Talha bin Yaqub ID:200991826
Material Selection and Specification for Blade, Disc and Shaft for the Turbine Stage of Static Gas Turbine Talha Bin Yaqub School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT UK
Table of Contents: 1.
Introduction..............................................................................................................................3
2.
Turbine Blade:.........................................................................................................................4 2.1 Performance Specification.....................................................................................................4 2.2 Possible Materials:.................................................................................................................7 2.1 Processing:.............................................................................................................................8 2.1.1 Forging (Avitzur, 1968):.................................................................................................8 2.1.2 Investment Casting (Kalpakjian, 2001):.........................................................................8
3.
Turbine Disc...........................................................................................................................11 3.1 Performance Specification...................................................................................................11 3.1.1 Possible Materials:........................................................................................................12 3.2 Process Specification...........................................................................................................14
4.
Turbine Shaft.........................................................................................................................17 4.1 Performance Specification...................................................................................................17 4.1.1 Bending stiffness:.........................................................................................................18 4.1.2 Torsional Stiffness:.......................................................................................................18 4.1.3 Possible Materials:........................................................................................................20 4.2 Process Specification:..........................................................................................................20
5.
Joining:..................................................................................................................................22
6.
Specification and Sourcing....................................................................................................22
7.
Inspection:..............................................................................................................................22
8.
Lifetime..................................................................................................................................23
List of Figures: Figure 1: Material Selection Chart (Yield Strength vs Density)...............................................7 Figure 2: Material Selection Chart (Young’s Modulus vs Density)..........................................7 Figure 3: Graphical Comparison of Processing Techniques (Discreet vs Capital Cost).......10 Figure 4: Graphical Comparison of Processing Techniques (Surface Roughness vs Capital Cost)..............................................................................................................................................11 Figure 5: Graphical Comparison of Processing Techniques (Discreet vs Relative Cost Index)............................................................................................................................................11 Figure 6: Material Selection Chart (Yield Strength vs Density).............................................13 Figure 7: Comparison of Tooling Cost for Primary Processes................................................16 Figure 8: Comparison of Capital Cost for Primary Processes................................................16 Figure 9: Comparison of Relative Cost Index for Primary Processes....................................17 Figure 10: Comparison of Section Thickness Range for Primary Processes.........................17 Figure 11: Material Selection Chart (Shear Modulus vs Density)..........................................20 Figure 12: Material Selection Chart (Young’s Modulus vs Density)......................................20 Figure 13: Comparison of Mass Ranges of Processes...............................................................22 Figure 14: Comparison of Relative Cost Index of Processes...................................................23
List of Tables: Table 1: Performance Specification for Turbine Blade..............................................................5 Table 2: Processing Specification for Turbine Blade..................................................................9 Table 3: Performance Specification for Turbine Disc..............................................................12 Table 4: Processing Specification for Turbine Disc...................................................................15 Table 5: Performance Specification for Turbine Shaft.............................................................18 Table 6: Processing Specification for Turbine Shaft.................................................................21
1. Introduction A gas turbine is a type of internal combustion engine in which air is used as working fluid. The gaseous energy of air is used to convert chemical energy of fuel to mechanical energy.
Output shaft power, which is needed to drive the propeller, is provided by the turbine section of the engine. The compressor and all other engine accessories are also driven by the power provided by this section. The gradual advancement in the material science has made it possible to manufacture gas turbine engines with high efficiency levels and high power ratings. It is the capability of a material to withstand higher temperature which leads to the higher efficiency; weight reduction can also be achieved if material has high elevated temperature strength to weight ratio (Muktinutalapati, 2011). In the start it was high temperature tensile strength which served as prime requirement for the materials development. Later on, stress rupture life and then creep properties became factors for material development. The choice of material was based on its capabilities to withstand higher loads (Muktinutalapati, 2011). This review gives an analysis on the advancement of materials and processes that are involved in the production of various components of gas turbine engine.
Although there are several components that join up to make a gas turbine engine, emphasis here has been on three main components i.e. blade, disc and shaft of turbine. These components are critical to the performance of the gas turbine engine.
2. Turbine Blade: Turbine blades are core component for the gas turbine engine as they are responsible for extracting the energy from high temperature and high pressure gasses (Gurajarapu, et al., 2014). Without turbine blades there will be no power at all while a slightest fault in the blades costs a fortune to repair.
2.1 Performance Specification During the operation, blades are subjected to significant gas bending and rotational stresses at very high temperatures. They also withstand sever thermo mechanical loading cycles during normal start-up and shutdown operation and unexpected trips. The turbine inlet is most difficult and challenging point as it is subjected to severe difficulties like, extreme temperatures in range of 9000 C- 12000C, high pressure, vibrations, small circulation area and high rotational speed (Bohidar, et al., 2013). Table 1: Performance Specification for Turbine Blade
Let suppose, the area of blade is rectangular, A=bh
And h=2b The objective function is an equation for the mass that we approximate as m=bhLρ While, h Stands for the length of blade, ρ
Stands for the density of material,
A Stands for the area of cross section, β
Is a constant multiplier
High strength of the turbine blades requires that:
FL =δ y 2 b2
Substituting value of b from this equation:
m=
Now for high bending stiffness,
S=
ρh L3 /2 21/ 2 σ 1/2
C1 EI L
3
≥S
¿
S* is the desired stiffness, E is Young’s modulus, C1 is a constant that b h3 depends on the distribution of load and I = 12 is the second moment of the area of the beam. Thus 12 S¿ 13 h= L C1 bE
(
)
Inserting this in equation gives equations for the performance metrics: the mass m2 of the blade m≥
¿ 1 3
12 S C1 b
( )
L
ρ
( ) 1
E3
Material indices for these equations:
M 1=
σ
1/ 2
ρ
1/3
&
M 2=
E ρ
Figure 1: Material Selection Chart (Yield Strength vs Density)
Figure 2: Material Selection Chart (Young’s Modulus vs Density)
2.2 Possible Materials: Zirconia, Nickel Chromium Alloys, Nickel based Super Alloys, Tungsten Carbides Out of these selected materials, I have chosen Nickel based super alloys using the information given in CESedupack. The reasons behind this selection are:
Zirconia is a ceramic material and it is highly brittle. Secondly, its process-ability is very difficult. Moreover at high temperatures, zirconia loses its stability thus affecting its
performance and properties (Namavar, et al., 2007). Nickel Chromium Alloys are very good corrosion and oxidation resistance but this is only up to 1000C. Above 1000C, these alloys are not stable and start to degrade (Davis, 2000). This
makes them unsuitable to be used in the gas turbine blades. Nickel based super alloys are one of the most advanced and best known alloys for high temperature applications. As their name suggests, they have been developed for some special applications. These are very stable up to 1300C and are capable of carrying high loads. They are most wildly used in aerospace applications. They also possess very good corrosion, oxidation and fatigue properties. Can carry loads at high temperatures. The most important benefit of nickel super alloys is that the melt can be directionally solidified to make single crystals which have exceptionally high performance and properties (Pollock & Tin, 2006). These are the alloys of nickel with titanium, chromium, aluminum and other trace alloying
elements. Tungsten Carbides are very hard and stable materials. They are best known for their high wear resistance property. These are best used where cutting or wear degradation is to be protected. Although tungsten carbide is very expensive material but its excellent performance as cutting tools make it very attractive to be used in cutting tips and other tools like drill bits. Most of the times WC is grinded using diamond cutter, that’s why they are too expensive to be processed. Another drawback with tungsten carbide is that it loses its stability above 1000C (Kurlov & Gusev, 2013). So these drawbacks make it unsuitable for gas turbine applications in most cases.
Considering all the facts mentioned above, I think Nickel based Super alloys are the best materials for turbine blade applications. It has maximum advantages or merits to be used as turbine blade.
2.1 Processing: After shortlisting the material to be used in turbine blades i.e., nickel based super alloys, we need to specify the processing routes by which blades should be manufactured using lowest cost. Important parameters to be considered are: Table 2: Processing Specification for Turbine Blade
Function Objective
Turbine blade Minimize cost Material: Nickel based super alloy Shape: solid 3D, Noncircular prismatic Constraints Mass: Depends on the size of turbine Surface roughness: A, very smooth Choice of the process Free Variables Mass The main processing routes found from CES using constraints are as below:
Forging Investment Casting
2.1.1 Forging (Avitzur, 1968): Forging is a shaping process in which the material in a rectangular or a billet shape is taken and it is then heated to a high temperature (usually austenitic range) and then it is pressed in the dies known as forging dies to make the required or desired shape. In some cases, cold forging is also carried out in which the preheating step is missing; this is normally done on relatively soft metals. In other words, material is squeezed by applying large plastic deformations. This process is widely used to shape steel or alloy parts. There are two kinds of forging dies used in industries: 1. Open die, in which the component limit is much higher but the precision is less. Blank is forced between two dies that are open and the material is exposed to the environment. The lower die is immoveable and the upper die comes and strikes the blank and shapes it. 2. Closed die forging is the one in which the dies join together and the materials is forced to take the internal shapes of the dies. This is the most precise form of forging but the component size is limited. Complex shapes can be given to the parts using closed die forging.
