- Email: [email protected]
PM technique for gas and steam turbines
Application of HIP/PM Technique for Gas and Steam Turbines Ragnar Ekbom (ABB STAL AB, S·612 20 Finspong, Sweden) In the gas and steam turbine industry strong efforts are being made to increase the output and the efficiency of the turbines and to lower the production costs. One way to achieve these goals is to increase the mechanical properties of the materials and to look for more efficient ways to fabricate parts for the turbines. In recent years the development in hot isostatic pressing and powder production has made it possible to produce materials used in gas and steam turbines with improved mechanical properties compared with the same type of materials produced as forgings. The benefit ofusing HIPIPM materials is not only improved properties but also in many cases cost reduction.
Due to the pre sent situation on the fossil fuel market with increasing prices and shortage of fuels in the future, there is a strong demand by the utilities to increase the output and efficiency of their turbines. This is valid both for gas turbines and steam turbines. On the other hand the turbine industry, which is today oversized and works under very strong competition, tries to meet that demand and to lower th eir production costs. One way to achieve these goals is to increase the mechanical properties of the mterials used for turbines and to look for more efficient and cost-effective ways to fabricate the ingoing parts. The properties of greatest interest for turbine parts are toughness, fatigue strength, good yield strength at elevated temperatures, good creep and creep rupture strengths and resistance to stress corrosion cracking. There are different methods of improving these properties. If one for instance increases the amount of alloying elements, it is pos sible to obtain better creep rupture properties, but at the same tim e the toughness or resistance to stress corrosion may be decreased. An increasing am ount of alloying elements will increase the costs so there will always be a compromise to find the most appropriate material for a given turbine part. Hot isostatic pressed (HIPed) powder materials have been used for more than ten years in disks for jet engines and in high speed steels. It has, however, been thought that the HIPed powder materials were too expensive to be used in more common applications. However, recent developments in facilities to produce powders and in hot isostatic presses has made it possible to produce clean powder at reasonable costs and to design presses big enough to make it possible to press large parts and with decreased pressing costs.
284
The benefits of HIP/PM material are: (1) Homogeneous material (2) Isotropic material (3) No segregations (4) Equal properties in all directions and in all locations (5) Fine grained and tough material (6) Possibility to make compound materials (7) Possibility to make parts with complicated design, for instance inner hollows and channels (8) Possibilty of cost reduction At ASEA Brown Boveri STAL [ABB STAL), we have done extensive research and development work during the last 10 years in
the field of HIP/PM technique. The goal of this work has been to find out the capability of using HIP/PM technique for parts for gas and steam turb ines with respect to improved mechanical properties and cost reduction. The development work has been done in close cooperation with our sister company ABB Powdermet (ASEA Powdermet). We hav e had PM parts in our turbines for five years and these parts are produced by ADD Powderrnet.
PRODUCfION PROCEDURE The manufacturing process of making PM parts used by ABBPowderrnet is shown in Fig. 1. The powder is produced from melting sto ck by nitrogen or argon gas atomization. The maximum powder diameter is 500 urn. The powder is in a transferred protected atmosphere to containers filled with nitrogen. Capacity of the powder production facility is 2.5 tons per heat. The powder is stocked in the containers under nitrogen protection. Cans are prepared from 1.5mm thick deep pressed steel sheet. The sheets are formed by spinning or pressing . After filling the cans with powder they are evacuated by vacuum pumping and thereafter sealed. Cans of varying sizes and forms are loaded in the hot isostatic press of the ASEA Quintus type. The press at Powderrnet has a loading diameter of 1450mm and a loading height of 3600mm. After loading the press is evacuated and filled with argon gas at a pressure of 50 MPa and the temperature is successively rais ed to 1150C-1250C depending on the type of powder. At full temperature the pressure is 100 MPa. The holding time at full temperature and pressure is 3 to 5 h. After that the pressure and temperature are reduced down to ambient pressure and temperature and the press is unloaded. The cans, which now have reduced in volume 30% and are 100% dense, are ready for ' thermal treatment and machining.
