Papers by Keyword: Retained Austenite

Abstract: In order to find optimal intercritical annealing treatment (IAT) temperature and alloy composition for simple process route of hot rolling followed by single-step IAT, the effects of IAT on three different medium-manganese steels were investigated. Nominal chemical compositions in wt.% were 1) 6Mn–0.3C, 2) 6Mn–0.4C and 3) 8Mn–0.4C(–2Al–1Si–0.05Nb–Fe). Materials were laboratory hot rolled to a thickness of 6 mm, and IAT was simulated with Gleeble 3800 and Linseis DIL L78 DQT / RITA dilatometer. Different variations of IAT included annealing temperatures of 650 °C, 675 °C, 700 °C and 725 °C, with holding time of 10 minutes, heating rate of 50 °C/s and cooling rate of 10 °C/s. Quasi-static tensile tests were performed parallel to rolling direction. XRD and EBSD phase mappings were performed to assess IAT temperatures effect on volume fraction of retained austenite. Most promising mechanical properties were obtained with material 6Mn–0.4C annealed at 700 °C. Product of strength and elongation well exceeded 40 000 MPa% for above-mentioned IAT-material variation, being distinguishable higher compared to other variations. However, investigated materials, especially 6Mn–0.4C, seems to be very sensitive to IAT temperature, which could inflict some challenges in industrial scale production. Also, all materials experienced some level of serrations during tensile testing, which is frequently encountered phenomena with medium-manganese steels. Further research is required, to evaluate the role of austenite stability on mechanical behavior of these materials and to determine effects of heating and cooling rates.

Abstract: This study examines the fatigue crack propagation behaviour of AISI 4140 steel subjected to multi-austempering heat treatments. Tensile test and fatigue crack propagation (FCP) specimens were prepared in accordance with ASTM E8 and ASTM E647, respectively. Multi-austempering was carried out by heating the specimen to an austenite temperature for 10 min using an induction heating coil. The specimen was then immersed in a salt bath for each isothermal transformation time of 60 min at three austempering temperature levels from 312°C to 412°C with a temperature increase of 50°C. Tensile and fatigue crack growth tests were performed on both annealed and multi-austempered specimens. It is observed that the multi-austempering heat treatment significantly improves the tensile properties and the FCP properties of AISI 4140 steel. Microstructural observations indicate that the bainitic phase and the retained austenite increase the tensile strength and reduce the fatigue crack propagation rate (da/dN). It is found that the bainitic structures are an effective barrier in reducing fatigue crack propagation as the fatigue loading cycle increases

Abstract: Double soaking (DS) has been proposed as an alternate processing route for medium manganese steels. DS consists of soaking in the intercritical annealing region to stimulate manganese enrichment of austenite by depletion of ferrite followed by secondary soaking at a higher temperature and cooling to room temperature to obtain a martensite/austenite microstructure. DS is different from more traditional medium manganese heat treating which usually involves a single soaking step in the intercritical region to generate a ferrite/austenite microstructure. DS has been shown effective at generating attractive tensile properties notably tensile strength levels in excess of typically observed levels in medium manganese steels. A review of properties and microstructural evolution obtained by DS of medium manganese steels is presented here.

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: 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: Laboratory rolling simulations, comprising of thermomechanically controlled rolling followed by ausforming and isothermal holding close to Ms temperature, were conducted on a medium-carbon (C) steel in order to understand the combined effect of prior straining and low temperature holding on the bainite transformation characteristics and resultant properties. Field emission scanning electron microscopy (FE-SEM) was employed to examine the morphology, size and volume fraction of different phase constituents and the observed microstructural features were correlated with the mechanical properties of the steels. The morphology of bainite and/or retained austenite (RA) after low temperature ausforming was found to be extremely fine compared to the sample ausformed at high temperature. An excellent combination of high yield (~1200 MPa) and tensile (~1800 MPa) strengths, good ductility (~18 %) and reasonable ambient temperature impact toughness (~15 J/cm²) were achieved in the low temperature ausformed steel that was attributed to significant refinement of bainite sheaves and presence of high fractions of film-like RA. The dislocations introduced by ausforming hindered the growth of bainite and promoted enhanced carbon diffusion, resulting in high fractions of finely divided film-like RA with high stability. Recent results obtained on a nanostructured /ultrafine medium-carbon bainitic steel elucidating the ausforming conditions and corresponding microstructures and properties are illustrated here.

