Thermomechanical processing of low carbon bainitic steels is used to obtain a bainitic microstructure with good strength and toughness by continuous cooling after forging without the need of further heat treating, hence reducing manufacturing costs. However, hot forging parameters can significantly influence the microstructure in the forged material. A series of heat treating and forging experiments was carried out to analyze the effect of austenitizing time and temperature on the grain growth and the effect of forging temperature on the Prior Austenite Grain Size (PAGS) and continuously cooled microstructure. The forged microstructures were characterized by optical microscopy, microhardness tests, and X-ray diffraction. The results indicate that at 1200 °C austenitizing temperature abnormal grain growth takes place. Forging temperature significantly affects the PAGS and the subsequently formed microstructure. At high forging temperature (1200 °C), an almost fully bainitic microstructure was obtained. As the forging temperature was reduced to 1100 and 1000 °C, the PAGS refined, while the polygonal ferrite faction increased and the amount of retained austenite decreased. Further evaluations showed that a decrease in the forging temperature results in a higher carbon concentration in solution in the retained austenite leading to a stabilization effect.
The effects of hot deformation on the bainitic transformation of a low carbon steel during continuous cooling were comprehensively studied through in situ high-energy synchrotron X-ray diffraction, dilatometry, and ex situ microstructural characterizations. The obtained results indicated that the prior deformation of austenite at 950°C accelerates the bainite formation at the early stages. During the ongoing of the transformation, both the overall kinetics of bainite and carbon enrichment of austenite are lower in deformed austenite. The bainitic microstructure developed from deformed austenite is more refined and presents the same retained austenite content at room temperature with slightly lower carbon content when compared with the undeformed sample. Besides, a significant higher dilatation strain was measured during the bainitic transformation in the deformed sample, which can be explained by the crystallographic texture in hot deformed austenite. The evolution of the peak broadening of the {220}c and {211}a reflections during bainitic transformation are discussed in detail.
In the field of massive forged components the mechanical engineering industry searches for processes with increasing energy and resource efficiency. The new generation bainitic steels are promising for such application because of the high strength, toughness and fatigue properties. In order to achieve the desired mechanical properties, the development of the bainitic microstructure depending on the parameters of the thermomechanical process and on the cooling procedure must be well-known. In the present work diverse experimental techniques were applied for the investigation of the microstructural development during thermomechanical treatment and subsequent continuous cooling through the bainitic transformation range. The thermomechanical processes were simulated using dilatometers and at the same time, the specimens were analyzed using an eddy current sensor or using in-situ X-ray diffraction measurements at synchrotron (DESY). The results show that the eddy current sensor is suitable for the monitoring of the microstructural development during cooling and during deformation. From the investigations suitable process parameters were deduced for achieving a possibly fine bainitic microstructure. The main factors are a relatively low deformation temperature in austenitic range, a fast cooling (> 2 K/s) into the bainitic range, bainitic transformation and/or a short deformation in the lower bainite range, and finally a slower cooling until room temperature.
The main aim of this work is to investigate the effects of combinative Ce and Zr additions (0.3 wt% Ce+0.16 wt% Zr; 0.3 wt% Ce+0.27 wt% Zr and 0.3 wt% Ce+0.36 wt% Zr) on the microstructure and mechanical properties in cast Al-Si-Cu-Mg alloy. The microstructures features were investigated by optical microscope, scanning electron microscope and hardness measurements. The microstructural analysis has shown that the increase of Ce and Zr contents increases the volume fraction of intermetallics formed during the solidification leading to grain refinement and changes in silicon morphology of the as-cast microstructure. The intermetallics formed do not dissolve during the solution heating treatment (T6). The mechanical behavior at room and high temperatures (175, 210, 245 and 275°C) was determined from uniaxial tensile tests. The high thermal stability of Al-Si-Cu-La-Ce and Al-Si-Zr-Ti-Mg phases found in microstructure, in particular for the alloy containing 0.3 wt% Ce+0.27 wt% Zr, is responsible for the increase to 6.7% and 5.1% the ultimate strength at 210°C and 275°C respectively, compared with the standard alloy.
In this work, the microstructural evolution during the dynamic transformation of austenite to bainite was directly observed by in-situ high energy synchrotron X-ray diffraction measurements during warm uniaxial compression performed at the P07 beamline of PETRA III, DESY (Deutsches Elektronen-Synchrotron). Plastic deformation triggers the phase transformation, which is continuously stimulated by the introduction of dynamic dislocations into the austenite. This scenario accelerates the kinetics of bainite formation in comparison with conventional isothermal treatment. No mechanical stabilization of austenite was observed during dynamic transformation. Evidence of carbon partitioning between phases during plastic deformation was obtained. Further post-process investigations suggest that the bainitic microstructure developed during compression is oriented perpendicular to the loading direction. The findings open up new possibilities to design carbide-free bainitic microstructures directly via thermomechanical processing.
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