Compression helical springs operate frequently under varying loads and deflections. High strength steels are typically used by the manufacturers, but high UTS value should not degrade toughness and fatigue resistance, which can be achieved by control of steel composition and proper heat treatment. In this paper was investigated failure of helical compression springs used on a train coach. Analysis included assessment of mechanical properties and observation of microstructure and fracture surfaces. Provided springs were made from 51CrV4 steel from different heat numbers. Results of tensile test showed high ultimate tensile strength, well above declared material properties but lower elongation. Nominal energies of Charpy impact tests were less than 20 % of required values. Chemical composition of steel complied with specification. Fractography analysis in SEM revealed brittle fracture on the whole surface without any striations otherwise indicating fatigue failure. Observation of microstructure revealed tempered martensite in whole cross-section without distinctive segregations, but prior austenite grain size was larger than specified almost by the factor of two. It was concluded that cause of failure was synergic effect of larger grain size, quenched microstructure and high UTS resulting in low fracture toughness and resistance to impact loadings.Keywords: Spring steel, fracture, mechanical testing, microstructure, fracture toughness
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