The main problem related to the development of fluorine‐free mould powders for slab casting is effectively controlling the heat transfer between the steel shell and mould. In commercial mould powders crystallization of cuspidine (Ca4Si2O7F2) from mould slag has a great effect on heat‐transfer control. In industrial process the crystallization rate for a fluorine‐free mould slag should be similar to the crystallization rate of cuspidine. To evaluate the crystallization rate for slags time‐temperature‐transformation (TTT) diagrams can be constructed using the Single Hot Thermocouple Technique by in situ observation. In the present work, fundamental information related to crystallization control in the CaO–SiO2–TiO2–Na2O–Al2O3 system was obtained. It was observed that the addition of Na2O in CaO–SiO2–TiO2 slags dramatically shortens the crystals' incubation times in TTT diagrams to the range of seconds. It is possible to control the crystallization kinetics in CaO–SiO2–TiO2 slags by changing the Na2O content. Some observations for the crystals' morphology are reported.
With the single hot thermocouple technique (SHTT) the solidification behavior of metallurgical slags has been studied by in situ observation, constructing timetemperature-transformation (TTT) or continuous-cooling-transformation (CCT) diagrams. The SHTT is a unique apparatus that enables measurement of the slag sample temperature using a thermocouple while the sample is heated or cooled simultaneously. Due to the low heat capacity of the system sample/thermocouple high heating or cooling rates can be easily obtained (>30008C/min). The following findings are reported in the present paper: (i) For the CaO-Al 2 O 3 slag -44% CaO, 56% Al 2 O 3 (wt%) -the CCT diagram shows large differences between liquidus and the temperature for first crystals precipitation, even at low cooling rates, for example, 1688C below the liquidus when cooling at a rate of 68C min À1 . (ii) For the CaO-SiO 2 slag -% CaO/% SiO 2 (wt%) ¼ 0.7 -no crystal is observed for continuous cooling, even at low cooling rates, such as 108C min À1 . During isothermal experiments crystallization was observed only at 10008C with an incubation time of 76 s (average of six experiments, standard deviation 27 s). However, crystallization becomes much more intense for the CaO-SiO 2 slag when increasing the temperature after reaching lower temperatures (<10008C), where probably the conditions for nucleation are better.[ Ã ] J. Figure 11. Viscosity measurements taken using a rotation viscometer, for a slag whose composition is close to that of the CA slag (46.1% CaO, 53.9% Al 2 O 3 ).
The Cr-Mn-Ni austenitic steel cast alloys containing 16 wt.-% chromium, 7 wt.-% manganese and 3 to 9 wt.-% nickel are used as matrix material to manufacture TRIP-Matrix-Composite containing MgO partially stabilised ZrO2ceramics. In the present work, these steel master alloys were atomised via a vacuum inert gas atomisation to provide fine grain sized metal powders for the assembling of TRIP-Matrix-Composite. The atomised steel powders where characterised according to their chemical composition and the d50mass median particle size determined by a laser diffraction analyses. The surface tension of the liquid steel alloys was experimentally investigated using the maximum bubble pressure (MBP) method. The reference austenitic steel alloy (AISI 304) has the highest surface tension and the highest mass median particle size compared to the Cr-Mn-Ni steel alloys, where d50values are lower due to lower surface tension values. Finally, it is concluded from the present investigation, that the d50size of the atomised steel powders decreases by a decrease of surface tension values for the liquid steel alloys.
With the Single Hot Thermocouple Technique the solidification behaviour of metallurgical slags has been studied, building TTT and CCT diagrams, with in situ observation for melting and solidification under various thermal conditions. Slags in the system CSTNA were studied, analysing the kinetics of crystals growth. Following findings are reported: (i) for the slag CS (%CaO/%SiO 2 = 0.7) no crystal is observed during contin uous cooling even to low cooling rates, but on the other hand intense crystallisation is obtained when increas ing the temperature after reaching a particular range of lower temperatures; (ii) it was observed that the longer the duration of the superheating is, the longer the incu bation times for a slag CST (41 %CaO, 29 %SiO 2 , 30 %TiO 2 ) becomes; (iii) the addition of Na 2 O into the CST slag system shortens intensely the crystals incubation times at TTT diagrams to the range of seconds.
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