AU630640B2

AU630640B2 - Process for producing a heat atmosphere by separation of air by permeation

Info

Publication number: AU630640B2
Application number: AU45562/89A
Authority: AU
Inventors: Eric Duchateau; Philippe Queille
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1988-11-24
Filing date: 1989-11-24
Publication date: 1992-11-05
Anticipated expiration: 2009-11-24
Also published as: PT92409A; EP0375477A1; ATE108752T1; ZA898876B; CA2003473A1; JPH02225303A; FR2639250B1; AU4556289A; JPH07112925B2; EP0375477B1; FR2639250A1; DE68916925D1; PT92409B; ES2057166T3; DE68916925T2

Abstract

The invention relates to a process for creating an atmosphere for the heat treatment of metals, the atmosphere being produced by introducing a stream of gaseous nitrogen, according to which there is first formed, by air separation by means of selective permeation, a crude gaseous nitrogen stream having a residual oxygen content, to which is added a stream of hydrogen which is at least sufficient to remove most of the residual oxygen by catalytic reaction to form water vapour, characterised by the combined implementation of the following measures: a) the air separation by selective permeation is carried out in such a way that the gaseous nitrogen stream has a residual oxygen content of between 0.5% and 3%; b) the catalytic reaction to form water vapour is brought about exclusively with catalysts which are used without external application of heat, and c) the catalytic reaction to form water vapour is carried out in such a way that the residual oxygen content is less than 30 vpm, with a water vapour content of between 1% and 6%, relative to the nitrogen stream, being created. Exclusive application to the annealing or brazing of copper, the decarburising and nitriding annealing of steels, and the sintering of copper, nickel and copper-nickel alloys where appropriate with lead, and copper-zinc alloys.

Description

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AUSTRALIA

Patents Act 630640 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: .a Priority 0 Related Art: 0 Applicant(s): L'Air Liquide, Societe Anonyme pour l'Etude et 1'Exploitation des Procedes Georges Claude Quai d'Orsay, 75007 Paris, FRANCE Address for Service is: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Complete Specification for the invention entitled: PROCESS FOR PRODUCING A HEAT ATMOSPHERE BY SEPARATION OF AIR BY PERMEATION Our Ref 154930 POF Code: 1290/43509 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): 1- 6006 i 1 A- PROCESS FOR PRODUCING A HEAT TREATMENT ATMOSPHERE BY SEPARATION OF AIR BY PERMEATION The present invention relates to a process for producing an atmosphere for heat treating metals formed by the supply of a gaseous nitrogen current with, if required, one or more of the following constituents: hydrogen, methanol, hydrocarbon.

The composition of such heat treatment atmospheres must be substantially devoid of oxygen and generally only permits 10 the presence of water vapour in relatively low contents, t which are moreover defined from one application to the other. This is the reason why, in the very larya mnjoriLy of applications of this type, one starts with very pure nitrogen produced by the cryogenic distillation of air whose residual oxygen content does not exceed 10 v.p.m. volumes per million). This nitrogen, termed cryogenic nitrogen, often has a high production cost and consequently, *in this heat treatment technique and in techniques employing nitrogen or oxygen, other industrial sources were '20 investigated, in particular the separation of the air by S selective adsorption or permeation. This latter type of production is described in the document "Materials and Processes Volume 132 No. 3 Sept. 1988, pages 100-107 Metal Park OHIO U.S. WAYNE C. QUANTZ Stealing nitrogen from the air" which mentions the production of 96.5% pure nitrogen.

But, in these cases, in order to obtain nitrogen production costs which are advantageous relative to cryogenic nitrogen, Co

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2 it has been necessary to privilege the ield to the detriment of the purity so that the nitrogen which is competitive as concerns adsorption or permeation usually contains a residual oxygen content of 0.5% to by volume.

This relative impurity however renders the use of this type of nitrogen very difficult in the production of a heat treatment atmosphere compatible with good results. In practice, nitrogen according to the selective permeation process has been proposed solely for the production of 10 atmosphere from a mixture of nitrogen and methanol, such as S described in the article "Heat Treating Processes with S Nitrogen and Methanol based atmospheres M. KOSTELITZ and S al Heat Treating". Volume 2 Number 1-35, and in the French patents Nos. 79.05599, 82.09328, 85.12380 and 85.12379 in the name of the Applicant.

