EP1280943B1 - Low-pressure carburising method - Google Patents

Abstract

The invention relates to a low-pressure carburising method comprising alternating low-pressure enrichment steps and diffusion steps in the presence of a neutral gas. During the enrichment steps, an enriching gas and neutral gas mixture is used, the proportion of the neutral gas being between 5 and 50% by volume of the enriching gas. The enriching gas can be, for example, acetylene (C<SUB>2</SUB>H<SUB>2</SUB>).

Description

The present invention relates to the treatment of metal parts and more particularly cementation, that is to say the introduction of carbon to a certain depth of the parts to improve their mechanical characteristics.

A particular process of low pressure cementation has already been described in the patent French No 2678287 the plaintiff (inventor: Jean Naudot). This patent provides for alternating enrichment steps and diffusion steps. It is specified that the carburizing gas can be any hydrocarbon likely to dissociate at the working temperatures to temper the parts to be treated. However, this method more particularly proposes the use of propane as a carburizing gas and nitrogen as the neutral gas between the carburizing phases.

In addition, an article Jelle H. Kaspersma and Robert H. Shay published in Metallurgical Transactions, Volume 13B, June 1982 , studies cementation rates related to the use of various enrichment gases and soot formation problems. It states that acetylene is the gas that allows the fastest cementation but that presents the disadvantage of generating the most soot in the treatment chamber.

Various attempts have been made to permit the use of acetylene while solving the problem of soot and tar generation.

The patent Russian version 6678978 filed on 2 June 1977 proposes to inject acetylene into the cement chamber at a temperature of 850 to 1000 ° C, by varying the pressure from 0.01 to 0.95 atmosphere (1 to 95 kPa) at a rate of change of pressure of 0.001 to 1 atmosphere per hour. It is explained that the amount of soot is reduced especially when the rate of increase of pressure is very low. However, this process is complex. To the knowledge of the applicant, the process described in this Russian patent has not been the subject of industrial exploitation and the results of the proposed solution could not be verified.

Another solution is proposed in the US patent n ° 5702540 (Kubota) in which it is suggested to use acetylene at a pressure below 1 kPa. Noticeable traces of soot appear from about 0.7 kPa and a significant amount of soot appears above 1 kPa. In addition, the description of this patent application indicates that the carburizing characteristics deteriorate between the outside and the inside of a room as soon as the pressure increases above 0.3 kPa. Experiments made by the applicant have confirmed the appearance of soot as soon as the pressure exceeds a value of the order of 0.5 kPa but, on the other hand, have indicated that, to obtain a satisfactory carburizing inside cavities, or when the charge of the carburizing reactor is very important, the pressure should be increased. It does not therefore seem that the solution proposed in the patent referenced above allows a satisfactory use of acetylene.

The present invention provides a novel process for the efficient use of acetylene and more generally any carburizing gas that may produce soot and tar.

To achieve this object, the present invention provides a low-pressure cementation method of using alternating low pressure enrichment steps and diffusion stages in the presence of a neutral gas in which, during the steps of enrichment, using a mixture of enrichment gas and carrier gas, the carrier gas being in a proportion of 5 to 50% by volume of the enrichment gas.

According to one embodiment of the present invention, the enriching gas is acetylene (C ₂ H ₂ ).

According to one embodiment of the present invention, the carrier gas is nitrogen.

According to one embodiment of the present invention, the carrier gas is hydrogen.

According to one embodiment of the present invention, the carrier gas comprises nitrogen and hydrogen in a proportion of 5 to 60%.

According to one embodiment of the present invention, the pressure in the carburizing chamber is greater than 1 kPa.

According to one embodiment of the present invention, the pressure in the cement chamber is between 1 and 2 kPa.

According to one embodiment of the present invention, the diffusion and enrichment steps are carried out substantially at the same pressure.

According to one embodiment of the present invention, the treatment temperature is of the order of 850 to 1200 ° C.

According to one embodiment of the present invention, each of the enrichment steps is divided into substeps of less than one minute durations separated by diffusion substeps of less than half a minute duration, preferably the order of ten seconds.

• la figure 1 représente une éprouvette en acier à laquelle on applique un procédé de cémentation ;
• la figure 2 est une courbe de pression en fonction du temps illustrant des phases successives d'un procédé de cémentation-diffusion ;
• les figures 3 à 6 illustrent des résultats expérimentaux d'expériences de cémentation :
  • en figure 3, le gaz de cémentation est C₂H₂ et la pression de 0,3 kPa,
  • en figure 4, le gaz de cémentation est C₂H₂ et la pression de 0,7 kPa,
  • en figure 5, le gaz de cémentation est C₂H₂ et la pression de 1,2 kPa, et
  • en figure 6, selon l'invention, le gaz injecté pendant les phases de cémentation est un mélange de C₂H₂ et d'azote et la pression est de 1,5 kPa ; et
• La figure 7 illustre des résultats expérimentaux caractérisant la formation de goudrons lors de cycles successifs de cémentation.