FIG. 1 Manufacturing PM process at ABB Powdermet
MPR April 1990
APPLICATIONS 12% Chromium Steels In the turbine industry 12% chromium steels are commonly used both in steam turbines and in gas turbines. The reasons for this are that they have good creep rupture strength up to about 600C as well as high yield strength and good toughness at lower temperatures. At the same time they have good thermal conductivity and low coefficient of thermal expansion. This is of importance because it gives low thermal stresses at start and stop of the turbines. With the right composition and heat treated in a proper manner, they also have reasonable resistance to stress corrosion cracking, which is essential for application in steam turbines. At ABBSTAL we use the 12% chromium steels both for parts in radial flow and axial flow steam turbines as well as in gas turbines.
Radial Flow Counter-Rotating Steam Turbine At ABB STAL we manufacture a special type of steam turbine called the counter-rotating radial flow turbine. the principle of which is seen in Fig. 2. There are two counter-rotating disks, each connected with an alternator. On each disk a number of blading rings are mounted. The steam flows from the centre in a radial direction through the blading rings, thus driving the disk to rotate. One ofthe disks rotates clockwise and the other rotates in the opposite direction.
FIG. 4 Sketch ofholV blading rings are built
2390 (req.)
%
min max
C
51
Mn P
0.20 0.25
0.10 O.SO
0.30 0.80
5
Cr
11.5 0.035 0.035 12.5
NI
Mo
V
O.W
1.0 1.4
0.25 0.35
Ni
Mo
V
0.39
1.09
0.29
Nb
N 0.030
2390 (test)
%
C
5i
Mn P
0.22
0.36
0.59
5
Cr
0.017 0.011 11.8
AS
Z
KV
~.
J
min 16
min SO
min 49
19.6
56.5
60
RpO.2
Rm
MPa
MPa
0;'
2390 (req.) 620/74{J
750/930
2390 (test) 708
838
Nb
N 0.060
HB 240/275
FIG. 5 Composition and room temperature properties of the steel 2390
FIG. 2 Principle of a radial flo IV steam turbine
..
FIG. 3 Blading ring MPR Apri11990
Blading Rings The blading rings are built from two rings between which the blades are mounted. Fig. 3 shows a blading ring and Fig . 4 a sketch of how it is built. The rings as well as the blades are made of a 12% chromium steel with the ABB ST AL designation 2390. The composition and room temperature properties of the steel are given in Fig. 5. For the manufacturing process and the function of the turbine it is important to have a material in the rings with good toughness at room temperature. good fatigue strength. good creep strength and creep rupture strength and relatively high yield strength. It is not so easy to combine high yield stress and good creep rupture strength with good toughness at room temperature. For forged rings we had difficulties in achieving the goals and this led to long delivery times and high costs. Consequently we tried to make the rings from PM/HIP material. The results have been very successful. Wt> not only increased the toughness of the ring material. but also the fatigue strength. and the creep rupture strength. In addition, the delivery times and costs were reduced. Up to now. about 700 rings have been manufactured with diameters from 300 to
1100mm. Defore selling any rings in operations, an extensive mechanical testing programme was performed. From the PM/HIP rings about 200 impact tests have been done. Fig 6 shows a comparison between the impact strength of PM/HIP rings and forged rings . As can be seen the PM/HIP rings have the same impact strength in all directions and also higher impact strength than the forged rings. The latter have a lower impact strength in test pieces taken in a radial direction compared with test pieces taken in longitudinal direction. As mentioned above. fatigue strength is essential for these rings. More than 400 test pieces from PM/HIP rings have been fatigue tested and the results are shown in Fig. 7. The fatigue strength is improved by about 25% for the PM rings compared with forged rings. The creep rupture strength of PM/HIP rings has been tested at550C and 600C. Up to now there are 122 test data and maximum time to rupture is 42,725 hr. In Fig. 8 a comparison is made between the creep rupture strength of PM/HIP material and forged material. As can be seen there is an increase in the creep rupture strength of the PM/HIP material both at 550C and 600C. 285
