EP0096602A1 - Method of heat treating metallic parts by carburization - Google Patents

Abstract

The method of carburizing steel workpieces comprises loading workpieces to be carburized in a furnace and maintaining them in a carbon enriching atmosphere comprising carbon monoxide, hydrogen and nitrogen. The treatment comprises a first phase carried out at a temperature from 850 DEG C. to 1050 DEG C. followed by a second phase carried out at a temperature from 700 DEG C. to 950 DEG C. During the first phase an atmosphere is used having a carbon potential from about 1.1% to about 1.6% by weight and during the second phase the amount of nitrogen in the atmosphere is increased from two to thirty times so that the carbon potential for the second phase is at least about 0.5% by weight less than the carbon potential for the first phase.

Description

The subject of the present invention is a method of heat treatment of metal parts, in particular steel parts, by carburetion.

It is known that the use of carbon enrichment atmospheres for the heat treatment of steels at temperatures from 1050 ° C. to 800 ° C. makes it possible to increase the carbon content over a certain thickness at the surface of the parts and thus increasing its hardness and resistance to wear.

The atmospheres generally used contain approximately 20% C0, 40% H ₂ , 40% N ₂ , and very small amounts of carbon dioxide and water vapor. These atmospheres are obtained either from so-called endothermic generators, or synthetically from a gas-gas or gas-alcohol mixture; the most common of these synthetic mixtures is nitrogen-methanol: in fact, at the treatment temperatures used, methanol decomposes according to the reaction CH ₃ OH → CO + 2H ₂ and it is possible to obtain a gaseous mixture having the composition given ci -above.

The carburetion process is carried out as follows: the carbon monoxide present in the treatment atmosphere reacts according to the relationship: 2CO⇄CO ₂ + C (1) and there is then transfer of the carbon atoms to the metal. The hydrogen present in the atmosphere also participates in carburetion from the point of view of the speed of the process because it reacts with carbon monoxide according to the reaction: CO + H ₂ ⇄ C + H ₂ O (2).

Some of the carburetion treatments implemented up to now, in particular the treatments carried out in ovens with several zones, comprise two successive phases: a first phase called "cementation" followed by a second phase called "diffusion". More precisely, such treatments consist in subjecting the part to be treated in the carburizing zone, at a temperature of 900 to 940 ° C, to a carbon enrichment atmosphere having a carbon potential of 0.9% to 1 , 2% by weight for a certain time; then the part is placed in the diffusion zone where the process is allowed to run its course: the temperature gradually decreases to 880 ° C to 800 ° C and the carbon potential of the atmosphere decreases to a value of 0.7% to 0.9% by weight. The part thus treated is then cooled by quenching in gas or liquid phase, for example in an oil bath. The drop in temperature obtained during the diffusion phase minimizes the problems of deformation during quenching.

The difficulties which arise during such treatments are that, on the one hand, it is advantageous, at the start of the process, to obtain a high carbon potential so as to increase the supply of carbon, but, however, this carbon potential must not exceed a limit value otherwise deposits of soot are formed on the part: this limit value, from 1% to 1.6%, depends on the temperature used; on the other hand, it is advantageous at the end of treatment to have a carbon potential on the surface otherwise, during the subsequent quenching, the metallurgical properties of the part are not satisfactory because there is then the presence of an austenite phase residual and therefore poor surface hardness of the treated product. As can be seen, during carburetion treatments, the problem arises of obtaining and controlling a well-determined carbon potential during each of the two phases of the treatment.

According to the processes used up to now, to obtain the desired carbon potential during each of said phases, a hydrocarbon such as methane is injected into the furnace, in addition to the mixture intended to form the C0, H2, N ₂ species. , propane, or butane in each of said zones and the flow rate of this hydrocarbon is adjusted as a function of the CO ₂ content of the atmosphere. Indeed, given, on the one hand the reaction of CO consumption by the part to be treated (see equation (1)), on the other hand the air inlets in the treatment enclosure, the C02 concentration of the atmosphere tends to increase and therefore the carbon potential to decrease. This is why the CO ₂ content of the atmosphere is monitored and the injection rate of the hydrocarbon is adjusted accordingly as a function of the desired carbon potential. This regulation can also be carried out by monitoring the H ₂ O or O _{2 content} in the atmosphere.

