Stratified enriched zones formed by the gas phase carburization and the slow cooling of cemented carbide substrates, and methods of manufacture

#1. Field of the Invention

#This invention relates to a WC-based cutting tool having a combination of toughness and deformation resistance which consists of a cemented carbide substrate with a stratified, cobalt-enriched surface zone and a double-layer coating of titanium carbide and titanium nitride. The term "stratified" refers to the layered appearance of the cobalt in the enriched zone. This substrate provides high thermal and mechanical shock resistance for maximum edge strength and increased insert toughness. By combining the enriched surface layer with the core substrate at 6% cobalt, the propagation of cracks from the coating is prevented and the cutting edge still maintains a minimum of deformation due to the heat and high forces generated at the cutting edge. The double layer of TiC and TiN, applied by chemical vapor deposition, enhances insert wear resistance and surface lubricity thereby enhancing tool life.

#This cemented carbide cutting grade is ideal for heavy roughing applications (that is, high metal removal rates) on carbon and alloy steels, tool steels, stainless steels and cast iron. The inherent toughness of this grade, as a result of the cobalt distribution through the enriched zone, provides reliable performance in interrupted cuts and in heavy-scale or out-of-round conditions found in castings and forgings. The coating protects the substrate from abrasion and chemical attack from scale and by the steel being machined.

#FIG. 1 is the Vickers microhardness of a treated substrate as a function of the distance from the coating/substrate interface.

#FIG. 2 is the cobalt content of a treated substrate as a function of the distance from the coating/substrate interface.

#FIG. 3 is the tungsten, cobalt and titanium contents of a treated substrate as a function of the distance from the coating/substrate interface.

#A stratified substrate is extremely difficult to manufacture. The stratification develops when the part, having a critical carbon content, is cooled slowly through the three phase WC-liquid binder-solid binder region of the phase diagram. This critical carbon content is a very narrow range in the free carbon region centered around the C08 porosity rating [The rating of porosities is in accordance with ASTM B276 ("Standard Test Method for Apparent Porosity in Cemented Carbides", B276, Annual Book of ASTM Standards, American Society for Testing and Materials, 1916 Race Street, Philadelphia Pa. 191030.] Our experiences have shown that for a given cooling rate--typically 1.5.degree. C./minute--acceptable enriched zones for optimum metal cutting performance are obtained with a carbon window of only .+-.0.007 weight percent carbon. This is less than the precision of carbon determinations by chemical analyses (.+-.0.02 weight percent) and considerably less than the carbon control required on conventional grades (.+-.0.05 weight percent). Such a fine control of the carbon level is extremely difficult to achieve.

#It is difficult to formulate weighed amounts of the constituent powders (WC, WTiC, TaC, cobalt and carbon), execute the many production steps (milling in an organic solvent, screening, addition of the fugitive binder and spray drying) and generate an alloy powder that will sinter to the precise requisite carbon level. Even if one is fortunate enough to achieve this in the freshly prepared powder, there is an aging phenomenon which lowers the sintered carbon level. Air and moisture slowly oxidize the finely divided cobalt. Then, during subsequent sintering of the pressed parts, carbon will be lost (as carbon monoxide) as these oxides react with the tungsten carbide and excess carbon in the part. Also, the pressed parts will slowly age while they are waiting for sintering.

#There is also a size effect; that is, larger size parts pressed from the same powder and sintered in the conventional manner in the same heat, will end up with slightly higher carbon contents. This size effect is noticeable even with the freshly prepared powders and is due, we believe, to the progressive difficulty of the paraffin wax to escape from the larger parts during the dewax portion of the cycle. This results in the cracking of the wax and the deposition of carbon in the interior of the parts. As the pressed parts age, the size effect becomes even more noticeable because the smaller parts will be influenced by air and moisture to a greater extent than the larger parts.