Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
  • B22F3/10—Sintering only
  • B22F3/1003—Use of special medium during sintering, e.g. sintering aid
  • B22F3/1007—Atmosphere
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F7/00—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression
  • B22F7/02—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of composite layers
  • B22F7/04—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of composite layers with one or more layers not made from powder, e.g. made from solid metal
• • 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
  • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
  • C23C10/30—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes using a layer of powder or paste on the surface
  • C23C10/32—Chromising
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12014—All metal or with adjacent metals having metal particles
  • Y10T428/12028—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, etc.]
  • Y10T428/12063—Nonparticulate metal component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12806—Refractory [Group IVB, VB, or VIB] metal-base component
  • Y10T428/12826—Group VIB metal-base component
  • Y10T428/12847—Cr-base component
  • Y10T428/12854—Next to Co-, Fe-, or Ni-base component

Definitions

• This invention relates to the formation of a chromiumcontaining coating on steel strip, and more particularly to the formation of an iron-chromium alloy coating.
• the principal object of this invention is to produce an adherent, protective iron-chromium alloy layer on the surface of steel articles such as strip, sheets, plates, rods, bars and wire.
• Another object is to produce a diffused chromium alloy coating on a steel base, wherein diffusion takes place in a protective atmosphere.
• a further object is to produce a diifused chromium alloy coating on a steel base, wherein diffusion takes place in a protective atmosphere, which is augmented with controlled amounts of chlorine gas.
• sintering should take place, preferably, in a substantially 100% pure hydrogen atmosphere. It is possible to maintain such an "atmosphere under ideal operating conditions, but in large scale operations, where, for example, a 36 inch wide strip, of a coil length weighing four tons .or more, is chromized by a compacted-sintered powder technique, the sintering atmosphere will frequently be contaminated with impurities which affect the sintering process adversely.
• Impurities may be introduced into the sintering atmosphere of a sintering, or annealing, furnace through leaks in the system, or from refractories used in the furnace base.'In a large scale coating operation, it may be difiicult to obtain sufficient hydrogen of the required purity. Contaminants most likely to be found in the sintering atmosphere are oxygen, carbon and nitrogen. Chromium powder, or chromium-containing powder, which has been compacted on the metal base prior to sintering, has a strong aflinity for any of the elements mentioned above as contaminants.
• Oxygen is particularly-troublesorne in a sintering operation of the sort utilized in powder coating of strip, because it has been found to interfere with the diffusion of the chromium into the iron of the base steel, and of the iron into the compacted chromium-containing powder.
• oxidation of the chromium in the powder prevents ready diffusion of the chromium and iron particles of the compacted material itself.
• a 9790 pound coil of 18 gage low carbon sheet steel (0.003% carbon) was filmed with a liquid, in this case tridecyl alcohol, and the filmed strip was passed through a fluidized bed of a chromium-iron alloy powder.
• the alloy powder used was that known as Simplex ferrochi'ome, having an analysis of 71.3% chromium, 0.35% manganese, 1.42% silicon, 0.01% carbon and the balance iron.
• the powder had a particle size represented by that which passes a 200 mesh screen (U.S'. Standard Sieve Series).
• the strip emerged from the fluidized bed compartment with a uniform coating of the alloy powder on both sides of the strip, the approximate mean thickness of the coating being 0.001 inch.
• the strip was next passed through the rolls of a temper mill, having 28 inch diameter rolls, to compact the powder on the strip. In this mill, the powder is pressed into a porous, semi-adherent metal shell, in which the particles of powder, immediately adjacent the strip base, are firmly forced into mechanical adherence with the base.
• the coil was open-wound with a 0.024- inch zig-zag spacing wire placed about 2 inches below the top edge of the coil.
• the coil was ready for the sintering step, and to this end was placed on edge on the pedestal, or base, of a Lee-Wilson type annealing furnace.
• a stainless steel inner cover fits over the base and coil.
• the inner cover which contains the atmosphere surrounding the treated coil, has a gas volume capacity of approximately 585 cubic feet.
• the inner cover is sealed at the base with a liquid metal (Woods metal).
• a steel, refractory-lined furnace'wall encloses the inner cover, and completes the furnace structure.
• Gas-fired radiant tubes in the furnace wall, supplying the heat to maintain the temperature within the inner cover which surrounds the charge. The temperature is checked by thermocouples inserted in the base, and extending into the coil.
• the inner cover atmosphere was purged with a 4% hydrogen96% nitrogen dry gas at a rate of 900 cu. ft. per hour, :until the concentration of oxygen was reduced to less than 1.0%, and the dew point of the exhaust gas had decreased to 40 F. At that point, the gas was turned OE, and substantially 100% hydrogen introduced at the rate of 900 cu. ft. per hour. After the concentration of hydrogen reached 75%, the furnace was placed on the base and the heating-up cycle begun.
• This 24 hour period represents the soaking period, during which diffusion takes place between the chromium in the powder and the iron in the steel strip base. Diffusion also takes place between the chromium in the powder and any iron or nickel contained therein.
• Chlorine and hydrogen continued to flow into the treatment area, at the aforesaid rates, during the first 12 hours of the soaking period, after which the flow of chlorine was discontinued. The flow of hydrogen was continued during the latter half of the soaking period, and during the major portion of the cooling cycle as well.
• the furnace heat was turned off at the end of the 24 hour soaking period, and the entire furnace unit permitted to cool.
• the coil had cooled to about 1300 F., at which point the furnace shell was removed and replaced with a cooling cover.
• introduction of hydrogen was discontinued, and gas containing 4% hydrogen-96% nitrogen introduced at a rate of 900 cu. ft. per hour. Cooling of the coil continued to 135 F., at which point the charge was uncapped.
• the introduction of chlorine gas into the hydrogen treating atmosphere is believed to reduce the oxides on the powder particles before diffusion begins, and, in addition, to prevent further oxidation during the heatingup and sintering steps.
• a 5000 pound coil of 37 /2 inch width, 20 gage strip was filmed with alcohol, as in the previous example, and coated with Simplex ferrochrornium powder.
• Another 5000 pound coil, of the same size and grade as the first coil was similarly filmed with alcohol and coated with regular low carbon ferrochromium powder (0.08% carbon, 70.9% chromium, 0.43% silicon and 0.50% manganese). The two coils were then joined into a 10,000 pound coil, open-wound with a 0.024 inch wire spacer, placed on the base of the annealing furnace,
• the inner cover was sealed and the furnace placed over it. Before firing, the inner cover atmosphere was purged with 4% hydrogen-96% nitrogen gas. During the heating-up period, the dew point of the exit gas was measured every hour up to a temperature of 800 F. To insure a dry atmosphere, the coil temperature was held at 800 F. for five hours. At the end of this five hour period, chlorine gas was introduced into the atmosphere surrounding the charge at the rate of 4.5 cu. ft. per hour, equivalent to 0.5% of the total volume of the sintering atmosphere. The temperature of the coil was raised gradually to 1700" F., and the flow of chlorine gas into the sintering atmosphere continued at the rate of 4.5 cu. ft. per hour.
