C. Capdevila

Application of dilatometric analysis to the study of solid–solid phase transformations in steels

Abstract This article outlines the use of dilatometry in solid–solid phase transformation research in steel. It describes how dilatometric data are interpreted, with an emphasis on continuous heating and cooling transformation diagrams. These diagrams show the microstructural constituents that result from given heating and cooling conditions, and are an invaluable tool for the metallurgist in characterizing steels with respect to their response to heat treatments. Several practical examples and applications of dilatometry in steel research are briefly described in this work.

Modelling of kinetics and dilatometric behavior of non-isothermal pearlite-to-austenite transformation in an eutectoid steel

European Coal and Steel Community (ECSC-7210. EC/939) and the Spanish Comisio´n Interministerial de Ciencia y Tecnologi´a (CICYTMAT95-1192-CE)

Metallographic techniques for the determination of the austenite grain size in medium-carbon microalloyed steels

Abstract Different techniques have been investigated to seek the best procedure to reveal the prior-austenite grain boundaries in three medium-carbon microalloyed steels. This study has been carried out over a wide range of temperatures (950–1250°C) and it has been found that thermal etching (TE) is the best technique to reveal the prior-austenite grain boundaries in these steels.

Revealing austenite grain boundaries by thermal etching: advantages and disadvantages

Abstract This article reviews the method of thermal etching for revealing the prior-austenite grain boundaries in steels. This method involves preferential transfer of material away from grain boundaries when the steel is exposed to a high temperature in an inert atmosphere. Thus, during austenitization of a prepolished sample, grooves are formed at the intersections of the austenite grain boundaries with the polished surface. These grooves remain intact after cooling and are clearly visible at room temperature outlining the austenite grain boundaries. However, at very high austenitization temperatures, those grooves might interfere with the advance of the austenite grain boundaries on the surface. In that case, this technique could lead to a wrong measurement of the austenite grain size. The aim of this work is to study the advantages and disadvantages of the technique to reveal the austenite grain boundaries in microalloyed steels.

Analysis of effect of alloying elements on martensite start temperature of steels

Abstract The stabilisation of austenite, a phenomenon that frequently occurs, renders the transformation from austenite to martensite difficult. The straightforward method of analysing the effect of a specific factor on the stabilisation of austenite is through its influence on the martensite start temperature M s. The present work outlines the use of an artificial neural network to model the M s of engineering steels based on their chemical composition and austenite grain size. The results are focused on elucidating the role in the stabilisation of austenite of alloying elements in steels, including less common elements such as vanadium and niobium, as well as the austenite grain size. Moreover, a physical interpretation of the results is presented.

Acicular ferrite formation in a medium carbon steel with a two stage continuous cooling

I. Madariaga*, I. Gutie ́rrez*, C. Garcı ́a-de Andre ́s** and C. Capdevila** * Centro de Estudios e Investigaciones Te ́cnicas de Guipu ́zcoa (CEIT) P Manuel Lardizabal, 15, 20009 San Sebastia ́n, Basque Country, Spain ** Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalu ́rgicas (CENIM), Consejo Superior de Investigaciones Cientı ́ficas (CSIC), Avda. Gregorio del Amo, 8. 28040 Madrid, Spain

Influence of bainite morphology on impact toughness of continuously cooled cementite free bainitic steels

Abstract The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size.

Dilatometric characterization of pearlite dissolution in 0.1C-0.5Mn low carbon low manganese steel

The authors acknowledge financial support from the European Coal and Steel Community (ECSC- 7210. EC/939) and the Spanish Comisio´n Interministerial de Ciencia y Tecnologi´a (CICYT-MAT95-1192-CE)

Modelling of kinetics and dilatometric behaviour of austenite formation in a low-carbon steel with a ferrite plus pearlite initial microstructure

The compiled knowledge in literature regarding the isothermal formation of austenite from different initial microstructures (pure and mixed microstructures), has been used in this work to develop a model for non-isothermal austenite formation in low-carbon steels (C < 0.2 wt%) with a mixed initial microstructure consisting of ferrite and pearlite. Likewise, calculations of relative change in length have been made as a function of temperature, and the differences between theoretical and experimental results have been analysed in 0.1C–0.5Mn low-carbon low-manganese steel. Experimental kinetic transformation, critical temperatures as well as the magnitude of the overall contraction due to austenite formation are in good agreement with calculations.

Effect of heating rate on reaustenitisation of low carbon niobium microalloyed steel

Abstract Austenite formation during a continuous heating in a low carbon niobium microalloyed steel with a pearlite and ferrite initial microstructure has been studied. Characteristic transformation temperatures, Ac 1, Ac θ and Ac 3 and the evolution of austenite formation have been determined by combining dilatometry and metallography in a range of heating rates from 0˙05 to 10 K s–1. It has been observed that nucleation and growth of austenite depends highly on the applied heating rate. At low heating rates (0˙05 K s–1) nucleation of austenite takes place both at pearlite nodules and at ferrite grain boundaries, while for higher heating rates (≥0˙5 K s–1), nucleation at grain boundaries is barely present compared to the nucleation at pearlite nodules. The heating rate also affects the austenite growth path and morphology and, thus, the distribution of martensite in the dual phase microstructure obtained at room temperature.