Mechanism of detrimental effects of carbon content on cleavage fracture toughness of low-alloy steel
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ALTHOUGH it is well known that carbon plays a dominant role in the mechanical behaviors of steels, some aspects of its microstructural and micromechanical mechanisms still remain obscure. While the strength of steel is remarkably increased by raising its carbon content, its toughness is definitely deteriorated. In a ferrite-pearlite steel, this is phenomenally attributed to the formation of carbides, which prohibits dislocations from moving and which induces the crack nucleation. McMahon and Cohen[1] first revealed that the cleavage cracks were initiated by the cracking of carbide. Following this work, Smith[2] developed a model incorporating both the grain size and the thickness of grain boundary carbide. Hodgson and Tetelman[3] further indicated that carbide particles may act as Griffith-type flaws and that the cleavage stress f is then determined by the maximum size of those particles. Based on compiled data showing a general relationship between the ferrite grain size and the size of the largest carbide particle in mild steel, Curry and Knott[4] modified Smith’s model and obtained a cleavage criterion involving only the thickness of the largest grain boundary carbide. But, as indicated by Chen et al.,[5] the critical event controlling cleavage in a notched specimen is the propagation of a ferrite-grain-sized crack rather than the propagation of a grain-boundary carbide crack into the matrix. What is the role played by carbon content in the context of grain-sized crack propagation-controlled cleavage? The effect of carbon content on the cleavage fracture toughness of low-carbon bainitic steels was explained by Brozzo et al.[6] It was found that the local fractures stress f was virtually independent of C content in the range from 0.025 to 0.05 pct and dependent only upon the average J.H. CHEN and G.Z. WANG, Professors, are with The Welding Research Institute, Gansu University of Technology, Lanzhou, Gansu, 730050, P.R. China. S.H. HU, formerly Graduate Student, The Welding Research Institute, Gansu University of Technology, is Doctorate Student, Zhe Jiang University of China, Zhe Jiang, P.R. China 310000. Manuscript submitted May 22, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
size of the bainitic packet dB (the microstructural unit size). Because f will not vary if dB is kept constant, any increase in yield stress due to an increase in C content will cause a decrease in fracture toughness. Recently a dual-criteria model for cleavage fracture of notched specimens was proposed,[6,7] in which the effect of plastic strain-induced initiation of crack on the fracture toughness was emphasized again, despite acceptance over the past 4 decades that the propagation of a crack nucleus under a normal stress is the dominant controlling factor for cleavage fracture.[8] In this work, the mechanism of the effect of carbon content on the fracture toughness was studied in detail by investigating the effects of carbon on yield strength; the critical plastic strain pc for crack nucleation; and the local fracture stre
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