US2231010A - Heat treating process - Google Patents

Heat treating process Download PDF

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Publication number
US2231010A
US2231010A US336178A US33617840A US2231010A US 2231010 A US2231010 A US 2231010A US 336178 A US336178 A US 336178A US 33617840 A US33617840 A US 33617840A US 2231010 A US2231010 A US 2231010A
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Prior art keywords
bath
activity
atmosphere
case hardening
oxidizing
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US336178A
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Donald A Holt
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EIDP Inc
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EI Du Pont de Nemours and Co
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Priority claimed from US192396A external-priority patent/US2231009A/en
Application filed by EI Du Pont de Nemours and Co filed Critical EI Du Pont de Nemours and Co
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/40—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 liquids, e.g. salt baths, liquid suspensions
    • C23C8/42—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 liquids, e.g. salt baths, liquid suspensions only one element being applied
    • C23C8/44—Carburising
    • C23C8/46—Carburising 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
    • C21D1/34—Methods of heating
    • C21D1/44—Methods of heating in heat-treatment baths
    • C21D1/46—Salt baths

Definitions

  • Heat treating operations for articles of various metals are often carried out by immersing the metal articles to be treated in a fused salt bath.
  • This method of heat treatment is advantageous because of the rapid transfer of heat from the bath to the metal articles, thus providing a rapid process Well adapted to large scale production.
  • Heat treating fused salt baths which contain no carburizing agent, for example alkali metal cyanide or other cyanogen compound, are known to have a more or less decarburizing effect upon steel surfaces, which effect has been overcome heretofore by introducing a carburizing material into the bath.
  • the carburi- Zation which occurs more or less compensates for the decarburizing effect of the bath.
  • This method of controlling the carburizing activity of baths containing a cyanogen compound is not particularly reliable or satisfactory.
  • a further object is to provide a method of operating fused salt baths containing a cyanogen compound whereby the case hardening activity of the bath may be inhibited or controlled as desired. Still further objects will be apparent from the ensuing description of the invention.
  • a fused salt bath which contains a cyanogen compound, such as sodium cyanide, may be rendered substantially inert as regards carburizing or decarburiz'ing activity, by maintaining the bath in an atmosphere of an oxygen-free, non-oxidizing gas, as described and claimed in my copending application Serial No. 192,396, filed February 24, 1938.
  • case hardening activity of such a bath increases as the amount of oxidizing constituent in the gaseous atmosphere maintained over the bath is increased, and that by varying the amount of oxidizing constituent in the gaseous atmosphere, the case hardening activity of the bath may be varied accordingly.
  • the fused case hardening baths to be used in practicing the present invention must contain a carburizing material such as an alkali metal cyanide.
  • a carburizing material such as an alkali metal cyanide.
  • Alkaline earth metal cyanides and cyanamides, as well as other cyanogen compounds, such as polymers of hydrocyanic acid, cyanamide, dicyandiamide and its polymers, may be used as carburizing bath ingredients.
  • the amount of carburizing material present in the bath may be varied over a wide range.
  • the bath may contain 0.5'to 40% by weight of sodium cyanide, or if desired, a bath of substantially pure sodium cyanide may be employed with good results.
  • the case hardening baths for use in practicing my invention preferably contain as the chief bath constituents a cyanogen compound and an alkali metal salt, an alkaline earth metal salt, or mixtures thereof.
  • a cyanogen compound and an alkali metal salt, an alkaline earth metal salt, or mixtures thereof.
  • the methods of mixing and fusing the salt ingredients of the bath, as well as the proportions used to obtain baths having melting points adapted for use at the desired temperature of operation will be known to those experienced in the preparation and use of fused salt baths for heat treating metals and need not be discussed here.
