JPH0432121B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of forming alloy parts by sintering compacts of particulate mixtures of component metals of the alloy and compounds of each of the metals.

The most common conventional method for forming metal alloy parts is to machine blocks of the alloy to the desired shape and dimensions. The cost of manufacturing alloy parts using this method varies depending on the accuracy required for the part and the number of processing steps required to form the part, and the cost increases if the shape of the part is complex. Furthermore, the cost is also affected by the fact that the parts of the alloy material other than the parts that have been made into products are discarded as scrap.

Another conventional method is powder metallurgy, in which alloy particles (powder) are pressed and sintered to form alloy parts. However, the properties of alloy parts formed by this method are not always satisfactory. Further, there are cases where it is difficult to obtain fine particles of the alloy or the price is very high. Attempts have been made to mix and sinter the powders of the constituent metals of the alloy in proportions corresponding to the composition of the alloy, but there is a problem in that oxides tend to form on the surfaces of the raw material particles and inhibit sintering.

The present invention relates to a method for forming alloy parts by sintering powder raw materials, and is a method for forming parts, especially precision parts, having characteristics equivalent to or better than parts obtained by machining alloy materials at a relatively low cost. The purpose is to provide a method that can be used.

The present invention, which achieves the above object, is a method for forming an alloy part of a desired shape by sintering a molded body of a fine particle mixture of metals and metal compounds, and is a method comprising the following steps.

(a) Particle size of two or more raw materials selected from the component metals of the target alloy and the oxides of each of the component metals, each containing at least one metal and at least one oxide, each having a grain size of 0.1-10 μm. (b) forming the mixture into a predetermined shape; (c) forming the mixture into a predetermined shape; (b) removing the binder from the molded body obtained in step (d) sintering the molded body after the binder has been removed in a reducing atmosphere, at which time the dew point of the atmosphere is set to one of the two or more types; The firing temperature is controlled so that it is always maintained in the reduction region of the dew point equilibrium curve for each raw material, and the firing temperature is kept below the sintering temperature until all the metal oxide particles in the compact are reduced to metal. A step of setting the firing temperature and firing time to be sufficient to achieve alloying through solid phase interdiffusion of metal between the fine particles constituting the molded body while keeping the firing temperature low, and gradually increasing the firing temperature to achieve sintering.

In the method of the present invention, an oxide of at least one of the constituent metals of the target alloy is used as a raw material, and a molded body (generally called a green body) of a mixture of fine metal particles and fine oxide particles is prepared in a reducing atmosphere. By firing while controlling the temperature as described above, all the metal oxides are first reduced to metals, and sintering is performed while alloying by solid-phase interdiffusion between metal fine particles progresses. in this way,
For example, for metals for which fine particle powder is expensive or metals that are difficult to handle in the form of fine particles, oxide powder can be used as a raw material, thereby reducing the manufacturing cost of alloy parts. By the firing step of the method of the present invention, it is possible to achieve complete alloying of raw metals (including those that were initially oxides), and the sintered product, which is a product, has the characteristics as an alloy. It is an alloy part with no defects.

The present invention will be explained in more detail below.

The particle size of the metals and metal oxides used in the method of the present invention needs to be sufficiently small, and the preferred range of particle size is from 0.1 μm to 10 μm.

For example, certain metals such as chromium become highly explosive when made into very small particles, so when particles of such metals are used, the particle size should be relatively large within the above range. Metal oxides have much lower surface energy than simple metals, so
It is easy to make fine particles with a particle size of 1 μm or less,
There are no particular problems with using such particles.

In the method of the present invention, a mixture of at least one type of metal particles and at least one type of metal oxide particles is used as a raw material, and the ratio (atomic ratio) of metal elements in the mixture is
is blended so that it corresponds to the composition ratio of the target alloy.
For example, when manufacturing stainless steel parts containing nickel and chromium, iron powder, a mixture of nickel powder and chromium oxide powder, a mixture of iron powder, nickel oxide powder and chromium oxide powder, or iron oxide powder, nickel powder and chromium oxide powder A mixture of these can be used as the raw material.

A suitable known binder such as paraffin wax, carnauba wax, polyethylene, etc. or a mixture thereof is added to the mixture of metal particles and metal oxide particles to form a uniform composition.

The composition containing the binder is formed into a desired shape by a known method to form a green body, and then the binder is removed from the green body by a suitable method.

After removing the binder, the green body is opened, placed in a kiln, etc., and fired in a reducing atmosphere. Dry hydrogen, which does not contain oxygen, is usually used as the reducing atmosphere. In the initial stage of firing, the temperature of the reducing atmosphere in the furnace is maintained at a temperature lower than the sintering temperature of the raw materials constituting the green body, but sufficient to reduce the metal oxide used to metal. At this time, the dew point of the reducing atmosphere in the furnace is always kept in the reducing region of the dew point equilibrium curve for each of the raw materials constituting the green body. For example, if a green body made of iron, nickel, and Cr ₂ O ₃ is fired in a hydrogen atmosphere to form a stainless steel part, the dew point of the furnace atmosphere is compared to the dew point equilibrium for Cr ₂ O ₃ in the attached drawing. Keep on the right side of the curve, ie on the reduction side. Equilibrium curves for iron oxide and nickel oxide are shown in Cr ₂ O ₃
far to the left of the equilibrium curve.

