WO2008083031A1 - Procédé d'optimisation d'un processus de traitement thermique sans oxygène - Google Patents

Procédé d'optimisation d'un processus de traitement thermique sans oxygène Download PDF

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Publication number
WO2008083031A1
WO2008083031A1 PCT/US2007/088352 US2007088352W WO2008083031A1 WO 2008083031 A1 WO2008083031 A1 WO 2008083031A1 US 2007088352 W US2007088352 W US 2007088352W WO 2008083031 A1 WO2008083031 A1 WO 2008083031A1
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Prior art keywords
concentration
gas
hydrocarbon gas
furnace
carbon
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PCT/US2007/088352
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English (en)
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Karen Anne Connery
Richard A. Novak
Simon Ho
Amitabh Gupta
Robert Marcel De Wilde
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Praxair Technology, Inc.
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Priority to EP07865917A priority Critical patent/EP1966398A1/fr
Publication of WO2008083031A1 publication Critical patent/WO2008083031A1/fr

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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
    • C23C8/28—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 one step
    • C23C8/30—Carbo-nitriding
    • C23C8/32—Carbo-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
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • 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
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces

Definitions

  • the present invention relates to methods of carburizing metal parts in an oxygen-free gas atmosphere, and more particularly to a method of varying in a controlled way the hydrocarbon and hydrogen gas concentrations in the furnace atmosphere to carburize metal parts with very low levels of inter-granular oxidation, and without formation of cementite or other metallic carbides in and/or soot on the carburized metal part.
  • Carburizing is a well-known process by which carbon atoms are adsorbed on the surface of a metal part and are introduced into the metal part via diffusion and is intended to make the surface of the metal part harder and more abrasion resistant.
  • Carburization of steel generally involves a heat treatment of the metallic surface using a gaseous or plasma source of carbon within a furnace.
  • Current carburization practice typically includes creating an endo-gas type of atmosphere containing nitrogen, hydrogen, a hydrocarbon, and an oxygen-containing compound such as carbon monoxide.
  • endo-gas atmospheres are somewhat difficult to generate and precisely control, are typically more costly to operate, and involve additional safety risks due to the gas atmosphere flammability and toxicity.
  • the endo-gas atmosphere includes oxygen-laden gases, the resulting formation of inter-granular oxidation (IGO) within the metal part cannot be avoided.
  • the present invention may be characterized as a method of controlling an oxygen free heat treating process using a heat treating model that optimizes the concentrations of gases in a controlled gas atmosphere of an atmospheric pressure furnace as a function of time.
  • the controlled gas atmosphere includes a reducing gas, such as hydrogen, and a carbon containing gas, such as a hydrocarbon gas, and is substantially free of oxygen.
  • the method comprises the steps of: (a) inputting selected parameters into the heat treating model including desired case depth, target temperature, and alloy composition of the part to be treated; (b) ascertaining a prescribed minimum concentration of a reducing gas, such as hydrogen, necessary to inhibit formation of metal-oxides in the part at the target temperature; and (c) ascertaining a concentration of a carbon containing gas as a function of time to achieve the desired case depth yet inhibit formation of cementite in the part at the target temperature.
  • a reducing gas such as hydrogen
  • the invention also may be broadly characterized as a method of controlling oxide formation in a controlled atmosphere oxygen free heat treating process.
  • the present method comprising the steps of: (a) identifying selected parameters including target temperature, level of residual oxygen in a heat-treating atmospheric pressure furnace, and alloy composition of the metal part to be treated; and (b) ascertaining a prescribed minimum concentration of a reducing gas, such as hydrogen, necessary to inhibit formation of metal-oxides in the part at the target temperature based on the level of residual oxygen in the furnace, and the alloy composition of the metal part to be treated.
  • a reducing gas such as hydrogen
  • Figure 1 is a schematic representation of the oxygen- free carburization system in accordance with the present invention.
  • Figure 2 is a graph that depicts the amount of oxygen required for the formation of chromium oxide compared to the equilibrium concentration of oxygen in the furnace atmosphere as a function of added hydrogen at about 900 degrees Centigrade;
  • Figure 3, Figure 4, and Figure 5 depict various hydrocarbon gas concentrations as a function of time in accordance with embodiments of the present invention.
