Effects of microstructure on cleavage fracture stress in steel

Abstract

It is shown, by compiling data from the literature, that there is a general relationship between the ferrite grain size and the size of the largest carbide particle in mild steels which are simply cooled after austenitization. By using this relationship, a cleavage fracture criterion derived by Smith is shown to predict a grain size dependence for the cleavage fracture stress of mild steel that is in good agreement with the results of many workers. These results indicate a value of 14 J m−2 for the effective surface energy of ferrite.Experimental results are presented showing the variation of the cleavage fracture stress of spheroidized steels with carbide particle radius. These support the suggestion that cleavage in such steels is due to the propagation of penny-shaped crack nuclei prod uced when spheroidal carbide particles crack. If the 95th percentile carbide radius is taken to represent the crack nucleus radius, an effective surface energy value of 14 Jm−2 is found to satisfy the fracture stress results. This value is in agreement with that deduced in the first part of the paper.