Abstract
The effects of various thermal treatments, i.e. oil quench and different tempering conditions, on quasi-static and impact fracture toughness, stress-strain characteristics, hardness and Charpy energy of 5140 H steel were examined. During quasi-static and impact loading notched round tensile specimens were used with a prefatigued crack. A specially designed device involving a pendulum hammer and electronic measuring system was used for measurement of the opening mode fracture toughness at loading rates of up to, KI = 3×106▪. It has been found that within the region of the lower tempering temperatures, 500 K≥T*≥650 K, the critical stress intensity factor KIc determined from impact testing is lower than that obtained during slow loading. Whereas at the higher tempering temperatures, 650 K≥T*≥ 900 K, dynamic KIc values have a tendency to be higher than their quasistatic counterparts. This behavior was analysed quantitatively using the Hahn-Rosenfield model which relates tensile properties to fracture toughness. A good agreement was found between quasi-static experimental results and the model. The relation between Charpy energy KV and the critical stress intensity factor KIc was also evaluated.
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