  • Sodium cyanide 15-30% by weight Remainder, sodium carbonate.
  • the bath during operation. If an entirely inert or non-oxidizing atmosphere is employed, the bath will be substantially neutral, i. e., it will exert no substantial carburizing or decarburizing activity. Illustrations of non-oxidizng gases which may be employed to render the bath inert are hydrogen, nitrogen, gaseous hydrocarbons; the rare gases such as argon, krypton, and helium; and ammonia. When it is desired that the bath have a definite carburizing action, a suitable amount of an oxidizing substance is added to such an inert atmosphere.
  • the carburizing activity of the bath may be increased by increasing the amount of the oxidizing material present in the gaseous atmosphere.
  • oxygen is the oxidizing material which is added to the gaseous atmosphere above the fused salt bath.
  • other oxidizing substances may be employed in accordance with my invention in place of oxygen.
  • gases such as carbon dioxide and water vapor are to be considered as oxidizing gases since their action upon the activity of a salt bath when they are added to the atmosphere above'such a bath is, generally speaking, the same as that produced when oxygen is added.
  • the carburizing bath may be provided with a hood or mufile which is filled with the gaseous atmosphere containing the desired amount of an oxidizing substance.
  • a convenient way of maintaning an atmosphere of suitable composition in the muflie is to pass continuously a mixture containing suitable proportions of an inert gas and an oxidizing gas into the aperture of the muflie through 'which the articles to be treated are inserted or removed from the bath.
  • the gas mixture should be passed into the hooded space at such a rate that it streams out of the aperture (or 60 similar openings in the muflie) through which work pieces are inserted or removed, thereby preventing entrance into the hooded space of amounts of air such as would be sufiicient'to change substantially the composition of the gaseous atmosphere over the bath.
  • suitable devices may be employed for passing the metal work into and out of the fused salt bath so that the treatment may be carried out in a continuous manner.
  • the steel strip may be passed continuously through a fused salt bath, by methods which are well known in the art, so as to insure that a given portion of the strip remains immersed in the baththe required length of time to efiect the desired case hardening.
  • the carburizing activity of the bath is controlled as desired by maintaining over the bath a gaseous atmosphere containing a predetermined suitable proportion of oxygen or other suitable oxidizing gas.
  • my invention may be practiced to effectively control the case hardening activity of case hardening baths in which cyanide or other cyanogen compound is used as the case hardening ingredient.
  • the case hardening activity of the bath may be varied accordingly.
  • a cyanide case hardening bath may be operated under an atmosphere of nitrogen or other nonoxidizing gas and a suitable proportion of oxygen or other oxidizing gas may be mixed with the nitrogen.
  • the atmosphere above the bath may for example consist of a nitrogen-oxygen mixture containing an oxygen concentration below that of the air, e. g., 1-5% by volume of oxygen.
  • the covering atmosphere may, for example, consist of air to which oxygen has been added.
  • My invention is not restricted to any particular temperature of heat treatment, but is generally applicable to the various heat treating temperatures commonly utilized for fused salt bath metal heat treating operations, e. g., 450-.
  • a method for controlling the activity of a fused case hardening bath. which contains a cyanogen compound comprising maintaining said bath under an atmosphere of a gas containing a predetermined amount of an oxidizing gas.
  • a method for controlling the activity of a fused case hardening bath which contains an alkali metal cyanide comprising maintaining said bath under an atmosphere of a gas containing a predetermined amount of oxygen.
  • a method for con-trolling the activity of a fused case hardening bath which contains an alkali metal cyanide comprising maintaining said bath under an atmosphere of a gas mixture composed of a non-oxidizing gas and less than 20% by volume of oxygen.
  • a method for controlling the activity of a fused case hardening bath which contains an alkali metal cyanide comprising maintaining said bath under an atmosphere of a gas mixture composed of a non-oxidizing gas and 1-5% by volume of oxygen.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Coating With Molten Metal (AREA)

