US3671297A - Method of chromizing in a fused salt bath - Google Patents

Method of chromizing in a fused salt bath Download PDF

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Publication number
US3671297A
US3671297A US120155A US3671297DA US3671297A US 3671297 A US3671297 A US 3671297A US 120155 A US120155 A US 120155A US 3671297D A US3671297D A US 3671297DA US 3671297 A US3671297 A US 3671297A
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chromium
bath
salt bath
fused salt
metal
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US120155A
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Noboru Komatsu
Tohur Arai
Masayoshi Mizutani
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Toyota Central R&D Labs Inc
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Toyota Central R&D Labs Inc
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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
    • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
    • C23C10/18—Solid state diffusion of only metal elements or silicon into metallic material surfaces using liquids, e.g. salt baths, liquid suspensions
    • C23C10/20—Solid state diffusion of only metal elements or silicon into metallic material surfaces using liquids, e.g. salt baths, liquid suspensions only one element being diffused
    • C23C10/24—Salt bath containing the element to be diffused

Definitions

  • 117-113 18 Claims ABSTRACT OF THE DISCLOSURE A method of treating metals and alloys in which a fused salt bath consisting of at least one boron-containing salt such as boron oxide (B203) or borax (Na2B4O7), and a chromium-containing material is prepared and a material to be treated is immersed in the fused salt bath as a result of which a chromium-impregnated layer with line structure and with excellent resistance to corrosion, oxidation and Wear is formed at the surface of the metal.
  • boron-containing salt such as boron oxide (B203) or borax (Na2B4O7)
  • the conventional techniques for coating or diffusionimpregnation of chromium on a metal surface are electrochemical plating, pack cementation and vapor-phase plating.
  • the corrosion resistance of deposits prepared by electrochemical plating is unsatisfactory owing to the presence of pin holes; moreover adhesion between the plated coating and the substrate is inadequate due to the lack of a diusion layer between the coating and the substrate, so that the coating is apt to peel.
  • This invention relates to the diifusion-impregnation of chromium into the surface of a substrate by dipping and maintaining the material to be treated within a salt bath comprising at least one boron-containing salt and a chromium-containing metal or alloy.
  • This type of bath is non-corrosive to the vessel, and the impregnation can be carried out in air.
  • the surface layer produced is structurally fine rather than porous and has excellent resistance to corrosion, oxidation and wear.
  • the boron oxide or borate forms a fused bath at relatively low temperature in which the chromium dissolves; also the boron oxide or borate dissolves any oxide on the surfaces of the substrate or the vessel so that the surfaces remain clean.
  • the method of the present invention may be carried out in an inert atmosphere.
  • a primary object of the present invention is to provide a method for forming a chromium-impreg nated layer with excellent resistance to corrosion, oxidation and wear and with ne structure in the surface of metals and alloys.
  • Another object of the present invention is to provide a. method for forming a chromium-impregnated layer of sufficient thickness in the surface of metals and alloys, which does not require complex apparatus and which can be carried out in air.
  • a further object of the present invention is to provide an economic, simple method for forming a chromium-impregnated layer in the sur-face of metals and alloys which is suitable for mass production.
  • Yet another object of the present invention is to provide a method for forming a chromium-impregnated layer in the surface of metals and alloys in a fused salt bath which does not corrode the vessel containing the bath.
  • the invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which will be exemplified in the method hereinafter disclosed, and the scope of the invention will be indicated in the claims.
  • FIG. 1 is a photomicrograph of chromized pure iron
  • FIG. 2 is a photomicrograph of chromized JIS S10C (structural carbon steel);
  • FIG. 3 is a photomicrograph of chromized IIS 845C (structural carbon steel).
  • FIG. 4 is a photomicrograph of chromized JIS SKZ (carbon tool steel).
  • FIG. 5 is a photomicrograph of chromized JIS FCZS (grey cast iron).
  • the method of the present invention is carried out by immersing a 'specimen in a fused bath which includes at least one compound containing baron-containing salt and a source of chromium metal.
  • a fused bath which includes at least one compound containing baron-containing salt and a source of chromium metal.
  • boron-containing salt boron oxide, -or borates of the alkali metals such as lithium borate, sodium borate, potassium borate or mixtures thereof may be employed.
  • metallic chromium metallic chromium may of course, be used; another 'source is ferro-chromium (Fe-Cr). It is economically advantageous to use the ferro-chromium compound.
  • At least about one percent of the chromium source must be present. In general the optimum amount is about 5% or more. As the amount of chromium added is increased, the viscosity of the salt bath mounts until it is so high that dipping of the specimen becomes dilicult so that a suitably impregnated layer of uniform thickness cannot be obtained. In view of this limitation it is undesirable in practice to add more than 60%.
  • Either stainless steel or graphite may be used for the vessel containing the bath.
  • a very small amount of the chromium may be dissolved out during the operation; however, the chromium removed in this way eventually diffuses into the substrate specimen without causing any damage.
  • the loss of chromium from the stainless steel vessel by this process is negligible and stainless steel vessels are suitable for industrial use.
  • the temperature of operation ranges from that at which the salt bath composition melts to the melting point of the substrate.
