mechanical properties were assessed on a forged disk heat-treated in industrial ... after solution heat-treatment but can be modified or adjusted with the aging ...
8th International Sym posium on Superalloy 718 and D erivatives E dited by: E ric Ott, A nthony Banik, X ingbo Liu, Ian Dempster, K arl Heck, Jo el Andersson, Jon Groh, Tim Gabb, R andy Helmink, and A gnieszka W usatowska-Sarnek
TM S (The Minerals, Metals & Materials Society), 2014
EFFECT OF AGING HEAT-TREATMENT ON MECHANICAL PROPERTIES OF AD730™ SUPERALLOY A. Devaux1, A. Helstroffer2, J. Cormier2, P. Villechaise2, J. Douin3, M. Hantcherli3, F. 3 Pettinari-Sturmel 1Aubert & Duval, Site des Ancizes, BP1, 63770 les Ancizes Cedex, France 2 Institut Pprime, CNRS - ENSMA - Universite de Poitiers, UPR CNRS 3346; Departement Physique et Mecanique des Materiaux, ISAE-ENSMA; 1 avenue Clement Ader, BP 40109; 86961 Futuroscope Chasseneuil Cedex, France 3CEMES/CNRS, 29 rue Jeanne Marvig, 31055 Toulouse Cedex, France Keywords: AD730TM, aging, gamma prime, tensile, creep Abstract AD730 tm was designed to be a cost effective superalloy for high temperature turbine disks. This superalloy, strengthened by gamma prime precipitates, presents a good ability for cast and wrought process route and a high combination between high temperature mechanical properties and cost. A study was performed to optimize the aging sequence considering tensile, creep and dwell-fatigue crack growth rate properties. Isothermal aging in the 730-790°C range with various durations in the 4-16h range were performed after a 1080°C/4h solution heat treatment followed by air cooling on coupons to investigate the effects of temperature and time on gamma prime precipitation and on mechanical properties. The effect o f a second additional aging step (700°C/8h) was also investigated and discussed. Results show that the strengthening peak during aging is close to 730°C. Above this aging temperature, tensile strength decreases due to a slight increase of secondary gamma prime diameter. Gamma prime precipitates remain however very fine for all aging sequences with a diameter in the 20-50nm range. Creep properties at 700°C were strongly decreased by aging temperatures above 760°C. A special attention was also paid to dwell-fatigue crack growth rate behavior. Relaxation tests at 650°C were performed to compare the different microstructures assuming that best behavior during crack propagation would be obtained with microstructures that promote the highest stress relaxation magnitude. Finally, mechanical properties were assessed on a forged disk heat-treated in industrial conditions with the selected aging 730°C/8h/Air and were at the expected level. Introduction Development of nickel-base superalloys for turbine disk applications has focused on increasing the volume fraction of the gamma prime phase. Such increases in the volume fraction, combined with increased levels of refractory elements, have led to difficulties in the processing of these alloys. Therefore, many modern turbine disk alloys are produced via the powder metallurgy (P/M) route. AD730TM is a commercial nickel-base superalloy designed for high temperature rotating applications designed to offer a unique combination between cost and mechanical properties [1-3]. As for many other nickel-base superalloys, AD730TM exhibits good mechanical strength at high temperatures thanks to a coherent precipitation of the L12 ordered gamma prime phase in a gamma matrix. A special attention was also paid to its ability for the Cast & Wrought processing route in order to avoid the expensive P/M route [3]. AD730™ presents therefore a lower gamma prime fraction and solvus than those of UdimetTM 720Li (denoted as U720 in the rest of the paper) in order to preserve a good workability and a good ability for the C&W route (see Fig. 1). y’ particles exist in this alloy as primary gamma prime (typically 1-10 p,m) present at 521
