Effect of Ni element on microstructure and properties of cold-rolled 316 L austenitic stainless steel

In this current investigation, the impact of Nickel (Ni) on the microstructural attributes and properties of a cold-rolled 316 L sheet was examined. The microstructure and phase configuration of austenitic stainless steels, specifically 316 L and 316LNi, were meticulously characterized through the utilization of metallography, X-ray Diffraction (XRD), and Electron Backscatter Diffraction (EBSD) techniques. Subsequent assessments were conducted to evaluate magnetic characteristics, microhardness, and tensile properties. The phase structure of both austenitic stainless steels conforms to a Face-Centered Cubic (FCC) crystal lattice, whereby the grain content oriented along the (110) plane progressively escalates with augmenting degrees of cold rolling. The magnetic conductivity of these austenitic stainless steels satisfactorily adheres to established standards. The incorporation of Nickel (Ni) into the alloy composition enhances the cold deformation capacity of 316 L stainless steel. However, substantial plastic deformation yields heightened dislocation density, thereby promoting enlarged grain dimensions upon solution treatment. Throughout subsequent cold rolling deformation sequences, the augmented grain size observed in 316LNi stainless steel leads to a reduction in dislocation density within the equivalently ordered cold-rolled plate. Simultaneously, this augmented grain size engenders a decline in grain boundary content coupled with an augmentation in twin content. Consequently, the interplay of grain coarsening, diminished dislocation density, and twin-induced softening collectively bestows upon 316LNi stainless steel a lower tensile strength compared to 316 L stainless steel, albeit accompanied by heightened plasticity.


Introduction
The 316 L austenitic stainless steel exhibits remarkable attributes encompassing corrosion resistance, favorable formability, and robust work hardening capability, rendering it extensively applied across engineering contexts and daily life scenarios [1][2][3].Presently, rolling technology serves as a pivotal avenue for optimizing the microstructural evolution kinetics of 316 L austenitic stainless steel and controlling its concurrent mechanical traits [4].By manipulating the rolling process and supplementary heat treatment protocols, the internal microstructure of 316 L stainless steel can be refined, thereby yielding commendable strength-plasticity synergy [5][6][7] and diminished friction coefficients [8].
Currently, research into 316 L stainless steel rolling technology primarily centers around three methodologies: hot rolling, warm rolling, and cold rolling.As the reduction rate progressively increases during hot rolling, the average grain size decreases gradually.This reduction in grain size initiates microstructural transitions, shifting from a nanocrystalline to a semicrystalline and eventually to a crystalline regime.These transitions play a significant role in enhancing mechanical properties, although it's important to note that they are more prominent with larger sheet thicknesses [9][10][11].Investigations regarding warm rolling of 316 L stainless steel are limited.Scholars such as Li [9] and Guo [12] have undertaken research in this domain.The microstructure of 316 L stainless steel post-warm rolling chiefly comprises austenite, ferrite, and minor NiC x phases, resulting in heightened yield strength albeit accompanied by complex rolling processes.Cold rolling technology for 316 L stainless steel yields thin to ultra-thin sheets with superior surface formability.Abundant research in this realm delves into the deformation structure and microstructural evolution kinetics of coldrolled stainless steel [13][14][15][16], processes of recrystallization and crystallization orientation (texture) evolution [17][18][19][20], augmentation of strength-plasticity synergy [21], and modifications in corrosion resistance [22].Beyond the rolling process, diverse rolling methods profoundly impact the microstructure and mechanical characteristics of 316 L stainless steel.Scholars such as Nezakat [23] and Tanhaei [24] achieved distinct texture types via synchronous and asynchronous rolling, respectively, leading to significant disparities in seawater corrosion, magnetic properties, and mechanical attributes of 316 L austenitic stainless steel.
Collectively considering the existing research on rolling of 316 L austenitic stainless steel, it is evident that the rolling processes and associated theories are well-established, facilitating the establishment of stable production lines that cater to the market demand.Consequently, enhancing the organizational structure and properties of rolled 316 L austenitic stainless steel can unlock latent potential within the rolling process, rolling methodologies, heat treatment, and related domains.Exploring adjustments in alloy system constituents is a noteworthy avenue.Schroder [25,26], for instance, utilized CrMnNi and X5CrMnNiMoN16 austenitic stainless steels as research subjects.By modulating Ni content, the cold formability and post-rolling plasticity of stainless steels were improved.Simultaneously, through the induction of martensitic phase transformation, stainless steel hardening capability was elevated, engendering a favorable plastic-strength synergistic equilibrium.
Recent advancements in stainless steel research have increasingly focused on the influence of alloying elements on its microstructure and properties.Elements like Chromium (Cr), Molybdenum (Mo), and Nickel (Ni) play pivotal roles in enhancing the material's performance.Notably, Cr is known for improving corrosion resistance, while Mo contributes to strength and hardness.Ni, in particular, has garnered attention due to its unique effects.It enhances ductility, forms and stabilizes the austenitic structure, and improves overall corrosion resistance.The addition of Ni in stainless steel, especially in types like 316 L, has shown to significantly influence these properties.This is critical in applications where both mechanical strength and corrosion resistance are paramount.Our study aims to delve deeper into this aspect, focusing on the effect of Ni on the microstructure and properties of cold-rolled 316 L austenitic stainless steel.The motivation for this study stems from the need to understand how varying Ni concentrations can be optimized to enhance the performance of stainless steel in demanding environments [27].Stainless steel alloys are widely used in various applications, and their mechanical properties are influenced by alloying elements.While the impact of alloying elements on stainless steel has been a subject of extensive research, our study explores a novel perspective by investigating the effects of adding Nickel (Ni) elements to 316 L stainless steel.This research aims to provide insights into the influence of Ni on the material's structure, properties, and performance.By introducing Ni as an alloying element, this study seeks to enhance the cold deformation capabilities of 316 L stainless steel, ultimately improving its plasticity and other mechanical characteristics.The study delves into the microstructural changes, mechanical properties, and magnetic conductivity of 316 L stainless steel with varying Ni content, shedding light on how alloying elements can be strategically employed to tailor material properties.

