T. Daniel, M. Smaga, T. Beck
The investigation of materials, the characterization of their properties as well as the accurate description of their microstructure are an aspect of continuously increasing importance in engineering and essential to exploit the full potential of materials as well as meet safety requirements in actual products and industrial processes.
In this context, the present study focuses on characterization of the microstructure morphology of the metastable austenitic stainless steel AISI 347, typically used for nuclear power plants applications. In this metastable austenitic alloy, a phase transformation from paramagnetic face-centered cubic (fcc) γ-austenite into ferromagnetic body-centered cubic (bcc) α‘-martensite can occur as a result of plastic deformation. Hence, a fully austenitic state and three states with various contents of α‘ martensite were investigated in detail. The phase states containing α‘ martensite were achieved by using defined quasistatic pre-deformation with in-situ measurement of ferromagnetic, i.e. α‘ martensite phase fraction via a FeritscopeTM sensor. To adjust small α‘-martensite contents, the pre-deformation was performed at ambient temperature and for high α‘-martensite contents, the pre-deformation was done with specimens cooled in liquid nitrogen to facilitate phase transformation.
In order to characterize the morphology of the microstructure of the investigated material systematically, a series of metallographic cross sections was prepared and examined by different high precision measurement methods. At first, microstructure investigations by light microscopy were performed to characterize grain size as well as area content and distribution of austenite and martensite. Additionally, qualitative and quantitative phase distribution of the specimen variants with different α‘-martensite contents were analyzed by X-ray diffraction. The X-ray phase analysis offered the opportunity to determine a calibration factor for the FeritscopeTM, such that the α‘-martensite content could be translated from FE-% in vol.-%. Furthermore, micromagnetic multiparameter microstructure and stress-analyze (3MA) technique was used for in-depth characterization of various magnetic properties. On this baseline, the 3MA method will be used in future for nondestructive characterization of microstructural changes and/or damage under service relevant loading conditions.