EP4317480A1 - Procédé de traitement par nitruration pour élément en acier - Google Patents

Procédé de traitement par nitruration pour élément en acier Download PDF

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Publication number
EP4317480A1
EP4317480A1 EP22781088.4A EP22781088A EP4317480A1 EP 4317480 A1 EP4317480 A1 EP 4317480A1 EP 22781088 A EP22781088 A EP 22781088A EP 4317480 A1 EP4317480 A1 EP 4317480A1
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EP
European Patent Office
Prior art keywords
gas
nitriding
treatment step
nitriding treatment
potential
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Pending
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EP22781088.4A
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German (de)
English (en)
Inventor
Yasushi Hiraoka
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Parker Netsushori Kogyo KK
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Parker Netsushori Kogyo KK
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Publication of EP4317480A1 publication Critical patent/EP4317480A1/fr
Pending legal-status Critical Current

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    • 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
    • C23C8/08—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 only one element being applied
    • C23C8/24—Nitriding
    • C23C8/26—Nitriding of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
    • 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 nitriding treatment method for a steel component which comprises at least three nitriding treatment steps.
  • a high pitting resistance and a high bending fatigue strength are required.
  • a carburizing treatment and/or a nitriding treatment are known as a technique for reinforcing such a steel component such as a gear.
  • JP-A-2013-221203 JP application number 2012-095035 ) (Patent Document 1) has disclosed that, in order to improve a pitting resistance and/or flexural fatigue strength of a steel component, it is effective to produce an iron nitride compound layer having a ⁇ ' phase as a main component on a surface of the steel component by a nitriding treatment.
  • JP-B-6755106 has disclosed a nitriding treatment method in which a nitride compound layer having a high ratio of a ⁇ ' phase (0.7 or more) is generated. Specifically, a gas nitriding treatment having three steps performed at a temperature within a range of 570 °C to 600 °C using two types of gases, which are an NH 3 gas and an AX gas, has been described.
  • a range of 0.1 to 0.25 is adopted for the nitriding potential in the first nitriding treatment step
  • a range of 1.0 to 2.0 is adopted for the nitriding potential in the second nitriding treatment step
  • 0.25 is adopted for the nitriding potential in the third nitriding treatment step.
  • Patent Document 1 cited in the present specification is JP-A-2013-221203 ( JP application number 2012-095035 ).
  • Patent Document 2 cited in the present specification is JP-B-6755106 .
  • Patent Document 2 JP-B-6755106
  • the action (reaction) in which the ⁇ ' phase is deposited in the nitride compound layer is affected by both the nitriding potential and the furnace temperature.
  • the nitriding potential at the third nitriding treatment step is set to be 0.25 or less, an ⁇ phase which is lower in hardness than the ⁇ ' phase is also deposited. This may result in an insufficient pitting resistance and/or an insufficient bending fatigue strength.
  • the present invention has been made based on the above findings. It is an object of the present invention to provide a nitriding treatment method to be performed in a temperature range of 500 °C to 590 °C, which enables a ⁇ ' phase to be deposited in a nitride compound layer in a suitable manner and thus achieves a high pitting resistance and a high bending fatigue strength.
  • the present invention is a nitriding treatment method for a steel component, including at least three nitriding treatment steps, i.e., a first nitriding treatment step in which a nitriding treatment is performed to a steel component under a nitriding gas atmosphere of a first nitriding potential, a second nitriding treatment step in which another nitriding treatment is performed to the steel component under another nitriding gas atmosphere of a second nitriding potential higher than the first nitriding potential, after the first nitriding treatment step, and a third nitriding treatment step in which a further another nitriding treatment is performed to the steel component under a further another nitriding gas atmosphere of a third nitriding potential lower than the second nitriding potential, after the second nitriding treatment step, wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590
  • the ⁇ ' phase can be deposited in the nitride compound layer in a suitable manner while an ⁇ phase, which is lower in hardness than the ⁇ ' phase, is inhibited to be deposited therein.
  • a high pitting resistance and a high bending fatigue strength can be achieved.