Although this type of atmosphere produced from S relatively impure nitrogen and methanol is usually intended for heating before hardening, carbonitriding and S carburizing steel, it is in fact only in carburizing that .20 nitrogen of the adsorption or permeation technique has been used industrially, owing to the high temperature, on the S order of 900 0 C, it requires, which temperature promotes the reaction of the residual oxygen carried along by the nitrogen with the chemical species of the hydrocarbon type provided in addition in the basic atmosphere. There may be mentioned in this respect the article "CARBURIZING with membrane N 2 Process and Quality issues P. MURZYN and Co.

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Further, systems exist for producing nitrogen of high purity which are other than those using the cryogenic method. These-relatively complex systems start with a generator of impure nitrogen of the type mentioned hereinbefore with which there is associated a unit known by the name DEOXO which permits attaining a purity similar to that of cryogenic nitrogen, i.e. a residual oxygen content of lower than 10 v.p.m. Such a process is disclosed in the 0 document US 3,535,074 NAKASHIMA).

These systems are not widely used, since this highly S pure nitrogen then results in a production cost which is close to that of the cryogenic nitrogen while the production plants employing adsorption or permeation do not have the advantages of flexibility and simplicity of the plants producing cryogenic nitrogen.

*~e cmIn view of this situation, the Applicant has however come to the conclusion that it is possible, by a series of production optimizations, to adapt the industrial permeation 20 nitrogen to a number of heat treatment applications selected in a strict and limitative manner, in starting with the "p roduction process in which there is first of all formed, by separation of air by selective permeation, a raw nitrogen gas current having a residual oxygen content to which there is added a hydrogen flow which is at least sufficient to eliminate, by a water vapour forming catalytic reaction, the essential part of the residual oxygen, and the invention is I -4 i j characterized by the following operating conditions: Sa) separating the air by selective permeation in such Si manner that the nitrogen gas current has a residual oxygen content of between 0.5% and 3%; b) ensuring the water vapour forming catalytic reation exclusively with catalysts which do not involve an exterior heat supply; c) conducting the water vapour forming catalytic reaction in such manner that the residual oxygen content is 0 lower than 30 with the establishment of a water 9o vapour content of between 1% and 6% relative to the nitrogen S flow; The whole being adapted to any one of the following >applications: dl) the addition of the hydrogen is effected with an excess flow so that the composite current after catalytic reaction has a hydrogen content of 2% to 5% relative to the total flow, and this gas current is employed exclusively for forming an atmosphere for annealing or brazing copper; d2) the hydrogen is added with an excess flow so that the composite current after catalytic reaction has a hydrogen content of 2% to 75% relative to the total flow and said gas current is employed exclusively for forming an atmosphere for decarburizing annealing steel; d3) the hydrogen is added at a minimum value just sufficient to ensure the conversion into water vapour and said atmosphere is produced by introduction of said gas I Y current and a 15% to 50% ammonia current relative to the total flow and 2% to 5% nitrogen protoxyde relative to the total flow, for nitriding steel; d4) the hydrogen is added with an excess flow so that the current formed has a hydrogen content of 2% to 15% and said gas current is employed exclusively for forming an atmosphere for sintering copper, nickel, alloys thereof, if required with lead; the hydrogen is added with an excess flow so that .10 the current formed has a hydrogen content of 2% to 15% while the hydrogen/water vapour ratio remains higher than four and said gas current is employed for forming an atmosphere for sintering a copper-tin alloy.

The complex compromise mentioned hereinbefore results from taking into account several factors. Thus, if the oxygen content of the permeator producing raw nitrogen is i limited to this permits covering a number of heat treatment applications allowing water vapour contents which might attain as is the case in the annealing or brazing i of copper, the decarburizing annealing, the nitriding of i steel, or the sintering of certain non-ferrous metals. If it is proposed to operate the permeator with a residual oxygen content at least equal to it is not only because the desire to increa e the separation quality of the permeator would result in a cost of the nitrogen which is incompatible with the envisaged applications, but also because these applications require, or at least support 6 without risk, water vapour contents equal to at least 1%.

It must also be noted that, in the applications proposed by the invention, it is not possible to benefit from the presence of hydrogen in the treatment atmosphere in the hope that the oxygen content is reduced in a certain manner in this region to a very low value compatible with the application, since, when in the course of treatment, the temperature becomes lower than 400-500° C the kinetics of the formation of water vapour from hydrogen and oxygen is :1 insufficiently rapid and the residual oxygen content is then sufficient to oxidize the metal in particular in the cooling zones of continuous furnaces. It is therefore essential to I eliminate the oxygen before the treatment gas is introduced into the furnace.

