Solid State Phenomena Vols. 172-174

Abstract: Metallic materials submitted to high strain rates upon dynamic loading can undergo phase changes induced by strains, stresses, and/or temperature increase associated with self-heating. Various mechanical and metallurgical assumptions have been proposed and implemented in numerical codes to deal with such complex interactions. In order to assess their respective influences, a simple nearly analytical model was developed and applied to the classical sphere expansion test carried out on a two-phase strain hardening, strain rate and temperature sensitive material. In this paper, classical homogenization assumptions are compared for deriving the overall material flow stress. Strain hardening transfer upon phase transformation is accounted for. Finally, the respective weights of the various contributions to the work rate, associated with stored energy, self-heating, and phase change, are analyzed.

Abstract: Nowadays more and more hot stamped steel sheets dedicated for the automotive body-in-white structure are pre-coated to prevent the steel surface against iron oxidation and decarburization during the austenitization step. For these applications, the coating is deposited by continuous hot-dipping the steel in an Al-Si bath. The Al-Si coating, at the delivery state, contains Al-grains, Al-Fe-Si ternary phases, Al-Fe binary phases. During the austenitization, the Al-Si coating transforms completely by inter-diffusion and solidification reactions. The mechanisms of Al-Fe-Si phase transformations at high temperature are almost unknown. The phase transformations occurring during austenitization define the final coating microstructure responsible for the in use properties of the product like spot welding, painting adherence or corrosion behavior. It is the aim of this paper to propose a new way of understanding the mechanisms of phase transformation in the Al-Si coating during the austenitization step (between 900 and 930°C) before the transfer into the hot-stamping press.

Abstract: Nanocomposite aerogel is proposed as a host matrix for the synthesis of glass ceramics. The large porosity is used as a sponge to incorporate chemical species getting a two phases material. We describe the steps of the synthesis of glass ceramics for nuclear waste containment, from nanocomposite aerogels loaded with actinides surrogates (Ce and Nd). The glass synthesis is obtained without melting, by a control of several solid phase transformations: sintering, viscous flow, crystallization and foaming. Thanks to their high resistance to thermal shock and water corrosion, these glass ceramics are certainly good candidates as actinides containment materials.

Abstract: We elaborated two carbide-free bainitic steels with different microstructures through specific heat treatments and alloy design. EBSD analysis was used to point out major differences in these microstructures. In-situ characterizations of the bainitic transformation were performed by high energy synchrotron diffraction to go further into the study of each phase characteristics. The elaborated microstructures exhibited various phase fractions of bainitic ferrite, retained austenite and blocks of martensite and retained austenite. Moreover, the volume fraction of retained austenite increased with higher austempering temperatures. On the other hand, the austempering temperatures showed a strong influence on the kinetics of the bainitic transformation. Isothermal transformation under Ms showed a two stage transformation which led first to the formation of self-tempered martensite and then to bainitic ferrite. Furthermore, the evolution of the austenitic cell parameter showed enrichment in carbon ruled by diffusional mechanisms.

Abstract: Two HSS grades (A and B) belonging to the complex system Fe-Cr-C-Si-X, where X is a strong carbide-forming element such as V, Mb or W, were studied. Samples in the as-received conditions came from an industrial spin casting process, with a varying cooling rate during processing. Chemical compositions of both alloys were closed to each other and were chosen to enhance their hardenability and to avoid less resistant phases such as pearlite and ferrite. Differential Thermal Analysis was performed on both alloys, in order to increase their crystallization behaviour. Light microscopy and SEM associated with EDS analyses were done to characterize the microstructure of both alloys in the as-received conditions and after DTA trials. The matrix of both HSS grades was composed of eutectic carbides, martensite and retained austenite, these phases exhibiting similar chemical compositions in both alloys. Unexpected pearlite was found in the as-cast HSS alloy B without W, this grade containing more Mo, more V and less Cr than the HSS grade A. It appeared from DTA tests that pearlite found in the alloy B arose more from the destabilisation of the Cr-rich retained austenite associated with the plate-like M₂C carbide, than from the matrix itself. In fact, pearlite zones located in the vicinity of M₂C are due to related isothermal solid phase transformations form the previous austenitic eutectic phase that is enriched with Cr and Mo.

Abstract: Non-isothermal austenite grain growth kinetics under the influence of several combinations of Nb, Ti and Mo containing complex precipitates has been studied in a microalloyed linepipe steel. The goal of these studies is the development of a grain growth model to predict the austenite grain size in the weld heat affected zone (HAZ). A detailed electron microscopic investigations of the as-received steel proved the presence of Ti-rich, Nb-rich and Mo-rich precipitates. Inter and intragranular precipitates of ~5-150 nm have been observed. The steel has been subjected to austenitizing heat treatments to selected peak temperatures of 950, 1150 and 1350°C at various heating rates of 10, 100 and 1000°C/s. Thermal cycles have been found to have a strong effect on the final austenite grain size. The increase in heating rate from 100 to 1000°C/s has a negligible difference in the austenite grain size irrespective of the austenitizing temperature. However, the increase in grain size has been noticed at 10°C/s heating rate for all the austenitizing temperatures. The austenite grain growth kinetics have been explained taking into account the austenite growth in the presence of precipitates.

