Bohuslav Mašek

Structure of miniature components from steel produced by forming in semi-solid state

Abstract To obtain new unconventional structures with specific mechanical and physical properties is possible not only by the development of new types of materials but also by treatment of conventional materials using unconventional innovative technological procedures. One of these technologies is the forming in semi-solid state involving rapid solidification of miniature components from steels. Production of such components is complicated by a number of technical problems. To explain phenomena of the process and structure development, the production of miniature components from the tool steel X210Cr12 difficult to form was experimentally tested. The structure of this originally ledeburite steel consisted of 95 % of metastable austenite after the treatment. Metastable austenite was located particularly in globular and polygonal grains while the remaining interspaces were filled by lamellar network. The detected high stability of extremely high fraction of metastable austenite was tested under different conditions of thermal exposition and mechanical loading.

Effect of Quenching and Partitioning Temperatures in the Q-P Process on the Properties of AHSS with Various Amounts of Manganese and Silicon

The use of the combined influence of retained austenite and bainitic ferrite to improve strength and ductility has been known for many years from the treatment of multiphase steels. Recently, the very fine films of retained austenite along the martensitic laths have also become the centre of attention. This treatment is called the Q-P process (quenching and partitioning). In this experimental program the quenching temperature and the isothermal holding temperature for diffusion carbon distribution for three advanced high strength steels with carbon content of 0.43 % was examined. The alloying strategies have a different content of manganese and silicon, which leads to various martensite start and finish temperatures. The model treatment was carried out using a thermomechanical simulator. Tested regimes resulted in a tensile strength of over 2000MPa with a ductility of above 14 %. The increase of the partitioning temperature influenced the intensity of martensite tempering and caused the decrease of tensile strength by 400MPa down to 1600MPa and at the same time more than 10 % growth of ductility occurred, increasing it to more than 20%.

Microstructure and Mechanical Behavior of a Mini-Thixoformed Tool Steel

We report on mini-thixoforming of a hard and wear-resistant crucible particle metallurgy tool steel. Significant microstructural modifications associated with the special semisolid forming process are characterized in detail by scanning electron microscopy and X-ray diffraction. The mechanical performance of the material is assessed both pre-thixoforming and post-thixoforming by nanoindentation of the constituent phases. The novel microstructural changes that result from mini-thixoforming, which are discussed in this article for the first time, are beneficial in further improving the hardness of the steel.

The Effect of Mn and Si on the Properties of Advanced High Strength Steels Processed by Quenching and Partitioning

The concepts new types of materials are, for economic reasons, focused mainly on low alloyed steels with a good combination of strength and ductility. Suitable heat and thermo-mechanical treatments play an important role for the utilization of these materials. Different alloying strategies are used to influence phase transformations. The quenching and partitioning process (Q-P Process) is one of the heat treatment methods which can result in a high ultimate strength as well as a good ductility. However, these good properties can be obtained only if a sufficient amount of retained austenite is stabilized. The influence of different contents of manganese, silicon and chromium on microstructural development and mechanical properties were experimentally tested. Alloying elements were used to stabilize the retained austenite in the final microstructure and also to strengthen the solid solution. Ultimate strengths of over 2000MPa with ductility over 10% were reached after the optimization of the Q-P Process. The microstructures were analyzed using several microscopic methods; mechanical properties were determined by a tensile test and the volume fraction of the retained austenite was established by X-ray diffraction phase analysis.

Physical Modelling of Microstructure Development during Technological Processes with Intensive Incremental Deformation

Convenient structure adjustment and thereby the achievement of suitable material and technological properties is one of the very important areas of technological as well as material research. In general, this issue includes a great number of parameters and variables. To find suitable technological conditions, it is possible to use various kinds of modeling processes. One of them is the utilization of thermomechanical simulators, which allow simulating the conditions of the real processes to be simulated with sufficient accuracy. It is then possible to perform the optimization on smaller specimens, while monitoring the real conditions with higher accuracy. This method was used for the optimization of unconventional technological processes for selected alloying strategies of low-alloyed multiphase steels. These strategies are designed to be applied to technologies, which combine anisothermal forming and thermomechanical treatment of quasimassive components using intensive plastic deformation. Incremental deformations allow a high amount of deformation to be reached. It is also possible to obtain very fine grained structures by a suitable choice of temperature. By a suitable choice of temperature it is also possible to obtain structures with very fine grain. At the same time, the morphology of the structure and thus also its final mechanical properties can be significantly influenced this way.