DISKS The disks for the radial 11 ow turbines are also made from the 12% chromium steel used for the rings. Disks have been manufactured in two ways. One way is to make a blank by PM/HIP which is subsequently forged to size. The other way is to HIP the powder material to near net shape. Disks were made in both ways and cut up tested. Test pieces arc taken at the rim and at the centre in both radial and tangential directions. Fig. 9 shows the yield stress as a function of temperature for the cut up tests and the request due to the material specification. As can be seen the properties are equal for the two ways of manufacturing and well within the specification. The impact temperature transition curves for the cut up test of the PM/HIP + forged disk and the PM/HIP near net shape disk are shown in Fig. 10. There is a higher transition temperature for the test pieces taken from the centre of the PM/HIP + forged disk. This is probably due to the higher yield stress of this disk and the heat treatment, and not to the manufacturing process. Fatigue tests have also been performed for the cut up disks. In Fig. 11 the results of these tests are compared with the results of the fatigue testing of forged rings and PM/HIP rings. The
99
2
at
FIG. 6 Impact strength offorged and PM/HIP rings
450 400 350
,
2390
STRESS-RUPTURE (LARSON-MILLER)
Fatigue strength
,
~
""
PowdermaL
F~DlnD
250 116-J-~~~i=$=ir==i==i=i=i~-i-+-++i-Hii-+-i:-H-HHi • 1000 - 10000 100 100000 10
10S
•
N cycles
FIG. 7 Fatigue strength of12% chromium steel PM/HIP rings and forged rings
FIG. 8 Creep rupture strength at 550C and 600C of PM/HIP and forged 12% chromium steel
800
700 -lIE- 239093
....... o
.... 239093CR7
600
...239093PT7
~5oo
-4-
'-'
239093PR7
~239093PT2
N
0a. 400
-e- 239093CT2
~
.... 239093TA2 300
2oo'-t---.----'--,r-----,----.------,r----.---,
o
100
200
JOO
400
temperature (C)
286
500
600
700
TIME TO RUPTURE (h)
FIG. 9 Yield stress as function of temperature for cut up tests from PMIHIP + forged disk and PM/HIP near net shape disk (239093 relates to 12% chromium steel. c =centre. P = periphery. R = radw/, T= tangential. 2 = PM/HIP near net shape, 7 = PM/HIP + forged)
PM/HIP + forged disk shows afatigue strength of the same order as the PM/HIP rings. The PM/HIP to near net shape disk shows a lower fatigue strength, but well above the fatigue strength of the forged rings. The lower fatigue strength of the PM/HIP ncar net shape disk may be due to some lower tensile strength of this disk. Fig. 12 shows a disk before being cut up. In our gas turbines we also use disks, but with some other design. The disks are without shafts. Fig. 13 shows a cut up view of a 15 MW gas turbine. Some of the disks are also made in a 12% chromium steel, but with a different composition to that used in the steam turbine disk and with a yield stress of minimum 780 , N/mm 2 at room temperature. Cut up tests are made from a forged disk and a disk PM/HIP + forged.
MPR April 1990
100
a
"U
N/mn2 ,....,. 80
....,
--
+
~
s:
239093CR7
... 239093PT7
~ 60
450 400
......239093PR7
Q)
L.
......239093PR2
CIl
0
-e- 239093PT2
40
c
-B- 239093CT2
E
+- 239093TA2
0-
.- 20
350 0PT7 Q)PR7, CR7 Q)PTZ
., ~
""
"
2390 Fatigue strength
=\..1...1 ~
_Powdermlt.
I
Ino
250
O+--r-----.-..---,--r---r-.---r--r-.------, -40 -20
0
20
40
60
80
100 120
140 160
180
temperature (C) FIG. 10 Impact temperature transition curves for cut up tests from PM/HIP + forged and PM/HIP near net shape disks
10S N cycles
FIG. 11 Fatigue strength ofcut up PM/HIP + forged and PM/HIP near net shape disks compared witli rings
FIr.. 12 Hodiol ll o.,' steotn turbine d isk
FIG. 1:1 Cut up ·ie .v uj 15.
I' go turbine
...
~.
...