  • the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace; two types of gases, which are an NH 3 gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the two types of gases constant; three types of gases, which are an NH 3 gas, an AX gas and an N 2 gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
  • the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH 3 gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the two types of gases constant; three types of gases, which are an NH 3 gas, an AX gas and an N 2 gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
  • the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace; two types of gases, which are an NH 3 gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the three types of gases constant; two types of gases, which are an NH 3 gas and an AX gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
  • the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH 3 gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the three types of gases constant; two types of gases, which are an NH 3 gas and an AX gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
  • the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH 3 gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH 3 gas and the AX gas while keeping an introduction amount of the other of the NH 3 gas and the AX gas constant; two types of gases, which are an NH 3 gas and an AX gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH 3 gas and the A
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
  • the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH 3 gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH 3 gas and the AX gas while keeping an introduction amount of the other of the NH 3 gas and the AX gas constant; three types of gases, which are an NH 3 gas, an AX gas and an N 2 gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
  • the one-chamber type of thermal processing furnace means a thermal processing furnace which does not include a chamber for a cooling step separately from a chamber for a heating step like in the batch type of thermal processing furnace (see Fig. 1 ) and in which both the heating step and the cooling step are conducted to only one chamber.
  • a pit type furnace see Fig. 3
  • a horizontal type furnace see Fig. 5
  • a time for which the third nitriding treatment step is performed is 60 minutes or longer. According to the inventor's finding, by setting 60 minutes or longer as the time for which the third nitriding treatment step is performed, replacement of the gas atmosphere in the thermal processing furnace (which is caused by the change of the nitriding potential) is so sufficiently achieved that the effects of increasing a ratio of the ⁇ ' phase are not undermined.
  • the ⁇ ' phase can be deposited in the nitride compound layer in a suitable manner while an ⁇ phase, which is lower in hardness than the ⁇ ' phase, is inhibited to be deposited therein.
  • a high pitting resistance and a high bending fatigue strength can be achieved.
  • An object to be processed is a steel component.
  • it is a steel component consisting of a carbon steel component used for a machine structure or an alloy steel component used for a machine structure, such as a gear used for an automatic transmission.
  • a plurality of cylindrical ring gears or a plurality of bottomed cylindrical ring gears are mounted on a plurality of stages of jigs, placed in a flat state in a case (described below), and subjected to a nitriding treatment.
  • the steel component is pre-cleaned to remove dirt and oil before being subjected to the nitriding treatment.
  • the pre-cleaning process is preferably, for example, a vacuum cleaning process for degreasing and drying by dissolving and replacing oil or the like with a hydrocarbon-based cleaning liquid and evaporating it, an alkali cleaning process for degreasing with an alkaline-based cleaning liquid, or the like.
  • Fig. 1 is a schematic view showing a structure of a batch type of thermal processing furnace 1 to be used for a nitriding treatment method according to the present invention.
  • the batch type of thermal processing furnace 1 includes a loading section 10, a heating chamber 11, a transfer chamber 12, and an unloading conveyor 13.
  • a case 20 is configured to be loaded into the loading section 10.
  • a steel component as an object to be processed (work) is configured to be contained in the case 20.
  • the maximum gross weight to be processed is 700 kg.
  • An inlet hood 22 having an openable and closable door 21 is attached to an inlet side (left side in Fig. 1 ) of the heating chamber 11.
  • the heating chamber 11 has a retort structure, and an outer periphery of the retort structure is configured to be heated by a heater (not shown) so that a temperature in the furnace (chamber) is controlled to a predetermined temperature.
  • a plurality of types of gases for the nitriding treatment are configured to be introduced into the heating chamber 11 while being controlled as described below.
  • a fan 26 is mounted on a ceiling of the heating chamber 11 to stir the gases introduced into the heating chamber 11 so that a heating temperature for the steel component is made uniform therein.
  • An openable and closable intermediate door 27 is attached to an exit side (right side in Fig. 1 ) of the heating chamber 11.
  • the transfer chamber 12 is provided with an elevator 30 for raising and lowering the case 20 that contains the steel component.
  • a lower part of the transfer chamber 12 is provided with a cooling chamber (oil tank) 32 in which a cooling oil 31 is stored.
