Manufacturing of Tungsten Carbide
Here we explain the different manufacturing methods we use
The powders used are mixed in a ball mill or an attritor and granulates are broken up. This requires a pressing agent, grinding body and grinding medium.
A pressing agent holds the pressed items together. Paraffin wax is suitable for this. As grinding body, balls made of carbide (ideally with the same composition as the granulate) with e.g. 8 mm, 4 mm or 1 mm diameter are used. A suitable grinding speed is crucial such that the grinding media does not damage the shell of the mill. The grinding medium serves to exclude air and as a solvent for the paraffin wax. Mostly hydrocarbons are used.
After grinding, the grinding media are sieved off. The granulation, which is required in die presses, is carried out industrially by spray drying. In the laboratory, the solvent is first removed in a vacuum. The ready-to-press carbide powder is squeezed through a sieve. The edges of the powder are sanded in a tumble mixer so that a powder that is easy to pour is created.
The indirect method of cold isostatic pressing and the direct methods of injection molding, extrusion molding and die pressing are available for shaping.
Cold isostatic pressing is suitable for large parts made of carbide. Pressure is exerted on the powder from all sides using a liquid medium and a rubber mold. The compacts can be processed further before sintering.
For extrusion and injection molding, thermoplastic plasticizers in the form of wax or organic polymers are kneaded into the vacuum-dried granulate for further processing. Now the plasticized mixture can be brought into the appropriate shape hydraulically and continuously depending on the desired diameter by screw (small diameter) or piston extrusion (diameter up to 30 mm) using a press nozzle. Threads and drills can be produced by extrusion. Injection molding is suitable for complex mass-produced components such as tungsten carbide watch cases.
The method of manufacturing large quantities of smaller tungsten carbide components is die pressing. The granulated powder is pressed into the desired shape under high pressure. The process is suitable for the production of indexable inserts and ballpoint pen balls.
Sintering takes place after the pressing process. It is mostly carried out in a vacuum oven. A specific temperature program is run here.
- At the beginning it is slowly heated to around 500°C. The existing plasticizer waxes are cracked and thus removed from the compound.
- After dewaxing, the temperature is slowly increased and further degassing takes place. Above all, the resulting carbon monoxide must be removed.
- The temperature is then slowly increased to around 1350°C-1500°C. The first melting processes begin at around 1310°C. A eutectic mixture of Co with some WC forms, which wets the WC crystallites and creeps along them. When the gas has been completely removed, the shrinking process starts, since a strong pressure acts on especially small pores. The material shrinks by up to 25%. When a coherent melt has formed, the cooling process can begin.
Almost all of the pores that were present in the tungsten carbide body before sintering can be removed in this way. In order to further increase the density, overpressure can now be applied using a protective gas (usually argon), which leads to further compression. This is known as HIP (hot isostatic pressing).
To analyze sintering, a dilatometer can be used to measure the distance covered during the shrinkage of the carbide metal sample. Using the DTA (differential thermal analysis) device, effects such as the melting of a component during sintering can be observed. Mass spectroscopy is also used to analyze the gases produced during sintering.
The W/C/Co phase diagram plays an important role in the manufacturing of tungsten carbide. The composition is applied against the temperature. The thermodynamically stable phases and the equilibrium curves for phase transition are important.
With the aid of a phase diagram, given a known composition of the mixture, it is possible to quickly find out which phases are present at a desired temperature and whether one is in the desired phase region.
Since the carbon content of the mixture can fluctuate due to the presence of atmospheric oxygen, it must be ensured that one is in the so-called "carbon window" of the three-component diagram W/C/Co. This is the two-phase area (here available: WC and Co mixed crystal), in which on the one hand no ternary carbide (W_3Co_3C), the so-called eta phase, is present (this would be the case with too little carbon), but in which on the other hand, there is also no elemental graphite (too much C). Both cases would have a negative impact on the desired material properties.
Inspection of the sinter batch begins with careful observation when removing the batch from the furnace. Uneven appearance of the furnace filling depending on the respective position, noticeable sintering distortion and other irregularities already give a person skilled in the art indications of the possibility of incorrect sintering or the need for extended sampling if necessary. Such externally recognizable differences already offer under certain circumstances the possibility of separating out parts of the sintering for corrective after-treatment.
The density and coercive force as a measure of the degree of sintering and the magnetic saturation as an indicator of the carbon balance are initially determined on these samples, mostly non-destructively.
The fracture structure examined with a 10 to 15 times magnifying glass is also an important indicator for a skilled person, especially for the occurrence of carbon precipitates, eta phase and other structural inhomogeneities, as well as selective grain growth.
Für die Härteprüfung wird am besten eine Bruchfläche angeschliffen und poliert.
For the hardness test, it is best to grind and polish a fracture surface.
Further tests, such as precise porosity and structure tests, can be carried out on a case-by-case basis depending on the results of previous determinations. A batch or parts of a batch released based on the metallurgical tests are passed on to further quality controls, if necessary tailored to the application of the hard metal parts concerned. Above all, this includes checking the dimensions, as well as a visual inspection for breakouts and other defects that can be identified with the naked eye or a magnifying glass.