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Grain Boundary Segregation Behavior of Boron in Low-Alloy Steel
GENICHI SHIGESATO, TAISHI FUJISHIRO, and TAKUYA HARA
The boron concentration proles around prior austenite grain boundaries in Fe-0.05C-0.5Mo-0.001B (mass pct) are examined using aberration-corrected STEM-EELS. In order to obtain the precise distribution of boron around the boundaries, tilt series measurements with thin specimens (<30 nm) are performed and the EEL spectra are analyzed by principal component analysis (PCA) and multivariate curve resolution (MCR). The boron concentration prole changes with the cooling rate from the solid solution temperature. The concentration at grain boundaries is maximized at a medium rate (30 C/s), where the concentration reaches 8 at. pct, and it decreases at a larger (250 C/s) or smaller (5 C/s) rate. On the other hand, the boron distribution becomes wider as the cooling rate becomes smaller. The current results suggest that the boron segregation in the alloy is formed by the non-equilibrium segregation mechanism.
DOI: 10.1007/s11661-013-2155-3 The Minerals, Metals & Materials Society and ASM International 2014
I. INTRODUCTION
SINCE boron highly improves hardenability of steel in a small amount, boron addition is a desirable method for producing high-strength steel products without much alloying of expensive elements. However, careful attention to the production process is required to control the boron eect on hardenability because it is quite sensitive to the form of boron. It is widely accepted that solute boron segregated to austenite grain boundaries retards the nucleation of allotriomorphic ferrite on austenite grain boundaries, resulting in the improvement of hardenability.[13] It has also been reported that boron can easily lose its potency with precipitation.[1,4]
Excessive addition of boron leads to precipitation of borocarbides and/or boron nitrides; as a result, boron becomes ineective. Therefore, for controlling the eect of boron, it is essential to understand the behavior of boron: the change of the form and quantity on the austenite grain boundaries with the chemical compositions of the steel and heat treatment conditions.
The segregation of solute boron to austenite grain boundaries is considered to occur by two dierent mechanisms: equilibrium and non-equilibrium segregation. These mechanisms show dierent dependences of the solid solution temperature and cooling rate. According to the equilibrium segregation mechanism, the concentration of solute atoms at grain boundaries decreases exponentially as the solid solution temperature increases.[5]...