Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 249-257; DOI: 10.3221/IGF-ESIS.37.33 251 EN 10270-1 SH (DIN 17223C) is high strength eutectoid spring steel, with a microstructure showing alternating ferrite and cementite lamellas. This steel is mainly used for manufacturing springs and other elastic elements. The biggest Bulgarian spring producer is the Company Feder Universal & Co Ltd Bulgaria (http://www.feder-u.com/ ) using imported spring steels with main supplier TREFILAC, France. Steel BS 250A53 (DIN 55Si7) is silico-manganese spring steel. This is a high carbon low alloy steel in the quenched and tempered condition used for the manufacture of railway track clip fastenings. Steel B experienced specific treatment, described by Murtaza [4] as follows. The hardening was conducted by heating the material to an austenitising temperature of 950 o С for 15 minutes followed by quenching in oil. Tempering was then carried out at a temperature of 450 o С for 30 minutes. The microstructures obtained after the given heat-treatments were analyzed by an image analysis system. A large number of non-metallic inclusions were seen. The average prior austenite grain sizes were found to be around 30.10 -6 m and 60.10 -6 m for standard and large grain size, respectively. Specimens of Steel A and Steel B are machined in hour-glass shape, and shown in Fig. 2. (a) (b) (c) Figure 2 : Specimen geometry and fatigue equipment: (a) steel A specimen; (b) fatigue machine FATROBEM 2004; (c) Steel B specimen; FATROBEM 2004 - electric engine 1, driving belt 2, ball-bearing unit 3, testing box 4, specimen 5, device for loading 6, counter 7, device for circulation and aeration of corrosion agent 8. Equipment and testing Steel A. Twelve hourglass-shaped specimens were subjected to symmetric cyclic rotating-bending fatigue (RBF) to fracture at different stress ranges (Δσ = 800, 1000, 1200, 1400 and 1500 MPa) and at the following testing conditions: R=-1, f=11 Hz, air environment and room temperature. Tests were performed on a table model Fatigue Rotating Bending Machine, FATROBEM-2004, designed and assembled in “Fracture and Fatigue” Laboratory in the University of Chemical Technology and Metallurgy–Sofia, UCTM, Fig. 2. A Scanning electron microscope (SEM) was used for having studied both, microstructure and fatigue-fractured surfaces, in the Laboratory LETAM (Laboratory of Study of Textures and Application to Materials), Paul Verlaine University–Metz, France. Steel B. Fourteen hourglass-shaped specimens were used in fatigue tests conducted by Murtaza in the Fracture and Fatigue Laboratory of the Institute SIRIUS, The University of Sheffield. The tests were carried out under fully reversed torsion loading conditions (TF) and frequency f=5 Hz for two sets of stress ranges applied in two different environments: in-air fatigue tests at Δτ = 915, 1080, 1106 MPa (respectively Δγ = 1.24, 1.57, 2.22%); and corrosion fatigue tests in 0.6M NaCl at Δτ = 404, 601, 815, 900 MPa (respectively Δγ = 0.49, 0.73, 1.03, 1.2 %).