FR2681332A1 - METHOD AND DEVICE FOR CUTTING STEEL IN A LOW PRESSURE ATMOSPHERE. - 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

METHOD AND DEVICE FOR CEMENTING A STEEL IN A

LOW PRESSURE ATMOSPHERE.

  In general, it is known that one of the main surface hardening treatments of steels most commonly used in the field of mechanics

  General is, without question, the cementation.

  This treatment makes it possible to increase the surface hardness, the mechanical characteristics, the superficial or rolling fatigue limit, and the resistance to wear of highly stressed mechanical members such as gears, transmission shafts, cams, etc. At present, this treatment is performed in a furnace (or not) within which the parts are brought to a treatment temperature in a

  atmosphere based on nitrogen and methanol.

  This technique, which is widely distributed,

  a number of disadvantages due, in large part

  part, oxygen present in the cementing atmosphere.

  In fact, this oxygen reacts on the surface of the steel by intergranular oxidation phenomenon.

  weakens the surface structure by decreasing local

  the hardness and especially the fatigue limit.

  2 - In addition, the oxygen present has the effect of limiting the transfer of carbon at the interface between the

  cementing gaseous phase and the solid to be cements This phenomenon

  nomene therefore limits the rate of cementation. To overcome these drawbacks, carburizing equipment is frequently provided with an oxygen sensor and an infra-red analyzer, so as to control the

  carbon potential in the atmosphere.

  However, although controlling the cementation phase, these systems can only limit, but without eliminating, the adverse effect of the presence of oxygen in the treatment atmosphere. Moreover, from a mechanical point of view, the superficial layer presenting the

  intergranular oxidation phenomenon, should generally be

  eliminated by costly rectification

tion.

  To these drawbacks are added those resulting from

  the law of carbon enrichment of a steel According to this law, the carbon enrichment which is linear as a function of time, below the saturation threshold of the austenite, becomes a function of the root

  square of time beyond this threshold (regime of

Fick's law).

  During this diffusional regime, a car-

  iron fur, very hard and very fragile called cementite

(Fe 3 C) as well as soot.

  In addition, taking into account the fact that the treatments are carried out at atmospheric pressure, and that the treatment gas content is difficult to accurately control, there is: a heterogeneity of treatment, 3 - an impossible control of the enrichment, a very low cracking rate of the process gas (usually CH 4) and a poor low carbon yield.

  In an attempt to avoid the harmful effects of -surcarbu-

  During this phase, it has been proposed to reduce the content of the dilution treatment gas by injecting a neutral gas into the furnace. However, this solution does not make it possible to obtain a total elimination of process gas and requires a relatively large amount of time to sufficiently reduce the

treatment gas rate.

  The invention therefore more particularly aims at eliminating

primer all these disadvantages.

  It proposes, for this purpose, a carburizing process

  wearing the parts to be treated at a temperature of

  a relatively high treatment temperature (between 800.degree. C. and 1100.degree. C., preferably greater than 9000.degree. C.) in an oxygen-free gaseous atmosphere maintained by low-pressure pumping (from 1 to 10 mbar, preferably mbar), and performing a plurality of successive carbon enrichment phases, each obtained by an injection, of limited duration, of a treatment gas comprising one or more pure hydrocarbons (for example of the CH 4, C 3 H 8 type), each enrichment phase having a duration less than the saturation phase transition time of the austenite, these enrichment phases being separated by vacuum diffusion phases of greater duration allowing

  to adjust the surface carbon content.

  The principle of this cementation process is based on

  mechanisms of dissociation of hydrocarbons at the

  and at the processing temperature which lead to simpler hydrocarbons and finally carbon and hydrogen, the main reactions being propane C 3 H 8 2 C + 2 H 2

C 3 H 8 <C 2 H 4 + CH 4

ethylene C 2 H 4 X 2 C + 2 H 2

C 2 H 4> C + CH 4

  methane CH 4 C + 2 H 2 ethane C 2 H 6 C 2 H 4 + H 2 In fact, the main cementing agent is ethylene resulting from "cracking", propane or ethane, methane

  being the least effective compound.

  The previously described method (reduced pressure / high temperature) makes it possible to overcome most of the drawbacks of the prior art, while allowing an increase in the kinetics of carburizing. Due to the fact that active carbon generation is achieved by simple dissociation of hydrocarbon molecules, the mechanism of carbon transfer is

greatly simplified.

  This phenomenon of dissociation is improved at a low

  sion, because of the law of displacement of balances

  (Le Chatelier law): a drop in pressure at tempera-

  ture, produces the reaction which leads to an increase

  system volume and vice versa It is noted that this process tends to favor reactions of the type:

C 3 H 8 I C 2 H 4 + CH 4

  Another important advantage of the process according to the invention is that it makes it possible to control the residence time of the gas in the oven: As soon as the injection stops (which has caused a rise in pressure), the gas is quickly pumped until the pressure returns to its initial value at which the process gas content is negligible It then becomes possible to avoid the passage below the saturation threshold of the austenite

  and the disadvantages that result.