Since the development of synthetic atmospheres, in particular based on nitrogen and methanol, we seek to increase the CO and H2 contents of the treatment atmospheres. In this regard, there may be mentioned more particularly the process described in American patent n ° 4,306,918. This process consists, in a first phase, to send pure methanol to the treatment furnace and to maintain the parts in an atmosphere having a carbon potential of 0.8% to 1.1%, then, in a second phase, to inject nitrogen (generally more economical than methanol) in the oven so that the treatment is less expensive and to maintain the parts in an atmosphere having a carbon potential of 0.7% to 0.9%. This process makes it possible to obtain significant fuel depths fairly quickly thanks to the increase in fuel species such as CO and H ₂ during the first phase. Note that, according to the process of this American patent n ° 4,306,918, the carbon potential ranges of the atmospheres used are conventional carbon potential ranges, ie from 0.7% to 1.1% and in any event less than 1.1%, and that the carbon potential difference between the two phases is small (0.2%).

The subject of the invention is a method of heat treatment of metal parts by carburetion which makes it possible to obtain a surface hardening and a carburetted depth of the treated parts satisfactory with a shorter treatment time.

The process according to the invention consists in placing the parts to be treated in an oven and in maintaining them, in a carbon enrichment atmosphere comprising in particular carbon monoxide, hydrogen and nitrogen, said treatment comprising a first phase carried out at a temperature of 850 ° C to 1050 ° C followed by a second phase carried out at a temperature of 700 ° C to 950 ° C (preferably 800 ° C to 950 ° C). It is characterized in that, during the first phase, an atmosphere is used containing approximately 20% to 50% by volume of CO and approximately 40% to 75% by volume of H ₂ and having a high carbon potential very close to the limit value leading to soot deposits, ie a carbon potential of approximately 1.1% to 1.6% by weight, and, during the second phase, to have a carbon potential significantly lower than that of the first phase , an increase in the nitrogen content of said atmosphere is increased from 2 to 30 times so that the difference in carbon potential between each of said phases is at least about 0.5% by weight.

According to a characteristic of the invention, during the first phase, the concentration of nitrogen in said atmosphere is approximately at most 40% by volume, and during the second phase, said concentration is about 30% to 80% by volume.

As will be understood, in order to improve the productivity of the carburetion treatments, the applicant sought to increase the carbon potential during the first phase to accelerate the carbon enrichment of the part.

However, to obtain good metallurgical properties, it is necessary to reduce the percentage of carbon close to the surface. For this, we must, during the second phase, significantly reduce the carbon potential of the atmosphere. However, it is difficult to vary the carbon potential in such a large and abrupt manner by the conventional means used, such as adjusting the rate of injection of a hydrocarbon into the furnace, and this all the more so in the case an atmosphere very rich in CO and H ₂ .

Given these observations, the applicant has imagined lowering the carbon potential, during the second phase, by diluting the atmosphere with an inert gas such as nitrogen.

. En diluant l'atmosphère par de l'azote, on diminue les pressions partielles de CO et CO ₂ dans les mêmes proportions ; par contre, le rapport

décroît et donc le potentiel carbone décroît.Indeed, as can be seen from equation (1), the potential decreases with the ratio

. By diluting the atmosphere with nitrogen, the partial pressures of CO and CO ₂ are reduced in the same proportions; however, the report

decreases and therefore the carbon potential decreases.

