EP0532386A1 - Process and apparatus for carburizing steel in an atmosphere of low pressure - Google Patents

Abstract

The carburising process according to the invention consists in heating the articles to be treated to a temperature of between 800 DEG C and 1100 DEG C, preferably above 900 DEG C, in an oxygen-free gaseous atmosphere maintained by pumping at a pressure of between 1 and 5 mbar and in performing a plurality of successive carbon-enrichment stages (C1, C2), each obtained by an injection for a limited period of a treatment gas containing one or a number of pure hydrocarbons, each enrichment stage (C1, C2) having a duration shorter than the time for the change of austenite into a saturation stage, these enrichment stages being separated by vacuum diffusion stages (D1, D2) of longer duration making it possible to adjust the surface carbon content. The invention makes it possible to simplify the mechanism of carbon transfer and to avoid the detrimental effects of overcarburising.

Description

In general, it is known that one of the main surface hardening treatments for steels which is the most used currently in the field of general mechanics is, without question, case hardening.

This treatment makes it possible to increase the surface hardness, the mechanical characteristics, the surface or rolling fatigue limit, and the resistance to wear of highly stressed mechanical components such as gears, transmission axes, cams, etc. ...

At present, this treatment is carried out in a charge oven (or not) inside which the parts are brought to a treatment temperature in an atmosphere based on nitrogen and methanol.

This technique, which is widely used, however has a number of drawbacks due, in large part, to the oxygen present in the cementing atmosphere.

Indeed, this oxygen reacts on the surface of the steel by phenomenon of intergranular oxidation. The latter weakens the surface structure by locally reducing the hardness and above all the fatigue limit.

In addition, the oxygen present has the effect of limiting the transfer of carbon at the interface between the cementing gas phase and the solid to be cemented. This phenomenon therefore limits the rate of carburizing.

To remedy these drawbacks, carburizing equipment is frequently provided with an oxygen sensor and an infrared analyzer, so as to control the carbon potential in the atmosphere.

However, although controlling the carburizing phase, these systems can only limit, but without eliminating it, the harmful effect of the presence of oxygen in the treatment atmosphere. Furthermore, from the mechanical point of view, the surface layer exhibiting the phenomenon of intergranular oxidation must generally be removed by costly rectification operations.

To these drawbacks are added those resulting from the particularities of the carbon enrichment law of a steel. According to this law, the carbon enrichment which is linear as a function of time, below the saturation threshold of austenite, becomes a function of the square root of time beyond this threshold (diffusion regime - Fick's law) .

During this diffusion regime, a very hard and very fragile iron carbide called cementite (Fe3C) is produced, as well as soot.

• une hétérogénéité de traitement,
• un contrôle impossible de l'enrichissement,
• un taux de "cracking" très faible du gaz de traitement (en général du CH4) et un mauvais rendement en carbone bas.

In addition, given the fact that the treatments are carried out at atmospheric pressure, and that the content of treatment gas is difficult to control with precision, it is found:

• heterogeneity of treatment,
• impossible control of enrichment,
• a very low "cracking" rate of the treatment gas (generally CH4) and a poor low carbon yield.

In an attempt to avoid the harmful effects of overfilling during the diffusion regime, it has been proposed to reduce, during this phase, the treatment gas content by dilution, by injecting a neutral gas into the furnace. However, this solution does not allow total elimination of the treatment gas and requires a relatively long time to sufficiently reduce the rate of treatment gas.

The invention therefore more particularly aims to eliminate all these drawbacks.

To this end, it proposes a case hardening process consisting in bringing the parts to be treated to a relatively high treatment temperature (between 800 ° C and 1100 ° C, preferably higher than 900 ° C) in a gaseous atmosphere free of oxygen maintained by pumping at low pressure (from 1 to 10 mbar, preferably 5 mbar), and carrying out a plurality of successive carbon enrichment phases, each obtained by an injection, of limited duration, of a gas treatment comprising one or more pure hydrocarbons (for example of the CH4, C3H8 type, etc.), each enrichment phase having a duration less than the time required to pass into the saturation phase of the austenite, these enrichment phases being separated by longer diffusion phases in vacuum allowing to adjust the surface carbon content.

The principle of this cementation process is based on the mechanisms of dissociation of hydrocarbons at the pressure and at the treatment temperature which lead to obtaining simpler hydrocarbons and ultimately carbon and hydrogen, the main reactions being:

In fact, the main cementing agent is ethylene from "cracking", propane or ethane, methane being the least effective compound.

The previously described process (reduced pressure / high temperature) overcomes most of the drawbacks of prior techniques, while allowing an increase in carburizing kinetics.

Thanks to the fact that the generation of active carbon is obtained by simple dissociation of hydrocarbon molecules, the mechanism of carbon transfer is considerably simplified.

