Patent Publication Number: US-6033789-A

Title: High speed cutting tool

Description:
TECHNICAL FIELD 
     The present invention relates to a high speed cutting tool, the material of which it is constructed, and the method of making such a tool. More particularly, the invention relates to materials formed by compaction of powdered materials. 
     BACKGROUND ART 
     It is known to produce metallic bodies by hot isostatic pressing (hip-ing) of powdered steel. In this procedure, the steel powder is compacted physically within a tube which is then evacuated of gas and sealed. The tube is then placed in a furnace and heated to a temperature in the region of 1050 to 1250° C., usally 1100-1200° C. An inert gas such as argon is supplied to the furnace at a desired pressure which may be in the region of 103 MPa. The cycle time may be in the region of 2 to 6 hours, allowing slow cooling. The powder in the tube is thereby compacted to form a unitary steel body which is cohesive, homogenous, and substantially free of potential stress fractures. 
     It is also known to make high speed and other cutting. tools from materials such as carbide containing steel, or from other steels. However, the high speed at which they operate, which may be 20,000 rpm, causes a high degree of wear at the cutting edges of the tool. Attempts have been made to extend the life of the tool by coating the edges with titanium nitride, which will lower the coefficient of friction by up to one third. However, this is not entirely satisfactory since the titanium nitride coating quickly wears away so as to leave an unlubricated utting edge, which becomes blunt even more quickly. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to provide a high speed cutting tool and a material from which it is constructed which enables the cutting tool to give improved performance for a longer period of use. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     According to a first aspect of the present invention there is provided a cutting tool comprising at least one cutting edge formed by a compacted mixture of carbide containing alloy steel and a ceramic material. 
     Preferably the mixture of at least one cutting edge comprises additionally particles of a hard or abrasive material. 
     The abrasive material may comprise 0.01-15 wt % of the mixture, optionally in the region of 1 to 10 wt %. 
     The abrasive material may comprise a carbide, such as silicon carbide or aluminium carbide or a boride/carbide such as aluminium titanium diboride-titanium carbide. 
     The ceramic material preferably comprises 0.01-15 wt % of the mixture, optionally in the region of 1 to 6 wt %. 
     In preferred embodiments, the amount of ceramic material may be 2 to 5 wt %, advantageously in the region of 3 wt %. 
     The ceramic material may comprise zirconium oxide, optionally stabilised by a minor amount of calcium oxide. 
     The ceramic material may have a particle size in the region of 1 to 15 μm, preferably 1 to 4 μm. 
     The steel may have a particle size of less than or equal to 500 μm. 
     The hardness of the steel body formed from the powder may vary slightly with the powder size but is generally in the region of 270 to 295 Hv20, which is increased after hardening. 
     Particles of carbide in the steel may have a size in the region of 3 to 5 μm. 
     According to a second aspect of the present invention there is provided a metallic body comprising a steel core zone and a peripheral zone comprising a compacted mixture of carbide containing alloy steel and a ceramic material. 
     The core zone may comprise a compacted mass of powdered alloy steel. 
     The core zone may alternatively or additionally comprise a core of mild or other steel. 
     The peripheral zone may additionally comprise particles of a hard or abrasive material. 
     The abrasive material may comprise 0.01-15 wt % of the mixture, optionally in the region of 1 to 10 wt %. 
     The abrasive material may comprise a carbide, such as silicon carbide or aluminium carbide or a boride/carbide such as aluminium titanium diboride-titanium carbide. 
     The ceramic material preferably comprises 0.01-15 wt % of the mixture, optionally in the region of 1 to 6 wt %. 
     In preferred embodiments, the amount of ceramic material may be 2 to 5 wt %, advantageously in the region of 3 wt %. 
     The ceramic material may comprise zirconium oxide, optionally stabilised by a minor amount of calcium oxide. 
     The density of zirconium oxide is approximately 6 g/cm3, rendering it compatible for powder metallargy for combination with steel powder having a density in the region of 8 g/cm3. 
     The ceramic material may have a particle size in the region of 1 to 15 μm, preferably 1 to 4 μm. 
     The steel may have a particle size of less than or equal to 500μm. 
     Particles of carbide in the steel may have a size in the region of 3 to 5 μm. 
     The size of the ceramic powder may be selected to be greater than that of the general size of carbide particles. 
     According to a third aspect of the present invention there is provided a method of manufacturing a metallic body comprising the steps of providing a core of steel material, locating said body substantially centrally within a tube and filling an annular space between the core and the tube with a powdered mixture of steel and ceramic material, substantially evacuating the tube, sealing the tube, heating the tube at a high temperature, preferably in the region of 1000° C.-1300° C., supplying an inert gas external of the tube at a high pressure, preferably in the region of 14000-16000 psi, whereby the annular mixture is compacted and bonded to the core to form a unitary body. 
     The powdered mixture may comprise 0.1 to 15 wt % ceramic material, preferably 1 to 6 wt %, most advantageously in the region of 3 wt %. 
     The powdered mixture may additionally comprise 0.1 to 15 wt % hard or abrasive material, preferably 1 to 10 wt %. 
     The core of steel material may be formed from a powdered steel which is compacted concurrently with the mixture of steel and ceramic and optionally abrasive material in the peripheral zone. 
     Preferably said core may comprise powdered alloy steel. 
     Alternatively said core may comprise an iron containing body, which may optionally be surrounded by an intermediate zone comprising powdered alloy steel. 
     The powdered steel/ceramic mixture, and where appropriate, the powdered steel may comprise particles preferably of diameter no more than 500 μm. 
     Advantageously, the powdered alloy steel when compacted, contains carbide particles of size within the range of 3-5 μm. 
     The ceramic material provided in the mixture may comprise zirconium oxide optionally stabilised with calcium oxide. 
     Preferably, such zirconium oxide has a particle size greater than that of the carbide particles, preferably within the range 1 to 4 μm. 
     The abrasive material may comprise 0.01-15 wt % of the mixture, optionally in the region of 1 to 10wt %. 
     The abrasive material may comprise a carbide, such as silicon carbide or aluminium carbide or a boride/carbide such as aluminium titanium diboride-titanium carbide. 
     According to a fourth aspect of the present invention, there is provided a method of manufacturing a cutting tool, comprising the steps of forming a unitary body as described in the third aspect above, compacting the body, rough forming an exterior surface of the body to have at least one cutting zone, annealing and heat treating said body to cause hardening, and forming said at least one cutting zone to have a cutting edge. 
     Where the cutting tool is a gear cutting hob, the thickness of the peripheral steel/ceramic zone may be in the region of 1 to 2 inches (2.5 to 5.1 cm), some of which is removed to leave outstanding cutting edges comprising the steel/ceramic mixture, or the steel/ceramic/abrasive mixture. 
     Embodiments of the present invention will now be more particularly described by way of example. 
     Steel used as the basis in this example comprises the following; by wt %: 
     
