Faculty of Georesources and Materials Engineering of the RWTH Aachen, Germany
Doi
Abstract
Boron (B) is the most effective alloying element for increasing the hardenability of steel. It achieves its remarkable effect on hardenability through the grain boundary (GB) segregation at austenite GBs. This suppresses the heterogeneous nucleation of ferrite. It is still fundamentally unclear how B achieves its remarkable effect on the ferrite nucleation and their mechanisms remain not fully understood. Carbon (C), on the other hand, is an important alloying element concerning the mechanical properties of steel. Though the carbon effects on various phase transformations are rather understood well, its GB segregation and local decoration effects at GBs remain not understood completely. The thesis aims to uncover the fine details about the GB segregation of B and C and then to relate its effect on the bulk phase transformation. In the present thesis, two systems of alloys are designed to study the GB segregation of B and C. One is binary Fe-B alloys with increasing B content in it. Another system is a typical low C steel with and without B. These alloys are subjected to various heat treatments in the dilatometer, in a controlled manner. Electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and atom probe tomography (APT) are employed on the heat-treated specimens for the site-specific correlative investigation to probe GBs. Through systematic study and careful investigation, the austenitization temperature and the cooling rate dependence of B segregation are unraveled. The corresponding segregation mechanisms are discussed in detail. It has been found that the precipitation of carbo-borides is more pronounced with a slower cooling rate compared with quenching. Additionally, B segregation and borides/carbo-borides precipitation are revealed through the APT analysis. Based on the observed segregation, GB energy reduction is estimated to be about 286 mJ/m2 in B-added low C steel due to B segregation. Thermodynamic and nucleation kinetics analyses reveal that this substantially affects the most favorable heterogeneous nucleation sites like grain corner and grain edge heterogeneous nucleation sites. However, it has been pointed that the effects of borides/carbo-borides precipitation on retarding the ferrite nucleation cannot be excluded. Detailed investigation of the corresponding mechanisms of B effects on ferrite nucleation is given based on the experimental and thermodynamic investigation. On the other hand, the difficulties in studying the GB segregation of C are discussed and possible routes to overcome these difficulties are highlighted. C-rich regions near the PAGBs, the packet, and the lath boundaries are commonly observed in various heat-treated specimens of low-C steels with and without B. The effect of these regions on the ferrite nucleation is excluded as these regions mostly appear due to the auto-tempering effects or due to the C redistribution in the austenite. Enhanced segregation of C leading to GB cementite formation is observed in steels that contain B. Through this, the importance of consideration of GB solute interactions is highlighted. It is pointed out that these solute interactions at GB can lead to GB (2D) phase formation/transition. Such low-dimensional phase formation/transition can promote the bulk phase transformations such as cementite, boride, or boro-carbides
