Thermomechanical Fatigue Resistant Dual Hardening Steels

Matthias Hofinger

    Research output: ThesisDoctoral Thesis

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    Abstract

    In hot-work applications, tool materials are subject to complex interacting cyclic mechanical and thermal loading conditions. Current hot-work tool steels consist of a tempered martensitic matrix, strengthened by the precipitation of secondary hardening particles. This allows for a high tempering resistance and red hardness, which in turn ensure resistance to thermomechanical fatigue. This fatigue causes the formation of a network of fine surface cracks, called heat-checks, which are currently the major life-limiting failure mechanism for hot-work tools. In the frame of this thesis, the performance of dual hardening steels was compared to hot-work tool steels in order to evaluate their eligibility for hot-work applications. Dual hardening steels combine the precipitation of secondary hardening carbides with the formation of intermetallic or metallic precipitates, thus, providing high strength levels whilst the amount of embrittling carbides is reduced. A test method was chosen in order to recreate the complex cyclical thermal and mechanical loading conditions present during hot-work applications on a laboratory scale. Special emphasis was put on the understanding of the influence of these loading conditions on the microstructure of hot-work and dual hardening steels in order to be able to correlate the resistance against thermomechanical fatigue to single and dual hardening. The thermomechanical fatigue tests were carried out by a servo-hydraulic test rig combined with an inductive heating system. In these tests, cylindrical specimens are heated cyclically whilst a certain percentage of the thermal expansion is mechanically suppressed. The decreasing yield strength at increasing temperatures leads to an accumulation of plastic compression, thus, a continuous shift of the mean stress in the tensile region, simulating the formation of residual tensile stresses in the surface region of a pressure casting die. The microstructural evolution of a dual hardening steel during heat treatment as well as early precipitation reactions during ageing were investigated utilizing electron backscatter diffraction and atom probe tomography. High resolution techniques including transmission electron microscopy were utilized in order to evaluate the influence of the thermomechanical loading conditions on the microstructure of single hardening hot-work tool steels and dual hardening steels. It could be shown that with increasing test temperatures, despite the lower solvus temperatures of the intermetallic phases, the dual hardening steel achieves higher resistance to thermomechanical fatigue compared to the hot-work tool steel due to its increased tempering resistance. The superposition of a mechanical strain amplitude had no effect on the softening caused by the Ostwald ripening and partial dissolution of the secondary hardening precipitates.
    Translated title of the contributionThermomechanisch ermüdungsresistente dualhärtende Stähle
    Original languageEnglish
    QualificationDr.mont.
    Awarding Institution
    • Montanuniversität
    Supervisors/Advisors
    • Leitner, Harald, Co-Supervisor (external)
    • Schnitzer, Ronald, Supervisor (internal)
    • Grün, Florian, Assessor B (internal)
    • Primig, Sophie, Assessor A (external)
    Publication statusPublished - 2020

    Bibliographical note

    embargoed until null

    Keywords

    • Dual Hardenine Steel
    • Hot-work Tool Steel
    • Thermomechanical Fatigue
    • Heat Checking
    • Atom Probe Tomography
    • Secondary Hardening

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