  • An outlet hood 36 having an openable and closable door 35 is attached to an outlet side (right side in Fig. 1 ) of the transfer chamber 12
  • the heating chamber 11 and the transfer chamber 12 may be the same processing space, and a configuration for air-cooling the thermally-processed steel component with a gas may be employed.
  • the heating chamber 11 may be divided into two or three chambers, and a three-stage nitriding treatment as described below may be performed in the two or three chambers.
  • the case 20 that contains the steel component is loaded into the heating chamber 11 from the loading section 10 by a pusher or the like.
  • the plurality of types of process gases are introduced into the heating chamber 11, and the process gases are heated to a predetermined temperature by the heater and stirred by the fan 26 (for example, rotating at 1500 rpm) so that the steel component loaded into the heating chamber 11 is subjected to the nitriding treatment.
  • Fig. 2 is a process diagram of an embodiment of the nitriding treatment method according to the present invention in a case wherein the thermal processing furnace 1 shown in Fig. 1 is used.
  • the heating chamber 11 is pre-heated to 550 °C in advance.
  • an N 2 gas is introduced at a constant flow rate of 70 (L/min)
  • a steel component (work) is loaded into the heating chamber 11.
  • the door 21 is opened, and thus the temperature in the heating chamber 11 is temporarily lowered as shown in Fig. 2 . Thereafter, the door 21 is closed and the temperature in the heating chamber 11 is heated again to 550 °C.
  • the NH 3 gas is introduced at a constant flow rate of 160 (L/min).
  • the total amount (flow rate) is also 160 (L/min).
  • a three-stage nitriding treatment is performed (Example 1-7, which will be described hereinafter). Specifically, at first, for example, a value of 0.30 (an example of a value within a range of 0.10 to 1.00) is employed as a first nitriding potential, and a first nitriding treatment step is performed at a temperature of 550 °C.
  • K N P NH 3 / P H 2 3 / 2
  • P(NH 3 ) i.e. a partial pressure of the NH 3 gas in the heating chamber 11 or P(H 2 ) i.e. a partial pressure of the H 2 gas in the heating chamber 11 is measured.
  • the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the first nitriding potential, which is a target nitriding potential.
  • P(H 2 ) i.e. a partial pressure of the H 2 gas in the heating chamber 11 is measured by a heat conduction type H 2 sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control.
  • the introduction amounts (flow rates) of the NH 3 gas and an AX gas are respectively increased or decreased while keeping the sum (total) amount of the two gases to be 160 (L/min).
  • the first nitriding treatment step is performed for 60 minutes.
  • a surface carbon content (which has an adverse effect on generation of a ⁇ ' phase) is reduced since a nitride compound layer is not generated on a surface of the steel component or since a nitride compound layer mainly consisting of an ⁇ phase is not generated on a surface of the steel component.
  • nitrogen can be effectively diffused inside the steel component.
  • a value of 2.00 (an example of a value within a range of 0.30 to 10.00) is employed as a second nitriding potential, and a second nitriding treatment step is performed at a temperature of 550 °C.
  • P(NH 3 ) i.e. a partial pressure of the NH 3 gas in the heating chamber 11 or P(H 2 ) i.e. a partial pressure of the H 2 gas in the heating chamber 11 is measured.
  • the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the second nitriding potential, which is a target nitriding potential.
  • P(H 2 ) i.e. a partial pressure of the H 2 gas in the heating chamber 11 is measured by the heat conduction type H 2 sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control.
  • the second nitriding treatment step is performed for 240 minutes.
  • a nitride compound layer consisting of a ⁇ ' phase, or an ⁇ phase, or mixture of a ⁇ ' phase and an ⁇ phase, is generated in the steel component.
  • a value of 0.30 (an example of a value within a range of 0.26 to 0.60) is employed as a third nitriding potential, and a third nitriding treatment step is performed at a temperature of 550 °C.
  • P(NH 3 ) i.e. a partial pressure of the NH 3 gas in the heating chamber 11 or P(H 2 ) i.e. a partial pressure of the H 2 gas in the heating chamber 11 is measured.
  • the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the third nitriding potential, which is a target nitriding potential.
