Non-destructive evalution of hardmetals using magnetic methods
Dr. David Whittaker reviews this presentation exclusively for ipmd.net
A primary aim of hardmetal processing is to reproducibly produce an optimum composition and geometrical arrangement of the binder phase (most commonly cobalt).
Assessment of the binder phase by microstructural analysis is very complex and the method is also a destructive one. The determination of characteristic magnetic parameters, such as magnetic saturation, Ms, and coercive force, Hcj, have therefore emerged as simple, rapid and non-destructive means of assessing hardmetals.
This paper discussed the significance of these parameters and the latest techniques for their measurement and data management for quality and process control.
Hardmetals comprise carbides, which are non-magnetic, and a binder (cobalt), which is the only ferromagnetic component in the material. Magnetic Saturation measurement allows the magnetisable content of a material to be determined. Each material has a defined and constant magnetic saturation value. Measurement is, within broad limits, independent of the shape and size of the specimen. If the material, as is the case with hardmetals, contains only one magnetisable component (cobalt), it is possible to determine its content in % with very high resolution (10μg) and accuracy (0.5%).
In hardmetal production, the ratio of the ferromagnetic cobalt content before and after the sintering process is of great significance. The sintering process causes some of the cobalt, tungsten and carbon to dissolve and this portion of the cobalt enters into intermetallic combinations and becomes non-magnetic. From the measurements before and after sintering, it is possible to deduce the composition of the binder phase and cobalt-ferrous compounds.
The magnetisable cobalt content after sintering should lie in the range 85 to 95% of the original value. Tungsten does not dissolve if WC is saturated with carbon. Tungsten dissolves in the binder phase if the carbide has an inadequate carbon balance. A mixed carbide forms Co2W4C ==> Co3W3C ==>Co4W2C with decreasing carbon content. Cobalt is extracted from the binder phase with increasing cobalt share in this mixed carbide. This leads to a clear drop in the saturation magnetisation and signifies formation of the η phase. This happens when the binder (cobalt) is absorbed into the tungsten, leaving material in which there is no binder for the carbide and this is detrimental to tooling performance.
Thus, Magnetic Saturation measurement allows the composition of the binder phase to be determined and also allows grade separation on the basis of cobalt content.
Fig. 1 Magnetising coil and parts holder for magnetic saturation measurement (Courtesy MPIF)
The principle of magnetic saturation measurement is shown in Fig. 1. The test piece is magnetised to saturation in a permanent magnet Hallbach array. Prompted by the software, a holder is used and the piece is pulled out of the coil manually or pneumatically. During the time that the piece is retracted, the coils measure its magnetic moment and a fluxmeter in the electronics integrates the result. If the weight of the piece is input to the software prior to the measurement, the weight-specific magnetic saturation and magnetisable content as a percent can be determined from the following relationships:-
% magnetisable material = σs/material constant
Fig. 2 Hysteresis loop (Courtesy MPIF)
The hysteresis loop is the characteristic curve for ferromagnetic materials. It shows the ratio between the magnetic field strength H and the magnetic polarization J (Fig. 2). The coercive field strength, Hcj, is the field strength at which the polarisation J, which has remained in saturation as the result of magnetisation, returns to zero again. The coercive field strength represents a measure of the resistance to magnetic reversal. Its value is dependent on the mobility of the Bloch walls representing a boundary between regions magnetised in the same direction (Weiss domains). As the particle size of a magnetic material decreases, the number of Weiss domains increases, as does the individual leakage fields and therefore also the coercive force.
Statements on the structural properties of hardmetals can be made with the aid of the coercive field strength Hcj :
- The carbide grain size. The smaller the grain size of the tungsten carbides, the higher will be the Hcj value.
- The cobalt distribution. The thinner the cobalt layers between the tungsten carbide grains, the higher will be the Hcj value.
- The quantity of dissolved tungsten carbide in the binder phase. The more tungsten is dissolved in the binder phase (inadequate carbon balance), the higher will be the Hcj value.
- Stresses in the cobalt phase, for example resulting from grinding the powder or resulting from the sintering process. Mechanical stresses increase the Hcj value
Fig. 3 Approximation of coercive field during sintering
(Courtesy MPIF)
Since the coercive field strength responds to several parameters, it is sometimes difficult to interpret. The optimum coercive field strength Hcj must be determined by trials for each grade of material (Fig. 3).
The coercive field strength initially drops significantly as sintering temperature is increased because of relief of the mechanical stresses resulting from grinding, but then increases to the first maximum owing to the incipient edge welds and resultant stresses.
The shrinkage, which now occurs, then relieves the stresses occurring at the edge welds. The progressive process of isothermal crystalline modification causes a renewed rise in the coercive field strength. The cobalt is propagated over all carbide particles as the result of adhesion and, as a result of this process, is subjected to stress again. The optimum density and hardness is achieved at this second maximum. In practice, crystalline modification is continued further (slight undersintering) until the coercive field strength drops again. Hardmetals sintered in this way achieve the best ratio of hardness to toughness. The toughness then increases at the expense of the hardness as crystalline modification progresses.
The coercive field strength is directly related to the abrasion resistance and hardness of the hardmetal.
Fig. 4 Sensor arrangement for coercive force measurement (Courtesy MPIF)
A proven type of technology and equipment for measuring coercive force uses an open magnetising circuit as is shown in Fig. 4. Here, the test piece is magnetised in a coil to saturation. Then, the polarisation, J, of the piece is measured using a pair of Förster probes and the opposition field is built up until the polarisation is reduced to zero. This opposing field strength at zero polarisation is the coercive field strength.
The equipment offered by Odyssey Technology has software containing a “learn” program that automatically determines the optimum parameters.
The components of the coercive force measurement apparatus consist of; a) the magnetising coil that houses the coils as well as the measurement probe (Förster probes), (b) the Electronics Measurement module that contains the magnetometer and system controller and (c) the PC and software for the virtual user interface, display and analysis databases. Parts to be tested are positioned on a paddle, which acts as a sample holder and is inserted into the magnetising coil.
Fig. 5 Series measurements (Courtesy MPIF)
Results can be displayed in “Piece” or “Series” modes. The use of series measurement allows statistical data and sorting limits to be enabled. An example of series measurement results is shown in Fig. 5.
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