Spark erosion processing of hard tungsten-cobalt alloy in water and subsequent carbonization

Abstract

The paper examines the advanced method of spark erosion for recycling hard-alloy scrap. It is shown that spark discharge over hard alloy in water leads to the decarburization of the initial WC phase in the surface layer and of the powder formed. After carburization, the powder can be used for the production of hard alloys. The structure and mechanical characteristics of the hard alloy made of the powder are studied. INTRODUCTIONTungsten carbide-cobalt hard alloys are widely used in modern industry in metal cutting, drilling, and woodworking. The world production of cutting tools was estimated at 2 billion dollars in 1992. About 60% of the total tungsten production is used to produce carbides for cutting tools.The great demand for tungsten and cobalt and the depletion of standard-quality raw materials cause 20-50% annual increase in prices. Hence, there is a need to find ways to recycle hard-alloy scrap.Grinding (dispersion) is the most complicated, labor-and energy-intensive operation in the regeneration of hard alloys. Spark erosion is one of the most promising methods to produce powder from lump scrap. B. R. Lazarenko and N. I. Lazarenko were the first to propose the method in 1943 [1]. It attracted much interest in the sixties-eighties since it involved no mechanical wear of equipment, any conducting materials could be disintegrated, and powder could be obtained in one operation [2][3][4][5].In view of the new trends in materials science, spark erosion is regarded as a method of producing nanocrystalline [6-8] and amorphous [9, 10] powders. Nevertheless, this method is not used to process hard alloys. All publications on spark erosion of various materials focus on research into the structure and chemical, grain-size, and morphological composition of the powder produced. In particular, it is noted in [11][12][13][14][15][16] that the grinding of hard alloys results in powders with modified phase and chemical composition and structure. We are aware of no research papers that would focus on the production of compact materials from dispersed powders. It is still to be decided how to produce hard alloys from spark-eroded powders.The objective of this paper is to optimize the spark-erosion conditions for hard alloys based on research into chemical, phase, and structural transformations that occur in the dispersion and further processing of the powder obtained. SPARK EROSION THEORYThe spark-erosion method is based on the erosion of a material caused by breakdown of a liquid dielectric. Zolotykh's thermal theory states that a gas bubble forms around the discharge channel after breakdown of a liquid dielectric (Fig. 1). The anode material rapidly heats up, melts, and partially boils. After the discharge stops, the gas bubble pulsates several times and collapses. The molten material is thrown from the craters into the interelectrode gap, cools down at a rate of 10 6 -10 9 deg/sec, and solidifies forming particles. At the same time, thermal stresses and pressure in the discharge channel (P ≤ 280 MPa) destruct the surface...

Electrochemical Production of Tungsten Powders from Tungsten Hardmetal Waste

“…Of particular interest are reduction processes in ionic melts, including the aluminothermic reduction of tungsten oxides in alkali salt melts [4]. The potential production of a tungsten powder in the pyrometallurgical conversion of the scheelite concentrate in ionic melts has been examined.…”

“…A series of techniques for reprocessing sintered tungsten hardmetal waste are currently employed in the national and foreign industry [1][2][3][4]. The production of metallic powders involves complex multistep operations 1 National Technical University 'Kharkiv Polytechnic Institute', Kharkiv, Ukraine.…”

“…These techniques are intended to recover tungsten from processed raw materials as a solution of sodium tungstate or other products and are only an intermediate stage in the overall process, followed by the reduction of tungsten compounds with hydrogen or a hydrocarbon gas phase to produce a metallic powder. The use of higher halides in high-temperature gas flows requires heating above 2273 K and involves a corrosive environment and complex equipment [4]. The plasma chemical method, being built on feedstock evaporation and condensation in plasma, requires heating above 4273 K.…”

Regeneration of Hard Alloy Wastes by Low Energy Consumption and Environmentally Friendly Hydrothermal Technology

“…There are several methods, and new methods for utilization of hard alloy wastes are developed from year to year, for example, exotic method based on the dispersion in water by electric discharge has been proposed [4]. At the same time methods that used in practice could be divided on 4 groups: 1) mechanical desintegration [5][6], 2) cobalt extraction by physico-metallurgical technique (Zn method -melting together with Zn, and subsequent sublimation in vacuum) [7][8],…”

“…Obvious characters of all processes are: a -high energy consumption (1,2,3 * ,4) b -long duration (3,4) c -high temperature (2,4) d -restricted purity of products (1,2) e -pollution of environment (2,3) f -expensive reagents (3) g -processing only for graded wastes (1,4) Despite the fact that latter process applied only for graded wastes, but it is very efficient in processing of waste generation, that are formed during the production of synthetic diamonds, for example. Generally here two kinds of hard alloys with 6% of Co for press dies and with 15% of Co for supporting plates are used.…”