Influence of High-Pressure Torsion on the Microstructure and Microhardness of Additively Manufactured 316L Stainless Steel

Abstract

High-pressure torsion (HPT) is known as an effective severe plastic deformation (SPD) technique to produce bulk ultrafine-grained (UFG) metals and alloys by the application of combined compressive force and torsional shear strains on thin disk samples. In this study, the microstructures and microhardness evolution of an additively manufactured (AM) 316L stainless steel (316L SS) processed through 5 HPT revolutions are evaluated at the central disk area, where the effective shear strains are relatively low compared to the peripheral regions. Scanning electron microscopy (SEM) analysis showed that the cellular network sub-structures in AM 316L SS were destroyed after 5 HPT revolutions. Transmission electron microscopy (TEM) observations revealed non-equilibrium ultrafine grained (UFG) microstructures (average grain size: ~115 nm) after 5 revolutions. Furthermore, energy dispersive x-ray spectroscopy (EDX) analysis suggested that spherical Cr-based nano-silicates are also found in the as-received condition, which are retained even after HPT processing. Vickers microhardness (HV) measurements indicated significant increase in average hardness values from ~220 HV before HPT processing to ~560 HV after 5 revolutions. Quantitative X-ray diffraction (XRD) patterns exhibit a considerable increase in dislocation density from ~0.7 × 1013 m−2 to ~1.04 × 1015 m−2. The super-high average hardness increment after 5 HPT revolutions is predicted to be attributed to the UFG grain refinement, significant increase in dislocation densities and the presence of the Cr-based nano-silicates, according to the model established based on the linear additive theory.

An Overview on the Corrosion Behavior of Steels Processed by Severe Plastic Deformation

“…Austenitic stainless steels have austenite (FCC) phase at room temperature, are low stacking fault energy (SFE) materials, and their plastic deformation at low temperatures occurs through several mechanisms simultaneously. These steels have been extensively studied using SPD (51,52,53,54,55,56) , and it has been shown that various deformation mechanisms are present, depending on the SFE of the material and the processing conditions such as deformation temperature. The initial phase (γ-austenite) gradually transforms to ɛ-and α'-martensite due to deformation.…”

“…The formation and adhesion of passive films on 316L SS processed using HPT have also been reported. The larger amount of grain boundaries, dislocation density, and higher residual stresses provide nucleation sites for the passive oxide layers to become more stable (52) . Yusuf et.…”

Enhanced Microhardness and Corrosion Performance of Additively Manufactured Inconel 718 Specimens through Nanostructuring by Severe Plastic Deformation

“…Up to now, several studies have been investigated on enhancing the hardness/strength of L-PBF AM metallic materials by SPD processes, specifically that of 316L SS [11,12], AlSi10Mg [13,14], and IN 718 [15]. However, there is a lack of study on the effects of SPD, particularly HPT on the combined microstructure, hardness, and corrosion performance, especially on AM-fabricated IN 718.…”