Cemented carbide rod with double coolant holes

Carbide production

The productionprocess of powder-metallurgical products basically includes the four steps of powder preparation, forming, sintering andfinishing.

Tungsten carbide production

The APT (ammonium para-tungstate) is calcined into tungsten oxide under high te-mperature. Subsequently the oxide is reduced to tungsten metal in a hydrogen atmosphere. The metal powder is then mixed with carbon and carburised under

inert atmosphere at high temperatures. The production parameters are decisive for the WC grain size in the sintered carbide.

Powder preparation

The tungsten carbide is intensely mixed with the binder metal

cobalt, nickel or iron, various grain growth inhibitors and materials,

which promote compaction, by wet grinding so that a

homogeneous suspension is created. Afterwards, the suspension

is dried in a spray tower to produce a granulate with good

flow characteristics. This granulate represents the basis for all

forming processes.

Metal forming

The objective of the forming process is to obtain a near net

shape sample. Pressing is normally carried out at room temperature

with pressures reaching up to several tons per square

centimetre.

There are several ways of pressing blanks:

During isostatic cold pressing the powder is filled into an elastic

flexible hose and pressed into a compacted form through

high liquid pressure. The powder blocks which are produced

in this way can then be processed mechanically. All common

machining methods like milling, cutting, drilling or turning may

be applied.

In uniaxial pressing the pressing tool consists of a die and an

upper and a lower punch. The carbide powder is filled into the

die and then compacted to create the so called green carbide,

which is ejected from the pressing die.

Extrusion pressing is mainly used to produce rectangular bar

or cylindrical rod, with or without axial hole(s). A plasticiser is

added to the powder. The resulting paste is pressed through

an extrusion nozzle. Before sintering, the plasticiser must be

evaporated in special drying furnaces.

Metal Injection Moulding (MIM) is a process used to produce

more complex forms which cannot be produced by direct

pressing. The paste preparation is similar to the extrusion

process.

Sintering process

The sintering process converts the blank into a homogeneous

and dense carbide with a high level of hardness. The material

is sintered at temperatures between 1,300 and 1,500 °C

(liquid phase sintering) and sometimes also at high pressure

(up to 100 bar). The volume is reduced by up to 50 % during

this process.

Finishing

In order to achieve the final requirements of surface finish, tolerances,

etc. carbide parts can be subjected to a series of finishing

processes such as grinding, spark erosion and coating.