Classifications

• • 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
  • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
  • C21D3/02—Extraction of non-metals
  • C21D3/04—Decarburising
• • 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
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S376/00—Induced nuclear reactions: processes, systems, and elements
  • Y10S376/90—Particular material or material shapes for fission reactors

Definitions

• My invention relates to a process whereby the carbon content of fabricated components, particularly nuclear reactor fuel rod grids, can be precisely and uniformly controlled to any desired level.
• the process was conceived and experimentally proven feasible during development of techniques for reducing the carbon content of AM-350 stainless steel.
• AM-350 is a semi-auste nitic iron-base alloy nominally containing 17 percent chronium, 4 percent nickel, 3 percent molybdenum, and 0.10 percent carbon.
• AM-350 sheet, annealed at 1900F. (Condition H) is austenitic with small amounts of delta ferrite and is ductile and can readily be formed into intricate components.
• the AM-350 composition is carefully balanced between austenite and ferrite formers in such a fashion that the solution of carbon in the austenite-ferrite matrix during the H anneal yields a stable austenite-ferrite structure at room temperature. If the material is given a lower temperature L anneal (1,710F.), part of the carbon in solution precipitates. The decrease of solid solution carbon content decreases austenite stability and results in a martensitic transformation on cooling to room temperature. AM- 350 processed in this fashion possesses all the desired nuclear and mechanical properties for LWBR fuel rod support systems with the exception of the degree of corrosion resistance required for core materials. This lack of corrosion resistance is caused by grain boundary carbide precipitates formed during the L anneal and during cooling through critical carbide precipitation ranges.
• the alloy must have a high enough carbon content for formability and after fabrication the carbon content must be reduced to impart corrosion resistance to heat treated components. Additionally, the carbon content must not be reduced excessively or required strength levels will not be attainable, and the carbon content must be maintained at an extremely uniform level to reduce nonuniform transformation induced distortion. Carbon gradients will result in nonuniform Ms temperatures causing warpage as local areas transform before others.
• Decarburization of ferrous sheet is an extremely common commercial process used, e.g., in the preparation of iron plus silicon alloys for transformer cores.
• all commercial decarburization processes are designed to reduce the carbon content of low alloy transformer steels to the lowest possible level to opti- C+2H2 CH,
• the second process requires exposing components to extremely wet hydrogen gas (5,000 to 10,000 ppmv H O) at temperatures in the range of 1,500F. to 1,800 F. Carbon is removed by the water gas reaction Neither of these processes is strictly applicable to the decarburization of intricate precision components formed from chromium bearing stainless steel.
• the first process for carbon removal by the methane reaction requires temperature far in excess of what can be tolerated from a distortion standpoint.
• the second process can be used but with significantly reduced water contents (approximately 200 ppmv H O at 1,900F.), to prevent chromium oxidization With reduced water contents, the reaction is sluggish and tends to be nonuniform due to water depletion at localized areas. Precision carbon control is virtually impossible. For these reasons, a new process for controlled carbon reduction was developed.
• the grid is an open cellular array assembled from 0.015 inch thick components. The assembly is approximately 20 inches in diameter, 2 inches high, and weighs 7 pounds.
• Decarburization is achieved by exposing the fabricated AM-350 stainless steel component to an oxidizing atmosphere of wet hydrogen having a dew point of from 23F. to -7F. at a temperature of from l,880F. to l920F. for about 1 hour.
• the component is exposed to a hydrogen atmosphere containing approximately 2,000 ppmv water for 28 to 40 minutes at a temperature of about 1,900F.
• the actual water concentration is not critical, provided it is in excess of 200 ppmv at 1,900F.
• Three reactions occur and are listed As indicated, some decarburization occurs during this step as a result of the water gas reaction (2) and the formation of hydrocarbons (3). However, the most significant reaction is the formation of Cr O on the AM- 350 surface.
