Slag in Semi-killed Steels

Name: Slag in Semi-killed Steels
Diagram No.: 972
Type of diagram: TTT
Chemical composition in weight %: 35.4% CaO, 25.2% Al2O3, 39.4% SiO2, CaO/SiO2=0.90
Group: Slags
Note: Top diagram: TTT diagrams of the beginning of crystallization obtained by in situ and ex situ observations using optical microscope and XRD analysis.
The TTT diagram was determined as shown in Fig. It modified the single crystal phase curve in the tentative TTT diagram obtained from in situ observation to the double noses TTT curves. The upper nose in the final TTT diagram obtained by XRD analysis would be the nose of anorthite and the lower one indicated that of gehlenite. The anorthite crystals precipitated at higher temperature were transparent or translucent, which were difficult to identify by the naked eyes. The nose of anorthite was located around 4800 s at 1300C and that of gehlenite was around 1200 s at 1200C. The reason why gehlenite precipitates faster than anorthite can be explained in terms of free energy functions of the compounds, which will be discussed in the following part.
The TTT diagrams obtained by using the above two techniques are almost identical. The observed crystalline phases for this composition are gehlenite (2CaOAl2O3 SiO2) and anorthite (CaO Al2O3 2SiO2). The TTT diagram from SHTT gives the nose position of gehlenite and anorthite of, respectively, 1200 s at 1200C and 4800 s at 1300C.
Bottom diagram: TTT diagrams displaying the crystallization start conditions obtained by heat cycle experiment.
Figure shows the final TTT diagram from heat treatment cycle which clearly contributed to two different crystal phases and the coexisting region. The dominant crystals evolved in this slag were also gehlenite and anorthite. The incubation time for gehlenite was 900 s at 1240C and that for anorthite was 2000 s at 1290C which were shorter time than the result of Hot thermocouple method. For the sake of reference, a series of experimental data.
Reference: Not shown in this demo version.

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