- Email: [email protected]
Microscopic Studies on the Characteristics of Different Alloys Suitable For Gas Turbine Components
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 11576–11584
www.materialstoday.com/proceedings
ICMMM - 2017
Microscopic Studies on the Characteristics of Different Alloys Suitable For Gas Turbine Components Rayapati Subbaraoa,*, Sadananda Chakraborty b a,b
Department of Mechanical Engineering, National Institute of Technical Teachers' Training & Research (NITTTR) Kolkata, Kolkata, India.
Abstract Gas turbine technology is growing at a faster pace, since it is a more viable option for power generation and related applications in the context of reducing conventional fuel resources. Experimental works on better materials increases hope on the life and extent of gas turbines that can be used with improved performance. In this work, experimentation is carried out about the characteristics of different materials that can be used in gas turbines and in particular, turbine blades. Alloys of different metal compositions of Nickel, Titanium and Steel are chosen. Apart from measuring hardness and surface roughness, microscopic studies are investigated, since mechanical properties depend much on the grain size. It is evident that alloys are crystalline in nature containing different grain structures and boundaries. Hardness test gives the index. After fine polishing, surface roughness test is carried out by measuring Ra and Rz. Metallurgical microscope of high caliber is used for studying the grain structure and average grain number is also calculated. Heat treatment process is carried out on the test specimen in order to identify the changes in grain structure. Study identifies that appropriate alloy could be recognized based on grain structure and other properties in order to ensure safe and long withstanding of gas turbine components. This will enable us with more options while meeting the power generation requirement in the days to come. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Keywords: Gas turbine materials; Turbine blade; Microscopic studies; Grain Structure; Inconel 718; Titanium alloy; Steel alloy
* Corresponding author. Tel.: +91-33-66251974; fax: +91-33-23376331. E-mail address: [email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Rayapati Subbarao, Sadananda Chakraborty / Materials Today: Proceedings 5 (2018) 11576–11584
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Nomenclature bcc fcc
body-centered cubic face centered cube
GT HRC
Gas Turbine Rockwell C Hardness
1. Introduction Gas turbine (GT) power plants are gaining importance in view of depleting fossil fuels and the advantages they can offer on working and performance along with wide range of applications. Gas turbine is most essential component in gas turbine power plant. It delivers a great amount of energy based on its size and its weight. From the past 1 or 2 decades, it is observed clearly that there is a rapid growth in the usage of gas turbine in power industries throughout the world [1]. In order to move in this direction and to meet challenges posed by the environment, Subbarao et al. [2] proposed the use of gas turbine in power generation. Now-a-days, several types of gas turbines are widely used for the purpose of power generation. In some cases, climatic conditions will also affect the performance of gas turbines [3]. Due to this reason, most of the countries are combining the gas turbine with steam cycle to get better power output, either to produce electricity or for industrial purpose. But, the efficiency and output of the gas turbine mainly depends on the temperature conditions. Previously, researchers reported that by varying the ambient temperature there is a significant change in power output of the turbine. Few others found that by varying the inlet parameters of the turbine, efficiency of gas turbine could be changed. In these cases, they need not depend on the governmental/main power source that involves huge amount of losses in transmission. Due to shortage of power from the power grids provided by governments, people are trying to setup their power generation sets. This falls under distributed power generation category. At any cost, it is need to improve the performance since this technology is gaining importance. The other way of improving the gas turbine efficiency is to have better materials for all the parts of the engine. By adapting this, not only improve the efficiency is improved, but also, we can have the plants that can run for longer duration. Hence materials research is important in the advancement of GT technology. Based on the availability of materials, they can be used for high and low stress areas. Wide range of materials are used for the gas turbine components. They vary from lighter to heavier or hard to soft, depending on the place of usage. Progress in materials research has paved the way for further improvement of gas turbine performance [4]. But, in the recent years, there have been only few works on gas turbine materials. Since the working fluid flows continuously in different components with varied temperatures, material selection is very important in view of the performance and power output of the system. Similarly, these gas turbine components are subjected to extremely high temperatures, causing significant mechanical and thermal stresses. Chromium rich, aluminide and platinum coating do solve the problem a little [5], but increase the cost drastically. To overcome such hurdles, gas turbine components are made of super alloys or advanced materials. There is a chance of withstanding high temperatures and increased efficiency, if suitable materials are chosen. Al alloys, which are lighter materials, may be helpful in reducing weight of the engine or system. Because of this, the manufacture of gas turbine needs high performance materials like special steels, titanium, nickel and super alloys. Dissimilar materials are used in different components. For the components of the GT system, Chromium, Nickel, Tungsten and Cobalt are the important metals that are necessary. Titanium and Tantalum are also used in advanced engines [6]. Also, turbine blades and gas turbine are manufactured by different methods, like conventional casting, differently solidifying and single crystal [7]. In these cases, studies on heat treatment provide crucial results in the selection of materials. In this context, present work chooses Ti, Ni and steel alloys for testing, based on their suitability in GT components. Here Ni based super alloy, Inconel 718 is considered, which has more industrial applications. Inconel 718 maintains superior tensile, fatigue and creep properties at high temperatures also. It is found to give better performance at high temperatures and pressures among the Ni based alloys [8]. Ti alloys are