         ______________________________________                                    
C         1.27   1.2         1.3  2.3                                     
Mn          0.27     0.3          0.3                                     
                                        0.4                               
Cr          4.04     4.0          4.2                                     
                                        4.0                               
Mo          4.52     4.8          5.2                                     
                                        7.0                               
V            2.03                                                         
                     2.9          3.2                                     
                                        6.5                               
Co          8.14     &lt;0.1     8.6      10.4                               
W            6.04                                                         
                     6.2          6.4                                     
                                        6.5                               
Si          0.27     0.3          0.55                                    
                                       0.5                                
S            0.03                                                         
                     &lt;0.1     &lt;0.1                                        
                                    &lt;0.1                                  
P            0.02                                                         
                     &lt;0.1     &lt;0.1                                        
                                    &lt;0.1                                  
Ni          0.09     &lt;0.1     &lt;0.1                                        
                                    &lt;0.1                                  
Cu          0.04     &lt;0.1     &lt;0.1                                        
                                    &lt;0.1                                  
Nb          0.01     &lt;0.1     &lt;0.1                                        
                                    &lt;0.1                                  
Ti          0.005                                                         
                    &lt;0.1      &lt;0.1                                        
                                    &lt;0.1                                  
______________________________________                                    
 
    
     In all cases remainder Fe and unavoidable impurities 
     Steels according to each of the above constitutions were powdered to a size of no more than 500 μm. The resulting powder was sieved to remove any oversized particles. The material was found to contain carbide particles, mostly, but not exclusively cobalt or tungsten carbide, which had a particle size of 3 to 5 μm. 
     The above powdered steel was then filled into a tube, located centrally within an outer tube. The annular space remaining was then filled with a mixture containing the same steel powder with the addition of 3 wt % zirconium oxide (stabilised by calcium oxide). This ceramic material had a particle size in a range of 1 to 4 μm. 
     The intermediate tube was then removed and the external tube and the contents thereof subjected to hot isostatic pressing (hip-ing). Gas from the tube is evacuated and the tube sealed. It is then placed in the furnace at a high temperature such as 1050 to 1250° C. and the furnace is subjected to a high pressure, such as 15,000 psi, by introduction of argon or some other inert gas. The powders are thereby compacted into a homogeneous unitary structure having a steel composition at its core and a steel/ceramic composition at its periphery. 
     In other Examples, the mixture contained additionally particles of a hard abrasive material such as silicon or aluminium carbide. 
     In some cases, it may be desirable to insert a central core of mild steel or other less expensive steel which may bond directly with the mixture of steel and ceramic, or may bond with an intermediate zone of compacted steel powder. Such a central core may be machined out if so required. 
     The material thus formed may then be converted into a high speed cutting tool, such as a gear cutting hob, a broach, a drill, a tap, a reamer, a shaper or any other similar cutting tool. One or more cutting edges may be formed roughly thereon, after which the material is annealed and hardened before final grinding is carried out to produce one or more cutting edges on the tool. 
     It has been found that tools embodying the present invention have a longer life, and it is thought that this may be due, in part, at least, to the heat absorbing properties of the ceramic material which enable the cutting edge to function at a lower temperature and thereby have a better edge retention. Given the high speed nature of the use of such tools (which may be as high as 20,000 rpm), the cooling effect should reduce or delay any tendency of the cutting edges to bluntness caused by frictional heating of the cutting edge. 
     Use of the invention also enables cutting tools to be manufactured from steels of lower hardness than is presently the case, for example from steel to British Standard M42, although it is equally applicable to harder steels such as those to BS T4.