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Streszczenie:
In this work, the very-high-cycle fatigue (VHCF) response of 10H2M steel in the as-received state and after 280,000 hours of service at 540 °C under pressure was analysed. Although after prolonged service, only a moderate decrease in yield strength was observed, fatigue performance was significantly reduced in the stress range applied. Fractographic analysis revealed that in both material states, fatigue cracks initiated near the surface, with carbides acting as preferential nucleation sites. In the as-received material, cracks propagated in a relatively uniform, transgranular manner with ductile overload fracture at final failure. In contrast, the prolonged-service material exhibited rougher fracture surfaces with more frequent crack deflections and secondary cracking, linked to carbide coarsening due to service. The results demonstrate that prolonged exposure does not fundamentally change the surface-dominated initiation mechanism but intensifies the role of carbides and boundary-related defects, thereby reducing fatigue resistance. EBSD-informed finite-element representative volume element (RVE) simulations further revealed that prolonged service exposure increased the local carbide/matrix stress concentration factor, providing micromechanical evidence for the experimentally observed deterioration in fatigue resistance. Additionally, X-ray diffraction (XRD) analysis confirmed thermodynamically driven carbide precipitation and crystallographic evolution, characterised by reduced carbon supersaturation, crystallite refinement, residual micro-stress relaxation and texture evolution following long-term service exposure.

Słowa kluczowe:
Very high cycle fatigue, Power plant steels , Failure mechanisms , Ultrasonic testing

Streszczenie:
This study investigates the influence of build orientation on the microstructure and early-stage deformation behaviour of austenitic stainless steel 316 L (SS316L) produced via laser powder bed fusion (LPBF). Three LPBF specimen orientations: horizontal (XY), inclined at 45° (ZX), and vertical (Z) were compared to conventionally produced wrought SS316L. Mechanical testing was conducted under uniaxial tension to fracture and to a controlled axial strain of 1 % to capture the onset of plasticity. Electron backscatter diffraction (EBSD) was performed before and after deformation to quantify grain boundary character, misorientation distribution, and grain morphology evolution. The LPBF material exhibited notable differences in yield strength and strain hardening, with the Z-oriented specimens exhibiting the lowest mechanical performance due to insufficient interlayer bonding and elongated melt pool boundaries aligned with the build direction. In contrast, the XY and ZX orientations showed relatively higher strength and more uniform deformation behaviour. EBSD revealed that early-stage plastic deformation led to the intragranular misorientation accumulation but the degree of it varied significantly with orientation. Wrought SS316L displayed the highest overall mechanical properties and more homogeneous deformation due to its equiaxed, recrystallized microstructure.

Słowa kluczowe:
Stainless steel, Microstructure, Additive manufacturing, Laser Powder Bed Fusion Melting (LPBF-M)

Streszczenie:
This study explores the influence of printing orientation on the yield surface characteristics and their evolution under plastic pre-deformation in additively manufactured (AM) stainless steel 316L produced via laser powder bed fusion (LPBF). Tubular specimens were fabricated in three orientations (XY, ZX, and Z) and subjected to multi-axial loading to experimentally determine initial and subsequent yield surfaces using a single-specimen probing approach. The yield surfaces were derived at two offset strain definitions (0.001% and 0.005%) and further analyzed after tensile pre-deformations of 0.35%, 0.5%, and 0.8%. Results revealed strong anisotropy in the AM specimens, with the Z-oriented samples displaying the lowest yield strength and most significant softening. In contrast, the XY and ZX orientations exhibited higher resistance to plastic deformation. The wrought SS316L showed superior mechanical performance. The evolution of yield surfaces highlighted the orientation-dependent hardening/softening mechanisms and directional stress redistribution. Electron backscatter diffraction (EBSD) analysis revealed that the microstructural anisotropy and grain morphology—particularly the presence of columnar grains in the Z-oriented samples—correlate strongly with the observed mechanical anisotropy and yield surface asymmetry

Słowa kluczowe:
Stainless steel, Yield surface, Additive manufacturing, Laser powder bed fusion melting (LPBF-M)

Streszczenie:
Stainless steel 316L tubes and bars were additively manufactured (AM) by using the Laser Powder Bed Fusion Melting (LPBF-M) method in three orientations. As-built specimens were then machined and the initial yield surface was determined for three printing orientations based on the yield stress definition for 0.005 % plastic offset strain. The as-received, wrought material was additionally tested using the same tension–compression-torsion conditions to compare the mechanical behaviour of AM and wrought SS316L. The sizes of yield surfaces elaborated for LPBF-M specimens increased along the tensile and compressive directions and shrunk when torsion was applied, as compared to the as-received specimen.

Słowa kluczowe:
Stainless steel ,Yield surface ,Additive manufacturing

Streszczenie:
The Laser Powder Bed Fusion Melting (LPBF-M) method was used to additively manufacture stainless steel 316L tubes in three different orientations. The yield surface approach was implemented to assess the variation of mechanical properties within the as-built specimens. Yield surfaces were determined for each build orientation based on the definition of yield stress for 0.005% plastic offset strain. The initial yield surfaces obtained for the as-built material exhibit anisotropic behaviour, possibly resulting from the preferred grain orientation developed during LPBF-M processing.