Microstructure, Residual Strain and Stress Corrosion Cracking Behavior in 316L Heat-Affected Zone

doi:10.1007/s40195-016-0461-7

Abstract

Abstract:

Austenitic stainless steels are usually chosen to make many components of nuclear power plants (NPPs). However, their microstructure in the heat-affected zone (HAZ) will change during the welding process. Some failures of the weld joints, mainly stress corrosion cracking (SCC), have been found to be located in the HAZ. In this research, the microstructure, micro-hardness, residual strain and SCC behavior at different locations of the 316L HAZ cut from a safe-end dissimilar metal weld joint were studied. However, traditional optical microscope observation could not find any microstructural difference between the HAZ and the base metal, higher residual strain and micro-hardness, and higher fraction of random high-angle grain boundaries were found in the HAZ than in the base metal when studied by using electron back-scattering diffraction scanning and micro-hardness test. What’s more, the residual strain, the micro-hardness and the fraction of random grain boundaries decreased, while the fraction of coincidence site lattice grain boundaries increased with increasing the distance from the fusion boundary in 316L HAZ. Creviced bent beam test was applied to evaluate the SCC susceptibility at different locations of 316L HAZ and base metal. It was found that the HAZ had higher SCC susceptibility than the base metal and SCC resistance increased when increasing the distance from the fusion boundary in 316L HAZ.

Key words: Austenitic stainless steels, Heat-affected zone, Microstructure, EBSD, Residual strain, Stress corrosion cracking

Hong-Liang Ming,Zhi-Ming Zhang,Peng-Yuan Xiu,Jian-Qiu Wang,En-Hou Han,Wei Ke,Ming-Xing Su. Microstructure, Residual Strain and Stress Corrosion Cracking Behavior in 316L Heat-Affected Zone[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(9): 848-858.

Figures/Tables 15

Fig. 1 Schematic illustration of the dissimilar metal weld joint and the positions of the CBB samples, marked by A, B, C and D

Fig. 2 Schematic diagram of the clamps used for CBB test

Fig. 3 Photograph of the sample and clamp assembly

Fig. 4 Schematic diagram of the CBB sample

Fig. 5 Optical microstructure of 316L base metal

Fig. 6 Side-view optical microstructure of 316L (from fusion boundary to base metal)

Fig. 7 Micro-hardnesses of the 316L HAZ and base metal

Fig. 8 a-d EBSD scanning results of the cross sections of samples A-D before bending. (a1)-(d1) Inverse pole figures, (a2)-(d2) kernel average misorientation figures, (a3)-(d3) grain boundary character maps

Fig. 9 a-d EBSD scanning results of the cross sections of samples A-D after bending. (a1)-(d1) Inverse pole figures, (a2)-(d2) kernel average misorientation figures, (a3)-(d3) grain boundary character maps

Fig. 10 Grain boundary character distribution of samples A-D before and after bending

Fig. 11 Kernel average misorientation values of samples A-D before and after bending

Fig. 12 a-d Surface morphologies of samples A-D after immersion test in the 3 wt% NaOH solution at 325 °C for 378.5 h. (a1)-(d1) Surface morphology at low magnification, (a2)-(d2) and (a3)-(d3) higher magnification SEM figures for (a1)-(d1)

Fig. 13 SEM images of fracture surface of samples a-d after immersion test. (a2)-(d2) Higher magnification of the SCC zone in (a1)-(d1), respectively

Fig. 14 Relationship between grain boundary character a and crack path b of sample A

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