MPR April 1990
287
1,000 100 900
..., .....,
........ 80
. . 239193 800
........
..... 239193 CR8
0..
... 239193 PT8
==g>
... 239193 PR8
~
c
:::E
700
0
600
'-' N
a.
-e- 239193
0
200
300
400
500
600
239193XPR
... 239193CR8
700
Iernpercture (C)
... 239193PT8
40
c
...... 239193PR8
a.
.~
500
100
*
CI>
C/)
SPT
+ 239193 SPR
0::
. . 239193XCR 60
20
O+---,,----,--,...--,--...,---,--r---,-.....----,,--. -40 -20
0
20
40
60
80
100
120
140
160
180
temperoture (C)
FIG. 14 Yield stress as a function of temperature for cut u'f tests ofgas turbine disks (P =rim, c =centre, R =radial, T =tangentia , 8 =PM/HIP + forged disk, S = forged disk)
FIG. 15 Impact strength-temperature transition curves for forged disk and PM/HIP + forged disk ( x = forged disk, 8 = PM/HIP + forged disk)
STEAM VALVE CASINGS The casings forthe steam valves are normally made from low alloy steel castings. Some casings have been made by PM/HIP near net shape. Fig. 16 shows a valve casing made from powder. The steel in this casing is a low alloy 2.25% Cr 1% Mo steel which has good creep rupture strength up to about 550C. Fig. 17 shows the yield stresses as a function of temperature forthe PM/HIP valve casing, and the properties are compared with the request of yield stresses as per the materials specification. There is a remarkable increase in yield stress for the PM material. For the creep rupture properties there is an even stronger increase especially at 550C. As can be seen from Fig. 18. the creep rupture strength at 550C and 20,000 hours rupture time for the PM material is as good as the creep rupture strength at 500C and 20,000 hours rupture time for forged or cast material of the same composition. Other valve parts made from PM/HIP material are bushings and valve seats. The bushings are made in an alloy with good wear and sliding properties and the valve seats are made as composites from a low alloy steel and a wear and erosion resistant material. Bushings are seen in Fig. 19 and
FIG. 16 Steam valve casing
I-+- FORGED MATERIAL! I "* POWDER MATERIAL I
611
~
511
ell
e;;-
)(
)(
o,
~ 311
'" '" ~
....
211
III .;
:-';
i
~1804.5OOC
Powder material
I+-.---,r--r--.--,--.---,r--r--.--,-----, I
11.
lH
211
2H
311
3H
Temperature (C)
FIG. 17 Yield stressesas a function oftemperature for PM/HIP material compared \I'ith the material specification properties
288
10 I--r-++ri+iri---l--+-ri-+~-t__;....;..+~;.-...;.-~~~---' , iii ill Ii i t diL, l I I 111111 I I I i IlHI 10
50 100
500, 1000
_~
recoo
50000 10ססoo
TIme to rupture (H)
FIG. 18 Creep rupture strength of PM/HIP 2.25% CR 1% Mo steel compared with [otged material MPR April 1990
FIG. 19 \'ah'e bushings
FIG. 21 Steam turbine
FIG. 20 \'aII'e seats valve scats in Fig. 20. In both cases the properties are increased and the costs are reduced. Fig. 21 shows a sectional view of an axial flow steam turbine comprising a rotor with its disks and blades. Between two rotating blade rows are stationary vanes to guide the steam in the right direction, A row of stationary vanes is also called a diaphragm. The diaphragms are normally fabricated by welding the vanes to an inner and an outer forged ring. By the PM/HIP route it is possible to make the rings and the vanes in one operation and without any welding. By using a special core technique it is possible to make the steam channel to very close tolerances without any machining after hot isostatic pressing. By making the diaphragms by PM/HIP the number of operations can be reduced from 27 to 13 and the costs reduced by between 40 and 50% depending on sizes. To prevent the steam entering the space between the rotor shaft and the diaphragms there arc a lot of shaft seals. Figure 22 shows a shaft seal which is divided in four parts, By a special technique these seal parts direct by the hot isostatic prossings. This means that the number of operations can be reduced from 13 to 7 and the cost reduced by more than 25% ..