These and other objects, features, and advantages of the present invention will be set forth in detail in the following description of particular embodiments given as a non-limiting example in connection with the accompanying drawings in which:

• FIG. 1 represents a steel test piece to which a cementation process is applied;
• Figure 2 is a pressure versus time curve illustrating successive phases of a cementation-diffusion process;
• Figures 3 to 6 illustrate experimental results of cementation experiments:
  • in FIG. 3, the carburising gas is C ₂ H ₂ and the pressure of 0.3 kPa,
  • in FIG. 4, the carburizing gas is C ₂ H ₂ and the pressure 0.7 kPa,
  • in FIG. 5, the carburising gas is C ₂ H ₂ and the pressure 1.2 kPa, and
  • in FIG. 6, according to the invention, the gas injected during the carburizing phases is a mixture of C ₂ H ₂ and nitrogen and the pressure is 1.5 kPa; and
• Figure 7 illustrates experimental results characterizing the formation of tars during successive cementation cycles.

The Applicant has carried out various cementation experiments on a specimen of the type shown in FIG. 1, consisting of a steel cylinder provided with a non-through bore, and measurements have been made as to the degree of carburizing of _ext at the outside of the specimen and at the carburization depth of _int within the bore formed in the specimen.

FIG. 2 represents a carburizing-diffusion cycle of the type described in the patent French 2678287 and used according to the present invention. The carburizing-diffusion operations are carried out at constant temperature and at constant pressure after an initial phase of setting temperature and pressure. E enrichment phases E are successively carried out during the course of time, during which a carburizing gas is injected into a carburizing enclosure containing charges, of which at least one specimen of the type shown in FIG. 1, and diffusion phases. in which a neutral gas is inserted into the enclosure. In order to vary the carburizing depth, the durations and the number of the respective stages of enrichment and diffusion are modified. Typically, the temperature is between 850 and 1200 ° C., the duration of each of the enrichment and / or diffusion phases being of the order of a few minutes.

First of all, the applicant carried out series of experiments on a test-tube of the type of that of FIG. 1 with acetylene gas (C ₂ H ₂ ) pure carburizing gas. The curves of FIGS. 3, 4 and 5 correspond to three particular pressures maintained during the carburizing-diffusion phases, ie respectively 0.3 kPa for FIG. 3, 0.7 kPa for FIG. 4 and 1, 2 kPa for Figure 5. Each of the curves represents the hardness as a function of the carburizing depth for a point taken outside (Ext) of the specimen and for a point taken inside (Int) of the specimen. The different points of each curve result from the testing of various test pieces having undergone distinct treatment periods.

As shown in Figure 3, for a pressure of the order of 0.3 kPa, there is a large difference between the carburizing depth inside the test piece and outside the test tube, that is to say that the result obtained is not satisfactory since the cementation is insufficient inside the test piece. For example, if one targets a carburizing depth of 1 millimeter, one realizes that, when this depth is obtained outside, the depth of carburizing is only 0.4 mm inside.

A bad result is also obtained in the case of Figure 4 where the pressure is 0.7 kPa. When the outside carburizing depth is 1 mm, the carburizing depth inside is only 0.6 mm.

On the other hand, we begin to obtain satisfactory results as regards carburizing from the moment when the pressure exceeds 1 kPa. For example, FIG. 5 represents results obtained for a pressure of 1.2 kPa: when the depth of carburization outside the test piece reaches 1 mm, the case-hardening depth inside reaches 0.8 mm, which corresponds to the generally accepted standards.

In addition, it is found that, if we distinguish the depth of carburizing inside the test piece upwards of the test piece and towards the bottom of the test piece, it is only from when the pressure exceeds 0.5 kPa that there is a cementation homogeneity inside the test piece.

The generation of soot and tars has been tested and it has been noted that the creation of soot and tar is negligible in the case where the pressure is 0.3 kPa, but becomes significant from 0, 7 kPa.

The present invention provides to use a cycle of the type shown in Figure 2 and to inject either a pure carburizing gas but a mixture of a carburizing gas and a carrier gas. Preferably the proportion of carrier gas will be chosen from about 25 to 50% of the amount of enrichment gas.

FIG. 6 indicates that, for example, a mixture of acetylene (C ₂ H ₂ ) and nitrogen N ₂ with a total pressure of 1.5 kPa and a proportion of approximately 30% of nitrogen, satisfactory cementation substantially identical to that illustrated in Figure 5. However, in this case the problem of the formation of soot and tars is solved.

Figure 7 shows the concentration of benzene (C ₆ H ₆ ) observed at the end of successive enrichment cycles. Indeed, it is known that the formation of tars implies a generation phase of aromatic compounds such as benzene and phenylethylene. Benzene generation is therefore a good indicator of the formation of soot and tar. In FIG. 7, the curves marked C ₂ H ₂ and C ₂ H ₂ + N ₂ respectively correspond to the cases described in relation to FIGS. 5 and 6. It can be seen that, using pure acethylene according to the prior art, the concentration of benzene increases noticeably at the end of each enrichment cycle, which corresponds to a significant formation of tars. On the other hand, in the case of a mixture of acetylene (C ₂ H ₂ ) and nitrogen N ₂ , according to the invention, the concentration of benzene remains substantially constant, at a low value, which corresponds well to negligible tar formation.

