Lecture
WEB On the grain boundary segregation of boron and its effect on phase transformations in steels
Thursday (24.09.2020) 16:10 - 16:25 S: Structural Materials 2 Part of:Boron (B) is the most efficient element in increasing the hardenability of low carbon steels even in small amounts (order of ppm) and is widely used in Press-Hardening steels. There are several proposed mechanisms for the effect of B on the hardenability. However, there is no systematic experimental approach to validate the proposed mechanisms in the atom-scale [1,2]. Here, we aim in understanding the B segregation on prior austenite grain boundaries (PAGBs) [3] and its effect on ferrite nucleation. To study this, we prepared two set of systems. One consists of pure Fe with different B contents (~20, 50 and 80 wt. ppm). The other set is of plain carbon steels (0.15 C-1.5 Mn-0.4 Si in wt.%) with B additions of 5 and 20 wt. ppm approx. Further, these alloys were heat treated in dilatometer. The heat treatments were designed such that the effect of B segregation on ferrite nucleation is evident. We studied B segregation at PAGBs using site-specific atom probe tomography (APT) with correlative transmission Kikuchi diffraction (TKD) [4]. These experiments revealed the grain boundary segregation quantitatively and its effect on phase transformation. In pure Fe systems, increase in B content (~20 to 50 ppm) delayed the transformation temperature upon quenching. However, further increase of B to 80 ppm increased the transformation temperature abruptly. The plain carbon steels showed higher B segregation compared with pure Fe-B systems emphasizing the effect of alloying elements.
Keywords: Boron, Grain boundary segregation, Atom Probe Tomography, Phase transformation
References
[1] C.R. Simcoe, A.R. Elseo, G.K. Manning, Study of the Effect of Boron on The Decomposition of Austenite, Trans. AlME. 203 (1955) 193–200.
[2] R.C. Sharma, G.R. Purdy, Nucleation Limitation and Hardenability, Metall. Trans. 5 (1974) 939–947.
[3] Y.J. Li, D. Ponge, P. Choi, D. Raabe, Segregation of boron at prior austenite grain boundaries in a quenched martensitic steel studied by atom probe tomography, Scr. Mater. 96 (2015) 13–16.
[4] J. Takahashi, K. Ishikawa, K. Kawakami, M. Fujioka, N. Kubota, Atomic-scale study on segregation behavior at austenite grain boundaries in boron- and molybdenum-added steels, Acta Mater. 133 (2017) 41–54.
