Electromagnetic Testing Chapter 3- Electromagnetic Testing

Eddy Current Testing at High Temperatures for Controlling Heat
Treatment Processes
1 Introduction
Increasing quality demands have made a reliable quality control getting more and more important for manufacturing and
processing industries. Especially if 100% testing is required, the advantages of the eddy current technique are used by a
wide range of applications to ensure constant quality: eddy current measurements require minimal testing time, offer the
possibility to measure without human intervention and are non-destructive [1]. For heat treating processes, the method is
used in various applications. Aircraft industry for example uses the eddy current technique to measure the conductivity of
heat treated aluminium components to check hardness and tensile strength [2]. To ensure that aluminium plates have been
properly heat treated and exhibit a satisfactory degree of homogeneity, systems are developed that automatically measure
conductivity on the top and the bottom of the plate as sensors scan across [3]. Multi-frequency systems make the control of
ferromagnetic workpieces possible, for example to check material structure or cracks [4-6]. For all of these test procedures,
the measurements are performed after the heat treatment has taken place, as there are no systems, which can be used at
the heat treatment temperatures and atmospheres. Measurements during the treatment would offer much more information
about the process, because not only the final state, but also all the changes during heat treatment could be monitored
continuously. This additional information makes the process saver and can be used to adapt the process more easily, if
changes become necessary. To perform measurements under such difficult conditions at high temperatures the eddy
current technique shows to be the most suitable NDT system because of its easy handling and the simple build-up of the
sensor. The article describes the development of an eddy current measuring system to characterise micro-structural
changes during heat treatment. It can be used at temperatures up to 500°C. The construction of the sensor and the method
of data analysis is illustrated. The system was used to characterise the aging process of the Al Zn Mg Cu alloy EN AW 7075.
The correlation between micro-structural changes and the eddy current signal are discussed.
2 Eddy Current Measurements
2.1 How to detect the heat treatment state with eddy-current testing Eddy currents can be used for material characterisation
because the sensor signal is dependent on two material properties: the electric conductivity and the permeability. These
parameters are often connected strongly to technological relevant quantities, e.g. hardness. Thus the use of eddy currents in
a heat treatment process makes it possible to monitor the quantities characterising the heat treatment state continuously and
in-situ. 2.2 Build up of the measuring system The eddy current measuring system consists of three components: a heat
resistant eddy current sensor an impedance analyser Solartron 1260A a standard PC to save and to evaluate the data. The
impedance analyser supplies a sinusoidal current to the transmitting coil. At the same time the voltage of the receiving coil is
measured and the complex impedance (inductance and resistance) is computed. The sensor is a radial symmetrical twocoil-system.
The receiving coil is placed between the sample and the transmitting coil where the strength of the secondary
field reaches the highest values. The sensor has to be made of heat resistant materials, because it will be used inside the
furnace. The winding wire of the coils is a fibreglass-insulated cooper wire. The coil former is made of machinable ceramic.
The coils are encapsulated in a ceramic tube filled with ceramic sealant (Fig. 1).
More: http://www.ndt.net/article/ndtce03/papers/p022/p022.htm
Charlie Chong/ Fion Zhang
http://www.ndt.net/article/ndtce03/papers/p022/p022.htm