EP0406047B1 - Process for heat-treating of metals - Google Patents

Description

The invention relates to the heat treatment of metals in a continuous furnace by continuous passage of metal parts longitudinally in an elongated treatment zone under a controlled atmosphere having an upstream part at high temperature where said controlled atmosphere comprises nitrogen and reducing chemical species. , in particular hydrogen, possibly carbon monoxide and a downstream part of cooling under atmosphere formed essentially by admission of nitrogen.

• soit on utilise un générateur exothermique assurant la combustion incomplète d'un hydrocarbure et de l'air et délivrant des gaz de combustion qui, après épuration éventuelle, contiennent de l'hydrogène et du monoxyde de carbone à des teneurs qui dépendent du rapport air/hydrocarbure admis dans le générateur. A titre d'exemple, une telle atmosphère exothermique peut contenir 5 à 10 % de monoxyde de carbone et 6 à 12 % d'hydrogène ;
• soit on réalise une atmosphère synthétique à partir de gaz industriels purs tels que l'azote et l'hydrogène. L'azote est produit par distillation cryogénique de l'air et contient très peu d'impuretés ; par exemple le total des impuretés vapeur d'eau et oxygène est généralement inférieur à 10 vpm. On adjoint à cet azote très pur de l'hydrogène, ou un hydrocarbure, ou de l'hydrogène et un hydrocarbure, ou du méthanol de façon à produire une atmosphère réductrice et le cas échéant non décarburante pour traiter les pièces métalliques.

This type of controlled atmosphere which is mainly used for the annealing of metal parts has so far been produced in the following way:

• either an exothermic generator is used ensuring the incomplete combustion of a hydrocarbon and air and delivering combustion gases which, after possible purification, contain hydrogen and carbon monoxide at contents which depend on the air / air ratio hydrocarbon admitted into the generator. For example, such an exothermic atmosphere may contain 5 to 10% of carbon monoxide and 6 to 12% of hydrogen;
• or a synthetic atmosphere is produced from pure industrial gases such as nitrogen and hydrogen. Nitrogen is produced by cryogenic distillation of air and contains very few impurities; for example, the total of water vapor and oxygen impurities is generally less than 10 vpm. To this very pure nitrogen is added hydrogen, or a hydrocarbon, or hydrogen and a hydrocarbon, or methanol so as to produce a reducing atmosphere and if necessary non-decarburizing to treat the metal parts.

This second way of doing things has the advantage of completely controlling the quality of the treatment atmosphere but has the drawback of using cryogenic nitrogen which is relatively expensive and therefore unsuitable for use in generally continuous ovens. not waterproof. This is the reason why we have been led to try to reduce the admitted gas flow rates by creating in particular at the outlet of the cooling zone a nitrogen buffer which makes it possible to avoid any rise of air through the zone. thus ensuring a significant reduction in the overall flow admitted. Despite this significant reduction in overall flow, it turned out that industrially pure gases are still far from being economically attractive compared to the gases produced by an exothermic generator.

This is the reason why, in certain applications where this has proved possible, it has been proposed to replace the cryogenic nitrogen by nitrogen produced by air separation according to the adsorption or selective permeation techniques which , under certain production conditions, lead to significantly reduced costs compared to cryogenic nitrogen to the detriment, however, of oxygen impurity since the nitrogen produced by adsorption usually contains a residual oxygen content of 0.5% to 5% whereas the residual oxygen content of the nitrogen produced by permeation generally exceeds 3% and can go up to 10%.

This oxygen impurity makes it very difficult to directly use this crude nitrogen to develop a suitable heat treatment atmosphere. In practice, the nitrogen produced according to the selective permeation process has been proposed only for the production of atmospheres produced from nitrogen and methanol, as described in the article "Heat treating processes with nitrogen and methanol based atmosphere "M. KOSTELITZ and al. in "Journal of Heat trating" volume 2 no. 1 - 35 and in documents US-A-4279406 and EP-A-0213011 in the name of the applicant. Such an atmosphere produced from nitrogen with residual oxygen content and methanol can in fact be used theoretically in different applications, namely heating before quenching, carbonitriding and case hardening of steel. But it is only in this latter field that the use of nitrogen with residual oxygen content has received industrial use and this because of the high temperature that carburizing implies, of the order of 900 ° C., this temperature favoring the reaction of the residual oxygen conveyed by the nitrogen with the chemical species of hydrocarbon type admitted simultaneously to form the basic atmosphere.

