Process and plant for thermal treatment of metals in protecting atmosphere

Abstract: Protective atmosphere for the heat-treatment of metals is obtained by heating a reactor (2) containing a Nickel-based catalyst to a temperature of between 1000°C and 1200°C, feeding the said reactor (2) with a stream of nitrogen having oxygen content of between 0.1% and 9% and a stream of hydrocarbons quantitatively substantially stoichiometric to obtain CO and H 2 , and sending the gas from the catalytic reactor (2) to a heat-treatment furnace (1).

Description: The present invention relates to a process and plant for the heat-treatment of metals in a protective atmosphere, e.g. annealing, normalization, pre-temper healing. In these types of process, the atmosphere used in the furnace must be neutral, not carburizing or decarburizing, to avoid modification of the surface composition of the treated metal; the atmosphere could be slightly reductive to eliminate any oxygen which enters the heat treatment furnace. Traditional heat treatment processes are known in which the protective atmosphere is produced by an exothermic generator in which a combustion reaction with a hydrocarbon takes place in a shortage of air, with comburant: fuel ratios (e.g. for methane) from 1:6 to 1:9. This process has the disadvantage of producing large quantities of CO 2 and H 2 O which must be at least in part removed from the mixture. There are also known processes which use an endothermic generator to obtain the desired atmosphere from a mixture of air and hydrocarbons. The comburant:fuel ratio for reaction is 2:1 when methane is used. The European Patent Application N°0482992 in the name of AIR LIQUIDE, describes a process for obtaining a protective atmosphere with a low content of reducing agents by passing nitrogen with O 2 content of between 1% and 7% through a catalytic reactor provided with a precious metal catalyst at a temperature of between 400°C and 900°C. On the one hand, this process has the advantage of producing an atmosphere with H 2 and CO contents in the same order as those of the exothermic reaction, but with low CO 2 and water contents; on the other, it presumes the use of fairly expensive catalysts and is poorly suited to the treatment of high-to medium-carbon steels. This same document refers to a possibility of operating at high temperatures with a Nickel-based catalyst, but judges such a process unsuitable for industrial production and advises against its use. SU-A-523144 discloses a method of forming a protective atmosphere for metal treatment plants according to which commercial nitrogen, containing O 2 , is mixed with natural gas in the amount of 2.0-2.5 volumes of the oxygen present in the nitrogen. The mixture is fed to a nickel catalyst, converted and fed to the furnace of the plant. This method is substantially corresponding to the method cited in above mentioned EP-A-0482992 and advised against by said application. Skakal'skii et al. ( Commercial Nitrogen - The Basis For A Universal Controlled Atmosphere) (1978) Plenum Publishing Corporation, discloses a process similar to the one described in SU-A-523144 and a plant wherein the composition of the atmosphere is detected by measuring the dew point or the content of CO 2 and is controlled to avoid changes of the carbon potential during a process of heat treatment of metals. The control of the carbon potential is obtained by changing the amount of hydrocarbons fed to the reactor; in this way different steels can be treated (in separate and different treatments) by slightly modifying the ratio of hydrocarbon to oxygen to have a condition of equilibrium of the steel surface with the atmosphere. Metals Handbook - Ninth Edition - vol.4, Heat Treating, 397-398, discloses the production of endothermic-base atmospheres by reaction of air and hydrocarbons in a reactor containing a nickel catalyst. This document also teaches that the maintenance requirements of endothermic-base atmosphere generators include weekly and/or monthly burn out of carbon deposits in generator, a clean and active generator being extremely important for accurate control of carbon potential in the atmosphere. To avoid soot formation in the catalyst, control of the dew point is suggested. The aim of the present invention is to overcome the aforementioned problems and provide a process for heat treatment in a protective atmosphere which is inexpensive, industrially applicable, has a controllable CO and H 2 contents and very low CO 2 contents. Such aim is achieved by the present invention which relates to a process for the heat treatment of metals in a protective atmosphere, characterized according to Claim 1. The invention also relates to a plant for the heat treatment of metals according to Claim 7. According to the invention, the stream of hydrocarbon into the catalytic reactor is interrupted periodically and/or by command, while the stream of nitrogen containing a measured and controlled oxygen content is maintained. According to a preferred aspect of the invention, during the interruption of the flow of hydrocarbons, the oxygen content of the nitrogen is maintained between 39% and 5%. According to another preferred aspect of the invention, the CO, hydrocarbon and CO 2 contents of the gas leaving the catalytic reactor are measured; a corresponding signal is generated and compared with a previously memorized value in a computer to regulate the rate and composition of the gas flow entering the catalytic reactor. The process according