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used for buckets and turbine wheels. Steel alloys are used in compressor blades and turbine wheels, depending on the application and in case of less amounts of power generation [9]. Hence, high grade Ti and steel alloys are considered here for testing and analysis. Even though coatings are suggested for higher temperature ranges, they are costly and the effect of their life is dependent on various other factors that are not controllable. 2. Experimental studies Materials chosen for studying are subjected to hardness, surface roughness and microscopic examination, before and after heat treatment. The specimen is prepared by sawing the sectional area to be examined. First coarse grinding is done to make the specimen finer. Further, normal grinding is done with emery paper up to the highest grade possible. This ensures proper fineness of the specimen. After that, polishing is done using alumina powder or diamond paste on the rotating wheel. For surface roughness and microscopic studies, etching is done in dilute acids. Later, they are washed in alcohol and dried before tested. 2.1. Hardness and surface roughness tests It is necessary to conduct these tests, since hardness and surface roughness are linked with the microstructure of the materials directly or indirectly. Hardness is the property of a material, which shows resistance to indentation. It is determined by measuring the permanent depth of the indentation. Hardness is obtained by measuring the depth or the area of the indentation. Proper hardness index means resistance to deformation, friction and abrasion. In the hardness test, force is applied mechanically on the specimen. Rockwell hardness tester is used here that presents direct reading of hardness number on a dial indicator. Its indentors are made of either hardened steel or diamond. For hardness testing of different materials, commonly ‘C’ scale is utilized with the applied force is about 150 kgf. Hardness is presented as HRC. Surface roughness is the component of surface texture that finds irregularities of small wavelength. It is measured by the variations in the direction of the normal vector of an existent surface from its principle form. If these deviations are large, the surface is rough and if they are small, the surface is treated as smooth. High-frequency, short-wavelength component of the surface measured is considered as roughness. Rough surfaces usually tend to wear more quickly and have higher friction coefficients than smooth surfaces. Mean surface roughness and surface roughness depth in terms of Ra and Rz are measured. As roughness is considered to be disadvantageous to part performance, this test also plays an important role. 2.2. Microscopic and heat treatment studies Refined grain size means improved mechanical properties and reduced wear and tear in response to thermal treatment. In this work, Leica Metallurgical microscopy is used to examine the micro-structure materials in order to understand the relationship between properties and structures that are coarse enough and are not visible to the naked eye. Magnifications in the range of 5x to 100x are used. Furthermore, heat treatment plays a vital role in order to either harden or soften the material, in which they are heated from normal to severe temperatures to attain the required result. Metals and alloys are made of small crystal structures, commonly known as grains. The mechanical behaviour of the metals and alloys is determined by the nature of the grain size and composition of metal. After heat treatment, these mechanical properties change due to the heat at high temperature and being cooled in a different medium. In the present work, specimens are heated at 950°C in a furnace for about 3 hours and made to cool in air indefinitely at room temperature. Keeping in view, the alloys considered in this work, the heating
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temperature and time are finalized. However, the maximum combustion section outlet temperature (turbine inlet temperature) in a GT engine is slightly above 1000°C. Specimens are prepared as shown in Fig. 1. The three alloy materials are cut in to 20x10 cross-sections by wire electric discharge machine. After that, coarse grinding operation is performed to remove any scale formed on the work surface. To remove any further scaling left on the specimen, fine grinding is performed. After that, polishing is done to measure surface finish and hardness. In the end, all the three specimens are etched with proper chemical solutions for microscopic studies.
Fig. 1. Specimens of Steel, Nickel and Titanium alloys.
Fig. 2. Preparation of the specimen and metallurgical microscope. 2.3. Etchant selection If we need to examine grain structure before and after heat treatment, the samples must be etched. The condition of an alloy and its heat treatment must be considered in the selection and application of etchants. Proper selection of etching agent plays an important role in the microstructure study of different metals and alloys. It largely depends on alloy composition, heat treatment and processing. Etchant has inherent corrosion resistance, hence using it will be a difficult task. Super alloys are very complex in structure and contain compositions of different elements, making their microstructure difficult to study. According to the alloy compositions, different chemicals are used as etchants. In this study three materials (Stainless steel alloy, Inconel 718, and Titanium alloy) are chosen for microstructure
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study. Nickel based super alloy have the same procedure of etching solution preparation as used in Steel alloy, because, both have face centered cube austenitic structure that indicates tremendous toughness and very low machinability. On the other hand, Titanium has a close-packed hexagonal crystal structure that is denoted by alpha phase and after heat treatment it converts to body-centered cubic structure. After fine polishing with alumina powder, the specimens are etched. Stainless steel and Inconel 718 are etched with Glycergia (15cc HCL+10cc Glycerol+5cc HNO3) and Titanium alloy is etched with the mixture of hydrofluoric acid and nitric acid (2cc HF+4cc HNO3 +100cc H2O) [10]. The etching process should be slow for these alloys. After performing the etching process, all the specimens are cleaned by clean water and alcohol. Then the specimens are expected to give the crystalline structure in optical microscope. 2.4. Experimentation details Specimens are prepared subjected to coarse grinding operation and then polished using proper slurries. After polishing, etching is done before subjected to microscopic studies. Etching unit and polishing machine are as shown in Fig.2. Figure 2 also depicts the Leica microscope with varied magnification range, which is used in the present study. Hardness tester of Brinell’s make and surface roughness tester of Marsurf PS1 make, which are used in the present study are shown in Fig.3. Surface roughness measurement is expressed in terms of Ra and Rz. These values include peak-to-valley profile measurement in combination with assessment of the frequency peaks within the sample area. Ra is the most common one, which is defined as the arithmetic average of absolute values. Rz is based on the five highest peaks and lowest valleys over the entire sampling length.