  • Figure 6 is a graph depicting the free energy of formation for selected hydrocarbons.
  • FIG. 1 there is shown a schematic representation of the present oxygen-free carburization control system 10.
  • the oxygen-free carburization control system 10 employs a gas control panel 12 to control the flow and mixing of a specific gas mixture and deliver that specific gas to an atmospheric pressure furnace 20.
  • atmospheric pressure furnace broadly means any non-vacuum furnace and more particularly means furnaces operating at pressures between 10 psig and slightly below atmospheric.
  • the controlled gas atmosphere includes a variable combination of: a reducing gas; and a carbon containing gas with the balance being an inert gas, all such constituent gases being substantially free of oxygen.
  • the specific gas mixture 60 includes a variable combination of nitrogen gas 14, hydrogen gas 16, and hydrocarbon gas 18 in prescribed concentrations and is delivered to the atmospheric pressure furnace 20 as a function of time. The gas atmosphere within the furnace is thus substantially oxygen free, with very small quantities of oxygen present as a result of leakage, impurities, etc.
  • a series of user inputs 22 defining the desired process parameters are collected via the control panel 12.
  • the desired process parameters help characterize the final carbon profile desired in the metal part to be carburized, and would preferably include the desired case depth, desired carburization temperature, initial carbon content, target saturation factor, maximum inter- granular oxidation (IGO) allowable, etc.
  • Other user inputs 22 may include results or analysis from test samples from the furnace during the calibration and operation of the furnace. Preferably such results or analysis would include actual carbon uptake realized in the furnace for given steel alloys at given atmosphere conditions.
  • a series of sensed or measured parameters including furnace temperature 24 as measure with a temperature sensor, and furnace oxygen content 26 as measured with an oxygen probe, are also preferably input to the gas control panel 12.
  • the control panel 12 uses a carburization model 30 to calculate a profile of nitrogen gas 14, hydrogen gas 16, and hydrocarbon gas 18 concentrations as a function of time.
  • the model 30 is preferably a software routine that uses selected inputs, including user inputs 22, furnace temperature 24, furnace oxygen content 26, as well as known parameters such as alloy composition, furnace type, etc. to calculate or ascertain the desired gas concentrations as a function of heat treatment time.
  • Metal parts 70 to be carburized are loaded into an atmospheric pressure furnace 20 and contacted with the prescribed gas mixtures 60 for the prescribed durations. Once heat treated, the treated metal parts 72 are removed from the atmospheric pressure furnace 20 and placed in a cooling or quench chamber 40.
  • the cooling or quench chamber 40 also comprises an atmosphere that is substantially free of oxygen so as to further minimize oxidation.
  • the carburized metal part 72 is cooled or quenched for a prescribed duration under controlled conditions after which the heat-treated metal part 74 is removed.
  • the gas atmosphere can be released or exhausted via a vent 42, as appropriate.
  • Flow of the nitrogen gas 14, hydrogen gas 16 and hydrocarbon gas 18 are controlled to the atmospheric pressure furnace 20 using control valves 34, 36, and 38 which are controlled or adjusted by the gas control panel 12 in response to the model 30 and feedback or inputs from the corresponding flow meters 44, 46, and 48.
  • the individually metered gas streams 54, 56, and 58 are then mixed and the resulting gas mixture 60 is introduced to the atmospheric pressure furnace 20 in accordance with the model 30.
  • the preferred oxygen-free heat treatment or carburization process involves the following typical sequence of steps: (1) Seasoning the atmospheric pressure furnace with a nitrogen, hydrogen, and hydrocarbon gas atmosphere to remove or reduce the oxygen and moisture content within the furnace and to equilibrate the furnace materials with the carburizing atmosphere; (2) Calibrating the carburization recipe or model for the nitrogen, hydrogen, and hydrocarbon gas atmosphere; (3) Establishing user inputs including case depth, treatment temperature, initial carbon content, saturation factor, etc.; (4) Generating an initial profile of desired hydrocarbon gas in the furnace as a function of time; (5) Changing the furnace atmosphere to a substantially nitrogen and hydrogen gas atmosphere; (6) Loading the metal parts into the furnace atmosphere until such metal parts achieve the desired treatment or process temperature; (7) Adjusting the hydrocarbon gas concentration, hydrogen gas concentration, and nitrogen gas concentration in the furnace in accordance with the model or recipe; (8) Removing the treated metal parts when the treatment cycle is complete and placing the treated metal parts into a quench or cooling chamber that, preferably, is substantially free of oxygen; and
  • Seasoning of a furnace is characterized as a process that uses an inert gas plus a carbon source to purge or reduce the oxygen and moisture content within the furnace as well as to eliminate or reduce the carbon uptake variability within the furnace prior to the start of the heat treating process.