  • P(H 2 ) i.e. the partial pressure of the H 2 gas in the heating chamber 11 is measured by the heat conduction type H 2 sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control. Specifically, the introduction amounts (flow rates) of the NH 3 gas and the AX gas are respectively increased or decreased while keeping the sum (total) amount of the two gases to be 160 (L/min).
  • the third nitriding treatment step is performed for 60 minutes. Thereby, a ⁇ ' phase is deposited in the nitride compound layer.
  • a cooling step is performed.
  • the cooling step is performed for 15 minutes (the case 20 is held in the oil bath (100 °C) for 15 minutes, the oil bath being provided with a stirrer).
  • the case 20 that contains the steel component is unloaded onto the unloading conveyor 13.
  • Fig. 3 is a schematic view showing a structure of a pit type thermal processing furnace 201 to be used for a nitriding treatment method according to the present invention.
  • the pit type thermal processing furnace 201 includes a bottomed cylindrical furnace wall 211 and a furnace lid 212.
  • a fan 213 is provided on a lower (inner) side of the furnace lid 212.
  • a rotation shaft of the fan 213 passes through the furnace lid 212, and is connected to a fan motor 214, which is provided on an upper (outer) side of the furnace lid 212.
  • a retort 221 is provided inside the furnace wall 211.
  • a gas guide tube 222 is provided further inside the retort 221.
  • An outer periphery of the retort 221 is configured to be heated by a heater (not shown) so that a temperature in the furnace (in the retort 221) is controlled to a predetermined temperature.
  • a case 20 is configured to be placed into the gas guide tube 222.
  • a steel component as an object to be processed (work) is configured to be contained in the case 20.
  • the maximum gross weight to be processed is 700 kg.
  • a plurality of types of gases for the nitriding treatment are configured to be introduced into the retort 221 while being controlled as described below.
  • the outer periphery of the retort 221 has a cooling function by a blower (not shown). When cooled, a temperature of the retort 221 itself is lowered, and thus the temperature in the furnace (in the retort 221) is lowered (furnace cooling).
  • the furnace lid 212 is opened, and the case 20 that contains the steel component is loaded into the gas guide tube 222.
  • the plurality of types of process gases are introduced into the retort 221, and the process gases are heated to a predetermined temperature by the heater and stirred by the fan 213 (for example, rotating at 1500 rpm) so that the steel component loaded into the gas guide tube 222 is subjected to the nitriding treatment.
  • Fig. 4 is a process diagram of an embodiment of the nitriding treatment method according to the present invention in a case wherein the thermal processing furnace 201 shown in Fig. 3 is used.
  • a three-stage nitriding treatment is performed (Example 5-7, which will be described hereinafter). Specifically, at first, for example, a value of 0.30 (an example of a value within a range of 0.10 to 1.00) is employed as a first nitriding potential, and a first nitriding treatment step is performed at a temperature of 550 °C.
  • K N P NH 3 / P H 2 3 / 2
  • P(NH 3 ) i.e. a partial pressure of the NH 3 gas in the gas guide tube 222 or P(H 2 ) i.e. a partial pressure of the H 2 gas in the gas guide tube 222 is measured (alternatively, a partial pressure of the NH 3 gas in the exhaust gas or a partial pressure of the H 2 gas in the exhaust gas may be measured).
  • the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the first nitriding potential, which is a target nitriding potential.
  • P(H 2 ) i.e. a partial pressure of the H 2 gas in the gas guide tube 222 is measured by a heat conduction type H 2 sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control.
  • the introduction amount (flow rate) of the NH 3 gas is increased or decreased while an AX gas is introduced at a constant flow rate of 50 (L/min). In this case, the total amount of the two gases is also increased or decreased.
  • the first nitriding treatment step is performed for 60 minutes.
  • a surface carbon content (which has an adverse effect on generation of a ⁇ ' phase) is reduced since a nitride compound layer is not generated on a surface of the steel component or since a nitride compound layer mainly consisting of an ⁇ phase is not generated on a surface of the steel component.
  • nitrogen can be effectively diffused inside the steel component.
  • a value of 0.50 (an example of a value within a range of 0.30 to 10.00) is employed as a second nitriding potential, and a second nitriding treatment step is performed at a temperature of 550 °C.
