This study presents a novel manufacturing process for open-cell stainless steel foams using a modified investment casting process and a novel approach to identify transitions between elastic-plastic, plateau, and densification region in the stress-strain history of compressed foams, based on its strain and structural hardening behavior. The influence of microstructure on the mechanical properties under quasi-static compression loading (plateau stress, energy absorption, and strain hardening) of austenitic (AISI 304, 316L) and super duplex (AISI F55) stainless steels is investigated. Microstructure is characterized prior and subsequent to mechanical testing using light microscopy and SEM. The manufacturing process yields open-cell foams with relative densities in the range of 14-20%, solid struts being circular in shape and defect-free surfaces. This morphology leads to improved yield strengths compared to open-cell steel foams produced by the powder metallurgical route. Among the manufactured open-cell steel foams, F55 foams with finegrained duplex microstructures show highest yield strength, strain hardening, and energy absorption with a sufficient ductility. Although a martensitic transformation is present in highly deformed struts of soft austenitic stainless steel foams, strain hardening in the plateau region is lower compared to duplex foams.
A wide range of methods and procedures exists to characterize microstructures and to quantify phases. This work presents a comparison between light microscopic examinations as a function of different etching methods and electron backscatter diffraction analyses. The procedures used in this context were etching with V2A solution, Kalling I and Beraha II etchants. The objective was to quantify the austenite phase in Q&P heat treated X46Cr13. The Q&P heat treatment allows to achieve high mechanical strengths (for X46Cr13 approx. Rp0.2 = 1750 MPa) alongside high elongations at break (A = 14 %). This can be attributed to a combination martensite tempering and the TRIP effect. The required specific ratio of martensite and austenite can be established by the heat treatment. In order to define the heat treatment's optimal parameters, the mechanical characteristic values need to be determined and comprehensive phase fraction determinations must be performed. Beraha II provided particularly promising results. Compared with EBSD, matching phase fractions of austenite could be determined with deviations of the absolute values between one and three percent. This method offers a possible alternative to, or supplements the microstructural characterization and assessment of the material development for a high sample throughput and at reduced cost and effort.
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