Analysis On Mechanism Of Cleavage Fracture At The Lower Shelf Of Ductile-to-brittle Transition Of A Low Alloyed Steel

This paper investigates the mechanism of cleavage fracture in precracked specimen made of modified C-Mn steel with three different grain sizes and carbide particles sizes (fine grain and fine carbide-particle microstructure,fine grain and coarse carbide particle microstructure,coarse grain and coarse carbide particle microstructure)at lower shelf region of ductile -brittle-transition temperatures. Based on observations of fracture surfaces and measurements of local critical parameters, detailed observations of configuration changes at precrack tips by metallographic cross sections in specimens unloaded at various applied loads and different low temperatures, sophisticated finite element method (FEM) calculations of distributions of stress, strain and triaxiality and analyses on the role of normal stressσL induced by the dislocation pile-up, the following main conclusions are drawn from this work. (1) According to series of Charpy-V impact tests of different low temperatures(-99℃ 20℃) for the three microstructures of C-Mn steel, it reveals that the dynamic yield strength increase with the test temperature decreasing ,the impact toughness decreases for different microstructures with the test temperature decreasing, ductile-brittle transition temperature(DBTT) of fine grain fine carbide microstructure is lower than that of coarse grain coarse carbide microstructure and fine grain coarse carbide microstructure, so the impact toughness of fine grain fine carbide microstructure is higher than that of the other microstructure .The effects of grain size on charpy toughness are higher than that of carbide size.(2) According to series of three point bending COD tests in specimens unloaded at different low temperature, it concludes that at the upper temperatures end (?90 to?60℃) of the low shelf transition temperature region:at the higher temperature (about?70℃), the cleavage fracture is controlled by the propagation of the ferrite crack acrossing the first grain boundary into the next grain ,the yield stress was high enough inside the first ferrite grain and the crack propagated into the next grain; at the lower temperature (about?90℃) , the cleavage fracture is controlled by the propagation of the carbide crack into contiguous grains, in this phase ,it should overcome an interfacial resistance between ferrite grain and carbide particle.(3) According to FEM ,it concludes that the lower shelf region can be divided into three phases and there different critical events at different phases: at the lower temperatures (?150 to ?130℃) , cleavage is controlled by crack nucleation in the microscopic scale and is driven by increasing applied load which blunts the precrack tip in the macroscopic scale; at moderate low temperatures (?130 to ?90℃) cleavage fracture is controlled by propagation of a second phase particle-crack into contiguous grains in the microscopic scale and is also driven by increasing applied load which further blunts the precrack tip in the macroscopic scale; at the upper temperatures (?90 to ?60℃) the cleavage fracture is controlled by the propagation of the carbide crack into contiguous grains in the microscopic scale, yet in addition to the crack tip blunting for compensating the drop of the yield stress a short fibrous crack is needed to extend from the precrack tip to increase the peak normal stress and to move it closer to the precrack tip. The results also reveal that the normal stressσL induced by the dislocation pile-up plays a key role for propagating the carbide crack into the matrix grain. With an increase in the applied load and the plastic strain at the site of an eligible crack-initiating particle, theσL increases through the work hardening. This not only compensates for the loss ofσ_yy but also further increases the total normal stressσyyt to trigger the cleavage fracture.