1. Introduction
The high cycle fatigue failures are most commonly occurring in turbine blades of a power plant which is experiencing high temperature heat input from the fuel source [1]. These kind of high cycle fatigue failures are influenced by resonance and excursion of the machine at operational speed especially at blade critical speed dry startup and dry shut down conditions [2]. Many researches were carried out to over-come the fatigue and wear failure of the turbine blades. From the literature review it was found that super alloys are providing better fatigue and wear resistance when comparing to other type of alloys, used for turbine blade applications. The Monel materials were highly used by the researchers because of its good thermal and mechanical properties [3]. The most commonly utilized material for turbine applications is Nickel 825 (CMSX4), but from the literature survey it was observed that the usage of nickel material exhibits poor wear, creep and fatigue resis-tance over time under different alternating temperature load conditions in actual service time [4]. The various materials properties were analyzed carefully and it was found that Monel 400 material which contains the composition of Ni 63%, Cu 28–34%, Fe 2.5%, and Mn 2.5% was used in various heat induced applications because of its high temperature resistance and fatigue resistance property in nature [5]. The various studies were also carried out in the aspect of replacing the Monel 400 material for different thermal applications [6]. The literature also reveals that the heat treatment of Monel 400 material will further enhances the high temperature and fatigue resistance along with hardness properties. Very few studies were attempted in the heat treatment of Monel 400 alloy and still for its effective utilization in turbine blades various aspects have to be studied detaily. In this study the investigation was carried out in the way of subjecting the Monel 400 material for heat treatment process followed by the testing of samples for various mechanical properties as per ASTM standards [7]. The results obtained from the various tests were used for modelling the turbine rotor blade in CATIA and same has been analyzed with the help of ANSYS workbench 16.0 for calculating mechanical stresses. The flow of heat over the rotor blades was carefully analyzed using Ansys CFD by assuming real time conditions. The major objectives of this study is to reduce the wear and tear nature over the blades as well as to withstand high temperatures. The study is also investigated for analyzing the maximum impact strength over the rotor blades for its effective implementations in real time conditions. The research gap of this study also exposed that very fewer studies have been conducted in heat treatment of Monel alloys for turbine application along with validation from finite element analysis software.
2. Experimentation
The various types of heat treatment techniques were available, but in this study quenching process was used to improve the hardness properties of Monel 400 alloy. The reason for choosing the quenching process is because of its ability to avoid unnecessary phase transformations because of its faster reaction time which prevents the possibility of thermodynamically favorable and kinetically accessible low temperature processes [8]. Initially the Monel 400 material is machined as per ASTM standards of hardness test, impact test, torsion test, wear test and tensile test. The machined specimens were heated in muffle furnace to a temperature of 850 C and kept inside the furnace at the same temperature for 2Hr, to improve surface hardness properties and the material is withdrawn from the muffle furnace and quenched in a salt bath solution [9].
2.1. Design of gas turbine blade
All the gas turbine blades, propeller blades, wind turbine blades follows certain standard design and sizes. The main purpose of the turbine is to expand the exhaust gases and to reduce the temperature and pressure, hence the blades should be effectively designed to ensure the flow of gases [10]. In this study N10 series type air foil was selected from the air foil tools section with reference to data book. The Fig. 1 shows the 3D model view of the blade. The Fig. 2 shows the inlet velocity triangle of the blades. Based on the requirements the calculations were made and by using the CATIA V5R20 software the required blade design was created. The assumptions used for the calculation of velocity triangle in the designed blade were as blade angle, (b) 155, nozzle angle, (a) 20, inlet jet velocity, (v) 500 m/s, blade velocity, (u) 250 m/s, mass flow rate, (ṁ) 100 kg/s, diameter of turbine, (D) 2 m, height of the blades, (h) 0.03 m.
3. Results and discussion
3.1. Experimental results
The various mechanical tests were carried out over the heat treated and non-heat treated Monel material to analyze and compare the effect of heat treatment over the different mechanical behaviour of the sample. The comparison results of Rockwell hardness test, Charpy impact test, wear test, torsion test and tensile test were detaily presented in the following Tables 1–5. The Rockwell’s hardness test clearly indicates that the hardness of quenched specimen showed a 25% improvement over the non - quenched specimen. The toughness of the quenched specimen is reduced upto 10.92% in sodium nitrate salt solution based quenching medium. The wear test results of the quenched specimen showed 27% reduction in wear rate when compared to unhardened specimen. The ultimate torque required to break quenched specimen is 12.06% more than unhardened specimen, which indicates that the quenched specimen possess higher shear modulus than the unhardened specimen. From the tensile test report it is clear that the heat treated alloy possess 13.27% higher Ultimate and Yield strength than the unhardened specimen. The heat treatment also showed 8.57% reduction in the ductile property of the specimens.