Effects of Carbon Variation on Microstructure Evolution in Weld Heat-Affected Zone of Nb-Ti Microalloyed Steels

We investigated the effects of C concentration variation from 0.028 to 0.058 wt pct on microstructure of the coarse grained heat-affected zone (CGHAZ) of low heat input girth welded Ti-Nb microalloyed steels by using electron microscope and atom probe tomography. It is found that the CGHAZ microstructure exhibits a systematic response to C variation. Increased C raises the temperature for precipitation of NbC. This leads to coarser (Ti, Nb)N-Nb(C, N) but finer delayed strain-induced NbC in the high-C steel than in the low-C steel. Fine strain-induced NbC are ineffective in preventing austenite grain coarsening in CGHAZ due to their fast dissolution upon heating. For a given inter-particle spacing originally determined by (Ti, Nb)N particles, increased epitaxial growth of Nb(C, N) on pre-existing (Ti, Nb)N in the high-C steel results in a smaller austenite grain size of 34 µm in the CGHAZ of the high-C steel than that of 52 µm in the low-C steel. Increased C promotes a microstructure consisting of bainitic lath structure with C Cottrell atmospheres at dislocation debris and martensitic layers of 30 to 100 nm in thickness at inter-lath boundaries in the CGHAZ. Increased C promotes configuration of crystallographic variants belonging to different Bain groups in the neighbors, preferentially twin-related variant pairs within a bainite packet.