Surface Roughness Tester Hardness Tester Fig. 3. Hardness and surface roughness testers used in the study. 2.5. Composition of alloys and their expected usage Compositions of alloys used in the present study are given in Table 1. Martensitic stainless steels used here, consist of less carbon, chromium about 10 -18% and remaining is iron. In order to provide desirable range of mechanical properties, these materials may be subjected to heat treatment, in a similar manner as that of conservative steels. But, they should offer higher hardenability and have different heat treatment temperatures. Their corrosion resistance may be described as moderate. Inconel 718 is a nickel-based alloy designed that has exceptionally high yield, tensile and creep-rupture properties at temperatures up to 800oC. This alloy has excellent weldability when compared to the nickel-base super alloys hardened by aluminium and titanium. This alloy is used for jet engines and aircraft parts. Strong, corrosion resistant, and light-weight titanium is used in Ti alloys, which are
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used for aerospace applications. Ti alloy considered here consists of aluminium, vanadium and titanium as major elements. This is considered here because of its application in GT components, which are used in power generation and jet engines. Table 1. Composition of the alloys used. Alloy
Composition
Stainless steel alloy
C (0.2-1 %), Cr (10.5 -18 %) and Fe (balance)
Ni alloy
C (0.08%), Ni+Co (55%), Cr (21%), Fe (balance)
Titanium Alloy
C (0.05%), Al(2.5 -3.5%), V (2-3%), Fe (0.3%) and Ti (balance)
3. Results and discussion 3.1. Hardness test Hardness is measure before and before and after heat treatment, whose indices are as shown in Table 2. Hardness of steel alloy is increased from 90 to 85, which is not a advantageous feature in the components applied in gas turbine systems. In the same way, hardness of Ni alloy showed rise with the rise in temperature. Only Ti alloy showed slight increment in hardness, which is a favourable component for usage in turbine blades. Finding proper hardness material is useful in producing components with increased resistance to erosion caused by airborne particles and pollutants, extending the life of the components. Usually, turbine blades, including airfoils, of aviation or industrial gas turbine engines are treated with boron to improve surface hardness and/or durability. It is time consuming and cost increasing process. By doing proper hardness test and finding a suitable material, such treatment stage can be avoided. 3.2. Surface roughness test Table 3 shows surface roughness results, before and after heat treatment. The surface roughness of steel alloy is increasing more with heat treatment. Ni alloy proved to be better than steel alloy, but not like Ti alloy, where the roughness is similar, before and after heat treatment. However, before and after applying heat, the roughness of steel alloy > Ni alloy > Ti alloy. This suggests that Ti alloys are suitable. Specifically, because of the withstanding nature of Ti alloys before and after subjected to higher temperatures, they are useful for turbine blades as well. This is visible in both the hardness and surface texture measurements. Table 2. Hardness before and after heat treatment. Alloy
Before heat treatment
After heat treatment
Stainless steel alloy
90.5
85
Ni alloy
25.5
82.5
Titanium alloy
35.75
40
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Table 3. Surface roughness before and after heat treatment. Alloy
Rz
Ra Before
After
Before
After
Stainless steel alloy
0.029
0.045
0.19
0.53
Ni alloy
0.025
0.024
0.20
0.26
Titanium alloy
0.023
0.027
0.22
0.22
3.3. Microscopic studies This section deals with the microscopic studies on Steel, Nickel and Titanium alloys. Table 4 shows the images obtained before and after heat treatment. Grain structure has got more clearer with heat treatment as observed in all the three cases studied. Before the heat treatment, the grain size area of Titanium is bigger than the two other alloys. Inter-molecular distance also plays crucial role in the strength of the material. This is seen improved with heat treatment. Many years before the hardened martensitic stainless steel is developed to overcome the restrain of fully austenitic and martensitic stainless steels because of the performance capability with respect to hardness, strength, toughness at high temperatures and pressures. Grain structures are refined after heat treatment. This shows that steels exhibit good corrosion resistance as well as excellent mechanical properties. These properties can be further refined by a suitable heat treatment process. Ni based super alloys are complex as observed in the microstructure point of view. The strength of the super alloys are high due to its fcc structure (commonly known as γ), which consists of nickel, chromium, molybdenum and cobalt as primary elements. At high temperatures and pressures, Inconel 718 has good corrosion resistance because of the structure. From the microstructure, it is clear that after heat treatment, the partial grains are converted into whole grains and they look like fine grains. That causes the higher hardness value. Also, the mechanical properties are affected by the altering grain sizes. Fine grain microstructure causes better tensile and fatigue properties, on the other hand, coarse grain microstructure causes good creep and fatigue crack growth properties at preeminent temperature. Titanium has multiline crystallographic form and is an allotropic element. Generally titanium has a close-packed hexagonal crystal structure that denoted as “alpha” phase. Above 850oC, the alpha phase of the structure changes to a beta phase bcc crystal structure. After heat treatment, the microstructure of titanium alloy is strongly influenced by the effect of different cooling rates depending on the application in gas turbine components. There are two types of alpha (primary and secondary) phases present in titanium alloy. Among them, secondary alpha is formed by transforming from beta phase. This may happen upon cooling from above the beta transus. Metallurgical microscopy can easily resolve the aged fine alpha. As the cooling rate increases, martensite (lamellar alpha) becomes finer. When the lamellar alpha in the Titanium alloy is heat treated below the beta transus, the structure is purely transformed to fine lamellar alpha as illustrated in the microscopic structure shown in Table 4. But, it shows the beta structure with some residual alpha before heat treatment. After the heat treatment, finer grain size is obtained, which plays role in increasing its toughness and hardness. This raises the capability of Ti alloys for usage in high temperature regions in gas turbine units. Overall, it can be concluded that Ti alloys show withstanding nature more than the remaining alloys and suitable for turbine and compressor blades. Whereas, steel alloys can be used where temperature effects are less, i.e. nozzle buckets or guide vanes. Ni alloys can be used in compressor or turbine buckets and as hub based structure materials.
Rayapati Subbarao, Sadananda Chakraborty / Materials Today: Proceedings 5 (2018) 11576–11584
Table 4. Microstructure before and after heat treatment. Alloy Stainless steel alloy
Nickel alloy
Titanium alloy
Before
After
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4. Conclusions Different gas turbine materials are tested for hardness, roughness and grain structure. Grain structure is observed using Metallurgical microscope. The major effect of heat treatment in steel alloy is to convert its past austenitic structure to martensitic, which has better surface finish with increased hardness. In Ni alloy, the grain boundaries are changed and refined with heat treatment. All grains are equally distributed on surface, decreasing surface roughness. Rockewell harness tester is used to measure and compare hardness. Ti alloy showed similar hardness number, before and after heat treatment. Hardness of steel and Ni alloys increased drastically with heat treatment. Surface roughness test is carried on all the three specimens after preparing them with grinding, etching and washing. Roughness of steel alloys increased more with application of heat. Surface roughness of Ni and Ti alloys is not much dependent on the heat treatment process. Microscopic studies revealed the grain structure of the three alloys. Grain structure of steel alloy is refined after heat treatment. This reveals that steels are good corrosion resistant and possess excellent mechanical properties. From the microstructure of Ni alloy, it is clear that after heat treatment, the partial grains are converted into fine grains causing higher hardness numbers. Study of the microstructure of Ti alloy, reveals that bcc structure is changed to fcc with heat treatment. It makes the alloy more tough and hard, increasing its ability to be used in high temperature areas of the gas turbines. The grain structure is more refined than the other two alloys. Results reveal that Ti alloy are more useful for taking higher loads with abilities to withstand high temperatures. Thus, the present work establishes the way of testing different alloy materials suitable for gas turbine components. References [1] Shalan H E M A, Hassan M A M, Bahgat A B G. Comparative study on modelling of gas turbines in combined cycle power plants, Proceedings of the 14th International Middle East Power Systems Conference, Cairo University, Egypt, 19-21, 2010. [2] Rayapati Subbarao, Senthil Vadivel V U, Subbarao P M V. Thermodynamic modeling of hybrid solid oxide fuel cell-gas turbine power plant. Journal of Institution of Engineers, 2009;1(1):26-30. [3] Farouk N, Sheng L, Hayat Q. Effect of ambient temperature on the performance of gas turbines power plant, International Journal of Computer Science Issues, 2013;10 (1) 3. [4] Nageswararao M. Materials for Gas Turbines – An Overview, Ch.13, http://www.intechopen.com. [5] Pomeroy M J. Coatings for gas turbine materials and long term stability issues, Materials and Design, 2005;26:225-231. [6] Small C. Strategic materials use in the gas turbine industry – challenges and opportunities, Rolls Royce report, 2011. [7] Kalpakjan, Schmid. Manufacturing processes for engineering materials, 5th ed., Pearson Education, 2008. [8] Thakur M. Thermal and structural analysis of gas turbine blade made of Ni-based superalloy, M.Tech Thesis, NITTTR Kokakta, 2016. [9] Shilke P W, Schenectady. Advanced gas turbine materials and coatings, Report, GER-3569 (08/04), 2004. [10] Small K B, Englehart D A, Christman T A. Guide to etching speciality alloys. Advanced materials & processes 2008;2:31-37.
