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A typical high-CSR Australian coke was subjected to high-temperature tests to determine its compression and creep-compression properties and the associated microstructural and mineralogical modifications. The major findings are as follows:
· The unique high-temperature facilities at ANSTO were capable of producing valid and reliable measurements of the high-temperature compression and creep-compression behaviour of high-CSR cokes with good repeatability. The rates of thermal expansion of the tested coke samples varied with the test temperature, with a significant decrease above 1200°C, followed by a constant rate to ~1550°C at which there was a second decrease. A testing protocol was developed to compare the compression strength and creep-compression behaviour of the cokes at different temperatures. On the basis of repeated preliminary trials (25°C to 1800°C) as well as experimental limitations, three test temperatures (1400°, 1550°, and 1700°C) were identified for creep-compression and compressive strength measurements.
· The present work confirmed the hypothesis that the compression strength of the coke increases significantly with increasing temperatures for at least high-CSR cokes such that the ultimate failure stress at 1400°C (30 MPa) was almost three times the value at room temperature (~13 MPa).
· The stress-strain behaviours observed at room and high temperatures are significantly different. At high temperatures, the samples showed steady elastic and plastic deformation prior to sudden failure while at room temperatures the steady-elastic part was followed by brittle failure and uneven load fluctuations owing to self-compression.
· Microstructural analysis showed an increase in the extent of porosity after high-temperature testing and this is attributed to crack and pore formation from the well-known in situ reduction of quartz and other oxide species in the coke. At 1400°C, the glassy silicate phases can undergo plastic flow, which can contribute to enhancement of the plastic deformation of the coke samples at these temperatures. However, at higher temperatures, the graphitisation degree of the coke would also increase, reaching a maximum at the highest testing temperature of 1800°C (as expected). Increasing graphitisation would result in decreasing load-bearing capacity owing to decreasing bonding and abrasion strength of the graphitic material.
· The stress relaxation test done at a constant stress of 13 MPa at 1550°C was able to identify the overall strain from elastic and plastic deformation to be ~1.19% and the elastic spring-back deformation on unloading was clearly seen from the deformation data.
In the published literature, brittle deformation of coke had been observed previously at room temperature and at 1000°C while plastic deformation has been observed during creep tests at constant stresses at 1300°, 1500°, and 1600°C and during compression testing in a Gleeble machine at 1600°C. However, studies reporting the effect of temperature on the failure stresses of cokes have not always shown consistent trends. This may be due partly to the variety of test methods used, specimen pre-conditioning, and definition of failure and temperature ranges assessed. Some axial compression tests have given higher strengths at 1400°C than at room temperature [Grant 1986, Grant et al. 1991]. However, a range of different failure modes has been observed and Haapakangas [Haapakangas et al 2014] has classified two different room-temperature and four different high-temperature deformation modes from fifty tests each on three grades of coke. In these tests, the coke specimens generally had lower strengths at 1600° and 1750°C than at room temperature while other work has found lower ultimate strength at 1600° than at 1000°C [Haapakangas 2013].
Failure strains observed in the current work were broadly similar at room and high temperature (~3.6 to 4.9%) and these were similar to other published data (e.g., 2.5 to 3.0% by Haapakangas et al. (2014) and 2.2 to 3.0% by Kim and Sazaki (2010)). The slopes of the initial elastic loading in these tests were similar between 1400° and 1700°C; however, these were almost twice the slope of the room temperature tests. Other published axial compression elastic moduli reported by Sato (1999) have been ~1500 MPa and ~1300 MPa. These were slightly lower than those of the present high temperature test data. The room-temperature tests by Amanat et al (2009) indicate an elastic modulus of ~900 MPa, which is broadly in agreement with the present room-temperature test data. The measured failure strains in the present work are consistent with values expected for foam-like materials at high temperatures and this further confirms the reliability of the data and the testing methods.