CONCLUSIONS After having produced several hundred of pieces of different sizes and shapes for steam and gas turbines by PM/HIP techniques. it is possible to draw the following conclusions: (1) It is possible not only to get equal
mechanical properties but in many cases to strongly increase the properties. (2) It is possible to reduce the production costs by more than 50%. (3) The production time can in many cases be reduced.
The PM/HIP technique gives the possibility to produce products not possible by any other technique. (4)
FIG. 22 Shaft sealing for steom turbine
MPR April 1990
(5) There is a very good potential for the PM/HIP technique in many areas.
289
Due to the pre sent situation on the fossil fuel market with increasing prices and shortage of fuels in the future, there is a strong demand by the utilities to increase the output and efficiency of their turbines. This is valid both for gas turbines and steam turbines. On the other hand the turbine industry, which is today oversized and works under very strong competition, tries to meet that demand and to lower th eir production costs. One way to achieve these goals is to increase the mechanical properties of the mterials used for turbines and to look for more efficient and cost-effective ways to fabricate the ingoing parts. The properties of greatest interest for turbine parts are toughness, fatigue strength, good yield strength at elevated temperatures, good creep and creep rupture strengths and resistance to stress corrosion cracking. There are different methods of improving these properties. If one for instance increases the amount of alloying elements, it is pos sible to obtain better creep rupture properties, but at the same tim e the toughness or resistance to stress corrosion may be decreased. An increasing am ount of alloying elements will increase the costs so there will always be a compromise to find the most appropriate material for a given turbine part. Hot isostatic pressed (HIPed) powder materials have been used for more than ten years in disks for jet engines and in high speed steels. It has, however, been thought that the HIPed powder materials were too expensive to be used in more common applications. However, recent developments in facilities to produce powders and in hot isostatic presses has made it possible to produce clean powder at reasonable costs and to design presses big enough to make it possible to press large parts and with decreased pressing costs.
284
The benefits of HIP/PM material are: (1) Homogeneous material (2) Isotropic material (3) No segregations (4) Equal properties in all directions and in all locations (5) Fine grained and tough material (6) Possibility to make compound materials (7) Possibility to make parts with complicated design, for instance inner hollows and channels (8) Possibilty of cost reduction At ASEA Brown Boveri STAL [ABB STAL), we have done extensive research and development work during the last 10 years in
the field of HIP/PM technique. The goal of this work has been to find out the capability of using HIP/PM technique for parts for gas and steam turb ines with respect to improved mechanical properties and cost reduction. The development work has been done in close cooperation with our sister company ABB Powdermet (ASEA Powdermet). We hav e had PM parts in our turbines for five years and these parts are produced by ADD Powderrnet.
PRODUCfION PROCEDURE The manufacturing process of making PM parts used by ABBPowderrnet is shown in Fig. 1. The powder is produced from melting sto ck by nitrogen or argon gas atomization. The maximum powder diameter is 500 urn. The powder is in a transferred protected atmosphere to containers filled with nitrogen. Capacity of the powder production facility is 2.5 tons per heat. The powder is stocked in the containers under nitrogen protection. Cans are prepared from 1.5mm thick deep pressed steel sheet. The sheets are formed by spinning or pressing . After filling the cans with powder they are evacuated by vacuum pumping and thereafter sealed. Cans of varying sizes and forms are loaded in the hot isostatic press of the ASEA Quintus type. The press at Powderrnet has a loading diameter of 1450mm and a loading height of 3600mm. After loading the press is evacuated and filled with argon gas at a pressure of 50 MPa and the temperature is successively rais ed to 1150C-1250C depending on the type of powder. At full temperature the pressure is 100 MPa. The holding time at full temperature and pressure is 3 to 5 h. After that the pressure and temperature are reduced down to ambient pressure and temperature and the press is unloaded. The cans, which now have reduced in volume 30% and are 100% dense, are ready for ' thermal treatment and machining.