It has been envisaged to purify the nitrogen with residual oxygen content produced by adsorption or permeation by reacting catalytically the oxygen with a corresponding supply of hydrogen sufficient to ensure the complete elimination of all oxygen, but this process relatively expensive leads to a production cost close to cryogenic nitrogen, which disadvantages this form of production of pure nitrogen, especially since the production of nitrogen by adsorption or permeation does not have the flexibility and simplicity of producing cryogenic nitrogen.

The present invention relates to a method of heat treatment of metals in a continuous furnace which makes it possible to substantially reduce the cost of the treatment atmosphere while ensuring the requisite qualities of said atmosphere which must be devoid of oxygen both in the zone of treatment at high temperature than in the cooling zone and this method according to the invention is characterized in that, in the upstream part at high temperature, the nitrogen constituting the treatment atmosphere is supplied by admission of nitrogen to residual oxygen content not exceeding 5%, and preferably greater than 0.5%, produced by air separation according to permeation or adsorption techniques, in that the reducing species in said treatment atmosphere are at all moment present in contents at least sufficient to eliminate the oxygen thus admitted with the nitrogen, while the nitrogen admitted in the downstream part of cooling t, with a flow rate lower than the nitrogen flow rate admitted to the upstream portion at high temperature, is of the type developed prior to admission with practically zero oxygen content.

Thus, in the high-temperature zone, by adding or creating in situ in sufficient quantities reducing species such as hydrogen and carbon monoxide, it ensures the almost instantaneous and almost complete elimination of the oxygen admitted. with nitrogen by transformation into water vapor and carbon dioxide, while maintaining, if necessary, a sufficient content of the said reducing species so that the H₂ / H₂O and CO / CO₂ ratios remain within the limits suitable for times to ensure the required treatment effect without causing the oxidation of the parts being treated. On the contrary, in the cooling zone, where the temperature is clearly lower and in any case insufficient to ensure the immediate reaction between the residual oxygen conveyed by the nitrogen and the reducing species possibly present, nitrogen is used industrially pure, that is to say nitrogen with practically zero oxygen content, which however only represents a flow rate of between 2% and 30%, preferably between 2 and 25%, of the total gas flow admitted in the treatment area. Thus, the admission of a low flow of deoxygenated nitrogen into the cold zone of the oven makes it possible to prevent the return of air and the use of less pure nitrogen in the hot zone makes it possible to reduce the operating costs without reducing the performances.

According to one embodiment, the nitrogen admitted downstream from the cooling zone is produced according to the technique of air separation by cryogenic distillation.

According to another form of implementation, the nitrogen admitted downstream from the cooling zone is produced according to the technique of air separation by permeation or adsorption producing crude nitrogen with residual oxygen content, which is eliminated. by catalytic reaction with a supply of hydrogen in an amount at least sufficient to ensure the elimination of residual oxygen.

The invention also relates to the application of the process to the annealing of metal parts.

The invention will now be illustrated by the following application examples:

First example: Annealing of low carbon steel tubes (≦ 0.3%)

• on admet, au niveau de la zone chaude, à température de l'ordre de 900°C, 108 m³/h (90 % du débit total) d'un mélange constitué de 76 m³/h d'azote à teneur résiduelle en oxygène de 0,5 % et de 18,8 l/h de méthanol qui, par craquage dans le four, donne environ 21,3 m³/h d'hydrogène et 10,7 m³/h de monoxyde de carbone ; l'oxygène se combine immédiatement aux espèces réductrices pour former de la vapeur d'eau et du gaz carbonique. Des mesures effectuées au niveau de la zone chaude du four ont permis de constater les teneurs suivantes dans l'atmosphère de traitement :
  • . H₂ = 19,5 %
  • . CO₂ = 0,3 %
  • . CO = 9,5 %
  • . H₂O = 0,6 %
  • . O₂ < 5 vpm

   Les rapports H₂/H₂O et CO/CO₂ sont tels que l'atmosphère de traitement n'est pas oxydante vis à vis du métal.

• on admet, à l'extrémité aval de la zone de refroidissement, afin de prévenir toute entrée d'air, 12 m³/h (10 % du débit total) constitués d'azote produit par distillation cryogénique à teneur en oxygène inférieure à 10 vpm.

