FIELD OF THE INVENTION

This application is patent of addition of Indian patent application No. 276/MUM/2005  granted on August 17  2011 with patent number 248740.

The present invention provides a method for the manufacture of a wear resistant metal part. More particularly  the present invention provides a method for the manufacture of wear resistant metal parts by embedding mineral grains therein on the wear surface. The present invention also provides an improved wear resistant metal part with mineral grains embedded therein on the wearing surface.
The present invention particularly relates to composite wear components manufactured by casting. Such wear components conventionally have a metal matrix whose wear face is provided with inserts to enhance the wear resistant characteristics.

BACKGROUND OF THE INVENTION

The use of porous abrasives to improve mechanical grinding processes is generally well known. Pores typically provide access to grinding fluids  such as coolants and lubricants  which tend to promote more efficient cutting  minimize metallurgical damage (e.g.  surface burn)  and maximize tool life. Pores also permit the clearance of material (e.g.  chips or swarf) removed from an object being ground  which is important especially when the object being ground is relatively soft or when surface finish requirements are demanding (e.g.  when back grinding silicon wafers).
Previous attempts to fabricate abrasive articles and/or tools including porosity may generally be classified into one of two categories. In the first category  a pore structure is created by the addition of organic pore including media (such as ground walnut shells) into the abrasive article. The media thermally decompose upon firing  leaving voids or pores in the cured abrasive tool. Examples of this category are U.S. Pat. No.5 221 294 to Carmen  et al.  and U.S. Pat.No. 5 429 648 to Wu  and Japan Patents A-91-161273 to Grotoh  etc. al.  A-91-281174 to Satoh  et. al. In the second category  a pore structure may be created by the addition of closed cell materials  such as bubble alumina  into an abrasive article. See for example U.S. Pat. No.5 203 886 to Sheldon  et al.
In an alternative approach  Wu et al.  in U.S. Pat. Nos.  5 738 696 and 5 738 697  disclose an abrasive article and method for fabricating the same including fiber-like abrasive grains having a length to diameter aspect ration of at least 5:1. The poor packing characteristics of the elongated abrasive grains resulted in an abrasive article including increased porosity and permeability and suitable for relatively high-performance grinding.
US Patent 6 755 729 discloses an abrasive article including from about 40 to about 80 volume percent interconnected porosity  the article being useful as a segment for a segmented girinding wheel  and a method for fabricating the same. The method includes blending a mixture of abrasive grain  bond material and dispersoid particles  the mixture including from about 40 to about 80 volume percent dispersoid particles. In one embodiment the mixture includes from about 50 to about 80 volume percent dispersoid particles. In another embodiment the mixture includes an organic bond material and from about 40 to about 80 volume percent dispersoid particles. The powder mixture is then pressed into an abrasive laden composite and thermally processed. After cooling the composite is immersed into a solvent  which dissolves substantially all of the dispersoid particles  leaving a highly porous  bonded abrasive article.
US Patent 6 749 653 discloses an abrasive article comprising a binder and a plurality of abrasive particles  wherein at least a portion of the abrasive particles are abrasive particles comprising at least 65.0 percent by weight sintered  polycrystal1ine zirconia  based on a total weight of the abrasive particle; wherein said abrasive article is selected from the group consisting of coated abrasive articles  bonded abrasive articles  non-woven abrasive articles  and abrasive brushes.
US Patent 5 552 353 discloses a process for the synthesis of homogeneous advanced ceramics such as SiC+AlN  SiA12O3  SiC+A12O3  and Si3N4 +AlN from natural clays such as kaolin  halloysite and montmorillonite by an intercalation and heat treatment method. Included are the steps of refining clays  intercalating organic compounds into the layered structure of clays  drying the intercalated mixture  firing the treated atmospheres and grinding the loosely agglomerated structure. Advanced ceramics produced by this procedure have the advantages of homogeneity  cost effectiveness  simplicity of manufacture  ease of grind and a short process time. Advanced ceramics produced by this process can be used for refractory  wear part and structure ceramics. The wear parts obtained are homogeneous in nature.
US Patent 6 399 176 discloses a process for manufacturing of composite wear component consisting of metal matrix whose wear surface comprising inserts being made of homogenous solid solution of 20 to 80% of alumina and 80-20% of zirconia by weight. The mixture disclosed in this prior art is homogeneous solid mixture.
