JPS5822538B2 - Heat treatment method for cobalt-based casting alloys - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat treatment method for improving the high temperature fracture resistance of cobalt-based cast alloys.

Cobalt-based cast alloys, which are made by adding elements such as C, W, and Cr to a Co-base, have excellent hardness and compressive strength at room and high temperatures, and are especially excellent in wear and deformation resistance at high temperatures. Therefore, it is currently widely used in high-temperature type steel tools.

In this way, cobalt-based cast alloys have high hardness and compressive strength at high temperatures, and excellent wear and deformation resistance, but on the other hand, their ductility is low even at high temperatures (700 to 800°C). This is extremely low, and for this reason, conventional alloys of this type have had the disadvantage of frequently cracking during use.

Looking at this cobalt-based cast alloy from the aspect of its crystal structure, it is effective to refine the microstructure of the cast product in order to improve its high-temperature compressive strength, but at the same time, it is effective to refine the microstructure of the cast product. However, making the microstructure of the cast product rougher contributes to improving the deformability of the cast product to some extent, but at the same time, the high temperature Since the compressive strength decreases, it becomes more prone to breakage, and in the end, the problem of cracking cannot be solved simply by adjusting the crystal structure.

Therefore, when we look at the main cause of this cracking, we find that the structure of the cobalt-based cast alloy contains C-Cr and C-W-
This is believed to be because a large amount of Co-based carbide is present, and this carbide is hard and brittle, has an angular shape, and is distributed in a directional or net-like manner.

In this way, the coarsely precipitated carbides in this special distribution state are divided and refined, the angular carbides that tend to cause stress concentration are made spheroidal, and the hardness of the austenite matrix that exists between the carbides is reduced. It is believed that the ductility of cobalt-based casting alloys can be improved without reducing the high-temperature compressive strength possessed by their fine microstructures, thereby eliminating the problem of cracking. It will become.

The present invention has succeeded in achieving such structural improvement through heat treatment, which is effective in preventing cracks.
The heat treatment method of the present invention includes C: 1.5 to 3.0%, Si: 1
．． 0% or less, Mn: 1.0% or less, Cr: 25-3
5%, W: 8-17%, Fe: 3.0% or less, P, S:
0.03%, Ni: 1.0% or less, remainder substantially C
4 to 1150 to 12000G
After heating and holding for 30 hours, the hardness of the austenite base was H.
It is characterized by having v500 or less.

This type of cobalt-based casting alloy has traditionally been heat treated for the purpose of strain relief and sintering.
This is not only different in processing conditions from the one according to the present invention, but also completely different in purpose.

In describing the present invention in detail below, the reason for limiting the range of components of the cobalt-based casting alloy to which the present invention is directed will first be explained.

C: 1.5-3.0% C produces various carbides in the structure, but 1
．． If the ratio is lower than 5, the amount of carbide is small and hardness is insufficient, and wear resistance and
Deformation resistance will deteriorate.

On the other hand, at C3.01 or higher, too many carbides are produced, and the cast product becomes noticeably brittle.

The desirable range for obtaining a cast product with good elongation and high hardness is C1.9 to 2.5%.

Si: 1.0% or less Si is an element that acts as a deoxidizing agent for molten metal, is effective in improving antioxidation properties, and improves castability, but is kept at 1.0% or less in order to reduce high-temperature strength.

Mn: 1.0% or less Mn is an element that improves castability and is effective as a deoxidizing agent, but it is kept at 1.0% or less because it reduces high temperature strength.

Cr: 25-35% If Cr is less than 25%, oxidation resistance at high temperatures is insufficient, and Cr
It is not preferable because the amount of r-based carbide crystallized and precipitated is small and the hardness is insufficient.

On the other hand, when the ratio exceeds 35, there are too many Cr-based carbides, and the resulting embrittlement becomes noticeable.

W: 8 to 17% If W is less than 8%, the amount of W-based carbides crystallized will be small, resulting in insufficient hardness.

On the other hand, if it exceeds 17 fb, the amount of W-based carbides crystallized is too large, resulting in significant embrittlement and at the same time making casting difficult.

The desirable range of W is 9 to 14φ.

Fe: 3.0% or less, P, S: 0.03fb or less, Ni
:□ 1.0φ or less These elements are treated as normal impurities, and the smaller the amount, the better.

Next, examples of the present invention will be described.

