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Investigation into thermal stresses in gas turbine transition-piece: Influence of material properties on stress levels
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
journal homepage: www.elsevier.com/locate/jmatprotec
Investigation into thermal stresses in gas turbine transition-piece: Influence of material properties on stress levels A.H. Mustafa a,b , M.S. Hashmi c , B.S. Yilbas d,∗ , M. Sunar d a
Mechanical Services Shops Department, Saudi Aramco, Saudi Arabia City University, Dublin, Ireland c School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland d Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, P.O. Box 1205, Dhahran 31261, Saudi Arabia b
a r t i c l e
i n f o
a b s t r a c t
Keywords:
Transition-piece accommodates hot gas emanating from the gas generator and received by
Transition-piece
the power turbine. The thermal stress developed in the outer wall casing of the transition-
Gas turbine
piece reaches high levels due to attainment of high temperature. In the present study, flow
Thermal stress
through a transition-piece resembling the actual gas turbine operation is considered and
Heat treatment
thermal stress levels in the outer casing of the transition-piece are computed. In order to demonstrate the material response to the high temperature, the substrate material (AISI 660 stainless steel or A286 iron-base superalloy) of the transition-piece is heat treated at elevated temperature for 2 h. Tensile and three-point bending tests are accommodated to determine the elastic modulus of the heat treated and as-received materials. In the numerical simulation of the flow and temperature field, the control volume approach is introduced while stress field is computed using the finite element model. It is found that the elastic modulus of the heat treated specimen is considerably lower than that of as-received material. This is because of the formation of -phase (hcp-Ni3 Ti) and dissolution of ␥ at the grain boundaries. Consequently, von-Mises stress level is significantly lower for the transition-piece subjected to the heat treatment than that of the as-received transition-piece material. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Super alloys such as A286 austenitic steel and Inconel 625 are widely used in gas turbine industry due to their good thermal resistance, superior mechanical properties, and ease of fabrication. A286 superalloy is used as gas turbine transition-piece material, which connects the gas turbine to the power turbine inlet. Due to the long duration of high temperature exposure and variation in flow properties, such as temperature, pressure and velocity, the transition-piece suffers from high thermal
∗
stresses and in some cases material failure occurs. Consequently, investigation into the mechanical properties of the alloy after exposure to elevated temperature and stress levels in the transition-piece become necessary. Considerable research studies were carried out to examine the mechanical and metallurgical properties of iron-base super alloys. Properties of A286 alloy after rotor forging were examined by Kohno et al. (1987). They indicated that good mechanical properties were caused by relatively lower Ti content. High temperature strengthening and toughening of
Corresponding author. E-mail addresses: [email protected] (A.H. Mustafa), [email protected] (M.S. Hashmi), [email protected] (B.S. Yilbas). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.11.264
370
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
iron-base superalloy were studied by Xie et al. (2003). They indicated that vacuum melting was good method to keep the high strength of the alloy and improve the ductility as well as reducing the crack propagation rates. The effect of high temperature on the inter-granular cracking of Nb–A286 alloy was investigated by Rho et al. (2002). They showed that the precipitation of the -phase at the grain boundary during the low cycle fatigue resulted in the grain boundary cavitations, which promoted the inter-granular failures at high temperature. The microstructural development and creep behaviour of A286 alloy were studied by De Cicco et al. (2004). They showed that the damage observed after creep tests was inter-granular and ageing modified the secondary strain rate and time to failure. The ageing response of a welded iron-base superalloy was studied by Strum et al. (1983). They showed that a persistent strength mismatch existed weldments between the base metal and weld metal regions, which was attributed to the microstructural coarseness and the chemical segregation of titanium. Gas turbine transition-piece in general, is in the form of an annular duct in which the hot gas flows through. Heat transfer rates from the hot gas to the casing of the transitionpiece are important due to high temperature development in the casing, particularly, in the outer casing. Consequently, investigation into thermal stress development in the