US 5 143 522 discloses abrasive grain comprising about 20 to about 50 per cent by weight of zirconia  reduced titania of 1.5 to about 10 percent in weight  carbon in an amount of 0.03 to about 0.5% by weight  impurities not greater than 3% by weight and balance of alumina. The abrasive grain of this patent has a high proportion of tetragonal zirconia. This patent also teaches about method for producing such abrasive grains. The abrasive grains of this patent may be produced by combining and melting alumina and zirconia; adding titania and carbon; melting and reducing the titania under reducing furnace conditions and solidifying the melt by means of suitable heat sink material. These abrasive grains may be used to produce coated abrasive products or bounded abrasive products. This patent teaches use of sufficient carbon in the said proportion in order that the titania in the product be properly reduced. Carbon is an essential ingredient of abrasive grain in the amount given and further process of manufacturing abrasive grains by the process provided in this patent result in impurities upto 3% in final composition.
The inclusion of grains into wear parts is known in the art. Conventional hard metal materials used to provide wear resistance to the underlying steel substrate usually comprises pellets or particles of cemented tungsten carbide (WC--Co) and/or cast carbide particles that are embedded or suspended within a steel matrix. The carbide materials are used to impart properties of wear resistance and fracture resistance to the steel matrix. Conventional hard metal materials useful for forming a hard faced layer on bits may also include one or more alloys to provide one or more certain desired physical properties. As mentioned above  the hard faced layer is bonded or applied to the underlying steel teeth by a welding process.
The hard faced layer is conventionally applied onto the milled teeth by oxyacetylene or atomic hydrogen welding. The hard facing process makes use of a welding "rod" or stick that is formed of a tube of mild steel sheet enclosing filler which is made up of primarily carbide particles. The filler may also include deoxidizer for the steel  flux and a resin binder. The relatively wear resistant filler material is typically applied to the underlying steel tooth surface  and the underlying tooth surface is thus hard faced  by melting an end of the rod on the face of the tooth. The steel tube melts to weld to the steel tooth and provide the matrix for the carbide particles in the tube. The deoxidizer alloys with the mild steel of the tube.
This hard facing process is effective for providing a good bond between the steel substrate and conventional hard metal material. However  it is a relatively crude process that is known to adversely impact performance properties of the hard faced layer. The hard facing welding process itself generates certain welding byproducts that can and do enter the applied material to produce an inconsistent material microstructure. For example  the welding process is known to introduce oxide inclusions and h-phases into the applied material  which then disrupt the desired material microstructure. Such disruptions or inconsistencies in the material microstructure are known to cause premature chipping  flaking  fracturing  and ultimately failure of the hard faced layer. Additionally  the welding process and associated thermal impact thereof can cause cracks to develop in the material microstructure  which can also cause premature chipping  flaking  fracturing  and ultimately failure of the hard faced layer.
Additionally  the hard facing process of welding the carbide-containing steel material onto the underlying substrate makes it difficult to provide a hard faced layer having a consistent coating thickness  which ultimately governs the rate of wear loss for the steel material  and the related service life of bit.
Examples of conventional hard metal materials  useful for forming a conventional hard faced layer  typically comprise in the range of from about 30 to 40 percent by weight steel  and include carbide pellets and/or particles having a particle size in the range from about 200 to 1 000 micrometers. Examples of conventional materials used for forming hard faced layers can be found in U.S patent numbers US 4 944 774; US 5 663 512; and US5  921 330.
The combination of relatively high steel content and large carbide particle size for such conventional hard metal materials dictate that the mean spacing between the carbide pellets within the steel matrix be relatively large  e. g.  greater than about 10 micrometers. It is believed that this relatively large mean spacing of carbide particles within the conventional hard metal material causes preferential wear of the steel matrix that is known to lead to uprooting and removal of the carbide particles. Such wear loss is known to occur along the milled tooth tip at high stress locations during drilling and functions to accelerate loss of the hard facing  which is a predominant failure mechanism for hard faced bit surfaces  thereby limiting the service life of such bits.
Another significant problem faced with wear components of the type described above is that a balance has to be drawn between the desired properties of ductility and resistance to abrasion. The common solution in the art is to provide a ductile metal part with wear inserts on the wearing face to impart toughness and abrasion resistance. The prior art has focused on improving wear resistance by improving the nature of the inserts.