Experimental procedures 2.1. Materials and fabrication processing
This study employs two distinct types of austenitic stainless steels, namely 316 L and 316 LNi, for investigation, and their precise chemical compositions are outlined in table 1.The cold rolling preparation procedure for both stainless steel variants entail initially cold rolling the 316 L plate from a 1 mm thickness to 0.285 mm, followed by a solid solution treatment at 1050 °C, wherein the samples are held for 1 min before undergoing air cooling to achieve room temperature.Subsequent to the solution treatment, the specimens are cold rolled again to attain thicknesses of 0.228 mm and 0.171 mm, respectively, thereby achieving cold rolling reduction rates of approximately 20% and 40%.Similarly, the 316 LNi plate, commencing with a 1 mm thickness, is cold rolled to a thickness of 0.18 mm, followed by the application of the identical solution treatment regimen employed for the 316 L stainless steel.Post solution treatment, the specimens undergo further cold rolling to attain thicknesses of 0.144 mm and 0.108 mm, thereby yielding cold rolling reduction rates of approximately 20% and 40%.Preliminary assessment of the two austenitic stainless steel variants prior to solution treatment already indicates that the incorporation of Ni element enhances the material's cold deformation capacity.

Microstructure characterization
Microstructural evolution within the plane oriented parallel to the rolling direction (RD) was carefully scrutinized.Metallographic specimens were meticulously prepared by sequential grinding with 80#, 120#, 240#, 400#, 800#, 1200#, 1500#, and ultimately 2000# abrasive papers.These specimens were subsequently subjected to standardized polishing and etching protocols.Nitrohydrochloric acid (at a 1:3 ratio of nitric acid to hydrochloric acid) was employed as the etchant, and etching durations were maintained at approximately 10-15 s.The microstructures within the RD planes were characterized through employment of an XJP300 optical microscope.Identification of phases was accomplished using a D8 advanced x-ray diffractometer (XRD), configured with Cu Kα radiation, operating at 40 kV and 40 mA with a scanning speed of 4 °/min.Origin Pro 8.5 software facilitated data analysis.Subsequent to the polishing stage, ion etching was executed for a duration of 30 min at a voltage of 6.5 V. To probe the microstructure at the atomic level, electron backscattering diffraction (EBSD) was conducted utilizing a TESCAN MAIA3 apparatus, equipped with Channel 5 software, operating at room temperature and employing step sizes of 0.15 μm and 0.25 μm.

Performance examination
The magnetic conductivity of both the 316 L and 316 LNi austenitic stainless steels before and after cold rolling was quantified utilizing a DP-RCY 2 instrument tailored for measuring weak magnetic material magnetic conductivity.To ascertain micro-hardness within the RD planes, a digital micro-hardness tester of MHV2000 type was deployed, with a loading force of 300 g and a 15-second dwell time.Microhardness values were recorded at five distinct points for each sample group and subsequently averaged.Employing the ISO 6892-1 Standard [28] as a framework, tensile tests were executed using a CSS-44100 electronic universal testing machine, applying a loading rate of 1 mm/min.Following three replicate tensile tests for each sample group, mean values were derived and employed to construct the tensile stress-strain curves.

Phase and microstructure
The grain dimensions of the 316 LNi stainless steel (approximately 58.6 μm in diameter) are notably more substantial than those of the 316L stainless steel (around 16.5 μm in diameter), as visually depicted in figures 1(a1) and (b1).This distinction primarily arises from the presence of Nickel (Ni), a quintessential stabilizing element within austenitic compositions, thereby elevating the material's cold deformation aptitude [29][30][31][32].Following cold rolling of stainless steel plates initially possessing equivalent thickness (1 mm), the degree of cold deformation in the 316 LNi stainless steel surpasses that of the 316 L variant, leading to greater internal energy accumulation.Under identical solution treatment protocols, the grains within the 316 LNi stainless steel are more prone to growth.Furthermore, as deformation magnitudes amplify, the grains within both stainless steel types assume elongated morphologies, evident in figures 1(a2), (a3), (b2), and (b3).
Through EBSD and XRD analyses, it is ascertained that the phase structure of the two austenitic stainless steels remains unchanged pre-and post-cold rolling, persisting as a conspicuous face-centered cubic (FCC) arrangement.The XRD outcomes further reveal that the augmentation of deformation corresponds with a gradual increase in grain content possessing the (110) orientation in both austenitic stainless steels, illustrated in figure 1(c).
Figure 2 illustrates the polar plot and inverse pole figure (IPF) representation of the two austenitic stainless steels under varying cold rolling conditions.In the case of solid solution-treated 316 L austenitic stainless steel, a distinct texture orientation is not evident, owing to the prevalence of annealing twins within grains, leading to the coexistence of multiple orientations within a single grain [33][34][35], as depicted in figure 2(a).As the extent of cold rolling augmentation unfolds, the 〈101〉 fiber texture emerges within the 316 L stainless steel, accompanied by an elevation in grain content within the 〈101〉 dominant orientation, as demonstrated in figure 2(b).Through continued cold rolling processes, the 〈101〉 fiber texture exhibits a slight diminishing trend, as showcased in figure 2(c).Notably, the microstructural evolution derived from EBSD slightly diverges from XRD outcomes.This discrepancy chiefly emanates from the relatively confined field of view dimensions inherent to EBSD sample detection, in contrast to the broader detection span encompassed by XRD, thus endowing the former's findings with potential perturbations.
The textural evolution within the 316 LNi austenitic stainless steel, vis-à-vis variations in cold rolling magnitude, mirrors the pattern observed in 316 L, as portrayed in figures 2(d)-(f).This substantiates that the introduction of Nickel (Ni) element does not exert a substantial influence on the textural alterations within 316 L stainless steel.During the course of cold rolling, the material's crystals tend to align in accordance with the rolling direction, thereby conferring superior mechanical properties along this orientation.Concurrently, the extensive plastic deformation inherent to the cold rolling process triggers a restructuring of the austenite crystal lattice, promoting its alignment towards a dominant direction, consequently enhancing the robustness of the 〈101〉 fiber texture in that specific direction [36][37][38][39].