grain boundaries and secondary gamma prime (50-500 nm) or occasionally very fine tertiary gamma prime (< 50 nm) both present within the grains. The primary y’ particles are formed during the forging operations and can control the grain size during solution heat-treatment if the temperature is below the y’-solvus. The secondary y’ particle size varies inversely with the overall cooling rate as well reported in literature [4-12]. At high cooling rates, only one size of y’ is formed. Slower cooling rates may result in a bimodal distribution of y’ particles. Larger precipitates form on cooling and are termed secondary y’ while finer tertiary y’ forms during the latter part of the cooling cycle. These secondary and tertiary y’ precipitates have a high strengthening effect and provide to the alloy its mechanical properties. For a given y’ volume fraction, there is an optimum precipitate size and distribution of y’ associated with maximum strength. The size and distribution of the precipitates is partly determined by the cooling rate after solution heat-treatment but can be modified or adjusted with the aging sequence made after the solution heat-treatment [13-17]. The aim of this paper is to study the effect of the aging heat treatment on y’ precipitation and mechanical properties of AD730TM
50%
< uiD Q. V-
o E O
30%
O
20%
4 -
re
10 %
0% 600
700
800
900
1000
1100
1200
Temperature (°C) Figure 1. Gamma prime fraction versus temperature calculated with ThermoCalc
Experimental procedure A double melt AD730TM 8’’ billet made at Aubert & Duval - Les Ancizes was used in this study. The chemical composition of the alloy is given in Table I. Table 1. Chemical analysis (wt%) of AD730TM 8’’ billet
AD730 tm
Ni
Fe
Base
4
Co 8.5
Cr 15.7
Mo 3.1
W
Al
Ti
2.7
2.25
3.4
3
Nb
B
C
1.1 0.01 0.015
Zr
S
0.03 0.0004
Coupons (15x15x70 mm ) were machined out from the billet and solution heat-treated at 1080°C during 4 hours and then cooled in air. Primary y’ precipitates were partly dissolved with a remaining fraction of particles close to 6%. The cooling rate after solution heat-treatment was estimated to be close to 200°C/min. Following cooling, isothermal aging in the 730-790°C range 522
with various durations in the 4-16 hours range were performed on samples and followed by air cooling. Observations were done after mechanical polishing up to a mirror finish. The y’ phase distribution was revealed by selective etching of the y’ particles, using a solution made of 1/3 HNO 3 , 2/3 HCl at 4°C for 5 to 10s. y’ precipitates were observed using a field emission gun SEM (SEM-FEG) JEOL JSM 7000F operating at 25 kV, using magnifications up to 200,000. Stereological analyzes of the gamma prime phase were performed using Visilog® software. Both area fraction and y’ particles size distribution were obtained using an algorithm described in a study [16]. For each studied condition, gamma prime size distribution was based on a statistic including at least 5000 particles. Tensile tests were carried out at 700°C under strain rate control, using a strain rate of 5.0 10- 3 min - 1 up to the 0.2% yield stress and a subsequent strain rate of 5.0 10- 1 min - 1 until failure. Creep and stress-rupture tests were respectively performed in air under constant load at 700°C under an initial stress equal to 690 MPa and 500 MPa. TEM observations were performed using standard foil preparation techniques and a JE0L-2010 microscope operating at 200 kV to observe the dislocation substructures. Foils were extracted from specimens crept at 700°C under a stress of 500 MPa and interrupted under load. Relaxation tests were performed at 650°C with an applied strain equal to 0.8% during one hour after having performed 1000 cycles under a 0.8% total strain control and strain ratio equal to 0, a frequency equal to 0.1 Hz and a triangular wave form. Relaxation tests were performed to compare the different microstructures assuming that slowest crack propagation rate would be obtained with microstructures that promote the highest stress relaxation during the dwell time.
