Parameters of load-displacement curves and their correlation with the fracture surface in medium-carbon steel

Impact tests are mandatory for a wide range of structural steel products to evaluate the required service life and operational safety. This test is especially relevant used in operation climatic areas with cold climate, since it allows one determine temperature ductile-brittle transition. Depending on structural-phase state metal, type relationship impact toughness its displacement axis may differ significantly. For ductile materials, transition shifted towards ultra-low negative temperatures whereas materials embrittled condition lies region higher temperatures. Ductile-brittle behavior medium-carbon 0.32 % C-1.35 Mn-0.003 B two conditions (after normalization after quenching tempering) by instrumented testing fracture analysis was studied. On descending branch force-displacement curves periods corresponding different zones propagation main crack distinguished: period I zone pure shear, II brittle fracture, III break fracture. Each can be quantitatively described set strength, plastic gradient parameters. It established that beginning linear (SIIb) slope curve (ΔF / ΔS)II have better correlation shear percentage (B) than other distinguished parameters curves.