Comparative study of fatigue crack growth rate of SS 304H Cu for two different heats

Abstract

"The current trend in the power generation industries is to operate at higher pressure and temperatures to meet the demand of power. In addition, the e ciency of the power production can be increased due to increase in the temperatures, at the same time pollution can also be reduced. Countries like U.S., China and U.K. are currently running full- edged programs to initiate Advanced Ultra-Super Critical (AUSC) Power Plants which operate at high temperatures and pressures above the critical point of water. Keeping energy production in the world wide, India also aims to develop such power plants. However, the material which can withstand at elevated temperatures is a challenging task in worldwide particularly for creep and fatigue properties. Thus, the materials development program has already begun in India in collaboration with R& D institutes and industries. Under this program, materials are being developed by Nuclear Fuel Complex (NFC) indigenously. These materials further characterised for various properties by Indira Gandhi Center for Atomic Research (IGCAR, Kalpakkam) in order to compare the various properties internally available grades of same materials. Under this program, various materials are being tested for properties like, creep, fatigue crack growth rate, fracture toughness etc. since, the thermo-mechanical, ow induced vibrations can lead to the crack nucleation and growth. Thus, the damage assessment procedure for structural integrity assessment is required. The fatigue crack growth rate studies are necessary for damage assessment. This thesis work was carried out to evaluate fatigue crack growth characteristics in Paris regime for the material Austenitic Stainless Steel 304H Cu. In this study, comparison was made between indigenously developed SS 304H Cu and imported SS 304H Cu. The experiments were carried-out in laboratory air at various temperatures: 300 K, 873 K, 923 K, 973 K and 1023 K. The fatigue crack growth rates were compared for both the materials. It was found out that, indigenously developed material was more resistant to fatigue crack growth than the mported material. The possible reason behind this di erence in the fatigue crack growth characteristics is attributed to presence of ne precipitates in the metal matrix which resulted in to crack branching. Moreover, it was observed that presence of twins is more in the case of indigenously developed material than in the imported material. This results in crack tip tress-shielding during the crack growth."