to the invention has numerous advantages over the present state of the art. The process according to the invention allows a protective atmosphere with reducing agent (H 2 and CO) content generally from 10% to 20%, similar to what can be obtained with an exothermic process, and with very reduced water and CO 2 contents. In this way, both the problems of lowering the water and CO 2 contents and the problems related to high content of carburizing substances which are typical of the exothermic process are solved. Furthermore, the oxidation reaction in the catalytic reactor can be controlled to give an atmosphere in which the CO 2 content is in equilibrium with the carbon content of the metal being treated and medium- to high-carbon content metals can be heat-treated also. A further important advantage is that the process according to the present invention does not require the traditional regeneration of the catalyst, which usually requires shutdown of the plant for all the time necessary to its completion. Another advantage is that the process allows copper and its alloys to be treated in bell furnaces. The invention will now be described in more detail with reference to the enclosed drawing which is by way of example and is not limiting, which shows a schematic embodiment of the plant according to the invention. Such plant comprises a furnace 1 for the heat-treatment of metal products, usually made of steel, copper and its alloys in a protective atmosphere. Upstream of furnace 1 there is a reactor 2 in which the required atmosphere is generated. The reactor 2 contains a Nickel-based catalyst 3 (e.g. of the type consisting of 6-7% of Nickel on alumina) and comprises a means 4 of heating it to a temperature of from 1000 to 1200°C. Two ducts 5 and 6 connect reactor 2 to a source 7 of nitrogen containing a controlled amount of oxygen, and a hydrocarbon source 8, respectively. The source of nitrogen with oxygen mixed in is of a type known to the art and is such as to provide a mixture whose O 2 content lies between 0.1% and 9.0%, preferably from 1% to 5% (by volume). A duct 9 takes the gas formed in the reactor 2 to the furnace 1. On duct 6 there is a valve 10 or similar means of regulating or interrupting the stream of hydrocarbons to the reactor 2. The means 10 is controlled by a computer 11, which comprises both a means of processing data and recording it. The computer 11 is linked by the line 14 to a means of analysis 13, which is connected to duct 9 by line 12. The plant according to the invention operates in the following manner. A value is set for the percentage of oxygen in the nitrogen stream feeding the reactor 2; as mentioned above, the N 2 -O 2 mixture comprises from 0.1% to 9.0%, preferably from 1% to 5% (by volume). Such a mixture is obtained by techniques known to the art, e.g. by absorption or permeation. The hydrocarbon stream is regulated so as to feed the reactor 2 a quantity of hydrocarbons substantially stoichiometrical with respect to the oxygen content to produce CO and H 2 . The desired reaction is shown below using methane (1) and propane (2) as hydrocarbon, by way of example: (100-x)N 2 + xO 2 + 2xCH 4 → (100-x)N 2 + 2xCO + 4xH 2 (1) (100-x)N 2 + xO 2 + 2/3xC 3 H 8 → (100-x)N 2 + 2xCO + 8/3xH 2 (2) The reactor 2 is maintained at a temperature of between 1000°C and 1200°C, preferably between 1050°C and 1100°C. The atmosphere thus obtained is sent to the furnace 1. As specified above, the hydrocarbon stream is regulated by means of the valve 10 to give the desired composition for the protective atmosphere. For example, analyzing the gas leaving the reactor 2 by means of the analyzer 13 (known per se to the art) and measuring the CO 2 content, the reaction can be controlled to have a CO 2 content in equilibrium with the carbon content of the steel present in the heat-treatment furnace 1. Valve means 10 also interrupt the hydrocarbon stream to the reactor 2 periodically and/or by command, while continuing to feed the nitrogen/oxygen stream to the same reactor 2. The O 2 content of the nitrogen stream fed to the reactor while the hydrocarbon stream is interrupted is usually less than 10% and is preferably within the range of 3% to 5%. Therefore, if the O 2 content of the nitrogen stream used at the same time as the hydrocarbon stream is within this range, this same N 2 /O 2 stream can be used during the said periods of interruption of the hydrocarbon. If the initial O 2 content is less, then it is preferably raised to the desired value. These interruptions are controlled by the computer 11 according to two distinct modes which can, however, be combined. The interruptions can be pre-programmed and actuated periodically according to a program run on the computer 11 which regulates their frequency and length based on pre-set data. As an alternative or in addition to the above, the interruptions could be triggered by a situation of incorrect operation of the reactor 2 being detected. In this case the means 13 measures the quantity of hydrocarbon in the gas leaving the reactor 2, generates a corresponding signal and sends it to the means of processing data in the computer 11. Here the values detected are compared to the values memorized in the computer which can - if necessary - interrupt the flow of hydrocarbons to the reactor 2. The length of each interruption can be pre-set (generally from 1 to 60 seconds) or linked to the values of CO and CO 2 detected in the gas leaving the reactor 2. In the latter case, the means 13 detects