outer casing of the transition-piece becomes essential. Considerable research studies were carried out to examine the heat transfer characteristics and the stress levels of the duct subjected to the internal flow. Yapici and Basturk (2006) studied transient temperature and thermally induced stress distribution in a solid disk heated radially. They indicated that the maximum temperature oscillated with the oscillations of the heat source and high levels of thermal stresses generated during the temperature oscillation. Laminar mixed convection in a horizontal annular duct was investigated by Nouar (1999). He showed that decrease in fluid viscosity with temperature lead to increase in axial velocity in the upper part of the annular duct. The turbulent flow in small angle diffusers using new wall treatment was considered by Brag and deLemos (2004). It was shown that, accelerated flows in the convergent ducts reduced turbulence level while turbulence increased in the divergent duct. Swirling turbulent flows and heat transfer in stationary annulus were considered by Zhang et al. (2003). They concluded that swirl number increase lead to increase in turbulent kinetic energy and heat transfer coefficient, while the gas axial velocity remained nearly unchanged. Alzaharnah et al. (2001) investigated flow through pipes and thermal stress development due to temperature gradient in the pipe wall. They showed that the pipe wall thickness and diameter were the most important parameters affecting the resulting stress distribution. In the present study, flow through a transition-piece is considered and temperature rise in the outer casing is predicted
numerically. Thermal stress developed in the casing computed using the finite element model. To resemble the high temperature environment, substrate material (A286) is heat treated to 930 ◦ C for 1 h. The tensile and three-point bending tests are carried out to determine the Young modulus of the heat treated and as-received materials. The Young modulus of the as-received and heat treated alloys are accommodated in the simulation to predict the stress field in the outer casing of the transition-piece. Microstructural changes in the alloy after the heat treatment are examined by SEM, EDS, and XRD.
2.
Experimental
A286 is austenitic iron-base superalloy with ␥ precipitation strengthening by addition of titanium and aluminium. The ␥ precipitation homogeneously distribute in the ␥-matrix and carbide MC exists in the ␥-matrix as inclusions. As-received sample is formed in the solution treated condition, and the elemental composition is given in Table 1. Experimental tests were carried out on test specimens of the A286 alloy to determine its modulus of elasticity and how these it varied for the two conditions; as-received and heat treatment conditions. The first condition was the as-received condition in which the specimen was solution annealed at 980 ◦ C for 1.5 h and then water quenched. Then the specimens were precipitation hardened at 710 ◦ C for 16 h and air cooled. 10 test specimens were produced in this condition. In the second condition is the heat treated condition, in which the as-received specimens were solution treated for 1 h at 930 ◦ C; air cooled and aged for 16 h at 730 ◦ C to retain the material original properties and relieve the residual stresses developed from the machining process of the preparation the test specimens, 10 test specimens were produced in this condition.
2.1.
Three-point bending testing
The test was carried out using the INSTRON 8801. The machine was equipped with a deflection measuring device where the error in the load measuring system was ±1% of the maximum applied load. There was no pre-loading in the machine, and each specimen was placed carefully into the test fixture to preclude possible damage and to ensure alignment of the specimen in the fixture. The bending test was carried out using the three-point bending test configuration according to the international standards (ASTM, 1996). The test was carried out at room temperature 25 ± 2 ◦ C and 50 ± 5% relative humidity. The strain rate used was 2.5 mm/min and to ensure accuracy of the results a total of four samples for each material condition were tested. During the test, the load and displacement characteristics were recorded. The tests were terminated when the sample fail or it reaches the 5% of its flexural strain. The tests
Table 1 – Elemental composition of as-received A286 iron-base superalloy Element wt%
Al
B
C
Cr
Mo
Fe
Mg
Ni
P
S
Si
Ti
V
0.17
0.0047
0.034
15.2
1.16
Balance
1.21
24.96
0.016
<0.003
0.3
1.96
0.25
371
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
Table 2 – Three-point bending test conditions and the results Sample
L (m)
I
d (m)
P (N)
E (GPa)
As-received (AR) analytical Heat treated (HT) analytical
0.068 0.068
2.0833E−10 2.0833E−10
0.0067 0.00255
4455 1126.556
2.08E+11 1.38E+11
are repeated for five times to ensure the repeatability. The measurement error based on the repeatability is within 5%.
