Recipe for improving the impact toughness of high-strength pearlitic steel by controlling the cleavage cracking mechanisms

Achieving superior impact toughness in high-strength pearlitic steels is extremely desirable and challenging due to the brittle character of the intervening cementite lamellae. Besides the 'Hall-Petch' type lamellar strengthening, a systematic understanding of various micro-toughening mechanisms through cleavage cracking resistance is essential to develop an integrated microstructure for toughness improvement. In this study, both thermal as well as thermo mechanical processing routes were adopted to correlate the processing and micro-structural parameters with the mechanical properties. The prime contribution of pearlite nodule size refinement towards improving the impact toughness is discussed in terms of frequent cleavage crack deflection at the nodule boundaries. Other microstructural parameters controlling the toughness are the pearlite morphology (lamellar or spheroidized), interlamellar spacing and the cementite lamellae orientation with the crack path. It appears that both spheroidized and fine lamellar pearlite are more effective crack arresters than their coarse lamellar pearlite counterparts. Besides, the variation of crack growth resistance with the pearlite lamellae orientation and the ferrite-cementite interface (habit) plane is also presented, relating to the mechanism of interface decohesion.