Table 2 below shows, as an example, the cobalt-based cast alloys whose chemical compositions are listed in Table 1, which were heat-treated under various treatment conditions, and their mechanical properties at room temperature and high temperature were measured.

To explain Table 2, 1,461 indicates the as-cast condition without heat treatment, and /462 indicates heat treatment for the purpose of stress relief and strain relief, which has been conventionally generally performed.

Therefore, based on the case of heat treatment of /16.2, in the case of heat treatment of 43 and A4 at 1100°C, the elongation increased by only 0.1%, but the improvement effect was not as large as that on the left. do not have.

In contrast, 1 of flat plate 6 and flat plate 7 corresponding to the embodiment of the present invention
In the case of heat treatment at a temperature of 150°C to 1200°C, although the overall hardness HRC (Rockwell hardness) slightly decreases, there is a significant increase in elongation, and at the same time, the tensile strength also increases6. , heat treatment temperature is 1200'C
In the case of grade 8, which exceeds 8, although the elongation greatly increases, the overall hardness HRC decreases significantly, and it is considered that the wear resistance and deformation resistance at high temperatures deteriorate.

That is, as the heat treatment temperature increases, the hardness Hv (Vickers hardness) of the austenite base existing between the carbides decreases, and the overall elongation increases due to the ductility improvement effect. 7. As shown in the case of No. 468, when the Vickers hardness of the austenite base is less than 500, an increase in elongation is noticeable, but when the heat treatment temperature is too high and exceeds 1200°C, the overall hardness decreases. The decrease in HRC becomes significant and the original characteristics are impaired.

Therefore, the present invention specifies a temperature range of 1150 DEG to 1200 DEG C. for heat treatment to improve the ductility or elongation of the cobalt-based cast alloy without reducing its excellent hardness.

Even within this heat treatment temperature range, if the heat treatment time is as short as 3 hr, as in the case of /I65, the effect is insufficient, and the heat treatment time must be at least 4 hr.

However, it is not preferable if the heat treatment time is too long, and the practical limit is 30 hours of heat treatment.

That is, even if heat treatment is performed for a longer period of time than this, no further beneficial effect of improving the structure can be obtained, but rather a significant decrease in hardness occurs.

When looking at the heat treatment method of the present invention from the perspective of improving the specific structure of cobalt-based cast alloys, the micrographs shown in Figures 1 to 4 are examples of the chemical composition shown in Table 3 below. It shows improvements in things.

In other words, FIG. 2 shows that before heat treatment, there is mainly a columnar crystal structure, that is, a first columnar crystal structure in which carbides have directionality and are distributed in elongated and straight lines.
The structure shown in the figure shows a structure in which the carbide is divided and granularized by heat treatment of 1200'CX4hr→furnace cooling.Figure 4 shows that before the heat treatment, the granular crystal structure, that is, the carbide is mainly distributed in a net shape. This shows a structure in which the carbides were made uniform and granular by the same heat treatment of <1200° CX4h, r→furnace cooling from the structure shown in FIG.

As described above, according to the heat treatment method of the present invention, carbides precipitated in a special distribution state in the structure of a cobalt-based cast alloy can be divided into fine particles and brought into relief.

As explained above, the heat treatment method of the present invention heat-treats a cobalt-based cast alloy at an extremely high temperature of 1150 to 1200°C for a specific period of time, thereby uniformly dispersing and granulating carbides in the structure. At the same time, the hardness of the austenite base existing between carbides is improved to less than 500, which is highly ductile, thereby further preventing cracks and crack propagation. It was decided that it would be possible to prevent this.

Therefore, according to the present invention, the problem of cracking can be effectively eliminated without impairing the excellent high-temperature wear resistance and deformability of cobalt-based casting alloys.

[Brief explanation of drawings]

Figures 1 to 4 are micrographs showing the structure of the cobalt-based cast alloy, with Figures 1 and 3 showing the structure before heat treatment, and Figures 2 and 4 showing the structure after heat treatment. .

Claims (1)

[Claims] IC: 1.5-3.0%, Si: 1.0% or less, M
n: 1.0% or less, Cr: 25-35%, W:
8-17%, Fe: 3.0% or less, P, S: 0.03
% or less, Ni: 1.0% or less, and the remainder substantially Co, is heated and held at 1150 to 12000 C for 4 to 30 hours to reduce the hardness of the austenite base to Hv500.
A method for heat treating a cobalt-based casting alloy, characterized by: