The AISI 316L stainless steel is a material widely used in the petrochemical industry, marine industry or in industries located in saline environments, due to its good corrosion, chloride, and oxidation resistance. This material contains a high content of Cr and Ni, which range for Cr between 16% and 18%, and for Ni between 10% and 14%. This combination provides good corrosion resistance, ductility and weldability. However, it has low hardness and mechanical strength, characteristics that cannot be improved by heat treatment [1] [2]. This steel contains low carbon content and other alloying elements that prevent the transformation of phases during the heating or cooling processes, which implies that the improvement in its mechanical behavior is based only on grain refinement processes, according to the Hall Petch equation [3] [4].

Severe plastic deformation (SPD) techniques are widely used methods to produce significant grain size refinement at the micro and nanometer scales [5]. Different SPD methods effectively modify the microstructure associated with defect generation, thereby improving the mechanical properties. Among the most common SPD methods include Constrained Groove Pressing (CGP) and Groove Pressing (GP), which consist of producing repetitive shearing of sheets between asymmetrical corrugation and flattening dies. Other techniques are High Pressure Torsion (HPT), Equal Channel Angular Pressing (ECAP), Twist Extrusion (TE), and Accumulative Roll Bonding (ARB) [6], [7]. These methods are performed using equipment and devices with special geometrical characteristics that do not allow free flow of the material during plastic deformation, resulting in the application of high hydrostatic pressure. High hydrostatic pressure combined with high shear stress results in a high density of defects (vacancies, dislocations) in the crystal lattice, especially dislocations that produce significant refinement of the grains in the material [8], [9]. Lei et al. [10] found that dynamic plastic deformation at high stresses facilitates deformation, leading to high tensile strength on the order of 1.40 GPa for 316L stainless steels. However, that process reduces ductility to the order of about 2%. In the search for a balance of properties, they also found that controlled annealing treatments reduced tensile strength from 1.4 to 1.0 GPa and improved ductility by 27%; this is due to the recrystallization of ultrafine grains that are located around the nanoscale grains.

On the other hand, Kim et al. [11] evaluated two types of treatments during the rolling at room temperature of a 316L austenitic stainless steel. In the first case, there was a thickness reduction of 20%, and in the second case of 80% with the addition of heat treatment at 760°C for 5 h, called R20 and R80+HT, respectively. They found that the RT80+HT deformation process allowed the formation of heterogeneous dual structures (austenite and phase (a)) compared to homogeneous structures (austenite) in the R20 process. Regarding the mechanical properties, the ultimate tensile strength (UTS) and ductility of the material subjected to the R80+HT process were higher than those obtained for the R20 process. They found that the balance between tensile strength, ductility, and work hardening ability was due to a combined effort between martensitic transformation and heterogeneous hardening induced by the presence of a dual behavior in the material. A similar result was obtained by Singh et al. [14] who determined that the improvement in the mechanical behavior of 316 stainless steel after CGP was due to the combined presence of high dislocation density, presence of martensitic transformation, formation of a substructure, and the reduction of grain size. Aragón-Lozano et al. [12] also reported significant reduction in austenitic grain size and the presence of deformation-induced martensite in a 316L stainless steel deformed by CGP with up to 4 passes. They reported an improvement in the corrosion resistance in chloride-rich environments after four passes of ~45%. This behavior was associated with the presence of a passive Cr₂O₃ layer favored by the exposure time and level of plastic deformation applied. The aim of this paper is to analyze the microstructural behavior of 316L austenitic stainless steel deformed by the severe plastic deformation technique (Groove Pressing), quantifying the effect of plastic deformation on the continuous and discontinuous dynamic recrystallization processes with emphasis on the influence of the stacking failure energy on the hardening processes, either by mechanical twinning, deformation-induced martensitic transformation, or dislocation glide.

AISI 316L austenitic stainless steel sheets of 20 mm x 96 mm x 2 mm were used in this study. The material composition was provided by the producer in the quality certificate and is summarized in Table 1.

Table 1

Element	C	Cr	Ni	Mo	Mn	Si	N	S	P	Fe
w.t %	0.018	16.8	10.2	2.04	0.74	0.59	0.01	0.001	0.035	Bal.

The sheets were subjected to homogenization annealing for 60 minutes at 1000°C with water quenching. Subsequently, two CGP dies made of AISI A2 type tool steel with dimensions of 96 mm x 96 mm, one corrugated and one flat, were used. The geometry of the corrugated matrix consists of 2 mm teeth and 45° angles (see Figure 1). The material was subjected to the constrained groove pressing technique without mass flow restriction (GP) at room temperature up to a maximum equivalent strain of £~4.64 and molybdenum disulfide (MoS₂) was used as a lubricant.

Figure 1

The microstructures of the samples were characterized by electron backscattered diffraction (EBSD) on the transversal section of the samples, which were cut from the center of the CGP specimens and mechanically polished from 1500 grit sandpaper to 0.02 urn colloidal silica suspension following standard metallographic procedures. The EBSD measurements were performed using a JEOL JSM-7001F field Emission SEM operating at 20 kV. The Oxford Instruments HKL Channel 5 software package was used to process the obtained results. Different step sizes of 0.2 um (annealed steel) and 0.1 um (deformed steel) were used. Misorientations below 3° were not considered in the postprocessing data procedure. Additionally, to corroborate the presence of martensite induced by deformation, the following techniques were used: X-ray diffraction using a PANalytical Multi-Purpose diffractometer model X'Pert PRO MRD with copper radiation (Ka=1.54187 Á) and Scanning Electron Microscopy using a Hitachi SU3500 SEM.

Based on the microstructural behavior of an AISI 316L stainless steel subjected to severe plastic deformation by the groove pressing technique, the following conclusions were generated:

It was observed that the undeformed 316L stainless steel presents an austenitic twinned grain with 25.9% of special grain boundaries of type 13 (60°<111>) and randomly oriented with some prevalence to the <111> direction and with average grain size of 6.9 ± 8.15 um with presence of x phase (Fe₃₆Cr₁₂Mo₁₀) in the hot rolling direction.