount of segregation occurs in the steel, resulting in a decrease in toughness.

For the above materials, firstly, at 800-1000°C,
Primary heat treatment of heating and holding for 6.0 hours and then cooling to the atmosphere;
After that, a structure adjustment heat treatment is performed by heating and holding at 670 to 760°C for 0.5 to 6.0 hours, followed by cooling in the air or in a furnace.
(Area ratio) is converted into granular iR-write. This greatly reduces the hardness, improving workability and toughness, and also makes it possible to ensure air hardenability in the tertiary heat treatment by performing thermal refining.

In this case, the heating temperature in the first heat treatment is set to 800 to 1000.
The reason for limiting the temperature range is that below 800°C, the casting structure cannot be made uniform, while above 1000°C, the crystal grains become coarse and the toughness decreases. In addition, limiting the heating holding time to a range of 0.5 to 6.0 hours is because if it is less than 0.5 hours, the casting structure cannot be made uniform, whereas if it is more than 6.0 hours, the casting structure cannot be made uniform. This is because productivity (decreases) as the temperature reaches saturation.In addition, the cooling method at that time was air cooling, but slow cooling (including furnace cooling) would reduce the amount required for secondary heat treatment. This is because granular pearlite cannot be obtained.

On the other hand, the heating temperature in the secondary heat treatment was 6.70 to 760°C.
The reason why it is limited to this range is that below 670'C, it takes a long time to decompose the nonorite, which reduces productivity, and it also makes it difficult to reduce the hardness, resulting in a drastic decrease in workability and toughness. This is because at temperatures above 760 DEG C., the /'-lite does not become granular and forms a two-phase mixed structure, which improves air hardenability but greatly reduces workability and toughness. In addition, the reason why the heating holding time is limited to the range of 0.5 to 6.0 hours is because if it is less than 0.5 hours, the decomposition of pearlite will be insufficient and the time required for austenite formation in the tertiary heat treatment will be short. However, the workability and toughness decreased significantly;
This is because if the heating time is exceeded, the granulation of pearlite will further progress and become ferrite, resulting in a decrease in air hardenability. Further, the cooling method at that time may be either air cooling or furnace cooling, but air cooling is preferable from the viewpoint of productivity.

Then, the material, which has been heat-treated as described above to have improved workability and toughness, is machined into a product shape.

In this case, this machining can be easily performed.

After that, 800 to 10
After heating to 00°C to austenite the base structure, a tertiary heat treatment is performed to uniformly form bainite by cooling in the atmosphere. As a result, the hardness can be greatly increased and the wear resistance can be improved.

At that time, the heating temperature for the tertiary heat treatment was set at 800 to 1000°C.
The reason why it is limited to the range of
This is because at temperatures above 00°C, toughness decreases due to coarsening of crystal grains.

Therefore, in the manufacturing method of the present invention, the hardness can be lowered by converting the base structure into granular pearlite through primary and secondary heat treatments, which improves workability and toughness and facilitates machining. Moreover, the finally obtained cast iron parts can be made to have uniform hardness and enhanced wear resistance through tertiary heat treatment without performing isothermal transformation treatment, which requires a large number of steps.

In addition, when manufacturing a cast part that requires fatigue strength, such as a gear, by the manufacturing method of the present invention, it is desirable to perform shot peening treatment to improve the fatigue strength after performing the above-mentioned tertiary heat treatment.

Next, specific examples will be described. Inventive materials I to (1) having the compositions shown in Table 1 below and a carburized material as a comparative material were prepared as test materials.

Among these, the present invention material ① had a temperature of 3.5
1st and 2nd stage, held for 4.5 hours and then cooled to the atmosphere.
Next, heat treatment was performed. In this state, the hardness is HRc15-19
This shows that it is possible to improve workability and toughness and facilitate machining. The volume fraction of graphite in the base structure was 10%10yx'', and the area fraction of granular barite was 50%)/cm2.

Next, this test material was subjected to a tertiary heat treatment in which it was held at a temperature of 900'C for 15 seconds and then forcedly air cooled.

As a result, the hardness was HRC52-54, which is much higher than that of conventional acicular cast iron (RC28-44).Also, the area ratio of bainite in the matrix structure was 60
~70%, area ratio of retained austenite is 20-30%
and were uniformly distributed.