Table 3 – Tensile test results Sample
(MPa)
ε (m)
E (GPa)
2.2.
As-received (AR) Heat treated (HT)
340.9574 312.0188
0.001829419 0.002288951
2.07E+11 1.37E+11
Tensile testing
The tensile test was performed using the INSTRON 5569 mechanical tester. The tester was equipped with instrument control and the data acquisition and analysis were performed through BLUEHILL software. The setup was equipped with a temperature chamber which had the temperature range of −20 to 350 ◦ C that allowed testing over a wide range of highly controlled temperatures. The tensile test was carried out using the international standards (ASTM, 2004). The test specimens employed for determining the stress–strain behaviour of the materials were dog-bone shape to reduce the cross-section area by dog-boning to force the failure to occur in the specimen mid-section. The test specimens were placed in between the upper and lower self-aligning mechanical grips, with the grips were placed 25 mm from each end. Minimum of 4 specimens were tested for each condition at room temperature of 25 ± 2 ◦ C and 50 ± 5% relative humidity. The test piece was setup in the testing machine with zero loads. The tensile tests were conducted at a cross-head speed of 3 mm/min. The elongation was measured at the parallel distance of the dog-boned test piece. The original gauge length is 25 ± 5% mm.
3.
Results and discussions
Fig. 1 shows SEM micrograph of untreated and heat treated A286 alloy. Untreated alloy shows almost equiaxed grain structure with the average grain diameter of about 30 mm. However, the grain size increases significantly after the heat treatment and uni-equiaxed grains with different sizes are formed. The finer grains are smaller than the original columnar grains. The increase in the grain size is responsible for low hardness after heat treatment process. It is also possible that ␥ depletion
near the grain boundary lowers the brittleness of the material after the heat treatment. The close examination of the grain boundaries suggests that locally scattered and discontinuous formation of carbides occur. Although grain boundary carbides formation increases the brittleness while lowering the ductility, locally discontinuous formation minimises this effect. Table 2 gives three-point bending test parameters and the results of the elastic modulus while Table 3 gives the tensile test results. It can be observed that the elastic module determined from the three-point bending and the tensile tests are in good agreement. The elastic limit of the alloy decreases significantly after the heat treatment process. The increase in ductility is attributed to the grain coarsening after the heat treatment. In addition, short duration heat treatment causes dissolution of ␥ and formation of -phase (hexagonal closed-packedhcp-Ni3 Ti phase) as seen from the XRD result in Fig. 2, lowering the mechanical performance of the material. The formation of -phase at the grain boundary can also act as the nucleation site of the crack where the material fails. This situation is observed during the three-point bending test, in particular at the tensile surface of the specimen. Fig. 3 shows temperature contours in the outer casing of the transition-piece obtained from the numerical computation. Temperature contours show that at the interface between the fluid and casing (gas–solid interface) temperature contours attains significantly high values while at the outer surface temperature is low due to the exposure of the outer surface to the atmospheric ambient. In the region close to the entry of the transition-piece (x = 0), temperature attains high val-
Fig. 1 – SEM micrograph of as-received and heat treated workpieces. As-received material. Heat treated material.
372
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
Fig. 2 – XRD results for the heat treated workpiece.
ues because of the convective heating by the hot gas and the radial heat conduction in this region. Consequently, temperature distribution varies significantly in the transition-piece because of the hot gas temperature variation.
3.1.
