An empirical model to describe the non-isothermal ageing behaviour of feroplug materials

Abstract

Feroplug is a new temperature indicator developed by the authers and co-workers. The device makes use of the characteristics of ferrite transformation in duplex stainless steels, and has the following advantages over conventional methods of temperature measurement: 1. (1) It is a piece of stainless steel, with no delicate parts, and can be used in very hostile environments; 2. (2) It does not require any cables such as those in the case of thermocouples; 3. (3) It is very much cheaper than thermocouples. This invention has been patented in the USA, UK and Europe with financial and legal support provided by the British Technology Group, UK. Patent application in Japan is still pending. The invention is particularly useful for life assessment purposes in power and petrochemical plants. The underlying physical principle of the Feroplug is based on the transformation of ferrite, which is ferromagnetic at room temperature, into other phases such as carbide and sigma which are nonferomagnetic at room temperature. The amount of transformation depends on the temperature and duration of exposure. If the amount of transformation and time of exposure is known, the temperature of exposure can be deduced. Thus, the Feroplug makes use of the change in the amount of ferrite, which can be easily detected by room temperature magnetic measurement, to indicate the temperature of exposure. A model has been developed which can describe the ferrite transformation behaviour of Feroplug materials under non-isothermal conditions. The agreement between experimental data and the model is good at temperatures above 550°C.