EP0465333A1 - Method and installation for the cementation of metallic alloy articles at low pressure - Google Patents

Abstract

A fuel mixture is employed, consisting of hydrogen and ethylene in a proportion of 2 to 60% by volume of ethylene and the furnace is heated between 820 DEG and 1100 DEG C. <??>The plant comprises a so-called double-vacuum furnace (50) consisting of a vessel (55) with its internal device for distributing the case-hardening gases, an annular space (56) surrounding the vessel, a cover through which pass conduits for pumping and delivering hydrogen (51) and ethylene (52) opening into the various stages of the vessel at a number of uniformly spaced places, thermocouples (TC) and a microcomputer (61). <??>Application to motor vehicle components. <IMAGE>

Description

The present invention relates to a low pressure carburizing process applied to metal alloy parts and more particularly steel as well as an installation allowing the implementation of this process.

Case hardening is a common practice in metallurgy when it comes to hardening metal parts on the surface to a certain depth to the exclusion of their internal parts which, themselves, must retain a certain flexibility so as not to break inadvertently.

According to a technique generally current in metallurgy, the carbon is incorporated by gas carburizing.

As described in particular in patent FR 2 154 398 in the name of HAYES, the articles to be case-hardened are placed in a vacuum furnace in which gaseous hydrocarbons are circulated essentially based on methane or propane and the treatment is only envisaged. '' at temperatures above about 950 ° C. We work at a pressure below atmospheric pressure, thus ensuring the absorption and thermal diffusion of carbon at the surface of the article. It can be noted that the implementation of this method implies the need to use a pulsation effect to ensure the diffusion at the desired depth of the carbon in the treated part.

According to another process described in patent BF 2,361,476 in the name of IPSEN, a methane-based fuel gas is also used. This gas has the disadvantage of dissociating by producing a lot carbon which turns into black smoke and hinders cementation by fouling the treated parts as well as the oven.

Other furnace manufacturers still use vacuum plasma discharge in an attempt to overcome the difficulties inherent in the use of the aforementioned hydrocarbons: it is ion carburizing.

The object of the present invention is to eliminate such drawbacks by implementing a process in which a fuel mixture consisting of hydrogen and ethylene is used, in a proportion of 2 to 60% of ethylene in volume and the oven is heated between about 820 ° C and about 1100 ° C depending on the nature of the metals constituting the parts and according to the desired content and depth of carbon on the surface of the parts.

The method according to the invention is particularly well suited to the treatment of parts used in advanced industries and the automotive industry such as bearings, gears, slides, cams, piston pins, etc.

Thanks to this process, it is possible to cement all the alloys treated by the processes currently known but under better conditions both in quality and more often in speed. It is also possible to treat certain alloys whose naturally very passive surface has hitherto required prior depassivation treatment. Other alloys which could not be treated even after depassivation can be treated using the methods of the invention.

• a) prévidage de la cuve du four jusqu'à une pression de 10⁻¹ hPa de façon à éliminer l'air,
• b) remplissage de la cuve par de l'azote purifié à la pression atmosphérique,
• c) enfournement de la cuve contenant les pièces métalliques,
• d) mise sous vide de la cuve à 10⁻² hPa,
• e) chauffage jusqu'à la température d'austénitisation et maintien à cette température pour l'homogénéisation des pièces,
• f) introduction d'hydrogène jusqu'à 500 hPa,
• g) enrichissement en carbone par introduction du gaz carburant à base d'éthylène à une pression de 10 à 100 hPa suivant les cas,
• h) diffusion sous vide à 10⁻¹ hPa,
• i) introduction d'azote pour défournement.

More specifically, the method according to the invention essentially comprises the following steps:

• a) preheating of the oven tank to a pressure of 10⁻¹ hPa so as to eliminate the air,
• b) filling the tank with purified nitrogen at atmospheric pressure,
• c) charging the tank containing the metal parts,
• d) evacuating the tank at 10⁻² hPa,
• e) heating to the austenitization temperature and maintaining at this temperature for the homogenization of the parts,
• f) introduction of hydrogen up to 500 hPa,
• g) carbon enrichment by introduction of the ethylene-based fuel gas at a pressure of 10 to 100 hPa depending on the case,
• h) vacuum diffusion at 10⁻¹ hPa,
• i) introduction of nitrogen for discharge.