Results and discussion Microstructure before aging Average grain size after solution heat-treatment 1080°C/4h/AC was close to ASTM 9 with some larger isolated grains (Fig. 2). Undissolved primary y’ precipitates, with a diameter included in the 1-10 |im range, were observed at grain boundaries. As expected, homogeneous secondary gamma prime precipitates were observed within the grains before aging. The average diameter of secondary y’ precipitates was estimated to be close to 27 nm. Effect of aging temperature Aging temperature was investigated for a constant duration equal to 8 hours in the 730-790°C temperature range (Table II). As shown in figure 3, the effect of aging temperature on the y’ precipitation is not obvious and average size of y’ precipitates is globally quite similar (25-40 nm) independently o f the aging temperature. y’ distribution calculated by image analysis (Fig. 4) indicated the presence of only one y’ population considered as secondary y’ precipitates due to their presence before aging. These analyses also indicate a slight coarsening of gamma prime precipitates above 730°C.
523
Figure 2. Microstructure after solution heat-treatment and before aging
Table 2. Results obtained with different aging temperatures for duration equal to 8 hours Aging sequence Gamma prime
None
730°C/8h/AC
760°C/8h/AC
790°C/8h/AC
Average diam (nm)
27
28
35
33
Tensile 700°C
None
730°C/8h/AC
760°C/8h/AC
790°C/8h/AC
UTS (MPa)
1236
1267
1224
1207
YS (MPa) El. 4d (%)
1022 16
1123 21
1102 22
1066 23
Creep 700°C/690MPa
None
730°C/8h/AC
760°C/8h/AC
790°C/8h/AC
Steady creep rate (h-1 )
1.6 E-5
2.14 E-5
4.61 E-5
2.48 E-4
t r (h) Elongation (%)
333
268
220
100
8
9
8
17
Creep 700°C/500MPa
None
730°C/8h/AC
760°C/8h/AC
790°C/8h/AC
Creep strain (%) 300h
ND
0.07
0.1
0.62
Steady creep rate (h-1 )
ND
1.65 E-6
3.22 E-6
1.97 E-5
Relaxation 650°C
None
730°C/8h/AC
760°C/8h/AC
790°C/8h/AC
Stress relaxation during 1 hour with s=0.8% (MPa)
ND
72
116
141
Tensile tests performed at 700°C shows that aging heat-treatment improves the tensile strength. Tensile strength decreases with the aging temperature but the yield strength (YS) remains always above 1050 MPa at 700°C. Tensile strength variations seem caused by the differences of y’ fraction which is increased during aging but decreased with the aging temperature (Figure 1). The optimum YS seems to be obtained for an aging heat treatment temperature close to 730°C in 524
AD730tm. Not surprisingly, the elongation varies inversely with the yield and ultimate tensile strengths.
SEI
25.0kV
X50.000
100nm
W U 4.4m m
Figure 3. Effect of the aging temperature the intragranular y’ size particles
0
10
20
30
40
50
60
In tragran ular gam m a prim e d iam eter (nm )
Figure 4. Effect of the aging temperature on the intragranular y’ size distribution 525
30
25
NO
ro C L
20 ^
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15
3 a. 3 L_
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2
250
l,0E-2 cl m aj
CU
E
>• -o ID ai
l,0 E -3 No aging (SHT)
730°C/8h/Air
760°C/8h/Air
790“C/8h/Air
Figure 7. Effect on aging temperature on creep properties at 700°C-690MPa
—
'j '
"T '
790°C /8h/A C -8=1.3%
|
200 nm
Figure 8. Dislocation substructures on crept specimens at 700°C-500 MPa 527
Relaxation tests performed after 1000 LCF cycle show that the relaxation during the hold time at 650°C under an applied strain equal to 0.8% varies inversely with the aging temperature and the yield strength. This tendency may suggest for the highest aging temperature (790°C/8h/AC) would lower fatigue crack growth rates with trapezoidal wave form [18]. Effect of aging time at 730°C and 760°C The impact of the heat treatment aging duration was investigated at 730°C and mainly at 760°C. First, similar intragranular y’ sizes were observed for agings ranging from 4h to 16h at 760°C (Fig.. 9). Intragranular y’ precipitates remain very fine (< 50 nm) independently o f the aging time at 760°C. A slower cooling rate after solution heat-treatment would have led to coarser y’ precipitates and would have been probably more suitable to investigate aging effect on microstructure, especially on the tertiary y’ precipitates [3, 16]. Table 3. Results obtained with different aging durations at 730°C and 760°C Aging sequence Gamma prime Average diam (nm) Tensile 700°C
