ABSTRACT Quenched and tempered 9%Cr grade 91 steels (9Cr1MoNbVN) display a lath martensitic matrix with M23C6 (M = Cr, Fe) carbides and fine precipitates named MX (M = Nb, V; X = C, N). MX particles provide the key to control the size and size distribution of austenite grains, which is significantly important in designing materials with specific mechanical properties. In previous works it was reported that samples of a T91 steel austenized at 1050ºCfor times between 0 and 40 minutes following heating at a rate of 50 ºC/s exhibit a heterogeneous austenitic grain size distribution after austenite holding for 20 to 30 minutes from the austenite plateaus tart. Besides, it was observed that all the second phase particles coming from the as-received state are present at the beginning of the austenite holding and that the dissolution of the MM23C6 carbides and a change of the chemical identity of MX precipitates occur within the first 5 minutes of holding. In the present work the detailed evolution of second phase precipitates over the 5 first minutes of austenite holding, obtained by means of a high speed, high resolution dilatometer is studied and followed by scanning and transmission electron microscopy. M23C6 precipitates were not observed from the first minute of austenitization. MX precipitates change progressively their character from V-rich to Nb-rich. The observed diminution of the Ms temperature values would be related to the M23C6 carbides and V-rich MX carbonitrides dissolution.

RESUMEN El acero 9 %Cr grado 91 (9Cr1MoNbVN) templado y revenido presenta una matriz martensítica con carburos del tipo M23C6 (M = Cr, Fe) y precipitados muy finos denominados MX (M = Nb, V; X = C, N). Los precipitados de la forma MX proporcionan la clave para el control del tamaño y la distribución de tamaño de grano austenítico, de suma importancia en el diseño de materiales con propiedades mecánicas específicas. En trabajos previos se informó que muestras de un acero grado T91 austenizadas entre 0 y 40 minutos a 1050 ºC luego de un calentamiento a 50 ºC/s presentan una distribución heterogénea de tamaño de grano al cabo de un mantenimiento en austenita entre los 20 y 30 minutos de iniciada la meseta isotérmica. Además, se observó que al inicio del mantenimiento en austenita están presentes todas las partículas de segunda fase del material en la condición de suministro, y que en los primeros 5 minutos del mismo se produce la disolución de los carburos del tipo M23C6 conjuntamente con un cambio en la identidad química de los precipitados MX. En el presente trabajo se informan los resultados obtenidos en el estudio de los precipitados de segundas fases que se forman durante los cinco primeros minutos del proceso de austenización de un acero ASTM A335 P91 utilizando un dilatómetro de alta velocidad y alta resolución y microscopía electrónica de barrido y transmisión. Los precipitados M23C6 no se observaron a partir del primer minuto de austenizado. Los precipitados MX pasan progresivamente de ser ricos en V a ricos en Nb. Las disminuciones observadas en los valores de la temperatura Ms estarían relacionadas con la disolución de los carburos M23C6 y carbonitrurosMX ricos en V.

ABSTRACT In the present work the microstructural characterization of a single-pass weld of 9Cr1MoVNb P91 steel performed by the flux-cored arc welding (FCAW) process in the “as welding” condition has been carried out by means of transmission electron microscopy. P91 steel offers a good weldability, but the mechanical properties of the welded joints are found to be inferior compared to the base metal. The heat affected zone (HAZ) of these types of steels is ~ 4 mm making it difficult to extract carbon replicas and thin foil to study the microstructure generated in each sub-zone of the ZAC: coarse grained ZAC (CGZAC), fine grained ZAC (FGZAC) and intercritical ZAC (ICZAC). Nev-ertheless, it was possible to extract replicas from each region to identify precipitates and using a focused ion beam (FIB-SEM), thin films could be extracted from a specific area to characterize the matrix microstructure. The precipitates present in each subzone of the ZAC were characterized as well as how the dissolution of M23C6 affects the matrix. At low peak temperatures and low heating / cooling rates such as those characterizing the fine grain zones (ICHAZ and FGHAZ) the M23C6 carbides were found to be partially dissolved and in contact with retained austenite. On the other hand, in the areas with the highest peak temperatures and high heating/cooling rates (CGHAZ and fusion zone) thin films of retained austenite were found on the austenitic grain boundaries and on the martensítica lath. On the other hand, in the areas with the highest peak temperatures and high heating / cooling rates (ZACGG and ZF) thin films of retained austenite on the austenitic grain edges and the martensite laths were found.

1. RESUMEN En el presente trabajo se llevó a cabo la caracterización microestructural de un cordón de pasada simple de un acero 9Cr1MoVNb P91 obtenido por el proceso de soldadura semiautomática bajo protección gaseosa (FCAW), en la condición “as welding” mediante microscopía electrónica de transmisión. El acero P91 tiene una buena soldabilidad, pero las propiedades mecánicas de las uniones soldadas son inferiores a las del material base. La zona afectada por el calor (ZAC) de este tipo de aceros es de ~ 4 mm lo que dificulta extraer réplicas y láminas delgadas para el estudio de las distintas microestructuras generadas en cada sub-zona de la ZAC: ZAC de grano grueso (ZACGG), ZAC de grano fino (ZACGF) y ZAC intercrítica (ZACIC). No obstante, fue posible extraer réplicas de cada región para identificar a los precipitados y por medio de la utilización de un haz de iones focalizados (FIB-SEM) se pudieron extraer láminas delgadas de un área específica para caracterizar la microestructura de la matriz. Se caracterizaron los precipitados presentes en cada sub-zona de la ZAC y cómo la disolución del M23C6 afecta a la matriz que lo rodea. A bajas temperaturas pico y bajas velocidades de calentamiento/enfriamiento tales como las que caracterizan a las zonas de grano fino (ZACIC y ZACGF) se encontró austenita retenida en contacto de los carburos M23C6 parcialmente disueltos. Por otra parte, en las zonas con más altas temperaturas pico y altas velocidades de calentamiento/enfriamiento (ZACGG y zona fundida) se encontraron películas finas de austenita retenida en los ex bordes de grano austenítico y en los listones de martensita.

ABSTRACT Hydrogen enters in Zry clads during fuel element life in service; when hydrogen exceeds the terminal solid solubility, circumferential hydrides are precipitated. In the presence of thermal gradients, produced for instance by oxide spallation, localized concentration of hydrides - known as blisters - can be formed. Due to the fact that zirconium hydrides have a lower density that zirconium alloys, stresses are produced around blisters. These stresses may overcome the necessary threshold to precipitate radial hydrides. Moreover, cracks inside blisters may act as initiators of delayed hydride cracking (DHC). In the dry storage facilities developed for Atucha 1, Zry - 4 clads of spent fuel elements (SFE) must withstand environmental conditions which include maximum temperature of 200°C and internal SFE pressure of ~ 75 bar (value at end of cycle). During dry storage, if blisters are present, stresses around them are superposed to circumferential stresses generated by the internal pressure due to fission gasses helping precipitation and/or growth of radial hydrides and DHC process. In order to assess this possibility, in the present work sections of Zry-4 cladding were hydrided. Blisters have been grown by thermal gradient under different conditions of temperature and shapes of thermal contact in order to obtain different blister dimensions and fraction of radial hydrides. After blister formation, some specimens were subjected to Hydride Reorientation Test (HRT) by applying an internal pressure of 75 bar at 200°C. Finally, tubes were sectioned and metallographically analyzed looking for radial hydrides. In some samples radial hydrides were observed, which effectively grew by the internal pressure of Zry-cladding during HRT.