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In this work, a high-performance maraging steel M300 was processed by laser powder bed fusion, where the layer thickness and the laser emission mode were modified. As-built microstructures were studied by considering the metallurgical phenomena taking place during printing: melting of the material deposited over previous layers; rapid solidification of the melt pool; and martensite-to-austenite reversion and precipitation of intermetallic phases induced by the combined effects of repeated heating and cooling cycles in the process. After printed, parts were subjected to ageing treatments at temperatures ranging from 480 to 540 °C. Microstructural characterization results evidenced the importance of layer thickness as a key parameter to modify the solidification cell size of the as-built samples. Two different phases were identified from the X-ray diffraction patterns in the microstructure: BCT martensite and FCC austenite. It was found that the tetragonality of martensite remained constant across different layers for all as-built conditions regardless of the successive reheating cycles, which are experienced during subsequent melting passes. No significant effect of the studied printing parameters was observed on the evolution of the microstructure during ageing. Finally, it was observed that the hardness of aged microstructures is comparable to that obtained by conventional manufacturing methods.

Mechanical properties of microalloyed steels are enhanced by fine precipitates, that ensure grain growth control during subsequent heat treatment. This study aims at predicting austenite grain growth kinetics coupling a precipitation model and a grain growth model. These models were applied to a titanium and niobium microalloyed steel. The precipitate size distributions were first characterized by TEM and SEM and prior austenite grain boundaries were revealed by thermal etching after various isothermal treatments. From CALPHAD database, a solubility product was determined for (Ti,Nb)C precipitates. A numerical model based on the classical nucleation and growth theories was used to predict the time evolution of (Ti,Nb)C size distributions during various isothermal heat treatments. The precipitation model was validated from TEM/SEM analysis. The resulting precipitate size distributions served as entry parameters to a simple grain growth model based on Zener pinning. The pinning pressure was calculated using the whole size distribution. The resulting austenite grain growth kinetics were in good agreement with the experimental data obtained for all investigated heat treatments.

The stability of the martensitic microstructure in ferritic/martensitic G91 steel at operating temperatures up to 700 °C might be improved by means of ausforming thermomechanical treatments. The goal sought is to promote the formation of a high number density of MX nanoprecipitates in the martensitic microstructure obtained after a subsequent tempering. This work is focused on the effect of the ausforming temperature. The results show that the lower the ausforming temperature is the higher is the dislocation density obtained in ausformed fresh martensite and the larger the number density of MX carbonitrides after tempering is. Creep strength, evaluated by Small Punch Creep Tests has allowed us to conclude that the best creep resistance was obtained for the steel ausformed at the lower temperature due to the higher pinning force for dislocation motion triggered by the distribution of MX. The creep results obtained on the ausformed samples were compared with those after the conventional heat treatment, showing that the high density of MX carbonitrides after an ausforming thermomechanical treatment is a promising processing to raise the operation temperature of this steel.

Rapid alloy prototyping is quickly emerging as a manufacturing approach to design novel structural alloys since allows producing low volume of high quality castings quickly at low cost. Thus, a large number of alloys can be produced and tested for finding the optimal composition in an alloy system. In this work, a new high throughput method for rapid alloy manufacturing has been developed and employed to cast AISI 304 stainless steel samples modified with an addition of Cu in the range of 1-4 wt%. The homogeneity of the microstructure and chemical composition of as-cast materials has been characterized at the macro- and micro-scale, using optical and scanning electron microscopy, x-ray fluorescence, x-ray diffraction and electron probe microanalysis. At the macro-scale, no compositional variations have been found, while variations in the distribution of phases in the microstructure were related to variations in the cooling rate and thermal path undergone across the solidifying ingot. At the micro-scale, it has been found that segregation of austenite and ferrite stabilizing elements occurs during solidification, as expected from the phase diagram. Additive Alloy Melting (ADAM) has been demonstrated to be a suitable, rapid and versatile casting method for manufacturing metallic samples on a laboratory scale with good chemical homogeneity at the macro-scale.

Small Punch Creep technique was used as a screening procedure to evaluate the creep properties of different microstructures developed in a thermomechanical simulator. The goal seek was to generate alternative microstructures in a conventional ferritic-martensitic G91 steel grade which boost thermal stability at temperatures as high as 700 °C. The developed microstructures allow studying the effect of the austenitization temperature optimized by thermodynamic calculations and the ausforming on the creep strength and ductility. The improvement in creep strength recorded was attributable to a higher number density of MX precipitates. By contrast, these microstructures showed an important reduction in creep ductility.

An analytical study of grain growth dynamics complemented with Vertex simulations is presented in this work. This has been done by including the effect of the two main drag forces in grain growth dynamics: those described by the Zener pressure (pinning particles) and Chan theory (solute drag force). The grain growth dynamics is usually addressed through fitting of the experimental measurements to a parabolic growth law, which is the solution of the mean grain size of a system free of drag forces, and the corresponding n-exponent derived. In our analysis, by contrast, analytical solutions of the grain growth dynamical equations explain and characterise the exponent variation with respect to the parameters that defines both drag forces. The Vertex simulation method completes the description of the grain size distribution, not only its mean, and allows us to characterise the evolution of the microstructure. In order to understand the results of the simulation, an analysis on the most commonly used statistical distributions describing the grain size has been carried out. It has been possible to conclude that the Weibull distribution, among all the others distributions, describes the nature of the simulated grain growth and its size distribution. As a consequence, the shape parameter k of Weibull distribution has been related with the pinning forces, being it itself a quantitative indicator of such forces. The results have been compared with experimental results with remarkable agreement, where the analytical solution and statistical relationship observed by simulation have been very useful describing and characterising the microstructure evolution. © 2012 Elsevier B.V. All rights reserved.

A new generation of steels has been designed, which on transformation at low temperature (200¿350 °C), leads to a nano-scale microstructure, known as NANOBAIN. The microstructure consists of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes (20¿40 nm). These advanced steels present the highest strength/toughness combinations ever recorded in bainitic steels. Their properties are mainly a consequence of the formation of nanoscale bainitic ferrite plates at very low temperatures. Transmission electron microscopy observations have shown that plastic relaxation in the austenite adjacent to the bainite plates may control the final size of the bainitic ferrite plates. The dislocation debris generated in this process resists the advance of the bainitic ferrite¿austenite interface, the resistance being greatest for strong austenite. The yield strength of the austenite must then feature in any assessment of plate size. In this scenario, the plates are expected to become thicker at high temperatures because the yield strength of the austenite will then be lower. The goal of this study is to evaluate the influence of yield strength of austenite to the nanoscale structural refinement of advanced bainitic steels. In this sense, in situ measurements of austenite strength before bainite formation using a deformation dilatometer Bähr 805D have been performed in a medium carbon high silicon steel transforming at intermediate temperatures (325¿400 °C) to a submicron structure of bainite and in a high carbon high silicon steel transforming at low temperatures (200¿350 °C) to nanostructured bainite. The role of the transformation driving force on the bainite plate thickness will be also discussed.

Curvature driven migration of a series of 〈1 1 0〉 tilt grain boundaries in a bcc Fe-20Cr alloy is simulated using molecular dynamics to investigate the relationship between the atomic migration mechanism and mobility at medium to high temperatures. The boundaries studied include low angle boundaries (LAGB), high angle boundaries (HAGB) and singular boundaries, such as coherent twins. The steady-state boundary shape and curvature are compared with a simple analytical model which incorporates the dependence of absolute mobility and free energy on boundary inclination. The comparison indicates that the 109.5° (11̄2) Σ3 coherent twin boundary will have relatively low energy but high mobility. This result is attributed to a particularly effective repeated shuffle mechanism which occurs on the twinning plane. Two other migration mechanisms are observed, one involving the motion of 〈1 1 1〉 glissile dislocations in LAGB and the other involving uncorrelated atomic shuffles in HAGB, sometimes associated with interfacial steps. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effect of different starting microstructures on the kinetics of Fe-rich (a) and Cr-rich (a0) phase separation during aging of Fe– Cr–Al oxide dispersion strengthened (ODS) alloys has been analyzed with a combination of atom probe tomography and thermoelectric power measurements. The results revealed that the high recrystallization temperature necessary to produce a coarse grained microstructure in Fe-base ODS alloys affects the randomness of Cr-atom distributions and defect density, which consequently affect the phase separation kinetics at low annealing temperatures.

The effect of chemical composition and processing parameters on the formation of acicular ferrite and/or bainite has been investigated. In particular, this paper deals with the influence that N through its combination with V, as V(C,N) precipitates, has on the decomposition of austenite. Likewise, the intragranular nucleation potency of V(C,N) precipitates is analyzed through the continuous cooling transformation diagrams (CCT) of two C-Mn-V steels with different contents of N. Results reported in this work allow us to conclude that acicular ferrite can only be achieved alloying with vanadium and nitrogen, meanwhile bainite is promoted in steels with a low level of nitrogen. It is concluded that higher strength values are obtained in acicular ferrite than in bainitic steel but a similar brittle-ductile transition temperature (BDT), and lower values of impact absorbed energy (KV) has been recorded in nitrogen-rich steel.

This paper presents a finite element modeling analysis of deformation on iron-base mechanically alloyed oxide dispersion strengthened alloy by spherical indentations (Brinell test). Results of the model are used to interpret the role of residual shear stresses on the development of recrystallized grain structure and the temperature at which recrystallization occurs.

Tensile specimens have been tested at room temperature, finding clear delamination in the rolling direction but only incipient delamination in the transverse direction. Delamination is found to be a strain rate-controlled process whose morphology depends on the material’s processing history, suggesting that its cause may be embrittlement induced by hydrogen occluded during processing. Delamination has only a small influence on mechanical properties in the rolling direction, but in the transverse direction a loss of ductility is detected particularly for aged specimens.

This paper discusses the influence of Particle Stimulated Nucleation (PSN) on the overall process of nucleation and subsequent grain growth in low carbon (LC) steels and proposes a global recrystallisation kinetics model based on fundamental physical metallurgy principles. The model takes into account the cementite content, which depends on the nominal carbon content, and the second phase particle size. The deformation field around cementite particles and its influence on processes is analysed. Finally, experimental evidence is given to show the accuracy of the theoretical predictions.

The coarsening of the three-dimensional microstructure resulting from phase separation during ageing at 748K of a Fe-based PM 2000TM oxide dispersion strengthened (ODS) steel has been investigated by atom probe tomography and hardness measurements. Phase separation resulted in the formation of isolated particles of the chromium-enriched phase. The aluminum and titanium were found to preferential partition to the iron-rich phase. The partitioning of aluminum is consistent with theoretical calculations. The change in the scale of the chromium-enriched phase was found to fit a power law with a time exponent of 0.32 in accordance with that predicted by the classical Lifshitz, Slyozov and Wagner (LSW) theory. The solute concentrations of the coexisting and phases were estimated from concentration frequency distributions with the Langer–Bar-on–Miller (LBM) method and proximity histograms. The hardness was linearly related to the chromium content of the phase. © 2008 Elsevier B.V. All rights reserved.

The variations of thermoelectric power (TEP) measurements during annealing of warm and cold rolled extra low carbon (ELC) steels were analysed. Where a prominent influence of recovery was detected in cold rolled steels during annealing, almost no variation of TEP due to both recovery and recrystallisation was observed during annealing of warm rolled steels. On the other hand, TEP measurements were proven to be a powerful technique to monitor dissolution of cementite and precipitation of aluminium nitrides. It was found that these two phenomena significantly affect TEP measurements and recrystallisation kinetics during annealing of warm rolled steels.

The possibility of obtaining multiphase steels with considerable fractions of retained austenite, which is of key interest for the occurrence of the transformation-induced plasticity (TRIP) effect, is investigated in steels with low amounts of Si and Al. Special emphasis is given to the microstructural observation of this phase, since its volume fraction, distribution and stability, in relation to the carbon content, will determine the efficiency of the TRIP effect and thus of the resulting mechanical properties. Mechanical testing was carried out in order to measure the efficiency of the retained austenite transforming to martensite.

During continuous heating of a cold rolled low carbon (LC) steel a dilatometric anomaly has been detected previous to the well known ferrite-to-austenite transformation. This anomaly is due to the dislocation annihilation and the texture change that accompany the recrystallization process.

In this work the different contributions to the thermoelectric power (TEP) measurement during the dissolution process of M23C6 carbides in an X45Cr13 stainless steel have been evaluated. Thus, the influences of carbon and chromium contents in solid solution in the austenite, the austenite grain boundaries and the microstructure on the TEP have been determined

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 which result from given heating and cooling conditions, and are an invaluable tool for the metallurgist in characterising 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

The influence of titanium on the transformation kinetics of allotriomorphic ferrite during the isothermal decomposition of austenite has been investigated by means of dilatometric and microstructural analysis in medium carbon–manganese steels. A careful comparison of the transformation kinetics curves of a C–Mn steel and a Ti microalloyed C–Mn steel shows that Ti enhances the allotriomorphic ferrite transformation.

The present article is concerned with a theoretical and experimental study of the growth kinetics of pearlite in a 0.4C-1.6Mn medium carbon steels. Factors controlling the isothermal formation of this microconstituent are explored in this work. In this sense, the transition temperature between local equilibrium (LE) and no partition local equilibrium (NPLE) growth mechanisms is theoretically determined. Moreover, the nucleation of pearlite has been considered as a cementite precipitation process on a moving austenite-ferrite interface. Finally, a theoretical model is presented in this work to calculate the evolution of austenite-to-pearlite transformation with time at a very wide temperature range.

The determination of the austenite grain size in medium-carbon microalloyed steel was investigated by metallographic techniques. Procedure based on the combination of heat treatment and chemical etching revealed the austenite grain boundaries for some steels in austentization conditions. The results showed that the thermal etching was the most adequated method for revealing the PAGS of the investigated microalloyed steel.

Mechanically alloyed iron-based ODS alloys have the potential for application in heat exchangers for biomass processing, with gas operating temperatures and pressures of approximately 1100°C and 15–30 bar. The yttria dispersion in such alloys improves the high-temperature creep and stress rupture life. The elevated temperature strength is enhanced by the development of a coarse-grained microstructure during recrystallisation. Factors controlling the evolution of this desirable microstructure are explored in this work, focusing specifically on PM2000, which is a yttria dispersion strengthened, mechanically alloyed material. The microstructure following mechanical alloying and consolidation is fine and uniform, making it difficult to nucleate recrystallisation. Therefore, the introduction of a strain heterogeneity in the microstructure is found to promote the nucleation of recrystallisation. In contrast, uniform deformation reduces the chances of nucleation and hence leads to the development of a coarser microstrutructure.

Acicular ferrite formation was achieved in medium carbon microalloyed forging steel by both isothermal treatments and continuous cooling. Inhibition of grain boundary nucleation of bainite was observed, which enabled the formation of fully acicular microstructures. High cooling rates were needed to completely avoid the formation of allotriomorphic ferrite. Lower cooling rates caused other phases to develop, but a high volume fraction of acicular ferrite could be obtained using two stage continuous cooling cycles.