Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/003—Making ferrous alloys making amorphous alloys
• • 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
  • C23C8/24—Nitriding

Definitions

• This invention relates to a process for the production of ferrochrome alloys having a high nitrogen content and a low carbon content.
• ferrochrome alloys are used on a vast scale in the metallurgic field.
• the numerous uses of ferrochrome alloys in this field include also the use of said alloys as a means for bringing the nitrogen content up to a 0.3% in certain kinds of chrome steels, with the aim of improving some of their properties (tensile strength, machinability, weldability, intercrystalline corrosion resistance, etc.).
• ferrochrome alloy In order for the ferrochrome alloy to bring about this result it is necessary that it have certain compositional characteristics, as for example a low carbon content, since the final content of carbon in and above-mentioned chrome steels does not commonly exceed 0.3%.
• nitriding ferrochrome in molten state such as is obtained by alumino-thermal or silicothermal processes and consequently having a very low carbon content.
• the nitriding is carried out by adding solid nitrates or by blowing in nitrogen. A nitrogen content not higher than 2% is reached by means of this process.
• One object of the present invention is to provide ferrochrome alloys with a very high nitrogen content which can reach a value as high as Such a nitrogen content is from two to seven times higher than the content of other ferrochrome alloy products presently available on the market.
• Another object of this invention is the employment, as the starting material for obtaining the desired nitrided ferrochrome alloys, ferrochrome alloys having a high carbon content-which indeed can be as high as 8% without the necessity for any preliminary decarburization.
• the high nitrogen content of the ferrochrome alloys obtainable by the process of the present invention offers the remarkable advantage of making the use of a ferrochrome alloy, as steel nitriding agent, substantially independent-at least within certain limits-of considerations of its carbon content. If in fact one compares the attainment of equal final nitrogen content in a certain steel by starting from the ferrochrome alloys commonly on the 3,595,709 Patented July 27, 1971 market (which therefore have a low nitrogen content) or by starting from the ferrochrome alloys produced according to the process of the present invention, the significantly smaller quantitative requirement of the latter alloys becomes quite clear.
• the carbon content need not necessarily be maintained at a very low value.
• the carbon increase in the steel will be maintained within acceptable limits even if use is made of a starting ferrochrome alloy with 1% carbon content.
• the reaction is carried out in thermoregulated tube furnaces, made of or lined with refractory material, at times varying in dependence on both the carbon content of the starting material as well as the carbon content desired in the final product and also on the operational conditions of the nitriding stage.
• the ferrochrome alloy starting material pulverized to less than 10 mesh (and preferably 70 mesh or less), is subjected to the action of ammonia either by simply heating the alloy within the gaseous stream using the fixed bed method, or by heating the alloy using the fiuidized bed method.
• the mesh sizes referred to herein are 0 the A.S.T.M. scale.
• the alloy starting material reposing on the bottom of the tube furnace, placed horizontally, is steadily kept stirred by means of suitable agitators in order to expose the maximum surface of contact to the action of the ammonia gas reactant.
• the duration of treatment of a common carburized ferrochrome, containing from 4 to 6% carbon ranges from a minimum of 6 hoursfor producing a final product containing about 2% carbon and 12% nitrogen-to the maximum of 40 hoursfor producing a final product with carbon below 1% and 15 nitrogen.
• the carburized ferrochrome alloy starting material is suspended in a fluidized bed, the gaseous stream used for maintaining said bed being the reacting gas itself or nitrogen.
• the duration of the treatment is remarkably reduced.
• reaction temperature is kept at a substantially constant value included between 500" C. and 800 C., and preferably between 600 C. and 700 C.
• carburized ferrochrome-to obtain a ferrochrome having a relatively low nitrogen content as well as a relatively low carbon content.
• a subsequent heat treatment at a temperature above 1300 C. in an inert gas atmosphere such as argon or hydrogen or under vacuum,
• the pulverized alloy was heated at 650 C. for 36 hours in a stream of dry ammonia at a flow rate of 12 m. /hr. 258 g. HCN and 66 g. hydrocarbons, expressed as CH were formed; outlet gases still contained about 15% by volume of NH and the remainder consisted of nitrogen and hydrogen.
• the fluidized bed was realized by causing the gaseous mixture to stream upwardly through a 5 cm. long porous diaphragm cemented at the center of a tube of refractory material having a diameter of 10 cm. and made of sintered quartz granules.
• the gas passed through this porous diaphragm and then through the layer of pulverized ferrochrome alloy while thus maintaining a rather stable fluidized bed.
• EXAMPLE 4 (This example describes the production of a ferrochrome alloy having a low carbon content and a low nitrogen content as mentioned above.)
• a process for the production of ferrochrome alloys having a high nitrogen content and a low carbon content comprising treating a pulverized ferrochrome having a carbon content up to about 8% by Weight in a stream of gaseous ammonia at a temperature between about 500 C. and 800 C. with consequent elimination of a large part of the carbon from the starting material substantially in the form of gaseous hydrocyanic acid and simultaneous nitrogen fixation.
• a process according to claim 1, wherein the reaction between the ferrochrome starting material and the ammonia is carried out by heating the alloy in an ammonia atmosphere with use of the fixed bed or the fluidized bed method, in this latter case the bed consisting of ferrochrome alloy particles kept in suspension by the reaction gas or nitrogen.
• reaction temperature ranges between about 600 C. and 700 C.
• a process according to claim 1, wherein the nitrogen content of the finished ferrochrome alloy. is up to about 15 6.
• a process for the production of ferrochrome alloys having a relatively high nitrogen content and a relatively low carbon content comprising treating a pulverized ferrochrome having an initial carbon content of from 1% to about 8% by weight in a stream of gaseous ammonia at a temperature between about 500 C. and 800 C. with consequent elimination of a large part of the carbon from the starting material and simultaneous nitrogen fixation to a final nitrogen content of up to about 15%.
• a process according to claim 1 for the production of ferrochrome alloys having a high nitrogen content of from about 12% to 15 and a low carbon content comprising treating a pulverized ferrochrome having a carbon content up to about 8% by weight in a stream of gaseous ammonia at a temperature above about 600 C. with consequent elimination of a large part of the carbon from the starting material and simultaneous nitrogen fixation.

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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)
• Powder Metallurgy (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

Applications Claiming Priority (1)

Publications (1)

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Cited By (2)

• 1968
  • 1968-03-01 NL NL6802941A patent/NL6802941A/xx unknown
  • 1968-03-04 US US709892A patent/US3595709A/en not_active Expired - Lifetime
  • 1968-03-05 FR FR1554543D patent/FR1554543A/fr not_active Expired
  • 1968-03-06 DE DE19681608252 patent/DE1608252A1/de active Pending
  • 1968-03-07 BE BE711816D patent/BE711816A/xx unknown

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