= 5 et

= 10, D étant le débit d'azote injecté dans le tour en m³/h et V le volume du four en m³. Ces courbes montrent qu'effectivement une dilution de l'atmosphère de traitement par de l'azote lors de la phase de diffusion, telle que mise au point par le demandeur, permet d'obtenir une diminution très rapide du potentiel carbone.Figure 1, attached, represents the theoretical evolution of the carbon potential in a constant temperature oven (without taking into account the influence of the tightness of the oven and the nature of its internal wall), under an initial atmosphere based on of C0, H ₂ and N ₂ , when high nitrogen flow rates are injected into this furnace. In this figure, two curves (I) and (II) are shown, giving the carbon potential as a function of time for respectively, a ratio

= 5 and

= 10, D being the nitrogen flow injected into the tower in m ³ / h and V the volume of the furnace in m ³ . These curves show that effectively a dilution of the treatment atmosphere with nitrogen during the diffusion phase, as developed by the applicant, makes it possible to obtain a very rapid reduction in the carbon potential.

Thus, thanks to the process according to the invention, an atmosphere with high carbon potential can be used during the first phase, even using an atmosphere rich in fuel species, and sufficiently reduce the carbon potential during the second phase.

According to a preferred embodiment of the invention, the treatment atmosphere is formed by introducing into the oven a mixture of nitrogen and methanol (the methanol being sprayed by the stream of nitrogen gas) in such proportions that the desired percentages of CO and H ₂ are obtained. According to the invention, the treatment atmosphere can also be formed by introducing an endothermic gas into the oven.

According to an alternative embodiment, a gaseous hydrocarbon such as methane, propane or butane is also introduced in a small percentage (from 0.5% to 5%) relative to the whole of the mixture introduced.

• - soit on mesure au fur et à mesure du traitement, c'est-à-dire de l'injection dans le four du mélange azote-méthanol et de l'hydrocarbure, les concentrations en CO et C0₂ ou CO et 0₂, ou CO et H 2 0, ou C02 et 02 , de l'atmosphère formée, ainsi qu'éventuellement la température, ce qui permet d'en déduire le potentiel carbone, et on fait varier le débit d'azote injecté dans le four de façon à obtenir les valeurs de potentiel carbone désirées pour chacune des deux phases.
• - soit on détermine tout d'abord lors d'essais préliminaires, compte tenu de l'acier à traiter, des dimensions du four, etc..., les valeurs de la concentration en azote que doit avoir l'atmosphère pour obtenir les valeurs de potentiel carbone désirées pour chacune des phases ; puis on effectue le traitement proprement dit en injectant le mélange d'azote et de méthanol au cours de chacune des deux phases en proportions telles qu'on obtienne les concentrations en azote ainsi fixées et on ajuste le potentiel carbone en réglant le débit de l'hydrocarbure injecté dans le four.

The implementation of the process can be carried out in two different ways:

• - either during the treatment, that is to say the injection into the oven of the nitrogen-methanol mixture and of the hydrocarbon, the concentrations of CO and C0 ₂ or CO and 0 ₂ are measured, or CO and H 2 0, or C02 and 02, of the atmosphere formed, as well as possibly the temperature, which makes it possible to deduce the carbon potential therefrom, and the flow rate of nitrogen injected into the furnace is varied. so as to obtain the desired carbon potential values for each of the two phases.
• - Either first of all during preliminary tests, taking into account the steel to be treated, the dimensions of the furnace, etc., the values of the nitrogen concentration that the atmosphere must have to obtain the values carbon potential desired for each of the phases; then the actual treatment is carried out by injecting the mixture of nitrogen and methanol during each of the two phases in proportions such that the nitrogen concentrations thus fixed are obtained and the carbon potential is adjusted by regulating the flow rate of the hydrocarbon injected into the furnace.

According to a characteristic of the invention, it is possible to inject, in addition into the furnace, gaseous ammonia in a proportion of 0.1% to 10% by volume relative to the whole of the gaseous mixture introduced; there is then carbonitriding. This variant embodiment provides additional surface hardening of the treated parts. The quantity of ammonia introduced into the furnace is chosen according to the steel treated and the degree of nitriding desired.

Two examples of implementation of the process of the invention are given below, without implied limitation, which will better show the characteristics and advantages of the invention.

EXAMPLE 1.

A treatment is carried out in accordance with the invention on steel parts of grade 18CD4 in an oven of the "batch" type shown diagrammatically in FIG. 2 attached.

