EP0013654A1 - Process for the heat treatment of steel and for controlling said treatment; steel obtained thereby - Google Patents

Abstract

The present invention relates to improvements to methods of treatment under a controlled atmosphere. An atmosphere formed of a carrier gas constituted by or containing industrially pure nitrogen and an active gas constituted by a hydrocarbon such as CH4, C4H10, C3H8, C2H4, C2H2, C2H6 and by oxide of carbon introduced in the form of pure gas or obtained by decomposition of methanol, the treatment being carried out between 750 ° and 1150 ° C. The invention applies to heating before quenching, annealing, carburetion and carbonitriding of steels.

Description

The present invention essentially relates to a process for the thermal treatment of steel, such as heating before quenching, annealing, carburizing (carburetion or carbonitriding), carried out in an oven in the presence of a continuously flowing atmosphere obtained by mixing a carrier gas including nitrogen and optionally hydrogen with an active gas consisting of a hydrocarbon with a proportion by volume of hydrocarbon of between 0.2 and 30% of said mixture.

The implementation of this known process can present certain difficulties, in particular in the case of case hardening of parts having deep bores or of parts of complex shapes. It may indeed occur, at the bottom of these bores or other hollow parts difficult to access, slight deposits of soot detrimental to the quality of the finished product.

An object of the present invention is to avoid the above-mentioned difficulties and to allow cementation of parts, even deemed difficult, which is homogeneous and free of soot.

Experiments for this purpose have made it possible to discover that the role of carbon monoxide is essential in the transfer of carbon atoms from the atmosphere to metal. Indeed, carbon monoxide makes it possible to carry out cementation on the surface of the metal part by a double layer effect. The carbon monoxide is absorbed in the form of a metal carbon-metal bond making it possible to cover the entire surface, even in the least accessible parts, of parts reputed to be difficult such as deep bores or hollows of parts of complex shapes. The hydrocarbon present in the mosphèrc can then form a double layer by a bond with the absorbed carbon monoxide radicals. It follows a continuous passage of the carbon atoms thus bridged by the double layer, which allows a homogeneous carburizing on all the parts of the treated parts.

Experience has also shown that it is important to exclude all traces of residual oxygen in the atmosphere, in order to avoid destabilization of the aforementioned double layer. The oxygen molecules cause a dominance effect, which authorizes them to capture hydrocarbons in the gas phase and thereby prevent the supply of hydrocarbon from the double layer formed on the surface of the metal, which results in the parts by areas insufficiently rich in carbon, that is to say heterogeneous cementation.

We also know that ordinary industrial nitrogen contains significant quantities (up to 2%) of oxygen, carbon dioxide and water vapor and experience has shown that it is this oxygen brought directly by ordinary industrial nitrogen, or coming from the decomposition of carbon dioxide or water vapor which it carries, which destabilizes the double layer described above.

The above-mentioned object is achieved by the process according to the present invention essentially by the fact that said atmosphere is produced by mixing with the abovementioned carrier gas, consisting of nitrogen or containing nitrogen having an O ₂ content < 10 ppm, an H ₂ O content <10 ppm and a CO ₂ content <10 ppm, a hydrocarbon chosen from one of the following compounds: CH ₄ , C ₄ H ₁₀ , C ₃ H ₈ , C ₂ H ₄ , C ₂ H ₂ , C ₂ H ₆ and carbon monoxide, the proportion of carbon monoxide in the total mixture being between 0.1 and 30% by volume and the temperature of the steel being between 750 and 1150 ° C.

The use of nitrogen with an O ₂ content <10 ppm, a C0 ₂ - content (10 ppm and an H ₂ 0 content <10 ppm (which corresponds to a dew point of - around 60 ° C) , i.e. nitrogen with industrial purity, ensures that an oxygen-free atmosphere is obtained; the carbon monoxide contained in this atmosphere can fully play its role, i.e. say facilitate the passage into the metal of carbon atoms of hydrocarbons, which results in carburizing, more precisely carburetion, homogeneous and without deposits of soot, whatever the configuration of the parts treated.

The value of the percentage of carbon monoxide in the total mixture, comprised in the range 0.1 to 30%, is chosen as a function of the characteristics of the steel of the part being treated, of the temperature of the treatment, itself a function, at least part of the characteristics of this steel, and of the configuration of the treated part, that is to say of the ratio between the area of its total surface and its volume, the percentage of CO having to be, for the latter parameter, all the more important as said ratio is higher.

According to another characteristic of the invention, the carbon monoxide can be introduced into the above-mentioned mixture in the form of pure gas or else in the form of a partial nitrogen-carbon monoxide mixture in the gaseous or liquid state.

ledit méthanol étant introduit dans le mélange précité.According to yet another characteristic of the invention, the carbon monoxide comes from the hot decomposition of methanol according to the reaction

said methanol being introduced into the above mixture.

