Refine
Document Type
Language
- English (2)
Has Fulltext
- yes (2)
Is part of the Bibliography
- yes (2)
Keywords
Groups
- Strukturbildung (2)
Research Field
Microalloyed steels contain small amounts of Nb, Ti, and V, which precipitate as carbides, nitrides, or carbonitrides at different stages of thermomechanical processing. Particle sizes and compositions vary depending on when they form; their size and position set their effect on the microstructure and hence mechanical properties. Alloy design, process control, and the production of steels with curtailed properties require an understanding of the correlations between processing conditions, particle formation, and mechanical properties. The relations can be derived from the analysis of statistically relevant data on the distributions of precipitate size and composition. This thesis provides a method to obtain such data through particle extraction by dissolution of the iron matrix in combination with colloidal analysis. The method is an alternative to commonly used electron microscopy of extraction replicas or thin films. Extraction protocols were systematically varied to obtain unagglomerated particles suitable for subsequent colloidal analysis with minimal particle losses. Colloidal analysis methods were evaluated by analysis of multimodal model particle systems for their suitability to analyze the broad size distributions of the precipitates. Analytical ultracentrifugation, field-flow fractionation, and single particle mass spectrometry were tested on particles extracted from steel and the results were compared to data from conventional metallography.
Different colloidal particle characterization methods are examined for their suitability to determine the particle size distribution of particles extracted from steels. Microalloyed steels are dissolved to extract niobium and titanium carbonitride particles that are important for the mechanical properties of these steels. Such particles have sizes ranging from several nanometers to hundreds of nanometers depending on the precipitation stage during the thermomechanically controlled rolling process. The size distribution of the particles is analyzed by dynamic light scattering (DLS), analytical ultracentrifugation (AUC), and hollow fiber flow field-flow fractionation (HF5) and compared to data obtained for reference particles as well as data from electron microscopy, the standard sizing technique used in metallurgy today. AUC and HF5 provide high-quality size distributions, average over large particle numbers that enables statistical analysis, and yield useful insights for alloy design; however, DLS fails due to a lack of resolution. Important aspects in the conversion and comparison of size distributions obtained for broadly distributed particle systems with different measurement principles and the role of surfactants used in sample preparation are discussed.
