Effect of High Strength Structural Steel Structural State on Fracture Resistance

Fracture resistance of low- and medium-carbon steels depending on type of microstructure (ferritepearlite, bainite, and martensite) and its volume fraction is studied. On the basis of studying the temperature dependence for impact strength of V-notched samples and pre-cracked specimens the contribution of work for crack initiation and propagation to the general specific impact energy of steels with a different structural type is revealed. It is shown, that the main part of failure energy in low-carbon steels with ferrite-pearlite structure is work for crack initiation. For low-carbon martensite steels the work for crack initiation and propagation is comparable. Failure resistance of tempered low- and medium-carbon steels depending on the quantitative relationship of martensite, bainite and ferrite structures is described. Characteristic structural features of lower and upper bainite influencing fracture resistance are detailed. It is shown that failure resistance of such complex structures is governed by properties of the components, and also their amount and location. Polygonal ferrite, whose separation occurs mainly at the boundaries of previous austenite grains, has a particularly unfavorable effect on ductility properties and cold resistance.