Buckling and crush resistance of high-density TRIP-steel and TRIP-matrix composite honeycombs to out-of-plane compressive load

AbstractThe mechanical and structural responses of high-density TRIP steel and TRIP-steel/zirconia composite honeycomb structures were studied under uniaxial compression in the out-of-plane loading direction over a wide range of strain rates. Their mechanical response, buckling, and failure mechanisms differ considerably from those of conventional thin-walled, low-density cellular structures. Following the linear-elastic regime and the yield limit of the bulk material, the high-density square honeycombs exhibited a uniform increase in compression stress over an extended range of (stable) plastic deformation. This plastic pre-buckling stage with axial crushing of cell walls correlates with the uniaxial compressive response of the bulk specimens tested. The dominating material effects were the pronounced strain hardening of the austenitic steel matrix accompanied by a strain-induced α’-martensite nucleation (TRIP effect) and the strengthening effect due to the zirconia particle reinforcement. The onset of critical plastic bifurcation was initiated at high compressive loads governed by local or global cell wall deflections. After exceeding the compressive peak stress (maximum loading limit), the honeycombs underwent either a continuous post-buckling mode with a folding collapse (lower relative density) or a symmetric extensional collapse mode of the entire frame (high relative density). The densification strain and the post-buckling or plateau stress were determined by the energy efficiency method. Apart from relative density, the crush resistance and deformability of the honeycombs were highly influenced by the microstructure and damage evolution in the cell walls as well as the bulk material’s strain-rate sensitivity. A significant increase in strain rate against quasi-static loading resulted in a measured enhancement of deformation temperature associated with material softening. As a consequence, the compressive peak stress and the plastic failure strain at the beginning of post-buckling showed an anomaly with respect to strain rate indicated by minimum values under medium loading-rate conditions. The development of the temperature gradient in the stable pre-buckling stage could be predicted well by a known constitutive model for quasi-adiabatic heating

Carbonized wood and sunflower seed hull pellets as a substitution for carbon black for the production of MgO–C refractories

Refractories based on MgO with up to 20 wt% carbon are the dominating lining materials for vessels in steelmaking and refining plants, where they have to withstand high temperatures, thermal shock, and aggressive steel/slag systems. To preserve limited resources and establish regional raw material supply chains, alternative resources for graphite as a carbon source are required. In this study, pellets from sunflower seed hull and wood were converted into carbon by a carbonization process and added to a MgO–C batch (3 % C) to replace up to 1.1 wt.-% of carbon black. After hardening and carbonizing the MgO–C samples at 1000 °C, their phase composition, microstructure, porosity, density, cold crushing strength (CCS), refractoriness under load (RuL), and oxidation resistance were investigated. The addition of carbonized wood and sunflower seed hull pellets reduced the CCS by more than 25 % due to an inhomogeneous microstructure with poor grain bonding to the matrix, but the porosity was not affected negatively. The RuL is at the same level as the reference samples, even if the carbonized pellets contain ashes with a high amount of K2O (50 %). The addition of carbonized biomass pellets has in particular a positive effect on the oxidation resistance, resulting in 30 % less carbon oxidation of wood pellet coke compared to carbon black at 1600 °C. Hence, sunflower seed hull and wood pellets were found to be prospective carbon sources for MgO–C refractories to support their evolution into a carbon-neutral material

Nano- and micrometre addition of SiO2, ZrO2 and TiO2 in fine grained alumina refractory ceramics for improved thermal shock performance

The present contribution investigates the influence of micro-metre- as well as nano-metre-additions of zirconia (ZrO2), titania (TiO2), silica (SiO2) and magnesia (MgO) into alumina-rich fine grained ceramic materials for refractory applications. Slip casted samples in the system alumina-zirconia-titania (AZT), alumina zirconia titania silica (AZTS) and alumina-zirconia-titania-magnesia (AZTM) were sintered and the physical as well as mechanical properties were investigated as fired and after thermal shock treatments. The generation of a micro-crack network after sintering due to the formation of phases with different thermal expansion coefficients and the formation and decomposition of aluminium titanate (Al5TiO5) before and after thermal shock exposure leads to higher strengths after thermal shock attack. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved