Oxide Spallation during Post-Isothermal High Temperature Oxidation Cooling of Cr-rich Cast Alloys highly alloyed with Hf
Cobalt, nickel and iron-based alloys containing 25wt.%Cr and strengthened either by chromium carbides or by HfC were oxidized at 1000, 1100 and 1200°C to obtain external oxide scales. The spallation of these oxides during the post-oxidation cooling was studied by exploiting the cooling part of the thermogravimetry curves. The best resistant alloys against scale spallation were the iron-based alloys which did not lose oxide. The nickel-based alloys well behaved too. The worst alloys were the cobalt alloys, principally due to the complex composition of the oxide scales. Beside the base element, the thickness of the oxide was also identified as a major parameter. The presence of Hf with contents much higher than usual was also beneficial for the resistance against spallation.
Kinetics of High Temperature Oxidation and Chromia Volatilization for HfC-Containing Nickel-Based Alloys
Many cast chromia-scale forming nickel-based superalloys are reinforced by carbides. In such alloys the primary chromium carbides or tantalum carbides rapidly lose their strengthening effect in service at high temperature. This is due to decrease of the volume fraction of the these carbides and to their morphology evolution. Other carbides, notably HfC, are more stable at high temperature and they can be candidates for the reinforcement of this type of superalloys. In this work, three nickel-based alloys containing 25 wt%Cr, 0.25 or 0.50 wt%C, and Hf with contents high enough (3.7 and 5.6 wt%) to promote the formation of numerous primary HfC carbides, were prepared in a foundry. They were tested by oxidation in air for 46 h at 1,200 °C with thermogravimetry measurement and subsequent metallographic characterization. All the mass-gain curves obtained are parabolic and the oxidation rates of the studied alloys are only slightly faster than for the corresponding Hf-free ternary nickel alloys containing the same chromium and carbon contents. The obtained values of the parabolic constant kp and of the chromia volatilization constant kv, deduced by applying the {m × dm/dt = kp–kv × m} method, are typical of a chromia-scale forming system. However, small quantities of HfO2 and NiCrTaO4 oxides are observed in addition to the chromia scale. In the bulk, the volume fraction and morphology of the HfC carbides only changed a little. These alloys appeared thus resistant enough against hot oxidation, this allowing first high temperature mechanical characterization in oxidizing atmosphere, before possibly considering these alloys for real applications.
Evaluation of Impact of Yarn Count and Stitch Length on Pilling, Abrasion, Shrinkage and Tightness Factor of 1 ? 1 Rib Cotton Knitted Fabrics
This study investigates the impact of yarn count and stitch length of 100% cotton knitted fabrics. 1 × 1 ribs structured fabrics were used in this research work. The stitch length 2.50 mm, 2.52 mm, 2.60 mm, 2.70 mm, 2.80 mm and 2.96 mm were used of the yarn count of 26Ne, 28Ne and 36Ne. The areal densities of the grey fabrics were in various amounts. The impacts of yarn count and stitch length were measured concerning pilling resistance, abrasion resistance, and shrinkage and tightness factor. The result showed that the fabric pilling grade decreased while number of cycle increased. Abrasion resistance decreased but length wise shrinkage increased despite the fact that the stitch length increased. The results also exposed that the tightness factor of the fabric increased with the increase of the stitch length in both dyeing and washing state and finished state compared to grey state for the yarn count 26Ne and homogeneous result not found for yarn count 28Ne and 36 Ne. Evaluation of Impact of Yarn Count and Stitch Length on Pilling, Abrasion, Shrinkage and Tightness Factor of 1 × 1 Rib Cotton Knitted Fabrics