Experimental Study On The Effects Of Welded Joints Microstructure On CTOD Of High-strength Steel And Flaws Assessment Based On BS7910

As the huge complex welded structure and being in harsh marine environment, the ocean engineering structures always suffered the brittle and fatigue failures, which resulted in great losses of life and property in past few centuries. It’s significant to research fracture toughness and assessment of flaws for welding metallic.The main work of this dissertation is based on the project of "Key technologies of total construction and installation of semi-submersible offshore oil platforms", supported by Ministry of Industry and Information Technology. In the present work, the toughness of welded joint are assessed based on the British Standards7448, while the microstructure and the fracture morphology are observed by SEM. At the end, three assessment methods of BS7910:2005"Guide to methods for assessing the acceptability of flaws in metallic structures" and its application on CTOD are described. Main contents and conclusions of this dissertation are as follows:1、This dissertation presents the social background and significance of CTOD tests, and assessing welded joint EQ56, EQ70/56and EQ70with CTOD toughness criterion. The results show that the CTOD values in weld centerline are qualified, but parts of CTOD value in the fushion line are little deviation and big dispersion.2、Make the scan electron spencimens by cutting from38mm thick steel plates of EQ56, and observe its fracture morphology. The fracture morphology of EQ56shows the ductile dimple fracture patterns, slight tearing edges and secondary cracks. It means that the fractograph with samller CTOD value have some toughness but unsatisfactory. Result shows that fracture morphology can’t reflect individual differences,and it need observed by SEM still further.3、The recent research progresses on microstructure of high strength steel welded joint are introduced. The microstructure of welded joint’s weakest zone-CGHAZ by cutting from44.5mm thick steel plates of EQ56is observed by SEM and relationship with CTOD toughness is discussion in dissertation.4、Three levels of fracture assessment in BS7910:2005are introduced. CTOD specimens were assessed through this guide with results of CTOD test. The assessments on welded joints of EQ70/56and EQ70show that their CTOD values are in the acceptable range. This method provides new way of thought in researches of allowed CTOD value.