In a continuous furnace forming an elongated heat treatment zone, a total gas flow of 120 m³ / h is admitted, which breaks down as follows:

• we admit, at the level of the hot zone, at a temperature of the order of 900 ° C, 108 m³ / h (90% of the total flow) of a mixture consisting of 76 m³ / h of nitrogen with residual oxygen content 0.5% and 18.8 l / h of methanol which, by cracking in the oven, gives about 21.3 m³ / h of hydrogen and 10.7 m³ / h of carbon monoxide; oxygen immediately combines with reducing species to form water vapor and carbon dioxide. Measurements carried out in the hot zone of the oven revealed the following levels in the treatment atmosphere:
  • . H₂ = 19.5%
  • . CO₂ = 0.3%
  • . CO = 9.5%
  • . H₂O = 0.6%
  • . WHERE <5 vpm

The H₂ / H₂O and CO / CO₂ ratios are such that the treatment atmosphere is not oxidizing with respect to the metal.

• we admit, at the downstream end of the cooling zone, in order to prevent any entry of air, 12 m³ / h (10% of the total flow) consisting of nitrogen produced by cryogenic distillation with an oxygen content of less than 10 vpm .

Second example: Decarburizing annealing of magnetic sheets

Such annealing is carried out here in a continuous oven at a temperature of the order of 800 ° C.

• on admet au niveau de la zone chaude, 85 m³/h (85 % du débit total) d'un mélange constitué de 68 m³/h d'azote à teneur résiduelle en oxygène de 3 % et de 10 litre/heure de méthanol qui, par craquage dans le four, produisent environ 11,3 m³/h d'hydrogène et 5,7 m³/h de monoxyde de carbone. L'oxygène résiduel se combine immédiatement aux espèces réductrices pour former de la vapeur d'eau et du gaz carbonique qui sont les agents décarburants des tôles magnétiques. Des mesures effectuées au niveau de la zone chaude du four ont permis de vérifier que la teneur en vapeur d'eau est suffisante pour assurer une décarburation du métal et que les rapports H₂/H₂O et CO/CO₂ restent suffisants pour protéger le métal contre toute oxydation en zone chaude, ce qui entraverait la décarburation.
  Valeurs mesurées :
  • . H₂ = 9,5 %
  • . CO = 5,0 %
  • . H₂ = 3,5 %
  • . CO₂ = 1,5 %
  • . O₂ < 5 vpm
• on admet, au niveau de la zone de refroidissement, 15 m³/h (15 % du débit total) d'azote cryogénique, ce qui permet d'obtenir un recuit décarburant sans bleuissement. Le fait d'utiliser de l'azote cryogénique prévient toute oxydation du fer constituant les tôles magnétiques, cet azote cryogénique ayant pour rôle essentiel de former un tampon à la sortie du four.

A total flow rate in the furnace of 100 m³ / h is allowed, which breaks down as follows:

• 85 m³ / h (85% of the total flow) of a mixture consisting of 68 m³ / h of nitrogen with a residual oxygen content of 3% and 10 liters / hour of methanol is admitted at the level of the hot zone. , by cracking in the furnace, produce approximately 11.3 m³ / h of hydrogen and 5.7 m³ / h of carbon monoxide. The residual oxygen immediately combines with the reducing species to form water vapor and carbon dioxide which are the decarburizing agents of magnetic sheets. Measurements carried out at the hot zone of the furnace made it possible to verify that the water vapor content is sufficient to ensure decarburization of the metal and that the H₂ / H₂O and CO / CO₂ ratios remain sufficient to protect the metal against any oxidation in a hot zone, which would hamper decarburization.
  Measured values:
  • . H₂ = 9.5%
  • . CO = 5.0%
  • . H₂ = 3.5%
  • . CO₂ = 1.5%
  • . WHERE <5 vpm
• 15 m³ / h (15% of the total flow) of cryogenic nitrogen is allowed at the level of the cooling zone, which makes it possible to obtain a decarburizing annealing without bluing. The fact of using cryogenic nitrogen prevents any oxidation of the iron constituting the magnetic sheets, this cryogenic nitrogen having the essential role of forming a buffer at the outlet of the oven.

If necessary, water vapor can be added to the cooling zone to obtain, on the contrary, a bluing of the parts.

Third example: Annealing of copper tubes

The annealing of copper tubes is carried out here in a continuous furnace at a temperature of the order of 650 ° C.

• on admet, en zone chaude, 170 m³/h (95 % du débit total) d'un mélange constitué de 165 m³/h d'azote à teneur résiduelle en oxygène de 0,5 % et de 5 m³/h d'hydrogène. Par réaction avec l'oxygène du four, il se forme environ 1,7 m³/h de vapeur d'eau, alors qu'il subsiste environ 3,3 m³/h d'hydrogène. De cette façon, on élimine quasi instantanément l'oxygène afin de ne pas oxyder le cuivre. La présence de vapeur d'eau n'a pas d'effet néfaste compte tenu de la teneur en hydrogène.
• on admet, en zone de refroidissement, 10 m³/h (5 % du débit total) d'un mélange d'azote, de vapeur d'eau et d'hydrogène, obtenu en ajoutant à de l'azote brut de perméation ou d'adsorption présentant une teneur résiduelle en oxygène de 0,5 %, de l'hydrogène en quantité au moins suffisante pour assurer l'élimination de l'oxygène par réaction catalytique.