The implementation of this method involves the use of a particular device whose characteristics are given in the remainder of this description.

This device, described in the case of a double vacuum oven, is also applicable in a cold wall oven.

• les figures 1a, 1b et 1c se rapportent à l'exemple 1 relatif à la cémentation sur une profondeur classique de 1,80 mm de pièces en acier 16 NCD 13.
• Les figures 2a, 2b, 2c et 2d se rapportent à l'exemple 2 relatif à la cémentation de pièces à géométrie difficile comportant des alésages borgnes ou ouverts en acier 14 NC 12.
• La figure 2c, se rapportant à l'exemple 2 est un schéma représentatif de la disposition des pièces en cours de traitement.
• Les figures 3a, 3b et 3c se rapportent à l'exemple 3 relatif à la cémentation sur une profondeur très faible de 0,25 mm de pièces en acier 16 NCD 13.
• Les figures 4a, 4b et 4c se rapportent à l'exemple 4 relatif à la cémentation de pièces en acier Z 15 CN 17.03.
• Les figures 5a, 5b et 5c se rapportent à l'exemple 5 relatif à la cémentation de pièces en acier Z 20 WC 10.
• Les figures 6a, 6b et 6c se rapportent à l'exemple 6 relatif à la cémentation de pièces en acier Z 38 CDV 5.
• Les figures 7a et 7b se rapportent à l'exemple 7 relatif à la cémentation de piéces en superalliage base Co : KC 20 WN.
• La figure 8 représente la cuve de cémentation comportant le dispositif de circulation du gaz carburant dans la cuve.
• La figure 9 représente un four de cémentation à double vide (paroi chaude).

Other advantages and characteristics of the invention will emerge from the following description of several nonlimiting exemplary embodiments of case hardening of different alloys given with reference to the appended drawings in which:

• FIGS. 1a, 1b and 1c relate to Example 1 relating to case hardening over a conventional depth of 1.80 mm of steel parts 16 NCD 13.
• Figures 2a, 2b, 2c and 2d relate to Example 2 relating to the hardening of parts with difficult geometry comprising blind holes or open in 14 NC steel 12.
• FIG. 2c, referring to example 2, is a diagram representing the arrangement of the parts being processed.
• FIGS. 3a, 3b and 3c relate to Example 3 relating to case hardening over a very shallow depth of 0.25 mm of 16 NCD 13 steel parts.
• Figures 4a, 4b and 4c relate to Example 4 relating to the case hardening of steel parts Z 15 CN 17.03.
• FIGS. 5a, 5b and 5c relate to Example 5 relating to the case hardening of Z 20 WC 10 steel parts.
• Figures 6a, 6b and 6c relate to Example 6 relating to the case hardening of Z 38 CDV 5 steel parts.
• FIGS. 7a and 7b relate to Example 7 relating to the case hardening of parts made of superalloy base Co: KC 20 WN.
• FIG. 8 represents the cementation tank comprising the device for circulating the fuel gas in the tank.
• FIG. 9 represents a double vacuum cementation furnace (hot wall).

To facilitate the reading of the 7 examples given below, some details are given here.

Use of cemented metal alloys Steel 16 NCD 13

Gears, hubs, shafts, ...

Bearing rings

Aviation safety parts in general

Steel 14 NC 12

Gears, hubs, shafts, ...

Steel Z 15 CN 17.03

Stainless bearing rings

Stainless track parts

integrated (aeronautics)

Steel Z 20 WC 10

Rolled tracks reported for

hot use (aeronautics)

Steel Z 38 CDV 5

Tool parts in general

Ex: Dies, punches, molds

Super alloy Cobalt KC 20 WN

Turbo machine parts in general

Reagent compositions used for micrographic attacks

In Figure 1a is shown the carbon profile of a piece cemented according to Example 1, we can thus observe the percentage of carbon incorporated as a function of the depth P.

In FIG. 1b is shown the microhardness HV 0.5 kg as a function of the depth for parts treated according to example 1.

In Figure 1c is shown a section of a cylindrical part 10 cemented on the surface according to Example 1 after attack at the nital 2% and respective magnification of 2 and 500 times showing the great regularity on the macrographic picture and the homogeneity of structure on the micrograph.