Abstract: This work is part of a collaborative study between CEA-Saclay and LMT-Cachan on the numerical simulation of multi-pass GTA-Welding of thick specimens made of X10CrMoVNb9-1 (ASTM 387 or “T91”) steel. This material is considered as a candidate for some components of future Very High Temperature nuclear Reactors. Some parts of these components should be manufactured by assembling thick components (typically 200 mm) using narrow groove multi-pass GTA-Welding process. This welding process generates complex thermo-mechanical cycles in the HAZ (Heat Affected Zone) inducing complex microstructural transformations and residual stresses which should affect the integrity of the vessels behaviour. In a previous study, G.-M. Roux [1] developed a first version of a Thermo-Metallurgical-Mechanical "TMM" model for the X10CrMoVNb9-1 martensitic steel. This model was validated regarding residual stresses on simple mono-pass spot-welding tests. In this paper, focus is made on the modelling of the complex austenitisation process of the tempered martensitic steel as induced by the multi-pass process. Three different approaches are presented, viz. a model first proposed by Brachet et al., second a new model based on JMA approach and last, the simple differential Leblond model that is implemented in various finite element codes. These models are identified from standard dilatometry tests performed over a large range of heating rates, viz. [0.1°C/s, 100°C/s]. Finally, the response of these models, and therefore, their predicting capabilities, are compared to the experimental response of the material for different transients that have been designed to be representative of the temperature history in different points of a multi-pass welding HAZ.

Abstract: Isothermal bainitizing of high carbon steel is used to obtain exceptionally high mechanical characteristics together with near zero distortion during heat-treating. Such heat-treatments are often carried out at temperatures close to but above the martensite start temperature, where transformation kinetics are unfortunately very sluggish. The formation of a small amount of martensite prior to isothermal holding is known to accelerate transformation kinetics. However, the influence of the amount transformed remains to clarify, and the consequence on final properties have not been explored. In the following, the influence of partial martensite formation on the kinetics of bainite formation is investigated in 100Cr6 (AISI 52100) and similar grades. It is shown that, although the formation of martensite allows for shorter heat-treatment durations, it does not seem to affect the bainite kinetics, inasmuch as the bainite formation kinetics after formation of X% martensite are identical to that which would be expected if the first X% transformation had been obtained through isothermal bainitic transformation.

Abstract: The novel heat treatment concept of Quenching and Partitioning (Q&P) offers exciting prospects for the production of higher strength steel products with enhanced formability from a microstructure containing retained austenite and martensite. The Q&P process hinges on an interrupted quench and partitioning step at intermediate temperatures whereby the untransformed austenite can be thermodynamically stabilised by enrichment of carbon from the supersaturated martensite. Although the concept is similar to that producing carbide-free bainite in TRIP-assisted steel, Q&P offers the advantage of separating the ferrite formation and austenite enrichment stages of the process. While the concept is readily understood, the details of microstructural evolution during interrupted quenching and partitioning steps are difficult to study and are generally inferred from dilatometry or metallographic examination after a final quench back to room temperature. Consequently, in this study, alloying has been used to develop a model alloy in which the sequential steps of heat treatment can be separated for closer, more direct inspection by neutron diffraction techniques.

Abstract: The Fe-Si-Ti system is known to show nanoscale precipitation of the Fe₂SiTi Heusler phase with potentially high volume fraction (~4%), very high density and a size ranging from 1 to 20nm after artificial aging. The strong hardening potential of these precipitates make these steels candidates for automotive applications; however no understanding of the precipitation sequence (competition with other phases) nor the precipitation kinetics are available. The present study presents a quantitative study of the precipitation kinetics (size, volume fraction and number density) in a wide temperature range (450-800°C), realised by coupling systematically Small Angle Neutron Scattering (SANS), Transmission Electron microscopy (TEM) and Tomographic Atom Probe (TAP). Tensile tests were also carried out so as to determine the microstructure/properties relationships. Along the complete temperature range, it is shown that a compromise between time for precipitation and small precipitate sizes can be reached around 550°C. At this intermediate temperature, precipitation is shown to occur in two steps, linked with a second nucleation process after nucleation & growth of the first family of Fe₂SiTi has been completed. This second precipitation step results in a temporary decrease in precipitate size and an increase in hardness. The nature of these precipitates is discussed in view of the TEM and TAP observations.