Christian Doppler Laboratory

          Early Stages of Precipitation

Christian Doppler Laboratory

          Early Stages of Precipitation

 

 

Project manager: Sabine Zamberger

 

The idea to use boron to increase hardenability of mild and low alloyed steels is facing a long history; first trials date back to the beginning of the 20th century. The peculiarity of this alloying element is that minute quantities are sufficient to show a significant hardenability effect, which offers the opportunity to reduce the amount of conventional and more expensive alloying elements like Mo, Cr, and Ni etc. It is assumed that, during the heat treatment, boron atoms segregate to the austenite grain boundaries. As a consequence, during the cooling process, the nucleation of ferrite at the grain boundaries is delayed. However, there are still open questions, as for example the lattice position of boron in ferrite, which mechanism, equilibrium or non‑equilibrium segregation, is the underlying one for the hardenability enhancement, or precipitation kinetics of boron alloyed steels. One reason is that, due to the low content of boron in steel, detection and quantification of boron enriched zones are rather difficult.
Therefore, one part of the project is focused on the evaluation of characterisation methods to find out where boron is located and in which condition, either as segregant or precipitate. The applied methods are light optical microscopy, scanning electron microscopy plus EDX‑analysis, electron probe microanalysis, time of flight secondary ion mass spectroscopy, differential scanning calorimetry, transmission electron microscopy and atom probe field ion microscope. The other part of the project deals with the numerical simulation of the boron behaviour as a function of time and temperature.

 

Figure: ToF-SIMS 3D reconstruction of MnS (blue) surrounded by BN (red+green) in a not Ti-stabilized boron alloyed steel.