Estudio de la transformación isotérmica austenita → ferrita en un acero 9Cr grado 92 candidato para la fabricación de componentes estructurales de reactores cuarta generación

Abstract

Tesis para optar al título de Magister en Ciencia y Tecnología de Materiales

The Time-Temperature-Transformation (TTT) diagram of an ASTM A335 P92 steel (9CrMoWVNNb) has been established starting from an austenitization temperature of 1050 ºC. Isothermal transformation was carried out at temperatures from 625 up to 750 ºC taking 25 ºC intervals, using a high-resolution dilatometer. Only two state fields (i.e., austenite and ferrite + precipitated second phases) were observed, in full agreement with previous results on similar steels. A subset of large austenite grains, with sizes significantly exceeding the mean, was observed in all of the tested samples. At temperatures below the nose of the TTT diagram, prior austenite grain boundaries were made visible by decorating them with carbides precipitated at the early stages of the transformation. Carbide decoration allowed to have an accurate picture of the size distribution of austenite grains under the prescribed conditions of thermal cycle. Above the nose, prior austenite grain boundaries are hardly seen due to a drastic change in carbide precipitation mechanisms. At the same time, the ferrite nucleation and growth is markedly different in these two temperature regions; there is a gradual transition between these two extreme behaviors. The dilatometric curves obtained at each temperature were fitted to the Kolmogorov-Johnson-Mehl-Avrami expression in order to extract kinetic information about the austenite-ferrite transformation. Fitting was accomplished so as to take into account the presence of the large austenite grains. At the same time, a thorough examination of the transformed samples was carried out by using optical and electron (FEG-SEM and TEM) microscopy. Carbon replicas were extracted from the surfaces of selected specimens and a detailed study of the second phase particles present in each case is presented. In all of the studied samples, M23C6, M2X and MX precipitates were observed that displayed differences in their compositions, sizes, nucleation sites and morphologies with temperature transformation.