2.1.2 Investment Casting (Kalpakjian, 2001): Investment casting also known as lost wax casting is a very old casting technique for complex parts with very high precision. A wax pattern is made and it is then dipped is a slurry. The slurry material sticks to the pattern. This is then dried and sintered. After this, the wax is melted by applying little heat and is removed. Then only ceramic mold is left behind. This mold is then fired to give it final compactness. After this the melt is poured into the mold. Simple shapes can be made easily without use of external forces in the melt but for making complex and intricate parts, air pressure or vacuum is sometimes necessary for high precision. This process is suitable for making parts with materials having a high melting temperature as the refractory material of mold is stable at very high temperatures. It can work with both large and small batch sizes as per requirement. Typically used for jewelry making and dental implants. Most advance use is to cast nickel and cobalt based alloys. Both of the processing techniques were analyzed and graphs were plotted by considering various parameters as shown below;
Figure 3: Graphical Comparison of Processing Techniques (Discreet vs Capital Cost)
Figure 4: Graphical Comparison of Processing Techniques (Surface Roughness vs Capital Cost)
Figure 5: Graphical Comparison of Processing Techniques (Discreet vs Relative Cost Index)
It is clear from the graphs that capital cost of investment casting is much lower than the forging processes in case of both discrete parts and low surface roughness products. While in case of comparing relative cost index with discrete parts production, forging seems better but the
difference is not very high thus I will prefer investment casting over forging for producing turbine blades.
3. Turbine Disc A gas-turbine is usually composed of a set of discs, fastened onto a shaft. The main functions of a turbine disc are to locate the rotor blades within the hot gas path and to transmit the power generated to the drive shaft (Bohidar, et al., 2013).
3.1 Performance Specification There are a number of forces acting on a turbine disc during operation. Therefore, the operational parameters require high integrity advanced materials having a balance of some key properties (Bohidar, et al., 2013) described in the table below: Table 3: Performance Specification for Turbine Disc
Function Objective Constraints
Free Variables
Fly wheel Maximizing K.E stored per unit mass Must possess high temperature strength to withstand high centrifugal forces > 100MPa Should have good corrosion resistance Must have high temperature stability, temperature range=9001400◦C(maximum for safety) Cross section of the disc Choice of material
Taking disc as a fly wheel and applying the equation to maximize the k.e stored/ unit mass. U=
I ω2 2
πρ R4 h 2 m=πρ R 4 h from objective : U R2 ω2 = m 4 but there will also be some centrifugal stress due to spinning of the disc
2
σ max =
2
R ω ρ 2
But this max stress should be less then the fracture stress. so, combining above two equations give us: U σf = m 2ρ
Constraints: M=
σf ρ
Figure 6: Material Selection Chart (Yield Strength vs Density)
3.1.1 Possible Materials:
Boron carbide Aluminum nitride Nickel chromium alloys Nickel based super alloys Tungsten carbide
Titanium Alloys Carbon Steels Cast iron
Out of all these possible materials, I have chosen nickel based super alloys for turbine disc. Some of the reasons are explained below: Boron carbide: Boron carbide although very hard and high temperature stable compound, and light but it cannot be used in the manufacturing of disc of turbine because it is very expensive to be prepared. The tooling cost is also very high as it can only be surface finished or surface modified using diamond grinding (Matkovich, 1977). Moreover, the toughness of boron carbide is also less then nickel super alloys. The maximum service temperature of titanium alloys is very less approx. 500C so they cannot be used here (Leyens, 2003). Same is the case with carbon steels and cast irons. Their maximum service temperature in around 400C. Aluminum Nitride: Here again the problem of manufacturing limits its use as it is very expensive to be grinded using diamond. Also the sintering temperature required for aluminum nitride is around 1900C which is too high and costly. There is another issue that aluminum nitride shows hydrolysis issues in presence of water. So moisture can affect its performance. Nickel based super alloys (Davis, 2000), nickel chromium alloys, tungsten carbide have already been explained in the turbine blades section. Except those general reasons of my selection, one other reason is that the yield strength of nickel super alloys is the highest among all these and its density is also lower then nickel chromium alloys, tungsten carbide and tungsten alloys. Density of tungsten alloys and tungsten carbides is also greater then super alloys. So considering all the manufacturing issues, general properties, density and mechanical properties so for me nickel based super alloys seem to be the best choice for disc manufacturing material.