  • the seasoning atmosphere within the furnace includes a small concentration of hydrocarbon gas (e.g. approximately 0.5% to 2.0% propylene gas), about 5.0 % hydrogen gas, and the balance of the introduced gas atmosphere being nitrogen gas.
  • the seasoning step should proceed for a time sufficient to allow the furnace to equilibrate at the desired process or treatment temperature. This is usually determined by steady measurements from an oxygen probe disposed within the furnace at the desired process or treatment temperature.
  • Calibration of the oxygen-free carburization model for the nitrogen, hydrogen, and hydrocarbon gas atmosphere is preferably accomplished using one or more test samples or shims.
  • the test sample or shim is introduced into the seasoned furnace atmosphere comprising constant volumes of nitrogen gas, hydrogen gas a small percentage of hydrocarbon gas.
  • the shim remains in the furnace atmosphere for about 30 minutes or a time sufficient to soak at the desired process or treatment temperature.
  • the test sample or shim is then removed from the furnace and the actual carbon uptake is determined and with other user inputs entered into the oxygen- free carburization model or recipe.
  • the step of adjusting the hydrocarbon gas concentration in accordance with the model typically involves varying the hydrocarbon gas flow to achieve a desired concentration of hydrocarbon gas in the furnace atmosphere as a function of time.
  • the desired range of hydrocarbon gas concentration is from about nil or 0.0% to about 10.0% and more preferably in the range of between about 0.3% to about 5.0%.
  • the actual hydrocarbon concentration is calculated using a carbon saturation factor within the process model.
  • a minimum concentration of hydrocarbon gas in the furnace atmosphere has been found to be preferred in order to achieve reasonable levels of carbon flux on the metal parts to be treated. It will be understood that some of the hydrocarbon gas is consumed through reacting with oxidizing species that are continuously introduced into the furnace through leaks, door openings, etc, and this can vary from one furnace and implementation to another.
  • the preferred minimum concentration of hydrocarbon gas within the furnace is likely in the range of 0.1% to 2.0%, and more preferably in the range of 0.3% to 1.5%. This minimum level of hydrocarbon gas within the furnace atmosphere is easily determined by shim tests as the present model is calibrated for a given furnace.
  • a maximum hydrocarbon gas concentration within the furnace atmosphere may be set due to practical flow rate limitations of the control valves and lines employed as well as economic or safety considerations.
  • Maximum hydrocarbon concentrations in the furnace atmosphere are typically in the range of about 4.0% to about 10.0%.
  • the preferred hydrocarbon is propylene and the gas concentration range of the propylene is preferably between about 1.0% to about 3.5% concentration of the substantially oxygen-free gas atmosphere in the furnace.
  • the step of adjusting the hydrogen gas concentration in the furnace in accordance with the model or recipe comprises setting the hydrogen gas flow to achieve a desired concentration of hydrogen gas in the furnace atmosphere.
  • the desired concentration of hydrogen gas is calculated within the model and is preferably between a prescribed threshold minimum concentration and a prescribed threshold maximum concentration.
  • the minimum concentration of hydrogen gas depends on several factors including the alloy elements within the metal part to be treated with potential for oxide formation at the process or treatment temperature; and the quantity of oxygen, moisture, and other impurities present in the furnace.
  • the maximum concentration of hydrogen gas is preferably determined based on consideration of several other process factors, such as flammability of the gas, potential for decarburization, potential for hydrogen embrittlement, and hydrogen gas cost.
  • the preferred range of hydrogen gas is between about 1.0% to about 10.0% and more preferably in the range of between about 1.5% to about 5.25% concentration of hydrogen gas in the furnace.
  • the final step of cooling e.g. quenching or tempering
  • the quenching atmosphere is vented and the heat treatment process is complete.
