WO2012152521A1

WO2012152521A1 - Process for the heat treatment of pressed mouldings

Info

Publication number: WO2012152521A1
Application number: PCT/EP2012/056639
Authority: WO
Inventors: Thomas Braun-Klabunde; Jens Mirschinka
Original assignee: L'air Liquide,Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude; Air Liquide Deutschland G.M.B.H
Priority date: 2011-05-11
Filing date: 2012-04-12
Publication date: 2012-11-15
Also published as: DE102011101264A1; DE102011101264B4

Abstract

Process for the heat treatment of at least one moulding 1, which has been pressed from at least one material powder and at least one pressing aid, in a continuous furnace 2 having an evaporation section 3 and a sintering section 4, said process comprising at least the following steps: a)the moulding 1 is introduced into the evaporation section 3, b)the moulding 1 is heated to a temperature which is suitable for the evaporation of the at least one pressing aid in the evaporation section 3, c)the moulding 1 is conveyed from the evaporation section 3 into the sintering section 4, d)the moulding 1 is heated to a temperature which lies below the melting temperature of the at least one material powder in the sintering section 4, e)at least one oxidizing gas, in particular carbon dioxide, is fed into the sintering section 4. With the teaching according to the invention, the targeted introduction of an oxidizing gas into the sintering section 4 of a continuous furnace 2 makes it possible to suppress the deposition of soot in the system and the carburization of metallic components and of the moulding 1 in the sintering section 4.

Description

Process for the heat treatment of pressed mouldings

The present invention relates to a process for the heat treatment of a moulding, which has been pressed from a material powder and a pressing aid, in a continuous furnace. The process is also referred to as sintering.

In sintering, first of all a moulding is pressed from a mixture of material powder, in particular metal or plastic powder, and a pressing aid. The pressed moulding is also referred to as a green compact. The moulding is heat-treated in a continuous furnace having an evaporation section and a sintering section. Before the actual sintering process in the sintering section, the pressing aid first of all has to be removed from the moulding. To remove the pressing aid, the moulding is heated to a temperature in the evaporation section at which the pressing aid evaporates. Then, the temperature of the moulding is increased to a temperature below the melting temperature of the material powder in the sintering section, in which case a bond is formed between the material powder particles by surface diffusion between the material powder particles and thus the moulding is bound. Both the evaporation of the pressing aid and the sintering itself take place in a suitable gas atmosphere, with the composition of the gas being adapted to the requirements in the various sections. The gases or gas mixtures are generally introduced counter to the conveying direction of the moulding. It is known that an oxidizing atmosphere is set in the evaporation section, such that the gases which form during the evaporation of the pressing aid, for example hydrogen (H₂), carbon monoxide (CO) and hydrocarbons (C_xH_y), can be oxidized. In order to generate an oxidizing atmosphere, humidified nitrogen (N₂), by way of example, is fed into the evaporation section. US 5,578,147 additionally discloses that the throughput rate of the mouldings or the furnace temperature is regulated during the evaporation process such that if possible all pressing aid constituents oxidize completely.

In order to obtain an optimum sintering result in the actual sintering process, it is necessary for a reducing atmosphere to be present in the sintering section, this being realized for example by feeding in hydrogen (H₂). In spite of the measures described above, such as feeding in the gas counter to the conveying direction of the mouldings and the regulated variation of the evaporation conditions, the evaporated, gaseous constituents of the pressing aid cannot always be prevented from passing into the sintering section. The presence of the evaporated pressing aid constituents and the high temperatures in the sintering section regularly lead to the carburization of metallic materials in the sintering section. Thus, for example, the conveyor belt of the continuous furnace, which is regularly produced from high-alloy, heat-resistant steel, is carburized in the sintering section and decarburized again outside. This intermittent carburization and decarburization of the conveyor belt leads to grain growth, depletion of alloying elements and embrittlement, which in turn can lead to considerable shortening of the service life of the conveyor belt and to loss of production, and this in turn leads to increased production costs. In addition, the sintering process is impaired by the inevitable carburization of the metallic material powder. Thus, inter alia, an excessive formation of soot is observed in the sintering section, as too are a reduction in the density and an increase in the hardness and brittleness of the sintered mouldings.

It is an object of the invention, therefore, to at least partially overcome the problems outlined with reference to the prior art, and in particular to specify a process in which the deposition of soot in the system and the carburization of metallic materials in a sintering section of a continuous furnace are avoided.

