Coupled microscopic and micromagnetic depth-specific analysis of plastic deformation and phase transformation of metastable austenitic steel AISI 304L by flow forming

Abstract

This paper presents the characterization of the microstructure evolution during flow forming of austenitic stainless steel AISI 304L. Due to plastic deformation of metastable austenitic steel, phase transformation from γ-austenite into α’-martensite occurs. This is initiated by the formation of shear bands as product of the external stresses. By means of coupled microscopic and micromagnetic investigations, a characterization of the microstructure was carried out. In particular, this study shows the distribution of the strain-induced α’-martensite and its influence on material properties like hardness at different depths. The microstructural analyses by means of electron backscattered diffraction (EBSD) technique, evidence a higher amount of α’-martensite (ca. 23 %) close to the outer specimen surface, where the plastic deformation and the direct contact with the forming tool take place. In the middle area (ca. 1.5 mm depth from the outer surface), the portion of transformed α’-martensite drops to 7 % and in the inner surface to 2 %. These results are well correlated with microhardness and micromagnetic measurements at different depths. EBSD and atomic force microscopy (AFM) were used to make a detailed characterization of the topography and degree of deformation of the shear bands. Likewise, the mechanisms of nucleation of α’-martensite were discussed. This research contributes to the development of micromagnetic sensors to monitor the evolution of properties during flow forming. This makes them more suitable for closed-loop property control, which offers possibilities for an application-oriented and more efficient production.

Detection of Phase Transformation during Plastic Deformation of Metastable Austenitic Steel AISI 304L by Means of X-ray Diffraction Pattern Analysis

“…Due to plastic deformation, shear bands appear in FZ1, which is the typical deformation mechanism of metastable austenitic steels. The 𝛼′-martensite nucleates at the intersection points of these shear bands, as was demonstrated and evidenced in [26]. The IPF map of FZ1 evidences the deformation-induced shear bands in a darker color, due to Using EBSD, a quantitative report of the amount of phase was carried out by means of the OIM analysis™ software.…”

Innovative Online Measurement and Modelling Approach for Property-Controlled Flow Forming Processes

“…The contact-less measurement is used due to the rotating mandrel during forming. For closed-loop control of metastable austenitic steel, Barkhausen noise will be used because in previous investigations a clear correlation between Barkhausen parameter 𝑀𝑀 𝑚𝑚𝑚𝑚𝑚𝑚 and α'-martensite content has been detected [8,9].…”