  • the quenching process may utilize various quenching techniques including intensive quenching with water or a cryogen or standard quenching with oil.
  • the cooling may be done more gradually at ambient temperatures in a nitrogen atmosphere or other oxygen free atmosphere for a prescribed period of time.
  • inter-granular oxidation For certain steel alloys, particularly those containing Cr, Ti, Mn, and/or Si, the presence of oxygen in the gas atmosphere within the furnace promotes the formation of inter-granular oxidation (IGO) within the treated part.
  • Introduction of hydrogen gas in sufficient concentrations into the gas atmosphere within the furnace negates the formation of the likely metal oxides in favor of reactions between hydrogen and oxygen.
  • the actual concentration of hydrogen gas necessary to reduce or negate the formation of inter-granular oxidation (IGO) is ascertained or calculated based on the thermodynamic equilibrium between the potential oxidized products and reactants within the system.
  • the oxidation potential observed with hydrogen varies based on the concentration of hydrogen gas in the furnace.
  • the reactants can include not only hydrogen, water, and oxygen, but also include species containing carbon, such as hydrocarbons, carbon monoxide, and carbon dioxide. For simplicity, the procedure described herein is limited to reactants including hydrogen, water, and oxygen.
  • Oxidation of the metals of interest, including Cr, Ti, Mn, and/or Si are well known and documented processes that depend, in part, on given furnace temperatures and operating conditions.
  • 'T' is the furnace temperature in Kelvin (K)
  • ⁇ G° is the Gibbs free energy (J/mole) for the reaction a Me + O 2 >» Me a 0 2 (assuming ideal behavior and pure metal).
  • a comparison is made between the partial pressure of oxygen required to form oxides of the metals of interest and the equilibrium concentration of oxygen in the furnace atmosphere with varying amounts of hydrogen gas in a given furnace under prescribed conditions. Included in these comparisons and calculations are the quantity of O 2 and moisture (i.e.
  • H 2 O initially present in the seasoned furnace atmosphere as adjusted to account for additional concentrations of O 2 and H 2 O attributable to impurities subsequently introduced in the furnace.
  • the adjusted quantity of O 2 and moisture (i.e. H 2 O) in the furnace at the given operating conditions is preferably estimated from past experience with the furnace or determined through experimentation or calibration testing of the furnace.
  • the minimum concentration of hydrogen gas to be used in the ambient gas atmosphere in the furnace is the calculated amount of hydrogen that results in a lower partial pressure of oxygen than is required to form oxides of the metals of interest found in the alloy of the part to be carburized.
  • inter-granular oxidation (IGO) formation may vary depending not only on the gas composition, but also physical construction and operation of the furnace and the actual composition of the steel alloy. For example, a furnace that allows a greater flow rate of outside atmosphere containing oxygen and/or moisture into the furnace would require a higher concentration of hydrogen gas to achieve the desired low value of oxygen partial pressure (p ⁇ 2 ). From a practical standpoint, minimizing inter-granular oxidation (IGO) to 0.0007 inches or below meets the SAE Aerospace Material Specification 2759/1 C for heat treatment of carbon and low- alloy steel parts.
  • the maximum concentration of hydrogen gas used in the ambient gas atmosphere in the furnace is determined based on consideration of several other factors, such as flammability of the gas, potential for decarburization, potential for hydrogen embrittlement, and economic impacts of such hydrogen gas use. It has been shown that hydrogen gas concentrations within a carburizing furnace up to about 40% can increase the carburization rate of hydrocarbons in nitrogen by several fold. However, when using the present
  • Figure 2 depicts a graph that compares the partial pressure of oxygen that is required to form chromium oxide in a gas atmosphere furnace to the partial pressure of oxygen available in the furnace as a function of added hydrogen.
  • the partial pressure of oxygen required to form the oxide of pure chromium (Cr) at 900 degrees C is about 6.0 x 10 "25 and is constant as the concentration of hydrogen gas within the furnace varies.
  • the partial pressure of oxygen available in the furnace as a function of hydrogen (H 2 ) at 900 degrees C ranges from about 2.59 x 10 "23 at about a 0.4% concentration of hydrogen gas within the furnace to about 4.14 x 10 "26 at about a 10% concentration of hydrogen gas.
