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
  • C23C8/06—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
  • C23C8/20—Carburising
  • C23C8/22—Carburising of ferrous surfaces

Definitions

• Carburization is the conventional process for case hardening of steel.
• gas carburizing the steel is exposed to an atmosphere which contains components capable of transferring carbon to the surace of the metal from which it diffuses into the body of the part.
• atmospheres A variety of atmospheres have been employed but the most commonly used one is the so-called endothermic (endo) atmosphere derived by partial combustion of natural gas in air. It is usually necessary to add a relatively small quantity of another constituent, usually natural gas, to the atmosphere to raise the carbon potential.
• U.S. Patent 4,049,472 also summarizes the prior art, the specification of which is herein incorporated by reference.
• the steel objects to be carburized are exposed at an elevated temperature, usually in the- range of about 1600°F (871°C), until carbon penetration to a desired depth has been achieved.
• the metal can then be cooled to room temperature by various known methods such as furnace, air, and media quench to develop the desired physical properties and case hardness in the finished article.
• the basic endothermic atmosphere produced by the incomplete combustion of natural gas in air consists of approximately 40% N 2 , 40% H 21 and 20% CO.
• the reaction by which carbon is generally believed to be deposited on the surface of the steel is represented by the following equation(l).
• Equations (1) and (2) may be added together to yield reaction (3).
• a further advantage of the methanol-nitrogen system is that the methanol is uniform in composition while natural gas contains, in addition to methane, widely varying amounts of ethane, propane and other higher hydrocarbons which affect the stoichiometry of the partial combustion reaction and may give rise to atmospheres of substantially varying composition which in turn leads to erratic and poorly controlled behavior of the carburization process itself.
• a pure methanol-based atmosphere is inherently more expensive both in terms of monetary value and the energy required to produce it, than is an atmosphere derived in part from methanol.
• total energy requirement to produce 100 SCF of base gas nitrogen at 1700°F (927°C) is 37,200 BTU's, while to produce the same volume of a base gas consisting of two-thirds H 2 and one-third CO by decomposition of methanol 61,800 BTU's are required.
• the atmosphere derived from pure methanol is advantageous in insuring that carburization proceeds uniformly and at a rapid rate, it is more expensive and consumes more energy than does an atmosphere derived from a combination of methanol and nitrogen.
• the more rapid carburization achieved with the pure methanol atmosphere is desirable since it results in a shorter cycle time to achieve a given case depth, and thereby lowers the amount of energy lost through the furnace walls.
• this gain in energy conservation is to some extent offset by the higher thermal conductivity of the pure methanol-derived atmosphere as compared to the synthetic endo atmosphere because of the greater hydrogen content of the former. It is estimated that this increased hydrogen concentration results in a heat loss rate ranging from about 9% to about 14% greater for the all-methanol derived atmosphere.
• an oxygenated hydrocarbon containing carbon, hydrogen, and oxygen having from 1 to 3 carbon atoms, no more than one carbon to carbon bond and a carbon to oxygen ratio of from 1 to 2 selected from the group consisting of alcohols, aldehydes, ethers, esters and mixtures thereof, and in particular the pure methanol-derived atmosphere during the first part of a carburization cycle provides the advantage of initially high carburization rate which is manifested in a reduced total cycle time. But it has also been found that after a period of time, part of the expensive methanol may be replaced by less expensive nitrogen without an accompanying increase in the time necessary to achieve a given case depth.
• a carrier gas mixture is obtained by catalytic partial oxidation of hydrocarbons (e.g. natural gas) resulting in a mixture which consists mainly of 20% CO, 40% H 2 and 40% N 2' Hydrocarbons (e.g. excess natural gas) are usually added to provide the carbon required.
