Lecture
On the grain boundary segregation of boron and its effect on phase transformations in steels
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.
