Advanced Materials Research Vols. 15-17

Abstract: Single discharge analysis and comparison of signals of multi-channels, two kinds of technique of Original position statistic distribution analysis, were used to analyze and evaluate the Al inclusion in continuous-casting thin slab. The samples are continuous casting slab . Datum of Al inclusions such as average concentration and concentration distribution , types of inclusion, concentration of the different type of Al inclusion, size distribution are obtained on different sect of cross section and the whole cross section of the sample.

Abstract: Interstitial free (IF) steels are widely used as thin sheet in the automobile industries because of their many favorable properties. Although, fatigue properties of IF steels do not have significant importance to auto body makers, however, they are very concerned about the tensile and fatigue strength of the steels used for structural purposes to ensure safety of passengers. So, fatigue results of this steel might help researchers to understand the behaviors of high strength steels. In this study cyclic and static properties have been studied at room temperature in the air. Initiative has also been taken to observe the fatigue fracture morphology of this steel. Experimental results show that the fatigue limit is corresponding to about 40% of tensile strength and 80% of the yield strength of this steel. Fractographic observations reveal a mixed type of fracture mode (intergranular and transgranular cracking) fractures.

Abstract: In the process of Original Position Statistic Distribution Analysis, many abnormal higher spark signals exist. They are believed as inclusion signals. Analyzing the time sequence and position of the abnormal spark, It is found that the higher spark signals for S and Mn will appear simultaneously . The reason is that MnS inclusion caused the element concentration enrichment at these zone. Mixing some pure MnS crystal in pure iron powder and pressed by Heat Isostatic Pressing technology, reference samples of MnS inclusion were made . Criteria of abnormal higher spark signals for Mn and S is the measured value from the “blank sample ”.After analyzing the spark behavior of these reference samples using OPA instrument, we can calculate the MnS inclusion concentration through such method. steel samples were analyzed and results is satisfactory.

Abstract: The presence of hydrogen in steel decreases its toughness and formability leading to hydrogen embrittlement. To understand the failure mechanisms of steel due to the presence of hydrogen, a profound insight in the hydrogen household of the steel is needed. This includes a study of the solubility, the diffusion and the trapping of hydrogen. Next, the absorption and desorption behavior during and after electrolytic charging must be well determined. This was investigated in this research for steels with various types of traps, e.g. dislocations, microcracks, grain boundaries and precipitates such as TiC and Ti4C2S2. The samples were cathodically charged at three different current densities: 0.8mA/cm2; 8.3mA/cm2 and 62.5mA/cm2. It was noticed that the cathodic current density used for hydrogen loading had a great influence on the results. Observation of the samples by scanning electron microscopy (SEM) showed that at the highest current density major damage of the surface had occurred. Hence it was decided to study more systematically the influence of charging current density on the resulting surface aspect and on hydrogen absorption and desorption. The hydrogen charging kinetics, maximum hydrogen solubility and hydrogen desorption behavior have also been evaluated for the different current densities during charging.

Abstract: The deformation mechanisms of an Fe-Mn-C TWIP steel have been investigated as a function of deformation and deformation temperature, using synchrotron X-ray diffraction at the European synchrotron radiation facility. Using the Warren theory, it is possible to reach a good qualitative understanding of the deformation mechanisms. We have confirmed that the deformation mechanisms shifted from the formation of martensite at very low temperature, to twinning around room temperature and dislocations at higher temperatures. Although some quantification of the density of crystalline defects can be reached using simple parameters such as peak shift and broadening, the complexity of defects present in this material require the development of more advanced data interpretation models. First results are shown, using shift and broadening of the peak and fit of intensity by a pseudo-voigt function, as well as the study of the asymptotic behavior of the intensity.

Abstract: This work presents the possibility of obtaining duplex stainless steels through powder metallurgy technology starting from austenitic X2CrNiMo17-12-2, martensitic X6Cr13 powders by controlled addition of alloying elements, such as Cr, Ni, Mo, Cu in the right quantity to obtain the chemical composition of the structure similar to biphasic one. Moreover the ferritic stainless steel X6Cr17 has been mixed to austenitic stainless steel in the ratio of 50%-50% in order to exam the deriving structure after sintering. In the studies behind the preparation of mixes, Schaffler’s diagram was taken into consideration. Prepared mixes of powders have been compacted at 800 MPa and sintered in a vacuum furnace with argon backfilling at 1260°C for 1 hour; after sintering rapid cooling has been applied in N2.

Abstract: In this work, three Zr microalloyed steels with different levels of Zr were compared with plain C-Mn, Nb and Nb-Ti steels. Austenitic grain size was compared as a function of temperature for these steels. A qualitative assessment of the potential of Zr to delay austenite recrystallization, was also undertaken. Of course, the actual use of Zr depends on many considerations: cost, availability and behavior during steel refining among others but this preliminary assessment was encouraging. It showed that the addition of Zr was able to prevent grain growth at typical reheating temperatures, around 1200oC. Also, Zr was able to delay austenite recrystallization.

Abstract: The dependence of Charpy toughness on the microstructure and texture of two industrial grades of API –X80 pipeline steel with a thickness of 15 and 17 mm, respectively, were studied in this work. Standard size Charpy samples with a long axis orientated at 0, 22.5, 45, 67.5 and 90° with respect to the rolling direction of the plate were tested at different temperatures varying from -196°C to 20°C. Microstructure and texture of the plates were studied by means of electron backscattering diffraction (EBSD) and XRD. The highest value of the absorbed energy (440 J) was measured at room temperature in the samples oriented with the long axis parallel to the rolling direction and the minimum values for the same temperature were observed in the samples enclosing an angle of 45° to RD (340J). Statistical model parameters obtained from Charpy impact tests were quantitatively correlated to microstructural parameters by means of a linear regression model with the aim to describe the Brittle-Ductile Transition Curve in terms of microstructural features. Based on dedicated experiments the Brittle Fracture Factor (BFF) parameter was introduced, which quantifies the fraction of the {100} planes oriented parallel to the crack propagation front. It was found that the Charpy toughness anisotropy of the plates in the lower shelf region can not be related only to their crystallographic texture, but to the microstructural anisotropy characterized by the grain size variations i.e. the density of grain boundaries in the different directions. Also the spatial distribution of the 2nd phase plays a determining role in the fracture behavior.

Abstract: Twinning-Induced Plasticity steels (TWIP steels) are extensively studied due to their ultra-high strain-hardening rate, that brings about a remarkable combination of ductility and strength. Twinning can be observed in high manganese-carbon steels. This paper considers hardening by combination of mechanical twinning with carbide precipitation. The kinetics of precipitation and the morphological evolution of carbides with annealing time were studied for two different TWIP steels with high manganese and carbon contents. The steels are first cold-rolled and then annealed at 800°C for recrystallization and carbide precipitation. Depending on the steel composition, the kinetics of precipitation and the morphology of the carbides are quite different. The influence of the annealing cycle on the mechanical properties has also been assessed. The results are used to discuss the influence of composition, stacking fault energy (SFE) and carbide precipitation on twinning. We show that the usual criteria based on the SFE only are not sufficient to characterize the twinning ability of a steel.