The decomposition of methanol according to the above reaction is that which occurs almost exclusively in the temperature range 700-1150 ° 0 which includes the temperature range of the treatment.

In the case of carbonitriding, said atmosphere is produced by mixing with the carrier gas, in addition to the aforementioned hydrocarbon and carbon monoxide, ammonia in a proportion of 0.1 to 30% by volume.

Parts are thus obtained having a homogeneous carbonitriding and without soot deposits whatever the configuration of the parts treated.

The value of the percentage of NH ₃ is chosen as a function of the steel treated and the degree of nitriding desired.

et

ce qui conduit à des contrôles très stricts en H₂0, CO₂ et CH₄ en sortie du four qui caractérisent réellement et très exactement la situation de carburation in situ.It should also be noted that the fact of introducing into the furnace an atmosphere practically free of oxygen, water vapor and carbon dioxide, allows a molecule to molecule reaction between the carbon dioxide or the water formed by the carburizing reaction and the hydrocarbon according to the following reactions (the hydrocarbon being assumed to be CHA):

and

which leads to very strict controls in H ₂ 0, CO ₂ and CH ₄ at the outlet of the furnace which really and very exactly characterize the situation of carburetion in situ.

The invention also relates to a process for controlling the heat treatment described above, that is to say carried out in an oven in the presence of a continuously flowing atmosphere obtained by mixing a carrier gas including nitrogen and optionally hydrogen with an active gas consisting of a hydrocarbon to which 00 is added.

This control process is characterized by the fact that the residual content of hydrocarbon, water vapor and carbon dioxide in the atmosphere leaving the oven is measured, using pre-established charts which determine a relationship between the residual hydrocarbon water vapor and carbon dioxide contents and the carbon levels of the steel, the hydrocarbon content of the atmosphere entering the furnace which is necessary for obtaining the desired carbon level and the flow rate is adjusted of hydrocarbon entering the furnace to obtain the abovementioned contents at the outlet of the furnace and therefore said desired carbon content.

Such a control method therefore makes it possible to control the treatment so as to prevent the formation, in the oven, of carbon dioxide and water vapor, the presence of which was carefully avoided in the atmosphere entering the oven; it therefore makes it possible to obtain a final product having the desired characteristics.

The invention also relates to the steels obtained by the above-mentioned treatment process.

Various tests relating to different types of treatment (carburetion, carbonitriding, heating before quenching) were carried out on steels of different compositions, the atmospheres entering the furnace were generally formed of industrially pure nitrogen, from one to minus the aforementioned hydrocarbons, possibly hydrogen, components to which was added either carbon monoxide or methanol. In the case of carbonitriding, the atmosphere also contained ammonia.

Regarding the processing temperatures, in the range 750-1150 ° C above When mentioned, it has been observed that the introduction of NH ₃ leads to a lowering of the preferential temperatures in the low range (750-1050 ° C) interval.

In the choir of these tests, the treatment being controlled by measuring the residual content of hydrocarbon, CO ₂ and H ₂ O in the atmosphere leaving the oven and by adjusting the flow rate of hydrocarbon introduced into the atmosphere entering the oven for l 'obtaining the desired fuel activity, that is to say by controlling the addition of hydrocarbon at the inlet of the furnace. To allow this control, charts were previously established which gave a relationship between on the one hand the residual hydrocarbon content, the residual H ₂ 0 content, the residual CO _{2 content} at the outlet of the oven and on the other hand the amount of carbon introduced into the steel. Reading these charts allows the adjustment of the carbon potential as indicated above.

Some examples of the tests carried out will be given below, in each of which the treatment carried out, the material of the steel treated and the composition of the atmospheres entering and leaving the furnace will be indicated.

Example 1 - (carburetion)

Parts of 20NCD2 steel (AFNOR standard) were treated for 4 h 30 at 900 ° C. in a batch type oven in which a nitrogen-hydrogen-methane-carbon oxide atmosphere was introduced. The atmospheric compositions at the entrance and at the exit of the oven were as follows:

The hardness of the carbureted layer then quenched in oil was 60 Rockwell in the C scale. The carburetted depth reached 0.7 mm with a carbon on the surface of 0.8% without residual austenite or carbides.

Example 2 - (carburetion)

Parts made of 16 CD4 steel (AFNOR standard) were treated for 2 hours at 900 ° C. in a batch type oven in which a nitrogen-propane-methanol atmosphere was introduced (the latter decomposing into carbon monoxide and hydrogen) . The composition of the entry and exit atmospheres was as follows:

Hardness measurements made on parts quenched in oil after treatment gave values of 87 Rockwells on the A scale. The carburetted depth reached 0.4 mm with a carbon on the surface of 0.8%.

