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
  • C23C8/02—Pretreatment of the material to be coated
• • 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
  • C23C8/40—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 liquids, e.g. salt baths, liquid suspensions
  • C23C8/52—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 liquids, e.g. salt baths, liquid suspensions more than one element being applied in one step
  • C23C8/54—Carbo-nitriding

Definitions

• the present invention relates to the heattreatment of steel. More particularly it relates to the heattreatment of all types of iron and steel which are capable of hardening on heating to the austenitising range and then quenching, without the need to introduce additional carbon during the heating step.
• steel of the class which can be hardened by heating and quenching This class of materi als comprises the plain carbon steels and cast steel of the mediumand high-carbon varieties and the alloy steels containing at least 0.25 percent by weight carbon.
• the present invention provides a method of heattreating steel of this class whereby the steel is provided with a surface layer that is resistant to scuffing and wear when subjected to friction under a heavy load and the underlying core is hardened so as to be resistant to indentation.
• a method for the heat treatment of steel of the class which can be hardened by heating and quenching which comprises heating the steel into its austenitizing temperature range in a heating medium which is noncarburizing towards the steel, then soaking the hot steel in a molten salt bath held at a temperature in the range 700770 C. and containing at least percent by weight and preferably at least percent by weight of alkali metal cyanate calculated as NaCNO for at least 3 minutes and in any case where the temperature of the salt bath is in the range 700-750 C. preferably for at least 5 minutes, and then rapidly cooling the steel.
• the austenitizing step may be carried out in a nonoxidizing gas atmosphere or in a neutral salt bath as known in the art.
• a conventional gas carburizing atmosphere or a salt bath containing cyanide salt may be employed.
• these media are generally known as carburizing media they are essentially noncarburizing in action towards high-carbon steels.
• a salt bath comprising alkali metal cyanide, alkali metal cyanate, alkali metal carbonate and optionally alkali metal chloride, as described in British Pat. specification Ser. No. 1,052,668 in connection with a socalled carbonitriding process.
• the cyanate content of the bath is to be maintained at the preferred minimum of 10 percent by weight or higher, this can be achieved in the manner taught in the said British Specification but since the treatment temperatures used in the present invention are in general higher than the carbonitriding temperatures taught in the said British specification the iron content of the alloy steel surfaces of the container and any electrodes in contact with the molten salt bath should not exceed 22 percent by weight and should preferably be between 10 percent and percent by weight.
• a treatment time of about 10 minutes in this type of bath is generally most useful and we have found that this treatment creates a compound layer rich in nitrogen and carbon which can be observed at the surface of an etched microsection of the treated steel.
• This layer at the immediate surface of the steel varies in depth from about 0.0003 inch to 0.002 inch, depending on the cyanate content and temperature of the bath, and there may also be some penetration of nitrogen further into the steel.
• the steel beneath the surface layer remains mainly or wholly in the austenitic state.
• An alternative procedure where the usual austentitizing temperature of the steel lies within the temperature range 750-770 C. is to carry out the austenitizing and to apply the carbon-nitrogen compound layer in the same bath. To do this the parts are heated for austenitizing by placing them directly into the said salt bath containing at least 5 percent by weight,.
• alkali metal cyanate calculated as NaCNO preferably 10 percent by weight, alkali metal cyanate calculated as NaCNO, soaked for at least 7 minutes after attaining bath temperature and then quenched to harden the steel.
• cyanate calculated as NaCNO in the salt bath used in the second treatment step, since this confers maximum resistance to scuffing on the surface of the treated article.
• Cyanate contents in the range below 10 percent but not less than 5 percent by weight are however quite acceptable for articles working under less arduous conditions; in fact the scuffing resistance achieved with cyanate contents in this range is surprisingly high and well within many commercial requirements.
• EXAMPLE l This example shows for purposes of comparison the results obtained with a conventional hardening treatment of a tool steel.
• Pieces of 0.9 percent carbon tool steel in the shape of discs 0.4 cm. in diameter suitable for testing on an Amsler weartesting machine were hardened in the conventional manner by heating for 10 minutes in a neutral slat bath consisting of a mixture of the chlorides of sodium and potassium at 780 C. and then quenching in water. Hardness tests showed Rockwell Hardness C65-66 on the treated discs.
• Two treated discs, A and B were then submitted to a wear test on an Amsler weartesting machine. In this test the discs were rotated in edge-toedge contact under a load of 20 kilograms at 400 and 440 r.p.m. respectively so that at the point of contact they moved in opposite directions, and they were lubricated with SAE 30 mineral oil. The discs were removed and weighed after L000, 9,000 and 25,000 revolutions. The test was stopped at 25,000 revolutions since by this time the discs were badly scuffed.
• the discs were austenitized by heating for 10 minutes at 800 C. in a neutral salt bath consisting of a mixture of the chlorides of sodium and potassium and were than transferred directly to a salt bath maintained at 730 C. and containing 26.1 percent NaCN and l3.7 percent NaCNO, the balance being alkali metal carbonate.
• the discs were held in this bath for 10 minutes and were then quenched in water.
• a method for the heat treatment of steel of the class which can be hardened by heating and quenching which comprises heating the steel into its austenitizing range in a heating medium which is noncarburizing towards the steel for a time sufficient to heat the steel throughout. then transferring the hot steel to a molten salt bath held at a temperature in the range 700-770 C. and containing at least 5 percent by weight of alkali metal cyanate calculated as N CNO and a remainder consisting essentially of alkali metal cyanide together with a salt selected from the group consisting of alkali metal carbonate and mixtures thereof with alkali metal chloride, and soaking the steel in said salt bath for at least 3 minutes and then rapidly cooling the steel.
• a method according to claim 1, wherein the step of heating the steel into its austenitizing range is carried out by immersing the steel in a neutral salt bath.

Landscapes

• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Articles (AREA)

Applications Claiming Priority (5)

Publications (1)

Family

ID=10133049

Family Applications (1)

Country Status (7)

Cited By (5)

• 1967
  • 1967-04-28 GB GB09659/67A patent/GB1191122A/en not_active Expired
• 1968
  • 1968-04-19 US US722573A patent/US3615908A/en not_active Expired - Lifetime
  • 1968-04-23 DE DE19681758208 patent/DE1758208A1/de active Pending
  • 1968-04-26 BE BE714333D patent/BE714333A/xx unknown
  • 1968-04-26 NL NL6805948A patent/NL6805948A/xx unknown
  • 1968-04-26 FR FR149648A patent/FR94664E/fr not_active Expired
  • 1968-04-27 ES ES353267A patent/ES353267A2/es not_active Expired

Also Published As

Similar Documents