Title of Invention

"A METHOD OF MANUFACTURING A CARBIDE INSERT GRINDING ROLL THEREFOR"

Abstract

The invention relates to a method of manufacturing a carbide insert grinding roll for crushing high ash coal in coal based thermal power plants comprising the steps of: -providing a plurality of inserts made of a highly wear-resistant chrome alloy m aterial; -installing said inserts directly on the periphery of an assembly die; -welding a plurality of steel rods through a plurality of holes provided to said inserts by main taing a gap representing a reinforcement of said inserts, and forming a shape like around cage; -transporting said insert cage to a pouring die and fixing said assembly to a centrifugal casting machine;, -preheating said assembly before simultaneous pouring of spherodised grey cast iron into said pouring die while set to a rotation to form said carbide insert roll; -removing said insert rolls from said centrifugal casting machine after cooling it to about 800° c, subjecting the said insert rolls to rolling upto 450° in air and cooling it further to ambient temperature.

Full Text

A METHOD OF MANUFACTURING A CARBID INSERT GRINDING ROLL BY CENTRIFUGAL CASTING PROCESS AND A CARBIDE INSERT GRINDING ROLL. Field of the Invention. The present invention relates to a method of manufacturing a Carbide insert ' I .si" grinding roll by Centrifugal casting process. The invention also relates to a grinding roll obtained by the implementation of this method. Background of the Invention. Grinding rolls consisting of an outer wear layer which is supported by a more ductile core are well known. These rolls are produced by successive casting of the wear layer and of the supporting core in order to form a metallurgical bond between the two materials. Given that this method of manufacture can be implemented only for a few pairs of compatible materials for a metallurgical bond, the choice of materials, both for the wear layer and for supporting core, is limited. To produce the metallurgical bond further more requires perfect synchronization of the casting operations and good control of the casting temperature. Document EP 0,476,4996 Al recommends the technique of using inserts for the manufacture of grinding rolls. According to this document ( EP- 04764996 Al), the inserts are cast Separately in the moulds first and subsequently placed in a sand mould into which the supporting core is poured in order to form a mechanical bond between the two materials. Contrary to metallurgical bond, the mechanical bond does not involve constraints from the choice of materials stand point, so that it is possible to choose harder alloys for the inserts. This technique also enables the variation of insert composition to have wear properties commensurating with the application particularly for high wear resistance to handle coals of high ash content i.e. the insert hardness can be increased at will without encountering roll cracking. However, this technique often comes up against other difficulties for example, when the second casting is carried out into the sand mould on cold inserts, these run the risk of cracking induced by heat shock. In order to avoid or at least to reduce such a risk, if it is preheated to about °400 500°c. Now, this preheating is not easy to perform given that it must entail bringing bulky heating equipment close to the sand mould and, in addition, the necessary temperature to be reached generally caused destroyal of the binder used for the mould sand. Document US5819838relates to the method of using a portable shell avoiding the sand casting and facilitating preheating of inserts and the die separately which is more time consuming. Also in this method , each time a shell gets destroyed a new shell has to be manufactured. The inserts are not bound by any steel rods and any cracks that may form during service will propagate much faster endangering the mill safety. It is therefore an object of the invention to propose a method for producing a carbide insert grinding roll which provides a wide selection range for both the wear layer and the supporting core. Another object of the invention is to propose a method which generates and retain an improved mechanical bond between the insert materials. Yet another object of the invention is to propose a method which allows preheating of the inserts directly on the die enabling better control of the preheating temperature. A further object of the invention is to propose a carbide insert grinding roll which prevents additional wear having improved grinding action and capable to withstand higher amount of shocks without failure. Summary of the Invention: According to the present invention there is provided a method of manufacturing carbide insert rolls using centrifugal casting method. The inserts are cast separately and fixed to the die. Spherodised grey iron is poured while the die is in rotation. The roll is removed from the die and kept in the pit for cooling. The invention further provides a carbide insert roll carrying out the method of the invention comprising a matrix of machinable ductile cast iron, with embedded inserts made of a material having a high wear resistance. The inserts are reinforced with steel rods through the specifically provided reinforcement holes in them and formed into a round cage shape. This assembly is done in the assembling die maintaining proper gaps. The assembled cage is then transported to the pouring die, fixed on the centrifugal casting machine. Preheating of the die is carried out along with cage to about 450 °c. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS. Fig. 1 shows the plan and front view of the centrifugally cast insert roll. The outer shell comprises inserts (1) made of high wear resistance material. These inserts are reinforced with a steel rods (2) through holes (3) provided for this purpose. The inside material is grey iron (4). Detailed Description of the Invention. In coal based thermal power plants, coal is generally crushed to required size in between bull ring segments of the bowl mill and rolls. Coal is abrasive in nature and hence material selected for manufacturing the rolls should invariably have very good wear resistance. Un alloyed and low-alloy white irons where the predominating carbides is cementite, have been used for many years because of their resistance to abrasive wear and still used for many industrial applications. However, in severe conditions their performance becomes unsatisfactory due to degeneration of carbide phase, forming a continuous network around grains making them brittle. The formation of other carbides through addition of carbide forming elements solves this problem. High chromium alloys have been developed and are found to possess high resistance to abrasive wear suitable for grinding rolls. The life of rolls presently in service is a matter of concern and the wear profile shows collar formation. After half life of the rolls, the output drops and there is increase in the mill rejects. In order to achieve this objective, the invention provides a method of the kind described in the preamble. Here assembly die is used which facilitates assembly of the inserts (1), The assembled insert cage is transported on to the die fixed on the centrifugal casting machine. The inserts are provided with steel rods (2) for reinforcement through specially provided reinforcement holes (3). The inserts are preheated along with the die to 450°c. The centrifugal casting machine is set in rotation. The spherodised grey cast iron (4) is poured and the roll is removed from the die at about 800°c. Forced air-cooling up to 450 °c followed by slow cooling is sandpit up to room temperature. The inserts are made up of high chromium cast iron with Molybednum and small additions of elements like Titanum, Cerium, Boron or Nibbium to make a highly wear resistant alloy. These inserts are embedded in soft ductile iron core on the working face. These inserts are arranged in such a way that a layer of ductile iron of about 5.0mm width is present between two hard inserts. During operation this thin layer wears out faster than the inserts resulting in groove formation. This groove helps the roll in biting the material positively avoiding slippage of material and thereby preventing additional wear. The grinding action is also improved due to improved retention of coal. The ductile core is capable of withstanding shocks without any danger of failure due to breakage. The centrifugal casting machine is driven at a speed of the order of 100 to 300 revolutions per minute. The speed is varied along with the pouring. The roll is removed from the die when the temperature falls to a value of the order of 800°c. This method makes it possible to keep all the advantages obtained with the insert technique, especially those, which are due to the presence of the mechanical bond between the inserts. Furthermore, the use of die for assembling the inserts eliminates the problems of preheating of inserts separately in a shell, given that the this preheating may now be carried out directly in the die in which insert cage is placed. This also enables the preheating temperature to be controlled better. In centrifugal casting, the centrifugal force generated by the rotation of the shell locks the inserts against the wall of the shell. Reinforcement through steel rods and welding contributes to better fixing of the inserts and reduction in the risk of movement during casting. Furthermore, under the effect of the centrifugal force, the ductile cast iron completely fills the spaces between the inserts in order to clamp them perfectly, without the risk of forming cavities due to the solidification of the cast iron before all the interstices are filled. A set of grinding rolls have been cast using this technology. WE CLAIM; 1. A method of manufacturing a carbide insert grinding roll for crushing high ash coal in coal based thermal power plants comprising the steps of: -providing a plurality of inserts made of a highly wear-resistant chrome alloy m aterial; -installing said inserts directly on the periphery of an assembly die; -welding a plurality of steel rods through a plurality of holes provided to said inserts by maintaing a gap representing a reinforcement of said inserts, and forming a shape like a round cage; -transporting said insert cage to a pouring die and fixing said assembly to a centrifugal casting machine; -preheating said assembly before simultaneous pouring of spherodised grey cast iron into said pouring die while set to a rotation to form said carbide insert roll. -removing said insert rolls from said centrifugal casting machine after cooling it to about 800° c, subjecting the said insert rolls to rolling upto 450° in air and cooling it further to ambient temperature. 2. The method as claimed in claim 1, wherein said cooling comprises subjecting to said insert rolls to a cooling up to 450° c in forced air. 3. The method as claimed in claim 2, wherein said cooling of insert roll comprises delivering the insert rolls to sandpits and allowing a slow cooling up to an am bient temperature. 4. A carbide insert grinding roll for crushing high ash content coals hi a coal based thermal plant produced by the method as claimed in claim 1, comprising a matrix of machinable ductile cast iron; and a plurality of highly wear resistant chrome inserts, said inserts being embedded therein having reinforced with steel rods arranged through a plurality of holes, said assembly of inserts in said matrix being preheated in a centrifugal casting machine and cooled in stages up to an ambient temperature.

Documents:

1196-del-2003-abstract.pdf

1196-del-2003-claims.pdf

1196-del-2003-correspondence-others.pdf

1196-del-2003-correspondence-po.pdf

1196-del-2003-description (complete).pdf

1196-del-2003-drawings.pdf

1196-del-2003-form-1.pdf

1196-del-2003-form-19.pdf

1196-del-2003-form-2.pdf

1196-del-2003-form-3.pdf

1196-del-2003-gpa.pdf