Issue34

B. Žužek et alii, Frattura ed Integrità Strutturale, 34 (2015) 160-168; DOI: 10.3221/IGF-ESIS.34.17 167 Fractured surfaces of specimens exposed to tensile-compression testing were analyzed using SEM and fatigue quasi- ductile fractured surfaces are shown in Fig. 7. The images of fractured surface are from the crack propagation zone, where obvious difference between fractured surface of CCC (Fig. 7A and 7C) and ESR (Fig. 7B and 7D) can be seen. In the case of CCC specimens fractured surface is smoother and consist of large number of separated short micro cracks being oriented perpendicular to the main crack patch through the specimen. It can be concluded that those micro cracks are formed during propagation of the main crack trough the specimen, when it reaches the softer negative segregation region in the material, containing sulphide-type non-metallic inclusions. At those inclusions the main crack changes its direction from transversal to longitudinal, thus retarding crack propagation rate. Energy needed for changes in the crack propagation direction coupled with longer crack propagation path also results in better fatigue resistance of CCC specimens. In the case of fractured surface of ESR specimens (Fig. 7B and 7D) very distinctive regions of fracture can be found, with staircase-like fracture regions being clearly visible. Fatigue test results show, that the absence of segregations, which leads to vivid larger staircase-like fracture regions and faster crack propagation result in reduced fatigue strength of the material, corresponding to reduced lifetimes of the spring steel. C ONCLUSIONS dditional process of electro-slag remelting contributes to more uniform and refined microstructure of spring steel as well as elimination of segregations. More uniform and refined microstructure mostly results in better repeatability and smaller scattering in mechanical and fatigue properties of examined spring steel. Results of this investigation revealed that ESR process increase mechanical properties of hot rolled and heat treated spring steel. It contributes to considerable increase in impact toughness, especially at sub-zero temperatures, while increase in hardness, tensile strength and fracture toughness is less pronounced. On the other hand, ESR results in reduced fatigue resistance and fatigue limit of the spring steel investigated. For both, tensile-compressive and bending loading conditions elimination of segregations and presence of some Al non-metallic inclusions contributes to reduced cracking resistance and deteriorated fatigue properties of spring steel. In the examined fractured surfaces additional ESR process leads to formation of staircase-like fracture regions with reduced crack propagation resistance and consequently to reduced fatigue properties of spring steel. In the case of the CCC specimens crack patch is changed at softer inclusions in the positive segregations, thus leading to formation of a large number of smaller cracks perpendiculars to the main crack direction. A CKNOWLEDGMENTS uthors would like to thanks Štore Steel company for the financial support and to Slovenian Research agency (ARRS) for the support through research program P2-0050. R EFERENCES [1] Perrard, F., Charvieux, F., Languillaume, J., A new spring steel with improved ductility dedicated for high strength parabolic leaf springs, 2nd Int. Conference Super-High Strength Steels, Peschiera del Garda, Italy, (2010). [2] Choi, S., Optimization of microstructure and properties of high strength spring steel, Ph.D. thesis, Posco, Korea, (2011). [3] Nam, W.J., Lee, C.S., Ban, D.Y., Effects of alloy additions and tempering temperature on the sag resistance of Si–Cr spring steels, Materials Science and Engineering, A289 (2000) 8-17. [4] Ona, H., Cold roll forming for high tensile strength steel sheet proposition on forming of thin spring steel sheet pipe, Journal of Materials Processing Technology, 153–154 (2004) 247-252. [5] Ai, J.H., Zhao, T.C., Gao, H.J., Hu, Y.H., Xie, X.S., Effect of controlled rolling and cooling on the microstructure and mechanical properties of 60Si2MnA spring steel, Journal of Materials Processing Technology, 160 (2005) 390-395. [6] Ardehali Barani, A., Ponge, D., Raabe, D., Refinement of grain boundary carbides in a Si–Cr spring steel by thermomechanical treatment, Materials Science and Engineering, A426 (2006) 194-201. [7] Nie, Y.-h., Hui, W.-j., Fu, W.-t., Weng, Y.-q., Effect of Boron on Delayed Fracture Resistance of Medium-Carbon High Strength Spring Steel, Journal of Iron and Steel Research, International, 14 (2007) 53-59. A A