PSI - Issue 19

Motoki Nakane et al. / Procedia Structural Integrity 19 (2019) 284–293 Author name / Structural Integrity Procedia 00 (2019) 000–000

286

3

2. Fatigue test of grinding finish material 2.1. Test material

The material subjected to the fatigue tests were austenitic stainless steel SUS316L of JIS G 4304 (2010) which is widely used in the nuclear power plants. The chemical compositions and mechanical properties of the material are shown in Table 1 and Table 2 respectively. The as-received material was plate of 250 mm × 250 mm × 12 mm, and the thickness of plates for grinding finish specimen were reduced to approximately 10.2 mm by milling. After that, the center area for two sides of plates were ground as shown in Fig. 1 with grinding condition presented in Table 3. Following three grinding scratch direction were considered in this study. Grinding type 1 : Parallel to specimen axis and cyclic loading direction Grinding type 2 : Perpendicular to specimen axis and cyclic loading direction Grinding type 3 : Simulating general grinding surface finish in routine work process. Final grinding scratch direction is perpendicular to specimen axis and cyclic loading direction The work processes of all the 3 types of grinding direction are also shown in Fig.1. Regarding type 3, grinding scratch was given to the material surface by rotating the plate as shown in Fig. 1. Since the thickness reduction per one time grinding was approximately 0.01mm, about 10 times grinding work was applied for two side of the plate so as to reduce the thickness of the plate to 10 mm. During grinding work, grinding scratch directions described above were kept all the time. In addition, emery polishing test specimens were also prepared to find out fatigue strength basis of SUS316L.

Table 1 . Chemical compositions (mass %). Material C Si Mn

P

S

Ni

Cr

Mo 2.07

Fe

SUS316L

0.011

0.38

0.82

0.025

0.001

12.06

17.57

Bal.

Table 2. Mechanical properties. Material

Proof strength  0.2 (MPa)

Elongation  (%)

Ultimate tensile strength  u (MPa)

SUS316L

550

278

59

2.2. Fatigue test specimen The outline of fatigue test plate specimen is shown in Fig. 2. The test specimens were taken from center of grinded plates as shown in Fig. 1 so that the grinding surface, indicated by shaded area in Fig. 2, is located at the center of the specimen for fatigue evaluation. The axial direction of test specimen corresponded to the rolling direction of the as received material. Therefore, the grinding scratch direction for Type 1 and Type 2 specimen were parallel and perpendicular to the axial direction of test specimen respectively. The gauge length of test specimen was 12.5 mm and the side of test portion was rounded and emery polished in order to avoid unexpected fatigue initiation from there. Fig. 3 shows example of surface profiles for grinding finished and emery polished specimens. With respect to grinding finished specimens, roughness for direction perpendicular to grinding scratch is a little bit bigger than that of direction parallel to grinding scratch. Compared with grinding finished specimens, surface roughness of emery polished specimen is much smaller. Maximum roughness heights for each specimen is summarized in Table 4. It can be found that the maximum grinding finish roughness perpendicular to grinding scratch direction is only a few micro meters bigger than that of parallel direction. The maximum roughness height of emery polishing is approximately as low as 1/10 of grinding finishing.