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Abstract
Three API 5L X70 steels with different Ti and N contents and otherwise identical chemistry were selected to investigate the effect of Ti/N ratio on the toughness in coarse grained heat affected zone (CGHAZ). A Gleeble 3500 thermomechanical simulator was used to simulate the thermal profile of CGHAZ of double submerged arc welding process. The microstructure was examined by optical microscopy. Statistics of CGHAZ grain coarsening were compiled by measuring the prior austenite grain size. Toughness of the simulated CGHAZ regions was evaluated by Charpy V-notch testing at −20 and −40°C. Morphology of the impact fracture surface was investigated using SEM. Steel B with Ti/N ratio of 3·22 (slightly below stoichiometric) showed slightly higher toughness in the simulated CGHAZ due to higher volume fraction of austenite grains less than 80 μm in diameter.
This work was funded by the Energy Pipelines CRC, supported through the Australian Government’s Cooperative Research Centres Program. The cash and in-kind support from the APIA RSC is gratefully acknowledged. BaoSteel, China is thanked for the materials to conduct this research. Special thanks are extended to Liang Chen and Bob De Jong for their great assistance on Gleeble simulation. One of the authors (Zhixiong Zhu) is also grateful for the financial support from China Scholarship Council (CSC).