• the atmosphere surounding the coil should be substantially free of moisture before the sintering temperature is reached, as any water vapor in the treating atmosphere increases the possibility of oxidation of chrominum particles. As shown in the examples, it is preferable to lower the dew point of the atmosphere to about -40 F. before sintering.
• Chlorine is found to be most effective during the heating-up period and the early stages of sintering, as it is during these heating stages that the chlorine reacts with the oxide film, or other compound film, on the compacted metal particles, and thus enables elemental chromium to diffuse rapidly into the strip.
• the amount of chlorine used represented, in one case 1.0% of the hydrogen treating atmosphere, while in the other, chlorine was introduced in an amount equivalent to 0.5% of the hydrogen atmosphere. Lesser amounts of chlorine may be used, down to about 0.10%, as obviously the amount of chlorine will depend on the amount of contaminants present.
• the chlorine introduced during sintering may be limited to a rate of 0.5% by volume of the total sintering atmosphere.
• chlorine may be introduced at a rate of 1.0% of the total volume of treating atmosphere, or higher if desired.
• the introduction of chlorine gas to the treating atmosphere should be discontinued before the charge is cooled. This may be done at the termination of sintering, or preferably at some point during sintering, as, for example, midway of the sintering cycle. Chlorine should be exhausted from the atmosphere surrounding the charge before the cooling cycle begins, to prevent formation of chlorine compounds which might condense and have a deleterious effect on the surface of the alloy coating.
• the type of metal powder which may be applied to the base metal includes chromium, iron-chromium alloy, a mixture of iron and chromium, iron-chromium alloy plus nickel, a mixture of chromium and nickel, iron-chromium nickel alloy, and a mixture of iron, chromium and nickel powders.
• the alloy powders iron, chromium or nickel may be added if desired. Any combination or mixture of these powders is also suitable as long as chromium is present in the mix in an amount suflicient to produce a chromiumiron alloy in the coating.
• the chromium content of the powder should be at least 20 percent by weight. Incidental amounts of inert material or other substance not adversely affecting the process can be tolerated.
• a truly continuous, pore-free, stainless-type coating will resist boiling 20 volume percent aqueous solution of nitric acid (based on 100% I-INO In order to obtain corrosion resistance of this magnitude, the coating must be relatively free of porosity.
• the composite article is decarburized before the sintering operation.
• the base strip should have not more than 0.01% effective carbon by weight at the time of sintering.
• the carbon content of the applied powder should not be more than 0.25% by weight in order to produce a desired coating.
• effective carbon refers to that carbon in either the base metal or the coating, which is available to produce deleterious chromium carbides. These carbides embrittle the coating, and thus limit the formability of the coated product. In addition a coating containing carbides has lower corrosion resistance than a coating free of carbide.
• a strip analyzing more than 0.01% carbon if a sequestering agent such as titanium is present in the strip. If titanium is present in suflicient quantity to combine with substantially all of the carbon in the strip, it will prevent the carbon in the strip from diffusing to the coating during sintering and forming undesirable chromium carbide in the coating.
• Other carbide formers which may be used to tie up the carbon are chromium and columbium.
• the total efiective carbon, in both the metal strip and the metal powder, should be sufliciently low during 7 sintering, so that the resultant alloy coating will meet the test of resisting attack of a boiling 20 volume percent solution of nitric acid.
• the coating produced is made up entirely of iron-chromium alloy, or, when nickel is used also as a starting material, of an ironchromium-nickel alloy. It is believed that the alloy is formed by a metal to metal diffusion or iron and chro mium, or of iron, chromium and nickel.
• the temperature during sintering should preferably range between approximately 1550" F. and 1900 F. At 1550 F. a minimum time cycle of 12 hours should be used, although considerably longer times may be desirable, depending on the amount of alloying desired, i.e. the amount of chromium in the alloy at the surface of the coating. There is no upper limit for sintering temperature, other than that which may be dictated by practical considerations. At temperatures above 1550 F., the minimum time required will be lowered in an inverse manner.
• An alloy coated product made by our method can be reduced in cross section by as much as This effect is of considerable importance in the manufacture of thin gage coated products, for the thickness of the base steel strip is limited during sintering to a gage which will prevent collapse of the coil at the sintering temperature, or a minimum strip thickness of about 0.018 inch.
• the stainless coating of this invention is produced through inter-diffusion of chromium into the steel base and of iron from the base into the compacted powder during sintering.
• the stainless coating relates to a coating formed by such diffusion during sintering.
• steel strip is meant to include steel sheet or plate as well, and all percentages refer to weight percent except for those relating to chlorine. Chlorine percentages represent volume.
• the method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, introducing the strip and compacted metal powder thereon into a sintering furnace, establishing a protective atmosphere therein, introducing chlorine gas into said furnace to bring the chlorine content of said protective atmosphere to not less than 0.10 percent by volume, and sintering said strip and compacted metal powder in said atmosphere for a time and at a temperature sufficient to cause dilfusion between the strip and the powder and to form an adherent stainless steel coating, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.
• the method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25 carbon, compacting the powder on the strip, heating up and sintering the strip and compacted powder in a sintering zone in a hydrogen atmosphere, introducing chlorine gas into said atmosphere during the heating up and at least the early stages of the sintering operation in an amount representing no less than 0.10% by volume of said hydrogen atmosphere, removing said chlorine from said sintering zone before sintering is completed and maintaining the effective carbon content of the strip at not over 0.01% during sintermg.
• the method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, heating the strip and compacted powder to a temperature not less than 1550" F. and sintering the strip and compacted powder in a continuously flowing hydrogen atmosphere, introducing chlorine gas into said atmosphere in which the chlorine represents not less than 0.10% by volume of said atmosphere during the heating and at least the early stages of sintering, maintaining the effective carbon content of said strip at not more than 0.01% during said sintering, and cooling the strip in a continuously flowing hydrogen atmosphere free of chlorine to a temperature at least as low as 500 F.
• the method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, heating up and sintering the strip and compacted powder in a sintering zone in a hydrogen atmosphere, introducing chlorine gas into said atmosphere during the heating up stage in an amount representing not less than 0.10% by volume of said hydrogen atmosphere, and continuing introduction of chlorine gas into said atmosphere during the sintering stage in'an amount representing not more than about 0.5% by volume of said hydrogen atmosphere, removing said chlorine from said sintering zone before sintering is completed, and maintaining the effective carbon content of the strip at not over 0.01% during sintering.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Composite Materials (AREA)
• Powder Metallurgy (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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Cited By (10)

