Fracture toughness correlation with microstructure and other mechanical properties in near-eutectoid steel

The variation of yield strength and fracture toughness was investigated for four different heat treatments attempted on specimens of a near-eutectoid steel. The aim of this study was to optimize the microstructure for simultaneous improvements in strength and toughness. Further, the fracture toughness deduced through empirical relations from tensile and charpy impact tests was compared with those measured directly according to ASTM Designation: E 399. Among the four different heat treatments attempted in this study, the plane strain condition was valid in the fracture toughness tests for (i) normalized and (ii) hardened and tempered (500°C for 1 h) treatments only. The latter of the two heat treatments resulted in simultaneous improvement of strength and plane strain fracture toughness. The finely-dispersed carbides seem to arrest the crack propagation and also increase the strength. The pearlitic microstructure of the former leads to easy crack propagation along cementite platelets and/or cementite/ferrite interfaces. The nature of variation of empirically determined toughness values from tensile tests for different heat treatments is similar to that measured directly through fracture toughness tests, although the two sets of values do not match quantitatively. On the other hand, the toughness data deduced from charpy impact test is in close agreement with that evaluated directly from fracture toughness tests.