Changes of properties before and after cold-rolling
Figure 3 elucidates the magnetic conductivity and microhardness profiles of the two austenitic stainless steels across distinct cold-rolled conditions.The magnetic conductivity of the 316 L stainless steel showcases an incremental trend commensurate with the amplification of rolling extent.However, it consistently resides within the stipulated range of magnetic conductivity specified for 316 L, as prescribed by the GB/T 35690-2017 standard (below 1.05) [40].This heightened magnetic conductivity observed in 316 L may potentially emanate from a minor presence of martensite [41], despite remaining unapparent through XRD, metallographic, and EBSD examinations.In contrast, the magnetic conductivity of 316 LNi exhibits minimal sensitivity towards   variations in rolling extent, consistently maintaining levels below 1.01, thereby fully conforming to the stipulations within the GB/T 35690-2017 standard.Furthermore, the microhardness values of the two austenitic stainless steels demonstrate analogous trends, both exhibiting progressive elevation in consonance with escalating degrees of cold rolling.Figure 4, alongside table 2, presents the outcomes of tensile tests conducted on the two distinct austenitic stainless steel variants across varying cold rolling conditions.As the magnitude of cold rolling augmentations escalates, the tensile stress exhibited by both 316 L and 316 LNi stainless steels experiences a gradual increase, concomitantly accompanied by a decrement in tensile strain.Under equivalent rolling conditions, the tensile strength of 316 L stainless steel marginally surpasses that of 316 LNi stainless steel, while the corresponding tensile strain in the former slightly trails that of the latter.