None
730°C/4h
730°C/8h
730°C/16h
760°C/4h
760°C/8h
760°C/16h
27
ND
28
ND
34
35
33
None
730°C/4h
730°C/8h
730°C/16h
760°C/4h
760°C/8h
760°C/16h
UTS (MPa)
1236
1255
1267
1240
1222
1224
1195
YS (MPa)
1022
1100
1123
1120
1074
1102
1088
El. 4d (%)
16
20
21
22
21
22
17
None
730°C/4h
730°C/8h
730°C/16h
760°C/4h
760°C/8h
760°C/16h
1.6 E-5
2.34 E-5
2.14 E-5
2.36 E-5
3.49 E-5
4.61 E-5
1.1 E-4
t r (h)
333
267
268
292
175
220
128
Elongation (%)
8
7
9
6
7
8
10
Creep 700°C/500MPa
None
730°C/4h
730°C/8h
730°C/16h
760°C/4h
760°C/8h
760°C/16h
Creep strain (% )300h
ND
ND
0.07
ND
0.06
0.1
0.17
Steady creep rate (h-1 )
ND
ND
1.65 E-6
ND
1.66 E-6
3.22 E-6
4.5 E-6
None
730°C/4h
730°C/8h
730°C/16h
760°C/4h
760°C/8h
760°C/16h
ND
ND
76
ND
142
116
214
Creep 700°C/690MPa Steady creep rate (h-1)
Relaxation 650°C Stress relaxation during 1 hour with s=0.8% (MPa)
528
Although microstructure variations with aging time are difficult to observe (Fig. 9), the effect of aging time on mechanical properties was well identified. Ultimate tensile strength remains almost equivalent whatever the aging time. Higher UTS values were obtained with aging made at 730°C for a given heat treatment duration (Fig. 10). The YS quickly increases for both aging temperatures and reaches a maximum value after 8 hours. After 16 hours, the YS remains stable for aging made at 730°C and slightly decreases for an aging performed at 760°C (Fig. 10). The increase of YS during aging is relatively limited compared to the YS before aging. This clearly indicates that 90% of strengthening is provided in AD730TM by the solution heat-treatment procedure (grain size and secondary y’ precipitates nucleating during solution cooling).
0
10
20
30
40
50
60
Intragranular gamma prime diameter (nm)
Figure 9. Effect of aging time at 760°C on the intragranular y’ size distribution
Figure 10. Effect of aging time at 730°C and 760°C on tensile properties at 700°C
529
As shown on Fig. 11, the aging duration at 760°C has an influence on steady creep rate at 700°C500 MPa. Similar results were obtained for a higher applied stress (see Fig. 12). Steady creep rate and time to rupture were negatively impacted by the increase in aging duration at 760°C. This tendency was not observed for agings at 730°C. The time to rupture remains actually close to 300 hours at 700°C-690MPa and the steady creep rate remains stable independently of the aging time at 730°C. It strongly suggests that microstructural variations within the grains are more important at 760°C than 730°C even if it was not properly observed by FEG-SEM examinations. Indeed, despite average y’ size hardly affected (see e.g. Figs. 4 and 9), it is observed slightly broader particles distributions with hotter and longer agings. In addition, longer agings at high temperature are also known to affect the y’ distribution close to grain boundaries. Long-term aging at these temperatures would be a good way to assess these observations.
sp C au u JS CL
Time (h)
Figure 11. Effect of aging time at 760°C on creep properties at 700°C-500 MPa
No aging (SHT)
4 hours
8 hours
16 hours
Figure 12. Effect of aging time at 730°C and 760°C on creep properties at 700°C-690 MPa 530
Relaxation tests made with different aging times at 760°C shows that the relaxation magnitude during the hold time does not vary significantly. Only an increase of stress relaxation magnitude was observed with the longest aging time (16h).