Examples 2 to 7 are illustrated by figures drawn up identically to the figures in Example 1.

FIG. 2c shows the arrangement in exploded view on three stages in the tank of the furnace of blind bores 11 and open bores 12. Remarkable results have been obtained using tubes of 85 mm in length, with external diameter 14 mm and a bore diameter of 8 mm.

FIG. 2a represents the dispersion band of the carbon profiles obtained on all of the parts shown in 2c.

FIG. 2b represents the dispersion band of the microhardness profiles obtained on all of the parts shown in 2c.

In FIG. 2d is shown a section of a tubular piece 20 cemented on the surface, at the periphery and in the bore, according to Example 2 after attack at the nital 2% and respective magnification of 2 and 500 times showing the great regularity and the homogeneity of the cemented layer.

The assembly represented in FIG. 8 comprises the tank 3 and the internal device as well as the cover 5. Gas inlet pipes 7, 8, 9 pass through the cover and open respectively to the first I, second II and third III stages of the tank in at least three outlets per stage regularly distributed such as 21, 22 and 23 for stage II in particular.

TC thermocouples installed on each floor are permanently connected to a microcomputer not shown which ensures the smooth running of all the operations of the installation.

Each stage has a perforated tray on which the articles to be cemented rest. At their entry, the gases circulate through the load in the direction of the two exhausts, one main at the top of the tank, the other derived at the bottom of the tank following the path indicated by the arrows to be finally sucked at the top of the cover by a large pipe 26 connected to a circulation pump 28. A relative flow curve as a percentage of the cementing gas is shown to the right of the furnace.

The installation shown in FIG. 9 comprises a so-called double vacuum oven 50 in the sense that a vacuum is established both in the tank 55 and in the annular space 56 surrounding the tank. The cementing gases arrive via lines 51 for hydrogen and 52 for ethylene and are directed to several stages where they are regularly distributed. The gas circulation takes place in the tank as described in FIG. 8. The gases are then directed to the pumping unit 62 by a line 59 with a sampling branch to a gas analyzer 60 in connection with a microcomputer. Two other lines, 53 for nitrogen, 54 and 57 for air, emerge respectively at the top of the tank 55 and the space 56. The various data such as temperatures, pressure, flow rates and gas composition are gathered by a acquirer connected to a microcomputer 61.

In addition to the indications given in the various examples, the following details should be provided:

Before starting the treatments, we proceed to the elimination of air from the tank, it is a filling which is carried out at a pressure of 10⁻¹ hPa and we fill the tank with purified nitrogen at atmospheric pressure.

Charging of the tank containing the parts to be treated then takes place and the first austenitization phase is carried out by heating to different temperatures depending on the case, and to a maximum vacuum of 10⁻² hPa.

The vacuum is broken by introducing hydrogen until a pressure of 500 hPa is obtained. Carbon enrichment is carried out by introduction of ethylene at a pressure generally close to 30 hPa and then diffusion at an absolute pressure less than or equal to 10⁻¹ hPa. The vacuum is then broken with nitrogen at atmospheric pressure and then a job treatment is carried out which makes it possible to obtain the desired final characteristics for the case-hardened parts. In the case of Examples 4, 5 and 6, after the diffusion, the vacuum is broken with hydrogen and a second carbon enrichment is carried out followed by a diffusion which precedes the vacuum breaking with nitrogen at pressure atmospheric.

The implementation of the method is carried out under the supervision of a microcomputer to which all the technical parameters programmed are supplied such as grades of steels, temperatures of the various places of the furnace, pressure in the enclosure, durations of the enrichment and diffusion sequences, general flow of gases on each stage, composition of gases and adjustment in depending on the analysis of the exit gases.