In addition, when the cooling method for the above-mentioned invention material I was natural cooling instead of forced air cooling in the tertiary heat treatment, the hardness (
It still outperforms conventional acicular cast iron. HRC47
~49 was shown.

A wear resistance test was conducted using the present invention material (1) and the carburized material Scr 22 (HRC62), which had been subjected to these two types of tertiary heat treatment. The test was carried out using two test materials, as shown in Figure 1, with a disk 2 formed of one test material and fixed on a rotating shaft 1 rotating at a constant speed, and a disk 2 formed of the other test material. This is based on a testing machine T consisting of a pin 3 fixed offset from the rotating shaft 1 and pressed against the upper surface of the disk 2 with a load P (Kg). The test conditions are that the friction speed is constant at 2.761 m/sec,
The friction area was 9 mm2, the test time was 5 minutes, the amount of offset between the rotating shaft 1 and the pin 3 was 4 Q 7 nm, and the contact condition was dry. The test results are shown in FIG. 2 in terms of the wear amount of the pin 3 with respect to the load P.

In FIG. 2, characteristic line A shows the case where both the disk 2 and the pin 3 are made of carburized material, and the load P is 4.
Burn-in occurs when using KIF.

On the other hand, characteristic lines B and B show the case where both the disk 2 and the pin 3 are made of the present invention material I,
The cooling methods in the tertiary heat treatment were forced air cooling and natural cooling, respectively.

According to this, there is no major difference in characteristics between forced air cooling and natural cooling, and both have the same wear resistance compared to when both the disk 2 and pin 3 are made of carburized material. However, the limit load P that causes sintering (-1
is on the hood. Furthermore, the characteristic lines C and C' are
A case is shown in which the disk 2 is made of a carburized material and the pin 3 is made of the material 1 of the present invention, and the cooling methods in the tertiary heat treatment are forced air cooling and natural cooling, respectively. According to this, although there is not much difference in characteristics between forced air cooling and natural cooling, both have better wear resistance than when both the disk 2 and pin 3 are made of the inventive material (■). Furthermore, the limit load P at which seizure occurs in both the disk 2 and the pin 3 is significantly higher than in the case where both the disk 2 and the pin 3 are made of carburized material.

Further, FIG. 3 shows the test results of the wear resistance test using the present invention materials 1 to ① and the carburized material in terms of the wear amount of the pin 3 with respect to two types of loads P.

Inventive products (1) to (2) were each subjected to first to third heat treatments under the same conditions as those applied to the above-mentioned inventive material (1), and cooling in the third heat treatment was performed by natural cooling.

Further, the same material was used for both the disk 2 and the pin 3 in the test. According to this, the products of the present invention I-II
It can be seen that the amount of wear due to I is equal to or less than the amount of wear due to carburized material.

As explained above, according to the manufacturing method of the present invention, a material of alloyed cast iron having a composition in the acicular cast iron region is heat-treated to transform the base structure into granular pearlite, then machined into a product shape, and then heat-treated again. As a result, the matrix structure is uniformly transformed into bainite, achieving both the contradictory requirements of workability, toughness, and wear resistance, without requiring a time-consuming isothermal transformation treatment, resulting in high toughness and high wear resistance. This has the excellent effect of simplifying the manufacture of cast iron parts having the same.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a testing machine for wear resistance testing, and FIGS. 2 and 3 are graphs showing the amount of pin wear against load P.

Claims (1)

[Claims] (1) Composition: Cu: 0.5 to 2.0% by weight, Mo
: 1.0-2.0% by weight, Ni: 1.0-3
,0% by weight and the balance is cast iron, 80% by weight.
After heating and holding at 0 to 1000°C for 0.5 to 6.0 hours, it is allowed to cool in the atmosphere, and then heated again at 670 to 760'C for 0.5 to 6.0 hours.
After being heated and held for 6.0 hours, a structure adjustment heat treatment is performed in which the material is cooled in the air or in a furnace, and then the material is machined into a product shape, and then heated to 800 to 1000'C to austenite the base structure. A method for manufacturing cast iron parts having high toughness and high wear resistance, which is characterized by forming bainite by cooling in the atmosphere.