FIG. 1 Manufacturing PM process at ABB Powdermet
MPR April 1990
APPLICATIONS 12% Chromium Steels In the turbine industry 12% chromium steels are commonly used both in steam turbines and in gas turbines. The reasons for this are that they have good creep rupture strength up to about 600C as well as high yield strength and good toughness at lower temperatures. At the same time they have good thermal conductivity and low coefficient of thermal expansion. This is of importance because it gives low thermal stresses at start and stop of the turbines. With the right composition and heat treated in a proper manner, they also have reasonable resistance to stress corrosion cracking, which is essential for application in steam turbines. At ABBSTAL we use the 12% chromium steels both for parts in radial flow and axial flow steam turbines as well as in gas turbines.
Radial Flow Counter-Rotating Steam Turbine At ABB STAL we manufacture a special type of steam turbine called the counter-rotating radial flow turbine. the principle of which is seen in Fig. 2. There are two counter-rotating disks, each connected with an alternator. On each disk a number of blading rings are mounted. The steam flows from the centre in a radial direction through the blading rings, thus driving the disk to rotate. One ofthe disks rotates clockwise and the other rotates in the opposite direction.
FIG. 4 Sketch ofholV blading rings are built
2390 (req.)
%
min max
C
51
Mn P
0.20 0.25
0.10 O.SO
0.30 0.80
5
Cr
11.5 0.035 0.035 12.5
NI
Mo
V
O.W
1.0 1.4
0.25 0.35
Ni
Mo
V
0.39
1.09
0.29
Nb
N 0.030
2390 (test)
%
C
5i
Mn P
0.22
0.36
0.59
5
Cr
0.017 0.011 11.8
AS
Z
KV
~.
J
min 16
min SO
min 49
19.6
56.5
60
RpO.2
Rm
MPa
MPa
0;'
2390 (req.) 620/74{J
750/930
2390 (test) 708
838
Nb
N 0.060
HB 240/275
FIG. 5 Composition and room temperature properties of the steel 2390
FIG. 2 Principle of a radial flo IV steam turbine
..
FIG. 3 Blading ring MPR Apri11990
Blading Rings The blading rings are built from two rings between which the blades are mounted. Fig. 3 shows a blading ring and Fig . 4 a sketch of how it is built. The rings as well as the blades are made of a 12% chromium steel with the ABB ST AL designation 2390. The composition and room temperature properties of the steel are given in Fig. 5. For the manufacturing process and the function of the turbine it is important to have a material in the rings with good toughness at room temperature. good fatigue strength. good creep strength and creep rupture strength and relatively high yield strength. It is not so easy to combine high yield stress and good creep rupture strength with good toughness at room temperature. For forged rings we had difficulties in achieving the goals and this led to long delivery times and high costs. Consequently we tried to make the rings from PM/HIP material. The results have been very successful. Wt> not only increased the toughness of the ring material. but also the fatigue strength. and the creep rupture strength. In addition, the delivery times and costs were reduced. Up to now. about 700 rings have been manufactured with diameters from 300 to
1100mm. Defore selling any rings in operations, an extensive mechanical testing programme was performed. From the PM/HIP rings about 200 impact tests have been done. Fig 6 shows a comparison between the impact strength of PM/HIP rings and forged rings . As can be seen the PM/HIP rings have the same impact strength in all directions and also higher impact strength than the forged rings. The latter have a lower impact strength in test pieces taken in a radial direction compared with test pieces taken in longitudinal direction. As mentioned above. fatigue strength is essential for these rings. More than 400 test pieces from PM/HIP rings have been fatigue tested and the results are shown in Fig. 7. The fatigue strength is improved by about 25% for the PM rings compared with forged rings. The creep rupture strength of PM/HIP rings has been tested at550C and 600C. Up to now there are 122 test data and maximum time to rupture is 42,725 hr. In Fig. 8 a comparison is made between the creep rupture strength of PM/HIP material and forged material. As can be seen there is an increase in the creep rupture strength of the PM/HIP material both at 550C and 600C. 285
