Transmission Electron Microscopy Studies of Plasma Arc-Welded DP600 Dual-Phase Steel in Keyhole Mode
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Microstructural characteristics of keyhole plasma arc-welded DP600 steel were analyzed using optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. The fusion zone (FZ) was observed to consist of allotriomorphic, Widmanstätten, and acicular ferrite along with bainite and martensite leading to enhancement in the hardness compared with the base metal. The coarse-grain HAZ consisted of bainite and martensite, while the fine-grain HAZ consisted of bainitic ferrite and tempered martensite. The sub-critical HAZ was found to consist of tempered martensite with reduced density of dislocations and carbide precipitation. This softening resulted in yield point phenomena and failure in the sub-critical HAZ during the transverse tensile test of welds. The non-isothermal tempering resulted in retarded cementite precipitation due to inadequate time for diffusion of carbon, which led to the lowest hardness of 168 HV0.5 in the sub-critical HAZ. The different features in the FZ and HAZ and their correlation with mechanical properties are discussed.
One of the authors (S.R. Bakshi) acknowledges funding from Institute Research and Development Award (Junior Level) of IIT Madras (MET/16-17/839/RFIR/SRRB) for carrying out the work. Authors also acknowledge funding received from Centre of Excellence in Steel Technology (MET/16-17/148/MSTE/HODX) for funding the studies.
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