Effect of high heat input welding on the microstructures, precipitates and mechanical properties in the simulated coarse grained heat affected zone of a low carbon Nb-V-Ti-N microalloyed steel

The significant influence of the welding heat input on the microstructural evolution, precipitation behavior and resultant properties of the coarse grained heat affected zone (CGHAZ) in low carbon Nb-V-Ti-N quaternary microalloyed steel was comparatively studied on a Gleeble 3800 thermomechanical simulator. Metallographic investigations revealed that the dominant microstructure transformed from lath bainite (LB) at heat input of 50 kJ/cm to intragranular acicular ferrite (IAF) at 100 kJ/cm, to IAF and intragranular polygonal ferrite (IPF) at 150 kJ/cm and to IPF, IAF and grain boundary ferrite (GBF) at 200 kJ/cm. In addition, Ti- and Nb-enriched submicron-scale precipitates promoted IAF and IPF nucleation with an orientation relationship 011 MC// [ ]  11 1 Ferrite and effectively pinned grain boundaries; V-enriched nanoscale precipitates acted as obstacles for dislocation movement, consequently improving CGHAZ performance. Moreover, the impact absorbed energy at 20 degrees C and -20 degrees C first increased from 216 & PLUSMN; 4.3J and 58 & PLUSMN; 2.7J to 219 & PLUSMN; 3.7J and 64 & PLUSMN; 3.0J, respectively, and then decreased to 184 & PLUSMN; 7.5J and 45 & PLUSMN; 2.1J, respectively, when the heat input increased from 50 kJ/cm to 200 kJ/cm. The maximum value was obtained at heat input of 100 kJ/cm, which was primarily attributed to the largest amount of high angle grain boundaries originating from IAF. The microhardness slightly reduced as heat input increased from 50 kJ/cm to 100 kJ/cm and did not obviously decrease as heat input varied from 100 to 200 kJ/cm, which was directly related to the microstructure constituents and nanoscale precipitates. Furthermore, the optimal heat input to obtain a good combination of strength and toughness was 100 kJ/cm, and Nb-VTi-N microalloyed steel was suitable for high heat input welding.