Example 3 - (carbonitriding)

27CD4 steel parts (AFNOR standard) were treated for 4 hours at 870 ° C in an oven batch into which was introduced a nitrogen-hydrogen-methane-carbon monoxide-ammonia atmosphere.

The compositions of the furnace inlet and outlet atmospheres were as follows:

Hardness measurements made on parts quenched in oil gave values of 857 Vickers in HV50g. The carbonitrided depth reached 0.4 mm at 650 HVl kg.

Example 4 - (carbonitriding)

Parts of 38C2 steel (AFNOR standard) were treated for 3 hours and 15 minutes at 890 ° C. in a batch oven into which a nitrogen-methane-methanol-ammonia atmosphere was introduced.

The compositions of the furnace inlet and outlet atmospheres were as follows:

Hardness measurements made on parts quenched in oil after treatment gave values of 62 Rockwells on the C scale. The carbonitrided depth reached 0.48 mm at 650 IIV 1 kg. The total cemented depth was 0.70 mm.

Example 5 - (heating before quenching)

Parts of 3OCD4 steel (AFNOR standard) were treated for 2 hours at 850 ° C. in a batch quenching oven, in a nitrogen-methanol atmosphere.

The compositions of the furnace inlet and outlet atmospheres were as follows:

The hardened parts had neither carburetion nor decarburization. The carbon potential of the atmosphere measured by a foil was 0.30% C. Example 6 - (comparative carburetion)

Tests have also been carried out on 20NCD2 steel parts; for 5 hours 30 minutes at 900 ° C. with two atmospheres, one of the type described in example 1, that is to say containing 15% of CO, the other identical but without CO. These parts had grooves or bores having a width of approximately 3 mm, and a depth also of approximately 3 mm. The results obtained are presented in the following table:

The carburetted depth on the side of the part is increased as well as that at the bottom of the groove. In addition, the homogeneity of treatment is better with a much more satisfactory relative difference.

The invention is in no way limited to the examples described above which have been given only by way of illustration but not limitation.

Claims (7)

1. - A method of heat treatment of steel such as heating before quenching, annealing, case hardening, carried out in an oven in the presence of a continuously flowing atmosphere obtained by mixing a carrier gas including nitrogen and optionally hydrogen with an active gas consisting of a hydrocarbon with a proportion by volume of hydrocarbon of between 0.2 and 30% of said mixture, characterized in that said atmosphere is produced by mixing with the abovementioned carrier gas, consisting of nitrogen or containing nitrogen having an O ₂ content <10 ppm, an H ₂ O content <10 ppm and a CO ₂ content <10 ppm, a hydrocarbon chosen from one of the following compounds: CH ₄ , C ₄ H ₁₀ , C ₃ H ₈ , C ₂ H ₄ , ^C ₂ ^H _{2 '} c ₂ H ₆ and carbon monoxide, the proportion of carbon monoxide in the total mixture being between 0 , 1 and 30% by volume and in that the steel is brought to a temperature between 750 and 1150 ° C. 2. - Method according to claim 1, characterized in that the carbon monoxide is introduced into the above mixture in the form of pure gas. 3. - Method according to claim 1, characterized in that the carbon monoxide is introduced into the above-mentioned mixture in the form of a partial nitrogen-carbon monoxide mixture in the gaseous or liquid state. 4. - Method according to claim 1, characterized in that the aforementioned carbon monoxide comes from the hot decomposition of methanol according to the reac-

said methanol being introduced into the above mixture. 5. - Method according to one of claims 1 to 4, characterized in that, in the case of carbonitriding, said atmosphere is produced by mixing in addition to the carrier gas, the aforementioned hydrocarbon and carbon monoxide, ammonia in a proportion of 0.1 to 30% by volume. 6. - Ther treatment control process. steel mique made, according to one of claims 1 to 5, in an oven in the presence of a continuously flowing atmosphere obtained by mixing a carrier gas including nitrogen and optionally hydrogen with a active gas consisting of a hydrocarbon and carbon monoxide, characterized in that the residual content of hydrocarbon, steam and water and carbon dioxide in the atmosphere leaving the oven is measured, using pre-established charts which give a relationship between the residual hydrocarbon water vapor and carbon dioxide contents and the carbon levels of the steel, the hydrocarbon content of the atmosphere entering the furnace which is necessary for obtaining the rate of desired carbon and the flow rate of hydrocarbon entering the furnace is adjusted to obtain the abovementioned contents at the outlet of the furnace and therefore said desired carbon content. 7. - Steel obtained by the process according to one of claims 1 to 5.