This dissociation phenomenon is improved, at low pressure, due to the law of displacement of equilibria (Le Chatelier law): a drop in pressure at constant temperature produces the reaction which leads to an increase in the volume of the system and vice versa. We see that this process tends to favor reactions of the type:

Another important advantage of the process according to the invention consists in that it makes it possible to control the residence time of the gas in the furnace: As soon as the injection stops (which caused a rise in pressure), the gas is quickly found pumped until the pressure returns to its initial value at which the treatment gas content is negligible. It then becomes possible to avoid passing below the saturation threshold for austenite and the resulting drawbacks.

In addition, process gas consumption is reduced, while system security is improved.

As previously mentioned, the invention also relates to a device for implementing the method described above, this device involving a heat or thermochemical vacuum treatment oven, equipped with heating means capable of bringing the parts to be treated to a temperature of the order of 800 ° C to 1100 ° C, pumping means making it possible to reduce the pressure inside the oven to a value of the order of 1 to 10 mbar, and means making it possible to inject periodically treatment gases inside the oven, for a determined period.

It turns out that in conventional installations of this type, one of the major difficulties to be resolved is to obtain homogeneity of the treatment on all the parts of a load and on all the shapes of each of these parts.

To achieve this industrial objective, consideration has been given to rotating the load containing the parts to be treated in front of the hydrocarbon injection nozzles.

However, this solution is difficult to implement and lacks industrial reliability due to the difficulty to rotate a load brought to high temperature.

In order to solve this problem, the invention proposes a gas injection system which makes it possible to rotate the gaseous flow of hydrocarbon in the enclosure of the furnace whatever its geometry, and this, in order to cement any type of parts , homogeneously, without introducing any complex mechanism in the hot part of the oven.

This result is obtained by using a plurality of injection nozzles judiciously placed as a function of the geometry of the furnace, each of these nozzles being associated with a solenoid valve controlled by a computer programmed so as to generate an opening and closing sequence, so as to obtain an atmosphere of pulsed cementing gas, in permanent, circular or helical movement.

• La figure 1 est un diagramme de température et de pression en fonction du temps, d'un cycle de cémentation à basse pression, conforme au procédé selon l'invention ;
• La figure 2 est une représentation schématique d'une installation de traitement utilisant un four cloche ;
• Les figures 3 et 4 sont des vues (respectivement axiale et radiale) du moufle du four utilisé dans l'installation de la figure 2, ces vues montrant l'implantation des buses d'injection de gaz de traitement ;
• La figure 5 est une représentation schématique du circuit d'injection du gaz de traitement ;
• La figure 6 est une représentation illustrant une séquence d'ouverture/fermeture des électrovannes du circuit de la figure 5 ;
• La figure 7 est une vue schématique montrant un mode d'implantation des buses d'injection dans un four présentant un moufle parallélépipédique ;
• La figure 8 montre l'implantation des buses dans la paroi latérale gauche, la paroi supérieure, et la paroi latérale droite du moufle représenté figure 7.

Embodiments of the invention will be described below, by way of nonlimiting examples, with reference to the appended drawings in which:

• FIG. 1 is a diagram of temperature and pressure as a function of time, of a low pressure carburizing cycle, in accordance with the method according to the invention;
• FIG. 2 is a schematic representation of a treatment installation using a bell furnace;
• Figures 3 and 4 are views (respectively axial and radial) of the oven muffle used in the installation of Figure 2, these views showing the location of the treatment gas injection nozzles;
• Figure 5 is a schematic representation of the process gas injection circuit;
• Figure 6 is a representation illustrating an opening / closing sequence of the solenoid valves of the circuit of Figure 5;
• Figure 7 is a schematic view showing a mode of implantation of the injection nozzles in an oven having a rectangular block;
• FIG. 8 shows the location of the nozzles in the left side wall, the upper wall, and the right side wall of the muffle shown in FIG. 7.

• une première phase P₁ de montée en température allant de la température ambiante à une température de 760°C, cette élévation de température s'effectuant à une vitesse de 15°C/mn ;
• un premier palier P₂ (par exemple de 1 heure) à la température de 760°C ;
• une deuxième phase P₃ de montée en température amenant les pièces de la température du palier (760°C) à la température de traitement (ici de 980°C) ;
• une phase P₄ de maintien en température à la température de traitement ; et
• une phase P₅ de refroidissement pouvant consister par exemple en une trempe.

As shown in FIG. 1, the thermal carburizing cycle successively comprises:

• a first phase P₁ of temperature rise going from room temperature to a temperature of 760 ° C, this temperature rise taking place at a speed of 15 ° C / min;
• a first stage P₂ (for example 1 hour) at the temperature of 760 ° C;
• a second temperature rise phase P₃ bringing the parts from the bearing temperature (760 ° C) to the treatment temperature (here 980 ° C);
• a phase P₄ of maintaining the temperature at the treatment temperature; and
• a cooling phase P₅ which may consist, for example, of quenching.

