Influence of Ti/N ratio on simulated CGHAZ microstructure and toughness in X70 steels

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 mm in diameter. © 2013 Institute of Materials, Minerals and Mining.