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B. Žužek et alii, Frattura ed Integrità Strutturale, 34 (2015) 160-168; DOI: 10.3221/IGF-ESIS.34.17 164 carbides in ferrite phase (Fig. 2A and 2B). Carbides are mainly distributed along previous martensitic laths, with some of the carbides being slightly coarsened, mostly those along primary ferrite grain boundaries (Fig. 2C and 2D). No obvious difference in size and distribution of carbides between CCC and ESR specimens can be noticed. Nevertheless, in the case of ESR specimens more homogeneous microstructure was obtained without distinctive segregations and reduced number of non-metallic inclusions (Fig. 2E and 2F). Additional remelting of steel trough ESR also slightly reduced the grain size, according to standard EN ISO 643 from G10 for CCC specimens to G11 for ESR specimens (Fig. 2C and 2D). Beside these, as indicated in Tab. 1, concentration of P and S, as well as other alloying elements has been slightly reduced by ESR. On one hand ESR resulted in refined microstructure of spring steel, but on the other hand ESR also caused the appearance of some Al 2 O 3 inclusions [27], otherwise absent in CCC specimens. All three different tempering temperatures during heat treatment resulted in the martensitic microstructure, with very distinctive segregations being observed only in the case of CCC specimens. Mechanical and toughness properties After vacuum heat treatment hardness of both CCC and ESR spring steel specimens was more or less the same. More uniform microstructure of ESR specimens with reduced number of inclusions increased yield and tensile strength. At the lowest tempering temperature (300°C) yield strength has been increased from 1735 MPa to 1755 MPa and tensile strength from 1960 MPa to 1990 MPa. For the highest tempering temperature (475°C) yield strength has been increased from 1410 MPa to 1420 MPa and tensile strength from 1480 MPa to 1485 MPa when using ESR refining method. However, the increase was less than 1.5%. On the other hand ESR greatly reduced scattering of results, and increased contraction for up to 40%, especially for the lowest tempering temperature. More pronounced effect of ESR refinement was observed in the case of impact toughness. For CCC spring steel, tempered at 475°C impact toughness at 20°C was 20.6 J. By decreasing testing temperature to 0°C impact toughness of CCC spring steel dropped to 19.9 J and below -20°C even to 16.0 J. On the other hand, impact toughness for ESR specimens at 20°C increased to 22.0 J, which even at the lowest testing temperature of -40°C didn't dropped below 21.5 J, thus showing much better resistance to low temperature brittleness. Additional ESR refinement of investigated spring steel had practically no influence on the steel fracture toughness level. However, through more uniform microstructure and reduced number of inclusions scattering was reduced and fractured surfaces differs significantly (Fig. 3). In the case of CCC spring steel specimens fracture toughness after tempering at 300°C was 25.8±1 MPa m 1/2 and increased with higher tempering temperature at 375°C to 29.2±0 MPa m 1/2 and finally increased to 92.3±1 MPa m 1/2 at 475°C. On the other hand, for ESR specimens fracture toughness improvement was only marginal, increasing to 25.9±7 MPa m 1/2 , 31.0±0 MPa m 1/2 and 92.8±1.8 MPa m 1/2 when tempered at 300°C, 375°C and 475°C, respectively. Figure 3 : Fractured surface of KIc-test specimens tempered at 475°C (A) CCC and (B) ESR. Fatigue resistance Microstructure refinement through ESR, in contrast to static mechanical properties, didn't had any positive effect on fatigue properties of the investigated spring steel. Both CCC and ESR spring steel specimens showed the same level of scattering and confidence at dynamic bending testing. Nevertheless, results of the performed tests show, that conventional continuous cast spring steel (CCC) has better fatigue resistance (Fig. 4). For CCC spring steel specimens tempered at A B