ScienceDirect Materials Today: Proceedings 5 (2018) 11576–11584
www.materialstoday.com/proceedings
ICMMM - 2017
Microscopic Studies on the Characteristics of Different Alloys Suitable For Gas Turbine Components Rayapati Subbaraoa,*, Sadananda Chakraborty b a,b
Department of Mechanical Engineering, National Institute of Technical Teachers' Training & Research (NITTTR) Kolkata, Kolkata, India.
Abstract Gas turbine technology is growing at a faster pace, since it is a more viable option for power generation and related applications in the context of reducing conventional fuel resources. Experimental works on better materials increases hope on the life and extent of gas turbines that can be used with improved performance. In this work, experimentation is carried out about the characteristics of different materials that can be used in gas turbines and in particular, turbine blades. Alloys of different metal compositions of Nickel, Titanium and Steel are chosen. Apart from measuring hardness and surface roughness, microscopic studies are investigated, since mechanical properties depend much on the grain size. It is evident that alloys are crystalline in nature containing different grain structures and boundaries. Hardness test gives the index. After fine polishing, surface roughness test is carried out by measuring Ra and Rz. Metallurgical microscope of high caliber is used for studying the grain structure and average grain number is also calculated. Heat treatment process is carried out on the test specimen in order to identify the changes in grain structure. Study identifies that appropriate alloy could be recognized based on grain structure and other properties in order to ensure safe and long withstanding of gas turbine components. This will enable us with more options while meeting the power generation requirement in the days to come. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Keywords: Gas turbine materials; Turbine blade; Microscopic studies; Grain Structure; Inconel 718; Titanium alloy; Steel alloy
* Corresponding author. Tel.: +91-33-66251974; fax: +91-33-23376331. E-mail address: [email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Rayapati Subbarao, Sadananda Chakraborty / Materials Today: Proceedings 5 (2018) 11576–11584
11577
Nomenclature bcc fcc
body-centered cubic face centered cube
GT HRC
Gas Turbine Rockwell C Hardness
1. Introduction Gas turbine (GT) power plants are gaining importance in view of depleting fossil fuels and the advantages they can offer on working and performance along with wide range of applications. Gas turbine is most essential component in gas turbine power plant. It delivers a great amount of energy based on its size and its weight. From the past 1 or 2 decades, it is observed clearly that there is a rapid growth in the usage of gas turbine in power industries throughout the world [1]. In order to move in this direction and to meet challenges posed by the environment, Subbarao et al. [2] proposed the use of gas turbine in power generation. Now-a-days, several types of gas turbines are widely used for the purpose of power generation. In some cases, climatic conditions will also affect the performance of gas turbines [3]. Due to this reason, most of the countries are combining the gas turbine with steam cycle to get better power output, either to produce electricity or for industrial purpose. But, the efficiency and output of the gas turbine mainly depends on the temperature conditions. Previously, researchers reported that by varying the ambient temperature there is a significant change in power output of the turbine. Few others found that by varying the inlet parameters of the turbine, efficiency of gas turbine could be changed. In these cases, they need not depend on the governmental/main power source that involves huge amount of losses in transmission. Due to shortage of power from the power grids provided by governments, people are trying to setup their power generation sets. This falls under distributed power generation category. At any cost, it is need to improve the performance since this technology is gaining importance. The other way of improving the gas turbine efficiency is to have better materials for all the parts of the engine. By adapting this, not only improve the efficiency is improved, but also, we can have the plants that can run for longer duration. Hence materials research is important in the advancement of GT technology. Based on the availability of materials, they can be used for high and low stress areas. Wide range of materials are used for the gas turbine components. They vary from lighter to heavier or hard to soft, depending on the place of usage. Progress in materials research has paved the way for further improvement of gas turbine performance [4]. But, in the recent years, there have been only few works on gas turbine materials. Since the working fluid flows continuously in different components with varied temperatures, material selection is very important in view of the performance and power output of the system. Similarly, these gas turbine components are subjected to extremely high temperatures, causing significant mechanical and thermal stresses. Chromium rich, aluminide and platinum coating do solve the problem a little [5], but increase the cost drastically. To overcome such hurdles, gas turbine components are made of super alloys or advanced materials. There is a chance of withstanding high temperatures and increased efficiency, if suitable materials are chosen. Al alloys, which are lighter materials, may be helpful in reducing weight of the engine or system. Because of this, the manufacture of gas turbine needs high performance materials like special steels, titanium, nickel and super alloys. Dissimilar materials are used in different components. For the components of the GT system, Chromium, Nickel, Tungsten and Cobalt are the important metals that are necessary. Titanium and Tantalum are also used in advanced engines [6]. Also, turbine blades and gas turbine are manufactured by different methods, like conventional casting, differently solidifying and single crystal [7]. In these cases, studies on heat treatment provide crucial results in the selection of materials. In this context, present work chooses Ti, Ni and steel alloys for testing, based on their suitability in GT components. Here Ni based super alloy, Inconel 718 is considered, which has more industrial applications. Inconel 718 maintains superior tensile, fatigue and creep properties at high temperatures also. It is found to give better performance at high temperatures and pressures among the Ni based alloys [8]. Ti alloys are
