TY - BOOK

T1 - Processing-structure-properties relationships of PH 13-8 Mo maraging steels

AU - Rosenauer, Andreas

N1 - embargoed until 07-02-2029

PY - 2024

Y1 - 2024

N2 - PH 13-8 Mo maraging steels are structural materials that are widely used in the aerospace industry for safety-critical components due to their good strength and toughness relation as well as their high corrosion resistance. The microstructure of these alloys consists mainly of a Fe-Ni lath martensite in which both reverted austenite and intermetallic NiAl precipitates are formed during an aging treatment. Owing it to complex microstructure of PH 13-8 Mo maraging steels, many questions with regard to the influence of the manufacturing process, i.e., heat treatment and hot forming, on the microstructure remain. Furthermore, knowledge about the effect of other microstructural constituents such as delta ferrite on the mechanical properties is insufficient and needs to be assessed. Therefore, the aim of the present work is to further investigate the relationships between the manufacturing process and the microstructure as well as between the microstructure and the mechanical properties using novel characterization techniques. The results will be used to deduce processing strategies for improving the mechanical properties of these alloys. Attention was given to delta ferrite and its influence on the toughness. Increasing the content of this phase resulted in a drastic decrease in impact toughness. Microstructural analysis revealed that the embrittlement is caused by cracks originating in delta ferrite. Moreover, multiscale in-situ techniques provided evidence for the austenite memory effect and recrystallization without prior cold deformation occurring in PH 13-8 Mo maraging steels. The latter effect causes a significant refinement of the microstructure, which has a positive effect on the mechanical properties. It was found that further refinement can be achieved by conducting ausforming. In addition, investigations on the effect of ausforming on the microstructure development during subsequent aging showed that finer precipitates and thus increased hardness can be reached by applying this thermomechanical treatment. Ultimately, a large part of the investigations was dedicated to intercritical annealing prior to aging. A detailed microstructural analysis by means of atom probe tomography and transmission electron microscopy revealed that Ni enriched zones, which promote the formation of reverted austenite during subsequent aging, result from this annealing step. In-situ high-energy X-ray diffraction tensile tests showed that this phase exhibits a high mechanical stability.

AB - PH 13-8 Mo maraging steels are structural materials that are widely used in the aerospace industry for safety-critical components due to their good strength and toughness relation as well as their high corrosion resistance. The microstructure of these alloys consists mainly of a Fe-Ni lath martensite in which both reverted austenite and intermetallic NiAl precipitates are formed during an aging treatment. Owing it to complex microstructure of PH 13-8 Mo maraging steels, many questions with regard to the influence of the manufacturing process, i.e., heat treatment and hot forming, on the microstructure remain. Furthermore, knowledge about the effect of other microstructural constituents such as delta ferrite on the mechanical properties is insufficient and needs to be assessed. Therefore, the aim of the present work is to further investigate the relationships between the manufacturing process and the microstructure as well as between the microstructure and the mechanical properties using novel characterization techniques. The results will be used to deduce processing strategies for improving the mechanical properties of these alloys. Attention was given to delta ferrite and its influence on the toughness. Increasing the content of this phase resulted in a drastic decrease in impact toughness. Microstructural analysis revealed that the embrittlement is caused by cracks originating in delta ferrite. Moreover, multiscale in-situ techniques provided evidence for the austenite memory effect and recrystallization without prior cold deformation occurring in PH 13-8 Mo maraging steels. The latter effect causes a significant refinement of the microstructure, which has a positive effect on the mechanical properties. It was found that further refinement can be achieved by conducting ausforming. In addition, investigations on the effect of ausforming on the microstructure development during subsequent aging showed that finer precipitates and thus increased hardness can be reached by applying this thermomechanical treatment. Ultimately, a large part of the investigations was dedicated to intercritical annealing prior to aging. A detailed microstructural analysis by means of atom probe tomography and transmission electron microscopy revealed that Ni enriched zones, which promote the formation of reverted austenite during subsequent aging, result from this annealing step. In-situ high-energy X-ray diffraction tensile tests showed that this phase exhibits a high mechanical stability.

KW - Maraging steel

KW - PH 13-8 Mo

KW - Precipitates

KW - Reverted austenite

KW - Delta ferrite

KW - Intercritical annealing

KW - Austenite memory effect

KW - Ausforming

KW - Mechanical properties

KW - In-situ characterization

KW - Atom probe tomography

KW - Maragingstahl

KW - PH 13-8 Mo

KW - Ausscheidungen

KW - Rückumgewandelter Austenit

KW - Delta-Ferrit

KW - Interkritisches Glühen

KW - Austenite memory effect

KW - Ausforming

KW - Mechanische Eigenschaften

KW - In-situ Charakterisierung

KW - Atomsondentomographie

M3 - Doctoral Thesis

ER -