Nano–Reinforced Al2O3–MgO–C Refractories

Abstract

Refractories in Al2O3–MgO–C system are an important class of oxide–carbon refractories. The structural property benefits of these refractories recommend their extensive usage in a range of steel making vessels (BOF’s, EAF’s, refining ladles), continuous casting components (ladle shroud, monotube, SEN), and many other flow–control devices (slide– gate plates, monoblock stopper). In steel making environments, these carbon based refractories work from ambient to 1600°C in oxidizing as well as non–oxidizing atmospheres. The structural properties of these high–temperature treated refractories in both oxidizing and non–oxidizing atmospheres are known to be essential for their increased service–life performance in steel making. Therefore, to gain the increased service–life performance, modern refractory processing strategies use nanoscale reinforcements as additional recipe parts for fabricating the fracture–resistant refractories with enhanced structural properties relative to the conventional or standard refractories (those systems that are fabricated without reinforcements). But to our knowledge such studies are very limited in the case of nano–reinforced Al2O3–MgO–C refractory. In the current work, nanoscale reinforcements, namely, YAG (Y3Al5O12; Yttrium Aluminium Garnet) nanopowder and EG (expandable graphite (EG)) hybridized powder were prepared in-house and studied their structural characteristics. Further, the nano–reinforced Al2O3–MgO–C refractories were fabricated by using these in-house prepared reinforcements (YAG; EG) with a reinforcement content of 0-2 wt.%. Subsequently, these nano–reinforced refractories were separately fired in oxidizing atmosphere as well as non–oxidizing (reducing) atmosphere at a maximum temperature of 1600°C. Then the structural properties of these fired refractories were further evaluated to determine the efficient function of a nanoscale reinforcement (YAG; EG) in promoting the structural property benefits. The nano-reinforced Al2O3–MgO–C refractories fired in oxidizing atmosphere exhibited significant enhancement of structural properties as compared to the standard components (R). Herein, both nanoscale reinforcements (YAG; EG) showed comparable structural property improvements in terms of oxidation resistance (73%–67%), hot-strength (10–11 MPa), and thermal shock performance (60– 67%) in Y20 (2 wt.% of YAG) and H20 (2 wt.% of EG) refractories, respectively. But the experimental data based on damage parameter calculations (DE) and energy–to failure characteristics (􀢢), indicated that the nano–reinforced refractory components fortified with EG (H20: DE~27%; 􀢢~36 kJ m-3) are more fracture–resistant than the nano–YAG reinforced refractories (Y20: DE~32%; 􀢢~29 kJ m-3). These structural property benefits with EG reinforcement were ascribed to the in–situ grown, bimodal microstructure with EG sintered framework in the nano–reinforced Al2O3–MgO–C refractory (H20) interior. Similarly, the nano-reinforced Al2O3–MgO–C refractories fired in non-oxidizing atmosphere, also showed a notable improvement in structural properties over the standard components (R). In this case, the nano-reinforced refractories (H20) with EG exhibited moderately enhanced structural properties as compared to the nano-YAG reinforced refractories (Y20). Herein, the structural property improvements are verified in terms of higher bulk density (Y20: 3.16 g cm-3; H20: 3.21 g cm-3 ), lower apparent porosity (Y20: 10.1%; H20: 9.2%), reduced stiffness (Y20: 84 GPa; H20: 62 GPa), increased load– bearing capacity (Y20: 0.423; H20: 0.591), enhanced mechanical reliability (Y20: m~22; H20: m~28), and greater dimensional 􀁖􀁗􀁄􀁅􀁌􀁏􀁌􀁗􀁜􀀃􀀋􀀼􀀕􀀓􀀝􀀃􀄮􀁡􀀚􀀑􀀘􀃮􀀔􀀓-6 °C-1􀀞􀀃􀀫􀀕􀀓􀀝􀀃􀄮􀁡􀀙􀀑􀀔􀃮􀀔􀀓-6 °C- 1), respectively. These reinforcement benefits with EG were attributed to the development of in-situ grown EG sintered framework as a core (YAG)–sheath (EG) microstructure in all parts of the nano–reinforced Al2O3–MgO–C refractory (H20). All these beneficial features confirm that the nanoscale EG powder as an efficient reinforcement than the YAG nanopowder. Additionally, this work proposes materials design strategies for the fabrication of fracture–resistant, nano–reinforced refractories in Al2O3–MgO–C system against oxidative damage and thermal shock failure with the application of our newly proposed scaling parameters. They are named as strength factor (fs) and reinforcement index (Ri), respectively. Furthermore, the guidelines for implications of these research findings to practical applications involved in various potential areas of steel making are discussed.