• the carbon potential which determines the degree of carburization, is controlled by monitoring either the C0 2 or the H 2 0 concentration in the furnace gas. Theoretically, the proper control parameters are Pco 2 /Pco 2 and PcoPH 2 /PH 2 o, but since Pco and PH 2 are kept virtually constant, one component control by Pco 2 or PH 2 0 is possible.
• the carrier gas may also be generated by thermal cracking of mixtures of nitrogen and oxygenated hydrocarbons (e.g. methanol).
• nitrogen and oxygenated hydrocarbons e.g. methanol
• Methanol is the preferred oxygenated hydrocarbon for this process however ethanol, acetaldehyde dimethylether, methyl formate and methylacetate have been shown to produce high CO and H 2 levels. So far efforts have been directed to imitating the composition of the endo gas mixture only, in order to achieve comparable results at temperature. This makes it possible to use exactly the same carbon control mechanism as used with the endo system, (i.e. conventional one component carbon control).
• the present invention resides in maintaining CO and H 2 concentrations higher than endo composition in the first-part of the cycle in order to speed up carbon transfer and to reduce CO and H 2 concentrations in the later part of the cycle to endo composition which will enable the use of conventional one component control.
• Higher CO and H 2 levels may be obtained by reducing the nitrogen content in a nitrogen-oxygenated hydrocarbon mixture to be thermally cracked.
• a closed batch heat treating furnace having a volume of 8 cu. ft. (0.227 cu. m) was used.
• the furnace was equipped with a circulating fan and thermostatically controlled electric heater. Provision was made for introduction of nitrogen gas and methanol liquid, the latter as a spray.
• the furnace was vented through a small pipe leading to a flare stack. There was also provision for admitting enriching gas (e.g. natural gas) to the furnace.
• enriching gas e.g. natural gas
• the exit line was fitted with a sampling device and analytical means which permitted measurement of the concentration of carbon monoxide and carbon dioxide in the exit stream.
• the carbon potential of the exit gas was calculated according to well-known chemical equilibrium equations and the amount of the enriching gas admitted to the furnace was varied so as to maintain a desired carbon potential (CP) in the furnace.
• An increase in enriching gas (e.g. natural gas) flow resulted in an increase in carbon potential while a decrease in enriching gas resulted in an corresponding decrease in carbon potential.
• Each test involved a total time cycle of three hours including a heat recovery period after loading of thirty minutes. At the end of this time, the rivets were discharged from the furnace, quenched and subjected to metallurgical testing to determine the case depth and hardness. The effectiveness of carbon potential control was determined by the analysis of a shimstock sample which had been placed in the furnace along with the rivets.
• Tests I-6 and 1-7 indicate that under the conditions of these tests (10% natural gas during warmup) little is accomplished after the first 1.5 hours of operation with the 100% atmosphere. However, this is not the most energy efficient mode of operation.
• the exact time and degree of dilution depends upon the carbon level desired at the surface of the workpiece, the case depth, and temperature at which carburization is carried out. In general, greater case depths and the correspondingly longer times involved, permit greater dilution of the atmosphere. With longer times and greater case depths, the rate of diffusion of carbon from the surface declines and an atmosphere capable of effecting rapid carbon transfer is not needed.
• gaseous ammonia can be added to the atmosphere to achieve carbonitriding of ferrous metal parts.
• Processes according to the present invention can be used in place of existing gas carburizing processes in batch type furnaces and with proper furnace control in continuous furnaces.
• Existing furnaces can be readily adapted to the present invention without altering systems used to measure carbon potential and with only minor furnace additions to accomodate the hydrocarbon and gas sources.

Landscapes

• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

Priority Applications (2)

Applications Claiming Priority (1)

Publications (2)

Family

ID=8188285

Family Applications (1)

Country Status (2)

Cited By (2)

Families Citing this family (2)

Citations (5)

• 1981
  • 1981-04-27 DE DE8181301834T patent/DE3174840D1/de not_active Expired
  • 1981-04-27 EP EP19810301834 patent/EP0063655B1/fr not_active Expired

Patent Citations (5)

Also Published As

Similar Documents

Legal Events