Mössbauer spectroscopy: A useful tool to follow the microstructure evolution in ASME 91/92 steels

J. I. BESOKY; C. A. DANÓN; C. P. RAMOS

Conferencia; International Conference on the Applications of the Mössbauer Effect (ICAME 2021); 2021

The so-called Creep Strength Enhanced Ferritic (CSEF) Steels, such as ASME grades 91/92, have found wide use in high temperature applications where creep is an important design consideration. Because of their main properties, they are excellent candidates for structural materials in the next generation of nuclear power plants [1]. The commercial heat treatment of these alloys usually involves normalizing and subsequent tempering at different temperatures. Lately, several modifications of the conventional manufacture process have been proposed-including the optimization of the chemical composition - [2] to improve the creep properties. These alternative routes require a thorough characterization of the microstructure, specially of the minor phases like alloy carbides and retained austenite (RA). The carbon content in RA is an important issue because it stabilizes the phase at low temperature, and it could be detrimental for the mechanical properties of these steels [3].Our work in the last years has been devoted to study the evolution of the microstructure in the as-received condition and in the continuous cooling step before the final tempering. Mössbauer spectroscopy (MS) has been a useful characterization tool because it enables to distinguish the matrix phases from the parent austenite phase by means of distinctive patterns (ferromagnetic and paramagnetic, respectively, at room temperature). In order to obtain a suitable Mössbauer absorber, two procedures are commonly used, taking into account the thin absorber approximation [4]. Powdering promotes the RA transformation to martensite; in particular, for our P91 steel samples, all the austenite transformed into martensite during the process [5]. Foil samples instead displayed RA whose carbon content was estimated when the weight fraction of this phase was appreciable [5]. Through MS it was also possible to characterize the matrix phase where the magnetic Fe surroundings are controlled by the alloy elements concentration and to detect a low volume fraction second phase -(Fe, Cr)3C-, which could not be conclusively determined by XRD [5-8].