Mid-plane
Fig. 4 shows the von-Mises stress along the axial direction in the outer casing of the transition-piece at the inner wall next to the inner surface. It should be noted that the elastic modulus of 207 GPa corresponds to as-received substrate material while 138 GPa is determined from the tensile and the threepoint bending tests after the heat treatment process. It can be observed that von-Mises stress attains high values at the distance corresponding to the edges of the transition-piece. This is because of the attainment of the high temperature gradient in these regions and the mechanical constrains at both ends of the transition-piece. In the simulation, both ends of the transition-piece are fixed to resemble the actual situation; in which case, one end of the transition-piece is joined to the gas generator exit and the other is joined to the power turbine inlet. However, the magnitude of von-Mises stress reduces considerably for the heat treated case, which is due to the low value of the elastic modulus. Moreover, the behaviour of the von-Mises stress for both elastic modulus are similar with dif-
Fig. 4 – von-Mises stress along the axial direction in the outer casing for heat treated and as-received materials.
ferent magnitudes. In the case of mid-plane (Fig. 5), von-Mises stress attains high values at the ends of the transition-piece. Therefore, similar observations can be made to those discussed for Fig. 4. This is because of temperature distribution at the mid-plane, which is similar to temperature distribution of the inner-wall. von-Mises stress distribution along the axial distance in the region close to the outer wall differs from that of the mid-plane in the region close to the inner-plane. This is more pronounced at the fixed ends and the region near to it. In this case, the magnitude of von-Mises stress reduces significantly and the second peak in the von-Mises stress is observed in the region next to the fixed ends before attainment of the steady value in the axial direction. This is attributed to the outer casing exposure to the free atmospheric ambient. This in turn, provides more uniform like temperature distribution along the axial length of the transition-piece. In addition, at the outer surface of the transition-piece, stress free condition is considered in the simulations. This enables the free expansion of the transition-piece in the radial direc-
Fig. 3 – Temperature contours in the outer casing of the transition-piece.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
373
at the edges of the transition-piece; this is attributed to the fixed ends of the transition-piece and attainment of the high temperature gradient in this region. von-Mises stress level is significantly lower for the heat treated workpiece than that of the as-received material, which is because of the low elastic modulus of the heat treated workpiece. von-Mises stress in the region close to the outer surface of the transition piece attains low values because the radial expansion of the transitionpiece from the outer surface to the free atmospheric ambient.
references
Fig. 5 – von-Mises stress along the axial direction in the mid-plane of the casing for heat treated and as-received materials.
tion in the outer region while lowering the stress levels in the region close to the free surface.
4.
Conclusions
Flow through a transition-piece is simulated in relation to operational condition of aero-derivative gas turbine engine used in the mechanical drive application. Temperature distribution in the outer casing of the transition-piece is predicted and experimental work was carried out. The elastic modules obtained from the tensile and three-point bending tests for as-received and heat treated workpieces are used for thermal stress simulations. In order to observe the actual operation conditions of the transition-piece, the substrate material (A286 iron-base superalloy) used in the transitionpiece casing is heat treated at elevated temperature under the open ambient conditions for 2 h. It is found that the elastic modulus of the heat treated specimen is significantly lower than that of the as-received material. This is because of the grain coarsening and formation of -phase (hexagonal closed-packed-hcp-Ni3 Ti phase), which in turn improves the ductility of the heat treated material. Moreover, ␥ depletion through dissolution at grain boundaries contributes to the low mechanical performance of the material after the heat treatment process. von-Mises stress attains high values
Alzaharnah, I.T., Hashmi, M.S., Yilbas, B., 2001. Thermal stresses in thick-walled pipes subjected to fully developed laminar flow. J. Mater. Process. Technol. 118, 50–57. ASTM C 1161-94 (Re approved 1996), 1996. Standard test methods for flexural strength of advanced ceramics at ambient temperature, American Society for Testing and Material Standards. ASTM E8M-00B, 2004. Standard method of tension testing of metallic materials, American Society for Testing and Material Standards, Philadelphia. Brag, E.J., deLemos, M.J.S., 2004. Numerical simulation of turbulent flow in small-angle diffusers and contractions using a new wall treatment and linear high Reynolds – model. J. Numer. Heat Transfer 45, 911–933. De Cicco, H., Luppo, M.I., Gribaudo, L.M., Ovejero-Garc´ıa, J., 2004. Microstructure development and creep behaviour in A286 superalloy. J. Mater. Char. 52, 85–92. Kohno, M., Honjo, T., Kinoshita, S., Suzuki, A., 1987. Properties of large A286 Alloy rotor forging. Kobelco Technol. Rev. vol. 2. Nouar, C., 1999. Numerical solutions for laminar mixed convection in a horizontal annular duct temperature dependent viscosity effect. Int. J. Numer. Methods Fluids 29, 849–864. Rho, B.S., Nam, S.W., Xie, X., 2002. The effect of test temperature on the intergranular cracking of nb-a286 alloy in low cycle fatigue. J. Mater. Sci. 37, 203–209. Strum, M.J., Summers, L.T., Morris Jr., J.W., 1983. Aging response of welded iron-based superalloy. Weld. Res. Suppl., 235–242. Xie, X., Mao, Z., Dong, J., Hu, Y., 2003. Investigation on High Temperature Strengthening and Toughening of Iron Base Superalloy, vol. 10. J. University of Science and Technology, Beijing, pp. 44–48. Yapici, H., Basturk, G., 2006. Reduction of thermally induced stress in a solid disk heated with radially periodic expanding and contracting ring heat flux. J. Mater. Process. Technol. 180, 279–290. Zhang, J., Dong, L., Zhou, L., Nieh, S., 2003. Simulation of swirling turbulent flows and heat transfer in an annular duct. J. Numer. Heat Transfer A44, 591–609.