Table 4. Results obtained with a second aging step 700°C/8h after various aging temperatures Aging sequence 730°C/8h 760°C/8h 790°C/8h 700°C/8h
760°C/8h 700°C/8h
790°C/8h 700°C/8h
33
34
29
1207
730°C/8h 700°C/8h 1281
760°C/8h 700°C/8h 1235
790°C/8h 700°C/8h 1217
1102
1066
1097
1074
1088
21
22
23
16
24
24
None
730°C/8h
760°C/8h
790°C/8h
730°C/8h 700°C/8h
760°C/8h 700°C/8h
790°C/8h 700°C/8h
Steady creep rate (h-1 )
1.6 E-5
2.14 E-5
4.61 E-5
2.48 E-4
1.35 E-5
4.37 E-5
2.3 E-4
t r (h)
333
268
220
100
331
208
119
Elongation (%)
8
9
8
17
12
16
22
Creep 700°C/500MPa
None
730°C/8h
760°C/8h
790°C/8h
730°C/8h 700°C/8h
760°C/8h 700°C/8h
790°C/8h 700°C/8h
Creep strain (% )300h
ND
0.07
0.1
0.62
0.06
0.07
0.42
Steady creep rate (h-1 )
ND
1.65 E-6
3.22 E-6
1.97 E-5
1.46 E-6
2.32 E-6
1.58 E-5
None
730°C/8h
760°C/8h
790°C/8h
730°C/8h 700°C/8h
760°C/8h 700°C/8h
790°C/8h 700°C/8h
ND
72
116
141
99
98
156
Gamma prime
None
730°C/8h
Average diam (nm)
27
28
35
33
Tensile 700°C
None
730°C/8h
760°C/8h
790°C/8h
UTS (MPa)
1236
1267
1224
YS (MPa)
1022
1123
El. 4d (%)
16
Creep 700°C/690MPa
Relaxation 650°C Stress relaxation (MPa)
Effect of a second step at 700°C Most of superalloys are aged with a double step aging sequence. The effect of each aging stage on the y’ precipitation is not fully understood. In case of a second step at a higher temperature than the first one, the first step at lower temperature is dedicated to the increase of tertiary y’ particles and the second one at high temperature to the tertiary y’ coarsening. A second step, at a lower temperature than that of the first one, provides higher tensile properties as reported for U720Li [16, 19-20]. It has been therefore decided to have a first step at a higher temperature than the second one. The temperature of the second temperature was defined to be lower than 730°C and close to the expected in-service temperature (700°C). As shown in Table IV, the effect of a second step of 8 hours at 700°C was studied after a first step o f 8 hours at 730°C, 760°C and 531
790°C. As shown in Figs. 13 and 14, the effect of a second aging step on tensile and creep properties is not very pronounced and appears as negligible compared to the aging temperature of the first step. However, it is interesting to note that the inelastic field (UTS minus YS) and the creep ductility are increased with a second step at 700°C.
