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Soma, Yasutaka; Ueno, Fumiyoshi
Zairyo To Kankyo, 67(5), p.222 - 228, 2018/05
Localized corrosion in crevice of SUS316 stainless steel after immersion in 288
C high purity water with dissolved oxygen concentration of 32 ppm for 100 h was analyzed. Two different types of localized corrosion initiated on grain boundary and inclusions. The former initiated on grain boundary and oxide grown into grain matrix. The oxidized area showed duplex structure composed of microcrystalline FeCr
O
and island-shaped residual metals. The latter initiated on inclusions containing Ca and S and microcrystalline FeCrO grown into metal matrix. These localized corrosion occurred selectively in oxygen depleted area indicated formation of macroscopic corrosion cell with the corroded area as anode and surrounding oxygenated area as cathode.
Kaji, Yoshiyuki; Aoki, So; Kondo, Keietsu; Yamamoto, Masahiro
no journal, ,
The influence of long term thermal aging on SCC susceptibility in L-grade austenitic stainless steels (SSs) has been investigated using Creviced Bent Beam (CBB) tests. Test materials were type 304L and 316L SSs, and aging heat treatments were conducted at 288C in air for up to 14000 hours followed by solution-annealing (SA) or 20% cold-working (CW). The evaluation of the SCC initiation susceptibility was conducted by CBB tests in BWR simulated high-temperature water at 288C for 1000 hours. It was revealed by CBB tests that long term aged 304L-SA, 304L-CW, and 316L-SA showed low susceptibility of cracking, whereas long term aged 316L-CW showed high SCC susceptibility. And so far, it is considered that the low temperature sensitization was not the main cause of SCC in long term aged 316L-CW, because the previous TEM/EDX analysis on the long term aged 316L specimen showed no chromium depletion in the vicinity of grain boundaries. Further investigations are in progress from the viewpoint of mechanical properties, cold-rolling texture, microstructure and michrochemistry after the long term thermal aging in order to discuss the factors increasing SCC susceptibility.
Kaji, Yoshiyuki; Aoki, So; Kondo, Keietsu; Yamamoto, Masahiro
no journal, ,
The influence of long term thermal aging (LTA) on SCC initiation susceptibility in L-grade austenitic stainless steels (SSs) has been investigated using Creviced Bent Beam (CBB) tests. Test materials were type 304L and 316L SSs, and aging heat treatments were conducted at 288C in air for up to 14000 hours followed by solution-annealing (SA) or 20% cold-working (CW). The evaluation of the SCC initiation susceptibility was conducted by CBB tests in BWR simulated high-temperature water at 288C for 1000 hours. It was revealed by CBB tests that 304L-SA, 304L-CW, 304L-SA+LTA, 304L-CW+LTA, 316L-SA 316L-CW and 316L-SA+LTA showed no SCC initiation susceptibility, whereas 316L-CW+LTA showed high SCC susceptibility. To clarify the cause of SCC initiation in 316L-CW+LTA, several investigations are in progress from the viewpoint of mechanical properties, cold-rolling texture, microstructure and michrochemistry after LTA. It is considered that the low temperature sensitization was not the main cause of SCC in 316L-CW+LTA, because the TEM/EDX analysis on the 316L+LTA and 316L-CW+LTA specimens showed no chromium depletion in the vicinity of grain boundaries. In this presentation, we will explain the further investigation results for effect of LTA on SCC initiation susceptibility in L-grade austenitic SSs.
Kaji, Yoshiyuki; Kondo, Keietsu; Aoki, So; Hirade, Tetsuya; Yamamoto, Masahiro
no journal, ,
To clarify the causal relationship between the LTA treatment and the SCC initiation susceptibility, microstructural analyses on 316L CW specimens before and after the LTA treatment have been conducted. One of viewpoints in our research is the deformation microstructure. Results of the surface examination by SEM on CBB tested specimens suggested the difference in the deformation microstructure of 316L CW and CW+LTA. Coarse and planar slip steps were frequently observed on the surface of 316L CW+LTA specimens, indicating the localization of dislocation motion to limited slip planes. On the other hand, surface morphology observed in 316L CW was mainly fine and wavy slip step, indicating the occurrence of the homogeneous plastic deformation. The detail mechanism of the change in the deformation microstructure by the LTA treatment is unknown yet. Researches on its effect on the deformation microstructure and, furthermore, the SCC susceptibility is now in progress.
Kaji, Yoshiyuki; Aoki, So; Fujimura, Yuki; Kondo, Keietsu
no journal, ,
In this study, the influence of the long term thermal aging (LTA) on SCC initiation susceptibility of L-grade SSs was examined. Results of CBB testing on Type 316L CW and CW + LTA under the BWR simulated water condition showed the increase of SCC susceptibility to initiation after the LTA treatment. The material examinations on those materials were conducted to identify the SCC accelerating factors. The initial microstructure in Type 316L CW and CW + LTA was characterized to dislocation tangles and dislocation cell structures, respectively, indicating that the LTA treatment induced the rearrangement of dislocations toward the energetically favorable state. Whereas, no sensitization of the grain boundaries of Type 316L CW + LTA was confirmed by means of TEM/EDX analysis. The examination of surface deformation microstructure by bending deformation was conducted by the consecutive SEM observation before and after CBB testing. The more heterogeneous deformation was enhanced by the LTA treatment, that is, the reduction in dislocation motion and increase of deformation twinning. It was considered from those obtained results that the concentration of local stress around grain boundaries caused by the more heterogeneous deformation might be one of the promoting factors for increasing the SCC susceptibility in non-sensitized Type 316L CW + LTA.