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used for buckets and turbine wheels. Steel alloys are used in compressor blades and turbine wheels, depending on the application and in case of less amounts of power generation [9]. Hence, high grade Ti and steel alloys are considered here for testing and analysis. Even though coatings are suggested for higher temperature ranges, they are costly and the effect of their life is dependent on various other factors that are not controllable. 2. Experimental studies Materials chosen for studying are subjected to hardness, surface roughness and microscopic examination, before and after heat treatment. The specimen is prepared by sawing the sectional area to be examined. First coarse grinding is done to make the specimen finer. Further, normal grinding is done with emery paper up to the highest grade possible. This ensures proper fineness of the specimen. After that, polishing is done using alumina powder or diamond paste on the rotating wheel. For surface roughness and microscopic studies, etching is done in dilute acids. Later, they are washed in alcohol and dried before tested. 2.1. Hardness and surface roughness tests It is necessary to conduct these tests, since hardness and surface roughness are linked with the microstructure of the materials directly or indirectly. Hardness is the property of a material, which shows resistance to indentation. It is determined by measuring the permanent depth of the indentation. Hardness is obtained by measuring the depth or the area of the indentation. Proper hardness index means resistance to deformation, friction and abrasion. In the hardness test, force is applied mechanically on the specimen. Rockwell hardness tester is used here that presents direct reading of hardness number on a dial indicator. Its indentors are made of either hardened steel or diamond. For hardness testing of different materials, commonly ‘C’ scale is utilized with the applied force is about 150 kgf. Hardness is presented as HRC. Surface roughness is the component of surface texture that finds irregularities of small wavelength. It is measured by the variations in the direction of the normal vector of an existent surface from its principle form. If these deviations are large, the surface is rough and if they are small, the surface is treated as smooth. High-frequency, short-wavelength component of the surface measured is considered as roughness. Rough surfaces usually tend to wear more quickly and have higher friction coefficients than smooth surfaces. Mean surface roughness and surface roughness depth in terms of Ra and Rz are measured. As roughness is considered to be disadvantageous to part performance, this test also plays an important role. 2.2. Microscopic and heat treatment studies Refined grain size means improved mechanical properties and reduced wear and tear in response to thermal treatment. In this work, Leica Metallurgical microscopy is used to examine the micro-structure materials in order to understand the relationship between properties and structures that are coarse enough and are not visible to the naked eye. Magnifications in the range of 5x to 100x are used. Furthermore, heat treatment plays a vital role in order to either harden or soften the material, in which they are heated from normal to severe temperatures to attain the required result. Metals and alloys are made of small crystal structures, commonly known as grains. The mechanical behaviour of the metals and alloys is determined by the nature of the grain size and composition of metal. After heat treatment, these mechanical properties change due to the heat at high temperature and being cooled in a different medium. In the present work, specimens are heated at 950°C in a furnace for about 3 hours and made to cool in air indefinitely at room temperature. Keeping in view, the alloys considered in this work, the heating
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temperature and time are finalized. However, the maximum combustion section outlet temperature (turbine inlet temperature) in a GT engine is slightly above 1000°C. Specimens are prepared as shown in Fig. 1. The three alloy materials are cut in to 20x10 cross-sections by wire electric discharge machine. After that, coarse grinding operation is performed to remove any scale formed on the work surface. To remove any further scaling left on the specimen, fine grinding is performed. After that, polishing is done to measure surface finish and hardness. In the end, all the three specimens are etched with proper chemical solutions for microscopic studies.
Fig. 1. Specimens of Steel, Nickel and Titanium alloys.
Fig. 2. Preparation of the specimen and metallurgical microscope. 2.3. Etchant selection If we need to examine grain structure before and after heat treatment, the samples must be etched. The condition of an alloy and its heat treatment must be considered in the selection and application of etchants. Proper selection of etching agent plays an important role in the microstructure study of different metals and alloys. It largely depends on alloy composition, heat treatment and processing. Etchant has inherent corrosion resistance, hence using it will be a difficult task. Super alloys are very complex in structure and contain compositions of different elements, making their microstructure difficult to study. According to the alloy compositions, different chemicals are used as etchants. In this study three materials (Stainless steel alloy, Inconel 718, and Titanium alloy) are chosen for microstructure
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study. Nickel based super alloy have the same procedure of etching solution preparation as used in Steel alloy, because, both have face centered cube austenitic structure that indicates tremendous toughness and very low machinability. On the other hand, Titanium has a close-packed hexagonal crystal structure that is denoted by alpha phase and after heat treatment it converts to body-centered cubic structure. After fine polishing with alumina powder, the specimens are etched. Stainless steel and Inconel 718 are etched with Glycergia (15cc HCL+10cc Glycerol+5cc HNO3) and Titanium alloy is etched with the mixture of hydrofluoric acid and nitric acid (2cc HF+4cc HNO3 +100cc H2O) [10]. The etching process should be slow for these alloys. After performing the etching process, all the specimens are cleaned by clean water and alcohol. Then the specimens are expected to give the crystalline structure in optical microscope. 2.4. Experimentation details Specimens are prepared subjected to coarse grinding operation and then polished using proper slurries. After polishing, etching is done before subjected to microscopic studies. Etching unit and polishing machine are as shown in Fig.2. Figure 2 also depicts the Leica microscope with varied magnification range, which is used in the present study. Hardness tester of Brinell’s make and surface roughness tester of Marsurf PS1 make, which are used in the present study are shown in Fig.3. Surface roughness measurement is expressed in terms of Ra and Rz. These values include peak-to-valley profile measurement in combination with assessment of the frequency peaks within the sample area. Ra is the most common one, which is defined as the arithmetic average of absolute values. Rz is based on the five highest peaks and lowest valleys over the entire sampling length.
Surface Roughness Tester Hardness Tester Fig. 3. Hardness and surface roughness testers used in the study. 2.5. Composition of alloys and their expected usage Compositions of alloys used in the present study are given in Table 1. Martensitic stainless steels used here, consist of less carbon, chromium about 10 -18% and remaining is iron. In order to provide desirable range of mechanical properties, these materials may be subjected to heat treatment, in a similar manner as that of conservative steels. But, they should offer higher hardenability and have different heat treatment temperatures. Their corrosion resistance may be described as moderate. Inconel 718 is a nickel-based alloy designed that has exceptionally high yield, tensile and creep-rupture properties at temperatures up to 800oC. This alloy has excellent weldability when compared to the nickel-base super alloys hardened by aluminium and titanium. This alloy is used for jet engines and aircraft parts. Strong, corrosion resistant, and light-weight titanium is used in Ti alloys, which are
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used for aerospace applications. Ti alloy considered here consists of aluminium, vanadium and titanium as major elements. This is considered here because of its application in GT components, which are used in power generation and jet engines. Table 1. Composition of the alloys used. Alloy
Composition
Stainless steel alloy
C (0.2-1 %), Cr (10.5 -18 %) and Fe (balance)
Ni alloy
C (0.08%), Ni+Co (55%), Cr (21%), Fe (balance)
Titanium Alloy
C (0.05%), Al(2.5 -3.5%), V (2-3%), Fe (0.3%) and Ti (balance)
3. Results and discussion 3.1. Hardness test Hardness is measure before and before and after heat treatment, whose indices are as shown in Table 2. Hardness of steel alloy is increased from 90 to 85, which is not a advantageous feature in the components applied in gas turbine systems. In the same way, hardness of Ni alloy showed rise with the rise in temperature. Only Ti alloy showed slight increment in hardness, which is a favourable component for usage in turbine blades. Finding proper hardness material is useful in producing components with increased resistance to erosion caused by airborne particles and pollutants, extending the life of the components. Usually, turbine blades, including airfoils, of aviation or industrial gas turbine engines are treated with boron to improve surface hardness and/or durability. It is time consuming and cost increasing process. By doing proper hardness test and finding a suitable material, such treatment stage can be avoided. 3.2. Surface roughness test Table 3 shows surface roughness results, before and after heat treatment. The surface roughness of steel alloy is increasing more with heat treatment. Ni alloy proved to be better than steel alloy, but not like Ti alloy, where the roughness is similar, before and after heat treatment. However, before and after applying heat, the roughness of steel alloy > Ni alloy > Ti alloy. This suggests that Ti alloys are suitable. Specifically, because of the withstanding nature of Ti alloys before and after subjected to higher temperatures, they are useful for turbine blades as well. This is visible in both the hardness and surface texture measurements. Table 2. Hardness before and after heat treatment. Alloy
Before heat treatment
After heat treatment
Stainless steel alloy
90.5
85
Ni alloy
25.5
82.5
Titanium alloy
35.75
40
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Table 3. Surface roughness before and after heat treatment. Alloy
Rz
Ra Before
After
Before
After
Stainless steel alloy
0.029
0.045
0.19
0.53
Ni alloy
0.025
0.024
0.20
0.26
Titanium alloy
0.023
0.027
0.22
0.22
3.3. Microscopic studies This section deals with the microscopic studies on Steel, Nickel and Titanium alloys. Table 4 shows the images obtained before and after heat treatment. Grain structure has got more clearer with heat treatment as observed in all the three cases studied. Before the heat treatment, the grain size area of Titanium is bigger than the two other alloys. Inter-molecular distance also plays crucial role in the strength of the material. This is seen improved with heat treatment. Many years before the hardened martensitic stainless steel is developed to overcome the restrain of fully austenitic and martensitic stainless steels because of the performance capability with respect to hardness, strength, toughness at high temperatures and pressures. Grain structures are refined after heat treatment. This shows that steels exhibit good corrosion resistance as well as excellent mechanical properties. These properties can be further refined by a suitable heat treatment process. Ni based super alloys are complex as observed in the microstructure point of view. The strength of the super alloys are high due to its fcc structure (commonly known as γ), which consists of nickel, chromium, molybdenum and cobalt as primary elements. At high temperatures and pressures, Inconel 718 has good corrosion resistance because of the structure. From the microstructure, it is clear that after heat treatment, the partial grains are converted into whole grains and they look like fine grains. That causes the higher hardness value. Also, the mechanical properties are affected by the altering grain sizes. Fine grain microstructure causes better tensile and fatigue