• Schnitzer, Ronald

Abstract

Maraging steels exhibit an excellent combination of high strength and ductility, which make them attractive for uses in machinery fields and aircraft applications. Their microstructure consists of a soft but heavily dislocated Ni martensite, intermetallic precipitates and reverted austenite which is formed during an aging treatment. Development of new materials with tailored properties requires a throughout understanding of the microstructure, its evolution and influence on mechanical properties. The knowledge of the individual influence of each microstructural constituent is of essential importance to develop materials apart from the trail and error method. The motivation of the present thesis is, therefore, to gain deep understanding of the structure-properties relationship in maraging steels in order to be able to design new high-performance steels. In a first step, appropriate methods had to be found, which enables exact characterization of the microstructure. A challenging task, thereby, was to find an appropriate method for exact characterization of the prior austenite grain size. This was achieved by a chemical etching method which allowed the evaluation of the grain size by light optical microscopy. Characterization of the microstructure opened a new understanding, especially for the formation of reverted austenite. Based on the experimental results, a model for the formation of reverted austenite was developed. Characterization of precipitates was performed by applying atom probe tomography in order to investigate the influence of the addition of alloying elements on the precipitation sequence in maraging steels. The influence of the microstructural constituents on mechanical properties was investigated by notched impact tests and tensile tests. It was demonstrated that the influence of reverted austenite on the mechanical properties is much more pronounced than differences in the prior austenite grain size. However, the austenite phase in maraging steels was found to be not stable during deformation, which was examined by in-situ X-ray diffraction using high-energy synchrotron radiation. In a final step, the mechanical properties, i.e. the yield strength in the present thesis, were modeled by using the experimentally determined microstructural data. Thereby, a new approach was used and compared to the classical theories of shearing and bypassing of precipitates. It was shown that the new approach, which takes the particle size distribution into account, leads to significantly improved predictions.

Topics

• impedance spectroscopy
• grain
• grain size
• phase
• x-ray diffraction
• strength
• steel
• impact test
• etching
• precipitate
• precipitation
• aging
• yield strength
• intermetallic
• optical microscopy
• ductility
• aging
• atom probe tomography