Characterization of Nb Interface Segregation During Welding Thermal Cycle in Microalloyed Steel by Atom Probe Tomography
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I.
INTRODUCTION
WELDING of niobium-bearing microalloyed steels was described as ‘‘Myths and Magic’’[1] due to many disagreements and some controversy on the role of niobium on the impact toughness of welding heat-affected zone (HAZ) in these steels.[2] Some researchers revealed that the small addition of Nb (0.01 to 0.04 wt pct) deteriorated the impact toughness of HAZ.[3–5] On the contrary, other researchers have demonstrated that Nb had beneficial effects or no significant influence on impact toughness, depending on the carbon level in the high-strength low-alloy (HSLA) steels and welding heat input.[6–10] It has been reported[11–13] that a small amount of Nb in the solid solution can lower c fi a transformation temperature leading to the formation of non-polygonal ferrite microstructure and increase in the yield strength. This effect has generally been attributed
H.H. WANG, Z. TONG, X.L. WAN, K.M. WU, and R. WEI are with The State Key Laboratory of Refractories and Metallurgy, Hubei Collaborative Innovation Center for Advanced Steels, International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan, China. Contact e-mail: [email protected] J. WANG, P.D. HODGSON, and I. TIMOKHINA are with the Institute for Frontier Materials, Deakin University, Geelong, Australia. Contact e-mail: [email protected] Manuscript submitted April 12, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
to the segregation of Nb to the prior austenite grain boundaries.[14,15] Moreover, Nb was found to promote the formation of martensite-austenite constituent (M-A), which decreased the absorbed energy.[11,13] Recent research has indicated that an addition of more than 0.10 wt pct Nb in the pipeline steel led to excellent impact toughness of HAZ even with higher welding heat input.[16] This was attributed to the grain refinement.[16] Our previous study has suggested that the addition 0.10 wt pct Nb promoted the bainite transformation, and increased the temperature of bainite transformation to ~ 700 C, which is higher than the conventional bainite transformation temperature. Furthermore, over a large range of cooling rate, bainite was the predominant transformation product in HSLA steel after continuous cooling.[17] The formation of bainite within a wide range of cooling rates can potentially increase the volume fraction of the stable, retained austenite that, in turn, improves toughness.[17,18] It was confirmed[19,20] that Nb as a solute has a strong tendency to segregate to the grain or interphase boundaries because of the large lattice misfit of Nb atoms in Fe. Although the equilibrium/nonequilibrium segregation of Nb has been studied,[21,22] there is still a lack of knowledge on segregation of Nb atom during welding thermal cycle. In the present study, the same Nb-bearing microalloyed steel that was used in our previous study[17] was employed. The distribution of Nb in the simulated coarse-grained heat-affected zone (CGHAZ)
was studied by atom probe tomography (APT). It provides an unde
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