Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Special medium during sintering
Reexamination Certificate
1999-11-24
2001-10-16
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Special medium during sintering
C073S023200, C073S031030
Reexamination Certificate
active
06303077
ABSTRACT:
The present invention concerns a method of sintering powder-metallurgically produced compounds. More specifically, the invention concerns a method of monitoring and controlling the composition of the sintering atmosphere.
Currently with the development of newer and better powder-metallurgical products there is a need of improved methods of controlling also the sintering atmosphere, and the object of the present invention is to meet this need.
In brief the present invention concerns a method of controlling and monitoring the furnace sintering atmosphere when sintering powder-metallurgical (PM) compacts, gases determining the carbon and oxygen potentials being measured continuously.
The invention is of special interest for monitoring and controlling the atmosphere during sintering of compacts of low-alloy iron-based materials including easily oxidable alloying elements selected from the group consisting of Cr, Mn, Mo, V, Nb, Zr, Ti, Al in order to keep the oxidation of these elements at a low level.
There is a wide variety of instruments for analysing and controlling the gases used in atmospheres for powder metallurgy, and the composition of the atmospheres used in sintering is determined either by insitu or by room temperature measurements. The measurements can also be performed in separated chamber, into which the furnace gases are extracted from the sintering furnace.
According to the invention, the oxygen potential is determined by using oxygen probes which are applied in the furnace muffle via the furnace wall or in the separate chamber or furnace and operate with a stabilised ZrO
2
cell. A reference gas (normally air) with a well defined partial pressure of oxygen penetrates one side of the cell, whereas the other side of the cell is in contact with the furnace atmosphere. The difference in partial pressure of oxygen creates an electric potential which is monitored, thereby defining the oxygen potential present. If the electric potential measured, which corresponds to the actual sintering atmosphere, differs from a set value, necessary atmosphere adjustments are performed. The set value for the sintering of a given material is decided empirically or theoretically and depends on the type and amount of the alloying elements. When using oxygen probes one has to consider that especially atmospheres with high carbon potentials tend to form soot on the ZrO
2
cell if necessary precautions are not taken, thereby preventing effective atmosphere control. Many producers have now foreseen such problems and equipped the oxygen probes with, for instance, mechanical brushes.
The oxygen probe can be applied in different places when controlling the atmosphere. In a belt furnace based on the countercurrent principle, the oxygen probe is preferably arranged in the end of the sintering zone where the “fresh” gas enters.
A second alternative is to arrange the probe close to the inlet of the furnace. For this alternative,it has to be taken into account that the oxygen potential might be higher due to possible reduction of oxides and burn-off of lubricants, and therefore the acceptable oxygen level in this part of the furnace has to be found by “trial and error” for each powder alloy.
As a third alternative the, oxygen probe can be arranged in a separate chamber or furnace into which the gases from the sintering furnace are extracted. In this alternative the oxygen probe is arranged in a separate chamber into which the gases from the sintering furnace are extracted. The temperature of the atmosphere in this chamber is optionally the same as the temperature of the furnace atmosphere. When the temperature of the atmosphere in the separate measuring chamber is different from the temperature of the sintering furnace atmosphere this temperature difference must be considered when determining the gas composition of the sintering furnace.
The natural constraint with regard to oxygen is that the measured oxygen potential shall be kept or set below the value for the equilibrium partial pressure of oxygen between the alloying elements and their oxides, e.g. Cr and Cr
2
O
3
. The equilibrium partial pressure of oxygen is well defined for any type of atmosphere used at a specific temperature. If the measured oxygen value is close to this set-point, a natural counteraction is to increase the flow of reducing gas, e.g. H
2
. As can be seen from Example 3 below, the oxygen level can also be controlled and adjusted to a required value by the introduction of a carbon-containing gas, such as methane.
It is still more common to monitor sintering conditions by room temperature measurement of the gas mixture. This measurement is generally based on either infrared analysis and/or dew point monitoring.
The infrared analysis is based on the principle that different gases absorb infraredenergy at characteristic wavelengths. If the concentration of a single component in a gas mixture is changed, it will result in a corresponding change in the total energy remaining in an infrared beam passed through the mixture. The energy changes, which are detected by an infrared analyser, are therefore a measure of the gas concentration. Each gas compound absorbs a certain portion of the infrared spectrum which no other gas absorbs, and the amount of radiation absorbed is proportional to the concentration of the specific gas. Typical applications of infrared analysers are in the field of gases with high carbon potential, and care has to be taken when the atmosphere is sampled in order to avoid soot formation and/or condensation.
The determination of the carbon potential comprises measuring the partial pressure of oxygen in combination with the measurement of one or more of the carbon-containing gases, such as carbon monooxide, thereby determining the carbon potential. Another alternative is to measure the concentration of all or all but one carbon-containing gases. The measurements are carried out on gases from the sintering zone, the cooling zone and/or the heat treatment zone.
The control and monitoring of the sintering atmosphere by measuring the oxygen and carbon potentials according to the present invention is preferably carried out by using a combination of an oxygen probe for measuring the oxygen potential and an IR instrument which concurrently measures the carbon-containing gases such as CO, CO
2
and methane. By using such a combination, the influence of the carbon-containing gases on the oxygen potential is taken into account and a superior method of controlling and monitoring the sintering atmosphere is obtained. By using this method, optimal sintering conditions can be maintained and the properties of the sintered materials will be improved.
Also the C potential is kept at a set value This set value depends on the desired carbon level in the sintered material.
The method according to the invention can be applied to all types of sintering atmospheres such as nitrogen-based atmospheres, dissociated ammonia, hydrogen-based atmospheres, endothermic gas etc. and within sintering temperatures between 1050 and 1350° C.
A preferred embodiment of the invention concerns a method of monitoring and controlling the atmosphere during sintering of compacts of low-alloy iron-based materials including easily oxidisable alloying elements selected from the group consisting of Cr, Mn, Mo, V, Nb, Zr, Ti, Al, in a belt furnace.
The invention is further illustrated by the following non-limiting examples.
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Arvidsson Johan
Eriksson Ola
Burns Doane Swecker & Mathis L.L.P.
Hoganas AB
Jenkins Daniel J.
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