3.2 Process Specification Table 4: Processing Specification for Turbine Disc
Function Objective
Turbine Disc Minimize cost Material: Nickel based super alloy Shape: solid 3D Mass: 20-70kg Constraints Tolerance 0.001-0.002 Surface roughness: A, B Process: primary shaping, discrete Choice of the process Free Variables Mass Depending on the constraints and the material shortlisted, there are two manufacturing options, 1) Forging 2) Hot isotactic pressing Forging (Avitzur, 1968) has already been explained in detail in turbine blade section. Hot isotactic pressing (Schatt, 1997) is the process in which the material is made powder and it is then mixed with little binder and it is then placed in a deformable container, heated and pressed. Very high pressure is provided using argon gas and also the compactness achieved during this process is very high. Parts achieved are normally 100% dense and isotropic properties. Material utilization fraction is around 0.9. The capital costs of this process may be high but once started, they can serve for years. Energy consumption is also lower.
Figure 7: Comparison of Tooling Cost for Primary Processes
Figure 8: Comparison of Capital Cost for Primary Processes
Figure 9: Comparison of Relative Cost Index for Primary Processes
Figure 10: Comparison of Section Thickness Range for Primary Processes
From the graphs, it is clear that the forging relative cost index is low as compared to the hot isotactic pressing techniques but section thickness range and tooling cost is better in case of hot isotactic pressing then that of forging. The capital cost is somehow comparable in both of the
cases. The choice is bit difficult but due to the ability of hot isotactic pressing of making precise and accurate parts with good surface finish, I will chose hot isotactic pressing. As in forging there may be some issues of surface oxidation and material softening at high temperature working. So, post forging treatments will also cause a lot of money addition to the process. Considering all these factors, hot isotactic processing seems to be the best choice for me.
4. Turbine Shaft The transmission shaft in a turbine is the first component to receive the power generated at the turbine blade. The role of the shaft in the hydrokinetic turbine system is to transmit the torque generated at the turbine blades to the generator. The torque and thrust at the blades are the major external forces acting on the system and therefore are considered as the primary forces acting on the shaft. The thrust does not lead to any bending moment because the horizontal component of thrust (acting parallel to the axis of the shaft) cancel each other due to the symmetry and accounts to only a normal force along the axis of the shaft. So the main forces acting on the shaft are the rotational forces.
4.1 Performance Specification Table 5: Performance Specification for Turbine Shaft
Function Objective Constraints Free Variables
Beam/Shaft Minimizing mass for greater power to weight ratio High Torsional Stiffness>50GPa High Bending Stiffness>25MPa High corrosion resistance High temperature stability. T=900-1400◦C(maximum for safety) Cross section A of shaft Choice of material m= ALρ
In this case of shaft, we will consider two types of stiffnesses one due to bending and other due to twisting.
4.1.1 Bending stiffness: SB=
EI L3
but here , 2
I=
A ∅ 12 Putting it in stiffness equation and eliminating A gives,
m=¿ [ 12 S B ¿
1/2
ρ L5/3 [ E 1/ 2 ]
4.1.2 Torsional Stiffness: ST =
A2G ∅ 7L
putting in m to eliminate A give: m=[
7 ST L
3
]
1/2
ρ L3/2 [ ( ∅ G )1/ 2 ]
1 /2
The constraints are:
M 1=
E ρ
1 /2
,
M 2=
G ρ
Figure 11: Material Selection Chart (Shear Modulus vs Density)
Figure 12: Material Selection Chart (Young’s Modulus vs Density)
4.1.3 Possible Materials:
Low alloy steel Low carbon steel Tungsten carbide Stainless steel Nickel based super alloys Nodular cast iron
WC and nickel based super alloy has been rejected due to their very high cost and very high density. Moreover the processing is also very difficult for these materials. Cast iron is too soft and it may bend easily. Low carbon steel degrades at high temperatures and also the low hardness of low carbon steel makes it unsuitable for this job. Low alloy steel is a good option as it has a comparable density and strength values. It can with stand the loads and pressure. Stainless steel is also a good option but it is brittle in nature and its formability is 2 while formability of low alloy steel is 3. Also the fracture strength of low alloy steel is greater than all of these shortlisted materials. Considering the ease of fabrication, density, strength and cost I will choose low alloy steel for turbine shaft.
