JPH05222482A - Method for sintering stainless steel powder - Google Patents

Abstract

PURPOSE:To obtain an attractive extenal appearance free from pinhole, clouding, and scratch of mirror-finished surface required of ornamental parts, such as time piece parts, at the time of producing ornamental sintered parts such as time piece parts for which stainless steel is used as a material by using an injection molding technique for metal powder. CONSTITUTION:This sintering method consists of a stage for which a green compact is heat-treated in vacuum at <=1200 deg.C, a stage heat-treatmented at 800-1200 deg.C in a gaseous mixture, such as methane, methane + argon, propane, propane + argon, butane, and butane + argon, and a stage heat-treatmented at >=1100 deg.C in a hydrogen, argon atmosphere. Since the sintering density can be increased to 7.8 (98% to the density of a melted material) and the structure of a section is free from oxides in the range between the surface and the position at the depth of 0.3mm from the surface, sintering is not hindered and also, as to the mirror-finished external appearance, and attractive mirror-finished external appearance free from pinhole, clouding, scratch, etc., can be obtained, and further, corrosion resistance to salt water and artificial sweat similar to that of the ordinary melted material, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sintering method for manufacturing a decorative sintered part such as a watch part made of stainless steel by using a metal powder injection molding technique. The manufacturing method of powder-injection molding of stainless steel parts is a technique suitable for mass-producing small-sized and complex-shaped precision sintered parts.

[0002]

2. Description of the Related Art In powder injection molding of stainless steel,
Usually, stainless steel powder with an average particle size of about 5μ to 20μ is mixed with organic binders that give plasticity and injection molding is possible, and the desired shape can be obtained in one step like ordinary plastic injection molding. To form. In this case, since the injection-molded body usually contains about 5 to 15% by weight of an organic substance, it is about 150% by volume with respect to the target sintered product.
~ 200%. This molded body is subjected to binder removal treatment to remove most of organic substances, and then sintered. The debinding process is usually by thermal decomposition of the binder. Depending on the type and processing conditions of the binder, 85% to 95% is removed by the debinding process, and the remaining binder maintains the shape of the molded body and burns stainless steel. It serves as an auxiliary material for binding.

Conventionally, in powder metallurgy of stainless steel, a method has been used in which powder is put into a mold, press-molded, and then sintered. The sintering atmosphere at this time is vacuum, hydrogen, argon, ammonia decomposition gas, or the like. However, in powder metallurgy by such a pressing method, the particle size of the powder used is 5
The sintered density is as low as 90% to 95%, and pinholes are scattered in the structure of the product after sintering. It was not sufficiently glossy and was inferior in corrosion resistance for application to decorative parts such as watch cases.

In powder injection molding, however, when the molded body is debindered and then sintered, the volume shrinks almost half, but the entire body contracts uniformly, so that the dimensional accuracy is high and the density of the sintered body is high. Has an advantage that it can be easily increased to 96% or more. The de-bindered molded product varies depending on the stainless steel material, but the final temperature is 1250 ° C.
Sintering is performed at a temperature of ~ 1350 ° C. In this case, the atmosphere of the sintering furnace greatly affects the characteristics of the product. Conventionally, as the sintering atmosphere, a vacuum atmosphere, a hydrogen gas atmosphere, an ammonia decomposition gas atmosphere, etc. have been used as in the case of ordinary powder metallurgy.

[0005]

However, heat treatment in such an atmosphere provides sufficient properties, such as tensile strength, dimensional accuracy, and toughness, when manufacturing general structural parts, mechanical parts, and the like. However, it is insufficient for parts such as a watch case, a band, and other decorative parts where the polished appearance is important.

The stainless powder used in this powder injection molding is often an atomizing method, especially a water atomizing method from the viewpoint of cost, but this powder is about 4000 to 60.
It contains oxygen of 00PPM. Therefore, the metal oxide combined with oxygen hinders sintering and causes many fine pinholes. Further, Cr, which is an element that maintains the high corrosion resistance of stainless steel, becomes Cr oxide, which causes poor corrosion resistance.

As a method of reducing such a metal oxide of Cr, there is a method of using high-purity hydrogen gas having a low dew point. In order to promote the reduction reaction at this time, it is necessary to reduce the partial pressure of water, and in particular, it is necessary to supply a large amount of hydrogen gas to remove oxygen from the inside of the sintered body. Yes and very expensive. In addition, when sintering is performed in a vacuum atmosphere, the evaporation of the metal is particularly severe at high temperatures, which reduces the weight of the sintered body, which makes it difficult to control the dimensional accuracy and causes an imbalance in the ratio of the evaporated metal. The structure of the sintered metal changed, which resulted in a loss of corrosion resistance.

For example, when a debindered molded product of stainless steel 316L was held at 1340 ° C. for 2 hours in vacuum, the weight loss was 5 to 6% by weight.

[0009]

The present invention has been made to eliminate the above drawbacks and is based on the following sintering treatment method. That is, from normal temperature to 1200 in vacuum
Heat treatment at 800 ° C to 1200 ° C in a mixed atmosphere of hydrocarbon gas such as methane, methane and argon gas, or mixed atmosphere of propane, propane and argon gas, butane, butane and argon gas. Further, heat treatment is performed at 1100 ° C. or higher in an atmosphere of hydrogen, argon or the like.

