United States Patent (19) 11) 4,049,472 Arndt (45) Sept. 20, 1977 (54) ATMOSPHERE COMPOST ONS AND 3,330,773 7/1967 De Hart ........... ... 252/372 METHODS OF USNG SAME FOR SURFACE 3,663,315 5/1972 Hoffman et al. .................... 148/16.5 TREATING FERROUS METALS Primary Examiner-Walter R. Satterfield Attorney, Agent, or Firm-James C. Simmons; Barry (75) Inventor: Edward J. Arndt, Lenhartsville, Pa. Moyerman 73) Assignee: Air Products and Chemicals, Inc., Allentown, Pa. 57 ABSTRACT Atmosphere compositions and processes utilizing the 21 Appi. No.: 643,348 compositions for heat treating ferrous metal articles (22 Filed: Dec. 22, 1975 under a controlled furnace atmosphere to either main 51 Int. Cl?........................ C21D 1/48; B23K 35/34; tain or change the surface chemistry of the article being CO1B 2/00 treated are disclosed in the following specification. 52 U.S. C. ..................................... 148/16; 148/16.5; The invention features atmosphere compositions or 148/16.6; 252/372; 148/27 mixtures which are blended from normally gaseous 58 Field of Search .................. 148/165, 30, 16.6, 16, components outside the furnace and the mixture or 148/27; 252/372 blend is injected into the furnace to provide a carburiz ing, decarburizing, carbonitriding or neutral hardening 56) References Cited atmosphere inside the furnace. U.S. PATENT DOCUMENTS 3,229,967 1/1966 Engelhard .............................. 148/16 19 Claims, 7 Drawing Figures O 2 3. 4. 5 6 7 CH/CO2 RATO OF INPUT BLEND U.S. Patent Sept. 20, 1977 Sheet 1 of 3 4,049,472 9 O a Q - 0 on a n to a v n e - o O % W NVILN3Lod NOVO 3DVN ???????????????????<t<<<<<<<<<<<<<<<<<<<<<<<< U.S. Patent Sept. 20, 1977 Sheet 3 of 3 4,049,472 CINE93Tl niºn94%98O qNangunani[06P QNE-igIndNI30oluyuºoo/ºHo O8W/ 99#7£2 •t?”----8-----0---• ONES)ET TV101 4,049,472 1. 2 the aforementioned sections of the Metals Handbook ATMOSPHERE COMPOSTIONS AND METHODS are incorporated herein by reference and will be re OF USING SAME FOR SURFACE TREATING ferred to from time to time in the specification. In par FERROUS METALS ticular, that portion of the section on control of surface carbon content appearing on pages 90 through 91 of the BACKGROUND OF THE INVENTION Metals Handbook referred to above, and dealing with 1. Field of the Invention the determination of carbon potential of a furnace atmo The invention pertains to the field of metallurgical sphere is pertinent to the invention herein disclosed. heat treating, and in particular, to the heat treating of As set out in the Metals Handbook, furnace atmo ferrous metal articles under controlled atmospheres. 10 spheres such as involved in the instant invention, fall Ferrous metal articles, and in particular, the conven broadly into six groups. The first of these is a so called tional grades of steel being denoted by grade according Exothermic Base Atmosphere which is formed by the to American Iron and Steel Institute (AISI) nomencla partial or complete combustion of a fuel gas/air mix ture contain carbon. As these articles are raised to ele ture. These mixtures may have the water vapor re vated temperature for thermal treatment, e.g., harden 15 moved to produce a desired dew point in the atmo ing, annealing, normalizing and stress relieving, under sphere. an ambient furnace atmosphere containing air, hydro The second broad category is the prepared nitrogen gen, water vapor, carbon dioxide, and other chemical base atmosphere which is an exothermic base with car compounds the surface of the article will become reac bon dioxide and water vapor removed. tive. It is well known that the presence of water vapor, 20 The third broad classification is Endothermic Base hydrogen (H2), and carbon dioxide (CO2) in the furnace Gas Atmospheres. These are formed by partial reaction atmosphere will cause carbon at the surface of the fer of a mixture of fuel gas and air in an externally heated rous metal article to react and thus be removed from the catalyst filled chamber. article. When the carbon is depleted from the surface of The fourth broad category is the charcoal base atmo the article, the article no longer has a homogeneous 25 sphere which is formed by passing air through a bed of cross section due to the change in chemistry and crys incandescent charcoal. tallography thus changing the physical properties such The fifth broad category is generally designated as as surface hardness and strength of the finished article. Exothermic-Endothermic Base Atmospheres. These In order to avoid this phenomenon, such articles are atmospheres are formed by complete combustion of a heated under a controlled atmosphere containing car 30 mixture of fuel gas and air, removing water vapor, and bon which is available for reaction with the article reforming the carbon dioxide to carbon monoxide by being treated, or under an atmosphere that is essentially means of reaction with fuel gas in an externally heated neutral (to either add a slight amount of carbon to the catalyst filled chamber. surface of the ferrous article being heated or prevent The sixth broad category of prepared atmospheres is removal of carbon from the surface). 