Description

Patented Feb. 11, 194i PATENT 2,231,010 nsa'r 'rnnarrno rnocEss Donald A. Holt, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Original application February 24,
1938, Serial N0. 192,396.
Divided and this application May 20, 1940, Serial No. $36,178
t Claims.
This application is a division of my copending patent application Serial No. 192,396, filed February 24, 1938.
Heat treating operations for articles of various metals are often carried out by immersing the metal articles to be treated in a fused salt bath. This method of heat treatment is advantageous because of the rapid transfer of heat from the bath to the metal articles, thus providing a rapid process Well adapted to large scale production. Heat treating fused salt baths which contain no carburizing agent, for example alkali metal cyanide or other cyanogen compound, are known to have a more or less decarburizing effect upon steel surfaces, which effect has been overcome heretofore by introducing a carburizing material into the bath. By this method the carburi- Zation which occurs more or less compensates for the decarburizing effect of the bath. This method of controlling the carburizing activity of baths containing a cyanogen compound is not particularly reliable or satisfactory. Moreover, it is often desired to employ baths which will exert a substantial and controllable carburizing action upon steel articles immersed therein. So far as I am aware, there has been no really satisfactory method available for practically controlling the carburizing activity of fused salt baths which contain a cyanogen compound.
It is an object of the present invention to provide a new and improved method for controlling the activity of fused salt case hardening baths which contain a carburizing material. A further object is to provide a method of operating fused salt baths containing a cyanogen compound whereby the case hardening activity of the bath may be inhibited or controlled as desired. Still further objects will be apparent from the ensuing description of the invention.
The above objects are accomplished in accordance with my invention by heating metal articles in a fused salt bath containing a cyanogen compound, while maintaining said bath in an atmosphere of a gas containing a predetermined amount of an oxidizing constituent. I have discovered that a fused salt bath which contains a cyanogen compound, such as sodium cyanide, may be rendered substantially inert as regards carburizing or decarburiz'ing activity, by maintaining the bath in an atmosphere of an oxygen-free, non-oxidizing gas, as described and claimed in my copending application Serial No. 192,396, filed February 24, 1938. I have also discovered that the case hardening activity of such a bath increases as the amount of oxidizing constituent in the gaseous atmosphere maintained over the bath is increased, and that by varying the amount of oxidizing constituent in the gaseous atmosphere, the case hardening activity of the bath may be varied accordingly.
The fused case hardening baths to be used in practicing the present invention of course must contain a carburizing material such as an alkali metal cyanide. Alkaline earth metal cyanides and cyanamides, as well as other cyanogen compounds, such as polymers of hydrocyanic acid, cyanamide, dicyandiamide and its polymers, may be used as carburizing bath ingredients. The amount of carburizing material present in the bath may be varied over a wide range. Thus, the bath may contain 0.5'to 40% by weight of sodium cyanide, or if desired, a bath of substantially pure sodium cyanide may be employed with good results.
The case hardening baths for use in practicing my invention preferably contain as the chief bath constituents a cyanogen compound and an alkali metal salt, an alkaline earth metal salt, or mixtures thereof. In particular, I prefer to use baths containing sodium cyanide together with alkali metal and alkaline earth metal halides and carbonates, or mxtures thereof. The methods of mixing and fusing the salt ingredients of the bath, as well as the proportions used to obtain baths having melting points adapted for use at the desired temperature of operation, will be known to those experienced in the preparation and use of fused salt baths for heat treating metals and need not be discussed here.
Specific examples of preferred fused salt baths for practicin my invention are:
Bath NO. 1
Sodium cyanide: 15-30% by weight Remainder, sodium carbonate.
Bath N0. 2
Sodium cyanide, 220% by weight Remainder, equal parts of sodium carbonate and potassium carbonate.
Bath N0. 3
Sodium cyanide: 220% by weight Remainder, the mixture:
Per cent by weight Barium chloride 50 Potassium chloride 30 Sodium chloride 20 Bath N0. 4
bath during operation. If an entirely inert or non-oxidizing atmosphere is employed, the bath will be substantially neutral, i. e., it will exert no substantial carburizing or decarburizing activity. Illustrations of non-oxidizng gases which may be employed to render the bath inert are hydrogen, nitrogen, gaseous hydrocarbons; the rare gases such as argon, krypton, and helium; and ammonia. When it is desired that the bath have a definite carburizing action, a suitable amount of an oxidizing substance is added to such an inert atmosphere. When there is present in the gaseous atmosphere over the bath a relatively small amount of an oxidizing, constituent, a definite but small carburizing activity will be exerted by the bath; the carburizing activity of the bath may be increased by increasing the amount of the oxidizing material present in the gaseous atmosphere.
I prefer to employ oxygen as the oxidizing material which is added to the gaseous atmosphere above the fused salt bath. However, other oxidizing substances may be employed in accordance with my invention in place of oxygen. For the purpose of the present invention, gases such as carbon dioxide and water vapor are to be considered as oxidizing gases since their action upon the activity of a salt bath when they are added to the atmosphere above'such a bath is, generally speaking, the same as that produced when oxygen is added.
