Grade 91 steels have been used in power generation for more than 20 years in high temperature, high pressure applications such as steam piping, headers and tubing because it provides superior creep and oxidation resistance at elevated temperatures. The mechanical properties of the material are dependent on the creation of a martensitic microstructure, however incorrect heat treatment during manufacture, installation or repair can result in a weak ferritic or semi-ferritic microstructure which can cause premature component failure. Currently, components with incorrect, weak microstructures are identified using hardness testing; a manual technique which is prone to error. This work details a series of tests carried out at the University of Manchester to assess the suitability of multi-parameter magnetic testing for the identification of incorrect microstructures. The tests stem from a workshop organized by the Electric Power Research Institute (EPRI) where three sets of samples (eight pipe sections, eight tube sections and eight unidentified tube sections) with different microstructures were circulated world-wide. The results of the work show that the magnetic measurement techniques employed in these tests have the potential to provide a basis for the development of a portable NDE system for the identification of incorrect microstructures in Grade 91 plant components. The developed system would enable fast scanning of components with very little surface preparation along with digital data storage, improving on current manual hardness testing.