  • the gas atmosphere of the seasoned furnace is estimated to have about 1 parts per million (ppm) O 2 and
  • the amount of hydrogen concentration in the furnace atmosphere required to substantially prevent oxidation of chromium in the metal part is about 3% or more concentration of hydrogen gas.
  • the primary objective of varying the concentration of hydrocarbon gas in the furnace is to maintain a desired net carbon flux from the furnace atmosphere that is less than the carbon flux required to saturate the surface of the metal part with carbon.
  • saturated we mean that the carbon concentration in the metal is at a specified level.
  • the specified level may be the maximum carbon concentration at a given temperature that would avoid formation of a certain phase, such as cementite or other metallic carbide.
  • the specified level may be a desired surface carbon concentration, for example the starting bulk carbon concentration in the case of a neutral hardening heat treatment.
  • the specified level denoting "saturation” may change depending on composition of the metal, temperature, or time in the heat treating process.
  • both the carbon flux required to saturate the surface of the metal part with carbon as well as the carbon diffusion of atoms within the metal part change as a function of time. Accordingly, the concentration of hydrocarbon gas within the furnace should likewise be adjusted as a function of time.
  • the desired hydrocarbon gas concentration is determined based on the actual measured carbon flux data from the furnace together with an estimated or calculated diffusivity of carbon in the treated metal part. Measuring the carbon flux is preferably accomplished using an appropriate methods such as shim testing, resistivity measurements of a thin wire disposed within the furnace, or other methods known to those skilled in the art and used to determine rate of carburization or carbon content of the metal parts being treated at known operating conditions.
  • 'C is the carbon atom concentration of the part
  • 'x' is the distance from the surface of the part that carbon atoms must diffuse into the part
  • 'D' is the diffusion coefficient of carbon atoms into the metal
  • 't' is time. As is well known in the art, the diffusion coefficient generally increases exponentially with the reciprocal of the temperature.
  • C s is the carbon concentration at the surface
  • C 0 is the original carbon concentration in the metal
  • C x is the carbon concentration at the distance x, from the surface of the metal part into the body of the metal part
  • the carbon concentration at the surface, C s is the specified level or saturation concentration.
  • the carbon concentration at the surface of the metal part is determined empirically by taking carbon flux measurements of samples in known furnace atmosphere conditions.
  • the carbon flux measurements are used to determine the amount of carbon (i.e. hydrocarbon gas) that is delivered to the surface of the metal part over time, at different atmosphere compositions, and different furnace temperatures. Carbon flux measurements are taken with foil samples of known surface area that are exposed to a known atmosphere compositions.
  • a carbon saturation factor ⁇ is then applied to assure that the amount of carbon delivered to the surface of the part is below that which would produce cementite, other metallic carbides, soot or, in the case of neutral hardening for example, would prevent decarburization on the surface of the metal part.
  • the carbon saturation factor ⁇ is represented by the equation:
  • FluxFuei is the carbon flux rate from the hydrocarbon gas to the surface of the metal part and is determined empirically from the measurements taken from the samples exposed to the known atmosphere compositions; and Fluxuetai is the sum of the calculated diffusive flux of carbon away from the surface of the metal part taken from the above carbon diffusivity estimate, and the carbon flux leaving the surface due to any decarburizing effect of reactions with the gas atmosphere.
  • the diffusive flux is usually the critical component of the flux away from the metal surface.
  • the diffusive flux is usually negligible due to a uniform carbon concentration throughout the metal, such that the decarburizing flux is the critical component of the flux away from the metal surface.
  • the decarburizing flux can be either calculated by methods known in the art based on estimates or measurements of decarburizing specie concentration in the gas atmosphere, or determined empirically by test.
  • the theoretical ideal carbon saturation factor, ⁇ is between about 0.75 and 0.99. However, given variation in accuracy of sample measurements and in precision of flow and temperature control, a practical range for the saturation factor is preferably between about 0.75 and 1.25, and more preferably between about 0.90 and 1.10. Using the preferred saturation factor range and the Fluxu eta i value calculated from the above diffusivity model, an optimal or target value of FluxFuei is determined. The preferred hydrocarbon gas concentration as a function of time is then determined based on the optimal or target value of Fluxp ue i so that the carbon saturation factor ⁇ , is maintained in the desired range during any time interval of the process treatment.
