Title of Invention

AN IMPROVED PROCESS FOR PRODUCTION OF FUSED TUNGSTEN CARBIDE

Abstract

An improved process for the production of fused tungsten carbide which comprises mixing carbon-reduced tungsten and carbon wherein total carbon content is in the range of 3.5 to 6 wt %, melting the said mixture using an extended thermal plasma of transferred mode and having an arc current range of 300 to 600 Ampers, arc voltage in the range of 30 to 60 volts and plasmagen gas preferably argon gas flow is in the range of 1 to 2 liters/minutes, quenching the resultant melt by conventional manner such as herein described, powdering the desired quenched product and sieving to get the desired powder of fused tungsten carbide.

Full Text

This invention relates to an improved process for the production of fused tungsten carbide. Fused tungsten carbide exhibits high hardness and superior resistance to wear and abrasion. This material finds extensive applications for the manufacture of wear resistance components such as drill bits, spraying, injection and blasting nozzles; thread guides in textile machinaries; micrometer anvils; burnishing tools; centres for lathe and grinders; ball point tips; teeth and jaws for excavators, etc.. The components can either be manufactured out of bulk fused tungsten carbide by adopting powder metallurgical technique or by casting from the liquid state. The fused tungsten carbide is also frequently used to develop extremely narci or wear resistance surfaces oy weiuing deposits and plasma spraying. The commercial production of tungsten carbon alloys by fusion has been a general practice since 1930. In this process a mixture of carbon-reduced tungsten powder and carbon or lampbelck is charged into a graphite tube furnace which is held at a tem- o perature of 3000-3200 C. The product is a molten mixture of WC & W C which is often quenched in an oil bath or on to a cold plate 2 and subsequently ground into powder for various uses. Reference may be made to "Tungsten sources, metallurgy, properties and applications" by S.W.H. Yih and C.T. Wang, Plenum Publication, NY & London, 1981, PP.391-392, 421. The drawbacks are that the graphite tube furnace is expensive and has low throughput and high thermal loss. The main object of the present invention is to provide an improved process for the production of fused tungsten carbide. Another object of the invention is to produce fused tungsten carbide in extended arc thermal plasma with high throughput. It is also the object of the invention to produce the said product at low cost. In the process of the present invention use is made of high enthalpy in thermal plasma generated in an extended arc device with a plasma forming gas such as argon to achieve high throughput by rapid melting with reduced heat loss. Thermal plasma can be generated by extended arc plasma. The nigh enthalpy available in the said plasma is effectively utilised for rapid melting. Thermal efficiency of arc plasma melting is further enhanced by generating the said plasma in the transferred arc mode. The said plasma melting process has higher thermal efficiency, higher throughput with low cost of production. The said plasma is formed using plasmagen gas such as argon which maintains a protective atmosphere for melting. Accordingly the present invention provides an improved process for the production of fused tungsten carbide which comprises mixing carbon-reduced tungsten and carbon wherein total carbon content is in the range of 3.5 to 6 wt % , melting the said mixture using an extended thermal plasma of transferred mode and having an arc current range of 300 to 600 Ampers , arc voltage in the range of 30 to 60 volts and plasmagen gas preferably argon gas flow is in the range of 1 to 2 litres/minutes , quenching the resultant melt by conventional manner such as herein described , powdering the desired quenched product and sieving to get the desired powder of fused tungsten carbide. The present invention relates to an improved process fcr zhe preparation of fused tungsten carbide (WC-W C) using an extendec 2 arc plasma furnace with graphite electrodes. The sequential steps of the process are as follows : a) Commercially available carbon-reduced tungsten and carbon (either in elemental or compound form of tungsten) with total carbon content varying from 3.5 to 6 (wt%) are used as starting material (charge) for preparation of fused tungsten carbide. b' The said charge is thoroughly mixed and taken lr. a graphite crucible either in loose form or in the form of pellets. The said crucible has a central tap hole at its bottom. The opening is plugged by placing a pre-sintered pellet of the same composition as the charge. c) An arc is struck on to the charge and an extended arc argon plasma in transferred mode is formed. The charge is melted in the said extended arc plasma where graphite crucible along with the charge forms one of the electrodes while a graphite rod acts as another electrode. The said graphite electrode has a provision for introducing plasmagen gas such as argon into the arc zone