Journal Journal of Chemical of Technology and and Metallurgy, 49, 49, 1, 2014, 1,

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1 Journal Journal of Chemical of Technology and and Metallurgy, 49, 49, 1, 2014, 1, FORMATION OF MOLYBDENUM CARBIDE DURING CARBIDIZATION OF MOLYBDENUM CHIPS WITH POWDERED CARBON AT ELECTROTHERMAL ROTARY BED CONDITION Boyan Yordanov 1, Dimitar Krastev 1, Aleksander Davidkov 2, Krastyo Klyavkov 1 1 Department of Physical Metallurgy and Thermal Equipment, University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria biyordanov@uctm.edu 2 Gent University, Department of Materials Science and Engineering, Technologiepark 903, B-9052 Zwijnaarde, Belgium Received 30 July 2013 Accepted 05 November 2013 ABSTRACT The ability of electrothermal rotary bed (ETRB) which is composed from metal chips and powdered carbon particles as an electroresistant self-heating mixture to accomplish a high temperature carbidization process has been studied. X-ray analysis and microstructure of the molybdenum carbide phase formed during carbidization of Mo-chips with powdered carbon in an ETRB condition process has been determined, and the microhardness of the final product (Hμ = MPa) was measured. The ability to use scrap-metal or chips from previous work as starting components to obtain molybdenum carbide in an elecrothermal rotary bed is proved. Keywords: electrothermal rotary bed, carbidization, molybdenum carbide. INTRODUCTION The modern technique and strategic technologies development are connected with research of new materials and improvement of many technical schemes. They have extreme parameters such as high temperatures, pressures, corrosion environment and long duration of the technological cycles. In these branches of science high temperature, high hardness and high chemical resistance materials such as carbides, nitrides, borides etc are used. These materials and technological schemes of their synthesis have been investigated for the past years. Carbides especially these of the transitional metals are the most perspective materials. They have high melting temperature, hardness, good electrical and heat conductivity, chemical resistance and good emission characteristic, etc. [1-4]. The problems of obtaining these materials are not completely solved. They have basically problems with the high temperature of obtaining and purity of the final products. Together with the commonly used carbothermal method of carbides synthesis, new methods such as plasma-chemical technologies and self-propagation of combustion over the reaction mass have been used. The problems such as reach of high temperature synthesis of these methods have already been solved but the problems with purity of the final carbides remain. The main reason for that problem is that the carbidization reaction does not flow to the end. These methods have been permanently improved but for the technical branches of industry new methods to carry out carbides synthesis have been searched too. At this direction the scientists pay attention toward heating methods which use internal source of heating. Private case of this heating process is carried out by electrical current passing through object. For this method it is typical to reach high speed and temperatures of heating 40

2 Boyan Yordanov, Dimitar Krastev, Aleksander Davidkov, Krastyo Klyavkov without using electroconductivity heating elements as in the cases of indirect heating. Electrothermal rotary bed (ETRB) is a method for direct electroconductivity heating of the reaction materials (mixture of metal powders and graphite or metal oxides and graphite). The process uses direct heating by electrical current passing through the rotating bed of reaction materials in argon or nitrogen atmosphere [5-8]. EXPERIMENTAL The starting components scrap materials from previous manufactures (molybdenum chips) and powdered graphite for obtaining molybdenum carbide in an electrothermal rotary bed conditions are shown in Table 1. Fraction of molybdenum chips is shown in Table 1 too. The molybdenium chips and powder carbon with fractions as pointed in Table 1 are mixed to reach stoi- chiometrical proportion of the final product (Mo 2 C). The amount of the graphite of 1,5 % more than its stoichio- metrical content in this carbides has been taken and the obtained mixture is placed in the reaction space of the furnace with reaction space at about % filled. The furnace working on ETRB principle has min 1 of rotation frequency and in its reaction space Ar or N 2 atmosphere with 0,01-0,02 MPa is supported (Fig. 1). The heating process of the reaction mixture at C/h is carried out, after that isothermal soaking is done. The experiments are completed at 1400 and 1500 C and soaking time 60 min (Table 1). X-ray diffraction and microstructure analysis of the final products have been made by apparatus TUR M62 working with characteristic K a -Co X-ray and an optical microscope Neophot 2. The principal scheme of an installation working of electrothermal rotary bed condition is shown in Fig. 2. Work cycle is conducted as follow: The furnace 1 with Electric power supply Fig. 1. ETRB furnace scheme for obtaining of molybdenum carbide: 1 - refractory tube; 2 and 3 - carbon electrodes; 4 and 5 - gas lead elements; 6 and 7 - current supply collector; 8, 9, 13, 14 - thermal insulation; 10 - temperature registration; 11 - layer from rotating dispersion electrical conductivity material (metal chips and powder carbon); 12 - fit elements. supporting flange 2 (left on the scheme) is positioned up side. After opening it the reaction powder mixture 5 is loaded in reaction space of the furnace between the refractory tube 7 and the carbon electrodes 3. After that the furnace is closed and the reaction volume is filled with argon (99,9999 % Ar) from bottles 9 for five minutes. The furnace operates at 0,01-0,02 MРa pressure over the atmosphere. The furnace with motor-drive device 12 with reverse rotation ability is rotated. In the reaction mixture using the carbon electrodes 3 industrial current from electric power supply 14 is switched on. The parameters of the heating process are controlled by control device 13. It works by signal from the temperature registration element 6. After soaking at previously determinate temperature, the furnace is cooled at a speed C/h in argon atmosphere. When the products reach temperature C the cooling process is Table 1. Starting component of carbidization process. Starting components Experiment, Molybdenum chips, Graphite, Soaking, min Temperature, No fraction, mm fraction, mm C 1-0,1-0,2 +0, / ,2 +0,1-0,2 +0, / ,315 +0,2-0,2 +0, /

