Further investigation of critical events in cleavage fracture of C-Mn base and weld steel
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INTRODUCTION
THE process of the fracture including cleavage cracking has been described as follows: When the applied load is increased, s l i p s occur in the grains in the vicinity of the notch root or precrack tip, and dislocations move and pile up at the boundaries of the second-phase particles or inclusions. Then the microcrack in the second-phase particle or inclusion is initiated, and extends stably through the matrix. With the increase in the length of the fiber crack, the normal stress Oryy ahead of it increases due to the increase of o-y by the strain hardening and the increase of stress intensification Q. When the normal stress exceeds the local fracture stress o-s, the criterion O'yy = Qo'y > o"i for the unstable propagation of a new microcrack initiated ahead of the fiber crack is reached and fracture occurs, m At a low temperature, the dislocationinduced microcrack could extend catastrophically without the stage of stable propagation. In C - M n steel, the controlling link, i . e . , the critical event of cleavage, is considered to be the unstable extension of the microcrack w i t h critical length.121 For some time, it has been disputed whether the critical event is the propagation of a ferrite grain-sized microcrack (FC) or that of a second-phase particle-sized microcrack (SC). In early work from the viewpoint of cracks induced by pileup of dislocation, the critical event was taken as the unstable extension of an FC.f3] In latter work based on Smith's model of cleavage induced by boundary carbide,t4~ Curry and KnotttS~ took the propagation of a crack of carbide (CC) as the critical event and considered that the available relationship between the local fracture stress o-s and the sizes of ferrite grains J.H. CHEN, Professor and President, L. ZHU, Lecturer, G.Z. WANG, Lecturer, and Z. WANG, Professor, are with the Welding Institute, Gansu University of Technology, Lanzhou, People's Republic of China. Manuscript submitted October 4, 1991. METALLURGICAL TRANSACTIONS A
was caused by the general relationship between the sizes of ferrite grains and that of carbides. In 1984, Hahn [6J proposed that the grain sizes were accorded a diminished role. The critical event could be the extension of CC, 15[ M - A constituent, t71 inclusion,t81 or pearliteJ91 Reference 10 identified with several evidences that the critical event of cleavage fracture in notched specimens of C - M n weld steel tested at - 6 0 °C was the extension of an FC (30 t o 40 /xm), though it could be nucleated in a particle of inclusion, M-A constituent, carbide-ferrite aggregate, and pearlite. In Reference 11, it was further found that the critical event was different between the notched and precracked specimens made of the identical weld steel. The critical event for the notched specimen tested at - 6 0 °C was the unstable extension of an FC (30 to 40 /zm), w i t h the corresponding value of o-i of 1700 MPa. However, the critical event for the precracked specimen tested at - 1 1 0 °C was the extension of an SC (< 10/zm), w i t h the correspo
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