Retained austenite phase detected by Mössbauer spectroscopy in ASTM A335 P91 steel submitted to continuous cooling cycles

Samples of ASTM A335 P91 steel submitted to continuous cooling at different rates were analyzed in the form of foils and powders by means of Mössbauer spectroscopy. The Continuous Cooling Transformation (CCT) diagram of steel ASTM A335 P91 displays two basic microstructural domains at low temperatures - ferritic and martensitic - whose limits depend on the austenite holding temperature, the precise chemical composition and the cooling conditions from the austenite mother phase. Under certain conditions, the martensitic transformation may not be completed, leading to a final microstructure with a non-negligible percentage of the austenite phase retained in a metastable state. This retained austenite could be detrimental for the mechanical properties of the steel. Mössbauer analysis suggested that powdering process promotes the retained austenite transformation to martensite; in particular, in the present case, all the austenite transformed into martensite during powdering. Foil samples instead displayed retained austenite whose relative fraction was determined as a function of the cooling rate. At the same time, the carbon content of retained austenite was estimated for the faster cooled samples; a preliminary explanation for the observed trends is given.Fil: Besoky, Jorge Ignacio. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Claudio Ariel Danón. Comisión Nacional de Energía Atómica; ArgentinaFil: Ramos, Cinthia Paula. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Constituyentes | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Constituyentes.; Argentin

Influencia del tiempo de revenido a 780oC sobre la resistencia al creep del acero ASTM A335 P91

Se investigó el efecto del tiempo de revenido a la temperatura de 780 °C sobre la resistencia al creep del aceroASTM A335 grado P91. La temperatura de revenido correspondió a la temperatura de revenido industrialy los tiempos evaluados se eligieron de tal manera de acumular, entre el revenido previo del material de suministro(40 minutos) y el tratamiento realizado en el laboratorio, tiempos de 3; 4; 5; 5,5; 5,7; 6 y 7 horas,por lo que en la práctica se aplicó un doble revenido. Posteriormente, las muestras fueron ensayadas a 600 ºCy 190 MPa hasta rotura. Los resultados muestran que el tiempo de revenido a 780 ºC, tiene un efecto muyimportante en la resistencia al creep del acero P91. Hasta 3 horas de revenido, el acero P91 mostró una buenaresistencia al creep con una velocidad mínima de creep de 7x10-9 s-1. Una marcada caída en la resistencia alcreep se observó para un tiempo de revenido de 5 horas (1.5x10-7 s-1), con una posterior recuperación a las5.5 horas (3x10-8 s-1). Este comportamiento al creep probablemente esté relacionado a la evolución de laspartículas MX durante el revenido. El tamaño promedio de las partículas de segunda fase en las probetas revenidasa 780º C con diferentes tiempos y sometidas a creep a 600º C, indicarían de manera indirecta un estadode disolución y re-precipitación de las partículas MX que ocurre durante el revenido. La rotura por creepocurre por la nucleación, crecimiento y coalescencia de cavidades en regiones próximas a los límites degrano de austenita previa, resultando en una fisura y propagación de la fisura hasta su rotura.Palabras clave: Acero 9% Cr, Comportamiento a creep, Resistencia a creep, Precipitación

Study of the behavior of the reduced activation martensitic ferritic steel F82H under continuous cooling

<p></p><p>ABSTRACT 9% Cr martensitic-ferritic steels are currently the privileged candidates to manufacture structural components of the so-called Generation IV nuclear fission reactors, because they exhibit very good thermophysical and mechanical properties under neutron irradiation. Along with them, the so-called Reduced Activation Ferritic-Martensitic steels (RAFM’s) have been in the same way selected for the future nuclear fusion reactors. In this contribution we introduce results involving the transformation behavior under continuous cooling of the F82H RAFM steel, obtained by applying the differential scanning calorimetry (DSC) technique. The metal-lurgical state of the as-received material was normalized and tempered. The applied thermal cycles were as follows: heating at 5 ºC/min up to 1050 ºC/min (austenite phase field), austenite holding for 15 min. and cooling at fixed rates between 1.5 and 50 ºC/min under constant pressure. After DSC test, samples were pre-pared by conventional techniques to be observed by optical microscopy. An algorithm under Matlab language was satisfactorily developed which allowed the simultaneous determination of the baseline and the transformed fraction as a function of temperature on the basis of a recursive procedure proposed in the previous literature. The transformation to martensite for the F82H steel under the prescribed thermal cycle conditions showed to be strongly dependent on the applied cooling rate, giving rise to a splitting phenomenon -or a multi-step transformation mechanism- as the cooling rate is lowered.</p><p></p

Study of the microstructural evolution during austenitization of an ASTM A335 P91 steel.

<p></p><p>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.</p><p></p

Influence of tempering time at 780°C on the creep resistance of ASTM A335 P91 steel

<p></p><p>ABSTRACT It has been investigated the effect of tempering time at 780°C on the creep strength of ASTM A335 grade P91 steel. The tempering temperature corresponded to the industrial tempering temperature and the times evaluated were chosen in such a way to accumulate, between the tempering prior as received (40 min) and the treatment carried out in the laboratory, times of 3; 4; 5; 5,5; 5,7; 6 and 7 hours, so that in practice a double tempering was applied. Subsequently, the samples were creep tested at 600 ºC and 190 MPa up to rupture. The results show that a tempering time to 780 ºC has a very significant impact on the creep strength of the P91 steel. Up to 3 hours of tempering, the P91 steel retains its creep strength, with a minimum creep rate of 7x10-9 s-1. This creep strength falls off sharply to the 5 hours of tempering (1.5x10-7 s-1), and retrieved to the 5.5 hours (3 x10-8 s-1). This creep behavior is probably related to the evolution of the MX particles during tempering. The average size of the particles of the second phase in the samples tempered to 780 °C during different times and subjected to creep to 600 °C, would indirectly indicate a state of dissolution and reprecipitation of MX particles, which occurs during the tempering. Creep rupture occurs by the nucleation, growth and coalescence of cavities, in regions close to the prior austenite grain boundaries, resulting in a crack and propagation up to fracture.</p><p></p

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 Mössbauer 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. Mössbauer 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.Fil: Carrizo, Denise Alejandra. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Unidad de Actividad de Materiales (CAC); ArgentinaFil: Besoky, Jorge Ignacio. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; ArgentinaFil: Luppo, María Inés. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Unidad de Actividad de Materiales (CAC); ArgentinaFil: Claudio Ariel Danón. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Unidad de Actividad de Materiales (CAC); ArgentinaFil: Ramos, Cinthia Paula. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentin