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

Andreas Rosenauer

Research output: ThesisDoctoral Thesis

Abstract

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.
Translated title of the contributionProzess-Struktur-Eigenschaften-Beziehungen von PH 13-8 Mo Maraging Stählen
Original languageEnglish
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Schnitzer, Ronald, Supervisor (internal)
  • Ebner, Reinhold, Assessor A (internal)
  • Leitner, Harald, Assessor B (internal)
  • Stockinger, Martin, Co-Supervisor (internal)
Publication statusPublished - 1800

Bibliographical note

embargoed until 07-02-2029

Keywords

  • Maraging steel
  • PH 13-8 Mo
  • Precipitates
  • Reverted austenite
  • Delta ferrite
  • Intercritical annealing
  • Austenite memory effect
  • Ausforming
  • Mechanical properties
  • In-situ characterization
  • Atom probe tomography

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