When extruding the casted rods from EN AW 2011 aluminium alloys, not only their homogenized structure, but also their extrudable properties were significantly influenced by the hardness of the alloy. In this study, the object of investigations was the EN AW 2011 aluminium alloy, and the effect of homogenisation time on hardness was investigated. First, homogenisation was carried out at 520 °C for different times, imitating industrial conditions. After homogenisation, the samples were analysed by hardness measurements and further characterised by microscopy and image analysis to verify the influence of homogenisation on the resulting microstructural constituents. In addition, non-equilibrium solidification was simulated using the program Thermo-Calc and phase formation during solidification was investigated. The homogenisation process enabled more rounded shape of the Al2Cu eutectic phase, equilibrium formation of the phases, and the precipitation in the matrix, leading to a significant increase in the hardness of the EN AW 2011 aluminium alloy. The experimental data revealed a suitable homogenisation time of 4–6 h at a temperature of 520 °C, enabling optimal extrusion properties.
Lightweight Fe-Mn-Al-C steels have low density, and high mechanical properties, which makes them a possibility for weight reduction in vehicles for road transport. In steel production, as-cast microstructure is an important parameter for further processing. The as-cast microstructure of five lightweight duplex steels was investigated: Fe-15Mn-10Al-0.8C, Fe-15Mn-10Al-1.7Ni-0.8C, Fe-15Mn-10Al-3.9Ni-0.8C, Fe-15Mn-10Al-5.6Ni-0.8C and Fe-15Mn-10Al-8.6Ni-0.8C. The influence of Ni was analysed through thermodynamic calculations and microstructural characterization. The samples were analysed through an optical and electron microscopy. The base microstructure of the studied steel consists of ferrite and austenite. Further investigation showed that the decomposition of austenite was accompanied by the formation of kappa carbides and the B2 ordered phase. The addition of Ni prevented the formation of a lamellar kappa ferrite morphology, but at 5.6 wt.% Ni, the decomposition of austenite was most severe, resulting in a large amount of kappa carbides and a B2 ordered phase.
Hot-work tool steels have very good mechanical properties, especially strength, hardness and wear resistance at high temperatures. Therefore, hot-work tool steels are used for different applications, such as the high-pressure die casting of light alloys, the extrusion of polymers and forging. Since all these processes are operating at high temperatures, the main focus of this research was to investigate the high-temperature oxidation resistance of tool steels. We investigated the high-temperature oxidation at two different temperatures: 500°C and 700°C. The following tool steels were analyzed: HTCS-130, W600, RavnexHD and Dievar. Tests were made in an air atmosphere, while the heating and cooling were made in a controlled argon atmosphere. Simultaneous thermal analysis (STA 449 C Jupiter), scanning electron microscopy (SEM and EDS) and X-ray diffraction were used as the investigation methods. The results showed that during high-temperature oxidation at 500°C Dievar steel has the best oxidation resistance, followed by RavnexHD, HTCS-130 and W600. However, at 700°C the results were different, HTCS-130 had the best oxidation resistance, followed by Dievar, W600 and RavnexHD.
In order to explore the influence of niobium, tantalum and titanium modification on grain growth in high-thermal-conductive hot-work tool steel during austenitisation at high temperatures, three types of modified steels (sample HTCS-130 + 0.06 w/% Nb, sample HTCS-130 + 0.03 w/% Ta, sample HTCS-130 + 0.006 w/% Ti) based on reference (sample HTCS-130 – 0) were prepared. The effect of different austenitisation temperatures (1030, 1060, 1080 and 1100) °C on hardness after quenching and grain size were investigated. The results show that there is a positive effect on the mechanical properties and a decreased grain-growth effect in the modified steel samples. The precipitation behaviour of the carbides was also investigated with electron microscopy. The Mo-W carbides were relatively weak at retaining grain size, but their pinning effect was increased with the incorporation of other carbide-forming elements like Nb and Ta. MC-type carbides on the grain boundary were effective at grain-boundary pinning. Nb even further increased the resistance by forming NbC carbides. The addition of Ti, on the other hand, proved to be ineffective due to the intergranular precipitation of the formed carbonitrides.
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