When all the metal oxides used are reduced to elemental metals, the entire green body of fine particles from which the binder has been removed becomes a homogeneous mixture of metal fine particles. Whether the reduction reaction is complete can be determined by checking the dew points at the inlet and outlet of the furnace. In other words, water is produced by the reduction reaction in hydrogen while oxides remain, so if the furnace is airtight, the dew point at the outlet will drop to the dew point at the inlet when all the oxides are reduced. Go down. Since the firing takes place in a reducing atmosphere, the metal particles in the green body will not be oxidized.

The time required to reduce all the metal oxides in the green body to metal varies depending on the type of raw material, the type of reducing atmosphere, the firing temperature, the shape and size of the green body, etc. Generally, firing in the reducing region to the right of the dew point equilibrium curve for one hour per 0.05 inch of green body thickness is sufficient.

As the metal oxide in the green body is reduced, alloying begins due to solid phase interdiffusion between fine metal particles. The firing temperature is increased if necessary for alloying by interdiffusion, but the dew point of the furnace atmosphere is maintained as described above. Finally, the firing temperature is raised to a temperature sufficient to sinter the fine particles that make up the green body. After sintering is achieved, the temperature inside the furnace is lowered to room temperature and the sintered body is taken out.

An alloy part formed by the method described above has superior properties to an alloy part formed by sintering fine particles of the alloy. The reason for this is thought to be that because the surfaces of the fine particles of the alloy are oxidized, it is difficult to achieve good and sufficient sintering using economically viable sintering methods.

Hereinafter, the present invention will be specifically described with reference to Examples.

Example: 1420 g of iron powder with a particle size of 2-4 μm, 200 g of nickel powder with a particle size of 2-4 μm, and 560 g of chromium oxide powder with a particle size of 1.5 μm were mixed, and 70.5 g of paraffin wax and 70 g of polyethylene were added as a binder to the mixture.
g was added and mixed at 150°C for 2 hours. The obtained homogeneous composition was molded into a rod-shaped green body using an injection molding machine.

Place an absorbent material made of ashless paper in an oven, place the green body on top of it, slowly raise the temperature of the oven to 200°C, and then heat it at 200°C for 3 hours.
The binder was removed from the green body by keeping it at °C.

The green body from which the binder had been removed was placed in a furnace, and hydrogen gas with a dew point of -59.4°C (75°C) at 1 atm was introduced into the furnace and circulated. The temperature inside the furnace was gradually increased to approximately 788℃ (1450〓) at a rate of 3.3℃ (6〓) per minute, and then gradually increased to approximately 9267℃ (1700〓) at a rate of 3.3℃ (6〓) per minute. and held at this temperature for 5 hours. Thereafter, the temperature was rapidly raised to 1260°C (2300°) while keeping the dew point of the atmosphere in the furnace 5.6°C (10°) lower than the dew point equilibrium curve, and this temperature was maintained for 1 hour. The dew point of the furnace atmosphere was continuously monitored and its changes were plotted in the accompanying drawings (to the right of the equilibrium curve for Cr ₂ O ₃ ). Thereafter, the operation of the furnace was stopped and the mixture was allowed to cool to room temperature.

The rod-shaped sintered body taken out from the furnace was entirely made of homogeneous stainless steel and was a non-magnetic material.

The method of the present invention described above is such that some of the component metals of the target alloy are available as fine particles of single metals, and some of the metal oxides can be reduced to metals at a temperature below the sintering temperature. If available as particulates, they can be used to manufacture parts made of almost any type of alloy.

Although the present invention has been described in detail with respect to its preferred embodiments, it goes without saying that various modifications are possible and the present invention is not limited to the preferred embodiments or examples described above.

[Brief explanation of drawings]

The drawing shows the dew point (〓) of hydrogen gas at 1 atm versus furnace temperature for Cr ₂ O ₃ , Fe ₂ O ₃ , WO ₂ and MoO ₂
This is a graph showing.

Claims (1)

[Scope of Claims] 1. A method for forming an alloy part of a desired shape by sintering a compact of a fine particle mixture of metals and metal compounds, comprising: component metals of the target alloy and each of the component metals. The composition of the target alloy is prepared by mixing fine particles of 0.1-10 μm in grain size of two or more raw materials containing at least one metal and at least one oxide with an appropriate binder. a step (a) of obtaining a homogeneous mixture containing the component metals in corresponding proportions; a step (b) of molding the mixture into a predetermined shape; and a step of extracting the binder from the molded body obtained in step (b). (c) of removing the binder, and firing the molded body after removing the binder in a reducing atmosphere, at which time the dew point of the atmosphere is set to all reducing side regions of the dew point equilibrium curve for each of the two or more types of raw materials. The sintering temperature and sintering time are controlled so that the sintering temperature is maintained at all times, and the sintering temperature and sintering time are controlled while keeping the sintering temperature lower than the sintering temperature until all the metal oxide particles in the compact are reduced to metal. A method for forming an alloy molded part comprising the following steps: (d) setting the temperature to be sufficient to achieve alloying through solid phase interdiffusion of metal between the constituent fine particles, and gradually increasing the firing temperature to achieve sintering. 2. The method according to claim 1, wherein the two or more raw materials consist of chromium oxide, iron, and nickel. 3. The method according to claim 1, wherein the two or more raw materials consist of chromium oxide, iron oxide, and nickel. 4. The method according to claim 1, wherein the two or more raw materials are chromium oxide, iron, and nickel oxide.