the content of said compounds in the gas leaving the reactor and the computer keeps valve 10 closed until the CO and CO 2 levels are below a pre-set threshold. As mentioned above, interrupting the flow as described above avoids the problem of having to regenerate the catalyst in the traditional way, with the plant shut down for not less than 12 hours once or twice a month. Without giving a complete scientific explanation of the phenomenon, it is believed that flushing with the N 2 /O 2 stream alone for short periods is sufficient to oxidize and remove carbon accumulations on the catalyst, without greatly varying the other operating parameters of the same. The invention will be further described with reference to the following examples. Example 1 Normalization of medium-high carbon steel pipes A stream of N 2 containing 3% (by vol.)O 2 and a stream of methane were fed into a catalytic reactor containing a Ni-based (7% on alumina) catalyst. The reactor was heated to 1050°C. The atmosphere generated by the reactor (which contained 6% of CO and 12% of H 2 ) was sent to the normalization furnace, heated to 900°C. The supply of methane was interrupted periodically for short periods during the production of the atmosphere. The treated pipes had a bright surface, without chemical alteration of the surface. Example 2 Treatment of copper products A stream of N 2 containing 2% of O 2 and a stream of methane gas was sent to a reactor according to Example 1. The atmosphere generated by the reactor comprised about 4% of CO and 8% of H 2 and was sent to a bell furnace heated at about 600°C. The products treated had a very bright surface without any surface oxidation.

Claims: A process for the heat-treatment of metals in a protective atmosphere, comprising the following steps: - heating a reactor (2) containing a Nickel-based catalyst to a temperature within the range of 1000°C to 1200° C; - feeding said reactor (2) with a stream of nitrogen containing from 0.1 to 9% oxygen; - feeding said reactor (2) with a stream of hydrocarbons in an amount substantially stoichiometric to give CO and H 2 ; - feeding the gas leaving the said catalytic reactor (2) to a heat-treatment furnace (1) to form the protective atmosphere inside the same; - interrupting periodically and/or by command said stream of hydrocarbons, while maintaining said stream of nitrogen, and resuming said hydrocarbons stream after a pre-set or calculated period time. A process according to claim 1, wherein the oxygen content of the nitrogen stream is varied during the interruption of the stream of hydrocarbons. A process according to claim 1 or 2, wherein the oxygen content of the nitrogen stream is within the range of 3% to 5% during the interruption of the stream of hydrocarbon A process according to any previous claim, wherein said catalytic reactor is heated to a temperature within the range of 1050°C to 1100°C. A process according to claim 4, wherein the oxygen content of the said nitrogen stream is within the range of 1% to 5%. A process according to any previous claim, further comprising the following steps: analysing the CO, hydrocarbon and/or CO 2 content of the gas leaving said catalytic reactor (2); generating a signal corresponding to said content and sendingthat signal to a means of data-processing (11) the said signal; comparing the value corresponding to the said signal with values memorized in the said means of data-processing; regulating ' the stream of hydrocarbons and/orthe oxygen content of the said nitrogen stream as a function of said memorized values. A plant for carrying out a process of heat-treatment of metals according to any previous claim, comprising a heat-treatment furnace (1) and means of generating a protective atmosphere, the said means comprising: a catalytic reactor (2) containing a Nickel-based catalyst (3); means (5,7) of feeding the said reactor with a stream of nitrogen containing oxygen within the range of 0.1 % to 9%; means (6,8) of feeding the said reactor with a stream of hydrocarbons; means (10) of regulating and of interrupting the flow rate of the said stream of hydrocarbons; and means (11) to control the operation of said regulating and interrupting means (10), characterized in that : said control means is a computer (11), comprising both means of processing data and of recording it, that is set to operate said interrupting means (10) periodically, according to a program run on said computer, while maintaining said stream of nitrogen and to resume said hydrocarbon stream after a pre-set period of time. A plant according to claim 7, further comprising means of increasing the oxygen content in the stream of nitrogen. A plant according to Claim 7 or 8, further comprising means (13) of measuring the CO, hydrocarbon and/or CO 2 content of the gas leaving the said catalytic reactor (2); means (13) of generating a signal corresponding to the said content; wherein said means of data-recording (11) memorize one or more values corresponding to pre-set CO, hydrocarbon and/or CO 2 contents; and said means of data-processing (11) process the said signal to compare the value corresponding to the said signal with the values memorized and operate said means (10) of regulating and interrupting the stream of hydrocarbons, as a function of the differences between the said measured values and those memorized values.

Inventors: VALTOLINA DANIELE, VALTOLINA, DANIELE
Applicants: SOL SPA, SOL S.P.A.
Application year: 1996
Main classification: C21D1
IPC codes: C21D1