Claims (11)

Method of carburizing under low pressure of metal alloy parts especially of steel in which the steel parts are treated in an oven by subjecting them to the action of a fuel mixture based on gaseous hydrocarbons, characterized in that the a fuel mixture consisting of hydrogen and ethylene is used in an amount of 2 to 60% of ethylene by volume and the oven is heated between approximately 820 ° C. and approximately 1100 ° C. depending on the nature of the metals constituting the parts and the desired depth of carbon incorporation. Case hardening method according to claim 1, characterized in that it comprises the following steps: a) preheating of the oven tank to a pressure of 10⁻¹ hPa so as to eliminate the air, b) filling the tank with purified nitrogen at atmospheric pressure, c) charging the tank containing the metal parts, d) evacuating the tank at 10⁻² hPa, e) heating to the austenitization temperature and maintaining at this temperature for the homogenization of the parts, f) introduction of hydrogen up to 500 hPa, g) carbon enrichment by introduction of the ethylene-based fuel gas at a pressure of 10 to 100 hPa depending on the case, h) vacuum diffusion at 10⁻¹ hPa, i) introduction of nitrogen for discharge. Case hardening method according to claim 2, in which the metal parts are made of 16 NCD 13 steel, characterized in that it comprises the following five steps: 1) austenitization under vacuum for half an hour at 980 ° C, 2) breaking the vacuum at 980 ° C with hydrogen until reaching a pressure of 500 hPa, 3) carbon enrichment at 980 ° C by the action of a fuel gas based on ethylene for 2 hours at a pressure of 35 hPa, 4) diffusion at 980 ° C for 3 hours 30 min at a pressure less than or equal to 10⁻¹ hPa, 5) breaking of the vacuum with nitrogen at atmospheric pressure followed by a job treatment at 825 ° C and carburizing is carried out to a depth of 1.80 mm, obtaining the percentage of carbon targeted as a function of the depth. Case hardening process according to claim 2, in which the metal parts are made of 14 NC 12 steel, characterized in that it comprises the following five steps: 1) austenitization under vacuum for half an hour at 880 ° C, 2) breaking the vacuum at 880 ° C with hydrogen until a pressure of 500 hPa is obtained, 3) carbon enrichment at 880 ° C. by the action of a fuel gas based on ethylene for 1 hour and 25 minutes at a pressure of 30 hPa, 4) diffusion at 880 ° C for 0 hours and 20 min at a pressure less than or equal to 10⁻¹ hPa, 5) breaking of the vacuum with nitrogen at atmospheric pressure followed by a job treatment at 825 ° C and carburizing is carried out to a depth of 0.55 mm, obtaining the percentage of carbon targeted as a function of the depth. Case hardening process according to claim 2, in which the metal parts are made of 16 NCD 13 steel, characterized in that it comprises the following five steps: 1) austenitization under vacuum for 30 minutes at 820 ° C, 2) breaking the vacuum at 820 ° C with hydrogen until a pressure of 500 hPa is obtained, 3) carbon enrichment at 820 ° C. by the action of a fuel gas based on ethylene for 1 hour at a pressure of 25 hPa, 4) diffusion (without), 5) breaking of the vacuum with nitrogen at atmospheric pressure followed by an employment treatment at 820 ° C. and the carburizing is carried out to a depth of 0.25 mm, obtaining the percentage of carbon targeted as a function of the depth. Case-hardening process according to claim 2, in which the metal parts are made of Co: KC 20 WN base superalloy, characterized in that it comprises the following five stages: 1) austenitization under vacuum for 30 minutes at 1100 ° C, 2) breaking of the hydrogen vacuum at 1100 ° C. until a pressure of 500 hPa is obtained, 3) carbon enrichment at 1100 ° C. by the action of a fuel gas based on ethylene for 4 hours at a pressure of 40 hPa, 4) diffusion at 1100 ° C for 2 hours at a pressure less than or equal to 10⁻¹ hPa, 5) breaking of the vacuum with nitrogen at atmospheric pressure, case hardening is carried out to a total depth of 0.8 mm. Case hardening process according to claim 1, characterized in that it comprises the following steps: a) preheating of the oven tank to a pressure of 10⁻¹ hPa so as to eliminate the air, b) filling the tank with purified nitrogen at atmospheric pressure, c) charging the tank containing the metal parts, d) evacuating the tank at 10⁻² hPa, e) heating