These obj ects are achieved by a process according to the features in the independent claim. Advantageous configurations of the process are indicated in the dependent patent claims. The features given individually in the claims can be combined with one another in any desired, technologically meaningful manner, and can be supplemented with explanatory information from the description, presenting further alternative embodiments of the invention.

These objects are achieved by a process for the heat treatment of at least one moulding, which has been pressed from at least one material powder and at least one pressing aid, in a continuous furnace having an evaporation section and a sintering section, said process comprising at least the following steps: a) the moulding is introduced into the evaporation section,

b) the moulding is heated to a temperature which is suitable for the evaporation of the at least one pressing aid in the evaporation section, c) the moulding is conveyed from the evaporation section into the sintering section,

d) the moulding is heated to a temperature which lies below the melting temperature of the at least one material powder in the sintering section, e) at least one oxidizing gas is fed into the sintering section.

The material powder preferably comprises metallic powders, in particular powders of iron, copper, molybdenum, bronze and/or stainless steels with incorporated hard materials, for example tungsten carbide or titanium carbide.

The pressing aids preferably comprise stearates which contain a very high proportion of bound hydrocarbons. Zinc stearate is used with particular preference as the pressing aid (see for example "Kieback, Bernd (Editor): Pulvermetallurgie. Technologien und Werkstoffe [Powder Metallurgy. Technologies and Materials], 2nd edition").

The continuous furnace can be formed by a plurality of individual furnaces arranged in succession, in which case a temperature which is independent of the other furnaces can be set in each furnace. Alternatively, the furnace can comprise precisely one furnace in which a predefinable temperature profile can be set, where the sections are defined by the temperatures which prevail during operation. If appropriate, the individual sections of the furnace may also be separated from one another by skirts or the like, for example in order to reduce the gas exchange between the individual sections. The mouldings are preferably conveyed from an inlet of the continuous furnace to an outlet of the continuous furnace with the aid of a conveying apparatus, e.g. a conveyor belt. The moulding therefore initially passes through the evaporation section and then the sintering section on the conveyor belt. The sintering section of the continuous furnace is generally followed by cooling zones, in which the sintered moulding is cooled, usually in a protective gas atmosphere. The temperature which prevails in step b) is at least so high that the pressing aid can evaporate completely, but not so high that a bond is formed with the material powder; in particular, the temperature in step b) is between 250°C and 700°C, preferably between 400°C and 6oo°C.

The temperature which prevails in step d) is in particular so high that a bond is formed with the material powder, but the latter just does not melt; in particular, the temperature in step d) is between 8oo°C and 1500°C, preferably between iooo°C and 1300°C.

According to the invention, and contrary to the opinion available to date, an oxidizing gas is fed into the sintering section. The intention of the oxidizing gas is to oxidize the gaseous pressing aid constituents which have passed into the sintering section, such that carburization of the metallic materials in the sintering section is suppressed. The carbon potential in the sintering section, which is a measure of the possible carburization, is therefore lowered to a non-critical level, and is in particular above the soot limit.

It is preferable for the oxygen partial pressure to be measured in the sintering section. On the basis of the oxygen partial pressure, it is possible to determine in particular the carbon potential in the sintering section, and therefore the amount of oxidizing gas fed in in step e) can preferably be regulated depending on the carbon potential. The oxygen partial pressure can be measured by means of known oxygen probes or known lambda probes. Therefore, the flow rate of the oxidizing gas fed in is increased or lowered until a predefined oxygen partial pressure is reached or until in particular a carbon potential of less than 0.2% is reached.

With very particular preference, carbon dioxide (C0₂) is fed in as the oxidizing gas in step e).

It is also preferable for a base gas comprising at least nitrogen (N₂), hydrogen (H₂) and/or methane (CH₄) to be fed into the continuous furnace counter to a conveying direction of the moulding. The composition of the base gas can vary depending on the requirements in the individual sections of the continuous furnace. It is therefore preferable for a gas mixture of at most 5% oxidizing gas with base gas as the remainder to be fed into the sintering section, where the base gas in the sintering section comprises at most 2% methane, 3% to 10% hydrogen and remainder nitrogen. In particular, the composition of the gas mixture depends on the nature of the mouldings, i.e. in particular on the geometry, on the material powder used, on the pressing aid used for the mouldings and on the conveying speed of the mouldings, and so on. It is particularly preferable for the pressing aid to be a stearate.

A further aspect of the invention proposes a continuous furnace for carrying out the process according to the invention, comprising an evaporation section, a sintering section, a conveying device for conveying the mouldings from an inlet to an outlet of the continuous furnace and at least one gas line for feeding gases into the sintering section.

