Micromechanism of cleavage fracture in fully pearlitic steels

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Abstract
The micromechanism of cleavage fracture in a fully pearlitic steel has been investigated. Uniaxial tensile and compression test specimens, together with single notched bend (SNB) and double notched bend (DNB) specimens, were heat treated such that the prior austenite grain size remained constant while the pearlite interlamellar spacing was varied. The SNB specimens were used to determine the cleavage fracture stress σfM, over the temperature range −25 to −196°C. The DNB specimens were used to study the initial stages of crack nucleation. The results indicate that pearlite can exhibit two different cleavage mechanisms which are dependent on the strength of the steel. For cleavage fracture stresses below about 2100 MN m−2, fracture is nucleation controlled and involves shear linking of carbide nucleated microcracks before unstable cleavage can occur. Under these conditions, the cleavage fracture stress is dependent on temperature and is proportional to the uniaxial proof stress. For cleavage fracture stresses above 2100 MN m−2, cracked carbides act directly as cleavage nuclei. Fracture is then propagation controlled and the cleavage fracture stress is independent of temperature. The transition from nucleation–controlled to propagation–controlled cleavage may be achieved by either a reduction in pearlite interlamellar spacing or a reduction in testing temperature. MST/355