Figure 13. Effect of a second aging step o f 8 hours at 700°C on the tensile properties at 700°C
Figure 14. Effect of a second aging step of 8 hours at 700°C on steady creep rate at 700°C
532
Mechanical properties o f a disk heat-treated with the selected aging sequence Aging temperature of 730°C/8h leads to the best combination of tensile and creep properties. Since the dwell-fatigue crack growth rates at 650°C are mainly governed by the cooling rate after solution heat-treatment and grain size, it is therefore impacted to a lesser degree by the aging conditions. It has been decided therefore to maintain high creep and tensile properties with an aging at 730°C and to adjust if needed the fatigue crack growth rates with the solution heattreatment. A second step at 700°C/8h was not selected because of a negligible improvement of the mechanical properties in regard to the duration of the aging that would be multiplied by a factor 2. A disk (Outer diameter = 620mm, Inner diameter = 460mm) was ring-rolled and close die forged from a 8’’ billet and was solution heat-treated at 1080°C during 4 hours then oil quenched. The disk was then aged during 8 hours at 730°C and then tested to evaluate the mechanical properties combination obtained with this selected aging. Average grain size was close to ASTM 10 and secondary y’ precipitates were evaluated to be close to 70 nm (figure 15). Tertiary y’ precipitates were also observed. The presence of tertiary y’ precipitates and coarser secondary precipitates can be mainly explained by a lower cooling rate after solution heat-treatment and also, to a lesser degree, by a different etchant which dissolves gamma matrix contrary to the previous one (see e.g. Figs. 2 and 3) and probably tends to over-estimate y’ size precipitates.
Figure 15. Microstructure observed on a disk after heat-treatment (aging = 730°C/8h/Air)
Temperature (°C)
Larson - Miller Parameter (log(tr) + Z0).(T°C+273)
Figure 16. Tensile (a) and creep (b) properties obtained on the disk aged 730°C/8h/Air 533
As shown in Fig. 16, tensile properties at 700°C (UTS = 1264 MPa, YS = 1105 MPa) are in good agreement with those obtained with the 730°C/8h/Air aging previously described in the article (Table II) in the study described previously. Creep rupture time at 700°C/690 MPa was close to 200 hours and is also in agreement with results obtained from heat treatments performed on specimens. LCF properties are good and in line with those obtained on DA718 fine grain material [21] (Fig. 17). Finally, fatigue crack growth rates were also at a good level and significantly better than those of DA718 in similar conditions [22].
_
o
1.E-04
6 5 0 °C, R=0.1 Trapezoidal wave form 10$-300s-10$
□
CJ
>.
✓**
1.E-05 -
/
■cO
10 0 0 0
100 000
Cycles to rupture (Nf)
Figure 17. LCF properties at 550°C/Re=0/f = 0.5 Hz (a) and dwell-fatigue crack growth rate at 650°C (b) for samples machined out from a disk heat treated 4h at 1080°C/0Q + 8h/730°C/AQ.
Conclusions The impact of the aging heat treatment after solution treatment on the mechanical properties of the newly developed AD730TM alloy has been investigated. A special attention has been paid to the intragranular y’ precipitates. These particles remain were very fine (< 50 nm) independently of the aging conditions and it was consequently difficult to observe a strong effect of the aging on the y’ precipitates of AD730TM alloy sub-solvus heat-treated with a fast cooling rate. A slower cooling rate would have been more suitable to investigate the consequences of the aging heat treatments. The aging temperature has a strong effect on tensile and creep properties. The strengthening peak was observed to be close to 730°C. A significant increase of the steady-state creep rate and a corresponding decrease of creep lifetime was observed with the highest aging temperature selected (790°C). Orowan looping seems to be the dominant deformation mechanism under during creep at 700°C for the hottest aging heat treatments. The effect of the aging duration was more important at 760°C compared to 730°C, especially on creep conditions, suggesting that a greater y’ evolution is occuring at 760°C compared to 730°C. A second step at 700°C after the initial aging made at different temperatures does not have a significant impact on mechanical properties. In addition to be a cost-effective heat treatment, the aging 730°C/8h after the solution treatment leads to the best combination of tensile and creep properties. Some stress-range variations, linked with tensile ones, were observed during relaxation tests suggesting that the highest tensile properties obtained with a 730°C aging heat treatment would be not favorable to dwell-fatigue 534
crack growth properties. However, as dwell-fatigue crack growth rates at 650°C is mainly governed by the cooling rate after the solution heat-treatment and by the grain size, 730°C/8h/Air was the selected aging sequence for further testing on turbine disk. All mechanical properties (tensile, creep, fatigue and fatigue crack growth) were at the expected level.
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