MXPA99010978A - Method of monitoring and controlling the composition of sintering atmosphere - Google Patents

Abstract

La invención se refiere a un método para supervisar y controlar la atmósfera de horno cuando se concrecionan materiales compactos de PM;de acuerdo con la invención, los gases que determinan los potenciales de carbono y oxígeno son medidos continuamente.

Description

METHOD TO MONITOR AND CONTROL THE COMPOSITION OF ATMOSPHERE OF CONCRETION DESCRIPTIVE MEMORY The present invention relates to a method for concreting metallurgically produced powder compounds. More specifically, the invention relates to a method for monitoring and controlling the composition of the concretion atmosphere. Concurrently with the development of newer and better metallurgical powder products there is a need for improved methods to also control the concretion atmosphere, and the object of the present invention is to satisfy this need. Briefly, the present invention relates to a method for controlling and monitoring the atmosphere of the concreting furnace when compact materials of metallurgical powders (PM) are concreted, with the gases that determine the carbon and oxygen potentials being measured continuously. The invention is of special interest for monitoring and controlling the atmosphere during the concretion of compact materials of low alloy iron-based materials including easily oxidizable alloy elements selected from the group consisting of Cr, Mn, Mo, V, Nb, Zr, Ti , Al in order to keep the oxidation of those elements at a low level. There is a wide variety of instruments to analyze and control the gases used in atmospheres for powder metallurgy, and the composition of the atmospheres used in the concretion is determined either by measurements in situ or ambient temperature. The measurements can also be made in a separate chamber, towards which the gases from the furnace are extracted from the concreting furnace. According to the invention, the oxygen potential is determined using oxygen probes that are applied to the furnace muffle via the furnace wall or in the separate chamber or furnace and operates with a stabilized Zr02 cell. A reference gas (usually air) with a well-defined partial pressure of oxygen penetrates one side of the cell, while the other side of the cell is in contact with the furnace atmosphere. The difference in partial pressure of oxygen creates an electrical potential which is monitored, thus defining the oxygen potential present. If the measured electrical potential, which corresponds to the current concretion atmosphere, differs from a set value, necessary atmosphere adjustments are made. The value established for the concretion of a given material is decided empirically or theoretically and depends on the type and quantity of the elements of the alloy. When using oxygen probes it should be considered that atmospheres especially with high carbon potentials tend to form soot on the ZrO2 cell if proper precautions are not taken, thus avoiding effective control of the atmosphere. Many producers have foreseen such problems and equipped the oxygen probes with, for example, mechanical brushes. The oxygen probe can be applied in different places when the atmosphere is controlled. In a band furnace based on the countercurrent principle, the oxygen probe is preferably arranged at the end of the concretion zone where the "fresh" gas enters. A second alternative is to arrange the probe near the oven entrance. For this alternative, it must be taken into account that the oxygen potential could be higher due to the possible reduction of oxides and burning of lubricants, and therefore the level of acceptable oxygen in this part of the furnace must be discovered by "test and error "for each powder alloy. As a third alternative, the oxygen probe may be arranged in a separate chamber or furnace towards which gases from the concreting furnace are extracted. In this alternative the oxygen probe is arranged in a separate chamber towards the which gases from the concreting furnace are extracted. The temperature of the atmosphere of 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 atmosphere of the concreting furnace this temperature difference must be considered when determining the gas composition of the concreting furnace. The natural restriction when oxygen is considered is that the oxygen potential measured must be maintained or set below the value of the partial pressure of oxygen balance between the alloying elements and their oxides, for example Cr and Cr2O3. The partial pressure of oxygen balance is well defined by any type of atmosphere used at a specific temperature. If the measured oxygen value is close to this set point, a natural counter action is to increase the flow of reduction gas, for example H2. As can be seen from Example 3 below, the oxygen level can also be controlled and adjusted to a required value by introducing a gas containing carbon, such as methane. It is even more common to monitor concretion conditions by measuring the ambient temperature of the gas mixture. This measurement is generally based on any infrared analysis and / or dew point monitoring. Infrared analysis is based on the principle that different gases absorb infrared energy 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 ray passing through the mixture. The energy changes, which are detected by an infrared analyzer, are therefore a measure of the gas concentration. Each gas compound absorbs a certain portion of the infrared spectrum which does not absorb another gas, and the amount of radiation absorbed is proportional to the concentration of the specific gas. The typical applications of infrared analyzers are in the field of gases with high carbon potential, and care must 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 monoxide, thereby determining the carbon potential. Another alternative is to measure the concentration of all or all except one of the gases that contain carbon. The measurements are carried out on gases from the concretion zone, the cooling zone and / or the heat treatment zone. The control and monitoring of the concretion atmosphere by measuring the oxygen and carbon potentials according to the present invention is preferably carried out using a combination of an oxygen probe to measure the oxygen potential and an IR instrument that measures the gases concurrently. which contain carbon such as CO, CO2 and methane. Using such a combination, the influence of the carbon-containing gases on the oxygen potential is taken into account and a superior method is obtained to control and monitor the concretion atmosphere. Using this method, the optimal concretion conditions can be maintained and the properties of the concreted materials will be improved. Also the potential C is maintained at a set value. This established value depends on the level of carbon desired in the material specified. The method according to the invention can be applied to all types of concreting atmospheres such as nitrogen-based atmospheres, dissociated ammonia, hydrogen-based atmospheres, endothermic gas etc. and within concreting temperatures between 1050 and 1350 ° C. A preferred embodiment of the invention relates to a method for monitoring and controlling the atmosphere during the concretion of compact materials of low alloy iron-based materials including easily oxidizable alloy elements selected from the group consisting of Cr, Mn, Mo, V , Nb, Zr, Ti, Al, in a band furnace. The invention is further illustrated by the following non-limiting examples.