journal homepage: www.elsevier.com/locate/jmatprotec
Investigation into thermal stresses in gas turbine transition-piece: Influence of material properties on stress levels A.H. Mustafa a,b , M.S. Hashmi c , B.S. Yilbas d,∗ , M. Sunar d a
Mechanical Services Shops Department, Saudi Aramco, Saudi Arabia City University, Dublin, Ireland c School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland d Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, P.O. Box 1205, Dhahran 31261, Saudi Arabia b
a r t i c l e
i n f o
a b s t r a c t
Keywords:
Transition-piece accommodates hot gas emanating from the gas generator and received by
Transition-piece
the power turbine. The thermal stress developed in the outer wall casing of the transition-
Gas turbine
piece reaches high levels due to attainment of high temperature. In the present study, flow
Thermal stress
through a transition-piece resembling the actual gas turbine operation is considered and
Heat treatment
thermal stress levels in the outer casing of the transition-piece are computed. In order to demonstrate the material response to the high temperature, the substrate material (AISI 660 stainless steel or A286 iron-base superalloy) of the transition-piece is heat treated at elevated temperature for 2 h. Tensile and three-point bending tests are accommodated to determine the elastic modulus of the heat treated and as-received materials. In the numerical simulation of the flow and temperature field, the control volume approach is introduced while stress field is computed using the finite element model. It is found that the elastic modulus of the heat treated specimen is considerably lower than that of as-received material. This is because of the formation of -phase (hcp-Ni3 Ti) and dissolution of ␥ at the grain boundaries. Consequently, von-Mises stress level is significantly lower for the transition-piece subjected to the heat treatment than that of the as-received transition-piece material. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Super alloys such as A286 austenitic steel and Inconel 625 are widely used in gas turbine industry due to their good thermal resistance, superior mechanical properties, and ease of fabrication. A286 superalloy is used as gas turbine transition-piece material, which connects the gas turbine to the power turbine inlet. Due to the long duration of high temperature exposure and variation in flow properties, such as temperature, pressure and velocity, the transition-piece suffers from high thermal
∗
stresses and in some cases material failure occurs. Consequently, investigation into the mechanical properties of the alloy after exposure to elevated temperature and stress levels in the transition-piece become necessary. Considerable research studies were carried out to examine the mechanical and metallurgical properties of iron-base super alloys. Properties of A286 alloy after rotor forging were examined by Kohno et al. (1987). They indicated that good mechanical properties were caused by relatively lower Ti content. High temperature strengthening and toughening of
Corresponding author. E-mail addresses: [email protected] (A.H. Mustafa), [email protected] (M.S. Hashmi), [email protected] (B.S. Yilbas). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.11.264
370
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
iron-base superalloy were studied by Xie et al. (2003). They indicated that vacuum melting was good method to keep the high strength of the alloy and improve the ductility as well as reducing the crack propagation rates. The effect of high temperature on the inter-granular cracking of Nb–A286 alloy was investigated by Rho et al. (2002). They showed that the precipitation of the -phase at the grain boundary during the low cycle fatigue resulted in the grain boundary cavitations, which promoted the inter-granular failures at high temperature. The microstructural development and creep behaviour of A286 alloy were studied by De Cicco et al. (2004). They showed that the damage observed after creep tests was inter-granular and ageing modified the secondary strain rate and time