Strengthening mechanism
A comprehensive analysis of the grain states and distributions within the two stainless steel variants was meticulously undertaken, yielding the findings depicted in figure 5. Following the solution treatment, an elevated prevalence of recrystallized grains is evident within both austenitic stainless steels, while the remaining grains assume subcrystalline characteristics, with a notable absence of deformed grains.Subsequent to extensive plastic deformation exerted upon the initial austenitic stainless steel, a pronounced accumulation of dislocations transpires within the austenite (FCC phase).The amplification in fault density contributes to the accumulation and entanglement of a substantial dislocation population proximate to the subgrain boundary, thereby engendering heightened stored energy within the material.To alleviate this stored energy, dislocation cells are generated through the mechanisms of sliding and climbing rearrangement, triggered once the dislocation plug reaches a critical accumulation threshold.During the solution treatment, a sequence of continuous dynamic recrystallization events unfolds in proximity to low-angle grain boundaries, characterized by dislocations [42][43][44][45].With a progressive augmentation in cold rolling magnitude, the plastic deformation grains within the two austenitic stainless steels, post solution treatment, demonstrably amplify, thereby diminishing the proportions of recrystallized grains and subcrystalline constituents.
The grain boundary and twin distributions of the 316 L and 316 LNi stainless steel samples are presented in figures 6 and 7.In 316 L stainless steel, the proportion of small-angle grain boundaries escalated from 3.09% to 65.6%, as depicted in figures 6(a1)-(c1), while in 316LNi stainless steel, the analogous boundaries increased from 0.81% to 70.5%, as displayed in figures 6(d1)-(f1).This augmentation in small-angle grain boundary content within both stainless steels is chiefly attributed to the significant plastic deformation induced by cold rolling, which, in turn, imparts a pronounced dislocation strengthening effect on the material's strength [46][47][48].Upon incorporation of Nickel (Ni), the grain size of 316 L stainless steel notably expands, concurrently leading to a reduction in the prevalence of large-angle grain boundaries within the field of view.Consequently, the small-angle grain boundary content within cold-rolled 316 LNi stainless steel marginally exceeds that of 316 L. Notably, prominent Σ3 (60°〈111〉) twin grain boundaries are evident within the two austenitic stainless steels.As the extent of cold rolling amplifies, the frequency of twins in 316 L stainless steel decreases from 54.2% to 24.7%, as illustrated in figures 6(a2)-(c2), and in 316 LNi, the frequency of  twins diminishes from 53.7% to 34.5%, as portrayed in figures 6(d2)-(f2).This reduction in twin occurrences in both stainless steels with escalating cold rolling is attributed to the surge in subcrystalline content engendered by deformation (as evidenced in figure 5), resulting in an elevation in large-angle grain boundary content.In the post-solution treatment scenario, annealing twins dominate within the two austenitic stainless steels, while cold-rolled materials predominantly feature deformation twins [33][34][35].Deformation twinning is attributed to a structural softening in the twinned region, establishing a favorable geometrical configuration for secondary deformation characteristics.Reasonably, it can be surmised that the enhancement of softening corresponds with an elevation in the fraction of deformation twins within the microstructure [49][50][51][52].Where 'b' represents the Burgers vector (2.35 × 10 −10 ) and 'μ' signifies the step size.The step sizes for 316 L and 316 LNi before and after cold rolling were 0.15 μm and 0.25 μm, respectively.The computed results for ρ GND are presented in table 3.In the case of 316L stainless steel following solution treatment, the presence of minimal KAM is observed, primarily attributed to the substantial occurrence of continuous dynamic recrystallization in the material post-solution treatment, contributing to a significant reduction in ρ GND (1.591 × 10 14 m −2 ).As the cold rolling procedure ensues, the distribution of KAM initially accentuates at grain boundaries before progressively extending intragranularly.With a cold rolling volume of 40%, ρ GND registers an approximately tenfold increment (11.002 × 10 14 m −2 ).A relatively sparse KAM distribution characterizes 316 LNi stainless steel in comparison to 316 L stainless steel, resulting in a ρ GND reduction of around fifty percent in the former relative to the latter.This phenomenon can be attributed to the magnified grain size and the concurrent decrease in the number of grain boundaries due to the Ni addition, as visually presented in figures 1 and 7.
In summary, the inclusion of Nickel (Ni) elements confers enhanced cold deformation capabilities upon 316 L stainless steel.Consequently, in comparison to the base 316 L variant (reduced from 1 mm to 0.285 mm), the Ni-enhanced counterpart can be subjected to a more extensive cold rolling process, reducing the thickness from 1 mm to 0.18 mm.Ultimately, maintaining equivalent solution treatment conditions, the 316 LNi stainless steel exhibits a more pronounced proclivity towards grain growth, approximately threefold that of 316 L. Consequently, the degree of grain refinement in 316 LNi is considerably attenuated [55][56][57].The decline in grain boundary content, resulting from the coarsening of grains, yields a reduction in dislocation density, consequently mitigating the potency of dislocation-induced strengthening.Notably, cold-rolled 316 LNi manifests a slightly augmented prevalence of twin boundaries relative to 316 L, which imparts a certain softening influence upon the material.As a result, while the strength of 316 LNi stainless steel is inferior to that of 316 L stainless steel, the former exhibits superior plasticity compared to the latter.(2) The magnetic conductivity of 316 LNi stainless steel falls below that of 316 L stainless steel; nevertheless, both comply with the designated usage specifications.

Conclusions
(3) The diminished strength of 316 LNi stainless steel, relative to 316 L stainless steel, can be attributed to grain coarsening, reduced dislocation density, and twin-induced softening, while concurrently exhibiting heightened plasticity.

Figure 3 .
Figure 3. Changes of magnetic conductivity and microhardness of two stainless steel samples.

Figure 4 .
Figure 4. Tensile curves of two stainless steel samples.

Figure 8
illustrates the EBSD-KAM diagram for both variants of 316 L stainless steels across varying degrees of cold rolling.The Kernel Average Misorientation (KAM) technique was applied to ascertain localized dislocations utilizing the EBSD dataset [53].The weighted average of KAM values enables the quantitative

Figure 7 .
Figure 7. Grain boundary and twin distribution of two stainless steel samples.

( 1 )
Incorporating Nickel (Ni) elements enhances the cold deformation capacity of 316 L stainless steel, albeit resulting in an increased grain size post solution treatment.

Table 2 .
Tensile test results of two stainless steel samples.

Table 3 .
KAM ave and ρ GND values of two stainless steel samples.