Materials Science Forum Vol. 1105

Abstract: High-temperature oxidation is a widely studied topic in the field of Solid Oxide Fuel Cells as it commonly affects the steels used in stacks and other system components. Considering the targeted lifetime of systems using this technology (> 60kh), long-term testing is required to certify material properties throughout the life cycle. The design of accelerated testing is often cited as a way to speed the development and validation of materials for these components. In this work, the effect of pressure (1 to 4 bar) at various operating temperatures (750 to 850°C) on the oxidation kinetics and electrical properties of AISI 441 steel was investigated. While oxide growth was affected by pressure at all test temperatures, electrical properties showed significant changes only at 850°C. The results were supported by theoretical calculations of the oxidation and chromium evaporation kinetics of the steel.

Abstract: Austenitic stainless steels produced by Laser Powder Bed Fusion (L-PBF) are interesting materials because of their excellent corrosion resistance. Due to their relatively low hardness, the tribological response of these materials is poor, which limits their use in applications where control of wear degradation is important. Nevertheless, low-temperature plasma-assisted carburisation is an interesting process for improving the wear resistance of austenitic stainless steels, as has been observed for wrought materials. In fact, the increase in hardness is guaranteed by a surface layer of expanded austenite (S-phase) with a thin top layer of amorphous carbon. In this work, AISI 316 L, produced by the L-PBF technique, was carburised using 5 different plasma gas mixtures (by varying the CH4/H2 ratio) at 475°C for 7 hours. The samples obtained were then subjected to a detailed microstructural characterisation in order to obtain information on surface modification. The morphological features of the surface were examined by SEM observations in top view and in cross-section. The tribological performance was evaluated by pin-on-flat tests (alumina sphere as counter-material) with 2 different applied loads and a stroke length of 5 mm. Friction coefficient, wear rate (stylus profilometer) and wear mechanisms (SEM) were also evaluated. Preliminary results show an increase in wear resistance of all plasma treated materials compared to the untreated material. The improved tribological performance was discussed in relation to the abrupt increase in surface hardness.

Abstract: The development of ultra-high-strength automotive steel sheets for lightweight automobiles is still an effective strategy from the material point of view. In the case of cold-rolled steels for BIW(Body In White), it has been a long time since steel with 980MPa of UTS or higher were commercialized, and the use of 1.2GPa and 1.5GPa of cold-rolled steels is also increasing. However, in the case of hot-rolled steel for chassis, the increase in strength is not as fast as that of cold-rolled steel, because chassis parts are the lower structure of a vehicle and are very sensitive to durability.

Abstract: To attain the aim of weight reduction and safety improvement of vehicles, some high strength steel sheets have been developed and investigated. TRIP-aided steel sheets with transformation-induced plasticity (TRIP) of the retained austenite have high strength and ductility, and excellent hydrogen embrittlement resistance. In previous study, as high strength TRIP-aided steel for forging parts, the volume fraction of retained austenite in the TRIP-aided steel could be increased by hot forging with austempering. Similarly, our research group reported that the thermomechanical process of hot rolling following by austempering could also increase the amount of retained austenite in the TRIP-aided steel sheet. The tensile properties and formabilities of TRIP-aided steel sheet subjected to the thermomechanical rolling just before austempering possess obvious advantages compared with those of TRIP-aided steel sheet without thermomechanical rolling process (with only austempering). These excellent mechanical properties may be caused by the finely dispersed retained austenite and refined bainitic ferrite and/or martensite brock by thermomechanical rolling process.

Abstract: This work deals with the prediction of time-to-rupture (TTR) diagrams of martensitic 9-12% Cr steels. Martensitic 9-12% Cr steels are state of the art materials for powerplants due to their high creep strength and oxidation resistance. Since the experimental determination of TTR diagrams is costly and time-expensive (minimum 10 years), it is of particular interest to be able to model TTR diagrams and gradually replace experiments. Here, we approach the question to what extent we can generate a TTR diagram of a material out of a fraction of experimental results plus detailed understanding of the underlying microstructural/physical phenomena during creep. Our model is based on dislocation creep and includes multiple interactions between the microstructural constituents. We show the applicability of our approach by reproducing a TTR diagram of the well-known material P92. Input parameters are basic material data from literature, the starting microstructure before creep, chemical composition, some model parameters determined on the similar material P91, and one single creep curve of P92. The precipitate evolution is simulated by the software MatCalc, the other microstructural constituents (dislocation densities, subgrain boundaries etc.) by our creep model. By varying the stress between individual creep simulations whilst keeping all input parameters (starting microstructure, temperature and material parameters) constant, we produce multiple creep curves and thus generate the complete dataset for a TTR diagram. The model is of particular interest when it comes to the development of new materials, as the application range of these materials can be estimated quickly and with good reproducibility.

