STUDY ON REJUVENATION HEAT TREATMENT OF A DIRECTIONALLY-SOLIDIFIED SUPERALLOYDZ125 DAMAGED BY CREEP
Jing ZHANG(),Yunrong ZHENG,Qiang FENG
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
Jing ZHANG,Yunrong ZHENG,Qiang FENG. STUDY ON REJUVENATION HEAT TREATMENT OF A DIRECTIONALLY-SOLIDIFIED SUPERALLOYDZ125 DAMAGED BY CREEP. Acta Metall Sin, 2016, 52(6): 717-726.
The degradation of microstructure and property in turbine blades of aircraft engines is inevitable during their service. Usually, rejuvenation heat treatment is applied to regenerate the original microstructure for extending the service life of blades and improve economic returns. To date, systematic investigations about rejuvenation heat treatment of the directionally-solidified superalloys are limited. In this work, the effect of rejuvenation heat treatment on the degraded microstructure and property of DZ125 superalloy damaged by creep was investigated. The interrupted creep test was first conducted on DZ125 superalloy to simulate the damage of turbine blades during their service. Three rejuvenation heat treatments with the solution temperature at 1230, 1240 and 1250 ℃ were applied to the interrupted creep specimen. Then, the rejuvenated specimens were retested, and their microstructures as well as creep properties were compared with those of the initial interrupted creep tests. The results showed that no recrystallization occurred after the interrupted creep tests at 1.0% and 3.5% strain followed by rejuvenation heat treatment, and the critical strain for the formation of the recrystallization was between 3.5%~10.0%. The solution treatment at 1230 ℃ partially dissolved the coarse γ' phase caused by creep deformation, caused a nonuniform microstructure of γ /γ' matrix after aging treatments, and resulting in partially recovering the baseline creep property. However, the solution treatment at 1240 and 1250 ℃ could not only dissolve the coarse γ' phase but also reduce the fraction of residual γ' eutectic significantly, and then regain a uniform microstructure of γ /γ' matrix after aging treatments. Hence, the effect of rejuvenation was further improved, maintaining or exceeding the baseline creep property.
Fig.1 Microstructure of γ /γ' matrix in dendrite core of DZ125 superalloy after standard heat treatment
Fig.2 Creep curves of DZ125 superalloy crept to different strains under 980 ℃ and 207 MPa(a) strain vs time (b) strain rate vs time
Fig.3 Cross (a, c) and longitudinal (b, d) sectional microstructures in the dendrite core of DZ125 superalloy after interrupted creep tests at strains of 1.0% (a, b) and 3.5% (c, d) under 980 ℃ and 207 MPa
Fig.4 OM image of DZ125 superalloy after interrupted creep test at 1.0% strain under 980 ℃, 207 MPa and then rejuvenated by RHT-1 process (RHT-1: 1230 ℃, 3 h, A.C., 1100 ℃, 4 h, A.C., 870 ℃, 20 h, A.C.; A.C.: air cooling)
Fig.5 OM images of DZ125 superalloy after interrupted creep test at strains of 3.5% (a) and 10.0% (b) under 980 ℃, 207 MPa and then heat treatment at 1240 ℃ for 3 h
Fig.6 Cross-sectional microstructures in the dendrite core of DZ125 superalloy after interrupted creep test at strain of 1.0% under 980 ℃, 207 MPa followed by rejuvenation heat treatments of RHT-1 (a, b), RHT-2 (c) and RHT-3 (d) (RHT-2: 1240 ℃, 3 h, A.C., 1100 ℃, 4 h, A.C., 870 ℃, 20 h, A.C.; RHT-3: 1250 ℃, 2 h, A.C., 1100 ℃, 4 h, A.C., 870 ℃, 20 h, A.C.)
Fig.7 Volume fraction of γ' eutectic in DZ125 superalloy after interrupted creep test at strain of 1.0% under 980 ℃, 207 MPa followed by three rejuvenation heat treatments
Fig.8 Cross (a) and longitudinal (b) sectional microstructures of dendrite core of DZ125 superalloy after interrupted creep test at strain of 1.0% strain under 980 ℃, 207 MPa followed by RHT-2 process and retested to strain of 1.0%
Before rejuvenation
After rejuvenation
T2/T1
Sample
Time to 1.0%
Minimum
Rejuvenation
Time to 1.0%
Minimum strain
strain T1
strain rate V1
heat treatment
strain T2
rate V2
h
10-8 s-1
h
10-8 s-1
No.1
43
4.7
-
27
7.5
0.628
No.2
38
5.6
RHT-1
33
6.5
0.868
No.3
27
7.1
RHT-2
31
7.0
1.148
No.4
27
7.4
RHT-2
37
5.4
1.370
Table 1 Experimental results of interrupted creep tests at strain of 1.0% under 980 ℃, 207 MPa for DZ125 superalloy before and after rejuvenation heat treatments
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