The non-isothermal ageing behaviour of the duplex stainless steel 7MoPLUS, which has been chosen as a new Feroplug/Sigmaplug alloy, has been studied. The rate of transformation of ferrite at a particular temperature depends on the amount already transformed during prior ageing at other temperatures and the transformation kinetics are dictated by the current ageing temperature only. This transformation behaviour is unaffected by initial solution-treatment temperature and plastic deformation. As long as ferrite decomposition is not complete, the Feroplug and the Sigmaplug, being novel temperature sensors, can possibly be used as indicators for thermal fluctuation. The usable time scale of these 'plugs' as temperature-fluctuation indicators can be extended by raising the initial solution-treatment temperature. This increases the initial ferrite content and retards its subsequent transformations. Samples have been thermally aged between 500 °C and 600 °C and between 650 °C and 800 °C, respectively. The morphologies of the (γ + σ) eutectoid aggregates are temperature-dependent and the sigma phase undergoes continuous changes in both dimension and composition. As ageing proceeds, the sigma phase is gradually enriched in Fe, Mo and Ni and depleted in Cr. The patterns of change in the contents of these elements are affected by other alloying elements. Mo, like Ni and Cr, may enhance deformation twinning in bcc ferrite. Once the spinodal microstructure reaches steady-state growth, its influence on structure-sensitive magnetic properties dies away. © 2006 Elsevier B.V. All rights reserved.