  • P(NH 3 ) i.e. a partial pressure of the NH 3 gas in the gas guide tube 222 or P(H 2 ) i.e. a partial pressure of the H 2 gas in the gas guide tube 222 is measured.
  • the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the second nitriding potential, which is a target nitriding potential.
  • P(H 2 ) i.e. a partial pressure of the H 2 gas in the gas guide tube 222 is measured by the heat conduction type H 2 sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control.
  • the introduction amount (flow rate) of the NH 3 gas is increased or decreased while the AX gas is introduced at a constant flow rate of 50 (L/min). In this case, the total amount of the two gases is also increased or decreased.
  • the second nitriding treatment step is performed for 240 minutes.
  • a nitride compound layer consisting of a ⁇ ' phase, or an ⁇ phase, or mixture of a ⁇ ' phase and an ⁇ phase, is generated in the steel component.
  • a value of 0.30 (an example of a value within a range of 0.26 to 0.60) is employed as a third nitriding potential, and a third nitriding treatment step is performed at a temperature of 550 °C.
  • P(NH 3 ) i.e. a partial pressure of the NH 3 gas in the heating chamber 11 or P(H 2 ) i.e. a partial pressure of the H 2 gas in the heating chamber 11 is measured.
  • the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the third nitriding potential, which is a target nitriding potential.
  • P(H 2 ) i.e. the partial pressure of the H 2 gas in the gas guide tube 222 is measured by the heat conduction type H 2 sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control.
  • the introduction amount (flow rate) of the NH 3 gas is increased or decreased while the AX gas is introduced at a constant flow rate of 50 (L/min). In this case, the total amount of the two gases is also increased or decreased.
  • the third nitriding treatment step is performed for 60 minutes. Thereby, a ⁇ ' phase is deposited in the nitride compound layer.
  • a cooling step is performed.
  • the same control as in the third nitriding treatment step is performed for the introduction amounts of the process gases. That is to say, the introduction amount (flow rate) of the NH 3 gas is increased or decreased while the AX gas is introduced at a constant flow rate of 50 (L/min).
  • the N 2 gas is introduced at a constant flow rate of 20 (L/min).
  • Fig. 5 is a schematic view showing a structure of a horizontal type thermal processing furnace to be used for a nitriding treatment method according to the present invention.
  • a horizontal type thermal processing furnace is basically a furnace in which a pit type thermal processing furnace is oriented horizontally.
  • the fan 213 and the fan motor 214 may be provided on a wall surface of the furnace wall 211 facing the furnace lid 212, instead of on the furnace lid 212.
  • the furnace lid 212 is opened, and the case 20 that contains the steel component is loaded into the gas guide tube 222.
  • the plurality of types of process gases are introduced into the gas guide tube 222, and the process gases are heated to a predetermined temperature by the heater and stirred by the fan 213 (for example, rotating at 1500 rpm) so that the steel component loaded into the gas guide tube 222 is subjected to the nitriding treatment.
  • the process diagram shown in Fig. 4 is also applicable in a case wherein the horizontal type thermal processing furnace is used. Specifically, the heating step (introduction manners of the process gases are different between a former half thereof and a latter half thereof), the first nitriding treatment step, the second nitriding treatment step, the third nitriding treatment step and the cooling step (introduction manners of the process gases are different between a former half thereof and a latter half thereof) may be performed. After the cooling step has been completed, the furnace lid 212 is opened, and the case 20 that contains the steel component is unloaded from the gas guide tube 222.
  • a nitrided steel component including an iron nitride compound layer which has a ⁇ ' phase as a main element (main component) on a surface thereof, regardless of whether a batch type of thermal processing furnace is used or a one-chamber type of thermal processing furnace is used.
  • the steel component obtained by the respective embodiments can achieve a sufficient pitting resistance and a sufficient bending fatigue strength because a nitrogen diffusion layer and a nitride are formed in the inside thereof to reinforce the same and an iron nitride compound layer rich in a ⁇ ' phase is formed on the surface thereof.
  • the nitriding treatment according to the present invention is performed at a temperature not higher than the austenite transformation temperature, so that an amount of strain is small.