to form extended/expanded arc piasma m transferred mode. d The melting starts at the surface of the charge where the arc route meets the charge and the melting proceeds, gradually downwards, ultimately melting the fusible plug. This allows the entire molten and superheated charge to drop down onto a chilled surface/bath for rapid quenching. e) The said product is crushed to powder of different size for wide range of applications. The plasma melting is carried out in argon plasma provides an excellent protective atmosphere. The said fused tungsten carbides is prepared by thermal plasma under the following process conditions. Arc current : 3 00-600 Amperes Arc voltage : 30-60 volts Argon flow : 1-2 litres/minute Time of fusion : 10-25 minutes The following typical examples are given to illustrate how the process of the present invention is carried out in actual practice and should not be construed to limit the scope of the invention . Example 1 200 g of charge was taken in a graphite crucible with a central tap hole at the bottom plugged with a pre-sintered WC pellet. WC being a good conductor of electricity, positive pclarity of arc was impressed on charge via the graphite crucible. Negative polarity was connected to top graphite electrode having a provi- sion of axial central hole to pass argon (plasmagen gas). Arc was struck in the transferred mode after passing argon at a rate cf 1.5 iitre/min. Thus thermal plasma was formed inside the graphite crucible and it started melting the charge due to high ion temperature. After 10 minutes the whole charge and the plugged pellet at the bottom of crucible were melted and started to flow and dropped on a big graphite block (with a cavity for collection of melt) for quenching/chi11ing. Also, without a hole m the bottom of crucible, the charge was melted and furnace coded. Voltage asing argon at a rate of 1.5 litre/min. Thus thermal plasma was formed inside the graphite crucible and it started melting the charge due to high ion temperature. After 10 minutes the whole charge and the plugged pellet at the bottom of crucible were melted and started to flow and dropped on. a big graphite block (with a cavity for collection of melt) for quenching/chilling. Also, without a hole in the bottom of crucible, the charge was melted and furnace cooled. Voltage ain transferred arc node. Argon was passed through the central axial hole of top electrode = t a rate of 2 litre/min. After 25 minutes of arcing the whole charge along with plug got melted. The melt was allowed to flow and dropped on a big graphite block (with a cavity for collection of melt) for quenching/chilling. Also, without making a hole in the bottom of crucible the charge was melted and furnace cooled. Voltage and current in these operation were maintained at 50 V dc and 550 A respectively at full arcing condition. The product • quenched/furnace cooled) was crushed to powder of different size cy impact hammering followed by sieving. Yield of product was round in 90-95% (by wt.) range. The main advantages are : 1. High quality product with high rate of production and low processing cost is obtained by utilising high enthalpy argon thermal plasma with protective atmosphere. 2. The process makes use of the low cost extended arc plasma formed by arcing between graphite electrode and the charge while argon is continuously fed through the graphite electrode. Fused tungsten carbide product obtained by melting a mixture of W and WC by using extended arc thermal plasma has superior properties in respect of hardness (2200-2800 VHN, 5 0g load) and low free carbon ( We Claim: 1. An improved process for the production of fused tungsten carbide which comprises mixing carbon-reduced tungsten and carbon wherein total carbon content is in the range of 3.5 to 6 wt % , melting the said mixture using an extended thermal plasma of transferred mode and having an arc current range of 300 to 600 Ampers , arc voltage in the range of 30 to 60 volts and plasmagen gas preferably argon gas flow is in the range of 1 to 2 litres/minutes , quenching the resultant melt by conventional manner such as herein described , powdering the desired quenched product and sieving to get the desired powder of fused tungsten carbide. 2. An improved process as claimed in claim 1 wherein the carbon reduced tungsten used is of commercial grade. 3. An improved process as claimed in claim 1 and 2 wherein the carbon used is either in elemental or compound form of tungsten . 4. An improved process as claimed in claim 1 and 3 wherein quenching method used is such as chilled bath method, chilled surface method. 5. An improved process for the production of fused tungsten carbide substantially as herein described with reference to the examples

Documents:

283-del-1995-abstract.pdf

283-del-1995-claims.pdf

283-del-1995-correspondence-others.pdf

283-del-1995-correspondence-po.pdf

283-del-1995-description (complete).pdf

283-del-1995-form-1.pdf

283-del-1995-form-2.pdf

283-del-1995-form-4.pdf

283-del-1995-form-5.pdf

283-del-1995-form-6.pdf

283-del-1995-form-9.pdf