to the austenitization temperature and maintaining at this temperature for the homogenization of the parts, f) introduction of hydrogen up to 500 hPa, g) carbon enrichment by introduction of the ethylene-based fuel gas at a pressure of 10 to 100 hPa depending on the case, h) vacuum diffusion at 10⁻¹ hPa, i) breaking of the hydrogen vacuum, j) carbon enrichment by introducing an ethylene-based fuel gas at a pressure of 10 to 100 hPa, k) dissemination, l) breaking of the vacuum with nitrogen at atmospheric pressure. Case hardening process according to claim 2, in which the metal parts are made of Z 15 CN 17.03 steel, characterized in that it comprises the following eight steps: 1) austenitization under vacuum for 30 minutes at 1020 ° C and cooling in the oven to 980 ° C, 2) breaking the vacuum at 980 ° C until a pressure of 500 hPa is obtained, 3) carbon enrichment at 980 ° C by the action of an ethylene-based fuel gas for 45 minutes at a pressure of 35 hPa, 4) diffusion at 980 ° C for 10 minutes at a pressure less than or equal to 10⁻¹ hPa, 5) breaking of the hydrogen vacuum at 980 ° C at a pressure of 500 hPa, 6) carbon enrichment at 980 ° C by the action of a fuel gas based on ethylene for 6 hours and 45 minutes at a pressure of 35 hPa, 7) diffusion at 980 ° C for 4 hours and 45 minutes at a pressure less than or equal to 10⁻¹ hPa 8) breaking of the vacuum with nitrogen at atmospheric pressure followed by a job treatment at 1020 ° C. and the carburizing is carried out to a depth of 1 mm, obtaining the targeted percentage of carbon as a function of the depth. Case hardening process according to claim 2, in which the metal parts are made of Z 20 WC 10 steel, characterized in that it comprises the following eight steps: 1) austenitization under vacuum for 30 minutes at 1010 ° C and cooling in the oven to 940 ° C, 2) breaking of the hydrogen vacuum at 940 ° C until a pressure of 500 hPa is obtained, 3) carbon enrichment at 940 ° C. by the action of a fuel gas based on ethylene for 45 min at a pressure of 30 hPa, 4) diffusion at 940 ° C for 10 min at a pressure less than or equal to 10⁻¹ hPa, 5) breaking of the hydrogen vacuum at 940 ° C until a pressure of 500 hPa is obtained, 6) carbon enrichment at 940 ° C. by the action of a fuel gas based on ethylene for 1 hour and 15 minutes, 7) diffusion (without), 8) breaking the vacuum with nitrogen at atmospheric pressure followed by a job treatment at 1100 ° C and carburizing is carried out to a depth of 1 mm, obtaining the target percentage of carbon as a function of the depth. Case hardening process according to claim 2, in which the metal parts are made of Z 38 CDV 5 steel, characterized in that it comprises the following eight steps: 1) austenitization under vacuum for 30 minutes at 1010 ° C and cooling in the oven to 960 ° C, 2) breaking of the hydrogen vacuum at 960 ° C until a pressure of 500 hPa is obtained, 3) carbon enrichment at 960 ° C. by the action of a fuel gas based on ethylene for 30 min at a pressure of 30 hPa, 4) diffusion at 960 ° C for 10 min at a pressure less than or equal to 10⁻¹ hPa, 5) breaking of the hydrogen vacuum at 960 ° C until a pressure of 500 hPa is obtained, 6) carbon enrichment at 960 ° C. by the action of a fuel gas based on ethylene for 1 hour, 7) diffusion at 960 ° C at a pressure less than or equal to 10⁻¹ hPa, 8) breaking the vacuum with nitrogen at atmospheric pressure
followed by a job treatment at 990 ° C and carburizing is carried out to a depth of 1 mm, obtaining the target percentage of carbon as a function of the depth. Installation for the carburizing of metal alloy, characterized in that it essentially comprises: - A furnace (50), called double vacuum, consisting of a tank (55) with its internal device for distributing the cementing gases, an annular space (56) surrounding the tank, a cover crossed by conduits pumping and supplying hydrogen (51) and ethylene (52) leading to the different stages of the tank in several regularly distributed locations, - thermocouples (TC) and other probes providing information on the pressure, the flow rate and the composition of the gases at different places in the oven in conjunction with a data acquisition device, itself connected to a microcomputer (61), - several stages for receiving the parts to be case-hardened with perforated plates to allow free circulation of gases.

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