  • the temperature of treatment ranges from 700 C. to 11100 C. depending on the rate of formation of the chromized layer, the desired thiokness of the layer, and the material of which the vessel is made.
  • materials are added to the bath in order to lower its melting point and thus prevent the strains resulting from the transformation.
  • the melting point of borax for example, is nearly 740 C.; where a steel having a .transformation point lower than 740 C. is to be treated,
  • alkali halides such as NaCl, KCl, and NaF, or oxides such P205, or hydroxides such as NaOH and KOH or sulfates,
  • the time of treatment depends upon the thickness of the chromized layer desired.
  • the minimum time of treatment is about one hou-r, the actual time depending on the temperature of the treatment.
  • the thickness of the impregnated layer increases as the time of treatment increases, in general it is not necessary to prolong the treatment beyond 30 hours, since the desired thickness can usually be achieved within this period.
  • EXAMPLE 1 A bath consisting of 300 grams of borax (67%) and 150 grams of ferro-chromium (67% chromium) in a graphite crucible was heated to 1000" C. A specimen of pure iron, 14 mm. in diameter and 10 ⁇ mm. in length was immersed for eight hours in this salt bath, removed, air cooled and immersed for 2 hours in boiling water to remove residual traces of the fused salt bath. Specimens of other types of steel were also prepared. The data are given in Table 1.
  • impregnated layers were formed on all of the specimens, and in all cases the layers were non-porous and the surfaces were smooth.
  • EXAMPLE 2 500 grams of a mixed powder consisting of 95% borax and 5% ferro-chromium (the ferro-chromium containing 67% of chromium was used in all the examples) were heated to 1l00 C. in a stainless steel vessel and a specimen 8K2 was immersed for six hours in the fused salt bath, removed, air cooled, and immersed in boiling water to remove the bath residue. Microphotography showed that the impregnated layer was about 20 microns thick. There was no visible corrosion on the inner surface of the stainless lsteel vessel. It follows that satisfactory impregnation with chromium can be achieved with a ferro-chromium content of 5% in a bath and that a stainless steel vessel is satisfactory for industrial use.
  • EXAMPLE 3 500 grams of a mixed powder consisting of anhydrous boric acid and 20% ferro-chromium were heated to 1000 C. in a graphite crucible. A specimen of SK2 was immersed for 8 hours therein, removed and immersed in boiling water to remove bath residue. Examination by microscope showed that a chormium-impregnated layer about 15 microns thick had been produced.
  • EXAMPLE 5 A bath consisting of 70% borax and 30% ferro-chromium was prepared in a graphite crucible. Specimens of tungsten, molybdenum, tantalum, niobium, nickel and cobalt measuring 5 mm. in diameter and 10 mm. in length were dipped therein. For the tungsten, molybdenum, tantalum and niobium, the bath temperature was 1000 C. and the time of immersion was 8 hours. For the nickel and the cobalt, the bath temperature was 900 C. and the time of immersion was 8 hours. After immersion and removal, the bath residue was removed by boiling water.
  • EXAMPLE 6 500 grams of a mixed powder containing 70% borax and 30% ferrochromium were brought to l000 C. in a graphite crucible. Two specimens each of SlOC and S45C were dipped into the crucible for 8 hours, removed and air cooled and cleaned of bath residue with boiling water.
  • Oxidation resistance of the treated specimens was compared with that of untreated specimens of SlOC, S45C and SUS27 (stainless steel) of the same size and shape as the treated specimens, where all were maintained for two hours in a furnace heated to 700 C.
  • EXAMPLE 7 500 grams of a mixed powder of 70% borax and 30% ferro-chromium were heated to 900 C. in a graphite crucible. A specimen of SKD l (alloy tool steel) of mm. thickness, 15 mm. width and 70 mm. length was immersed for 24 hours in the fused salt bath, removed, air cooled, and cleaned of bath residue by boiling water. Microscopic observation of a cross section showed an impregnated layer 25 microns thick, and X-ray analysis showed Carbides of M23C6 and M7C3 types.
  • the treated specimen of SKDl and a control specimen of SKDl having the same size and shape were subjected t0 a wearing test of Ogoshi type where the control specimen had been quenched and tempered in a salt bath furnace.
  • normalized 545C ⁇ was used as the opposing sliding material, at a sliding distance of 600 m., under a load of 3.3 kg. and without lubrication.
  • the amount worn from the treated specimen of SKDl was from one half to one fifth that of the comparative sample as is shown in Table 4.
  • the method of the present invention wherein the surfaces of both ferrous and non-ferrous metals are treated by dipping and holding in a fused salt bath containing chromium and boron oxide or borate are remarkably improved with respect to oxidation and corrosion resistance and with respect to wear resistance. Moreover both the intensity and the depth of chromizing can be controlled by choice of the operating temperature and time of immersion.
  • the method is broadly applicable for treatment of machine parts where resistance to corrosion, oxidation and Wear resistance is required.
  • a method of chromizing a metal work-piece comprising the steps of preparing a fused salt bath consisting essentially of at least one boron-containing salt and a chromium-containing metal and immersing said metal work-piece in said fused salt bath until a diffusion layer containing chromium is formed on the surface of the work-piece.
  • boroncontaining salt is selected from the group consisting of B203 and borates.
  • borates are selected from the group consisting of L2B4O7, N32B407 and K2B407.
  • said fused salt bath is formed by heating and fusing at least one boron-containing salt and adding thereto one chromiumcontaining metal.
  • ferrous material is selected from the group consisting of iron, structural carbon steel, carbon tool steel, grey cast iron, stainless steel and alloy steel.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Treatment Of Metals (AREA)
  • Chemically Coating (AREA)
  • Other Surface Treatments For Metallic Materials (AREA)
US120155A 1970-03-06 1971-03-02 Method of chromizing in a fused salt bath Expired - Lifetime US3671297A (en)