Abstract: Flat low carbon microalloyed structural steels processed by controlled rolling have excellent mechanical properties, particularly regarding toughness, since austenite is rolled with full recrystallization between rolling passes (roughing stage at high temperatures), and, subsequently, with no recrystallization at all between passes (finishing stage at low temperatures), maximizing nucleation of ferrite grains during austenite transformation after hot rolling. However, rolling must be prompted in an intermediate temperature range where recrystallization of austenite between rolling passes occurs partially, as this can lead to great heterogeneity in the grain size distribution of the product, a potential condition to impairs its low temperature toughness. On the other hand, this holding period that is necessary to avoid rolling within this temperature range can last several minutes, reducing the productivity of the rolling line and, in this way, potentially harming the financial performance of the plant. This work was developed to analyze several proposed rolling conditions for the finishing stage during controlled rolling of 16 mm thick strips of a low C Nb-Ti microalloyed steel strip, 380 MPa structural grade, processed in a Steckel mill, to identify the best condition that provides maximum productivity without affecting the required microstructural, toughness and mechanical characteristics of the final product.

Abstract: Phase boundaries of the pseudo-binary Fe-C diagram are key inputs for the prediction and understanding of matrix phase transformation in steels. The mechanical properties, of such steels, however, are often not dictated by the individual phase fractions, accessible through CALPHAD calculations, but by the arrangement of the phases, i.e., the steel’s microstructure. The prediction of these microstructural constituents requires the application of additional models, which are reviewed in the present contribution. Additionally, the current use and limitations for industrial application are presented together with an outlook to future challenges and opportunities in this field of research.

Abstract: Impact of first annealing on the formation and evolution of the microstructure during second annealing and on the final mechanical properties was investigated. Simple Fe-C-Mn-Si steel was used in this study. Dilatometry tests coupled with metallographic examinations were carried out to monitor the evolution of phase transformations and associated microstructure. Difference in the austenite evolution between simple and double annealing was highlighted. Based on the obtained results, conditions were selected for the annealing trials on bigger sample. Mechanical properties of heat-treated steels were assessed through the standard tensile tests. Double annealing treatment resulted in a better strength-ductility balance and in a good stability against soaking and quenching temperature variation. Complex ultra-fine multiphase microstructure containing at least 5 different microstructural constituents was revealed and observed using Scanning Electron Microscopy. As well, retained austenite fraction was estimated through magnetization saturation method.

Abstract: In this work a new kind of thermal treatment, called Quenching and Bainite (Q&B), was proposed and studied. A rather standard Fe-C-Mn-Si composition was used for this study. Annealing trials were performed using a combination of salt pots on relatively big samples allowing to perform the standard tensile and hole expansion tests. The obtained results were compared with the properties obtained using more known Q&P annealing. Generated microstructures were also compared. Characterization was done using optical and Scanning Electron Microscopy as well as magnetization saturation method for measuring retained austenite fractions. The Q&B heat treatment provides an alternative way to obtain 3G AHSS with promising strength-ductility-formability compromise.

Abstract: The effects of Mo and V on impact toughness in martensitic steels tempered at low temperatures were investigated using three low-alloy medium-C steels. Previous examination of these alloys had identified differences in impact toughness without a clear cause. In this work, the Base alloy with a reduced Mo addition experienced a significant loss in hardenability leading to the formation of small fractions of bainite during quenching even at relatively high quench rates. The use of different quench media to simulate cooling rates throughout a heavy section demonstrated that the variation in previously reported Charpy V-notch impact absorbed energies was readily explained by some regions cooling fast enough to avoid bainite while others formed some small fraction of upper bainite leading to increased cleavage fracture and decreased impact toughness. Small amounts of bainite transformation were not detected by dilatometry or tensile properties. These results emphasize the importance of effective through-hardening and careful microstructure evaluation in alloys that are meant to maintain good toughness and strength in thicker sections.