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: The influence of small contents of nitrogen present as an impurity in 0.3C Al-bearing steels, which were processed through thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP), was examined in respect of room temperature tensile ductility and impact toughness. Two similar chemical compositions (in wt.%): Fe-0.3C-0.6Si-1.1Al (High-Al) with different N contents of 10 and 30 ppm were selected for this study. In addition, two other DQP steels with compositions: Fe-0.3C-1.0Si (High-Si) and Fe-0.3C-0.5Si-0.5Al (Al-Si), both containing about 30 ppm nitrogen, were also included in the study to compare the properties. Detailed metallographic studies using FESEM-EDS, TEM, EPMA and XRD combined with tensile testing and fractographic analysis indicated that already 30 ppm of nitrogen could impair tensile ductility of TMR-DQP processed High-Al steel in comparison to that with 10 ppm nitrogen. Similarly, the effect was adverse also in Al-Si steel (30 ppm N) despite its reduced Al content (0.5 wt.%), but High-Si steel (Al < 0.002 wt.%, N 30 ppm) did not show any such detrimental effect on tensile ductility. Extensive material characterization verified that even 30 ppm of nitrogen could impair ductility of Al-bearing steels, essentially due to the presence of AlN inclusions, despite that TMR-DQP processing enabled stabilization of 6–10% retained austenite (RA) in the steels. The capacity of RA in promoting improved ductility and strain hardening capacity was impaired by the presence of these inclusions. In contrast, impact toughness transition temperature T_28J was not clearly affected with Al-Si when compared to low-N High-Al steel, although excessive splitting in Al-Si caused pronounced scatter in the results and increase in upper shelf impact toughness.

Abstract: The article is aimed at studying the effect of austempering temperature below and above Ms temperature on the phase-structural state and mechanical properties of 0.2 wt.% C TRIP-assisted steel micro-added with Nb, V, Mo, Cr. The samples were austenitized at a temperature close to the Ac₃ point (900 °C) and held at 300 °C (below M_s), 350 °C (close to M_s) and 400 °C (above M_s) for 5-20 min. The work was performed using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction, and tensile/impact testing. It was found that austempering at the aforementioned modes ensures the multiphase structure consisting of carbide-free bainite, tempered martensite, ferrite and retained austenite (in different combinations). The optimal was austempering at a temperature close to Ms which provided an advanced complex of tensile properties (PSE of 23.9 GPa×%) and V-notched impact toughness (95 J/cm²). TRIP-effect contributed to these properties while the strain hardening process tended to be prolonged with increasing the austempering temperature.

Abstract: Carburized-quenched steel has a hard layer on the surface and a soft layer in the core. Internal fatigue cracks are observed around the boundary between these two layers under cyclic stress. In this research, we investigated the microstructures (carbon content, prior austenite grains and retained austenite) in the carburized-quenched chromium molybdenum steel bar (JIS-SCM415, diameter = 10 mm) failed by rotating bending test (RBT) at nominal stress amplitude of 716 MPa. After the investigations, we obtained three conclusions: the carbon content in the area from the surface to 0.1 mm depth was higher than other area; the prior austenite grain (PAG) sizes at 0.1 mm depth from the surface was almost the same as that of 0.6 mm depth; and the retained austenite which was indicated from the ratio of γ to α in the cross section ranging from the surface to 0.1 mm depth was decreased by rotating bending fatigue.