Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 325 that the stress increase observed in nonproportional loading mainly results from the reduction in the mean cell size, and the following Hall-Petch relationship holds. 1/2 R R     k d (3) The constant, k , takes the value of 1.2. M ICROSTRUCTURE AND HARDENING BEHAVIOR OF TYPE 304 STAINLESS STEEL AT HIGH TEMPERATURE icrostructure of 304SS was studied on the specimen after cyclically loaded at 823K using strain paths shown in Fig. 8. The chemical composition of the steel was 0.38 Si, 1.13Mn, 0.008P, 8.74Ni, 18.52Cr, 0.06C reminder Fe in weight percent ratio with ASTM No.3.5 grain size. In Fig. 8, (a) is the push-pull loading (P), (b) the reversed torsion loading (T) and (c) the alternating push-pull and reversed loading in each cycle (APT). Fig. 8 (d) is the strain path cyclically pre-strained in APT loading and switched to P loading, and Fig. 8 (e) is the strain path cyclically prestrained in P loading and switched to T loading. These (d) and (e) strain paths were motivated to study the re-arrangeability of the dislocation structure formed in the prestrain loading to that in the following loading. The (f) and (g) strain paths in Fig. 8 are the strain path to investigate the hardening characteristic of the material cyclically loaded in P and APT strain paths. The material was cyclically prestrained in P loading followed by many directional loadings in the figure to draw the hardening ellipse in (f), and (g) is the same loading path for the prestrained material in APT loading. Strain controlled tests were performed using the specimen geometry as shown in Fig. 1 with a triangular strain waveforms at 0.1Hz at 823K. Stress amplitudes in P, T and APT loadings at a Mises strain range of 1.0% are shown in Fig. 9. The P and T loadings, which are proportional strain path, give almost the same stress amplitude, but APT loading that is nonproportional strain path yields larger stress amplitude than P and T loadings. Clear cyclic strain hardening was observed in the three strain paths. Fig. 10 shows dislocation structures at N 1 , N 2 and N 3 in the three loading paths at a Mises equivalent strain range of 1%, where N 1 is the number of cycles where the cyclic work hardening almost saturated ( 0.05 0.1 f N   ), N 2 the number of cycles where rapid cyclic work hardening saturated ( 0.2 0.3 f N   ) and N 3 the life ratio of 0.5 f N . Dislocation structures in P loading are mostly ladder, partly maze and less cells. APT loading yields only cell structure. The photographs in column N 1 show that in all the loading modes dislocation structures form in the early stage of straining and that they are firmly established as cycling progresses. Fig. 11 shows geometrical orientation of ladder and maze structures at N 3 , where the equivalent strain range was 1.0%. Schematic figures on the right of Fig. 11 show {111} planes, i.e. the primary slip planes of the grains on which the maximum shear stress operates. They are distinguished from other slip planes according to the loading direction. Since the electron beam penetrates from the top to the bottom of the schematic figure, it is clear that the dense parts of ladder or maze structures are on {111} planes. This was also confirmed at N 1 and N 2 . Figure 9 : Stress amplitudes in P, T and APT loadings. M