Use of a CCD camera for the thermographic study of a transient liquid phase bonding process in steel

The bonding of steel pieces and the development of novel soldering methods, appropriate to the extended variety of applications of steels nowadays, bring the sensing of temperature an outstanding role in any metallurgical process. Transient liquid phase bonding (TLPB) processes have been successfully employed to join metals, among them steels. A thin layer of metal A, with a liquids temperature Tu, is located between two pieces of metal B, with a liquids temperature T-LB higher than TLA The joining zone is heated up to a temperature T (T-LA<T<T-LB) and a bonded product with a near homogeneous composition is obtained. The most relevant parameter of the process is the bonding temperature T. The TLPB process for steel is performed in a 30 Kw induction furnace at temperatures in the range 800 degreesC to 1400 degreesC depending on the layer metal. A small window was opened between the central loops of the coil in order to observe the radiation emitted by the hot steel zone. A low price black and white CCD camera with 752x582 pixels has been adapted for temperature measurements through the coil of the furnace. The output of the camera is digitized and visualized in a 14" monitor. The temperature is calculated using the correlation with the gray tone present in the monitor, which is measured by means of suitable software. The technical specifications of the camera and the modifications introduced to adapt it for this work are presented. The calibration of the camera and the method employed in the measurements are described. The measured temperatures are corrected by the effect of emissivity of the materials surfaces and the environment radiation reflected. Thermographs obtained are shown and results are discussed. We conclude that a low priced camera may be used to measure temperature in this range with acceptable accuracy.