FRACTURE-TOUGHNESS CORRELATION WITH MICROSTRUCTURE AND OTHER MECHANICAL-PROPERTIES IN NEAR-EUTECTOID STEEL

The variation of yield strength and fracture toughness was investigate d for four different heat treatments attempted on specimens of a near- eutectoid steel. The aim of this study was to optimize the microstruct ure for simultaneous improvements in strength and toughness. Further, the fracture toughness deduced through empirical relations from tensil e and charpy impact tests was compared with those measured directly ac cording to ASTM Designation: E 399. Among the four different heat trea tments attempted in this study, the plane strain condition was valid i n the fracture toughness tests for (i) normalized and (ii) hardened an d tempered (500 degrees C for 1 h) treatments only. The latter of the two heat treatments resulted in simultaneous improvement of strength a nd plane strain fracture toughness. The finely-dispersed carbides seem to arrest the crack propagation and also increase the strength, The p earlitic microstructure of the former leads to easy crack propagation along cementite platelets and/or cementite/ferrite interfaces. The nat ure of variation of empirically determined toughness values from tensi le tests for different heat treatments is similar to that measured dir ectly through fracture toughness tests, although the two sets of value s do not match quantitatively. On the other hand, the toughness data d educed from charpy impact test is in close agreement with that evaluat ed directly from fracture toughness tests.