EXAMPLE 1 This example illustrates that the influence of oxygen potential as measured with an oxygen probe is in accordance with the theoretical calculations. The oxygen probe used was Econox Type 1000 from Econox S.A. (Switzerland). The powder compacts containing pre-alloyed iron powder containing 3% Cr and 0.5% Mo were concreted 45 minutes in an atmosphere based on various ratios of H2 (g) / H2O (g) at 1 120 ° C. The oxygen probe was disposed near the oven entrance. The results of the three tests with different concreting gas compositions are described in the following table.

The results of the three tests show that a more pronounced oxidation occurs for oxygen potentials exceeding 3.4.10"17 atm, which is in accordance with the theoretical calculations that show that the oxygen potential should not exceed 4.6. 10"17 as can be seen from the following equations: Reaction number 1: 2Cr (s) + 3/202 = Cr2? 3 cal mol G0! = 62.1 T-267750 T = temperature (K) Reaction number 2: 2C_r + 3/202 = Cr2O3 According to "Treatment of Metallurgical Problems", p.256, the change in Gibbs energy due to the dissolution of chromium in An iron matrix is described and quantified by the equation: ? G (Cr) = 6000-Npe Ncr-T (2.4-3.6 Ncr) for reaction number 3 Cr (s) = Cr (pure, Cr solid? Cr in solid solution). The pure reaction number 2 is obtained by subtracting reaction number 3 from reaction number 1, which in turn means that? G ° 2 =? G °? -2-? G (Cr). Applying this on a material containing 3% chromium; NFe = 0.95 Ncr = 0.031; ? G (Cr) = 6000 NFe Ncr-T (2.4-3.6Ncr) cal? G (Cr) = 3.001 -10? G? = 59730.3 mol lime? G ° 2 =? G °? - 2? G (Cr)? G ° 2 = -1 .752 10 mol Approximate ideal solution: Balance between metal and oxide Acr = Ncr = 0.032 pO2 pO2 = 4.614 10- 117 'atm? G ° 2 = change of Gibbs free energy for reaction number 2 formation of Cr2O3 from dissolved Cr gas and oxygen. Abbreviations:? G ° 1 = change in Gibbs free energy for reaction number 1 formation of Cr2O3 from pure Cr gas and oxygen (cal / mol). G (Cr) = change of Gibbs free energy to dissolve Cr in iron matrix. NFT and Ncr denotes molar fraction of Fe and Cr, respectively. acr denotes chromium activity.

EXAMPLE 2 This example illustrates the invention for an on-line control of the atmosphere in a production furnace. The example shows the possibility of extracting gas from the concretion zone and carry out the analysis in a small separate furnace placed near the production furnace or chambers (see figure 1). Data for the production furnace, atmosphere and specific material used: a) Meshbelt oven manufactured by Efco, 200 KW, band width = 450 mm, approximately 40 m length, b) 5 temperature zones 600, 650, 700, 1120, 1 120 and 1120 C. c) concreted material: iron powder 0.7% C, 1.5% Cu and 0.8% wax H-, 150 kg / h. d) atmosphere: 10% H2 (g) / 90% N2 (g) + X% CH4 (g) (0 <X <2%) depending on the desired carbon potential. e) Realization time: approximately 25 minutes at 1 120 ° C. For the mentioned concretion test, the addition of CH (g) was for the purpose of producing concreted material with a carbon content of 0.7% (uniformly through each specified part). A thin steel tube 7 m long (6 mm outside diameter and 3 mm inside diameter) was inserted into the opening of the furnace opening. The tube was connected to the sampling system through a pump and the length of the tube allowed the extraction of the gas in the high temperature zone of the furnace (1120 ° C). The assembly is illustrated in Figure 1. The gas composition and the carbon potential were monitored continuously by measuring the oxygen potential and the concentration of CO (g) (see figure 2). At 11.20 (marker 1) it was discovered that% CO ~ 0.41 and EMK ~ 1215mV, which according to the calculation immediately give a carbon potential = 0.22. In order to increase the carbon potential, the amount of CH4 (g) was increased and consequently higher CO- and EMK values were measured after a certain time. At 13.15 we discovered that the level of CO- high = 0.85 and EMK = 1230mV leading to a carbon potential of = 0.6. The material material of the two periods mentioned was analyzed with respect to the carbon content and the results revealed the difference with respect to the atmosphere conditions. As expected, the decarburization effect was more pronounced for material concreted in an atmosphere with a carbon potential = 0.21 compared to material materialized at a carbon potential = 0.6.

