CCT and TTT diagrams to characterize crystallization behavior of mold fluxes
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The isothermal and non-isothermal experiments were performed to construct the continuous cooling transformation (CCT) and temperature time transformation (TTT) diagrams of four industrial mold fluxes through visual observations in an experimental apparatus based on the single hot thermocouple technique (SHTT). The results of the CCT diagrams indicate that ➀ the crystallization temperature of mold fluxes lowers as the cooling rate increases, ➁ the mold fluxes have larger critical cooling rate, higher crystallization temperature, and less onset time of crystallization when the basicity increases or the viscosity decreases, ➂ the influences of the melting points of the mold fluxes on their crystallization tendency are not significant. Isothermal tests show that the onset time of crystallization decreases at first, and then increases, and finally represents a “C” shape with increasing isothermal temperature. The TTT diagrams of four industrial mold fluxes were divided into two separate “C” shape regions. The crystal phase of C20A selected was analyzed by X-ray diffraction, which is cuspidine (Ca4 Si2 O7 F2) over 1100 °C and calcium silicon oxide fluoride (Ca2 SiO2 F2) below 1100 °C. When compared with the TTT diagram, the CCT diagram can provide a more realistic estimate of the critical cooling rate of the mold fluxes. Thus, both the CCT and TTT diagrams can unambiguously describe the crystallization phenomena of the mold fluxes.
Key wordsmold flux CCT TTT critical cooling rate crystallization
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