Investigations on the Accelerated Creep Testing of Alumina-Forming Austenitic Stainless Steel
LIU Tian, LUO Rui(), CHENG Xiaonong, ZHENG Qi, CHEN Leli, WANG Qian
School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
Cite this article:
LIU Tian, LUO Rui, CHENG Xiaonong, ZHENG Qi, CHEN Leli, WANG Qian. Investigations on the Accelerated Creep Testing of Alumina-Forming Austenitic Stainless Steel. Acta Metall Sin, 2020, 56(11): 1452-1462.
The heat transfer component is a major component of a nuclear power plant, the safety and service life of which are determined based on the long-term creep performance of the heat-transfer pipe material. Several long-term creep tests are usually required to determine the creep life of the heat-transfer pipe materials, which considerably restrict the evaluation efficiency of the material service performance. The objective of this study is to investigate the feasibility of accelerated creep test (ACT) to reduce the time required for evaluating the creep properties of materials. A alumina-forming austenitic (AFA) stainless steel was prepared, and the ACT was performed on a Gleeble thermal simulator. Based on the ACT developed and realized on the Gleeble thermal simulator, damage accumulation was realized by applying elastic-plastic tensile and compressive strains on the ACT specimen to simulate the accelerated changes in the microstructure of the alloy that can be usually observed during a conventional creep test (CT). The average stress with respect to all the cyclic stress relaxation stages in the ACT was considered to be the initial stress of the conventional CT, and a creep fracture test was conducted on the alloy sample. Results revealed that the ACT accelerated the microstructure evolution of the precipitated phases, dislocations, twins, and so on in a short time. Nevertheless, the repeated generation and annihilation of a large number of dislocations in the AFA alloy during the ACT provided the nucleation point and reduced the driving force associated with the nucleation of the precipitated second phase, including the Laves phase. In addition, the cyclic strain applied during the ACT will reduce the strengthening effect of the nanoscale deformation twins in the AFA alloy, resulting in differences in the evaluation effect. Thus, ACT is useful for the efficient evaluation of the creep properties of materials; however, the optimal range of test parameters must be further investigated.
Fund: China Postdoctoral Science Foundation(2019M661738);Jiangsu Provincial Key Research and Development Program(BE2017127);Natural Science Foundation of the Jiangsu Higher Education Institutions of China(19KJB430001)
Fig.1 Load curve of accelerated creep test (ACT) (εa—cyclic strain, th—time for stress relaxation, t0—time from unloading to uploading) (a) and morphologies of failed samples after ACT (b)
Fig.2 XRD spectrum of the alumina-forming austenitic (AFA) stainless steel
Fig.3 OM image (a), SEM images (b~d) of AFA stainless steel after solution treatment at 1050 ℃ for 45 min, and EDS annlysis of the intracrystalline second phase (inset in Fig.3d)
Fig.4 Stress-time curve after ACT at 600 ℃ and a cyclic strain of 0.135
Fig.5 Stress-time curves after ACT at 650 ℃ and cyclic strains of 0.120 (a), 0.135 (b) and 0.150 (c)
Fig.6 Stress-time curve after ACT at 700 ℃ and a cyclic strain of 0.120
Fig.7 Stress-time curves during load holding process of ACT (a) typical stress relaxation curve (b) stress relaxation curve with precipitation platform (c) relaxation curve in several cycles (d) increasing stress relaxation curve
Sample No.
Temperature / ℃
Cyclic strain
σa (σc) / MPa
PACT / MPa
PCT / MPa
PCT/PACT
1
600
0.135
340
163.64
178.91
1.09
2
650
0.120
290
171.58
190.41
1.11
3
650
0.135
320
169.28
186.97
1.10
4
650
0.150
340
158.96
180.27
1.13
5
700
0.120
260
173.60
190.93
1.10
Table 1 Comparisons between ACT and conventional creep test (CT) results
Fig.8 TEM images of precipitates and corresponding SAED patterns (insets) in the AFA stainless steel after ACT (a) NbC after ACT at 600 ℃ and a cyclic strain of 0.135 (Inset in Fig.8a shows the corresponding SAED pattern) (b) NbC after ACT at 700 ℃ and a cyclic strain of 0.120 (c) Laves after ACT at 600 ℃ and a cyclic strain of 0.135 (d) Laves after ACT at 700 ℃ and a cyclic strain of 0.120 (Inset in Fig.8d shows the corresponding SAED pattern)
Fig.9 TEM images of AFA stainless steel after ACT (a) dislocation bundles after ACT at 600 ℃ and a cyclic strain of 0.135 (b) dislocation networks after ACT at 650 ℃ and a cyclic strain of 0.120 (c) dislocation cell and particle on the boundary (showed by arrow) after ACT at 650 ℃ and a cyclic strain of 0.120 (d~f) subcrystalline structures after ACT at 700 ℃ and a cyclic strain of 0.120
Fig.10 TEM images of twins structures in AFA stainless steel after ACT at 650 ℃ and a cyclic strain of 0.150 (a, b) annealing twins (c, d) deformation twins
Temperature / ℃
Experiment (strain)
MC
M23C6
M6C
Laves
B2
γ'
600
ACT (0.135)
√
√
√
√
√
CT
√
√
√
√
650
ACT (0.120)
√
√
√
√
ACT (0.135)
√
√
√
√
ACT (0.150)
√
√
√
√
√
CT
√
√
√
√
700
ACT (0.120)
√
√
√
CT
√
√
√
Table 2 Comparisons of precipitates in AFA stainless steel after ACT and CT
Fig.11 Equilibrium phases of the AFA stainless steel calculated by JMatPro
Fig.12 HRTEM analyses of small-scale deformation twins structure in AFA stainless steel after ACT at 650 ℃ and a cyclic strain of 0.150 (a) low magnification image of deformation twins (b) fast Fourier transformation (FFT) result of region A in Fig.12a (c) local high magnification image of region A in Fig.12a
Fig.13 Dislocation configurations of deformation twins on () planes (inset) (a) dislocation locks after ACT at 650 ℃ and a cyclic strain of 0.120 (b) edge dislocation accumulation after ACT at 650 ℃ and a cyclic strain of 0.150
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