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Erscheinungsjahr
- 2017 (3) (entfernen)
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- Beitrag zu einem Tagungsband (3) (entfernen)
Schlagworte
- Additive Fertigung (1)
- Component Testing (1)
- Eigenspannungen (1)
- IN718 (1)
- Martensitic stainless steel (1)
- Neutronenstreuung (1)
- Residual Stress (1)
- Reversed austenite (1)
- Sectioning Method (1)
- Selektives Laserschmelzen (1)
Residual stresses and distortions in welded I-girders for steel construction are relevant when evaluating the stability of steel beams and column members. The application of high strength steels allows smaller wall thicknesses compared to conventional steels. Therefore, the risk of buckling has to be considered carefully. Due to the lack of knowledge concerning the residual stresses present after welding in high strength steel components conservative assumptions of their level and distribution is typically applied. In this study I-girders made of steels showing strengths of 355 MPa and 690 MPa were welded with varying heat input. Due to the dimension of the I-girders and the complex geometry the accessibility for residual stress measurement using X-ray diffraction was limited. Therefore, saw cutting accompanied by strain gauge measurement has been used to produce smaller sections appropriate to apply X-ray diffraction. The stress relaxation measured by strain gauges has been added to residual stresses determined by X-ray diffraction to obtain the original stress level and distribution before sectioning. The combination of both techniques can produce robust residual stress values. From practical point of view afford for strain gauge application can be limited to a number of measuring positions solely to record the global amount of stress relaxation. X-ray diffraction can be applied after sectioning to determine the residual stresses with sufficient spatial resolution.
The novel heat treatment concept, “quenching and partitioning” (Q&P) has been developed for high strength steels with enhanced formability. This heat treatment involves quenching of austenite to a temperature between martensite start and finish, to receive a several amount of retained austenite. During the subsequent annealing treatment, the so called partitioning, the retained austenite is stabilized due to carbon diffusion, which results in enhanced formability and strength regarding strain induced austenite to martensite transformation. In this study a Q&P heat treatment was applied to a Fe-0.45C-0.65Mn-0.34Si-13.95Cr stainless martensite. Thereby the initial quench end temperature and the partitioning time were varied to characterize their influence on microstructural evolution. The microstructural changes were analysed by dilatometer measurements, X-ray diffraction and scanning electron microscopy, including electron back-scatter diffraction. Compression testing was made to examine the mechanical behaviour. It was found that an increasing partitioning time up to 30 min leads to an enhanced formability without loss in strength due to a higher amount of stabilized retained and reversed austenite as well as precipitation hardening.
Die Schicht-für-Schicht additive Fertigung (AM) in Form des selektiven Laserschmelzens (SLM) bietet einerseits Vorteile bezüglich des Probendesigns, andererseits sind thermische Eigenspannungen (ES) aufgrund des hohen Temperaturgradienten unvermeidbar. Diese Eigenspannungen wurden in zwei SLM-gefertigten Proben aus IN718 zerstörungsfrei mit Neutronenstreuung hinsichtlich der folgenden zwei Einflüsse analysiert: der Einfluss der Hatch-Länge und das Abtrennen des Werkstücks von der Bauplatte. Begleitet wurden die Messungen der Eigenspannungen durch optische Mikroskopie und die taktile Vermessung der Oberfläche. Eine Korrelation zwischen den Eigenspannungen und der Hatch-Länge konnte beobachtet und erklärt werden.