  In addition, the consumption of process gas is

  reduced, while the security of the system is improved

SOE.

  As previously mentioned, the invention also relates to

  a device for carrying out the pre-

  described above, this device involving a vacuum heat treatment or thermochemical furnace, equipped with heating means able to carry the workpieces to a temperature of about 8000 C to 11000 C, pumping means for lower the pressure inside the oven to a value of about 1 to

  mbar, and means for periodic injection

  treatment gases inside the oven, during

a fixed term.

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

of these pieces.

  To achieve this industrial goal, it was thought to rotate the load containing the parts to be treated

  in front of the hydrocarbon injection nozzles.

  However, this solution is difficult to implement

  and lack of industrial reliability because of the diffe-

  6 - F iculty to rotate a load carried at high temperature.

  In order to solve this problem, the invention proposes a system

  gas injection system that allows to turn the gaseous flow of hydrocarbon in the enclosure of the furnace whatever its geometry, and this, in order to coementer any type of parts, in a homogeneous way, 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 according to 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 a pulsating cemented gas atmosphere, in permanent movement, circular or helical.

  Embodiments of the invention will be described below.

  BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of temperature and pressure as a function of time, of a cycle of

  low pressure cementation, in accordance with

  assigned according to the invention; Figure 2 is a schematic representation of a treatment plant using a bell oven; 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 implantation of the treatment gas injection nozzles; Figure 5 is a schematic representation of the process gas injection circuit; FIG. 6 is a representation illustrating an opening / closing sequence of the electro valves of the circuit of FIG. 5; FIG. 7 is a schematic view showing a mode of implantation of the injection nozzles in

  an oven with a parallelepiped muffle

  dique; Figure 8 shows the nozzle implantation

  in the left side wall, the upper wall

  and the right side wall of the muffle

represented in FIG.

  As shown in FIG. 1, the thermal conditioning cycle comprises, successively, a first phase P 1 of raising the temperature from room temperature to a temperature of 7600 C, this temperature rise being effected at a speed of 150 C / min. ;

  a first level P 2 (for example 1 hour) at the time

  7600 C; a second phase P 3 of temperature rise bringing the parts of the temperature of the bearing (7600 C) to the treatment temperature (here of 980 'C);

  a phase P 4 for maintaining temperature at room temperature

  treatment status; and a cooling phase P 5 which may consist of

example in a quench.

  8 - During this thermal cycle, the pressure inside

  oven (indicated in thick lines) is maintained by

  page to a relatively low value until a time ti following a phase P 6 of thermal homogenization of the parts at the processing temperature (for example 30 minutes after the temperature has reached the treatment temperature). Indeed, at time t1, a first carbon enrichment phase C1 is initiated by performing a first injection of treatment gas for a short period (from 1 s to 5 minutes). This injection has the effect of slightly increase the pressure for a duration calculated so as to avoid exceeding the saturation threshold of

  austenite Due to the pumping of the injected gas, the pressure

  sion then quickly returns to its initial value (instant t 2) The duration of this enrichment phase C1 is, in general, of the order of a few seconds to

a few minutes.

  From time t 2, a first diffusion phase D1 is initiated (diffusion of carbon towards the core of

  steel) during which the temperature is now

  at the processing temperature, and the low-pressure atmosphere contains practically no more

  treatment to allow carbon enrichment.

  At the end of this diffusion phase D1, time t 3, a second carbon enrichment phase is triggered.

  C 2 by performing a second injection of process 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 is terminated by vacuum quenching with oil or supercharged gas (P 5 phase) which intervenes (instant t 5) after a diffusion phase D 2 (time t 4) 9 - of a substantially equal duration to that of the phase of

diffusion Dl.

  During the quenching phase P 5, the temperature rises

  processing temperature at room temperature corresponding to the quenching operation of the steel

  or hardening of case hardened steel.

  Of course, the invention is not limited to the treatment cycle previously described: Thus, for example, the number of enrichment phases and the number of phases

  could be greater than two depending on the

  tion of the desired cemented depth.

  FIG. 2 shows an installation capable of carrying out a low pressure carburizing treatment 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 sealingly mounted and disconnectable on a circular base 2 forming the oven floor on which are placed the

parts to be treated.

  The body 1 contains a cylindrical muffle 3 made of refractory material, inside which are disposed electric heating resistors 4 allowing

  to ensure heating of the rooms by radiation.

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

a control cabinet 6.

  This control cabinet 6 contains, moreover, the usual electronic instruments such as displays

  or recorders and the programming device

  heating regulation.

_ 10 -

  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 Gl, G 2 connected to injection nozzles 7 which pass through the whole body 1 / muffle 3, via a circuit successively comprising a flow meter 8 and solenoid valves (block 9) each associated with one or more nozzles

injection 7.

