ABSTRACT
This work presents the results of metallographic studies and the tensile, impact, and fatigue crack growth (FCG) resistance tests of 17H1S main gas pipeline steel in the as-received (AR) state and after a long-term operation (LTO). A significant number of non-metallic inclusions forming chains stretched along the direction of pipe rolling were found in the microstructure of the LTO steel. The lowest values of elongation at break and impact toughness of the steel were determined for the lower part of the pipe close to its inner surface. FCG tests at a low stress ratio (R = 0.1) did not reveal a significant change in its growth rate in degraded 17H1S steel compared to steel in the AR state. During tests at a stress ratio R = 0.5, the effect of degradation was more pronounced. The Paris' law region of the da/dN-∆K diagram for the LTO steel corresponding to the lower part of the pipe close to its inner surface was higher than those for the steel in the AR state and the LTO steel corresponding to the higher part of the pipe. Fractographically, a significant number of delaminations of non-metallic inclusions from the matrix were recognized. Their role in the embrittlement of steel, especially steel from the lower part of the pipe close to its inner surface, was noted.
ABSTRACT
Yttria-stabilized zirconia (YSZ) is well-known as a material with perfect mechanical, thermal, and electrical properties. It is used for manufacturing various high-temperature components for aerospace and energy generation, as well as wear- and corrosion-resistant devices in medicine. This work investigated the effect of a Y2O3 addition to ZrO2 on the microstructure and mechanical properties of YSZ ceramics produced by one sintering schedule. ZrO2 ceramics doped with 3, 4, 5, 6, 7, and 8 mol% Y2O3 (designated 3YSZ through to 8YSZ) were prepared by using conventional sintering at 1550 °C for 2 h in argon. The effect of yttria content was analyzed with respect to grain size, morphology of the microstructural features, phase composition, parameters of fracture surface, and flexural strength. The 7YSZ ceramics sintered at 1550 °C for 2 h showed the highest level of flexural strength due to the formation of the fine-grained microstructure containing mainly the monoclinic and tetragonal zirconia phases. The fracture micromechanism in the studied YSZ ceramics is discussed.
ABSTRACT
It is known that the yttria-stabilized zirconia (YSZ) material has superior thermal, mechanical, and electrical properties. This material is used for manufacturing products and components of air heaters, hydrogen reformers, cracking furnaces, fired heaters, etc. This work is aimed at searching for the optimal sintering mode of YSZ ceramics that provides a high crack growth resistance. Beam specimens of ZrO2 ceramics doped with 6, 7, and 8 mol% Y2O3 (hereinafter: 6YSZ, 7YSZ, and 8YSZ) were prepared using a conventional sintering technique. Four sintering temperatures (1450 °C, 1500 °C, 1550 °C, and 1600 °C) were used for the 6YSZ series and two sintering temperatures (1550 °C and 1600 °C) were used for the 7YSZ and 8YSZ series. The series of sintered specimens were ground and polished to reach a good surface quality. Several mechanical tests of the materials were performed, namely, the microhardness test, fracture toughness test by the indentation method, and single-edge notch beam (SENB) test under three-point bending. Based on XRD analysis, the phase balance (percentages of tetragonal, cubic, and monoclinic ZrO2 phases) of each composition was substantiated. The morphology of the fracture surfaces of specimens after both the fracture toughness tests was studied in relation to the mechanical behavior of the specimens and the microstructure of corresponding materials. SEM-EDX analysis was used for microstructural characterization. It was found that both the yttria percentage and sintering temperature affect the mechanical behavior of the ceramics. Optimal chemical composition and sintering temperature were determined for the studied series of ceramics. The maximum transformation toughening effect was revealed for ZrO2-6 mol% Y2O3 ceramics during indentation. However, in the case of a SENB test, the maximum transformation toughening effect in the crack tip vicinity was found in ZrO2-7 mol% Y2O3 ceramics. The conditions for obtaining YSZ ceramics with high fracture toughness are discussed.
ABSTRACT
The YSZ-NiO ceramics for solid oxide fuel cells (SOFCs) anode have been investigated. A series of specimens were singly reduced in a hydrogenous atmosphere (Ar-5 vol% H2 mixture) at 600 °C under the pressure of 0.15 MPa or subjected to 'reduction in the mixture-oxidation in air' (redox) cycling at 600 °C. The YSZ-Ni cermets formed in both treatment conditions were then aged in 'water vapor in Ar-5 vol% H2 mixture' atmosphere at 600 °C under the pressure of 0.15 MPa. Additionally, the behaviour of the as-received material in this atmosphere was studied. It was revealed that small amount of water vapor in Ar-5 vol% H2 mixture (water vapor pressure below 0.03 MPa) does not affect the reduction of the nickel phase in the YSZ-NiO ceramics, but causes some changes in the YSZ-Ni cermet structure. In particular, nanopore growth in tiny Ni particles takes place. At higher concentration of water vapor in the mixture (water vapor pressure above 0.03-0.05 MPa), converse changes in the kinetics of reduction occur. The best physical and mechanical properties were revealed for the material treated by redox cycling after holding at 600 °C in water depleted gas mixture. The dual effect of water vapor on nickel-zirconia anode behaviour is discussed basing on scanning electron microscopy analysis data, material electrical conductivity, and strength.
ABSTRACT
The cyclic treatment technique (redox cycling) comprising stages of material exposition in reducing and oxidizing high-temperature environments and intermediate degassing between these stages has been developed to improve the structural integrity of YSZ-NiO ceramic anode substrates for solid oxide fuel cells. A series of specimens were singly reduced in a hydrogenous environment (the Ar-5 vol% Ð2 mixture or hydrogen of 99.99 vol% H2 purity) under the pressure of 0.15 MPa or subjected to redox cycling at 600 or 800 °C. The influence of redox cycling at the treatment temperatures of 600 and 800 °C on the structure, strength and electrical conductivity of the material has been analysed. Using the treatment temperature 600 °C, a structure providing improved physical and mechanical properties of the material was formed. However, at the treatment temperature 800 °C, an anode structure with an array of microcracks was formed that significantly reduced the strength and electrical conductivity of the material.
