HARD METAL COATING

Abstract: The present invention relates to a method for coating a portion of a metal alloy work piece, such as a debarking tool, which contains carbon, with hard metal, such as tungsten carbide, wherein a hard metal rod, preferably a drill or mill blank, containing tungsten carbide, is applied to the work piece; and said hard metal rod is heated to about 1000-3000 DEG C to partly transform with the work piece, obtaining a transform area and a surface area, whereby the surface area contains essentially tungsten and the transform area contains essentially metal alloy and tungsten.

Description: HARD METAL COATING FIELD OF INVENTION The present invention relates generally to coating, and specifically to hard metal coating. BACKGROUND It has long been desired to be able to coat carbon steel with hard metal, since hard metal is much more durable than carbon steel but more expensive than carbon steel. With a base of carbon steel and a surface of hard metal a reasonably inexpensive work piece with a durable surface is achieved. Present solutions for achieving a hard metal surface are e.g. to solder specially treated hard metal material to a work piece, or to weld with special electrodes, such as crushed hard metal moulded m nickel. The different solutions have different problems. A soldered coating has lower endurance to heating than a welded coating, whereby a work piece with soldered coating withstands high working temperature less than a welded coating. Welding with a special electrode is expensive since such electrodes are expensive per se (to purchase) , and further a special electrode, such as an electrode of crushed hard metal moulded m nickel, achieves a non-homogenous joint of low hardness. SUMMARY OF THE INVENTION An object of the present invention is to provide a hard metal coating, which overcomes the above-mentioned disadvantages . This object is achieved by a method as claimed in claim 1. According to an aspect of the present invention a hard metal rod, preferably a drill or mill blank, is applied to a work piece and heated to partly transform the hard metal rod into the work piece and thereby achieving a durable hard metal surface in an inexpensive way. Further features and advantages of the present invention will be evident from the following description. BRIEF DESCRIPTION OF THE DRAWING The present invention will become more fully understood from the detailed description of embodiment of the present invention given below and the accompanying figure, which is given by way of illustration only, and thus are not limitative of the present invention, wherein : Fig. 1 schematically depicts degree of hardness at increasing depth in a hard-metal coated work piece. DETAILED DESCRIPTION OF EMBODIMENT In the following description, for purpose of explanation and not limitation, specific details are set fourth, such as particular techniques and applications in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiment that depart from these specific details. In other instances, detailed descriptions of well-known methods and apparatuses are omitted so as not to obscure the description of the present invention with unnecessary details. An embodiment of the present invention will now be described. A debarking tool made of carbon steel has a cutting edge that after about 16 hours use in a barking machine needs to be recoated or the whole tool needs to be exchanged. The debarking tool is preferably preheated to about 200- 250 °C. The preheating is mainly dependent on the amount of carbon in the carbon steel, which the debarking tool is manufactured of. A hard metal rod, comprising tungsten carbide, which preferably is a drill or mill blank, is applied to the cutting edge of the debarking tool. The hard metal rod is heated to about 1500-1600 °C, which melts the hard metal rod and coats the cutting edge with a layer of tungsten carbide. The heating is preferably performed by TIG-welding, but may also be performed by plasma welding. The surface of the debarking tool now consists of tungsten carbide. The coated debarking tool is preferably cooled slowly by wrapping the coated debarking tool in wool, preferably specially treated wool preventing putting the wool on fire. The cooling is performed in dependence on the composition of the carbon steel. The coated cutting edge is preferably grinded to obtain a desired sharpness. The debarking tool with a coated cutting edge of hard metal is still effective after over 200 hours use. The debarking tool obtains three main layers at the cutting edge. A surface area containing mainly hard metal, i.e. tungsten (WC) , a transform area containing mainly tungsten and carbon steel, and the rest containing mainly carbon steel. For illustration Fig. 1 schematically depicts the hardness against the depth of a carbon steel work piece coated with hard metal, where a rod containing 80% tungsten carbide and 20% cobalt (as binder) , was used as hard metal rod. The carbon steel work piece coated with the hard metal mentioned above contains three main layers: I, II and III. The first layer I is the surface layer, which contains mainly tungsten with a hardness of about 1000 HV. The third layer III is the carbon steel work piece, which contains mainly carbon steel with a hardness of about 250 HV. The second layer II, i.e. the transform area, is an area where the carbide has transformed with the carbon in the carbon steel and formed a transient area where the hardness decreases from the layer I to the III. The layer II may have deviations from the decreasing hardness dependent on how smooth the transition is. Changing the composition of the hard metal rod, i.e. by choice of drill or mill blank, may vary the hardness of the coated debarking tool. The more binder, such as cobalt or nickel, a hard metal rod contains the lower hardness is obtained. Different blanks may have different grain sizes. It is as mentioned above possible to alternatively heat the hard metal rod by plasma welding. Plasma welding obtains higher temperature than TIG-welding and is preferably utilized with blanks that have large grain sizes, whereas TIG-welding obtains lower temperature and is preferably utilized with blanks that have smaller grain sizes. Another possible method of heating the hard metal rod is to use gas welding. The method described above achieves a hardness of the surface of the coated work piece of at least 80% of which pure hard metal has, of which was used for coating that work piece. It has been described above how a debarking tool is coated with hard metal. Another tool that advantageously may be coated with hard metal according to the method above is an axle. Other surfaces that advantageously may be coated with hard metal according to the method above, with the exception that grinding is not necessary, are wearing plate, dipper ladle, and at a excavation machine. A layer of hard metal on a work piece has a thickness of about 0.5-3 mm. If a thicker layer is desired it is possible to coat the already coated work piece with another layer, obtaining a thicker layer of hard metal on the work piece. It is further possible to partly coat a work piece, e.g. a string, a spot, or a joint, which still improves the hardness of the surface, but with less hard metal. All wearing, cutting, grinding and crushing surfaces that contains carbon steel may benefit from a hard metal coating. Additionally slide, bearing, and slide bearing surfaces may benefit from a hard metal coating. It will be obvious that the present invention may be varied in a plurality of ways. Such variations are not to be regarded as departure from the scope of the present invention. All such variations as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.

Claims: CLAIMS 1. A method for coating a portion of a metal alloy work piece, containing carbon, with hard metal, c h a r a c t e r i z e d b y the steps of a) applying a hard metal rod, containing tungsten carbide, to the work piece; and b) heating said hard metal rod to 1000-3000 °C to partly transform with the work piece, obtaining a transform layer and a surface layer, whereby the surface layer contains substantially tungsten and the transform layer contains substantially metal alloy and tungsten. 2. The method as claimed in claim 1, wherein said hard metal is heated to 1300-1800 °C, preferably to 1400-1700 °C, and more preferably to 1500-1600 °C . 3. The method as claimed in claim 1 or 2, wherein the applying step is preceded by a step: preheating the work piece to 50-500 °C . 4. The method as claimed n claim 3, wherein said work piece is preheating to 100-350 °C, and preferably to 200- 250 °C. 5. The method as claimed m any of claims 1-4, wherein said heating step is followed by a step: cooling the coated work piece. 6. The method as claimed in claim 5, wherein the cooling is performed by wrapping the coated work piece m wool. 7. The method as claimed in claim 5 or 6, wherein said coo.ling step is followed by a step: grinding the coated work piece. 8. The method as claimed in any of claims 1-7, wherein the heating is obtained by TIG-welding, plasma welding, or gas welding. 9. The method as claimed in any of claims 1-8, wherein the hard metal rod contains of about 80% tungsten carbide and about 20% cobalt, whereby a hardness degree of at least 900 HV is obtained. 10. The method as claimed in claim 9, wherein a hardness degree of at least 950 HV is obtained, preferably at least 975 HV, and more preferably at least 1000 HV. 11. A method for coating a portion of a metal alloy work piece, containing carbon, with hard metal, c h a r a c t e r i z e d b y the steps of a) applying a hard metal rod, containing tungsten carbide, to the work piece, wherein the hard metal rod is a drill or mill blank; and b) heating said hard metal rod to about 1000-3000 °C to partly transform with the work piece, obtaining a transform layer and a surface layer, whereby the surface layer contains substantially tungsten and the transform layer contains substantially metal alloy and carbide. 12. A metal alloy work piece, containing carbon, coated with a hard metal by a method as claimed in any previous claim, wherein the surface of said hard metal coating has a hardness of at least 80% compared to the hardness of the hard metal per se.

Inventors: GUSTAVSEN STEINAR, GUSTAVSEN, STEINAR
Applicants: GUSTAVSEN STEINAR, MAELAR TRADING AB, GUSTAVSEN, STEINAR
Application year: 2000
Main classification: B23K9
IPC codes: B23K31, B23K35