to failure. The ageing response of a welded iron-base superalloy was studied by Strum et al. (1983). They showed that a persistent strength mismatch existed weldments between the base metal and weld metal regions, which was attributed to the microstructural coarseness and the chemical segregation of titanium. Gas turbine transition-piece in general, is in the form of an annular duct in which the hot gas flows through. Heat transfer rates from the hot gas to the casing of the transitionpiece are important due to high temperature development in the casing, particularly, in the outer casing. Consequently, investigation into thermal stress development in the outer casing of the transition-piece becomes essential. Considerable research studies were carried out to examine the heat transfer characteristics and the stress levels of the duct subjected to the internal flow. Yapici and Basturk (2006) studied transient temperature and thermally induced stress distribution in a solid disk heated radially. They indicated that the maximum temperature oscillated with the oscillations of the heat source and high levels of thermal stresses generated during the temperature oscillation. Laminar mixed convection in a horizontal annular duct was investigated by Nouar (1999). He showed that decrease in fluid viscosity with temperature lead to increase in axial velocity in the upper part of the annular duct. The turbulent flow in small angle diffusers using new wall treatment was considered by Brag and deLemos (2004). It was shown that, accelerated flows in the convergent ducts reduced turbulence level while turbulence increased in the divergent duct. Swirling turbulent flows and heat transfer in stationary annulus were considered by Zhang et al. (2003). They concluded that swirl number increase lead to increase in turbulent kinetic energy and heat transfer coefficient, while the gas axial velocity remained nearly unchanged. Alzaharnah et al. (2001) investigated flow through pipes and thermal stress development due to temperature gradient in the pipe wall. They showed that the pipe wall thickness and diameter were the most important parameters affecting the resulting stress distribution. In the present study, flow through a transition-piece is considered and temperature rise in the outer casing is predicted
numerically. Thermal stress developed in the casing computed using the finite element model. To resemble the high temperature environment, substrate material (A286) is heat treated to 930 ◦ C for 1 h. The tensile and three-point bending tests are carried out to determine the Young modulus of the heat treated and as-received materials. The Young modulus of the as-received and heat treated alloys are accommodated in the simulation to predict the stress field in the outer casing of the transition-piece. Microstructural changes in the alloy after the heat treatment are examined by SEM, EDS, and XRD.
2.
Experimental
A286 is austenitic iron-base superalloy with ␥ precipitation strengthening by addition of titanium and aluminium. The ␥ precipitation homogeneously distribute in the ␥-matrix and carbide MC exists in the ␥-matrix as inclusions. As-received sample is formed in the solution treated condition, and the elemental composition is given in Table 1. Experimental tests were carried out on test specimens of the A286 alloy to determine its modulus of elasticity and how these it varied for the two conditions; as-received and heat treatment conditions. The first condition was the as-received condition in which the specimen was solution annealed at 980 ◦ C for 1.5 h and then water quenched. Then the specimens were precipitation hardened at 710 ◦ C for 16 h and air cooled. 10 test specimens were produced in this condition. In the second condition is the heat treated condition, in which the as-received specimens were solution treated for 1 h at 930 ◦ C; air cooled and aged for 16 h at 730 ◦ C to retain the material original properties and relieve the residual stresses developed from the machining process of the preparation the test specimens, 10 test specimens were produced in this condition.
2.1.
