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

Ma, Xiaoping; Li, Xueda; Langelier, Brian; Gault, Baptiste; Subramanian, Sundaresa; Collins, Laurie

doi:10.1007/s11661-018-4751-8

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

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Gault, Baptiste
Atom Probe Tomography, Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung GmbH, Max Planck Society;

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Citation

Ma, X., Li, X., Langelier, B., Gault, B., Subramanian, S., & Collins, L. (2018). Effects of Carbon Variation on Microstructure Evolution in Weld Heat-Affected Zone of Nb–Ti Microalloyed Steels. Metallurgical and Materials Transactions A, 49(10), 4824-4837. doi:10.1007/s11661-018-4751-8.

Cite as: https://hdl.handle.net/21.11116/0000-0001-E5C4-F

Abstract

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. © 2018 The Minerals, Metals Materials Society and ASM International