properties, on the other hand, coarse grain microstructure causes good creep and fatigue crack growth properties at preeminent temperature. Titanium has multiline crystallographic form and is an allotropic element. Generally titanium has a close-packed hexagonal crystal structure that denoted as “alpha” phase. Above 850oC, the alpha phase of the structure changes to a beta phase bcc crystal structure. After heat treatment, the microstructure of titanium alloy is strongly influenced by the effect of different cooling rates depending on the application in gas turbine components. There are two types of alpha (primary and secondary) phases present in titanium alloy. Among them, secondary alpha is formed by transforming from beta phase. This may happen upon cooling from above the beta transus. Metallurgical microscopy can easily resolve the aged fine alpha. As the cooling rate increases, martensite (lamellar alpha) becomes finer. When the lamellar alpha in the Titanium alloy is heat treated below the beta transus, the structure is purely transformed to fine lamellar alpha as illustrated in the microscopic structure shown in Table 4. But, it shows the beta structure with some residual alpha before heat treatment. After the heat treatment, finer grain size is obtained, which plays role in increasing its toughness and hardness. This raises the capability of Ti alloys for usage in high temperature regions in gas turbine units. Overall, it can be concluded that Ti alloys show withstanding nature more than the remaining alloys and suitable for turbine and compressor blades. Whereas, steel alloys can be used where temperature effects are less, i.e. nozzle buckets or guide vanes. Ni alloys can be used in compressor or turbine buckets and as hub based structure materials.
Rayapati Subbarao, Sadananda Chakraborty / Materials Today: Proceedings 5 (2018) 11576–11584
Table 4. Microstructure before and after heat treatment. Alloy Stainless steel alloy
Nickel alloy
Titanium alloy
Before
After
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4. Conclusions Different gas turbine materials are tested for hardness, roughness and grain structure. Grain structure is observed using Metallurgical microscope. The major effect of heat treatment in steel alloy is to convert its past austenitic structure to martensitic, which has better surface finish with increased hardness. In Ni alloy, the grain boundaries are changed and refined with heat treatment. All grains are equally distributed on surface, decreasing surface roughness. Rockewell harness tester is used to measure and compare hardness. Ti alloy showed similar hardness number, before and after heat treatment. Hardness of steel and Ni alloys increased drastically with heat treatment. Surface roughness test is carried on all the three specimens after preparing them with grinding, etching and washing. Roughness of steel alloys increased more with application of heat. Surface roughness of Ni and Ti alloys is not much dependent on the heat treatment process. Microscopic studies revealed the grain structure of the three alloys. Grain structure of steel alloy is refined after heat treatment. This reveals that steels are good corrosion resistant and possess excellent mechanical properties. From the microstructure of Ni alloy, it is clear that after heat treatment, the partial grains are converted into fine grains causing higher hardness numbers. Study of the microstructure of Ti alloy, reveals that bcc structure is changed to fcc with heat treatment. It makes the alloy more tough and hard, increasing its ability to be used in high temperature areas of the gas turbines. The grain structure is more refined than the other two alloys. Results reveal that Ti alloy are more useful for taking higher loads with abilities to withstand high temperatures. Thus, the present work establishes the way of testing different alloy materials suitable for gas turbine components. References [1] Shalan H E M A, Hassan M A M, Bahgat A B G. Comparative study on modelling of gas turbines in combined cycle power plants, Proceedings of the 14th International Middle East Power Systems Conference, Cairo University, Egypt, 19-21, 2010. [2] Rayapati Subbarao, Senthil Vadivel V U, Subbarao P M V. Thermodynamic modeling of hybrid solid oxide fuel cell-gas turbine power plant. Journal of Institution of Engineers, 2009;1(1):26-30. [3] Farouk N, Sheng L, Hayat Q. Effect of ambient temperature on the performance of gas turbines power plant, International Journal of Computer Science Issues, 2013;10 (1) 3. [4] Nageswararao M. Materials for Gas Turbines – An Overview, Ch.13, http://www.intechopen.com. [5] Pomeroy M J. Coatings for gas turbine materials and long term stability issues, Materials and Design, 2005;26:225-231. [6] Small C. Strategic materials use in the gas turbine industry – challenges and opportunities, Rolls Royce report, 2011. [7] Kalpakjan, Schmid. Manufacturing processes for engineering materials, 5th ed., Pearson Education, 2008. [8] Thakur M. Thermal and structural analysis of gas turbine blade made of Ni-based superalloy, M.Tech Thesis, NITTTR Kokakta, 2016. [9] Shilke P W, Schenectady. Advanced gas turbine materials and coatings, Report, GER-3569 (08/04), 2004. [10] Small K B, Englehart D A, Christman T A. Guide to etching speciality alloys. Advanced materials & processes 2008;2:31-37.