Effect of substituting SiO 2 with TiO 2 on viscosity and crystallisation of mould flux for casting titanium stabilised stainless steel

Sample preparation

Analytical grade TiO₂, SiO₂, MgO, Al₂O₃, Na₂CO₃, CaCO₃ and CaF₂ were taken as raw materials, with Na₂CO₃ and CaCO₃ being substitutes for Na₂O and CaO respectively. Table 1 shows the compositions of the samples investigated.

Glass samples with different compositions were prepared by conventional melting and quenching methods. Raw materials were mixed, placed in a platinum crucible and then melted in air in a furnace at ∼1473 to 1573 K. The samples were held at the set temperature for nearly 2 h to ensure complete melting and homogenisation. After melting, the melts were quenched by water and bulk glass samples formed. These samples are shown to be amorphous by X-ray diffraction (XRD) (Fig. 1).

OPEN IN VIEWER

Figure 1.

Typical XRD pattern of quenched glass sample

Viscosity measurements

Viscosity measurements were carried out using a rotating cylinder method with a rotary viscometer (model: RTW-10). Before measurements, the viscometer was calibrated at room temperature by using standard oil with known viscosity. About 130 g melted slag for each sample was subject to the viscosity measurement which was carried out in a graphite crucible. The materials and dimensions of the crucible and spindle are listed in Table 2. As the temperature reached 1673 K, the sample was held in the crucible for ∼30 min to ensure that the slag was completely melted and homogeneous. The bob was then immersed in the molten slag and rotated at 200 rev min⁻¹. The furnace was then cooled at a rate of 5 K min⁻¹ and the measurement of viscosity started. Argon was used as the protective gas during the whole experimental process.

Differential thermal analysis

The glass samples were pulverised into powder (150–200 mesh) for differential thermal analysis (DTA), performed in the range of 373–1473 K using a Netzsch STA 449 C TG-DTA calorimeter at a heating rate of 10 K min⁻¹ and with Al₂O₃ as a reference material.

X-ray diffraction analysis

The powdered glass samples were heated in a Pt crucible, held for ∼1·5 h at the exothermic peak temperature and then quenched by air. In order to identify the crystallisation products, the heat treated samples were subjected to XRD analysis which was performed using a MAC M21XRHF22 (21KW) X-ray diffractometer with Cu K_α radiation.

Microstructural of crystal phase

After polishing and coating with carbon, the heat treated glass samples were subjected to scanning electron microscopy (SEM) examinations using Zeiss EVO MA18 to obtain the microstructure of the crystallisation products. The working voltage was 20 kV.

Viscosity characteristics

Figure 2(A) shows the viscosity changes of the studied system and Fig. 2(B) is an enlarged part at a high temperature. It can be seen from Fig. 2(B) that at the high temperature (∼1563 to 1623 K) the viscosity of slag slightly decreases with the TiO₂ content increasing from 0 to 10 mass-%. This result is in line with work by Johnston and Brooks.8 They also investigated a similar decrease in viscosity due to the addition of TiO₂. However, as the temperature decreases, the viscosity of the slag with 10%TiO₂ increases more than that of the TiO₂ free slag. TiO₂ could act as a network breaker in the system and have some destructive efforts to the Si–O complex structure groups. Simultaneously, the increase in the TiO₂/SiO₂ ratio can lead to basicity increase [w(CaO)/w(SiO₂)] and results in the reduced degree of polymerisation of the system. Also the Si–O bond is stronger than the Ti–O bond, which leads to the decrease in viscosity to some extent. However, with the addition of TiO₂, solid particles, e.g. CaTiO₃, could form at a lower temperature; the viscosity critically increases and becomes greater than that of TiO₂ free slag. X-ray diffraction results of the slag with 10%TiO₂ cooled from molten to ∼1273 K has verified this speculation by demonstrating that CaTiO₃ crystallises (Fig. 3). The present results demonstrate that small substitution (<5%) of TiO₂ to SiO₂ in mould fluxes would not deteriorate the lubricity function of mould fluxes. However, further substitution of TiO₂ leads to increasing viscosity due to crystallisation of CaTiO₃, and bring risks of failure in casting practice.

OPEN IN VIEWER

Figure 2.

Viscosity curves of mould fluxes

OPEN IN VIEWER

Figure 3.

X-ray diffraction patterns of 10%TiO₂ sample cooled from molten state to 1273 K

Crystallisation characteristics

There are two kinds of crystallisation in mould flux films during casting. One is direct crystallisation from the liquid layer, which is close to the strand. The other is crystallisation from the glass layer formed near the mould. The latter crystallisation critically affects the thickness of the crystalline layer, and thereby has a strong effect on horizontal heat transfer. The present study investigated the effect of TiO₂ substitution to SiO₂ on crystallisation of mould glass quenched by employing various experimental techniques, such as DTA, XRD and scanning electron spectroscopy energy dispersive spectroscopy (SEMEDS).