  In this example, the oven includes six groups of three

  nozzles (B1-B18) arranged vertically one above the other

  others, these groups being angularly shifted

  600 to each other (Figures 3 and 4).

  Figure 6 is an uncoiled view of the cylindrical surface

  the mittle 3, in which the locations

  injection nozzles Bl to B 18, while FIG. 5 shows a mode of execution of a circuit

  injection valve comprising nine solenoid valves E1 to E9,

  the sound of a solenoid valve for two injection nozzles

  each holding two different groups.

  The control * of the opening and closing of these solenoid valves is performed by a microcomputer 11 suitably programmed so as to obtain, during the enrichment phases, a stream of cementing gas

pulsed in permanent motion.

  By performing an opening / closing sequence of

  solenoid valves associated with the injection nozzles

  FIG. 6 in the following order (1, 9, 17 2,, 18 3, 11, 13 4, 12, 14, 15, 7, 5, 16, 8, 6).

  gets a propeller up and down Advantage-

  Each solenoid valve can work for 2,77 hundredths of a second during a loop lasting about 0.5 seconds. The cementing gas can, for its part, have at the outlet of the injection nozzles a speed

1.48 m / s.

  Moreover, two additional solenoid valves El O and Ejl are provided at the output of the two sources Gl, G 2 so as to send into the injection circuit, either the

  cementing gas (propane), ie the neutral gas (nitrogen) used

  to clean the nozzles after each phase of

  tation. As previously mentioned, FIG. 7 shows a mode of implantation of the injection nozzles in an oven of which the muffle 12 has merely been schematically represented.

parallelepipedic shape.

  In this example, the right and left side faces FD, FG of the muffle 12 are traversed 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, in turn, crossed by three batteries of five injection nozzles

  parallel to the batteries of the side faces.

  In FIG. 8, the nozzles of 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 bears the same number on each of the three faces of the oven, opens at the same time and that 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.

  With this arrangement, one obtains inside the muffle a horizontal gas helix at each cycle.

  opening / closing solenoid valves.

  12 - For example, in a bell oven of the type of that

  described in Figure 2, the treatment on a circle of dia-

  meter 40 mm, thickness 12 mm, steel type MC MC 5, can be carried out under the following conditions temperature on part = 960 'C, propane flow rate = 5 1 / min, total propane injection time = 423 s,

pressure = 3.7 mbar.

  The following results were obtained:

  cemented conventional depth = 5 / 10th of a milli-

  meter, surface hardness (HV 0.1) = 690 to 724 HV,% superficial carbon = 0.75%, carbon flux (mg / h / cm 2) = 15,

grain size = 7-8.

13 -

Claims (7)

claims   Process for the cementation of steel parts, characterized in that it consists in bringing the parts to be treated to a temperature of between 800 ° C. and 1100 ° C., preferably greater than 900 ° C., in a gaseous atmosphere free from oxygen maintained by pumping at a pressure of between 1 and 5 mbar, and to carry out a   plurality of carbon enrichment phases successively   sives (Cl, C 2), each obtained by an injection of limited duration, a treatment gas comprising one or more pure hydrocarbons, each enrichment phase (Cl, C 2) having a duration less than the time of passage into saturation phase of the austenite, these enrichment phases (Ci, C 2) being separated by vacuum diffusion phases (D1, D2) of greater duration to adjust the content superficial carbon.   2 Process according to claim 1, characterized in that the treatment temperature is greater than 9000 C.   3 Method according to one of claims 1 and 2   characterized in that the pressure is equal to or close to mbar.   Process according to one of the preceding claims   cedentes, characterized in that the injected gas is propane or methane, the "cracking" of which produces ethylene and   methane which act as the primary cementing agent pal.   Method according to one of the preceding claims   cedeentes, 14 - characterized in that the injection is carried out so as to rotate the gas flow around the load to be treated. 6 Device for implementing the method   according to one of the preceding claims, this device   involving a vacuum heat treatment furnace of the type comprising, inside a sealed enclosure (1, 2) connected to a pumping station (5), a thermally insulating muffle (3) equipped with heating means ( 4), inside which are arranged the   loads to be treated, and means (Gl, G 2, 7, 8, 9)   injecting into the furnace a process gas   characterized in that the pumping station (5) is designed so as to obtain inside the enclosure a pressure of the order of 1 to 10 mbar, in that the heating means (4) ) are designed to carry the charge placed inside the oven at a temperature of between 8000 C and 11000 C, and in that the injection means (Gi, G 2, 7, 8, 9) are designed for   to carry out injection phases of limited duration calculated   so as not to exceed the saturation threshold of austenite.   7 Device according to claim 6, characterized in that the duration of each phase   injection is between 1 s and 5 min.   8 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) whose flow rate is controlled by solenoid valves (9), the   arrangement of the nozzles inside the oven and the pilot   solenoid valves (9) being designed to obtain a rotational movement of the gas flow around load. 2681332   9 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 stream is a horizontal helix.