35 the Ammonia Base Atmosphere. This atmosphere can Under certain conditions it is desirable to add substan be raw ammonia, dissociated ammonia, or partially or tial but controlled amounts of carbon to the surface of completely combusted dissociated ammonia with a reg the article to increase its surface hardness and wear ulated dew point. resistance. This is normally accomplished by heating In situ generation of carburizing atmosphere in the the article to an elevated temperature in a controlled 40 furnace by decomposition of a hydrocarbon liquid at carbonaceous atmosphere that adds a desired percent elevated temperature, is disclosed in U.S. Pat. Nos. age by weight of carbon to the surface of the article. In 2,056,175. U.S. Pat. No. 2,161,162 discloses in situ cre the same manner, if ammonia is added to the controlled ation of a carburizing atmosphere in the furnace and use carbonaceous atmosphere, nitrogen as well as carbon is of the spent furnace atmosphere as a carrier gas. U.S. added to the surface of the article to produce additional 45 Pat. No. 3,413,161 discloses creation of a carburizing hardness and wear resistance of the surface of the arti atmosphere by in situ combustion of a hydrocarbon fuel cle. in the presence of less than stoichiometric amounts of In certain manufacturing operations, it is desirable to air in the furnace. remove controlled amounts of carbon from the surface Other aspects of carburizing are disclosed in U.S. Pat. of the article to achieve a predetermined lower percent 50 Nos. 2,287,651; 2,955,062; 3,356,541 (reissued as U.S. age of carbon in the surface of the article. This is ac Pat. No, Re, 26,935) and U.S. Pat, No. 3,397,875. complished by heating the article to an elevated temper U.S. Pat. No. 2,786,003 discloses a method of nitrid ature in a controlled carbonaceous atmosphere that ing a chromium steel by spiking the furnace atmosphere removes carbon from the surface of the article. with carbon monoxide to control the depth of nitriding. In its broad aspect then, the present invention pertains 55 All of the foregoing are representative of the state of to heating ferrous metal articles under an atmosphere the art of protective furnace atmospheres, as well as which is created to control the surface chemistry of the furnace atmospheres for carburizing, decarburizing, article being treated. carbonitriding or other carbon control in the surface of 2. Description of the Prior Art a ferrous metal article being heat treated. The prior art is adequately summarized in the section 60 entitled "Furnace Atmospheres and Carbon Control' SUMMARY OF THE INVENTION found at pages 67 through 92, and that portion of the The present invention is drawn to gaseous composi section entitled "Case Hardening of Steel" appearing at tions that are blended at ambient temperature and in pages 93 through 128 of volume of 2 of the Metals jected into a metallurgical furnace maintained at an Handbook published in 1964 by the American Society 65 elevated temperature (e.g. in excess of 1500 F), the for Metals, Metals Park, Ohio. The particular volume of furnace being used to provide a thermal treatment to a the Metals Handbook is referred to as Heat Treating, ferrous article while the article is maintained under a Cleaning and Finishing. All of the material set forth in protective atmosphere. Specific processes are disclosed 4,049,472 3 4. as part of the present invention for performing carburiz FIG. 7 is a plot of carbon potential against methane/- ing, decarburizing, carbon restoration, carbonitriding carbon dioxide ratio for carburizing compositions of the or neutral hardening of a ferrous article by a combina present invention injected into a furnace at 1750 F. tion of the thermal history of the article being treated and control of the furnace atmosphere. 5 DETAILED DESCRIPTION OF THE Broadly, the preferred atmosphere compositions are a INVENTION gaseous nitrogen base to which is added natural gas Furnace Atmosphere compositions suitable for use which is substantially methane, carbon dioxide, and in during heat treating of ferrous articles can be accom the case of a carbonitriding atmosphere, ammonia. In plished by blending individual gases outside of the fur order to effect the processes, it has been discovered that 10 nace and then injecting these gases into the furnace for the ratio of natural gas (methane) to carbon dioxide either protecting the surface of the ferrous articles, must be controlled within specified limits.