MICRO STRUCTURAL CHARACTERIZATION & EDX ANALYSIS OF NICKEL BASED SUPER ALLOY UDIMET 500 UNDER DIFFERENT SET OF ENVIRONMENTAL CONDITIONS

Abstract

Nickel based super alloys are the most widely used high temperature materials used in the construction of hot sections of aircraft engine components, due to their outstanding strength at elevated temperatures and resistance to hot corrosion. In this research those service and maintenance conditions are simulated which these turbine blades actually encounter during actual flight and during engine overhauling. The response of turbine blades made up of nickel based super alloy UDIMET 500 at 900 0C and 950 0C temperatures and under the influence of acidic and alkaline media are investigated and discussed in this work. This project focuses primarily on the micro structural changes taking place in the turbine blade under these conditions, although additional testing (hardness testing and grain size measurements) is also performed upon turbine blade samples subjected to elevated temperature condition. This research work would serve as an aid to Pakistan Air Force for taking suitable remedial measures to prevent failure of turbine blades made up of this particular alloy. During high temperature exposure, turbine blade samples with different flying hours are subjected to normal (900 0C) and aggressive (950 0C) temperature exposure in furnace for one minute followed by air cooling. Subsequently, each sample is analyzed on SEM and EDX spectrometer. In turbine blades subjected to 900 0C temperature, excessive Cr carbide precipitation at GBs. This phenomenon is more prominent in samples subjected to aggressive conditions and can lead the blade susceptible to Intergranular corrosion. Hardness is found increasing from top to bottom region of blades and also found increasing with the increase in flying hour of turbine blades. With the increase in hardness of turbine blade, the average grain size decreases. During acidic medium exposure, turbine blade samples having different flying hours are immersed in acidic solution (composition: 302.8 ml HCL, 10.3 ml HNO3 & 101.75 gm FeCl3) for 25 seconds (normal condition) & 50 seconds (aggressive condition) at room temperature followed rinsing and neutralization with Na2CO3 (53 gm / lit of water) for 2 minutes at room temperature. Final rinsing is done with hot water and drying via compressed air application. Samples are subsequently analyzed on SEM and EDX spectrometer. Samples subjected to normal acidic environment are generally having coarser grain structure while those subjected to aggressive acidic environment are having fine grain structure; GB dissolution is also enhanced under aggressive conditions. During alkaline medium exposure, turbine blade samples having different flying hours are immersed in alkaline solution (composition: NaOH 187 gm/lit, KMnO4 75 gm/ lit, H2O) for 45 minutes (normal condition) and 90 minutes (aggressive condition) at 85 to 95 0C temperature followed by cold water rinsing. Subsequently all the samples were immersed in Phosphoric acid solution containing inhibitor for 15 to 20 minutes at 20 to 40 0C temperature, followed by cold water rinsing and air drying. Samples are subsequently analyzed on SEM and EDX Spectrometer. Extensive pitting is observed under normal as well as aggressive conditions.