Effect of welding heat input on microstructural evolution, precipitation behavior and resultant properties of the simulated CGHAZ in high-N V-alloyed steel

Abstract

The effect of welding heat input characterized by the cooling time taken from 800 °C to 500 °C (t8/5) on the microstructural evolution, V(C,N) precipitation and resultant mechanical properties of the simulated CGHAZ in high-N V-alloyed steel was comparatively investigated using a Gleeble-1500D thermomechanical simulator. Metallographic analysis indicated that the dominant microstructure transformed from lath bainite to granular bainite when t8/5 increased from 30 s to 90 s and then changed to intragranular ferrite at t8/5 of 180 s. The nanoscale V(C,N) precipitates were coarsened as t8/5 increased, coupled with a slightly increased number density. Furthermore, the orientation relationship [001]α//11¯0V(C,N) reduced the interfacial structural energy (MC/α) and enhanced the V(C,N) precipitation in the ferrite matrix. Moreover, the microhardness progressively decreased due to the combined effect of microconstituents, precipitates and grain boundaries. The impact toughness first decreased and then increased, and the optimal value was obtained at t8/5 of 180 s. Furthermore, the formation of intragranular ferrite, especially acicular ferrite, and the increased fraction of high angle grain boundaries could completely remedy the detrimental effect caused by the increased content of grain boundary ferrite and the coarsened effective grain size and precipitates. In addition, the high nitrogen content of 240 ppm accelerated V(C,N) precipitation in the austenite region, and the submicron V(C,N) precipitates could promote the formation of intragranular ferrite by providing heterogeneous nucleation sites, in combination with the larger prior austenite grain size caused by higher welding heat input.