Review of temperature indicators and the use of duplex stainless steels for life assessment

This paper presents an overview of the recent development of life assessment techniques for high temperature plants, with special emphasis on a novel temperature-measuring technique, i.e. the ‘Feroplug’.Improved life assessment techniques will enable components to be replaced before failure, so reducing the risk of industrial accidents, as well as minimizing financial loss caused by unscheduled outages. The Feroplug is a new invention and has been patented by the US, UK and European patent bodies, with financial and legal support provided by the British Technology Group. It has been developed to supplement conventional temperature-measuring methods, such as the thermocouple, in extremely hostile environments, to provide information on the operating temperature, which is a major concern for life assessment. The principle of the Feroplug, which consists of piece of duplex (austenite + ferrite) stainless steel with specially designed composition, is based on the transformation of ferrite in the material on heating in the range 300–1000 °C. The extent of the transformation, being an indication of the temperature to which the Feroplug has been exposed, can be easily monitored using magnetic measurement techniques. The characteristics of the ferrite transformation are affected—in addition to the ageing temperature—by many metallurgical parameters, such as the composition of the alloy, thermomechanical pretreatments, etc. The recent progress of research in ferrite transformation is also reviewed in detail to provide the ground for understanding the principles of the Feroplug.