Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low allo
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2/20/04
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Mechanisms and Modeling of Cleavage Fracture in Simulated Heat-Affected Zone Microstructures of a High-Strength Low Alloy Steel A. LAMBERT-PERLADE, A.F. GOURGUES, J. BESSON, T. STUREL, and A. PINEAU The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductileto-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.
I. INTRODUCTION
HIGH-STRENGTH low alloy (HSLA) steels are now widely used for structural applications. These materials combine excellent tensile strength and ductile-to-brittle transition (DBT) properties. However, this combination of high strength and high fracture toughness can be deteriorated after welding thermal cycles. The degradation of the fracture toughness of HSLA steels after welding is attributed to the formation of “local brittle zones” in the welded joint.[1,2] Significant embrittlement can be encountered in the coarse-grained heataffected zone (CGHAZ) and, in particular, in the intercritically reheated CGHAZ (ICCGHAZ) of multipass welded joints (e.g., References 3 through 7). According to literature data, the fracture toughness of these local brittle zones is influenced by metallurgical factors such as prior austenite grain size, bainite packet size, and distribution of second phases such as carbides and martensiteaustenite (M-A) constituents (e.g., References 5 and 8 through 10). The cleavage-facet size has been related to the prior austenite grain size.[9] The bainite packet boundary influences the brittle fracture properties of the heat-affected zone (HAZ) in a HY-80 grade steel[11] as well as in other bainitic steels (e.g., References 12 through 15). On the other hand, M-A constituents have a deleterious effect on both crack initiation and propagation (e.g., References 2, 8 through 10, 16, and 17). However, correlation between welding thermal cycles, the resulting microstructures, and fracture toughness properties has only been scarcely evidenced in detail yet. Better understanding of the fracture micromechanisms operating in these complex microstructures is still needed, which is the aim of the present study. A. LAMBERT-PERLADE and T. STUREL, Research Engineers,
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