The correlation of microstructure with the strength and fracture toughness of pearlitic steels
The role of composition, heat treatment and microstructural variables on the hardness, flow stress and fracture toughness has been investigated for a range of C-Mn steels. The study is divided into two parts. In Part 1, the influence of various ferrite-pearlite microstructures on the hardness and flow stress has been examined with respect to a Hall-Petch analysis. In contrast with previous reports it was found that a Hall-Petch equation can be applied satisfactorily in describing these properties provided care is taken in obtaining the appropriate mean slip distance for a given microstructure. The mean random interlamellar spacing was found to best quantify the microstructure when account is taken of the ferrite volume fraction, ferrite grain size, prior-austenite grain size and calculated cementite thickness. These parameters have been combined in a simple law of mixtures model to evaluate the mean slip distance in ferrite for a range of pearlite volume fractions between 20 and 100%. By substituting the mean slip distance for the effective ferrite grain size in a Hall-Petch equation both a positive friction stress and a very good correlation was obtained with measured hardness and flow stress data. In Part 2, the effect of changes in microstructure on the toughness of high carbon pearlitic steels was studied using standard Charpy, instrumented-impact and plane strain fracture toughness tests. Controversy in the literature regarding the influence of pearlite colony and prior-austenite grain boundaries in obstructing cleavage crack propagation has been resolved by close examination of the microstructure and fracture surface. The pearlite nodule size was found to be directly related to the cleavage facet size when proeutectoid ferrite is considered in hypo-eutectoid steels. Refining the pearlite nodule size by low austenitising temperatures and accelerated cooling gives improved toughness. Although the pearlite nodule size was shown to primarily determine the ductile-brittle transition temperature, it is suggested that the pearlite spacing may be of more importance as regards fracture toughness.