B. Žužek et alii, Frattura ed Integrità Strutturale, 34 (2015) 160-168; DOI: 10.3221/IGF-ESIS.34.17 160 Focussed on Crack Paths Effect of segregations on mechanical properties and crack propagation in spring steel B. Žužek, M. Sedlaček, B. Podgornik Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Slovenia A BSTRACT . Considerable efforts have been made over the last decades to improve performance of spring steels, which would increase the service time of springs and also allow vehicles weight reduction. There are different possibilities of improving properties of spring steels, from modifying the chemical composition of steels to optimizing the deformation process and changing the heat treatment parameters. Another way of improving steel properties is through refining the microstructure and reducing amount of inclusions. Therefore, the focus of the current investigation was to determine the effect of more uniform and cleaner microstructure obtained through electro-slag remelting (ESR) of steel on the mechanical and dynamic properties of spring steel, with special focus on the resistance to fatigue crack propagation. Effect of the microstructure refinement was evaluated in terms of tensile strength, elongation, fracture and impact toughness, and fatigue resistance under bending and tensile loading. After the mechanical tests the fracture surfaces of samples were analyzed using scanning electron microscope (SEM) and the influence of microstructure properties on the crack propagation and crack propagation resistance was studied. Investigation was performed on hot rolled, soft annealed and vacuum heat treated 51CrV4 spring steel produced by conventional continuous casting and compared with steel additional refined through ESR. Results shows that elimination of segregations and microstructure refinement using additional ESR process gives some improvement in terms of better repeatability and reduced scattering, but on the other hand it has negative effect on crack propagation resistance and fatigue properties of the spring steel. K EYWORDS . Spring steel; Crack propagation; Electro-slag remelting; Segregations; Fracture toughness. I NTRODUCTION ncreasing demands of industry, especial automotive industry on performance improvement of steels put a lot of pressure on the steel and steel parts producers. The pressure is in weight reduction and cost savings what governs the producers in new design concepts and further material development. Weight reduction is very important because it reduces costs and even more importantly it reduces fuel consumption and CO 2 pollution in automotive industry. One of the biggest fuel consumers and polluters are trucks, where redesign and use of lighter high strength leaf springs can bring considerable benefits. Beside lower weight of springs, reduced dimensions also allow increased flexibility in design, enabling more room for additional safety components. Traditional parabolic leaf springs used in suspension systems of truck front axles are usually made of two leafs, and serve two main purposes: support the weight of the trailer and also I