Hard metal is a powder metallurgical material that is made up of:
- WC hard particles (tungsten carbide)
- A binder metal, cobalt (Co)
- Ti, Ta, Nb hard particles (titanium carbides, tantalum, niobium)
A grade represents the hardness or toughness of the insert and is determined by the mix of ingredients that make up the substrate.
Hard metal microstructure
Hard metal is made up of hard particles (carbides) in a binding matrix.
The binder is in almost all cases cobalt (Co), but it can also be nickel (Ni). The hard particles are mainly made up of tungsten carbide (WC) with the possible addition of a gamma phase (Ti, Ta, Nb nitrides and carbides).
Gamma phase (TiC)
The gamma phase has better heat resistance and is less reactive at high temperatures, so it is often used in grades where the cutting temperature can
be tall WC has better resistance to abrasion wear.
Fundamental characteristics of Hard Metal
In addition to the grain size of WC, the amount of binder phase (cobalt) is an important factor in determining the characteristics of the carbide.
An increase in Co content and WC grain size increases the toughness in the core, but also reduces the hardness. As a result, the substrate will have less resistance to creep and this means less wear resistance/practical tool life.
Hard metal fabrication
Carbide insert manufacturing is a carefully designed process where geometry and quality are balanced to deliver a product that is perfectly suited to the application.
There are two main elements of a carbide insert:
– WC = tungsten carbide
– Co = cobalt
Other commonly used elements are titanium, tantalum and niobium carbides. The design of different types of spraying and different percentages of each element is what differentiates the qualities.
The powder is crushed and spray-dried, sieved and poured into containers.
Tungsten Carbide Grain Size
The main raw material for the manufacture of hard metal is the concentrate
of tungsten ore. Tungsten powder is produced from chemically derived tungstic oxide from the raw material. By modifying the reduction conditions it is possible to prepare tungsten powder with different grain sizes. The carbide granulate after evaporation drying is small in size and varies in size depending on the quality.
Basic properties of hard metal
In addition to the grain size of WC, the amount of binder phase is an important factor in determining the characteristics of the carbide. An increase in the Co content and in the grain size of WC increases the toughness, but also reduces the hardness and with it the wear resistance of the substrate.
compacted powder pressing
The pressing operation has several tools:
– Upper and lower stuffers
– Center pin
– The powder is poured into the cavity
– Upper and lower stuffers are joined (20-50 tons)
– A robot picks up the insert and deposits it on a granite tray
– A random SPC control is carried out, to check the weight
The insert has a porosity of 50% in this phase
Sintering of pressed inserts
Sintering consists of the following phases:
– Loading of insert trays in a sintering furnace
– Temperature rises up to ~1400 °C
– This process melts the cobalt that acts as a binder
– The insert shrinks by 18% in all directions during the sintering phase, resulting in a 50% volume reduction