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3833968A (en) * 1971-03-09 1974-09-10 Toyoda Chuo Kenkyusho Kk Metallic card clothing
US3885064A (en) * 1973-04-12 1975-05-20 Toyoda Chuo Kenkyusho Kk Method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article
US3912827A (en) * 1973-11-13 1975-10-14 Toyota Chuo Kenkyusko Kk Method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article
US3959092A (en) * 1972-11-16 1976-05-25 Kabushiki Kaisha Toyota Chuo Kenkyusho Method for a surface treatment of cemented carbide article
US4158578A (en) * 1977-05-09 1979-06-19 Kabushiki Kaisha Toyota Chuo Kenkyusho Method for forming a carbide layer of a Va-Group element of the periodic table or chromium on the surface of a ferrous alloy article
US4202705A (en) * 1977-06-30 1980-05-13 Kabushiki Kaisha Toyoto Chuo Kenkyusho Treating bath, forming a mixed carbide layer of Va-Group elements on a ferrous alloy surface and resulting product
US4230751A (en) * 1977-08-11 1980-10-28 Kabushiki Kaisha Toyota Treating composition, forming a mixed-carbide layer of Va-Group elements and of chromium on a ferrous-alloy surface and resulting product
US4250208A (en) * 1978-06-19 1981-02-10 Kabushiki Kaisha Toyota Chuo Kenkyusho Method for forming a two-layered carbide surface on a ferrous-alloy article and resulting product
US4440581A (en) * 1980-07-02 1984-04-03 Degussa Aktiengesellschaft Process for the production of vanadium carbide coatings on iron
CN105506545A (zh) * 2016-02-24 2016-04-20 哈尔滨汽轮机厂有限责任公司 一种用于超超临界汽轮机1Cr11MoNiW1VNbN材质高温快速渗铬的渗铬剂及其制备方法

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3833968A (en) * 1971-03-09 1974-09-10 Toyoda Chuo Kenkyusho Kk Metallic card clothing
US3959092A (en) * 1972-11-16 1976-05-25 Kabushiki Kaisha Toyota Chuo Kenkyusho Method for a surface treatment of cemented carbide article
US3885064A (en) * 1973-04-12 1975-05-20 Toyoda Chuo Kenkyusho Kk Method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article
US3912827A (en) * 1973-11-13 1975-10-14 Toyota Chuo Kenkyusko Kk Method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article
US4158578A (en) * 1977-05-09 1979-06-19 Kabushiki Kaisha Toyota Chuo Kenkyusho Method for forming a carbide layer of a Va-Group element of the periodic table or chromium on the surface of a ferrous alloy article
US4202705A (en) * 1977-06-30 1980-05-13 Kabushiki Kaisha Toyoto Chuo Kenkyusho Treating bath, forming a mixed carbide layer of Va-Group elements on a ferrous alloy surface and resulting product
US4230751A (en) * 1977-08-11 1980-10-28 Kabushiki Kaisha Toyota Treating composition, forming a mixed-carbide layer of Va-Group elements and of chromium on a ferrous-alloy surface and resulting product
US4250208A (en) * 1978-06-19 1981-02-10 Kabushiki Kaisha Toyota Chuo Kenkyusho Method for forming a two-layered carbide surface on a ferrous-alloy article and resulting product
US4440581A (en) * 1980-07-02 1984-04-03 Degussa Aktiengesellschaft Process for the production of vanadium carbide coatings on iron
CN105506545A (zh) * 2016-02-24 2016-04-20 哈尔滨汽轮机厂有限责任公司 一种用于超超临界汽轮机1Cr11MoNiW1VNbN材质高温快速渗铬的渗铬剂及其制备方法

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GB1305681A (enrdf_load_stackoverflow) 1973-02-07
DE2110414A1 (de) 1971-09-16
DE2110414B2 (de) 1976-09-23
JPS4938416B1 (enrdf_load_stackoverflow) 1974-10-17

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