Method of providing decarbonization protection for metallic surfaces
BACKGROUND OF THE INVENTION 
     Several techniques are presently known for the decarbonization protection of metallic surfaces (particularly those of ferrous materials) during heat treatment of such materials. In one arrangement of this type, a coating of borax is placed directly on the surface to be protected, and serves during the heat treatment to form accelerated scaling on such surface. When the scale is removed, the exposed surface is relatively free of decarbonizing. 
     An additional arrangement, which is in some respect superior to the borax treatment, employs as the coating material a ceramic frit which has a high concentration of boron oxide or the like. It has been found that such substance provides a more highly viscous and adherent film on the relevant surface than does borax. 
     SUMMARY OF THE INVENTION 
     The method of the present invention concerns a new way of providing a surface coating to afford the required decarbonizing protection of a metallic surface, particularly a ferrous surface. Basically, the coating is formed as a very thin (1 mm max.) layer of a paste-like mixture of ingredients whose properties, when applied in relatively thick layers, are already known for their boronizing capabilities. In particular, such thick coatings have already been used to increase the hardness of steel surfaces, e.g., by converting the first 20-400 microns or so of such surface into FeB or Fe 2  B via the diffusion of boron from the mixture into the adjoining steel surface. 
     The mixture consists basically of (1) one or more known boron-emitting substances, such as boron carbide (2) an activator such as fluoro borate and/or a halogen compound of an earth or alkaline earth metal or ammonium, and (3) a liquid binder. 
     A particularly effective formulation of this type consists of 10 - 80% by weight (preferably 20 - 60% by weight) of boron carbide, 2 - 10% by weight of alkali fluoro borate, and the remainder an aqueous binder such as waterglass, methylcellulose, or resin glue. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The method of the invention is further set forth in the following detailed description taken in conjunction with the appended drawing in which: 
     FIG. 1 is a photomicrograph taken after the heat treatment of a steel workpiece whose surface has been afforded decarbonization protection in accordance with the invention; and 
     FIG. 2 is a photomicrograph, taken under similar conditions to that of FIG. 1, illustrating the condition of an untreated surface of a steel workpiece after heat treatment. 
    
    
     DETAILED DESCRIPTION 
     The characteristics of the inventive method can be best illustrated by an exemplary sequence of steps for providing decarbonization protection of a steel bar having a 0.6% carbon concentration and measuring 80 mm on a side. 
     The surfaces of the bar are first suitably degreased, and are then sprayed, in accordance with the invention, with a thin coating (1 mm max.) consisting of 30% by weight of boron carbide, 3% by weight of potassium boron fluoride, and the remainder liquid sodium silicate. The so-protected steel bar sample is then subjected to a 4-hour heat treatment, during which the bar is heated to incandescence at about 850°C. 
     After the heat treatment, the bar is permitted to cool down and the adhering thin protective layer is removed by suitable means, e.g. a dilute 10% solution of phosphoric acid. 
     During one typical test run under these conditions, by which a thin coating of 0.8 mm was obtained, the sample, after removal of the coating as indicated above, was etched transversely with a 2% solution of nitric acid. Thereafter, photomicrographs having a 250 to 1 enlargement were taken. One such photomicrograph is shown in FIG. 1. 
     A careful inspection of FIG. 1 indicates that no significant boron diffusion, which would lead to undesirable changes in hardness in the steel surface, was obtained when the technique of the invention was used. This fact was also verified by means of separate metallographic probes, wherein microhardness measurements, measured with 50 ponds indicated that the hardness of the structure was constant within a very narrow tolerance from the surface to a significant extent into the interior of the sample. 
     For comparison, photomicrographs were also taken, as shown in FIG. 2, of the end zone of an unprotected steel bar sample that was heat treated under the same conditions as the test sample leading to the results in FIG. 1. In particular, the characteristics of FIG. 2 illustrate a strong decarbonizing of the test sample, such decarbonizing extending to a depth of about 0.2 mm into the interior of the sample. 
     In the foregoing the method of the invention has been described in connection with an illustrative sequence of steps. Many variations and modifications of such sequence will now occur to those skilled in the art. It is accordingly desired that the scope of the appended claims not be limited to the specific disclosure herein contained.