The present work describes a new methodology designed to characterize the microstructures of tool steels containing carbide hard phases, with the focus set on their abrasive wear resistance. A series of algorithms were designed and implemented in MATLABÒ to (i) recognize each of the features of interest, (ii) measure relevant quantities and (iii) characterize each of the phases and the alloy in function of attributes usually neglected in wear description applications: size distribution, shape and contiguity of the hard phases. The new framework incorporates new parameters to describe each one of these attributes, as observed in SEM micrographs. All three aforementioned stages contain novel contributions that can be potentially beneficial to the field of materials design in general and to the field of alloy design for severely abrasive environments in particular. Models of known geometry and micrographs of different powder metallurgy steels were analyzed, and the obtained results were compared with the obtained by the linear intercept method. The relation between the new parameters and the ones available in the scientific literature is also discussed.
Hot isostatic pressing (HIP) units are worldwide used for the compaction of metal alloy powders. The cooling rate in a HIP unit is usually comparatively low. This lengthens cycle times and requires an additionally heat treatment for quenched and tempered steels. Novel cooling HIP concepts in HIP units feature high quenching rates. In this study, tool steels were investigated with respect to their time-temperature-transformation behaviour for different cooling parameters. The paper shows that encapsuled powdered tool steels can be compacted and hardened in the HIP unit. The examined steels exhibit a comparable or even a higher hardness and a finer microstructure. HIP units with high-quenching rates enable to compact and heat treat materials in one step.
Increasing requirements concerning the operational conditions and durability of tools create a demand for the optimization of tool steels. High-speed steels (HSSs), for example, contain high amounts of carbides embedded in a secondary hardenable martensitic matrix. The wear behavior and the mechanical properties of HSS can be optimized for a certain application by adjusting the type and amount of carbides, as well as their compositions and the composition of the matrix. Computational thermodynamics based on the calculation of phase diagrams method allow the estimation of arising phases as well as phase compositions during the solidification or the heat treatment of a steel. However, in complex alloy systems, for example, HSS, the relationships between the content of alloying elements and the stability and the composition of phases can be complicated and nonlinear. Therefore, it can be difficult to find alloy compositions that are suitable to achieve a desired microstructure with iterative calculations. To handle this difficulty, a computational tool is developed, which determines compositions to obtain predefined HSS microstructures. The computational tool is based on a neural network that was previously trained with a thermodynamically calculated database. The efficiency of this approach is experimentally verified by producing and investigating laboratory melts of different HSS.
Metal‐supported silica membranes are attractive candidates for CO2 capture from the exhaust of coal‐fueled power plants. Compared to their full ceramic counterparts, the introduction of the metal support facilitates sealing of the membrane by established technologies, such as welding, and enhances the robustness of the membrane in the harsh environment of the power plant. As well‐known from other steel components in flue gas desulfurization units, long‐term corrosion resistance of the metal support is mandatory for the success of this new membrane concept. In the present work, a research concept is introduced enabling a systematic benchmark of stainless steels regarding their suitability to be used for the metal support of the CO2 selective silica membranes. The study combines field tests of porous samples in direct contact with the exhaust gas of a lignite‐fueled power plant and standardized corrosion tests of dense and porous samples in the laboratory according to DIN 50918 using exhaust gas condensate as the corrosive medium. Preliminary results are achieved on austenitic steel (AISI 316L) as well as on two ferritic steels (Crofer22APU, Plansee ITM). Ferritic steels are chosen due to their availability as substrates with well‐defined porosity and with adapted thermal expansion coefficient enabling successful coating of the CO2 selective silica membrane.
Herein, the abrasive wear behavior of different high‐alloyed powder metallurgical (PM) tool steels is investigated at elevated temperatures (400–600 °C) in a dry‐pot wear tester containing Al2O3 particles. To identify the influence of the microstructure, PM tool steels with different hot hardnesses, carbide types, and carbide volume contents are selected. Wear tracks are analyzed by scanning electron microscopy (SEM) to clarify wear mechanisms. The results show that there is no direct correlation between wear resistance and only one material property such as hot hardness, carbide content, or carbide type. More important seems to be the best possible compromise between a sufficient hot hardness of the metallic matrix and a high volume content of carbides that are harder than the attacking abrasive particles at the respective temperature. When the test temperatures surpass the tempering temperature of the investigated steels, there is a pronounced change in wear behavior due to the stronger embedding of abrasive particles into the wear surface. It is thus necessary to discuss the microstructural properties as a function of temperature, considering interactions with the abrasive particles.
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