662
T. Ebner et al.
Fig. 1 Microstructure of straight WC-Co grades with different Co contents (Co: white area) and
WC grain sizes (WC: gray area) [9]
material and strongly affects the characteristics of the composite. In commercially
available WC-Co grades 4–16 wt% binder metal is applied typically. The grain size
of tungsten carbides commonly accounts to values between submicron to tens of
micrometers. Figure 1 depicts the microstructure of two different cemented carbide
grades [1–8].
Cemented carbides are manufactured by a powder metallurgical route. At first,
the required amounts of tungsten carbide and cobalt powders are blended and milled.
Subsequently, a consolidation of the mixtures into various shapes by a wide range of
methods takes place. The further processing of these green parts exhibits particular
importance for the investigations of the present study, therefore, a more detailed
description of the sintering is given subsequently [5, 10].
Sintering converts the pressed powder into a dense and compact material by
treating the green parts at temperatures far below the melting point of the high-melting
hard phase (WC, T M,WC = 2870 °C) exceeding the eutectic point between WC and
the binder material (Co, T M,Co = 1493 °C) under vacuum conditions. A typical,
commercially applied WC-10%Co cemented carbide is sintered at 1400 °C, because
according to the WC-Co binary system, a liquid phase appears at temperatures above
1320 °C due to the occurrence of a low-melting eutectic. At these temperatures solid
WC and liquid cobalt, in which WC is dissolved, are present. Upon cooling, the
liquid phase solidifies containing 20–25% WC in solid solution. With decreasing
temperature, the WC content of the binder declines, because WC precipitates on the
surface of the undissolved hard phase resulting in a growth of the WC grains. In
principle, the binder contains less than 1% WC in solid solution at room temperature
[5, 10, 11].
The amount of remaining WC in the binder strongly depends on the carbon balance
of the cemented carbide. Only in a narrow range of a molar W:C ratio of about 1,
WC precipitates upon cooling. At a W:C ratio < 1, the formation of WC and C as
graphite occurs, at ratios > 1 Co 3 W 3 C, the so-called η phase, forms. Both cases
lead to undesired changes of the mechanical properties of the manufactured product.
Hence, it is of particular importance to control the carbon content and keep it within
T. Ebner et al.
Fig. 1 Microstructure of straight WC-Co grades with different Co contents (Co: white area) and
WC grain sizes (WC: gray area) [9]
material and strongly affects the characteristics of the composite. In commercially
available WC-Co grades 4–16 wt% binder metal is applied typically. The grain size
of tungsten carbides commonly accounts to values between submicron to tens of
micrometers. Figure 1 depicts the microstructure of two different cemented carbide
grades [1–8].
Cemented carbides are manufactured by a powder metallurgical route. At first,
the required amounts of tungsten carbide and cobalt powders are blended and milled.
Subsequently, a consolidation of the mixtures into various shapes by a wide range of
methods takes place. The further processing of these green parts exhibits particular
importance for the investigations of the present study, therefore, a more detailed
description of the sintering is given subsequently [5, 10].
Sintering converts the pressed powder into a dense and compact material by
treating the green parts at temperatures far below the melting point of the high-melting
hard phase (WC, T M,WC = 2870 °C) exceeding the eutectic point between WC and
the binder material (Co, T M,Co = 1493 °C) under vacuum conditions. A typical,
commercially applied WC-10%Co cemented carbide is sintered at 1400 °C, because
according to the WC-Co binary system, a liquid phase appears at temperatures above
1320 °C due to the occurrence of a low-melting eutectic. At these temperatures solid
WC and liquid cobalt, in which WC is dissolved, are present. Upon cooling, the
liquid phase solidifies containing 20–25% WC in solid solution. With decreasing
temperature, the WC content of the binder declines, because WC precipitates on the
surface of the undissolved hard phase resulting in a growth of the WC grains. In
principle, the binder contains less than 1% WC in solid solution at room temperature
[5, 10, 11].
The amount of remaining WC in the binder strongly depends on the carbon balance
of the cemented carbide. Only in a narrow range of a molar W:C ratio of about 1,
WC precipitates upon cooling. At a W:C ratio < 1, the formation of WC and C as
graphite occurs, at ratios > 1 Co 3 W 3 C, the so-called η phase, forms. Both cases
lead to undesired changes of the mechanical properties of the manufactured product.
Hence, it is of particular importance to control the carbon content and keep it within