DISKS The disks for the radial 11 ow turbines are also made from the 12% chromium steel used for the rings. Disks have been manufactured in two ways. One way is to make a blank by PM/HIP which is subsequently forged to size. The other way is to HIP the powder material to near net shape. Disks were made in both ways and cut up tested. Test pieces arc taken at the rim and at the centre in both radial and tangential directions. Fig. 9 shows the yield stress as a function of temperature for the cut up tests and the request due to the material specification. As can be seen the properties are equal for the two ways of manufacturing and well within the specification. The impact temperature transition curves for the cut up test of the PM/HIP + forged disk and the PM/HIP near net shape disk are shown in Fig. 10. There is a higher transition temperature for the test pieces taken from the centre of the PM/HIP + forged disk. This is probably due to the higher yield stress of this disk and the heat treatment, and not to the manufacturing process. Fatigue tests have also been performed for the cut up disks. In Fig. 11 the results of these tests are compared with the results of the fatigue testing of forged rings and PM/HIP rings. The
99
2
at
FIG. 6 Impact strength offorged and PM/HIP rings
450 400 350
,
2390
STRESS-RUPTURE (LARSON-MILLER)
Fatigue strength
,
~
""
PowdermaL
F~DlnD
250 116-J-~~~i=$=ir==i==i=i=i~-i-+-++i-Hii-+-i:-H-HHi • 1000 - 10000 100 100000 10
10S
•
N cycles
FIG. 7 Fatigue strength of12% chromium steel PM/HIP rings and forged rings
FIG. 8 Creep rupture strength at 550C and 600C of PM/HIP and forged 12% chromium steel
800
700 -lIE- 239093
....... o
.... 239093CR7
600
...239093PT7
~5oo
-4-
'-'
239093PR7
~239093PT2
N
0a. 400
-e- 239093CT2
~
.... 239093TA2 300
2oo'-t---.----'--,r-----,----.------,r----.---,
o
100
200
JOO
400
temperature (C)
286
500
600
700
TIME TO RUPTURE (h)
FIG. 9 Yield stress as function of temperature for cut up tests from PMIHIP + forged disk and PM/HIP near net shape disk (239093 relates to 12% chromium steel. c =centre. P = periphery. R = radw/, T= tangential. 2 = PM/HIP near net shape, 7 = PM/HIP + forged)
PM/HIP + forged disk shows afatigue strength of the same order as the PM/HIP rings. The PM/HIP to near net shape disk shows a lower fatigue strength, but well above the fatigue strength of the forged rings. The lower fatigue strength of the PM/HIP ncar net shape disk may be due to some lower tensile strength of this disk. Fig. 12 shows a disk before being cut up. In our gas turbines we also use disks, but with some other design. The disks are without shafts. Fig. 13 shows a cut up view of a 15 MW gas turbine. Some of the disks are also made in a 12% chromium steel, but with a different composition to that used in the steam turbine disk and with a yield stress of minimum 780 , N/mm 2 at room temperature. Cut up tests are made from a forged disk and a disk PM/HIP + forged.
MPR April 1990
100
a
"U
N/mn2 ,....,. 80
....,
--
+
~
s:
239093CR7
... 239093PT7
~ 60
450 400
......239093PR7
Q)
L.
......239093PR2
CIl
0
-e- 239093PT2
40
c
-B- 239093CT2
E
+- 239093TA2
0-
.- 20
350 0PT7 Q)PR7, CR7 Q)PTZ
., ~
""
"
2390 Fatigue strength
=\..1...1 ~
_Powdermlt.
I
Ino
250
O+--r-----.-..---,--r---r-.---r--r-.------, -40 -20
0
20
40
60
80
100 120
140 160
180
temperature (C) FIG. 10 Impact temperature transition curves for cut up tests from PM/HIP + forged and PM/HIP near net shape disks
10S N cycles
FIG. 11 Fatigue strength ofcut up PM/HIP + forged and PM/HIP near net shape disks compared witli rings
FIr.. 12 Hodiol ll o.,' steotn turbine d isk
FIG. 1:1 Cut up ·ie .v uj 15.
I' go turbine
...
~.
...