This oven 1 consists of a metal enclosure coated internally with a refractory lining. It has a treatment zone 2 provided with a loading door 3 for the parts to be treated and an airlock 4 provided with a tank 5 for quenching in oil and an outlet door 6 for the treated parts. The treatment zone 2 and the airlock 4 are separated by an interior door 7. The parts to be treated are placed in a basket 8 resting on the bottom of the treatment zone 2. A turbine 9, the function of which is to continuously brew the atmosphere of the oven is placed at a distance above the basket 8. Reservoirs of nitrogen, methanol and methane, symbolized respectively in 10, 11 and 12, are connected by means of conduits 13, 14 and 15, fitted with valves 16, 17 and 18, to a pipe 19 which opens into the upper part of the treatment zone 2. The gaseous treatment mixture is discharged by burning a flare 20.

The oven is heated to a temperature of 920 ° C. and then a nitrogen-methanol mixture is introduced into it in proportions such that the atmosphere formed in the oven contains in main species approximately 10% N ₂ , 30% CO and 60% H ₂ . After a certain time, the parts to be treated are placed in the treatment zone 2, we wait until the temperature rises to 920 ° C and the carbon potential of the atmosphere reaches 1.3% and we maintain the parts in this atmosphere for 2 hours 25 min. During this phase, the rate of C0 ₂ in the atmosphere is 0.20%.

The second phase is then carried out at 860 ° C., increasing the quantity of nitrogen injected into the treatment zone 2 so that the atmosphere formed in the enclosure contains in cash main about 70% N ₂ , 10% CO and 20% H ₂ and that the carbon potential is 0.7%, and the parts are kept in this atmosphere for 45 min. During this phase, the CO _{2 level} in the atmosphere is 0.095%.

During the two phases, a small percentage of methane is injected into zone 2 of the furnace (0.5 to 5% relative to the total amount of gaseous mixture introduced) and the injection rate of said methane is adjusted to adjust the carbon potential at the expected value.

The significant variation in carbon potential between the two phases could be obtained without difficulty thanks to the dilution effect with nitrogen, the variations in the CO ₂ rate necessary to obtain the desired carbon potentials logically moving in the direction of dilution.

• Dureté Vickers en surface : 890 HV
• Profondeur pour laquelle on a une dureté Vickers de 550 HV: 0,86 mm.

After quenching of the parts thus treated in the oil tank 5, the hardness measurements of the carbureted layer are carried out. The results obtained are as follows:

• Vickers surface hardness: 890 HV
• Depth for which we have a Vickers hardness of 550 HV: 0.86 mm.

COMPARATIVE TREATMENT IN EXAMPLE 1.

For comparison, a treatment is carried out using an atmosphere rich in fuel species having exactly the same composition as that of the first phase of the treatment according to the invention described in Example 1 above, but without modify the nitrogen injection during the second phase, that is to say at constant atmosphere; this comparative treatment is carried out in the same furnace and on 18CD4 steel parts identical to those of Example 1. The first phase is therefore carried out at a temperature of 920 ° C. with an atmosphere whose concentration in main species is 10% N ₂ 30% CO and 60% H ₂ , and with injection of a small quantity of methane and adjustment of the injection rate of said methane to adjust the value of the carbon potential to 1%; the CO _{2 level} in the atmosphere is 0.28%; the parts are kept in this atmosphere for three hours. The second phase is then carried out at 86 ° C., with the same atmosphere as above, for 1 hour, by adjusting the carbon potential to 0.8%; the rate of C0 ₂ in the atmosphere is 0.72%.

According to this treatment, an atmosphere with carbon potential of more than 1% cannot be used during the first phase; in fact, since the two phases are carried out at constant atmosphere, it would not be possible to obtain an increase in the rate of C0 ₂ sufficient to cause the reduction of the carbon potential, during the second phase, necessary to obtain good metallurgical properties at the surface.

• Dureté Vickers en surface : 887 HV
• Profondeur à laquelle on a une dureté Vickers de 550 HV : 0,85 mm.

