Differentiation of martensite and bainite in hot work tool steels using EBSD

Abstract

In aluminum die casting for automotive applications, there is a trend toward increasingly large structural components in order to reduce the number of individual parts and associated processing steps. This development significantly lowers manufacturing time and cost, but at the same time imposes extreme thermal and mechanical loads on the tooling. As a result, a new class of tooling, commonly referred to as Giga-Dies, has emerged. The demanding service conditions, combined with the substantial material volume required for such tools, necessitate the use of high-quality hot work tool steels. However, due to limited hardenability, these steels do not form a fully martensitic microstructure throughout the entire cross-section. Instead, a mixed microstructure develops, with significant amounts of bainite forming in the center of large dies. For a reliable assessment of microstructural evolution and its influence on mechanical properties, particularly in the center of Giga-Dies, a clear and quantitative distinction between martensite and bainite is essential. In practice, differentiating these phases remains challenging and is often based on qualitative metallographic interpretation. The objective of this work is therefore to develop a robust methodology for quantifying the fraction of martensite and bainite in hot work tool steels using electron backscatter diffraction (EBSD). This task is particularly demanding due to the close similarity in crystal structure and resulting EBSD signatures of the two phases. To address this, model microstructures with predominantly martensitic, bainitic, and mixed characteristics have been produced by controlled heat treatments using dilatometry. These well-defined samples served as a basis for systematic EBSD investigations. The analysis focused on extracting distinguishing features from EBSD data, such as local misorientation distributions and pattern quality, as well as derived output properties such as the morphology of the martensite/bainite.