  • the relationship between FluxFuei and hydrocarbon gas concentration is determined empirically and periodically updated using actual foil sample data from the actual furnace.
  • the preferred hydrocarbons useful with the present embodiments of the invention include those high purity hydrocarbons having intermediate stability at typical carburization temperatures (e.g. 1550 to 1700 degrees F). Hydrocarbons of intermediate stability can be selected based the value of free energy of formation per gram-atom of carbon.
  • hydrocarbons with a free energy of formation per gram-mole carbon between about 64 and 85 kJ/gmol over the carburization temperature range of 850 - 1000 0 C provide the best balance of carburization rate and avoidance of soot or carbide formation.
  • hexane 111, Cyclohexane 112, methane 113 and benzene 114 are also shown in Figure 6, and even more preferred sources of hydrocarbon for use with the present methods.
  • Even more preferred sources of hydrocarbon for use with the present methods include unsaturated hydrocarbons and propylene is the most preferred hydrocarbon.
  • the oxygen-free carburization process Upon completion and execution of the carbon diffusivity model, the oxygen-free carburization process, as set forth above is continued.
  • the final result of the oxygen- free carburization process performed in accordance with the model should achieve the desired case depth and carbon profile in the treated metal part with little or no inter-granular oxidation (IGO), soot formation or cementite or other metallic carbide formation.
  • IGO inter-granular oxidation
  • Some testing and adjustment of operating conditions may be used to adapt the model and associated procedures to the conditions and variability of a particular furnace, material, surface finish, etc.
  • An optimum hydrocarbon gas concentration profile may involve three or four phases including: (a) an initial high hydrocarbon concentration phase; (b) a moderate hydrocarbon concentration phase; (c) a lower hydrocarbon concentration phase; and (d) zero or nil hydrocarbon concentration phase.
  • the most preferred profile would be a continuously varying hydrocarbon concentration, beginning with the high concentration phase and ending with the low or zero concentration phase. This would provide the closest match to the diffusive flux and minimize carburization time.
  • the initial high hydrocarbon phase is intended or adapted to bring the carbon content at the surface of the part rapidly up to at or near the constraint level (i.e. 1.2% C, or as high as practical so as not to form carbides on the surface of the metal part).
  • the moderate hydrocarbon concentration phase is intended or adapted to maintain the carbon content at the surface at the desired percent (e.g. about 1.2% C) while the carbon atoms diffuse into the body of the metal part. Put another way, the moderate hydrocarbon concentration phase attempts to match the rate of carbon delivery to the surface of the metal part to the rate of carbon diffusion into the body of the metal part.
  • the lower hydrocarbon concentration phase is intended or adapted to allow the carbon content at the surface of the metal part to decline to a lower level, for example where the process conditions or carburization requirements call for a surface carbon content of less than 1.2% C.
  • the zero or nil hydrocarbon concentration phase is intended or adapted to allow the carbon profile to reach the desired end state. This zero or nil phase may occur near the end of the carburization process to remove any trace of free carbon from the surface of the treated metal part or may also be used during the cooling or quenching operations.
  • a minimum concentration of hydrocarbon gas in the furnace atmosphere has been found to be needed to achieve reasonable rates of carbon flux on the metal parts to be treated. Some of the hydrocarbon gas feed is consumed by reacting with oxidizing species that are continuously introduced into the furnace through leaks, door openings. This minimum concentration of hydrocarbon gas varies depending on rates of impurities in the furnace, but is likely in the range of 0.1% to 2.0%, and more probably in the range of 0.5% to 1.5%. The level is easily determined by shim tests when the model is calibrated for a given furnace or with an oxygen probe.
  • a maximum hydrocarbon concentration may be set due to practical flow rate limitations of the hardware utilized, economic considerations, or more preferably safety considerations.
  • the lower explosive limit (LEL) for propylene in air is about 2.4%, and the minimum oxygen concentration required for flammability is about 5.0%. If the carburizing atmosphere contains propylene at about 3.15% or less, then no mixture of furnace atmosphere and ambient air (i.e. outside the furnace) would produce a propylene concentration above the LEL coincident with an oxygen concentration above the minimum required for flammability. Therefore from a safety aspect, the preferred embodiment would set a maximum propylene concentration of 3.15% to ensure a non-flammable atmosphere during operation. Broadly speaking, hydrocarbon concentrations in the range of 0% to about 10% are recommended, more preferably in the range of about 1% to about 5%.
