From raw material to final product

Tungsten carbide, commonly referred to as “carbide”, is a common material in shops. This tungsten and carbon compound has completely changed the world of metal cutting in the past few decades, increasing speed and feed rate and prolonging tool life. Tungsten carbide was first studied as a tool material in 1925. Later, Ge set up a special department to produce tungsten carbide cutting tools. In the late 1930s, Philip M. McKenna, the founder of Kennametal, found that adding titanium compounds to the mixture could make tools work better at higher speeds. This began to move towards today’s lightning cutting speed.

“Cemented carbide”, the materials constituting tools and blades, are actually tungsten carbide particles along with other materials, which are cemented together with metal cobalt as binder.

Beginning in the ground

There are several tungsten ores that can be mined, refined into tungsten or made into tungsten carbide. Wolframite is the most famous. The ore is crushed, heated and chemically treated into tungsten oxide.

Then, the fine tungsten oxide is carburized into tungsten carbide. In one method, tungsten oxide is mixed with graphite (carbon). Heating the mixture to 1200 ? C（2200 ? F) Above, a chemical reaction occurs to remove oxygen from the oxide and combine carbon with tungsten to form tungsten carbide.

Grain size defines properties

The size of carbide particles determines the mechanical properties of the final product. The particle size will depend on the size of tungsten oxide particles and the time and temperature of treating the oxide / carbon mixture.

Tungsten carbide particles are a small fraction the size of a grain of sand. They can range in size from half a micron to 10 microns. A series of sieves sort out different particle sizes: less than 1 micron, 1.5 micron, etc.

At this point, tungsten carbide is ready to be mixed into “grade powder”. In the tungsten carbide industry, people speak of grade rather than alloy, but they mean the same.

Tungsten carbide enters a mixing vessel together with other components of this grade. Powdered cobalt metal will act as a “glue” to bond the materials together. Other materials such as titanium carbide, tantalum carbide and niobium carbide are added to improve the properties of the material during cutting. Without these additives, when cutting ferrous materials, tungsten carbide tools may react chemically between the tool and workpiece debris, leaving pits in the tool, especially in high-speed cutting.

Mix it up

All these ingredients are blended with a liquid such as alcohol or hexane and placed in a mixing vessel, often a rotating drum called a ball mill. In addition to the grade ingredients, cemented balls 1/4″ to 5/8″ in diameter are added, to help the process of adhering the cobalt to the carbide grains. A ball mill may be as small as five inches in diameter by five inches long, or as large as a 55-gallon drum.

When the mixing is complete, the liquid must be removed. This typically happens in a spray dryer, which looks like a stainless steel silo. An inert drying gas, nitrogen or argon, is blown from the bottom up. When all the liquid is removed, the remaining dry material is “grade powder,” which looks like sand.

For cutter inserts, the grade powder goes into insert shaped molds specially designed to allow for the shrinkage that will happen later on in the process. The powder is compressed into the molds, in a process similar to how pharmaceutical tablets are formed.

燒結(jié)

The powder compacts are heated to a certain temperature (sintering temperature) and to maintain a certain time, then cool down, to obtain the required properties of materials, this process is called sintering. In the process of sintering, the bonding between particles is realized by heating by means of atomic migration. When the particles are bonded, the strength of the sintered body increases, and in most cases the density increases.

After the inserts are removed from the furnace and cooled, they are dense and hard. After a quality control check, the inserts are usually ground or honed to achieve the correct dimensions and cutting edge. Honing to a radius of 0.001″ is typical, though some parts receive a cutting-edge radius of half a thousandth or as large as 0.002″, and some are left “dead sharp,” as sintered.

Some types and designs of inserts come out of the sintering furnace in their final shape and in-spec, with the correct edge, and don’t need grinding or other operations.

The process for manufacturing blanks for solid carbide tools is very similar. The grade powder is pressed to shape and then sintered. The blank or stock may be ground to size afterward before shipping to the customer, who will form it by grinding or perhaps EDM.

Inserts bound for most non-ferrous applications may be ready to package and ship at this point. Those destined for cutting ferrous metals, high temperature alloys or titanium, will need to be coated.

coatings drop the scene

To prolong tool life under challenging cutting conditions, many types and combinations of coatings have been developed. They can be applied in two ways: by chemical vapor deposition (CVD) or physical vapor deposition (PVD). Both types are applied in furnaces.

Chemical vapor deposition

For CVD, the coating is usually 5-20 microns thick. Milling and drilling blades typically achieve a hardness of 5 – 8 microns because these operations require better surface finish and more impact, so greater edge toughness is required. For turning applications, the coating is often in the range of 8-20 microns. When cornering, heat and wear are often more worrying.

Most CVD coatings consist of multiple layers, usually three layers.

Each company has its own coating “formula”. This is a typical scheme, which consists of three layers.

? a layer of titanium carbide with hardness and wear resistance

? a layer of alumina, which maintains hardness at high temperature and has very stable chemical properties

? a layer of titanium nitride to prevent metal accumulation caused by workpiece fragments welded to the tool. This coating is golden and edge wear is easily observed. In order to apply CVD coating, the parts are placed on pallets and sealed in the furnace. The furnace was evacuated.

Physical vapor deposition

PVD coating is usually about 2-4 microns thick. Different manufacturers use different layers. These PVD coatings are very suitable for cutting high temperature, nickel based, cobalt based or titanium based materials, and sometimes steel and stainless steel.

Titanium carbonitride, titanium nitride and titanium aluminum nitride are widely used as PVD coatings. The latter is the hardest PVD coating with the highest chemical stability.

The inserts are mounted on the frame so that they are separated from each other. Each rack rotates and the entire rack assembly rotates in the furnace so that each surface of the insert is exposed to the deposition process. The stove was emptied.

A strong negative charge is applied to the plug-in. Install a piece of titanium or titanium and aluminum on the wall or floor of the furnace. Metals evaporate through an arc or electron beam, releasing positively charged metal ions. These ions are attracted by negatively charged inserts. Nitrogen and methane are added appropriately to obtain different types of coatings.

After the insert is removed from the furnace, it can be ground again or packaged and shipped directly.

By continuously improving the design of tungsten carbide tools and developing better and better coating technology, tool manufacturers are coping with the pressure of increasing feed rate and speed, as well as the need to prolong tool life and reduce cost.