Results a) carbon potential = 0.21 Surface hardness = 160 Vichers (HV5), surface carbon content on the 0.2-0.3 scale. b) carbon potential = 0.6 Surface hardness = 185 Vickers (HV5), surface carbon content on the scale of 0.4-0.55.

Calculations 1) LogPo2 = -0.678-EMK / (0.0496 * T) where T is the temperature of the probe (Kelvin). Relationship between carbon concentration (% by weight) and carbon activity. 2) ac =? Xc / (1-2 Xc) where Xc is the mole fraction of carbon in a Fe-C alloy and? = Exp ((51 15.9-8339.9 Xc / (1-Xc) /T-1.9096 ) 3) For the reaction C + 1/2 O2 (g)? CO (g) the following equation can be deduced (C = ac in gas phase).

Po2 * acce where K = f (T). Using equation 1 -3 and measuring Po2 and% CO, it is possible to calculate the carbon activity (ac) as shown in example 2. For a mixture of N2-H2-CH the carbon activity is almost independent of the temperature (see figure 3) and therefore the mentioned relationships are very easy to apply to a sampling system where gas monitoring is conducted in a separate small furnace at a different temperature than the one used for concretion.

EXAMPLE 3 This example describes the influence of the addition of methane on the oxygen potential in a concretion atmosphere consisting of 97/3 nitrogen / hydrogen. As can be seen from Figure 4, the oxygen potential is clearly influenced by the addition of methane to the concretion atmosphere. As in Example 1, the oxygen potential was measured by the Econox Type 1000 probe. The methane concentration was measured by an IR analyzer supplied by Maihak (Germany).

It is obvious that the contemporary measurement of the C and O potentials according to the invention allows a superior control of the concretion atmosphere, which is especially advantageous when low alloy components containing easily oxidizable elements are concreted. This careful control is necessary, inter alia, to obtain a small variation of the dimonal change during the concretion as well as a negligible dispersion in mechanical properties of the concreted components.

Claims (11)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for monitoring and controlling furnace atmosphere when compacting metallurgical powder materials, characterized in that the gases that determine the oxygen and carbon potentials are continuously measured in a selected furnace zone of the concretion zone, the cooling zone and / or the heat treatment zone.

2.- The method to monitor and control furnace atmosphere when compact metallurgical powder materials are made, characterized in that the gases that determine the oxygen and carbon potentials are measured continuously in a separate chamber, towards which the gases are extracted from the furnace of concretion.

3. The method according to claim 1 or 2 with the condition that the concretion is not carried out at a reduced pressure.

4. The method according to any of claims 1-3, characterized in that the oxygen potential is determined by an in situ measurement.

5. The method according to any of claims 1-4, characterized in that the determination of the oxygen and carbon potentials comprises measuring the partial pressure of oxygen.

6. - The method according to any of claims 1-5, characterized in that the oxygen partial pressure is measured with an oxygen probe.

7. The method according to any of claims 1-6, characterized in that the measurement of the carbon potential comprises measuring the oxygen partial pressure with an oxygen probe and the concentration of at least one gas containing carbon with an analyzer GO.

8. The method according to any of claims 1-7, characterized in that the oxygen level is maintained at a value below the equilibrium value for the formation of the metal oxide, and that the potential C is maintained at a value established depending on the carbon potential desired in the material specified.

9. The method according to any of the preceding claims, characterized in that the compact materials are low-alloy iron-based materials that include easily oxidizable alloy elements selected from the cosiste group of Cr, Mn, Mo, V, Nb, Zr, Ti, Al.

10. The method according to any of the preceding claims, characterized in that the concretion is carried out in a band furnace.

11. - The method according to claim 2, characterized in that the temperature of the separated chamber is different from the temperature of the concreting furnace.