engleski

Experimental investigation of fracture toughness parameters in quenched and tempered, and in austempered 65 Si 7 steel

The results of comparative investigations of two different heat treatment procedures on the magnitude of some fracture mechanics parameteres for a characteristic kind of steel are reviewed in this paper. The experimental investigations were performed on the specimens made of 65 Si 7 steel. The processes which arise by tempering of two different microstructures, i.e. martensite and lower bainite, and their influence on fracture toughness of the material were investigated. An advantage of austempering over hardening and tempering is the achievement of bainite microstructure. The austempering procedure is much simpler in comparison with the procedure of hardening and tempering, especially in continuous heat treatment, and a product is less inclined to occurrence and accumulation of microcracks because of lower residual stresses. Steel of bainite microstructure has a greater toughness, ductility, contraction, fatigue strength and a better fracture toughness than a tempered martensite for the same steel at the equal level of strength (hardness). A bainite microstructure also gives a better resistance to thermal fatigue in comparison with martensite microstructure. The above mentioned greater values of mechanical properties refer to untempered state of bainite. In order to investigate the influence of two different heat treatment processes, i. e. hardening and tempering vs. austempering, on the value of some fracture mechanics parameters in steel, a laboratory testing of fracture toughness of steel was carried out and diagrams of loading relationships - CMOD, CTOD-Da and J-integral -Da were recorded.

fracture mechanics; fracture toughness; tempering; austempering

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