During this thermal cycle, the pressure inside the furnace (indicated in thick lines) is maintained by pumping at a relatively low value until an instant t₁ which follows a phase P₆ of thermal homogenization of the parts at the temperature treatment (for example 30 minutes after the temperature has reached the treatment temperature).

In fact, at time t₁, a first carbon enrichment phase C₁ is initiated, by carrying out a first injection of treatment gas, for a short duration (from 1 s to 5 min). This injection has the effect of slightly increasing the pressure for a period calculated so as to avoid exceeding the saturation threshold of the austenite. Due to the pumping of the injected gas, the pressure then quickly returns to its initial value (instant t₂). The duration of this enrichment phase C₁ is, in general, of the order of a few seconds to a few minutes.

From the instant t₂, a first diffusion phase D am (diffusion of carbon towards the core of the steel) is initiated during which the temperature is maintained at the treatment temperature, and the atmosphere at low pressure does not contains almost more process gas to allow carbon enrichment.

At the end of this diffusion phase D₁, instant t₃, a second carbon enrichment phase C₂ is triggered by carrying out a second injection of treatment gas. The duration of this second enrichment phase may be different from that of the first, provided that the saturation threshold previously mentioned is not exceeded.

The treatment ends with the vacuum quenching using oil or pressurized gas (phase P₅) which occurs (instant t₅) after a diffusion phase D₂ (instant t₄) of a duration substantially equal to that of the diffusion phase D₁.

During the quenching phase P₅, the temperature suddenly changes from the treatment temperature to the ambient temperature corresponding to the quenching operation of the steel or hardening of the case-hardened steel.

Of course, the invention is not limited to the previously described treatment cycle: Thus, for example, the number of enrichment phases and the number of diffusion phases could be greater than two depending on the desired cemented depth.

FIG. 2 shows an installation capable of carrying out a cementation treatment at low pressure using a vacuum heat treatment furnace of the bell type, that is to say comprising a sealed enclosure comprising a cylindrical body 1, oriented vertically and open in its lower part, this body being movable and mounted in a sealed and disconnectable manner on a circular base 2 forming the bottom of the oven on which the parts to be treated are placed.

The body 1 contains a cylindrical muffle 3 made of refractory material, inside of which electrical heating resistors 4 are arranged making it possible to provide heating of the parts by radiation.

The interior volume of the oven is connected to a suction circuit comprising a vacuum pump 5 controlled by a regulation circuit at least partially housed in a control cabinet 6.

This control cabinet 6 also contains the usual electronic instruments such as displays or recorders as well as the device for programming and regulating the heating.

Furthermore, the interior volume of the enclosure is connected to a treatment gas injection system comprising one or more (here, a source of propane and a source of nitrogen) gas sources G₁, G₂ connected to nozzles injection 7 which pass through the body 1 / muffle 3 assembly, by means of a circuit successively comprising a flow meter 8 and solenoid valves (block 9) each associated with one or more injection nozzles 7.

In this example, the oven comprises six groups of three nozzles (B₁ to B₁₈) arranged vertically one above the other, these groups being angularly offset by 60 ° with respect to each other (Figures 3 and 4).

FIG. 6 is an unrolled view of the cylindrical surface of the muffle 3, in which the locations of the injection nozzles B₁ to B a have been indicated, while FIG. 5 shows an embodiment of an injection circuit comprising nine solenoid valves E₁ to E₉, at the rate of one solenoid valve for two injection nozzles each belonging to two different groups.

The opening and closing of these solenoid valves is controlled by a microcomputer 11 suitably programmed so as to obtain, during the enrichment phases, a current of pulsating cementing gas in permanent movement.

By carrying out an opening / closing sequence of the solenoid valves associated with the injection nozzles shown in FIG. 6 in the following order (1, 9, 17 - 2, 10, 18 - 3, 11, 13 - 4, 12, 14 - 15, 7, 5 - 16, 8, 6), a rising and falling propeller is obtained. Advantageously, each solenoid valve can work for 2.77 hundredths of a second during a loop of duration of the order of 0.5 seconds. The cementing gas may, as for it has a speed of 1.48 m / s at the outlet of the injection nozzles.

Furthermore, two additional solenoid valves E₁₀ and E₁₁ are provided at the outlet of the two sources G₁, G₂ so as to send into the injection circuit either the cementing gas (propane) or the neutral gas (nitrogen) used to clean the nozzles after each carburizing phase.

As previously mentioned, FIG. 7 shows a mode of implantation of the injection nozzles in an oven of which the muffle 12 has simply been shown diagrammatically.

In this example, the right and left lateral faces FD, FG of the muffle 12 are crossed by three batteries of five injection nozzles aligned horizontally on three respective levels, each nozzle being indicated by a point.