Mini-thixoforming of a Steel Produced by Powder Metallurgy

Semi-solid processing is complicated by various inherent technical problems. However, once these problems are solved, thixoforming allows intricately shaped components to be manufactured very effectively – often with microstructures that cannot be produced by any other techniques. The recently introduced mini-thixoforming method is an example of such a novel technique for semi-solid processing of steel. The wall thicknesses of resulting parts are about 1 mm. Microstructures of semi-solid-processed steels typically consist of a high proportion of globular particles of metastable austenite embedded in a carbide network, the latter being much harder and more brittle. This paper illustrates that mini-thixoforming allows inverting that microstructural configuration. As an experimental material, powder steel with increased content of vanadium and chromium was used. The post-thixoforming microstructure consisted of a dispersion of carbides and high-vanadium and high-chromium eutectic in an austenitic matrix. Applying optimised processing parameters, complex-shaped parts could be produced. According to the high hardness of resulting microstructural components, the new materials are likely to exhibit extraordinary strength and wear resistance.

Rapid Spheroidization and Grain Refinement Caused by Thermomechanical Treatment for Plain Structural Steel

The cold formability of ferritic-pearlitic steels is one of the base parameters for material choice for different forming parts. One of the key factors is the pearlite morphology, which is strongly dependent on chemical composition and previous treatment history. The carbides in pearlite occur mainly in the lamellar form. One of the ways of improving the ductility along with formability is the change of lamellar carbides to globular carbides. This can be conventionally done by soft annealing, which is characterised by long processing times and high energy costs. This paper presents a new processing modification which can lead on the one hand to significant shortening of carbide spheroidization times and on the other hand to intensive refinement of grain size even for low-carbon steels. Low temperature thermomechanical treatment with variation of the heating temperature around Ac1 and incremental deformation was examined on low carbon plain RSt-32 steel. After the thermomechanical treatment conditions were optimized, the refinement of the ferritic grains from an initial 30 μm to circa 5 μm took place, and the time necessary for carbide spheroidization was shortened from several hours to several seconds.

The Influence of Thermomechanical Treatment of TRIP Steel on its Final Microstructure

Higher demands are currently being made on material properties and their combinations, for example, high strength steel with good cold formability. With increasing strength, material formability generally worsens, but with some kinds of steels, for example, with TRIP steel, it is possible to obtain an excellent combination of strength and ductility. However, these materials are interesting not only for use as sheets but also for other applications. Semi-products with thin walls represent the best way of further extending TRIP steels into the bulk forming area. This trend supposes that suitable technological conditions will be found and the process for a chosen production procedure will be optimized. The model optimization of the process for TMT with incremental deformations was experimentally carried out on a thermomechanical simulating machine. Microstructures obtained by the model treatment were analyzed by means of optical and electron microscopy.

The Effect of Alloying Elements on Microstructure of 0,2%C TRIP Steel

Three low alloyed transformation induced plasticity (TRIP) steels with 0.2%C were used in this work. The first one was based on the most common and popular 0.2%C - 1.5%Mn - 1.8%Si concept and was used as a reference material. The second steel was further micro-alloyed by 0.06% of Niobium. The third steel was designed with lower manganese content of 0.6% and additional alloying by 0.8% of Chromium. Thermo-mechanical processing with incorporated incremental deformation was applied to each steel. Various cooling rates and numbers of deformation steps were tested with regard to final microstructure and properties. After this optimization, microstructures with the potential to utilize TRIP effect were achieved for all steels. Very good mechanical properties were obtained with ductility typically in the interval of 30-40% and the tensile strengths in the range of 680-835 MPa.

Static and Dynamic Induction Heating — Experiment and Numerical Simulation

Abstract Induction heating is widely used in industry for melting, metal heat treatment, quenching, surface hardening, tempering and also as a heating method in hot forming processes. In this paper, results of preparatory experiments of induction heating for the newly developed technology of rotary spin extrusion are presented. The measured temperatures during static and dynamic induction heating are compared with the results computed using the finite element code ANSYS. Parameters influencing the accuracy of the numerical solution are introduced and discussed.