MPR April 1990
287
1,000 100 900
..., .....,
........ 80
. . 239193 800
........
..... 239193 CR8
0..
... 239193 PT8
==g>
... 239193 PR8
~
c
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700
0
600
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200
300
400
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Iernpercture (C)
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0
20
40
60
80
100
120
140
160
180
temperoture (C)
FIG. 14 Yield stress as a function of temperature for cut u'f tests ofgas turbine disks (P =rim, c =centre, R =radial, T =tangentia , 8 =PM/HIP + forged disk, S = forged disk)
FIG. 15 Impact strength-temperature transition curves for forged disk and PM/HIP + forged disk ( x = forged disk, 8 = PM/HIP + forged disk)
STEAM VALVE CASINGS The casings forthe steam valves are normally made from low alloy steel castings. Some casings have been made by PM/HIP near net shape. Fig. 16 shows a valve casing made from powder. The steel in this casing is a low alloy 2.25% Cr 1% Mo steel which has good creep rupture strength up to about 550C. Fig. 17 shows the yield stresses as a function of temperature forthe PM/HIP valve casing, and the properties are compared with the request of yield stresses as per the materials specification. There is a remarkable increase in yield stress for the PM material. For the creep rupture properties there is an even stronger increase especially at 550C. As can be seen from Fig. 18. the creep rupture strength at 550C and 20,000 hours rupture time for the PM material is as good as the creep rupture strength at 500C and 20,000 hours rupture time for forged or cast material of the same composition. Other valve parts made from PM/HIP material are bushings and valve seats. The bushings are made in an alloy with good wear and sliding properties and the valve seats are made as composites from a low alloy steel and a wear and erosion resistant material. Bushings are seen in Fig. 19 and
FIG. 16 Steam valve casing
I-+- FORGED MATERIAL! I "* POWDER MATERIAL I
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Powder material
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211
2H
311
3H
Temperature (C)
FIG. 17 Yield stressesas a function oftemperature for PM/HIP material compared \I'ith the material specification properties
288
10 I--r-++ri+iri---l--+-ri-+~-t__;....;..+~;.-...;.-~~~---' , iii ill Ii i t diL, l I I 111111 I I I i IlHI 10
50 100
500, 1000
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50000 10ססoo
TIme to rupture (H)
FIG. 18 Creep rupture strength of PM/HIP 2.25% CR 1% Mo steel compared with [otged material MPR April 1990
FIG. 19 \'ah'e bushings
FIG. 21 Steam turbine
FIG. 20 \'aII'e seats valve scats in Fig. 20. In both cases the properties are increased and the costs are reduced. Fig. 21 shows a sectional view of an axial flow steam turbine comprising a rotor with its disks and blades. Between two rotating blade rows are stationary vanes to guide the steam in the right direction, A row of stationary vanes is also called a diaphragm. The diaphragms are normally fabricated by welding the vanes to an inner and an outer forged ring. By the PM/HIP route it is possible to make the rings and the vanes in one operation and without any welding. By using a special core technique it is possible to make the steam channel to very close tolerances without any machining after hot isostatic pressing. By making the diaphragms by PM/HIP the number of operations can be reduced from 27 to 13 and the costs reduced by between 40 and 50% depending on sizes. To prevent the steam entering the space between the rotor shaft and the diaphragms there arc a lot of shaft seals. Figure 22 shows a shaft seal which is divided in four parts, By a special technique these seal parts direct by the hot isostatic prossings. This means that the number of operations can be reduced from 13 to 7 and the cost reduced by more than 25% ..
CONCLUSIONS After having produced several hundred of pieces of different sizes and shapes for steam and gas turbines by PM/HIP techniques. it is possible to draw the following conclusions: (1) It is possible not only to get equal
mechanical properties but in many cases to strongly increase the properties. (2) It is possible to reduce the production costs by more than 50%. (3) The production time can in many cases be reduced.
The PM/HIP technique gives the possibility to produce products not possible by any other technique. (4)
FIG. 22 Shaft sealing for steom turbine
MPR April 1990
(5) There is a very good potential for the PM/HIP technique in many areas.
289