• the carbon content of the component has now been reduced to a very low level and can be used in applications where corrosion resistance is paramount and strength is of little importance. However, generally it is desirable to raise the carbon content to a level great enough to impart adequate strength to the component.
• the following carburizing step will also be explained with particular reference to AM-35O stainless steel.
• the final step of the process requires equilibrating the charge in reducing C O H mixtures exhibiting a carbon activity equal to an equilibrium AM-350 carbon concentration of 0.05 w/o at 1,900F.
• Equation 6.C when solved indicates that a hydrogen atmosphere containing 31 ppmv CH has a carburizing potential of 0.05 w/o for the AM-350 composition at l,900F.
• Equation 7.C cannot be solved directly without first selecting a moisture content.
• the equilibrium partial pressure of water for the oxidization of chromium, Equation 1 is approximately 200 ppmv. It was found experimentally, that the carburization reaction occurs most rapidly at this moisture concentration, and is the basis for selecting this concentration.
• Equation 7.C indicates that a hydrogen atmosphere containing 105 ppmv CO and 200 ppmv H O also has a carburizin g potential of 0.05 w/o for the AM-350 composition at 1,900F.
• One variation of the process which may substantially reduce the length of the cycle would be to combine the purging and equilibration steps. Rather than first reducing the overall carbon level to less than 0.05 w/o and then increasing the level to 0.05 w/o one could establish a floor, e.g., 0.05 w/o below which the carbon content could not be reduced. This can be accomplished by exposing the charge after oxidation to a purging and equilibrating hydrogen mixture of the same CO/I-I O ratio (105/200 but containing less than 200 ppmv H O. This gas mixture could be, e.g., 52.5 ppmv CO, ppmv H 0, and hydrogen.
• the initial oxidizing treatment, decarburization deoxidizing treatment (autodecarburization) and equilibration treatments were carried out each for 1 hour at 1,900F. These times and temperatures are conservative and were chosen based on calculations that the diffusion time for a carbon atom to traverse one-half the 0.015 inch AM-3 50 thickness at 1900F. is approximately 1 hour. Times and temperatures could be adjusted for different materials and thicknesses, and to increase or decrease the reaction rates.
• the carbon content of the entire cross section of every component treated in this fashion can be con trolled to 0.05 w/o or virtually any other carbon level desired. Increasing the desired carbon level would simply require increasing the CO content of the hydrogen gas as necessary; the opposite would be true for lower carbon contents. Thicker gauge components would require longer times at temperature or increased temperatures. Additionally, Steps A and B alone can be used to decarburize chromium bearing stainless steels.
• EXAMPLE It was desired to reduce the as-received carbon content (about 0.10 w/o) of an AM-350 grid to about 0.05 w/o.
• the grid is an open cellular array assembled from 0.015 inch thick components. The assembly is approximately 20 inches in diameter, 2 inches high, and weighs 7 pounds.
• the grid was exposed to an oxidizing H O H mixture containing approximately 2,000 parts per million 6 by volume (ppmv) for 1 hour at 1,900F. Some decarburization occurred during this step as a result of the water gas reaction and the formation of hydrocarbons, however, the most significant reaction was the formula tion of Cr O on the AM-35O surface.
• the oxidizing H O H mixture was purged with dry hydrogen (less than 200 ppmv H 0) and the grid was maintained in this atmosphere for about 10 minutes at a temperature of l,900F.
• the grid was found to have a carbon content of about 0.005 w/o.
• the dry hydrogen was then replaced with a H CO H O mixture containing 350 ppmv CO and 200 ppmv H O.
• the grid was maintained in this atmosphere for 1 hour at l,900F. After this treatment the grid was found to have a carbon content of 0.054 w/o.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

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Cited By (12)

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Citations (2)

• 1974
  • 1974-01-29 US US438148*A patent/US3925109A/en not_active Expired - Lifetime
• 1975
  • 1975-01-08 SE SE7500183A patent/SE7500183L/xx not_active Application Discontinuation
  • 1975-01-29 DE DE19752503634 patent/DE2503634A1/de active Pending
  • 1975-01-29 JP JP50011479A patent/JPS50131812A/ja active Pending
  • 1975-01-29 FR FR7502781A patent/FR2259154A1/fr not_active Withdrawn

Patent Citations (2)

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