But not only the oxygen content of the raw nitrogen must not exceed 3% if the obligation of adding the hydrogen in economically inadmissible quantities is to be avoided, but also the catalytic water vapour forming reaction must be carried out with a relatively good efficiency so that the residual oxygen content does not exceed 30 v.p.m. It is however unnecessary and very costly to employ reactors of the DEOXO type with very high performances which eliminate almost Pll of the oxygen and in any case deliver a very pure treated gas. Such catalytic reactors are expensive to employ, since they require an appreciable exterior supply of heat. This is why the present invention proposes the use of catalytic reactors having an energetic self-sufficiency, as e r r 7 some already known and employed in other applications, such as those employing palladium on an alumina support as a catalyst. Such catalysts operate without an exterior supply of heat and permit correct performances, whereby it is possible to avoid in a sure manner residual oxygen contents higher than 30 v.p.m.

The invention will now be described in more detail by first of all examining the nitrogen generator.

This nitrogen generator is of the selective membrane :10 type. A semi-permeable membrane is employed which may be in I the form of hollow fibres for separating the compressed air S into nitrogen and oxygen. A gas enriched with oxycn i..n water vapour is discharged at the end of the separation module, whereas a dry gas enriched with nitrogen is on the 'i 15 side of the module. Such generators are well known and Kii permit generating a nitrogen gas whose purity varies from 97% to 99.5%, depending on the regulation employed.

There may also be employed a continuously operating V analyzer of the oxygen content of the raw nitrogen and a i S" 20 tank of pure liquid nitrogen, for guaranteeing the security S *of supply and for, if necessary, modifying the quality of the permeation nitrogen.

The catalytic reaction requires the prior intervention of a raw nitrogen and hydrogen mixer advantageously associated with a buffer chamber. The catalyst is so chosen as to permit an immediate and complete reaction of the oxygen at ambient temperature with a residual oxygen content 8 of lower than 30 v.p.m. The catalyst which may be employed is of the alumina type with 0.5% of palladium which is capable of treating an hourly flow of about 5,000 to 10,000 times the volume of the reactor. This type of catalyst requires no prior heating of the gas and moreover does not involve a reactor starting up sequence with an initial rejection of gas to the open air.

This catalytic reactor may be inserted in a plant in different ways, for example: :10 each heat treatment furnace includes a catalytic pot whose volume is adapted to the flow of the furnace. This arrangement permits deliveriny to the furnace hyrrogeniJat 0 nitrogen with a water vapour content which is a function of the initial oxygen content of the nitrogen generator. The reactors may be supplied at low pressure (lower than 1 relative bar), which simplifies the realization.

j or one and the same catalytic reactor supplies a plurality of furnaces and this reactor is then supplied with pressurized gas, generally of 5 to 7 bars, and the de- 20 oxygenated gas issuing from the reactor has a water vapour I content which is a function of the regulation of the generator.

The addition of excess hydrogen relative to the stoichiometric quantity is then a function of the treatments to be carried out.

The applications concerned by the invention will now be examined.

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9 Annealing and brazing of copper The treatment is usually carried out with cryogenic nitrogen and hydrogen (2 to With copper, it is essential to have a very low residual oxygen content in the treatment gas in order Lo avoid oxidation problems. On the other hand, the control of the water vapour is unimportant.

The cryogenic nitrogen is therefore advantageously replaced by a treatment gas from a permeation generator which delivers raw nitrogen to which hydrogen is added so that .0 there is 2 to 5% of hydrogen after the catalytic reaction, this gas mixture containing water vapour whose quantity is a function of the initial oxygen content in the nitrogen i produced by the generator.

Decarburizing annealings of steels These annealings are usually achieved with cryogenic t nitrogen and hydrogen. An injection of water vapour at high temperature permits increasing the dew point and promotes I! *the decarburizing. The hydrogen and water vapour contents are so adjusted as to decarburize the parts without oxidizing them. The cryogenic nitrogen is advantageously replaced by a treatment gas generated by a permeation f nitrogen generator to which hydrogen is added so that there is obtained between 2 and 75% of hydrogen after the catalytic reaction, the gas mixture containing water vapour whose content is a function of the initial oxygen content in the nitrogen.

