Corrosion resistant grades

This group contains cemented carbide grades in which the binder phase has been specifically designed to raise corrosion resistance to a level exceeding that of the grades that contain cobalt (Co) alone as the binder phase. This is achieved by alloying Co with elements such as nickel (Ni) and chromium (Cr) or completely replacing it with a more corrosion-resistant alloy.

 

The binder phase has been specifically designed to raise corrosion resistance

 

The susceptibility of the binder phase of cemented carbides to wet corrosion can result in wear problems. Corrosion mechanisms give rise to surface depletion of the binder phase, permitting the carbide grains to become detached relatively easily by the wear process. Awareness of this situation is important to the selection of the correct cemented carbide for a particular application, like carbide cutting tools.

Cobalt is unsuitable as a binder phase in wet corrosion conditions. Hyperion Materials & Technologies has developed a series of highly corrosion resistant grades for these applications (i.e., carbide cutting tools). As illustrated, straight WC-Co grades are corrosion resistant down to pH 7. This is also valid for WC-Co grades containing g- phase (i.e., TiC, TaC, and NbC). The highest corrosion resistance is obtained for the TiC-Ni grades, which are resistant down to pH 1. However, compared with the straight WC-Co grades, they have low strength and inferior thermal conductivity. In addition, they are difficult to grind and have poor brazeability. Thus, they are used only when corrosion resistance requirements are high, combined with low demands in terms of mechanical strength and thermal shock resistance.

In most corrosion-wear situations, an optimum choice is the WC-Ni grades, which are resistant down to pH 2 to 3. These grades retain WC as the hard phase and substitute Co for Ni; thus, they exhibit mechanical and thermal properties similar to those of the WC-Co grades.

 

The first choice material in hostile environments

 

Hyperion grade DZ10 is the choice for outstanding performance, compared to standard grades, in the production of both aluminum and steel cans.

The failure mechanism of classical WC-CO carbide punches used for production of two piece beverage cans by Draw and Wall Ironing (DWI) is primarily due to leaching of the binder phase. DZ family grades (sub-micron carbide grains combined with appropriate binder) have been tailored to fulfill the specific requirements of the can tooling industry. This means a material with very high wear and corrosion resistance. The shift from the classical grade used in the field to the specific DZ10 grade has brought many valuable improvements. Lifetimes have tripled and wear has been reduced, allowing more regrinds per punch. After 19 million cans, a wear of 0.002 mm (0.0000079 in) was found for the DZ10 punch while 0.007 mm (0.00000275 in) was found for standard grades after only 14 million cans.