MACRO- AND MICROSCOPIC PROPERTIES OF GRADIENT ULTRA-HIGH-STRENGTH SEAMLESS STEEL TUBES PRODUCED BY DEDICATED THERMO-MECHANICAL TREATMENTS

摘要

Thermo-mechanical rolling of high-strength steels is standardly used to adjust mechanical characteristics of engineering components. In the case of seamless steel tubes, the treatment results in the formation of non-homogeneous distributions of phases, microstructure and residual stresses. These yield gradients influence the overall mechanical response of tubes decisively. In this contribution, properties of seamless steel tubes produced, using a set of thermo-mechanical rolling treatments, and are analyzed at macro and micro scale in order to reveal the correlation between local and average mechanical behavior, heat treatment conditions and local microstructure as well as residual stress state. The gradient tubes are characterized by moessbauer spectroscopy, transmission electron microscopy, indentation and X-ray diffraction. Tensile and fracture tests are performed on specimens out of tube wall sections as well as on miniaturized specimens isolated from distinct regions. The results document that local mechanical properties vary continuously across the tube walls. The investigated properties of these seamless steel tubes correlate (i) with the phase distribution of ferrite, perlite, bainite and martensite as well as (ii) the applied vary cooling intensities during the manufacturing process. The multi-scale characterization approach opens the possibility to understand the mechanical behavior of complex gradient materials.