Tungsten Carbide Cobalt Carbide
Tungsten carbide-cobalt hard metals ( WC-Co hard metals ) are hard metals that mainly consist of particles of tungsten carbide (WC) that are held together by a binding phase made of cobalt (Co). This carbide grade is the standard grade that makes up the largest proportion of hard metals. They are used for various tools, including cutting tools . Since they react chemically with steel at high temperatures, they are not used for machining (turning, milling, drilling) steel (there are special types for this ), but they are suitable for cast iron . When cutting materials they belong to the application group K. The abbreviation DIN is HW ( H style metal, W olframcarbid basis) or if they were coated HC ( H style metal C oated = coated). The smaller the grain size, the better the strength and hardness . Both are also influenced by the cobalt content.
Composition and structure
The tungsten carbide-cobalt hard metals consist mainly of tungsten carbide , which gives the material its hardness, and cobalt , which holds the individual WC grains together and improves toughness and strength. Tungsten carbide is a very hard material that is still hard enough even at 1000 ° C to be used as a tool material. It melts at 2600 ° C and has a very high compressive strength, but is brittle and breaks easily under bending stress. Cobalt makes up about 4% to 30% of the total mass - up to 12% for the types used for machining - and significantly improves the flexural strength compared to pure tungsten carbide. Various metals have been researched for this purpose, cobalt forms the strongest bonds with tungsten carbide and wets it very well, as both solidify in a hexagonal structure. The WC grains have a diameter of 10 µm to 0.5 µm; small ones improve both hardness and strength. They have prismatic shapes. Between them is the cobalt matrix . Ideally, only tungsten carbide and cobalt are used; if there is too little carbon in the material, the eta phase, a carbide with the composition Co 3 W 3 C, is formed, which reduces the strength. Too much carbon leads to elemental carbon ( graphite ) being present, which also reduces strength. Some of the carbon and tungsten is dissolved in the cobalt.
In some cases, small amounts of vanadium carbide (VC, up to 0.8%), chromium carbide (Cr 3 C 2 ) or tantalum - niobium carbide (both up to 2%) are used as doping additives , which ensure a fine-grained structure.
properties
The essential properties for tools are hardness, compressive strength and flexural strength. The compressive strength and the hardness decrease roughly proportionally with the increasing cobalt content, the bending strength increases sharply up to about 5%, after that only slowly. The WC-Co hard metals therefore all contain at least 4% cobalt.
They are classified in the application group "K" of cutting materials . The number below indicates how hard and firm the varieties are. Small numbers stand for great hardness and low flexural strength.
| HW-K05 | HW-K10 | HW-K25 | HW-K40 | |
|---|---|---|---|---|
| composition | 4% Co (WC-4Co) | 6% Co | 9% Co | 12% Co |
| Density [g / cm 3 ] | 15.1 | 14.9 | 14.6 | 14.2 |
| Compressive strength [N / mm²] | 5700 | 5400 | 5000 | 4500 |
| Flexural strength [N / mm²] | 1600 | 2000 | 2350 | 2450 |
| Modulus of elasticity [1000 N / mm²] | 650 | 630 | 590 | 580 |
| Fracture toughness [N m 1/2 / mm²] | 6.9 | 9.6 | 12.3 | 12.7 |
| Poisson's number [-] | 0.21 | 0.22 | 0.22 | 0.22 |
| Thermal conductivity [W / (mK)] | 80 | 80 | 70 | 65 |
| Thermal expansion coefficient [1 / 1,000,000 K] | 5 | 5.5 | 5.6 | 5.9 |
Grain sizes
The smaller the grain sizes, the stronger and harder the material is. According to DIN, a distinction is made between normal grain (HW) and fine grain (HF <2 µm).
| Cobalt content | Grain size of tungsten carbide [µm] | Vickers hardness | Breaking strength [N / mm²] | Compressive strength [N / mm²] |
|---|---|---|---|---|
| 6th | 0.7 | 1800 | 1750 | 4550 |
| 6th | 1.4 | 1575 | 2300 | 4250 |
| 9 | 1.4 | 1420 | 2400 | 4000 |
| 9 | 4.0 | 1210 | 2770 | 4000 |
Applications
WC-Co hard metals are mainly used for the machining of short-chipping materials, i.e. those that form short chips. This includes cast iron , non-ferrous metals , non-metals such as fiberglass-reinforced plastic , wood or stone .
wear
With WC-Co hard metals, different forms of wear occur. Diffusion is of particular importance .
Plastic deformation
In contrast to high-speed steels , hard metals do not undergo plastic deformation because of their high hot hardness and high temperature strength.
diffusion
The machining of steel with tools made of WC-Co hard metal can be impaired by diffusion. Diffusion is a phenomenon that only appears at high temperatures. Carbon and tungsten dissolve in the tool and migrate into the chips during machining. Conversely, atoms from the chip get into the tool, which reduces its strength. To avoid the effect, the cutting speed can be reduced, which leads to lower temperatures at which no diffusion takes place. However, these cutting speeds can also be achieved with high-speed steel. Diffusion does not play a special role in materials other than steel.
Bonding (adhesion)
At a low cutting speed, parts of the chip may stick to the tool due to adhesion . The following workpiece material pulls these adhesions with it and separates smaller parts of the tool from the surface. This wear mechanism occurs in many metallic materials, including cast iron, but it is relatively weak and the tool life is long.
Abrasion
Due to the high hardness of tungsten carbide particles are seldom affected by mechanical abrasion ( abrasion ). It can play a role when machining materials that contain large amounts of substances that are harder than tungsten carbide. Adhering grains of sand to cast parts are also problematic, but they are usually too rare to lead to significant abrasive wear.
literature
- Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, pp. 177-202.
- Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, pp. 119-137.
Individual evidence
- ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, pp. 177-179.
- ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, pp. 125f.
- ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 127.
- ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 127.
- ↑ (Including table) Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 180.
- ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 128.
- ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 129.
- ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 186.
- ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, pp. 186f.
- ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, pp. 188-195.
- ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, pp. 195-200.
- ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 200.