Three-point bending testing
The test was carried out using the INSTRON 8801. The machine was equipped with a deflection measuring device where the error in the load measuring system was ±1% of the maximum applied load. There was no pre-loading in the machine, and each specimen was placed carefully into the test fixture to preclude possible damage and to ensure alignment of the specimen in the fixture. The bending test was carried out using the three-point bending test configuration according to the international standards (ASTM, 1996). The test was carried out at room temperature 25 ± 2 ◦ C and 50 ± 5% relative humidity. The strain rate used was 2.5 mm/min and to ensure accuracy of the results a total of four samples for each material condition were tested. During the test, the load and displacement characteristics were recorded. The tests were terminated when the sample fail or it reaches the 5% of its flexural strain. The tests
Table 1 – Elemental composition of as-received A286 iron-base superalloy Element wt%
Al
B
C
Cr
Mo
Fe
Mg
Ni
P
S
Si
Ti
V
0.17
0.0047
0.034
15.2
1.16
Balance
1.21
24.96
0.016
<0.003
0.3
1.96
0.25
371
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
Table 2 – Three-point bending test conditions and the results Sample
L (m)
I
d (m)
P (N)
E (GPa)
As-received (AR) analytical Heat treated (HT) analytical
0.068 0.068
2.0833E−10 2.0833E−10
0.0067 0.00255
4455 1126.556
2.08E+11 1.38E+11
are repeated for five times to ensure the repeatability. The measurement error based on the repeatability is within 5%.
Table 3 – Tensile test results Sample
(MPa)
ε (m)
E (GPa)
2.2.
As-received (AR) Heat treated (HT)
340.9574 312.0188
0.001829419 0.002288951
2.07E+11 1.37E+11
Tensile testing
The tensile test was performed using the INSTRON 5569 mechanical tester. The tester was equipped with instrument control and the data acquisition and analysis were performed through BLUEHILL software. The setup was equipped with a temperature chamber which had the temperature range of −20 to 350 ◦ C that allowed testing over a wide range of highly controlled temperatures. The tensile test was carried out using the international standards (ASTM, 2004). The test specimens employed for determining the stress–strain behaviour of the materials were dog-bone shape to reduce the cross-section area by dog-boning to force the failure to occur in the specimen mid-section. The test specimens were placed in between the upper and lower self-aligning mechanical grips, with the grips were placed 25 mm from each end. Minimum of 4 specimens were tested for each condition at room temperature of 25 ± 2 ◦ C and 50 ± 5% relative humidity. The test piece was setup in the testing machine with zero loads. The tensile tests were conducted at a cross-head speed of 3 mm/min. The elongation was measured at the parallel distance of the dog-boned test piece. The original gauge length is 25 ± 5% mm.
3.
Results and discussions
Fig. 1 shows SEM micrograph of untreated and heat treated A286 alloy. Untreated alloy shows almost equiaxed grain structure with the average grain diameter of about 30 mm. However, the grain size increases significantly after the heat treatment and uni-equiaxed grains with different sizes are formed. The finer grains are smaller than the original columnar grains. The increase in the grain size is responsible for low hardness after heat treatment process. It is also possible that ␥ depletion
near the grain boundary lowers the brittleness of the material after the heat treatment. The close examination of the grain boundaries suggests that locally scattered and discontinuous formation of carbides occur. Although grain boundary carbides formation increases the brittleness while lowering the ductility, locally discontinuous formation minimises this effect. Table 2 gives three-point bending test parameters and the results of the elastic modulus while Table 3 gives the tensile test results. It can be observed that the elastic module determined from the three-point bending and the tensile tests are in good agreement. The elastic limit of the alloy decreases significantly after the heat treatment process. The increase in ductility is attributed to the grain coarsening after the heat treatment. In addition, short duration heat treatment causes dissolution of ␥ and formation of -phase (hexagonal closed-packedhcp-Ni3 Ti phase) as seen from the XRD result in Fig. 2, lowering the mechanical performance of the material. The formation of -phase at the grain boundary can also act as the nucleation site of the crack where the material fails. This situation is observed during the three-point bending test, in particular at the tensile surface of the specimen. Fig. 3 shows temperature contours in the outer casing of the transition-piece obtained from the numerical computation. Temperature contours show that at the interface between the fluid and casing (gas–solid interface) temperature contours attains significantly high values while at the outer surface temperature is low due to the exposure of the outer surface to the atmospheric ambient. In the region close to the entry of the transition-piece (x = 0), temperature attains high val-
Fig. 1 – SEM micrograph of as-received and heat treated workpieces. As-received material. Heat treated material.