The upper face FS of the muffle 12 is, for its part, traversed by three batteries of five injection nozzles oriented parallel to the batteries of the side faces.

In FIG. 8, the nozzles on each of these faces are numbered from +1 to +18 in the order of their opening, during an injection cycle, it being understood that the solenoid valve which has the same number on each of the three faces of the oven opens at the same time and the sequence takes place in ascending order of the numbers. The gas injection speed at the outlet of the nozzles can be, here, of the order of 4.71 m / s.

Thanks to this arrangement, a horizontal gas propeller is obtained inside the muffle at each opening / closing cycle of the solenoid valves.

• température sur pièce = 960°C,
• débit de propane = 5 l/mn,
• temps total d'injection de propane = 423 s,
• pression = 3,7 mbar.

For example, in a bell oven of the type described in FIG. 2, the treatment on a round with a diameter of 40 mm, a thickness of 12 mm, a steel of type 16 MC 5, can be carried out in the following conditions:

• room temperature = 960 ° C,
• propane flow = 5 l / min,
• total propane injection time = 423 s,
• pressure = 3.7 mbar.

• profondeur conventionnelle cémentée = 5/10ème de millimètre,
• dureté superficielle (HV 0,1) = 690 à 724 HV,
• % carbone superficiel = 0,75 %,
• flux carbone (mg/h/cm²) = 15,
• grosseur de grain = 7-8.

The following results were obtained:

• conventional cemented depth = 5 / 10th of a millimeter,
• surface hardness (HV 0.1) = 690 to 724 HV,
• % surface carbon = 0.75%,
• carbon flux (mg / h / cm²) = 15,
• grain size = 7-8.

Claims (10)

A method of carburizing steel parts, this method consisting in bringing the parts to be treated to a temperature between 800 ° C and 1100 ° C, preferably higher than 900 ° C, in a gaseous oxygen-free atmosphere maintained by pumping at a low pressure, and to carry out a plurality of successive carbon enrichment phases (C₁, C₂), each obtained by an injection of limited duration, of a treatment gas comprising one or more pure hydrocarbons, each phase of enrichment (C₁, C₂) having a duration less than the time of transition to the saturation phase of the austenitic phase of the steel brought to high temperature, these enrichment phases (C₁, C₂) being separated by diffusion phases under vacuum (D₁, D₂) of longer duration making it possible to adjust the surface carbon content,
characterized in that the above pressure is maintained at a value between 1 and 10 mbar with a gas flow such that the cementing atmosphere is saturated or has a high cementing potential, and in that the injection phases of treatment gas are obtained by means of a plurality of injection nozzles implemented in a sequence making it possible to generate a moving gas flow on the parts to be treated. Method according to claim 1,
characterized in that the treatment temperature is higher than 800 ° C. Method according to one of claims 1 and 2,
characterized in that the pressure is between 1 and 10 mbar and, preferably, equal to 5 mbar. Method according to one of the preceding claims,
characterized in that the gas injected is propane or methane, the "cracking" or dissociation of which produces ethylene and methane as well as the main cementing agent, namely atomic carbon, creating a saturated atmosphere in carbon by means of an adapted initial gas flow. Method according to one of the preceding claims,
characterized in that the injection is carried out so as to rotate the gas flow around the load to be treated. Device for implementing the method according to one of the preceding claims, this device involving a vacuum heat treatment oven of the type comprising, inside a sealed enclosure (1, 2) connected to a pumping (5), a thermally insulating muffle (3) equipped with heating means (4), inside which the loads to be treated are arranged, and a plurality of means (G₁, G₂, 7, 8, 9) allowing injecting a treatment gas inside the furnace, the pumping station (5) being designed so as to be able to obtain inside the enclosure, a pressure of the order of 1 to 10 mbar, and the heating means (4) being designed so as to bring the charge placed inside the oven to a temperature between 800 ° C and 1100 ° C,
characterized in that the injection means (G₁, G₂, 7, 8, 9) are designed to carry out injection phases of limited duration calculated so as not to exceed the saturation threshold of the austenite, each of injection phases comprising a successive implementation of the injection means in an appropriate sequence to cause a displacement of treatment gas flow on the parts to be treated. Device according to claim 6,
characterized in that the duration of each injection phase is between 1 s and 5 min and that the duration of the diffusion phases is always greater than the previous cementation gas injection time. Device according to one of claims 6 and 7,
characterized in that each injection phase is carried out by means of a plurality of injection nozzles (7) the flow of which is controlled by solenoid valves (9), the arrangement of the nozzles inside the oven and the control solenoid valves (9) being designed so as to obtain a rotational movement of the gas flow around the load. Device according to claim 8,
characterized in that the movement of the gas flow is a rising and falling propeller. Device according to claim 8,
characterized in that the movement of the gas flow is a horizontal helix.

Images