In Situ DIC Study on LCF Behavior of Retired Weld Joint Subjected to Prolonged Service at Elevated Temperature

doi:10.1007/s40195-021-01353-8

Abstract

Abstract:

By using digital image correlation (DIC), low-cycle fatigue (LCF) behavior of CrMoV weld joint taken from a retired gas turbine rotor after 15-year service was investigated at 500 °C and 540 °C. The most remarkable plastic strain was observed in the weld metal (WM), which was up to 6 times of the global strain at mid-life cycle. Due to the cyclic accumulation of local deformation, the stress-strain hysteresis loops relative to tensile or compressive strain concentration area in WM displayed the ratchetting shape. The local deformation accumulation of WM was attributed to the effect of the equiaxed grain zone near the WM center. The accumulated plastic strain was considered as the main fatigue failure mechanism.

Key words: Low-cycle fatigue, Digital image correlation, Local deformationWeld joint, Failure mechanism

Anqi Zuo, Xia Liu, Chendong Shao, Mingzhe Fan, Ninshu Ma, Fenggui Lu. In Situ DIC Study on LCF Behavior of Retired Weld Joint Subjected to Prolonged Service at Elevated Temperature[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(8): 1317-1328.

Figures/Tables 11

Table 1 Chemical compositions of BM and WM (wt%)

Material	C	Cr	Mo	Ni	Mn	V	Cu	Pb	Fe
BM	0.23	1.29	0.94	0.65	0.40	0.38	0.08	0.03	Bal.
WM	0.09	1.80	0.91	0.11	1.43	0.04	0.13	0.02	Bal.

Fig. 1 Schematic of sampling specimens and in situ DIC setup: a weld joint of rotor; b fatigue test specimen (unit: mm); c in situ DIC setup for fatigue tests at high temperature

Fig. 2 Microstructures of the CrMoV weld joint: a overall microstructure of the weld joint; b equiaxed grains between two layers of weld bead; c equiaxed grains near the center of WM; d microstructure of CGZ in WM; e microstructure of EGZ in WM

Fig. 3 Micro-hardness distribution of the CrMoV weld joint before the LCF tests at RT

Table 2 Average values of the micro-hardness of CGZ in WM and EGZ near WM center at different temperatures (HV)

Test location	RT	500 °C	540 °C
CGZ	215	147	148
EGZ	208	143	145

Fig. 4 Stress-strain hysteresis loops of weld joint from different life periods: a, b 500 °C, c, d 540 °C; a, c results from DIC, b, d results from the extensometer

Fig. 5 Axial strain evolution of weld joint in 0.5% strain amplitude at 500 °C: a evolution of the strain distribution at tensile peak; b evolution of the strain distribution at compressive peak; c evolution of stress-strain hysteresis loops of point 1; d evolution of stress-strain hysteresis loops of point 2

Fig. 6 Axial strain evolution of weld joint in 0.5% strain amplitude at 540 °C: a evolution of the strain distribution at tensile peak; b evolution of the strain distribution at compressive peak; c evolution of stress-strain hysteresis loops of point 3; d evolution of stress-strain hysteresis loops of point 4

Fig. 7 Evolution of stress-strain hysteresis loops of weld joint and the axial strain distribution across weld joint at 540 °C: a, b the 1st cycle; c, d the 10th cycle; e, f the mid-life cycle

Fig. 8 Microstructure of the specimen tested at 540 °C: a macrostructure of the specimen; b detailed microstructure in a; c inverse pole figure (IPF) of crack initiation area in b; d IPF of crack propagation area in b; e detailed microstructure in a; f IPF of crack initiation area in e; g IPF of crack propagation area in e

Fig. 9 Relationship between the failure locations and the ratchetting strain: a distribution of axial ratchetting strain along weld joint at 500 °C; b distribution of axial ratchetting strain along weld joint at 540 °C; c fracture location at 500 °C; d fracture location at 540 °C

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