3 Journal of Chemical Technology and Metallurgy, 49, 1, 2014 Fig. 2. ETRB reactor for obtaining of molybdenum carbide: 1 - furnace, 2 - supporting flanges, 3 - graphite electrodes, 4 - thermal insulation, 5 - tungsten-graphite mixture, 6 - thermocouple, 7 - refractory tube, 8 - thermocouple collector, 9 - argon and nitrogen reservoir, 10 - current supply collector, 11 - belt-drive, 12 - motor-drive device, 13 - control device, 14 - electric power supply. done at room temperature. The final product is unloaded when the supporting flange 2 is opened and the furnace is rotated down. RESULTS AND DISCUSSION The final products at 1400 С and 1500 С temperature after 60 min of isothermal maintaining in ETRB condition are given in Table 2. The product is molybdenum carbide (Mo 2 C) and unconverted Mo. The quantity of molybdenum carbide in the final products increases permanently with decrease of the fraction of starting particles. Fig. 3. Work parameters of ETRB during carbidization process at 1500 С. The work parameters of ETRB during carbidization process at 1500 С are shown on Fig. 3. Electrical power and temperature in the reaction bed slowly increase their values. The moment when the temperature of the reaction mixture reaches 1500 С soaking time begins and the electrical power and temperature of layer stay permanent. The electrical resistance of the reaction mixture (molybdenum chips and powdered carbon) decreases very fast at first but in 20 minutes it become very low with value of Ω. The electrical conductivity of the bed after 800 С increases 7-8 times in comparison to the starting value. This is the moment when carbidization process begins. X-ray diagrams of the phase composition of final products at 1400 C and 1500 C, 60 min isothermal maintaining, fraction 0.1 mm, are given in Fig. 4 and Fig. 5. The formation of Mo 2 C begins from out side of the Table 2. Final products at 1400 С and 1500 С after 60 min of isothermal maintaining in ETRB condition. No Product, Phase composition of the end Soaking, Temperature, C fraction, mm product from X-Ray analyses min 1-0,1 Mo 2 C + not reacted Mo ,2 +0,1 Mo 2 C + not reacted Mo ,315 +0,2 Mo 2 C + not reacted Mo ,1 Mo 2 C ,2 +0,1 Mo 2 C ,315 +0,2 Mo 2 C

4 Boyan Yordanov, Dimitar Krastev, Aleksander Davidkov, Krastyo Klyavkov Mo Mo θ Fig. 4. X ray diffraction diagram of carbidized molybdenum chips with powdered carbon particles at Fig. 4. X ray diffraction diagram of carbidized molybdenum chips with powdered carbon particles at 1400ºC, size 0.1 mm and soaking time 60 min. a) θ Fig. 5. X ray diffraction diagram of carbidized molybdenum chips with powdered carbon particles at 1500ºC, size 0.1 mm and soaking time 60 min. molybdenum chips particle to their inner space (Fig. 6). The carbon diffuses from the surface of the particles to their central zone and molybdenum carbide layer is formed. This carbidization process goes according to phase diagram of the Mo - C system. On the outside of the particles the phase Mo 2 C is situated, which remain not etched, after the etching procedure. The Mo 2 C has high corrosion resistance [13-15]. Not carbidized molybdenum is situated under Mo 2 C, whose grain boundaries, are etched and they are visible by light microscope observation. Reaction temperature 1400 C and 60 minutes soaking are not sufficient to convert the entire starting quantity of molybdenum chips into Mo 2 C. The fractions from 0.1 mm to mm contain not reacted Mo, which is innerly situated under molybdenum carbide on the surface of the molybdenum chips (Fig. 6). At reaction temperature 1500 C and 60 minutes soaking time the starting reaction mixture can be converted into the final products containing Mo 2 C and retained carbon. The microstructure and the formation of molybdenum carbide phase during carbidization of Mo - chips with powdered carbon in an ETRB condition process is shown in Fig. 6. Microhardness Hμ = MPa of Mo 2 C is measured. Such high hardness for carbides of transitional elements such as Ti, W, Cr, Mo, is typical [9-12]. CONCLUSIONS b) Fig. 6. Microstructure of carbidized molybdenum chips particles (a) at 1400ºC and (b) at 1500ºC, size 0,1 mm and 60 minutes soaking time, etched condition. The ability of electrothermal rotary bed which is composed of metal chips and powdered carbon particles as an electroresistant self-heating mixture to accomplish a high temperature carbidization process has been proved. X-ray analysis and microstructure of the formation of molybdenum carbide phase during carbidization of Mo-chips with powdered carbon in an ETRB condition process is studied and the microhardness of the final product (Hμ = MPa) was measured. The ability to use scrap-metal or chips from previous work as starting components to obtain molybdenum carbide in an elecrothermal rotary bed is proved. 43

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