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Citations (7)

• 1964
  • 1964-07-10 GB GB28567/64A patent/GB1067126A/en not_active Expired
  • 1964-07-22 DE DE1964B0077788 patent/DE1287401C2/de not_active Expired
  • 1964-07-24 NL NL646408519A patent/NL149858B/xx not_active IP Right Cessation
• 1965
  • 1965-05-28 NL NL6506808A patent/NL6506808A/xx unknown
  • 1965-05-28 DE DE1521146A patent/DE1521146C3/de not_active Expired
  • 1965-05-28 BE BE664627A patent/BE664627A/xx unknown
  • 1965-05-28 FR FR18761A patent/FR87899E/fr not_active Expired
• 1966
  • 1966-08-10 GB GB35762/66A patent/GB1160121A/en not_active Expired
  • 1966-08-17 NL NL6611585A patent/NL6611585A/xx unknown
  • 1966-08-19 DE DE1521169A patent/DE1521169C3/de not_active Expired
  • 1966-09-20 US US580607A patent/US3340054A/en not_active Expired - Lifetime
  • 1966-10-19 NL NL6614736A patent/NL6614736A/xx unknown
  • 1966-10-20 GB GB47027/66A patent/GB1163947A/en not_active Expired
  • 1966-10-20 DE DE19661521173 patent/DE1521173A1/de active Pending
  • 1966-10-21 BE BE688728D patent/BE688728A/xx unknown

Patent Citations (7)

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