After activating the surface of the powder by performing vacuum treatment on the low temperature side, a hydrocarbon gas is introduced to react the oxide on the surface with the hydrocarbon gas and take out as CO gas. Carbon C remaining in the debindered compact
Is the oxide MO on the surface of the stainless powder MO + C →
It has a function of reducing in the form of CO + M, but it is difficult to control the carbon content compared to the oxygen content. If carbon is contained in excess in the binder, the sintered density becomes low. Furthermore, when the amount of carbon is less than that of oxygen, the amount of oxygen increases, which causes poor corrosion resistance.

Therefore, by introducing a hydrocarbon gas such as methane, it is possible to obtain a beautiful structure containing a small amount of oxides and carbon from the surface by a certain thickness or more. In addition, the evaporation of the metal components during the final sintering at high temperature is hydrogen,
It was controlled by argon.

[0012]

According to the present invention, 120 in vacuum with a low oxygen partial pressure is used.
Heat treatment is performed at 0 ° C. or lower, and at 800 ° C. to 1200 ° C. in a hydrocarbon gas methane, a mixed atmosphere of methane and argon gas or a mixed atmosphere of propane, propane and argon gas, butane, butane and argon gas. 1
In a method of sintering a stainless powder characterized by performing a heat treatment in an atmosphere of hydrogen, argon, etc. at 100 ° C. or higher, the metal oxide of stainless is reduced in a vacuum atmosphere to reduce oxides, etc. Then, methane or a mixed gas of methane and argon, or propane or a mixed atmosphere of propane and argon is used to reduce carbon by invading carbon from the surface and reduce the remaining oxide by the reaction of MO + C → CO, and hydrogen is generated at a higher temperature side. + Prevents evaporation of metal components by completing the sintering in an atmosphere of argon gas, etc., and has the mirror surface appearance required for decorative parts such as watch parts, improving corrosion resistance and stabilizing dimensional accuracy There is.

[0013]

【Example】

(Example 1) S having an average particle size of 10 μ as stainless powder
US 304 powder was used, and as an organic binder, ethylene-vinyl acetate copolymer, methacrylic acid ester copolymer, sorbitan monoalkylate, and paraffin wax were mixed together while heating.
The amount of binder was 9.1% of the total weight. This mixture was injection molded into a watch case.

The molded body was heated to 500 ° C. in a nitrogen atmosphere at atmospheric pressure to remove the binder. The binder remaining in the molded body after debinding was 0.45%. The debindered molded body was heated to 1150 ° C. at a rate of 6 ° C./min in a vacuum atmosphere (10 ⁻⁴ Torr). After that, a mixed gas in which the ratio of methane and argon was 1: 3 was introduced into the sintering furnace and kept at 1150 ° C. for 1 hour. The heat treatment temperature is preferably 800 ° C to 1200 ° C. After that, it is converted into ammonia decomposition gas, and it is 1 at a rate of 10 ° C./min in a hydrogen atmosphere.
The temperature was raised to 340 ° C., and the temperature was maintained for 2 hours for sintering.

The watch case manufactured in this way is
The sintered density is 7.81 (98% of the density of the ingot material),
When the structure of the cut surface was confirmed, an oxide-free layer was confirmed up to a thickness of 0.3 mm from the surface. Further, when it was mirror-polished, it did not cause pinholes, cloudiness, and scratches, and had good corrosion resistance against salt water and artificial sweat, similar to ordinary ingot materials. (Example 2) SU having an average particle size of 5μ as stainless powder
Using the S316L material, a binder having the same composition as in Example 1 was mixed. At this time, the amount of binder is 10% of the total weight.
And

The binder remaining in the sintered body after injection molding and debinding as in Example 1 was 0.56%. This molded body is placed in a vacuum atmosphere (10 ^-4 Torr) for 2 minutes.
Held for hours. Then, the temperature was raised to 1200 ° C. at a rate of 5 ° C./min. When the temperature reached 1200 ° C., a mixed gas of argon and propane was introduced at a mixing ratio of 2: 3, and the mixture gas was held for 1 hour. After that, the gas was changed to argon gas alone, and the atmosphere was argon. At a rate of 10 ° C./min, 1330 ° C. The temperature was raised to 2 hours and held for sintering. The case manufactured in this way has a sintered density of 7.86 (about 98% of the density of the molten material).
%), And when confirming the structure of the cut surface, it is 0.5 from the surface.
A beautiful structure without oxides and pinholes can be confirmed up to a thickness of mm. Further, when it was mirror-polished, it did not cause scratches or fogging, and showed the same corrosion resistance to salt water and artificial sweat as ordinary ingots.

[0017]

As described above, if the sintering conditions shown in the present invention are used, it is possible to manufacture a stainless part having excellent appearance and good corrosion resistance, which is very useful especially for making a decorative stainless part. ..

Claims (1)

[Claims] 1. Debinding of a molded body by injection molding after adding an organic binder to stainless steel powder,
In the sintering method for manufacturing parts by sintering, after debinding, heat treatment at 1200 ° C or less in vacuum
At ~ 1200 ° C, methane, mixed gas of methane and argon,
Heat treatment in an atmosphere of propane, a mixed gas of propane and argon, butane or a mixed gas of butane and argon,
A method of sintering a stainless powder, which comprises performing a heat treatment in an atmosphere of hydrogen and argon at 1100 ° C. or higher.