  • Figure 3 shows a graph depicting the model hydrocarbon concentrations 90 as a function of time based on the outputs of the model compared with the actual hydrocarbon concentrations 92 employing the practical or prescribed threshold minimum concentrations 94 and maximum concentrations 96 as discussed herein.
  • This example involved the carburization of a piece of %" x 2" SAE 9310 steel alloy rod to a desired effective case depth of about 0.050 inches using the oxygen-free carburization model.
  • a nitrogen atmosphere containing 0.6% propylene and 5.0% hydrogen was equilibrated in a laboratory tube furnace atmosphere at about 1700 degrees F until the O 2 concentration stabilized.
  • the carbon uptake in the tube furnace was empirically determined to be about 1.0% based on a SAE 1010 shim sample (0.001" thick) placed in the furnace and contacted with the oxygen- free gas atmosphere for about 15 minutes.
  • This example involved the carburization of a SAE 4140 steel alloy sample to a desired effective case depth of about 0.047 inches in an operating atmosphere pressure carburization furnace using the oxygen-free carburization model.
  • a nitrogen, propylene and hydrogen gas atmosphere was equilibrated in an operating atmosphere pressure carburization furnace at about 1700 degrees F until the O 2 concentration stabilized.
  • the relevant process inputs were entered into the oxygen-free carburization model and a desired hydrocarbon gas concentration profile as a function of time was established to achieve the desired effective case depth.
  • the upper hydrocarbon limit was constrained by the hydrocarbon injection equipment.
  • the hydrocarbon gas concentration profile 99 used is depicted in Figure 5. Hydrogen gas concentration during the carburization process was maintained between about 1.19 % and about 2.99%.
  • the sample was placed in a loading vestibule and purged with nitrogen to remove residual oxygen. The sample was then introduced to the furnace atmosphere at about 1700 degrees F and allowed to come to temperature. The propylene gas flow was then introduced according to the predicted hydrocarbon gas profile 99.
  • the samples were placed in a loading vestibule and purged with nitrogen to remove residual oxygen.
  • the samples were then introduced to the furnace atmosphere at about 1700 degrees F and allowed to come to temperature.
  • the propylene gas flow and hydrogen gas flows were then introduced into the furnace.
  • the hydrocarbon gas concentration profile was kept constant at about 1.7% throughout the 4 hour oxygen-free carburization treatment. Hydrogen gas concentration during the carburization process was also maintained at a constant concentration of about 5.0%.
  • the hydrogen and propylene were turned off, the samples were removed from the furnace and quenched.
  • Example 3 Analysis of the sample in Example 3 showed an effective case depth of about 0.034 inches, no visible sooting, and an IGO level of less than or equal to 0.0001 inches (See Table 2). Analysis of the sample in Example 4 showed an effective case depth of about 0.034 inches, no visible sooting, and no detectable level of IGO (See Table 2). These results compare beneficially with the expected IGO level of about 0.0004 inches in a conventional endo-gas atmosphere and a case depth of 0.035 inches.
  • This example involved the carburizing of a SAE 1010 steel alloy sample (1/8" thick x 2") to a desired effective case depth of about 0.030 inches in a laboratory tube furnace using the oxygen-free carburization model.
  • a nitrogen atmosphere containing 0.3% ethane and 5.0% hydrogen was equilibrated in an operating atmosphere pressure carburization furnace at about 1700 degrees F until the O 2 concentration stabilized.
  • the presently disclosed oxygen-free carburization process uses a precisely controlled, low- oxygen, nitrogen-based, carbonaceous atmosphere in an atmosphere pressure furnace to minimize the formation of soot, unwanted carbides, and inter-granular oxides, and avoid hydrogen embrittlement.
  • Utilization of the above-described carburization process provides a high-quality and cost-effective process for the carburization of many steel alloys that offers many features and advantages when compared to commercially available prior art carburization processes and systems.
