Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/02—Pretreatment of the material to be coated
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/34—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in more than one step
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/80—After-treatment

Definitions

• the present invention relates to a method of thermochemical treatment of steel parts to reinforce them, and a gearbox gear for a motor vehicle reinforced with such a type of treatment.
• thermochemical treatment of this type of parts by carbonitriding.
• These treatments comprise first a carbon and nitrogen temperature diffusion over a certain thickness of the material, before quenching the part making it possible to increase the surface hardness while keeping certain original characteristics at heart. .
• One type of known treatment method presented in particular by the document FR-B1-284523, comprises a first stage of temperature rise to reach 930 ° C., then a second stage of equalization of this temperature, followed subsequently by a succession similar cycles comprising first a cementation phase and then a nitriding phase, and finally the thermal quenching. It remains well during all cycles of supply elements at a constant temperature.
• a problem with this type of process is that with the carburization temperatures usually used, which are around 900 ° C, the nitriding efficiency is low. It is then necessary to have significant nitriding times to introduce enough nitrogen into the room. In addition nitriding the workpiece with a reducing gas such as ammonia causes a decarburization of the metal surface by the hydrogen released during the decomposition of this ammonia, which is reducing and consumes the carbon. This decarburization reduces the hardness, and degrades the functional performance of the room.
• a reducing gas such as ammonia
• the control of the nitriding at the end of the cycle is delicate because the austenitic steel grains are already saturated with carbon and nitrogen by the preceding cementation phases. Grains can no longer absorb nitrogen and release it into grain boundaries. This nitrogen combines with the additive elements such as chromium and nickel, which charges the gaskets into precipitates of nitride of chromium and silicon, as well as carbonitrides.
• Another type of known process of treatment comprises a first stage of rise in temperature to reach 800 ° C followed by a first phase of nitriding at this temperature, then a second step of temperature rise to 980 ° C, then a succession of cycles each comprising a cementation phase followed by diffusion by maintaining this temperature, then a temperature descent step at 850 ° C followed by a second nitriding phase at this temperature. temperature, and finally the quenching phase of the metal.
• a cementation time is obtained which comprises a succession of cementation and diffusion cycles, which is relatively long.
• the second nitriding phase after cementation also causes a decarburization of the layer on the surface of the metal.
• the present invention is intended to avoid these disadvantages of the prior art.
• thermochemical treatment of steel parts to improve the mechanical characteristics, comprising a carbon diffusion by carburizing and nitrogen nitriding in a low pressure furnace according to a temperature cycle, characterized in that it first has a rise to a first temperature relatively low to begin the nitriding of the metal, then a single nitriding phase at a temperature between this first temperature and a second carburizing temperature, then a single carburizing phase, then a diffusion at this second temperature which remains constant, and finally the quenching of the metal.
• An advantage of this treatment process is that the cementation is not followed by a nitriding phase, there is no decarburization of the surface of the workpiece, and its hardness remains at the best level.
• the metallurgical quality is also better controlled, avoiding the formation of various nitrides in the grain boundaries that would reduce the mechanical characteristics of the part.
• the only nitriding phase being made between the first relatively low temperature allowing this operation, and the second carburizing temperature, it is possible in certain cases to produce temperature rises simultaneously during this nitriding, which saves time on the complete cycle of treatment.
• thermochemical treatment process according to the invention may further comprise one or more of the following characteristics, which may be combined with each other.
• the nitriding phase is carried out at the first temperature which remains constant.
• the nitriding phase can be followed before the carburizing phase, by a diffusion phase comprising a rise in temperature from the first temperature to the second carburizing temperature.
• the nitriding phase can be followed immediately after the carburising phase, first comprising a temperature rise from the first temperature to the second carburizing temperature.
• the nitriding phase comprises a rise in temperature from the first temperature to the second carburizing temperature.
• the nitriding phase can be followed before the carburizing phase by a diffusion phase carried out at the second temperature which remains constant.
• the first relatively low temperature for starting the nitriding of the metal is of the order of 700 ° C.
• the second carburizing temperature is of the order of 940 ° C.
• the invention also relates to a steel gear for a motor vehicle gearbox, reinforced by a thermochemical treatment comprising any one of the preceding features.
• FIG. 1 is a micrographic section of an austenitic steel treated by a carbonitriding process according to the prior art
• FIG. 2 is a micrographic section of this steel treated by a carbonitriding process according to the invention.
• FIG. 3 is a graph showing, as a function of time, the treatment method according to the invention.
• FIG. 4 is a graph showing the treatment method according to a first variant.
• FIG. 5 is a graph showing the treatment method according to a second variant.
• FIG. 1 shows the micrographic section 6 of a low carbon austenitic steel, made to a height of a few tens of micrometers below the surface 2.
• This steel has undergone a low pressure carbonitriding treatment in accordance with the process presented by FIG. FR-B1 -2884523, comprising successive phases of carburizing and nitriding before final quenching.
• a low pressure carbonitriding treatment in accordance with the process presented by FIG. FR-B1 -2884523, comprising successive phases of carburizing and nitriding before final quenching.
• large precipitates of nitrides 4 forming black spots are observed, following the supply of ammonia throughout the duration of the treatment phase giving an enrichment of carbon and nitrogen. These precipitates reduce the mechanical strength of the surface layer, which can cause premature wear of highly stressed parts with stress cycles, such as gears of gearboxes for motor vehicles.
• Figure 2 shows the micrographic section of a similar austenitic steel, having undergone a low pressure carbonitriding treatment according to the process according to the invention. For the layer a few tens of micrometers below the surface 2, there is an absence of nitride precipitate in the grain boundaries.
• FIG. 3 shows a first carbonitriding treatment cycle according to the invention carried out in a low pressure furnace, indicating the temperature T ° as a function of time t.
• the piece of steel to be treated is at time t0, at room temperature.
• a first step M of progressive temperature rise of the furnace is made to reach a first temperature level T1 of nitriding, which in this example is 700 ° C.
• the ideal nitriding temperatures are between 750 and 800 ° C. However, it can be started at 700 ° C and continued up to 900 ° C if you tolerate a little less nitrogen absorption in the room, to save processing time by reducing the rise time in temperature.
• a third diffusion step D comprises a rise in temperature under a neutral atmosphere, to reach a second temperature plateau T2 of carburizing, which in this example is 940 ° C.
• diffusion D of nitrogen is carried out in the metal surface layer.
• the carburizing temperature may vary, with a treatment depth which increases as a function of this temperature.
• a treatment depth which increases as a function of this temperature.
• E650 the same processing depth corresponding to the standardized treatment "E650”
• This second diffusion step D is optional, it depends on the desired carbon content at the end of treatment.
• FIG. 4 alternatively shows a second carbonitriding treatment cycle according to the invention.
• This variant makes it possible to optimize the treatment time, by reducing the temperature step between the nitriding phase N and the cementation phase C.
• the nitriding being carried out with a variable temperature rising above its optimum level, the concentration Nitrogen on the part is weaker with the same nitriding time.
• FIG. 5 alternatively presents a third carbonitriding treatment cycle according to the invention.
• the first step M of temperature rise is identical, to reach at the first time t1 the first nitriding temperature T1.
• a nitriding phase N comprising a temperature which initially remains at the first temperature stage T1, and which then gradually rises to reach the second temperature stage T2.
• this process specially developed by the company carrying out the invention, makes it possible not to use particular carbonitriding processes which may be the property of suppliers of low pressure furnaces intended for this type of treatment.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

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• 2013
  • 2013-04-18 FR FR1353536A patent/FR3004731B1/fr active Active
• 2014
  • 2014-03-13 CN CN201480022094.0A patent/CN105264106A/zh active Pending
  • 2014-03-13 EP EP14720174.3A patent/EP2986750A1/de not_active Withdrawn
  • 2014-03-13 WO PCT/FR2014/050581 patent/WO2014170566A1/fr active Application Filing

Non-Patent Citations (2)

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