After soaking the parts thus treated in the oil bath, the hardness of the carbureted layer is measured. The results obtained are as follows:

• Vickers surface hardness: 887 HV
• Depth at which we have a Vickers hardness of 550 HV: 0.85 mm.

It can therefore be seen that, by virtue of the process of the invention, the fact of diluting the atmosphere, during the diffusion phase, with nitrogen makes it possible to obtain results, as regards the properties of the carburetted layer. , analogous to those obtained with an atmosphere of constant composition, rich in fuel species, but that, on the other hand, a gain of 20% is obtained over the overall duration of the treatment.

EXAMPLE 2.

A treatment is carried out in accordance with the invention on steel parts of grade 18CD2, in a continuous pushing oven, shown diagrammatically in FIG. 3 attached.

This oven 21 comprises an entry airlock 22 provided with a loading door 23 for the parts to be treated, a carburizing zone 24, a diffusion zone 25, and an exit airlock 26 provided with an exit door 27 of the parts processed. The entry airlock 22, the cementation zone 24, the diffusion zone 25 and the exit airlock 26 are separated from each other by internal doors 28. The parts to be treated are arranged in baskets 29 which move on the bottom of the furnace 21. The carburizing zone 24 has two parts A and B: in part A, the parts to be treated are brought to temperature, and in part B the carburizing itself. Turbines 30, the function of which is to continuously stir the atmosphere of the furnace, are placed at a distance above the baskets 29, in part B of the cementation zone 24 and in the diffusion zone 25. Reservoirs 'nitrogen, methanol and methane, symbolized respectively at 31, 32 and 33, are connected via conduits 34, 35 and 36, provided with valves 37, 38 and 39, to a conduit 40 which opens into part B of the cementation zone 24. A pipe 41, fitted with a valve 42 and connected to a nitrogen tank symbolized at 43, opens into the diffusion zone 25. The treatment gas mixture is evacuated by burning by means of a flare 44. The treated parts are then cooled in an oil bath (not shown in the figure).

The carburizing zone 24 is brought to a temperature of 900 ° C. A nitrogen-methanol mixture is injected into this zone in proportions such that the atmosphere formed in said zone contains approximately 10% N ₂ , 30% CO and 60% H ₂ in main species, as well as a small amount of methane (0 , 5% to 5% relative to the total amount of the gaseous mixture introduced) so as to obtain a CO _{2 level} of 0.27%, which corresponds to a carbon potential of 1.2%. Part B of the carburizing zone 24 can contain five baskets.

Diffusion zone 25 is at 860 ° C. In this zone only nitrogen is injected (fuel species such as CO and H ₂ coming directly from the cementation zone), in an amount such as the atmosphere in this zone diffusion contains about 10% CO, and 20% H ₂ . The CO ₂ rate in the atmosphere is 0.115%, which corresponds to a carbon potential of 0.6%. The diffusion zone 25 can contain two baskets.

The baskets containing the parts to be treated are introduced into the oven every 11 minutes 15 seconds; the pieces therefore remain approximately 56 minutes 15 seconds in the cementation zone and 22 minutes 30 seconds in the diffusion zone.

• Dureté Vickers en surface : 925 HV
• Profondeur à laquelle on a une dureté Vickers de 550 HV : 0,45 mm.

After quenching the parts thus treated in an oil bath, the hardness measurements of the carbureted layer are carried out, which give the following results:

• Vickers surface hardness: 925 HV
• Depth at which we have a Vickers hardness of 550 HV: 0.45 mm.

COMPARATIVE TREATMENT IN EXAMPLE 2.

For comparison, a treatment is carried out using an atmosphere rich in fuel species, similarly composition of N ₂ , CO and H (10% N ₂ , 30% CO and 60% H ₂ ) than the atmosphere used in the carburizing zone during the treatment according to the invention in Example 2 above; but, in this comparative treatment, the atmosphere is the same as in the carburizing zone and in the diffusion zone.

