Coal Age

FEB 2012

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operating ideas Brazing of Carbide Mining Tools BY DR. YEHUDA BASKIN temperature refers to the temperature to which a material assem- blage is heated to enable the filler metal to spread and adhere to or wet the joining metals. The mining tool parts to be brazed are the steel shank and the cemented carbide bit. The most common brazing filler alloys used are shown in Table 1. These melt in the temperature range suitable for joining of the tools. All three contain manganese, in addition to nickel, copper and other minor constituents. Manganese is impor- tant for bonding to cemented carbides. The most common form for the filler metal is a flat coupon or shim, which is placed between the steel and the cemented carbide at the bottom of the formed pocket. Sometimes, additional alloy wire is added to the upper part of the carbide at peak temperature for additional strength. A miner changes conical bits on a continuous miner. (Photo courtesy of Kennametal) Mining tools, such as conical bits, roof bits, flats radials, etc., have evolved over the years. Currently, these tools consist of cobalt- bonded, tungsten carbide bits brazed to steel shanks. When proper- ly processed, long-lasting tools are produced which combine great strength, toughness and high hardness. Early coal mining tools were made of steel, eventually leading to different types of hardened steel. Then came the carbide revolu- tion, which began in the 1920s, but really took off after World War II. The earliest tools with cemented carbide bits increased the life- time of rock drills by at least a factor of 10 over steel-based drills. With improvements in cemented carbide technology, this advan- tage has increased. Cemented carbides, or hard metals as they are also called, are produced by cementing ultra-hard tungsten carbide (WC) grains in a binder matrix of cobalt metal by liquid-phase sintering. The high solubility of WC in cobalt at elevated temperature and the excellent wetting of WC by the liquid cobalt results in superior densification during liquid-phase sintering, and a pore-free struc- ture. As a result, material is produced which combines great strength, toughness, high hardness and wear resistance, and is also readily brazed, which is not the case for pure tungsten car- bide. The high hardness is necessary for the tools to function effectively in their working environment. While the various vari- eties of coal are generally soft, many of the associated minerals are abrasive, causing excessive wear to metal tools. Carbide mining tools are produced by a joining process called brazing, which is a well-established commercial process capable of producing strong joints. It is widely used in industry because of its ability to join most metallic materials, including dissimilar metals. The American Welding Society (AWS) defines brazing as a group of joining processes that cause the coalescence of materials in the presence of a brazing filler metal that has a liquidus temperature above 840°F/450°C, but below the solidus temperature of the base materials. The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action. The term brazing 46 Table 1: Filler Alloys Used in Mining Tools Composition Aloy %Ni %Cu %Mn %Zn Solidus Liquidus Nicuman 23 9.00 67.50 23.50 — 1,700°F/925°C 1,750°F/955°C Nicuman 37 9.50 52.50 38.00 -— 1,573°F/855°C 1,680°F/915°C Alloy 548 6.00 55.00 4.00 35.00 1,620°F/880°C 1,691°F/920°C Prior to final assembly the carbide bits are coated with flux, by putting them in containers filled with water-based flux. The coated bits are then placed on top of the alloy shims within the formed pocket. Recently a new dispensing machine has become available specifically for water-based flux; this allows precise and automated deposits up to 3,600 per hour. The system provides superior resis- tance to high abrasive constituents, and maintains the flux in opti- mal condition. Just such a system provides the user with a successful alternative to petroleum-based product. Cemented carbide bits are brazed to a steel shank. (Photo courtesy of Kennametal) February 2012

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