It is preferable for the continuous furnace to comprise a measuring sensor for measuring the oxygen partial pressure in the sintering section.

It is preferable for the continuous furnace to furthermore comprise a control apparatus which is connected at least to the measuring sensor or a controllable valve in the at least one gas line. The details disclosed in relation to the process according to the invention can be transferred and applied to the apparatus according to the invention, and vice versa.

The invention and the technical field will be explained by way of example below on the basis of the figure. It should be pointed out that the figure shows a particularly preferred alternative embodiment of the invention, but the latter is not limited thereto. Schematically:

Fig.: shows a continuous furnace for carrying out the process according to the invention. The figure schematically shows a continuous furnace 2 having an evaporation section 3, a sintering section 4, a cooling section 7 and a conveying device 5 for conveying at least one moulding 1 through the continuous furnace 2 from an inlet 12 to an outlet 13. The continuous furnace 2 furthermore comprises gas lines 6, by means of which gases or gas mixtures can be fed into the sections of the continuous furnace 2 via valves 10. To measure the oxygen partial pressure in the sintering section 4, provision is made of a measuring sensor 11, which is connected to a control apparatus 8 via a data line 9. The control apparatus 8 is furthermore connected to the valves 10 for providing a gas atmosphere with a gas composition in the sections of the sintering furnace 2.

The moulding 1 which has been pressed from a material powder and a pressing aid passes through the continuous furnace 2 from the inlet 12 to the outlet 13. The moulding 1 is first of all introduced into the evaporation section 3, where the pressing aid evaporates at temperatures of between 400°C and 6oo°C. Then, the moulding 1 passes into the sintering section 4, where the material powder is sintered at temperatures of between 1030°C and 1250°C. The sintered moulding is cooled in the cooling section 7 with the supply of a protective gas atmosphere.

The constituents of the pressing aid evaporate in the evaporation section 3, in particular with the release of hydrogen, carbon monoxide and hydrocarbons. However, since evaporated pressing aid constituents pass into the sintering section 4, carbon dioxide is added to the inherently reducing gas mixture of, for example, hydrogen and nitrogen in the sintering section 4. As a result, the pressing aid constituents which have passed into the sintering section 4 are oxidized, and carburization of the metallic constituents and the deposition of soot in the system in the sintering section 4 are avoided.

With the teaching according to the invention, the targeted introduction of an oxidizing gas into the sintering section 4 of a continuous furnace 2 makes it possible to suppress the deposition of soot in the system and the carburization of metallic materials, in particular of mouldings 1 in the sintering section 4. List of reference symbols

1 Moulding

2 Continuous furnace

3 Evaporation section

4 Sintering section

5 Conveying device

6 Gas line

7 Cooling section

8 Control apparatus

9 Data line

10 Valve

11 Measuring sensor

12 Inlet

13 Outlet

Claims

Process for the heat treatment of at least one moulding (1), which has been pressed from at least one material powder and at least one pressing aid, in a continuous furnace (2) having an evaporation section (3) and a sintering section (4), said process comprising at least the following steps:

a) the moulding (1) is introduced into the evaporation section (3), b) the moulding (1) is heated to a temperature which is suitable for the evaporation of the at least one pressing aid in the evaporation section (3),

c) the moulding (1) is conveyed from the evaporation section (3) into the sintering section (4),

d) the moulding (1) is heated to a temperature which lies below the melting temperature of the at least one material powder in the sintering section (4),

e) at least one oxidizing gas is fed into the sintering section (4).

Process according to Claim 1, wherein the oxygen partial pressure is measured in the sintering section and the amount of oxidizing gas fed in in step e) is regulated depending on the oxygen partial pressure.

Process according to Claim 1 or 2, wherein carbon dioxide (C0₂) is fed in in step e).

Process according to one of the preceding claims, wherein a base gas comprising at least nitrogen (N₂) and hydrogen (H₂) is fed into the continuous furnace (2) counter to a conveying direction of the moulding (1).

Process according to one of the preceding claims, wherein the pressing aid is an amide wax.

6. Continuous furnace (2) for carrying out the process according to one of the preceding claims, comprising an evaporation section (3), a sintering section (4), a conveying device (5) for conveying the at least one moulding

(I) and at least one gas line (6) for feeding gases into the sintering section (4).

Continuous furnace according to Claim 6, comprising a measuring sensor

(II) for measuring the oxygen partial pressure in the sintering section (4).

8. Continuous furnace according to Claim 6 or 7, having a control apparatus (8) which is connected at least to the measuring sensor (11) or a valve (10) in the at least one gas line (6).