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Experimental characterization of the crack growth rate by creep/fatigue under melt-welded stainless steel G-NiCr28W-MA


The search for the increase in the production of petroleum products due to the high world demands requires that materials with better mechanical properties at high temperature should be used. Therefore, the present work aims to use different tests to evaluate the mechanical behavior and the rate of propagation of fatigue cracks under the creep of a stainless steel produced by static casting G-NiCr28W-MA, where MA is the modified nomenclature with presence of niobium, which are made the pipes of oil cracking furnaces, connections, structural elements that remain long periods of time exposed to the operating temperature of a maximum of 927ºC. The results will provide useful parameters to determine, from the point of view of fracture mechanics, the remaining life of the component in case of pre-existence of cracks or cracks from the creep mechanism and propagated by static creep, i.e, measurement of creep crack growth times in metals. The mechanical tests to be performed will be tensile at different temperatures, impact at high temperatures, hardness, creep propagation (da/dt x C*), with constant load, according to ASTME1457, high temperature fatigue (ASTME647), creep-fatigue crack growth testing (ASTME2760), the latter with dwell time and different frequencies. The study will be developed in two phases: the first will be carried out the most basic tests such as tensile testing at high temperatures, hardness and fractographic and micrographic analyzes. In the second and last phase, the main objective will be to establish parameters that allow the best possible correlation between the experimental results and the real situation of the component in service, which will allow to determine the critical crack size in the component that will be associated with the same value of C * critical, which is a correlation parameter between creep and fatigue crack growth under stable creep conditions determined in the laboratory. In summary, the main idea is to obtain a curve of the da/dt versus C *, with the equipment recently machined and installed in the laboratory of materials of USP de Lorena, for test specimens standardized according to ASTM E1457-15-, and after the results and with the help of finite elements obtain the constant H which is a kind of f (a/w) for the real components using for that the knowledge of fracture mechanics at room temperature and at high temperature. At the same time to this main objective mentioned will be testing the fatigue/creep and crack growth rate at high temperatures to know the complete fatigue behavior at high temperatures of the material G-NiCr28W-MA (AU)