  • a quenching step which is necessary for a carburizing treatment or a nitrocarburizing treatment, can be omitted, so that an amount of strain variation is small. As a result, a high-strength and low-strain nitrided steel member can be obtained.
  • the temperature of each nitriding treatment step is 500 °C to 590 °C. It is said that, when the temperature of a nitriding treatment step is higher, the productivity thereof is better. However, according to the inventor's verification, if the temperature of a nitriding treatment step is higher than 590 °C, an amount (degree) of hardening is reduced and an austenite layer is formed on the surface. Thus, it is preferable that 590°C is the upper limit. On the other hand, according to the inventor's verification, if the temperature of a nitriding treatment step is lower than 500 °C, a formation speed of the nitride compound layer is slow, which is not cost effective. Thus, it is preferable that 500°C is the lower limit.
  • the temperature difference between both the nitriding treatment steps is controlled to be preferably 50 °C or less, more preferably 30 °C or less.
  • a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same batch type of thermal processing furnace 1.
  • a thickness of the compound layer was measured as a thickness of a surface compound layer from a tissue observation result of a cross section which was cut in a depth direction of the nitrided steel component. It is preferable that a thickness of the compound layer rich in the ⁇ ' phase is 2 ⁇ m to 20 ⁇ m. When it is less than 2 ⁇ m, i.e., too thin, the fatigue strength is not sufficiently improved. When it is more than 20 ⁇ m, a porous layer in the compound layer which may be an origin of fatigue crack is too thick, which deteriorates the fatigue strength.
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 1-1 to 1-14.
  • the comparative examples 1-1 to 1-8 have proved that an ⁇ phase which is lower in hardness than the ⁇ ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
  • a three-stage nitriding treatment was performed according to the conditions of Table 2 shown in Fig. 7 .
  • a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 2-1 to 2-14.
  • the comparative examples 2-1 to 2-8 have proved that an ⁇ phase which is lower in hardness than the ⁇ ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
  • a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same batch type of thermal processing furnace 1.
  • the two types of gases which are the NH 3 gas and the AX gas
  • a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
  • the two types of gases which are the NH 3 gas and the AX gas
  • a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 3-1 to 3-14.
  • the comparative examples 3-1 to 3-8 have proved that an ⁇ phase which is lower in hardness than the ⁇ ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
  • a three-stage nitriding treatment was performed according to the conditions of Table 4 shown in Fig. 9 .
  • a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
  • the two types of gases which are the NH 3 gas and the AX gas
  • a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
  • the two types of gases which are the NH 3 gas and the AX gas
  • a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of each of the NH 3 gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 4-1 to 4-14.
  • the comparative examples 4-1 to 4-8 have proved that an ⁇ phase which is lower in hardness than the ⁇ ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
  • a three-stage nitriding treatment was performed according to the conditions of Table 5 shown in Fig. 10 .
  • a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
  • a nitriding potential during the first nitriding treatment step was controlled to be close to the first nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of one of the NH 3 gas and the AX gas while keeping an introduction amount of the other of the NH 3 gas and the AX gas constant.
  • K N the first nitriding potential
  • the two types of gases which are the NH 3 gas and the AX gas
  • a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of one of the NH 3 gas and the AX gas while keeping an introduction amount of the other of the NH 3 gas and the AX gas constant.
  • the two types of gases which are the NH 3 gas and the AX gas
  • a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of one of the NH 3 gas and the AX gas while keeping an introduction amount of the other of the NH 3 gas and the AX gas constant.
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 5-1 to 5-14.
  • the comparative examples 5-1 to 5-8 have proved that an ⁇ phase which is lower in hardness than the ⁇ ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
  • a three-stage nitriding treatment was performed according to the conditions of Table 6 shown in Fig. 11 .
  • a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
  • the two types of gases which are the NH 3 gas and the AX gas
  • a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K N ), which is a target nitriding potential, by changing an introduction amount of one of the NH 3 gas and the AX gas while keeping an introduction amount of the other of the NH 3 gas and the AX gas constant.
  • the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 6-1 to 6-14.
  • the comparative examples 6-1 to 6-8 have proved that an ⁇ phase which is lower in hardness than the ⁇ ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.

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