Characterization of an ASTM A335 P91 ferritic-martensitic steel after continuous cooling cycles at moderate rates

In this contribution some aspects of the behavior of the ASTM A335 P91 (9Cr1MoVNbN) steel subjected to continuous cooling cycles under fixed austenitization conditions are studied. Representative samples of the structures obtained after cooling at moderate rates (i.e. pure martensitic and mixed martensitic-ferritic) were analyzed in order to incorporate information to the Continuous Cooling Transformation (CCT) diagram of this material. The characterization was carried out by means of scanning and transmission electron microscopy (TEM), X-ray diffraction and Mossbauer spectroscopy. For the working conditions here employed, the results showed the existence of retained austenite within both, the pure martensitic and the mixed martensitic-ferritic domains of cooling rates, adding important information to the CCT diagram since retained austenite presence could be detrimental to the mechanical properties of the steel. Second phase precipitates, being relevant to fix the mechanical behavior of the material, were identified by means of TEM on carbon replicas. Mossbauer spectroscopy was very helpful to detect a low volume fraction of cementite-type phase in all of the samples, considering that it could not be conclusively determined by XRD due to orientation effects.