MICROSTRUCTURAL EFFECTS ON THE CLEAVAGE FRACTURE OF PEARLITIC EUTECTOID STEEL (TOUGHNESS, EFFECTIVE SURFACE ENERGY, WORK HARDENING).

摘要

An experimental study was conducted to determine the effect of large, independent variations of the prior austenite grain size and the pearlite interlamellar spacing on the mechanical properties of fully pearlitic eutectoid steel, with an emphasis on the cleavage fracture properties.; The yield strength increased as the interlamellar spacing was decreased, or as the test temperature decreased from 20(DEGREES)C to -90(DEGREES)C, independent of changes in the prior austenite grain size. The work hardening exponent increased as the interlamellar spacing increased, while the rate of work hardening was independent of the interlamellar spacing; both phenomena were independent of the prior austenite grain size. The tensile reduction of area increased as either the prior austenite grain size or the interlamellar spacing decreased.; Cleavage fracture in blunt notched specimens initiated at inclusions well beneath the notch surface, and appeared to occur at a critical tensile stress level which increased as the interlamellar spacing decreased, independent of the austenite grain size. This critical stress, called the cleavage fracture stress, was independent of the temperature for fine interlamellar spacings, while it increased as the temperature decreased for coarse interlamellar spacings. The effective surface energy for cleavage crack growth seemed to increase as the interlamellar spacing decreased, independent of both the prior austenite grain size and the test temperature.; Cleavage fracture in fatigue precracked specimens did not originate at inclusions, but began slightly ahead of the precrack tip. Of the microstructures studied, only the combination of a coarse prior austenite grain size and coarse interlamellar spacing showed higher fracture toughness values at room temperature; all other microstructures had similar and lower toughnesses. The fracture toughness behavior was described by a model which attributes the toughness to two factors: first, the decrease in the local crack tip stress intensity, which depends on the magnitude of the crack deflections during the initial crack growth from the fatigue precrack; and second, the work required to fracture ligaments created between adjacent portions of the crack front as it propagates through the pearlite. Both the prior austenite grain size, through its control of the cleavage fracture facet size, and the interlamellar spacing, through its control of the yield strength and thus the plastic zone size, influenced the fracture toughness.