Magnetic and ageing behaviour of 7MoPLUS and the viability of monitoring ferrite decomposition using AC magnetic susceptibility

The objectives of this work were to assess the possibility of adopting 7MoPLUS as a new Feroplug/Sigmaplug alloy for temperature measurement and investigate the viability of using AC magnetic susceptibility to monitor ferrite decomposition. A series of 7MoPLUS samples were subjected to thermal ageing between 500 and 800 °C and the attendant changes in their microstructures and magnetic properties were studied by using optical microscopy, XRD, vibrating sample magnetometry (VSM) and AC susceptibility. 7MoPLUS can be used satisfactorily as a Feroplug/Sigmaplug alloy. AC magnetic susceptibility is a viable tool for monitoring ferrite decomposition and so it can serve as a complement to XRD and VSM for this purpose. Plastic deformation will enhance ferrite decomposition and thus the attendant changes in magnetic properties will be accelerated. Magnetic measurements and XRD have shown that cold rolling will not induce martensitic transformation in the austenite of 7MoPLUS. Extension of the use of the Feroplug and Sigmaplug techniques from long-term temperature measurement (several thousand hours) to intermediate-term temperature measurement (a few hundred hours) may be achievable by using plastically deformed 7MoPLUS. © 2005 Elsevier B.V. All rights reserved.