Figure 4 shows the DTA curves of powdered samples from which the glass transition temperatures T _g, onset crystallisation temperature T _x, crystallisation peak temperatures T _c and liquidus temperature T _l can be obtained. These characteristic temperatures have been marked in Fig. 4 based on the definition of Boettinger et al.,18 as shown in Table 3. It can be seen that the increase in TiO₂ makes a slight effect on glass transition temperatures, 565, 557 and 555°C for samples with 0, 5 and 10%TiO₂ substitution respectively. The introduction of TiO₂ leads to a decrease in the crystallisation peak temperature, from 791 to 765°C. With the increase in TiO₂, the crystallisation peaks decrease in intensity and become smoother, which indicates that the substitution of SiO₂ with TiO₂ weakens the crystallisation of mould flux. As the content of TiO₂ reaches 10%, there appears a new crystallisation peak at 870°C except for the peak at 760°C, indicating that a new kind of crystal has crystallised. The peak area of DTA curve could indicate the enthalpy of crystallisation, and thereby reflects the crystallisation ability of sample. Figure 5 shows the integral area of the first crystallisation peak for each sample. It can be also seen that area of crystallisation peak decreases with increasing TiO₂.

OPEN IN VIEWER

Figure 4.

Differential thermal analysis curves of powdered samples

OPEN IN VIEWER

Figure 5.

Integral of crystallisation peak with different TiO₂ contents

To identify the phase crystallising out at each crystallisation peak temperature in the DTA curves for each sample, sample nos. 1–3 respectively, was heated in Pt crucible to the exothermic peak temperature, 791, 765 as well as 870 and 760°C, held for ∼1·5 h (5 h for sample no. 3; since the crystalline particles are too small and crystallisation is too weak to detect by SEM when it was held for 1·5 h) and then quenched by air. The treated samples have been investigated by XRD and SEMEDS ( ^{Figure 6} Figs. 6 and 7 respectively). From the XRD results in Fig. 6 it can be seen that cuspidine (Ca₄Si₂O₇F₂) is the only crystallite for the TiO₂ free slag samples, while for the slag samples with 5 and 10%TiO₂, two kinds of crystallites appear, i.e. cuspidine and perovskite (CaTiO₃). Figure 7 shows the backscatter electron microscopy micrographs of the crystallised slag samples with 0, 5 and 10%TiO₂ respectively and 1 represents the cuspidine phase. Cuspidine crystallite can be observed for the slag samples with 0 and 5%TiO₂; however, differently from the XRD results, perovskite is almost undetectable by SEM for samples with 10%TiO₂. It can be explained that with the substitution of TiO₂ below 5%, cuspidine is the dominant crystallite and the crystallisation of perovskite is too weak to detect by SEM. Figure 7c shows small crystal particles other than cuspidine, indicating that when the substitution of TiO₂ is as high as 10%, different crystal phases appear. However, it is difficult to detect and observe these small crystal particles accurately and clearly by SEM, which can be understood as that the increase in TiO₂ substitution has an inhibiting effect on crystal growth and reduces crystallisation ability of mould fluxes.

OPEN IN VIEWER

Figure 6.

X-ray diffraction patterns of heat treated samples at crystallisation temperatures

OPEN IN VIEWER

Figure 7.

Images (SEM-EDS) of crystallised samples for a 0%TiO₂, b 5%TiO₂ and c 10%TiO₂

Many indices have been proposed ¹⁹ 19,20 to estimate the stability of glass, and the glass forming ability index K _gl, introduced by Hruby,20 is expressed as the following equation (2) According to Hruby,20 a short interval T _x–T _g signifies that the glass contains some structural units with a higher crystallisation tendency, i.e. the original melt is unfavourable to the formation of glass, while a short temperature interval T _l–T _x indicates that the crystalline phase, which is formed at T _x, has a relatively lower melting point, that is to say, the original melt has a relatively low solidification temperature, i.e. it is favourable to formation of glass. Hruby suggests that T _x–T _g is directly proportional to the glass forming tendency and T _l–T _x is indirectly proportional to the glass forming tendency. It is reported that the bigger difference T _x–T _g and the smaller temperature interval T _l–T _x result in a greater K _gl, which indicates the lower tendency of crystallisation and more stable for glasses.21 The value of K _gl is shown in Table 3. It can be seen from Table 3 that with increasing TiO₂ content, the value of K _gl increases from 0·207 to 0·382. This indicates that substitution of TiO₂ for SiO₂ leads to more stable glasses and reduces the crystallisation tendency of the investigated glass system.

The influence of TiO₂ substitution for SiO₂ on the crystallisation behaviour of this system could be explained by the following consideration:

The strong influence of TiO₂ on crystallisation of mould fluxes would inevitably bring some problems during casting of stainless steel. Since cuspidine is a major type of crystal in the solid flux film and has a critical effect on horizontal heat transfer from mould to strand, the inhibition of cuspdine formation could lead to a decrease in thermal resistance and reduce cast product quality. The direct measurement of thermal resistance of slag films would be performed in a future study.