Hardmetal is the term used to signify a group of sintered, hard, wear-resisting materials based on the carbides of one or more of the elements tungsten, tantalum, titanium, molybdenum, niobium and vanadium, bonded with a metal of lower melting point usually cobalt. Tungsten carbide is the most widely used.
Cermet helical milling cutters can weigh up to 20kg (Kennametal Hertel AG)
Tungsten carbide is the most widely used.
These materials are commonly referred to as cemented carbides or simply as carbides as, for example, carbide tools.
- By varying the carbide particle size, the amount of binder metal, and the sintering conditions, the properties such as wear resistance, impact strength, resistance to cratering, and hot hardness may be optimised for a given application.
For example in the case of a wire drawing die wear resistance is the major requirement, but for a cutting tool, especially if subject to intermittent loading, high impact strength is required.
Hardmetal indexable inserts for turning made from the universal coated grade Widadur TN 250. (Widia)
- Other materials having metallic borides or nitrides cemented with a binder metal are also known, but these have limited special applications, and, although they are hard materials, they are not normally included in the term hardmetal as it is now defined.
- Diamond cutting tools are made by similar processes. In this case it is important to provide a matrix which gives maximum support to the diamonds in order to keep the tool sharp. Each cutting application requires, therefore, separate consideration, and matrix materials range from bronzes of different compositions to cemented carbidesy.
Diamond is still the hardest substance available for cutting but only in a few cases are large stones used on their own.The majority of diamond impregnated tooling is made using the PM techniques.
Currently they are extensively employed in stone processing, road repair, machining of glass, ceramics and metals, but one of the main and rapidly growing uses is cutting non-metallics, e.g. natural stone, concrete etc.A modern stone saw is basically a steel wheel with diamond impregnated segments brazed, or laser welded, to projections on the circumferenced.
It is normally made in three stages:
The mixing process, whereby the matrix powder-diamond blend is prepared, is usually carried out in three-dimensional, pendulum motion mixers.
- Both the careful selection of components and their efficient intermingling, to produce a uniform blend, have a great effect on the quality of the final product .
2. Dosing; manual mould filling; cold pre-pressing.
When the mixture is ready, carefully pre-calculated segment portions are weight out.
- The weighing is either performed by the automatic dosing machines, and followed by manual filling of the graphite mould, or carried out in the course of the automatic cold pre-pressing route.
- The latter option allows to produce multi-layer segments . It also increases productivity of the subsequent hot pressing stage since the purpose-designed graphite mould takes more segments than the conventional one, filled with loose powder.
The hot pressing cycle consists of the simultaneous application of heat and pressure so as to obtain, in a relatively short time, a product nearly free from internal porosity.
Heating rate, sintering temperature and holding time, as well as the applied pressure, will mainly depend on the matrix composition but also on the equipment available.
3. Second Operations
When the diamond containing components are made, there are still three additional non-PM operations to complete the saw production process.
- Grinding and sharpening
- Tension rolling and testing.
First the segments are brazed onto the steel core.
The automatic, high-frequency brazing machines make this process cost effective and eliminate the risk of overheating both segments and the steel core as well.
After grinding the segment sides and brazing zone as well as rounding off and sharpening the circumference the saw blade has to be properly tensioned and straightened by rolling and, when necessary, hammering.
The stresses built in to the blade in this way will adequately compensate the stresses created in use, by heat and the cutting forces, and cause the saw to run true.