An ultrasonic device was designed to fabricate relatively small (about 0.7 kg) vacuum chill castings of FeAl-based alloys with improved microstructure. The influence of ultrasound on the solidification microstructure of the Fe-40 at.% Al based castings was investigated. The subsequent hot-rolling of such an as-cast alloy was made using a special stainless steel bandage of the casting. This effectively prevents fraying and cracking caused by the thermal shock arising at the surface of the hot casting, when it comes in contact with a cold roller. The efficiency of ultrasonic vacuum casting, based on the influence of the acoustic cavitation on the solidification of the melt, is manifested by a refined microstructure of the prepared iron aluminides Fe-40 at.% Al with addition of C or Zr and B. First results of improved mechanical properties of these alloys are presented and discussed.
The iron aluminide containing 45 at. % Al, 4 at. % C and 1 at. % Si (Pyroferal) was considered as a structural material without any possibility of forming. The brittle character of the material is the reason for cleavage, which takes place on the surface or in the whole volume of the casting during forming, even at high temperatures. A special rolling procedure of such material is newly described, which enables to roll the casting at high temperatures.The sample is enclosed with a special can to prevent the direct contact of the cold rolls with the surface of the hot casting. The positive effect of the can is documented and the structure of the rolled sample (FeAl + Al 4 C 3 composite) is described.
The paper reports a study of temperatures of phase transitions of nine samples of real grades of low-alloyed steels, including one sample of pure iron, using differential thermal analysis. Temperatures of phase transitions were measured in the low-temperature interval (20 – 1 000 °C). Investigated systems were analysed at selected rates of heating/cooling and then temperatures of phase transitions for zero heating/cooling rate were determined. The results obtained are compared with temperatures of the equilibrium metastable diagram Fe – C, with results from the software IDS and with temperatures of phase transitions calculated according to relations published in available literature. It follows from the results that differences, sometimes considerable ones, still exist between experimental and theoretical data, which implies the necessity for further systematic research in this area.
The paper summarizes the experiences acquired from on-line acoustic emission monitoring (herafter AE) of heavy castings during their manufacturing (solidification and following cooling in the mould). They are usually monitored elastic waves generated above all by stress changes in the solid state. In order to exactly determine plastic-elastic transition state the investigation was focused on raising the sensitivity of detection. The suitable experimental technique is discussed in the first part of this article. The main problem of the measurement by high temperatures was solved by using waveguides. It is very important in this case to select useful signal sources from mechanical and electromagnetical disturbances. Some laboratory experiments were done for studying the signal origin in the first state of solidification. The results from on-line monitoring of two types cast rolls during manufacturing were compared. Each type of casting has its typical AE histogram. For the quality evaluation ( in our case) is significant the time period of approximate 5 days after pouring . The time delayed stress induced cracking generates high level AE signal in this time period and the presence of such signal indicates defective product. The study of high temperature tensile tests, structural phase transformation and solidification processes using AE is very important for analysis of AE sources. The use of the laboratory results for the AE source analysis on real products will be subject to futher research.
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