A total flow rate of 180 m³ / h is allowed in the oven, which breaks down as follows:

• 170 m³ / h (95% of the total flow) of a mixture consisting of 165 m³ / h of nitrogen with a residual oxygen content of 0.5% and 5 m³ / h of hydrogen is allowed in the hot zone . By reaction with oxygen from oven, approximately 1.7 m³ / h of water vapor is formed, while approximately 3.3 m³ / h of hydrogen remains. In this way, oxygen is eliminated almost instantaneously so as not to oxidize the copper. The presence of water vapor has no harmful effect taking into account the hydrogen content.
• we admit, in the cooling zone, 10 m³ / h (5% of the total flow) of a mixture of nitrogen, steam and hydrogen, obtained by adding to raw nitrogen permeation or d adsorption having a residual oxygen content of 0.5%, hydrogen in an amount at least sufficient to ensure the elimination of oxygen by catalytic reaction.

Fourth example: Annealing bronze pieces at 500 ° C

Conditions identical to example 3 are applied.

Claims (9)

1. A method for the heat treatment of metals by continuously passing metal components longitudinally through an elongate treatment zone under a controlled atmosphere having an upstream high-temperature portion where this controlled atmosphere comprises nitrogen and chemical reducing species, in particular hydrogen and possibly carbon monoxide, and a downstream cooling portion under an atmosphere formed substantially by a supply of nitrogen, characterised in that, in the upstream high-temperature portion, the nitrogen forming the atmosphere is provided by supplying nitrogen obtained by air separation using permeation or adsorption techniques with a residual oxygen content of no more than 5%, the reducing species being present at all times in quantities at least sufficient to eliminate the oxygen supplied in this way with the nitrogen, and in that the nitrogen supplied to the downstream cooling portion, at a rate of flow which is lower than that of the nitrogen supplied to the upstream high-temperature portion, is of the type processed prior to supply and has an oxygen content which is practically zero.
2. A method for the heat treatment of metals as claimed in claim 1, characterised in that the residual oxygen content of the nitrogen forming the atmosphere of the upstream high-temperature portion is greater than 0.5%.
3. A method for the heat treatment of metals as claimed in claim 1 or 2, characterised in that the nitrogen supplied to the downstream cooling portion is processed using the technique of air separation by cryogenic distillation.
4. A method for the heat treatment of metals as claimed in claim 1 or 2, characterised in that the nitrogen supplied to the downstream cooling portion is processed using the technique of air separation by permeation or adsorption producing crude nitrogen with a residual oxygen content that is eliminated by catalytic reaction with a supply of hydrogen in a quantity at least sufficient to ensure that the residual oxygen is eliminated.
5. A method for the heat treatment of metals as claimed in any one of claims 1 to 4, characterised in that the flow of nitrogen supplied to the downstream cooling portion of the treatment zone is between 2% and 25% of the overall gas flow supplied to the treatment zone.
6. Application of the method for the heat treatment of metals as claimed in any one of claims 1 to 5 to the annealing of metal components.
7. Application of the method for the heat treatment of metals as claimed in claim 6 to the annealing of steel components with a supply, in the upstream portion of the treatment zone which is at high temperature, of nitrogen with a residual content of oxygen and methanol producing, by cracking, hydrogen and carbon monoxide such that the annealing treatment atmosphere has H₂/H₂0 and CO/CO₂ content ratios providing this treatment atmosphere with its reducing nature.
8. Application of the method for the heat treatment of metals as claimed in claim 6 to the decarburization annealing of magnetic components with a supply, in the upstream portion of the treatment zone which is at high temperature, of nitrogen with a residual oxygen content providing, by reaction with the reducing species, water vapour and carbon dioxide in quantities sufficient to ensure decarburization, while the H₂/H₂0 and CO/CO₂ content ratios remain sufficient to prevent any oxidation of the metal in this upstream high-temperature portion.
9. Application of the method for the heat treatment of metals as claimed in claim 6 to the annealing of copper or bronze components at a temperature of between 350°C and 700°C with a supply of nitrogen with a residual content of oxygen and hydrogen to the upstream high-temperature portion of the treatment zone.