The treated parts and the temperatures of the cementation and diffusion zones are the same as those of Example 2. On the other hand, the C0 ₂ rate of the atmosphere is 0.37% in the cementation zone, which corresponds to a carbon potential of 0.9%; and the CO _{2 content} of the atmosphere in the diffusion zone is 0.85%, which corresponds to a carbon potential of 0.7%. It is not possible to envisage a greater variation in the CO ₂ rate between the two zones, therefore a higher carbon potential in the first zone, because there is no modification of the global atmosphere.

With these processing conditions, it is necessary to lengthen the cycle times. The baskets containing the parts to be treated are introduced into the oven every 15 minutes; the pieces therefore remain for 1 hour 15 minutes in the cementation zone and 30 minutes in the diffusion zone.

• Dureté en suriace : 923 hV
• Profondeur à laquelle on a une dureté Vickers de 550 HV : 0,45 mm.

After soaking the parts thus treated in an oil bath, the hardness measurements of the carburetted layer are carried out, measurements which give the results below:

• Surface hardness: 923 hV
• Depth at which we have a Vickers hardness of 550 HV: 0.45 mm.

Thus, it can be seen that, thanks to the process according to the invention, results are obtained, as regards the metallurgical properties of the treated parts, similar to those obtained with an atmospheric treatment, rich in fuel species, constant in the two zones. of the oven, but that, on the other hand, we obtain a time saving of 25%.

Claims (8)

1. - A method of heat treatment of metal parts, in particular of steel parts, by carburetion according to which said parts are placed in an oven and they are kept in a carbon enrichment atmosphere comprising in particular carbon monoxide, hydrogen, 'and nitrogen, said treatment comprising a first phase carried out at a temperature of 850 ° C to 1050 ° C, followed by a second phase carried out at a temperature of 700 ° C to 950 ° C, characterized in that, during the first phase, an atmosphere is used containing approximately 20% to 50% by volume of CO and approximately 40% to 75% by volume of H ₂ and having a carbon potential of approximately 1.1% to 1.6% by weight, and during the second phase, in order to have a significantly lower carbon potential than that of the first phase, the nitrogen content of said atmosphere is increased by 2 to 30 times so that that the carbon potential difference between each of said phases is at least about 0.5% by weight. 2. - Method according to claim 1, characterized in that the second phase is carried out at a temperature of 800 ° C to 950 ° C. 3. - Method according to one of claims 1 or 2, characterized in that, during the first phase, the concentration of nitrogen in said atmosphere is about at most 40% by volume, and during the second phase, the concentration of nitrogen in said atmosphere is about 30% to 80% by volume. 4. - Method according to one of claims 1 to 3, characterized in that said carbon enrichment atmosphere is formed by introducing into said oven a mixture of nitrogen and methanol. 5. - Method according to claim 4, characterized in that one additionally introduces a gaseous hydrocarbon such as methane, propane, or butane in a proportion of 0.5% to 5% by volume relative to the all of the mixture injected into the oven. 6. - Method according to one of claims 1 to 5, characterized in that one measures, as the treatment, the concentrations of CO and C0 ₂ , or CO and 0 ₂ , or CO and H ₂ 0, or C0 ₂ and 0 ₂ , of the atmosphere formed, which makes it possible to deduce the carbon potential therefrom, and the nitrogen flow injected into the furnace is varied so as to obtain the desired carbon potential values for each of the two phases. 7. - Method according to one of claims 1 to 5, characterized in that one determines first of all, during preliminary tests, the values of the nitrogen concentration that the atmosphere must have to obtain the values of carbon potential desired for each of the phases, then the actual treatment is carried out by injecting the mixture of nitrogen and methanol during each of the two phases in proportions such that the nitrogen concentrations thus fixed are obtained and the carbon potential by adjusting the flow rate of the hydrocarbon injected into the furnace. 8. - Method according to one of claims 1 to 7, characterized in that, in the case of carbonitriding, is also introduced into the furnace of gaseous ammonia in a proportion of about 0.1% to 10% by volume relative to the totality of the mixture injected into the oven.

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