Achieving nanostructured superaustenitic stainless steel by severe plastic deformation

Owing to the combination of favorable properties such as strength-ductility balance and corrosion resistance, the austenitic stainless steels are considered as desirable engineering materials. At the end of the twentieth century, the increasing demand for steels with superior mechanical and functional properties led to the introduction of a new class of austenitic stainless steels, known as the superaustenitic stainless steels. The AISI 904L steel is one of the most common superaustenitic stainless steels, which is widely adopted for applications in oil and gas transportation, energy, petrochemical, and marine sectors. Generally, the mechanical properties of austenitic stainless steels are controlled by their microstructure, especially the average grain size, and thus, grain refinement is known as a viable technique to improve mechanical properties. In the meantime, severe plastic deformation (SPD) has emerged as a promising method to produce nanostructured and ultrafine-grained materials with attractive properties. In this work, the effects of high-pressure torsion (HPT) on the grain refinement and mechanical properties of an AISI 904L alloy were investigated. HPT was performed under the pressure of 6 GPa at room temperature. The HPT deformation was continued for three revolutions (N) of 1/2, 1, and 10 turns. For microstructural analysis, field-emission scanning electron microscopy (FESEM), electron backscatter diffraction (EBSD) analysis and transmission electron microscopy (TEM) were used. Mechanical properties were studied by hardness and tensile testing. The average grain size of the as-received sample was around 21 µm and the Vickers hardness and the Yield stress for this sample were 173 and 390 MPa respectively. For N=1/2, the Vickers hardness of the center and near the edge of the disk were 277 and 498 respectively. Therefore, there was a significant heterogeneity in the strain hardening of the disk due to the nature of the HPT process. However, by increasing the number of revolutions up to N=10, considerable homogeneity in strain hardening was obtained in the disk, and Vickers hardness in the center and near the edge of the disc were 450 and 512, respectively. EBSD and TEM images of the N=10 sample showed the equiaxed nanostructure with an average grain size of 50 nm. The tensile test from the N=10 sample showed a very high yield stress of 2120 MPa. On the other hand, the XRD patterns of the N=10 sample revealed that it was fully austenitic. Despite the high-strain hardening in the N=10 sample, the microstructure remained fully austenitic and the deformation-induced martensite did not appear. This is ascribed to the ultra-high mechanical stability of this superaustenitic stainless steel. Consequently, the nanostructured superaustenitic stainless steel with excellent mechanical properties was obtained by severe plastic deformation.