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The Borofuse Process™ is a commercially available method whereby boron is diffused into the surface of a wide variety of metals in order to substantially improve surface properties. Boron reacts with the base material to form a fully dense reaction zone of metal borides, increasing surface hardness to very high levels. This effectively becomes a "case-hardening" process producing an exceptionally wear resistant case on a broad range of metal alloys. In addition, surface chemistry is altered, often resulting in improved corrosion resistance relative to the base alloy. Most Metals react with boron to form boride intermetallic compounds. Thus, the Borofuse Process is applicable to many commercially significant materials, including ferrous alloys, nickel and cobalt alloys, metal bonded carbides, and most refractory alloys. In the case of ferrous alloys, the diffusion of boron into the surface results in the formation of Fe2B, with a hardness in the range of 1500-1700 KHN. This hardness is compatible to that of cobalt bonded tungsten carbide (typically Ra 90), a hardness which is significantly higher than that achievable by case hardening processes such as carburizing (about Rc62=775 KHN) or nitriding (about 1000 KHN).
Fig. 1 is a photomicrograph of Borofuse Processed 4140 with included microhardness indentations in the boride zone and the base metal. The interface between the boride and the base metal is essentially a phase boundary, resulting in excellent adhesion. Most important, the boride zone is in biaxial compression, a desirable stress condition for a high hardness surface treatment. The Borofuse Process is not a line of slight process, making possible the treatment of complex shapes. Thus, the I.D. of small diameter high aspect ratio holes, as well as highly complex external shapes, can be borided. Since this is a diffusion process, the original surfaces are maintained, and surface finishes of approximately 16 micro inch and above are retained. The significant increase in surface hardness results in a substantial reduction of wear and galling as was demonstrated using a Falex Wear Test (Table 1) and ASTM Method D-2596 employing a Shell Four-Ball Extreme Pressure Test Apparatus (Fig. 2)
In many environments, borides can provide increased corrosion resistance. The ability of a Borofuse Processed metal to resist corrosion is dependent upon the base metal composition as well as the specific chemical environment involved. Generally, it can be stated that substantial improvements are achieved for Borofuse Processed components exposed to hydrochloric acid, hydrofluoric acid, and sulphuric acid, while no protection is afforded in nitric acid or against the normal corrosion (rusting) of ferrous alloys. |
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