M. Eskandari

Effect of different microstructural parameters on hydrogen induced cracking in an API X70 pipeline steel

In this study, the surface and cross section of an as-received API X70 pipeline steel was studied by SEM and EDS techniques in order to categorize the shape and morphology of inclusions. Then, an electrochemical hydrogen charging using a mixed solution of 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate has been utilized to create hydrogen cracks in X70 steel. After hydrogen charging experiments, the cross section of this steel has been accurately checked by SEM in order to find out hydrogen cracks. The region of hydrogen cracks was investigated by SEM and EBSD techniques to predict the role of different microstructural parameters involving hydrogen induced cracking (HIC) phenomenon. The results showed that inclusions were randomly distributed in the cross section of tested specimens. Moreover, different types of inclusions in as-received X70 steel were found. However, only inclusions which were hard, brittle and incoherent with the metal matrix, such as manganese sulfide and carbonitride precipitates, were recognized to be harmful to HIC phenomenon. Moreover, HIC cracks propagate dominantly in transgraular manner through differently oriented grains with no clear preferential trend. Moreover, a different type of HIC crack with about 15-20 degrees of deviation from the rolling direction was found and studied by EBSD technique and role of micro-texture parameters on HIC was discussed.

Comprehensive Deformation Analysis of a Newly Designed Ni-Free Duplex Stainless Steel with Enhanced Plasticity by Optimizing Austenite Stability

A new metastable Ni-free duplex stainless steel has been designed with superior plasticity by optimizing austenite stability using thermodynamic calculations of stacking fault energy and with reference to literature findings. Several characterization methods comprising optical microscopy, magnetic phase measurements, X-ray diffraction (XRD) and electron backscattered diffraction were employed to study the plastic deformation behavior and to identify the operating plasticity mechanisms. The results obtained show that the newly designed duplex alloy exhibits some extraordinary mechanical properties, including an ultimate tensile strength of ~900xa0MPa and elongation to fracture of ~94xa0pct due to the synergistic effects of transformation-induced plasticity and twinning-induced plasticity. The deformation mechanism of austenite is complex and includes deformation banding, strain-induced martensite formation, and deformation-induced twinning, while the ferrite phase mainly deforms by dislocation slip. Texture analysis indicates that the Copper and Rotated Brass textures in austenite (FCC phase) and {001}〈110〉 texture in ferrite and martensite (BCC phases) are the main active components during tensile deformation. The predominance of these components is logically related to the strain-induced martensite and/or twin formation.

Development of Ultra-Fine-Grained Structure in AISI 321 Austenitic Stainless Steel

Ultra-fine-grained (UFG) structure was developed in AISI 321 austenitic stainless steel (ASS) using cryogenic rolling followed by annealing treatments at 923xa0K, 973xa0K, 1023xa0K, and 1073 K (650xa0°C, 700xa0°C, 750xa0°C, and 800xa0°C) for different lengths of time. The α′-martensite to γ-austenite reversion behavior and the associated texture development were analyzed in the cryo-rolled specimens after annealing. The activation energy, Q, required for the reversion of α′-martensite to γ-austenite in the steel was estimated to be 80xa0kJxa0mol−1. TiC precipitates and unreversed triple junction α′-martensite played major roles in the development of UFG structure through the Zener pinning of grain boundaries. The optimum annealing temperature and time for the development of UFG structure in the cryo-rolled AISI 321 steel are (a) 923 K (650xa0°C) for approximately 28800 seconds and (b) 1023xa0K (750xa0°C) for 600 seconds, with average grain sizes of 0.22 and 0.31 µm, respectively. Annealing at 1023 K (750xa0°C) is considered a better alternative since the volume fraction of precipitated carbides in specimens annealed at 1023 K (750xa0°C) are less than those annealed at 923 K (650xa0°C). More so, the energy consumption during prolonged annealing time to achieve an UFG structure at 923 K (650xa0°C) is higher due to low phase reversion rate. The hardness of the UFG specimens is 195 pct greater than that of the as-received steel. The higher volume fraction of TiC precipitates in the UFG structure may be an additional source of hardening. Micro and macrotexture analysis indicated {110}〈uvw〉 as the major texture component of the austenite grains in the UFG structure. Its intensity is stronger in the specimen annealed at low temperatures.

Effect of Hot Deformation on Texture and Microstructure in Fe-Mn Austenitic Steel During Compression Loading

Hot compression tests of a new high-Mn austenitic steel were carried out at deformation temperatures of 700, 800, 900, and 1000xa0°C under strain rate of 0.01xa0s−1. The hot deformation behavior was investigated by the analyses of flow curves, texture, and deformed microstructures. Microstructures of the deformed specimens and macrotexture were examined using electron backscatter diffraction and x-ray diffraction methods, respectively. The results showed that the flow stress depended strongly on the deformation temperature and decreased by increasing deformation temperature. The microstructural evidence indicated that the dynamic recrystallization (DRX) process of experimental steel was initiated at 800xa0°C with necklace structure. The volume fraction of DRX grains was considerably increased by increasing deformation temperature to 1000xa0°C. Texture of the DRX grains tended to become a weak texture and was associated with the formation of Goss and R-Cube components. Meanwhile, martensitic transformation was detected in the hot-deformed austenite. The martensitic transformation was the most difficult in the DRX grains because of the effect of small grain size. The tendency of transformation was decreased after compression at 1000xa0°C.

Alleviation of Mechanical Anisotropy in Ultrafine/Nano-grained AZ31 Magnesium Alloy

AbstractAn ultrafine/nano-grained AZ31 magnesium alloy was processed through multipass accumulative back extrusion at 280 and 380xa0°C. Texture analysis was conducted to determine preferred crystallographic orientations developed during multipass deformation. The anisotropy in mechanical properties of the processed materials was studied by compressive and tensile testing. The results implied that the accumulative back extrusion effectively reduces the anisotropy of yield strength, ultimate strength and strain-to-fracture during compressive and tensile loading. The operations of different twinning/slip systems during compressive and tensile deformation were discussed using Schmid factors measurements. A decrease in twinning-related anisotropy, as well as the contribution of different slip systems in strain accommodation, could reduce the mechanical anisotropy of ultrafine/nano-grained AZ31 alloy. The compressive strength of the two-pass processed material was lower along extrusion direction than transverse direction; however, the result was reverse after four passes. The tensile behavior implied that the contribution of basal slip might accommodate strain along length as well as the thickness of the specimen, while prismatic slip may assist in narrowing the width of the specimen and elongating the length.n