372
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
Fig. 2 – XRD results for the heat treated workpiece.
ues because of the convective heating by the hot gas and the radial heat conduction in this region. Consequently, temperature distribution varies significantly in the transition-piece because of the hot gas temperature variation.
3.1.
Mid-plane
Fig. 4 shows the von-Mises stress along the axial direction in the outer casing of the transition-piece at the inner wall next to the inner surface. It should be noted that the elastic modulus of 207 GPa corresponds to as-received substrate material while 138 GPa is determined from the tensile and the threepoint bending tests after the heat treatment process. It can be observed that von-Mises stress attains high values at the distance corresponding to the edges of the transition-piece. This is because of the attainment of the high temperature gradient in these regions and the mechanical constrains at both ends of the transition-piece. In the simulation, both ends of the transition-piece are fixed to resemble the actual situation; in which case, one end of the transition-piece is joined to the gas generator exit and the other is joined to the power turbine inlet. However, the magnitude of von-Mises stress reduces considerably for the heat treated case, which is due to the low value of the elastic modulus. Moreover, the behaviour of the von-Mises stress for both elastic modulus are similar with dif-
Fig. 4 – von-Mises stress along the axial direction in the outer casing for heat treated and as-received materials.
ferent magnitudes. In the case of mid-plane (Fig. 5), von-Mises stress attains high values at the ends of the transition-piece. Therefore, similar observations can be made to those discussed for Fig. 4. This is because of temperature distribution at the mid-plane, which is similar to temperature distribution of the inner-wall. von-Mises stress distribution along the axial distance in the region close to the outer wall differs from that of the mid-plane in the region close to the inner-plane. This is more pronounced at the fixed ends and the region near to it. In this case, the magnitude of von-Mises stress reduces significantly and the second peak in the von-Mises stress is observed in the region next to the fixed ends before attainment of the steady value in the axial direction. This is attributed to the outer casing exposure to the free atmospheric ambient. This in turn, provides more uniform like temperature distribution along the axial length of the transition-piece. In addition, at the outer surface of the transition-piece, stress free condition is considered in the simulations. This enables the free expansion of the transition-piece in the radial direc-
Fig. 3 – Temperature contours in the outer casing of the transition-piece.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 1 ( 2 0 0 8 ) 369–373
373
at the edges of the transition-piece; this is attributed to the fixed ends of the transition-piece and attainment of the high temperature gradient in this region. von-Mises stress level is significantly lower for the heat treated workpiece than that of the as-received material, which is because of the low elastic modulus of the heat treated workpiece. von-Mises stress in the region close to the outer surface of the transition piece attains low values because the radial expansion of the transitionpiece from the outer surface to the free atmospheric ambient.
references
Fig. 5 – von-Mises stress along the axial direction in the mid-plane of the casing for heat treated and as-received materials.
tion in the outer region while lowering the stress levels in the region close to the free surface.
4.
Conclusions
Flow through a transition-piece is simulated in relation to operational condition of aero-derivative gas turbine engine used in the mechanical drive application. Temperature distribution in the outer casing of the transition-piece is predicted and experimental work was carried out. The elastic modules obtained from the tensile and three-point bending tests for as-received and heat treated workpieces are used for thermal stress simulations. In order to observe the actual operation conditions of the transition-piece, the substrate material (A286 iron-base superalloy) used in the transitionpiece casing is heat treated at elevated temperature under the open ambient conditions for 2 h. It is found that the elastic modulus of the heat treated specimen is significantly lower than that of the as-received material. This is because of the grain coarsening and formation of -phase (hexagonal closed-packed-hcp-Ni3 Ti phase), which in turn improves the ductility of the heat treated material. Moreover, ␥ depletion through dissolution at grain boundaries contributes to the low mechanical performance of the material after the heat treatment process. von-Mises stress attains high values
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