| Traditionally, pearlitic steels have been treated as homogeneous materials with properties that reflect their assumed homogeneity. In reality, pearlitic steels are two phase, lamellar microstructures containing ferrite, which is commonly considered to be ductile, and cementite, which is commonly considered to be brittle. Because of this microstructure, the properties such as the tensile strength and the cleavage fracture stress are dependent on the dimensional characteristics of the pearlite. Therefore, improvement of the mechanical performance of pearlitic steels resides with understanding the relationship between the microstructural characteristics and the mechanical properties.; The primary objective of this study was to define the microstructural and cleavage fracture stress relationship which exists for hypo-, hyper-, and eutectoid steel compositions with cementite contents ranging from 7 to 15 volume percent. By controlling the quench conditions, predominately pearlitic microstructures were achieved in plain carbon steels with carbon concentrations ranging from 0.45 to 1.0 weight percent.; Utilizing continuous cooling procedures, mechanical specimens were fabricated from AISI 1045, 1065, 1080, and 10100 grade steels. Mechanical testing to determine bend fracture strengths and tensile properties was performed at temperatures ranging from 125 to 250°C. From the nominal bend fracture strength and the yield strength, a determination of the cleavage fracture stress was made.; SEM analysis of polished and picral etched specimens allowed for the measurement of the pearlite dimensional characteristics including the spacing and cementite thickness. These parameters were then linked to the mechanical performance. Fractography on the bend specimens was utilized to identify the fracture initiation sites, and link the experimental and theoretical fracture behaviors.; The results of the mechanical testing and the correlation to the pearlite dimensional characteristics revealed that the cleavage fracture stresses of the five different pearlitic microstructures were inversely proportional to the square root of the pearlite spacings. The inverse square root relationship between spacing and cleavage fracture stress was linked to the increase in the effective surface energy for cleavage fracture which accompanies refinement of the pearlite spacing. Effective surface energies of 60 and 100 J/m 2 were determined for pearlite spacings of 0.11 and 0.08 micrometers, respectively. |
