Carbide grades for steel processing

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Carbide types for steel processing are special types of hard metals that not only consist of tungsten carbide ( phase) and cobalt ( phase) like the tungsten carbide-cobalt hard metals (WC-Co hard metals), but also of other carbides , especially of Titanium carbide , tantalum carbide or niobium carbide ( phase). There are no uniform names. There are the abbreviations "WC-TiC-Ta- (Nb) -Co alloys" and "WC- (Ti, Ta, Nb) C-Co". They are used almost exclusively for machining (turning, milling, drilling) steel . With other materials, other types of carbide are more suitable, but these are usually not very suitable for machining steel. Carbide grades with a proportion of gamma phase below 10 to 11 percent by mass are used as transition grades for machining rustproof, acidic and heat-resistant steels with an austenitic structure and for corresponding cast materials (e.g. cast iron with nodular graphite , malleable cast iron in the tempered state). Grades with larger contents are used for other types of steel. In the standardization of cutting materials they have the initials HW ( H style metal W olframcarbid basis), a distinction to pure WC-Co grades via the application group: The low-transition group belonging to the application group M alloyed to the P group.

The grades described here are not the only hard metals that are suitable for machining steel. Cermets - tungsten carbide-free types - are also possible .

Composition and structure

Both the composition and the structure are similar to the common WC-Co hard metals. In addition, parts of the gamma phase are also included. The main mass is made up of the various carbides. Tungsten carbide is in the form of prismatic particles, the other carbides form round (globulitic) grains. Between the carbides is the matrix of cobalt with dissolved tungsten and carbon.

With proportions of titanium carbide of more than 25 ( mass)% , no more tungsten carbide can be seen in the structure. This is because tungsten carbide has more than three times the density than titanium carbide, while the volume fraction can be seen in the structure. For equal proportions of mass, titanium carbide fills three times the volume.

No titanium carbide is dissolved in tungsten carbide, but conversely, up to 50 percent by mass of tungsten carbide is dissolved in titanium carbide. If tantalum carbide is contained, it can also be detached from the titanium carbide, since both solidify in the same lattice modification ( face-centered cubic , in contrast to the hexagonal tungsten carbide and cobalt). The dissolved structures are also known as mixed crystals . Since this microstructure was first discovered in Germany in hard metals, the literal translation in English is the titanium carbide components of hard metals as mixed crystals, while other mixed crystals are referred to as solid solution (literally: solid solution).

properties

The steel types of the hard metals, like all hard metals, are very hard and pressure-resistant. They are inferior to pure WC-Co types, but they are more chemically resistant to steel. Grades with a high proportion of gamma phase are more resistant to chemical reactions with steel and allow higher cutting speeds, but reduce the strength, which is why grades with low contents are more likely to be used. The achievable cutting speeds are about three times higher than with pure WC-Co hard metals.

Tungsten carbide (WC) [%] 60 64.5 72.7 78.5 84.5 82.5
(Ti, Ta, Nb) C (gamma phase) [%] 31.0 25.5 17.3 10.0 9.5 11.0
Cobalt (Co) [%] 9.0 10.0 10.0 11.5 6.0 6.5
Density [g / cm 3 ] 10.6 11.7 12.6 13.0 13.3
Vickers hardness [-] 1560 1500 1490 1380 1700 1550
Compressive strength [N / mm²] 4500 5200 4600 4450 5950 5500
Flexural strength [N / mm²] 1700 2000 2200 2250 1750 1900
Modulus of elasticity [1000 N / mm²] 520 500 550 560 580 570
Fracture toughness [N m 1/2 / mm²] 8.1 9.5 10.0 10.9 9.0 10.5
Poisson's number [-] 0.22 0.23 0.22 0.23 0.22 0.22
Thermal conductivity [W / (mK)] 25th 20th 45 60 83 90
Thermal expansion coefficient [1 / 1,000,000 K] 7.2 7.9 6.7 6.4 6.0 6.0

literature

  • Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, pp. 202-210.

Individual evidence

  1. ^ Fritz, Schulze: Manufacturing Technology , 11th Edition, p. 297.
  2. ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, pp. 126, 129.
  3. ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 125 f.
  4. ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 204.
  5. ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 205.
  6. ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 130.
  7. ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 205.
  8. ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 206.
  9. ↑ Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 130.