  • the present carburization process employs a substantially oxygen-free furnace atmosphere with a prescribed minimum concentration of hydrogen gas to reduce oxidation potential with trace metals in most steel alloys and to minimize the formation of inter-granular oxides (IGO).
  • IGO inter-granular oxides
  • Hydrogen gas is also used as the energizer to promote dissociation of hydrocarbon fuel near and on the metal surface.
  • the hydrogen gas concentration is preferably kept low to avoid hydrogen embrittlement and to improve operational safety of the carburization process by eliminating the possibility of gas explosion with low oxygen and low hydrogen concentrations.
  • the hydrocarbon concentrations within the nitrogen-based furnace atmosphere are continuously varied to match the carbon-iron diffusion flux within the treated part for optimized carburization.
  • the required fuel concentration to provide proper carbon flux is estimated using a flux/diffusion model together with empirically derived data obtained during calibration and operation of the oxygen-free carburization process.
  • the disclosed oxygen-free carburization process advantageously provides a means or technique to optimize speed of carburization through control of carbon flux.
  • the hydrocarbon gas could comprise propylene, propane, methane or other suitable hydrocarbon.
  • helium or argon gas may be used as a substitute for the nitrogen gas and dissociated ammonia may be used as a source of nitrogen gas or hydrogen gas.
  • the heat treating process can be performed in any atmospheric pressure furnace including, for example, continuous type furnaces, batch furnaces, or other furnace types (e.g. rotary retort, humpback, bell, annealing, pit, etc.) and the furnace may have a single or multiple control zones.
  • the present invention thus provides a method of oxygen-free carburization in an atmospheric pressure furnace. While the invention herein disclosed has been described by means of specific embodiments and processes associated therewith, numerous modifications and variations can be made thereto by those skilled in the art without departing from the scope of the invention as set forth in the claims or sacrificing all its material advantages. For example, similar such methods could be employed for nitrocarburizing or ferritic nitrocarburizing by including an additional source of nitrogen such as ammonia.
  • the present process can be adapted for use with other heat treating processes such as neutral hardening, ferritizing, annealing, normalizing, spheroidizing, tempering, sintering and sinter hardening.

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Abstract

L'invention concerne un procédé de contrôle d'un processus de traitement thermique sans oxygène dans un four à pression atmosphérique. Le présent procédé utilise une atmosphère gazeuse sans oxygène comprenant du gaz hydrogène en des concentrations comprises entre environ 1,0 pour cent et 10,0 pour cent, un hydrocarbure gazeux, tel que du propylène, en des concentrations comprises entre environ 0,1 pour cent et 10,0 pour cent qui changent en fonction du temps, l'équilibre de l'atmosphère gazeuse étant de l'azote. Le présent procédé de carburation sans oxygène utilise une atmosphère gazeuse contrôlée avec précision pour minimiser l'oxydation intergranulaire, pour éliminer la formation de suie et de cémentite, et pour éviter la fragilisation d'hydrogène.
PCT/US2007/088352 2006-12-26 2007-12-20 Procédé d'optimisation d'un processus de traitement thermique sans oxygène WO2008083031A1 (fr)

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US9109277B2 (en) 2011-01-10 2015-08-18 Air Products And Chemicals, Inc. Method and apparatus for heat treating a metal
JP6162077B2 (ja) * 2014-06-11 2017-07-12 日本碍子株式会社 熱処理炉
CN104846171A (zh) * 2015-06-04 2015-08-19 马钢(集团)控股有限公司 一种电工钢退火还原用氢等离子发生装置及其还原方法
CN106011731B (zh) * 2015-12-10 2018-07-10 贵州西南工具(集团)有限公司 一种不锈钢零件的气体软氮化工艺
JP6583600B1 (ja) * 2018-03-22 2019-10-02 日本製鉄株式会社 真空浸炭処理方法及び浸炭部品の製造方法
CN114341392B (zh) * 2019-08-29 2024-03-26 日本制铁株式会社 真空渗碳处理方法和渗碳部件的制造方法
CN110923420A (zh) * 2019-12-17 2020-03-27 常熟市恒丰机械制造有限公司 一种导纱器的生产工艺
CN113774207B (zh) * 2021-08-31 2022-11-11 北京科技大学设计研究院有限公司 一种辐射管热处理炉炉内保护性气氛控制的方法

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