More generally, the present invention provides, in all cases where it is carried out a cementation in the presence of an aliphatic hydrocarbon under conditions where there are problems of generation of soot and tars, to add a neutral gas. Preferably the proportion of neutral gas will be chosen from about 5 to 50% of the amount of enrichment gas. These problems of generation of soot and tars rise very strongly in the case of acetylene in which the present invention is particularly useful but also arise in particular in the case of other hydrocarbons, for example propane (C ₃ H ₈ ).

The neutral gas is not necessarily nitrogen but may be any other type of gas not involved in the carburizing reaction, for example argon or a mixture of gases. Nitrogen will preferably be chosen because of its low cost. But, for specific requirements, or if the costs of gas come down, we can choose another neutral gas or carrier gas to solve the problem of soot generation and tars.

The applicant has also shown that it may be advantageous to add hydrogen during the cementation phases. If we add a neutral gas comprising a proportion of 5 to 40% by volume of hydrogen, a perfectly satisfactory characteristic curve is obtained such as that of FIG. 6 (to be compared with that of FIG. 4 in the case where acetylene alone is used).

it is conceivable that the dissolution of acetylene by the carrier gas during the enrichment phases reduces the polymerization reactions of acetylene and its derivatives, resulting in the significant decrease observed in the amount of tars formed in inside the oven and possibly at the pumping unit.

The use of a hydrogenated nitrogen mixture has the additional advantage of promoting decomposition kinetics or thermal cracking of acetylene, resulting in better penetration into cavities and regular carburizing. Indeed, it is then possible, even for a low pressure, to obtain a uniform cementation of the walls of deep cavities. An advantage of this solution is that the amount of carburizing gas and thus the pollution and the gaseous effluents are reduced.

According to another variant of the present invention, the Applicant has shown that the formation of tars can be further reduced by reviewing the relative duration of enrichment (E) and diffusion (D) cycles described in connection with FIG. Conventionally, provision is made, for example, for six enrichment and diffusion cycles having durations of the order of those indicated in the table below (in seconds). E1 D1 E2 D2 E3 D3 E4 D4 E5 D5 E6 D6 520 100 190 150 150 300 100 350 80 450 60 600

The applicant proposes to split each of the enrichment cycles into brief steps followed by short diffusion times. For example, enrichment steps of up to 50 s followed by a step broadcasting time of the order of 10 s. The first enrichment cycle E1 will then comprise 10 or 11 enrichment steps, each of which is followed by a diffusion step of about ten seconds, the final diffusion phase D1 being maintained substantially at its initial duration indicated in the table. above. The second enrichment cycle E2 will comprise 4 enrichment stages, each of which is followed by a diffusion step of about ten seconds, the final diffusion phase D2 being maintained substantially at its initial duration indicated in the table above. . And so on. The concentration of benzene at the end of each enrichment cycle for this pulsed mode of operation is indicated in FIG. 7 by the curve C ₂ H ₂ + N ₂ (pulse). It can be seen that the concentration of benzene is substantially divided by two compared to the case where uninterrupted cycles are conventionally used.

Other modifications of the cycles, for example the choice, for a given pressure of variable flow rates, progressively lower, may provide additional improvements.

Claims (10)

1. A low-pressure cementation method consisting of using an alternation of low-pressure enrichment steps and of steps of diffusion in the presence of a neutral gas wherein, during enrichment steps, a mixture of an enrichment gas and of a carrier gas is used, characterized in that the carrier gas is in a proportion of from 5 to 50% in volume of the enrichment gas.
2. The low-pressure cementation method of claim 1, characterized in that the enrichment gas is acetylene (C₂H₂).
3. The low-pressure cementation method of claim 1, characterized in that the carrier gas is nitrogen.
4. The low-pressure cementation method of claim 1, characterized in that the carrier gas is hydrogen.
5. The low-pressure cementation method of claim 1, characterized in that the carrier gas comprises nitrogen and hydrogen in a proportion of from 5 to 60%.
6. The low-pressure cementation method of claim 1, characterized in that the pressure in the cementation chamber is greater than 1 kPa.
7. The low-pressure cementation method of claim 1, characterized in that the pressure in the cementation chamber ranges between 1 and 2 kPa.
8. The low-pressure cementation method of claim 1, characterized in that the diffusion and enrichment steps are carried out substantially at the same pressure.
9. The low-pressure cementation method of claim 1, characterized in that the processing temperature is on the order of from 850 to 1200°C.
10. The low-pressure cementation method of claim 1, characterized in that each of the enrichment steps is divided into sub-steps of a duration shorter than one minute separated by diffusion sub-steps of a duration shorter than one half-minute, preferably on the order of some ten seconds.

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