Kesan rawatan haba terhadap sifat mekanikal inconel 718 pada suhu tinggi

Omar, Bapokutty (2015) Kesan rawatan haba terhadap sifat mekanikal inconel 718 pada suhu tinggi. PhD thesis, UKM.

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Inconel, a nickel based superalloy is one of the best advanced engineering materials for the purpose of working in the extreme temperature environments. Inconel 718 which commonly used as a turbine disc in turbine power generation, holds turbine blades made of Inconel 738 and operate at very high temperature. High temperature may degrades mechanical properties of Inconel 718 due to weakening of the grain boundaries and enhancing the cavity formation which lead to cracking. This research is aimed to determine the mechanical properties and fracture behaviour as well as the microstructural evolutions of Inconel 718 at high temperature conditions. The as-received Inconel 718 were heat treated to develop its high temperature strength. To compare as-received and heat treated Inconel 718, tensile and creep tests, have been conducted at 550 and 650oC which give the material properties of as received and after heat treatment. Then stress relaxation and low cycle fatigue (LCF) for heat treated Inconel 718 at 550, 650 and 750oC has been investigated. The stress relaxation test was conducted at 1% and 2% strain and carried out for 3 days (72 hours). For 1% strain, the tests were stop at 3 and 48 hours to investigate the evolution of microstructure and changes in material properties by using scanning electron microscope (SEM) and x-ray diffraction (XRD), respectively. The results showed that heat treatment significantly improved the tensile strength and creep rupture properties. However, the ductility or elongation of heat treated samples drastically reduced to 4 to 5% compared to that of the as-received materials which exhibited more than 30% elongation. It was found that precipitations of gamma prime (γʹ), gamma double prime (γʺ) and long needle-like delta (δ) phase observed at the grain boundary which restricted the grain growth and block grain boundary sliding. These mechanisms are believed to be the reasons for increase in mechanical properties of Inconel 718. However, the presence of these precipitates caused the material to become harder and more brittle. Moreover, the increase in loads from 70% to 90% of the ultimate tensile strength and in temperature significantly accelerated the creep rate. From stress relaxation tests, it was found that thermal dependent stress increased with decreasing temperature. In contrast, stress relaxation rate increased with increasing temperature. Microstructure observation by SEM shows that recovery process has occurred. This was further supported by the evidence of decreased in dislocation density with increase in time and temperature which in-line with the Vickers micro-hardness results obtained. The dislocation density level for 2% strain measured at 72 hours was higher compared to that of 1% for 550oC temperature condition. However, the difference of the dislocation density decreased as the temperature increased to 750oC. The result from LCF showed that increasing temperature reduced the life of Inconel 718. The LCF conducted in temperature 650oC showed that increasing frequency will increased the life cycle for Inconel 718 but time to failure decreased with increasing frequency. As a conclusion, heat treatment improved mechanical properties at high temperature of Inconel 718 until beyond half melting temperature up to optimum. The finding of this study can be used as a basic reference for designing of turbine disc to reduce the failure of component during service at high temperature.

Item Type: Thesis (PhD)
Uncontrolled Keywords: aluminum alloys - heat treatment, metals - heat treatment
Subjects: T Technology > TN Mining engineering. Metallurgy
Divisions: Library > Tesis > FKM
Depositing User: Noor Rahman Jamiah Jalil
Date Deposited: 19 Nov 2015 00:43
Last Modified: 19 Nov 2015 00:43
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