Effect of alloy design on improving toughness for X70 steel during welding

Highlights: • Microalloying precipitates were statistically analysed and correlated with Ti/N ratio. • CGHAZ toughness was evaluated using CVN and CTOD testing. • Significance of Ti/N ratio was found with respect to CGHAZ properties. • Correlation of Ti/N ratio and mechanical properties of CGHAZ was discussed. - Abstract: To identify and determine the optimal alloy design of Ti, the toughness in simulated coarse-grained heat affected zone (CGHAZ) of API 5L grade X70 steels with Ti/N ratios ranged from 1.9 to 4.9 was evaluated using Charpy V-notch (CVN) testing and crack tip opening displacement (CTOD) testing. A Gleeble 3500 thermo-mechanical simulator was utilised to produce the simulated CGHAZ with equivalent heat input of 2.5 kJ/mm. The CGHAZ microstructures including prior austenite grain size and precipitation were examined using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the microstructural constituents in the simulated CGHAZ for different Ti/N ratios consisted of bainitic ferrite and a small amount of martensitic-austenitic (M-A) constituents. In the studied range of Ti/N ratios, the statistical analysis of precipitates revealed that Ti/N ratio has little influence on the average precipitation size, which was controlled in a tight range (52–57 nm). However, at near-stoichiometric Ti/N ratio, higher number density of precipitates in the simulated CGHAZ certainly contributed to the finer austenite grain size and hence improved CGHAZ toughness performance.