Care should be exercised in choosing the gases to be used in making up the gaseous atmosphere having a predetermined amount of an oxidizing constituent. Thus, mixtures of oxygen with hydrogen, carbon monoxide or ammonia should be avoided because of the explosive hazards involved. I prefer to employ mixtures of nitrogen with suitable amounts of oxygen as gaseous coverings for the fused salt baths when operated in accordance with the present invention.
Methods for maintaining an atmosphere having a predetermined oxidizing value over the case hardening bath will be obvious to those who are familiar with the art of heat treating metals and need not be described in detail here. It will be sufficient to state that the carburizing bath may be provided with a hood or mufile which is filled with the gaseous atmosphere containing the desired amount of an oxidizing substance. A convenient way of maintaning an atmosphere of suitable composition in the muflie is to pass continuously a mixture containing suitable proportions of an inert gas and an oxidizing gas into the aperture of the muflie through 'which the articles to be treated are inserted or removed from the bath. The gas mixture should be passed into the hooded space at such a rate that it streams out of the aperture (or 60 similar openings in the muflie) through which work pieces are inserted or removed, thereby preventing entrance into the hooded space of amounts of air such as would be sufiicient'to change substantially the composition of the gaseous atmosphere over the bath. If desired, suitable devices may be employed for passing the metal work into and out of the fused salt bath so that the treatment may be carried out in a continuous manner. Thus when the case hardening of steel strip is involved, the steel strip may be passed continuously through a fused salt bath, by methods which are well known in the art, so as to insure that a given portion of the strip remains immersed in the baththe required length of time to efiect the desired case hardening. During operation, the carburizing activity of the bath is controlled as desired by maintaining over the bath a gaseous atmosphere containing a predetermined suitable proportion of oxygen or other suitable oxidizing gas.
In general, my invention may be practiced to effectively control the case hardening activity of case hardening baths in which cyanide or other cyanogen compound is used as the case hardening ingredient. By varying the proportion of oxidizing gas in the atmosphere above the case hardening bath, the case hardening activity of the bath may be varied accordingly. For example, a cyanide case hardening bath may be operated under an atmosphere of nitrogen or other nonoxidizing gas and a suitable proportion of oxygen or other oxidizing gas may be mixed with the nitrogen. To decrease the activity of the bath, the atmosphere above the bath may for example consist of a nitrogen-oxygen mixture containing an oxygen concentration below that of the air, e. g., 1-5% by volume of oxygen. To increase the activity of the bath, the covering atmosphere may, for example, consist of air to which oxygen has been added. This method of control is generally applicable to case hardening baths containing cyanogen compounds and is not restricted to baths activated with cyanide.
My invention is not restricted to any particular temperature of heat treatment, but is generally applicable to the various heat treating temperatures commonly utilized for fused salt bath metal heat treating operations, e. g., 450-.
I claim:
1. A method for controlling the activity of a fused case hardening bath. which contains a cyanogen compound comprising maintaining said bath under an atmosphere of a gas containing a predetermined amount of an oxidizing gas.
2. A method for controlling the activity of a fused case hardening bath which contains an alkali metal cyanide comprising maintaining said bath under an atmosphere of a gas containing a predetermined amount of oxygen.
3. A method for con-trolling the activity of a fused case hardening bath which contains an alkali metal cyanide comprising maintaining said bath under an atmosphere of a gas mixture composed of a non-oxidizing gas and less than 20% by volume of oxygen.
4. A method for controlling the activity of a fused case hardening bath which contains an alkali metal cyanide comprising maintaining said bath under an atmosphere of a gas mixture composed of a non-oxidizing gas and 1-5% by volume of oxygen.
DONALD A. HOLT.
US336178A 1938-02-24 1940-05-20 Heat treating process Expired - Lifetime US2231010A (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2568860A (en) * 1948-01-22 1951-09-25 Du Pont Process for the carburization of ferrous metals
US3203386A (en) * 1962-06-06 1965-08-31 Birch Brothers Inc Tacking aligner construction
US4019928A (en) * 1973-03-05 1977-04-26 Duetsche Gold- Und Silber-Scheideanstalt Vormals Roessler Process for nitriding iron and steel in salt baths regenerated with triazine polymers
EP0072525A1 (en) * 1981-08-10 1983-02-23 Kabushiki Kaisha Toyota Chuo Kenkyusho Indirect heating furnace for the surface treatment of a metal or the like employing a salt bath

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2568860A (en) * 1948-01-22 1951-09-25 Du Pont Process for the carburization of ferrous metals
US3203386A (en) * 1962-06-06 1965-08-31 Birch Brothers Inc Tacking aligner construction
US4019928A (en) * 1973-03-05 1977-04-26 Duetsche Gold- Und Silber-Scheideanstalt Vormals Roessler Process for nitriding iron and steel in salt baths regenerated with triazine polymers
EP0072525A1 (en) * 1981-08-10 1983-02-23 Kabushiki Kaisha Toyota Chuo Kenkyusho Indirect heating furnace for the surface treatment of a metal or the like employing a salt bath

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