OBJECTS OF THE INVENTION

The main object of the invention is to provide an improved wear resistant metal part.
Yet another object of the invention is to provide a method for the manufacture of an improved wear resistant metal part.
Yet another object of the invention is to provide a method for improving wear characteristics of metallic parts used by various industries like cement  mining and thermal power.
SUMMARY OF THE INVENTION

The invention accordingly provides mineral grains at the wearing surface  wherein mineral grain comprises 38 to 42 % Zirconia and balance of Alumina and less then 2 % of impurities. The invention also provides a method for the manufacture of a wear resistant metallic part comprising introducing mineral grains at the wearing surface  said mineral grains comprising a eutectic mixture of discovered advantageous ratio of zirconium oxide (zirconia) and aluminium oxide (alumina) wherein  38 to 42 % Zirconia and balance of Alumina is present. The mineral grains are introduced into the wearing surface of the metallic part by forming cakes of the desired shape. The mixture of mineral particles and an organic or inorganic binder is filled in boxes of suitable shapes and optionally treated with gases  to develop adequate strength. The cakes are heated to a temperature in the range of 80 - 220°C. The cakes are placed on the surface of refractory mold or metal die corresponding to wearing surface of the casting being produced  and then liquid metal is poured into the cavity. The metallic die/mold being then disturbed after cooling and the wear parts then removed and subjected to a heat treatment to provide wear resistance and toughness to the metallic portion.
In one embodiment of the invention  the mixture comprises alumina  alumina/zirconia eutectic.
In one embodiment of the invention  the said cakes being imparted strength by addition of fine aluminium oxide powder along with sodium silicate.
In one embodiment of the invention  the liquid metal composition being selected to enhance wear resistance of resultant metal matrix ceramic composite. In another embodiment of the invention  the metallic part comprises a grinding roll for a thermal power station.
In another embodiment of the invention  inserts are provided in the metallic die  followed by pouring of iron into the die to develop the composite casting  the casting thus obtained being heat treated to improve wear resistance and toughness.
In another embodiment of the invention  the metallic wear part is a table liner for a vertical mill  said method comprising introducing cakes of mineral grains at the wear surface of the part of the casting  the casting being produced by a conventional foundry casting process  the casting thus obtained being heat treated and machined to impart and improve wear resistance.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has succeeded in improving wear characteristics of Metal matrix ceramic composite used by various industries like cement  mining and thermal power. The improvement has been achieved by introducing mineral grains at the wearing surface. Minerals which have been proved to improve the wear resistance have been identified as Zirconia  and Alumina in the range of 38 to 42 % Zirconia and balance Alumina plus some impurities less then 2 %. The Zirconia and Alumina is present as eutectic mixture which provides exceptional combination of toughness and wear resistance. This is due to very fine intermixing of the grains. Minor amount of Alumina or Zirconia may be present as separate phases.

In order to able to introduce such grains at desired locations  it is necessary to form cakes of required shape. Moreover  such cakes have to possess adequate strength to withstand eventual engulfing by liquid metal. Addition of very fine Aluminium Oxide powder along with Sodium Silicate is aimed to achieve high strength. Addition of fine Aluminium Oxide powder imparts thixotropic properties and improves wear resistance. The mixture of previously described mineral particles  Alumina powder and organic or inorganic binder filled in boxes of suitable shapes and treated with gases  if required  to develop adequate strength for handling. After removing the cake from the boxes  the same are heated to temperature between 80-220 Deg C so that adequate strength is developed for handling.

The cakes are located at the surfaces of moulds or metallic dies. After closing the mould assemblies /die assemblies  liquid metal is poured into the cavity. The moulds/
dies are disturbed after adequate cooling time and the wear parts thus produced are subjected to special heat treatment so that metallic portion develops better wear resistance and toughness.
In one embodiment of the invention  a wear resistant metallic part  comprising a metallic body with one or more wearing surfaces  said wearing surfaces being provided with mineral grains comprising substantially a eutectic mixture of zirconium oxide and aluminium oxide wherein Zirconia is 38 to 42 % and balance is Alumina and less then 2% of impurities.
The zirconium oxide and aluminium oxide wherein Zirconia is 38 to 42 % and balance is Alumina and less then 2 % of impurities” is substantially a eutectic mixture.
As pure eutectic mixture is challenging to obtain hence  the composition is substantially a eutectic mixture and about 0.1 to 10 % of non-eutectic mixture of zirconium oxide and aluminium oxide is present in the composition.
In yet another embodiment of the invention  a wear resistant metallic part wherein wear resistant metallic parts are provided in the form of inserts.
In yet another embodiment of the invention  a wear resistant metallic part as wherein the mixture of alumina and alumina/zirconia is eutectic mixture.

Example

A metal matrix ceramic composite (MMCC) was produced using Alumina/Zirconia grains as described above. The particular wear component was used in one atox mill. Metal composition was 26% Cr / 2.7 % C alloyed with 1.2 % Ni and 0.92 Mo. This component showed 23 % improvement over conventional MMCC parts.

The weight percent of alumina and zirconia was specifically selected in order to obtain improved wear resistance over the prior art disclosed so far.

WE CLAIM:

1. A wear resistant metallic part  comprising a metallic body with one or more wearing surfaces  said wearing surfaces being provided with mineral grains comprising substantially a eutectic mixture of zirconium oxide and aluminium oxide wherein Zirconia is 38 to 42 % and balance is Alumina and less then 2% of impurities.

2. A wear resistant metallic part as claimed in claim 1  wherein wear resistant metallic parts are provided in the form of inserts.

3. A wear resistant metallic part as claimed in claim 1  wherein the mixture of alumina and alumina/zirconia is eutectic mixture.