Determination of the Energetic Parameters Controlling Cleavage Fracture Initiation in Steels

Abstract

The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies ypm and y&, which defme the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the yp value remains constant around 7 J/mz, it has been observed that y-is dependent on temperature. At -196°C the value is lower than 50 J/mz and at room temperature it is higher than 200 J/m2. This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.Keywords-Cleavage fracture; Ti micro-alloyed steels; Effective surface energy; Critical failure stress. NOMENCLATURE a = particle size D = grain size E = Young's modulus f. = ferrite volume fraction rn = Weibull modulus n = Hollomon hardening coefficient UTS = ultimate tensile strength fi =misorientation angle 4 = correction for ellipticity v = Poisson's ratio u-, = maximum and minimum particle size D , , D-=maximum and minimum first cleavage facet size fuTa = coarse TiN particle volume fraction y-, yp = effective surface energy matrix-matrix and particlematrix, respectively u,,,, =yield stress uF, OF* = critical failure stress, macroscopic and local, respectivelyu-= stress necessary for crack to cross matrix-matrix boundary up = stress necessary for crack to cross particle-matrix boundary

Micromechanics of cleavage fracture initiation in ferritic steels by carbide cracking

“…However, the outcome of the present analysis depends strongly on the physical entities put in and also on the stress range under consideration. For example, there is a notable uncertainty as regards the surface energy, f , of the ferrite, and this value should perhaps be lower, as discussed by Linaza et al (1997). Furthermore, the initial yield stress used here, 200 MPa, is representative for pure ferrite and should be regarded as a lower bound for ferritic steels.…”

Failure of metals I: Brittle and ductile fracture

“…Schematically, cleavage of ferritic steels most frequently results from the succession of three elementary steps illustrated in Fig. 5 (See also [24][25][26][27][28][29]):…”

Micromechanical modeling of grain boundary resistance to cleavage crack propagation in ferritic steels

“…Therefore, the behavior of a polycrystal was prescribed to grain B, while grain A was given the behavior of a single crystal since it is assumed that cleavage initiates there along a plane perpendicular to the main loading direction. Thus the ratio B / A was set to 4 and thereby B = 160 N / m. Similar values are also reported from experimental studies by Hahn (1984); Martín-Meizoso et al (1994) and Linaza et al (1997) for cleavage crack growth across a ferrite grain boundary.…”