Fracture mechanism and toughness of the welding heat-affected zone in structural steel under static and dynamic loading
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I. INTRODUCTION
A special region denoted as the heat-affected zone (HAZ) forms as a result of thermal cycle experience in the parent metal during welding. For single-pass welding, the region of lowest toughness is generally associated with the coarsegrained heat-affected zone (CG HAZ).[1,2] During multipass welding, the HAZ formed by the previous welding heat cycle is modified by the subsequent thermal cycles, forming localized and discontinuous zones. In the HAZ formed by multipass welding, the IC CG HAZ is found not only to demonstrate low toughness,[3–6] but also to have lower toughness than CG HAZ.[3,6,7] The low toughness of the CG HAZ and the IC CG HAZ was attributed to local brittle microstructure M-A constituent (high-carbon martensite with some retained austenite).[3,5,6,8–10] Further studies discovered that for submerged arc welding, brittle fracture initiation points are associated with the intersections of a0B areas with different orientations rather than with the M-A constituent.[7,11,12] Moreover, other microstructures, such as a banding structure comprised of ferrite and cementite similar to pearlite, precipitated in high heat input welding, also affect toughness.[13] Therefore, for a given steel, its fracture mechanism should relate to its microstructures and welding parameters. In this work, the brittle fracture initiation and crack propagation of the IC CG HAZ of a structural steel under static and dynamic loading were examined. Through the analysis
of its fracture mechanism, the effect of strain rate on fracture toughness and the dependence of fracture toughness on fracture mechanism were revealed. The influence of thermal experience caused by welding on fracture toughness under static and dynamic loading was also investigated. II. EXPERIMENTAL A hot-rolled steel, SN490, with chemical compositions 0.11C/0.29Si/1.39Mn/0.01P/0.02S wt pct was selected for this work. SN490 consists of ferrite and pearlite, as shown in Figure 1. In fact, the actual IC CG HAZ in a welded joint is so narrow that it is sometimes impossible to prepare test specimens. To overcome this problem, welding thermal-cycle simulation techniques were applied in this work. Since simulated HAZ tests give the same toughness ranking, although not the same absolute values,[3] as those on actual welding HAZs, a simulated IC CG HAZ can be used to substitute for an actual IC CG HAZ. The typical welding thermal cycles are shown in Figure 2. The terms Tp1 and Tp2 are the peak temperatures of the first and second thermal cycles, respectively. The time cooling from 800 8C to 500 8C is denoted as t8/5. Two thermal series, TA (Tp1 5 1350 8C, Tp2 5 750 8C, and t8/5 5 30 s) and TB (Tp1 5 1350 8C, Tp2 5 750 8C, and t8/5 5 75 s), were applied. For the welding of thick plate, t8/5 is approximately proportional to heat input E t8/5 5 kE
H. QIU, Research Associate, is with the National Research Institute of Metals, Ibaraki 305-0047, Japan. H. MORI, Senior Research Engineer, is with the Railway Technical Research Institute, Tokyo 185-8540, Japan. M.
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