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D. Wipp, L. Volgger, H. Hutter, A. Gramlich, M. Weiss, A. Limbeck, S. Zamberger, M. Galler, E. Povoden-Karadeniz:
"Successful determination of trace element boron in steel by combined high-resolution analytical techniques";
Talk: Euromat 2019, Stockholm; 2019-09-01 - 2019-09-05.

Introduction/Purpose
Analysis of light elements in the order of parts-per-million (ppm) in microalloyed steel still presents
tremendous challenges. To control a complex microstructure and consequently the mechanical
properties, a deep understanding of the distribution and concentration of elements in steel is essential.
Boron, in the order of ~30 ppm, is used to increase hardenability of steel. This effect occurs only if
Boron is in solid solution throughout the whole material and is not forming precipitates. Therefore, it is of
great interest to evaluate the Boron distribution and to assert the nature of its occurrence: elemental
versus compound-bounding. A set of analytical methods was applied to make a substantiated statement.
Methods
Scanning Electron Microscope (SEM) was used to analyse the microstructure of the cross-section of a
steel wire. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) showed alloying
element concentration profiles in detail. Elemental-mapping by Time-of-Flight Secondary Ion Mass
Spectrometry (ToF-SIMS) was utilized and compared to SEM images. With these analysis techniques, a
Boron enriched region was defined, which was chosen for exact examination. This close up was
possible by Atom Probe Tomography (APT or 3D Atom Probe) which led us to full clarity.
Results
LA-ICP-MS results showed a heterogeneous Boron distribution in the bainitic steel matrix, therefore a
deeper understanding of the Boron profile was necessary. By ToF-SIMS BO2
- was characterized on
grain boundaries. With APT, Boron oxide and Boron nitride were found as segregates at and next to
grain boundaries. Although no elemental Boron has been found by APT, additional ToF-SIMS analysis
with high-current-bunched-mode (HCBU) proved the prevalence of elemental Boron. This indicates that
measured Boron oxide in APT was mainly a consequence of atmospheric oxygen, affecting specimens
during preparation.
Conclusions
By application of complementary analysis methodologies, we unambiguously clarified the condition in
which Boron exists in the microstructure of Boron-containing microalloyed steel.

Project Head Erwin Povoden-Karadeniz:
CDL-IPE