Issue34

G. Lesiuk et alii, Frattura ed Integrità Strutturale, 34 (2015) 290-299; DOI: 10.3221/IGF-ESIS.34.31 293 Figure 4 : Microstructure of investigated puddled steel (1899-1904) – marked as S-steel, in (a) post-operating state (light microscopy) – presented nonmetallic inclusions typical for puddled steel, (b) post-operating state; ferrite grains with numbers of nonmetal inclusions (A) and degradation separations (C) inside ferrite grains and also thick envelope Fe 3 C III on the grain boundary (light microscopy, etched 3%HNO 3 ), (c) ferrite grains structure with nonmetallic inclusions – mainly silicates (A) and thick envelope of Fe 3 C III on the grain boundary (B) with numbers of degradation separations brittle phase inside ferrite grains (C), post-operating state SEM, etched 3%HNO 3 , (d) magnified ferrite grains with degradation symptoms; thick enveloper of Fe 3 C III (B) and separations inside ferrite grain (C), post-operating state SEM, etched 3%HNO 3 , (e) magnified area of ferrite grain with brittle separations (C), post-operating state SEM, etched 3%HNO 3 , (f) microstructure of ferrite grains with nonmetal inclusion (A) (after normalisation) with significantly reduced number of degradation symptoms; (B) – remains of the brittle separations on the grain boundary, (C) – minor amounts of separations inside ferrite grain, SEM, etched 3%HNO 3 , [2], [6]. B-steel [P/N] S-steel [P/N] W-steel [P/N] Typical values for puddled steel, [5] YTS [MPa] 264.8/257.4 257.6/242 263/294 220-280 UTS [MPa] 372.8/379.5 376/340 359.7/368.6 330-400 Elongation at break A [%] 20.8/24.9 20.8/22.7 15.l/27.1 8-25 Necking Z [%] 28.2/27 34/36.8 33.9/39 variable Impact resistance test results, Charpy value [J/cm 2 ] 35/49 37/58 n/a variable Table 2 : Strength properties of the investigated materials in post-operating state (P) and after heat treatment – normalising (N). F ATIGUE PROPERTIES Low-cycle tests results low cycle fatigue test for the puddled steel (S-steel) – 5 round specimen ϕ 12 mm for each test step, controlled total strain amplitude  t =0.2%, 0.25%, 0.3%, 0.35%, 0.4%, f=0.2Hz – was performed with accordance to the standards ASTM E606. The stabilized hysteresis loops were selected for analysis, corresponding to half of the number of cycles to destruction. The graphs of fatigue life of specimens in the after-operation and normalized states are presented in Fig. 5. These curves are known in the literature as Coffin and Manson equation:                   ' ' 2 2 2 c b f t N N f f f E , (1) where: A