4.2 Process Specification: Table 6: Processing Specification for Turbine Shaft
Function Objective
Shaft Minimize cost Material: low alloy steel Shape: solid 3D Mass: 50kg(min) Constraints Tolerance 0.001-0.002 Surface roughness: A, B Process: primary shaping, discrete Choice of the process Free Variables Mass The possible processes that appear in the software are
Hot isotatic process Forging
(both of these are already explained in above sections) And after analyzing the graphs shown below, it can be seen that forging appears to be the best method for the manufacturing of the shaft as it is the low cost process relative to hot isotactic pressing and also there will be a requirement of a very large and complex setup for hot isotactic pressing. Forging also allows the manufacturing of shafts with greater mass ranges. There will be some post forging treatments required like oxide layer removal of high surface finish. They will be carried out after words. The grans structure after forging is also suitable for the manufacturing of shaft.
Figure 13: Comparison of Mass Ranges of Processes
Figure 14: Comparison of Relative Cost Index of Processes
5. Joining: To join blades with disc, material is the same for these i.e nickel based super alloys, the joint is taken as tie joint and the forces acting on the blades are taken as bending forces. Now the best joining method found using CESedupack is laser welding. Joining of blades and disc reference: (Singer & Arzt, 1986)
6. Specification and Sourcing Hastealloy X: (International, 1997) Inconel 738: (Inc., n.d.) Edupack: (CES, 2015)
7. Inspection: Overhauling of the gas turbine engines must be carried out with a gap of 1 to 3 years. Besides this there must be a setup of regular inspections. For these parts of the gas turbine, I would go for the NDT inspection methods. In this, Ultrasonic can be used for shaft and also DPT testing can be carried out to inspect the blades and disc. Besides this, radiography is a very good technique to inspect the component with high accuracy. Real time monitoring of rotary equipment during service can be done. While the machine or system is in service, the device is brought near the equipment and it automatically monitors the condition of the components. It has a built in stroboscope, camera and laser pyrometer. It detects the crack automatically and generates the results.
8. Lifetime References 1. Avitzur, B., 1968. METAL FORMING.. PROCESSES AND ANALYSIS. NEW YORK: MCGRAW-HILL, INC., NEW YORK. 2. Bohidar, S. K., Dewangan, R. & Kaurase, K., 2013. Advanced Materials used for different components of Gas Turbine. International Journal of Scientific Research and Management, pp. 1-7. 3. CES, 2015. CES EduPack 2015, s.l.: Granta Design. 4. Davis, J. R., 2000. Nickel, cobalt, and their alloys.. U.S.: ASM international.
5. Gurajarapu, N., Rao, V. N. B. & Kumar, I. N., 2014. Selection of a Suitable Material and Failure Investigation on a Turbine Blade of Marine Gas Turbine Engine using Reverse Engineering and FEA Techniques. International Journal of u-and e-Service, Science and Technology, 7(6), pp. 297-308. 6. Inc.,
T.
I.
N.
C.,
n.d.
Alloy
IN-738.
[Online]
Available
at:
http://www.nipera.org/~/Media/Files/TechnicalLiterature/IN_738Alloy_PreliminaryData_497 _.pdf 7. International,
H.,
1997.
Hastelloy
X
Alloy.
[Online]
Available at: https://www.haynesintl.com/pdf/h3009.pdf 8. Kalpakjian, S., 2001. Manufacturing engineering and technology. s.l.:Pearson Education India. 9. Kurlov, A. S. & Gusev, A. I., 2013. Tungsten carbides: structure, properties and application in hardmetals. 184 ed. s.l.:Springer Science & Business Media. 10. Leyens, C. &. P. M., 2003. Titanium and titanium alloys. Weinheim: Wiley-VCH, Weinheim. 11. Matkovich, V. I., 1977. Boron and refractory borides. s.l.:s.n. 12. Muktinutalapati, N. R., 2011. Materials for Gas Turbines-An Overview. s.l.:INTECH Open Access Publisher. 13. Namavar, F. et al., 2007. Thermal stability of nanostructurally stabilized zirconium oxide. Nanotechnology, 18(41), p. 415702. 14. Pollock, T. M. & Tin, S., 2006. Nickel-based superalloys for advanced turbine engines: chemistry, microstructure and properties. Journal of propulsion and power, 22(2), pp. 361374. 15. Schatt, W. &. W. K. P., 1997. Powder metallurgy: processing and materials. 3 ed. s.l.:European Powder Metallurgy Association. 16. Singer, R. F. & Arzt, E., 1986. High temperature alloys for gas turbine and other applications. In: 97 ed. s.l.:Betz, W., Ed, 97., p. 160.