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10 Nitriding of steel The t: atmsait s for nitriding steels may be carried out with ni -ogen, ammonia (15 to 50%) and 2 to 5% nitrogen protoxide. These treatments are principally carried out in fdiscontinuous furnaces.. The injection of a nitrogen produced by a permeation generator would be liable to result in an oxidizing of the parts upon cooling and chis may be avoided by addiny hydrogen in a quantity which is just sufficient to ensure the catalytic conversion of the oxygen 10 into water vapour.

Sintering of copper, nickel and alloys thereof with if required lead Cryogenic nitroyen with hydrogen are usually employed for sintering these metals. There may be advantageously 15 substituted therefor a mixture of permeation nitrogen and hydrogen so that 2 to 15% of hydrogen is obtained after the catalytic reaction.

Sintering ot copper-tin alloys *0 6 6 o~

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In the case of Cu-Sn alloys, the tin being a rather oxidizable element, it is necessary in addition to limit the water vapour content so that the ratio H 2 (v.p.m)/H 2 0(v.p.m) is higher than four.

Claims

1. Process for producing an atmosphere for heat treating metals formed by supplying a nitrogen gas current, if required with one or more of the following constituents: hydrogen, methane, hydrocarbon, comprising first :adit forming, by separation of air by selective permeation, a raw nitrogen gas current having a residual oxygen content, to which there is added a hydrogen flow which is at least Su_ s O-A I sufficient toeliminate, by water vapour forming catalytic reaction, the s-ia--part of the residual oxygen, characterized by the combination of the following steps: a) separating the air by selective permeir ioii i ui manner that the nitrogen gas current has a residual oxygen content of between 0.5% and 3%;b vo\uL\e. S a. 00 CI 0 too a e 40 0 00 b) exclusively effecting the water vapour forming catalytic reaction with catalysts whose use is exempt from an exterior supply of heat; c) conducting the water vapour forming catalytic reaction in such manner that the residual oxygen content is 20 lower than 30 with establishment of a water vapour content of between 1% and 6% relative to the nitrogen flow; d) said nitrogen gas current purified in this way serves as a basis for the production of an atmosphere for annealing copper, brazing copper, decarburizing annealing of steel, sintering of copper, of nickel or alloys thereof, if required with lead, the sintering of copper-tin alloy.

2. Process for producing an atmosphere for heat 7 _1_ 12 treating according to claim 1, comprising adding the hydrogen with an excess flow so that the composite current after catalytic reaction has a hydrogen content of 2% to relative to the total flow, and employing said gas current exclusively for forming:an atmosphere for annealing copper or brazing copper. Proc&s fr rducing- an atmcshere for hecait treating according to claim 1, comprising addi he hydrogen with an excess flow so that e composite current after catalytic reaction h a hydrogen content of 2% to relative to the t flow, and employing said gas current exclusi y for forming an atmosphere for annealing copper oe* r brazing copper. Process for producing an atmosphere for heat 15 treating according to claim 1, comprising adding the hydrogen with an excess flow so that the composite current after catalytic reaction has a hydrogen content of 2% to relative to the total flow, and employing said gas current Sexclusively for forming an atmosphere for decarburizing i 20 annealing steel. I 4 Process for producing an atmosphere for heat treating according to claim 1, comprising adding the hydrogen at a minimal value which is just sufficient to ensure the conversion into water vapour, and producing said atmosphere by introducing said gas current and a current of to 50% ammonia relative to the total flow, and 2% to p nitrogen protoxide relative to the total flow, for nitriding ".o 13 steel. Process for producing an atmosphere for heat treating according to claim 1, comprising adding the hydrogen with an excess flow so that the current formed has a hydrogen content of 2% to 15%, and employing said gas current exclusively for forming an atmosphere for sinteriny copper, nickel, alloys thereof, if required with lead. S A process for producing an atmosphere for heat treating substantially as hereinbefore described with reference to any one of the e* examples. DATED: 24 November 1988 PHILLIPS ORMONDE FITZPATRICK Patent Attorneys for: L'AIR LIQUIDE, SOCIETE ANONYME POUR L'ETUDE ET L'EXPLOITATION DES PROCEDES GEORGES CLAUDE II i 4 9 *we