316LN Austenitic Stainless Steel Research Articles

Investigations on corrosion behavior of 316LN and 316L austenitic stainless steel under corrosion-deformation interactions

PurposeThis paper aims to explore the influence of corrosion-deformation interactions (CDI) on the corrosion behavior and mechanisms of 316LN under applied tensile stresses.Design/methodology/approachCorrosion of metals would be aggravated by CDI under applied stress. Notably, the presence of nitrogen in 316LN austenitic stainless steel (SS) would enhance the corrosion resistance compared to the nitrogen-absent 316L SS. To clarify the CDI behaviors, electrochemical corrosion experiments were performed on 316LN specimens under different applied stress levels. Complementary analyses, including three-dimensional morphological examinations by KH-7700 digital microscope and scanning electron microscopy coupled with energy dispersive spectroscopy, were conducted to investigate the macroscopic and microscopic corrosion morphology and to characterize the composition of corrosion products within pits. Furthermore, ion chromatography was used to analyze the solution composition variations after immersion corrosion tests of 316LN in a 6 wt.% FeCl3 solution compared to original FeCl3 solution. Electrochemical experiment results revealed the linear decrease in free corrosion potential with increasing applied stress. Electrochemical impedance spectroscopy results indicated that high tensile stress level damaged the integrity of passivation film, as evidenced by the remarkable reduction in electrochemical impedance. Ion chromatography analyses proved the concentrations increase of NO3− and NH4+ ion concentrations in the corrosion media after corrosion tests.FindingsThe enhanced corrosion resistance of 316LN SS is attributable to the presence of nitrogen.Research limitations/implicationsThe scope of this study is confined to the influence of tensile stress on the electrochemical corrosion of 316LN at ambient temperatures; it does not encompass the potential effects of elevated temperatures or compressive stress.Practical implicationsThe resistance to stress electrochemical corrosion in SS may be enhanced through nitrogen alloying.Originality/valueThis paper presents a systematic investigation into the stress electrochemical corrosion of 316LN, marking the inaugural study of its impact on corrosion behaviors and underlying mechanisms.

Effects of Ce on the precipitation behaviors and creep properties in 316LN austenitic stainless steel

The effects of Ce on the precipitation behaviors and creep properties in 316LN austenitic stainless steel (316LN) were studied by experiments and first-principles calculations. It was found that Ce in 316LN induces intragranular Laves phase precipitates with an average diameter of 166 nm. Aside from Ce rich in TiN nanoparticles facilitating Laves phase precipitation by heterogeneous nucleation, the solute Ce atom attracting its surrounding Mo atoms favors Laves phase precipitation by homogeneous nucleation. These intragranular Laves phases strongly impede the dislocation climbing and thus enhance the creep resistance. Here, 0.032 wt% Ce addition in 316LN considerably extends the creep life from 313 h to 556 h at 700 °C/150 MPa. These results provide a new insight into profoundly understanding the microalloying mechanism and effects of rare earth elements in heat-resistant steels.

Dislocation-density-related constitutive model and its applicability to cyclic deformation and damage behavior of 316LN SS at 823 K

In the present investigation, an in-depth understanding of the relationship between dislocation kinetics and cyclic plasticity, along with the description of yield asymmetry behavior during loading and unloading in nuclear grade 316LN austenitic stainless steel at 823 K, was examined by employing a constitutive framework using the evolution of physically-based internal-state variables. The formulation encompassed a set of first-order simultaneous differential equations that governed the evolution of dislocation density, mean free path of dislocations, and back stress. These variables were interdependent with the power law which described the kinetics of plastic strain rate. The evolution equations were numerically integrated, and the simulated data were fitted to the experimental data up to stabilized cycle at a temperature of 823 K and total strain amplitudes of ±0.004, ±0.006, ±0.008 and ± 0.010. The predicted cyclic stress response as a function of the cyclic number displayed the continuous cyclic hardening from the first cycle to the initiation of stabilized cycle. Further, good agreement between the predicted and experimental hysteresis loops was found at different cycle numbers. The predicted dislocation density and tensile peak values of back/effective stress increased with cyclic hardening and approached the value of saturation at the stabilized cycle. On the other hand, a rapid decrease in mean free path with cycle number was noticed. An increase in total strain amplitude resulted in a systematic increase in dislocation density, peak stress, back stress and effective stress. At the total strain amplitudes above ±0.006, the predicted higher rates of dislocation accumulation and annihilation, and the faster evolution rate of back stress towards saturation led to the development of stable dislocation substructure configurations with a clear distinction between dislocation high/low dense regions. Furthermore, the applicability of the model was extended by including an empirical relationship between the damage variable and cycle number within the power law, thereby enabling predictions of the cyclic stress-strain response beyond the stabilized cycle.

Effect of Defocusing Distance on Microstructural, Hardness, and Tribological Behavior of Ni-Cr-Si-B-C Powder Deposit on 316LN Austenitic Stainless Steel Substrate Using Laser Hard-Facing

The Ni-based alloy powder has been melted and bonded to an AISI 316LN austenitic stainless steel (ASS) base material (substrate) using a high-energy disc laser with a maximum power of up to 12 kW. The substrate’s hard-faced reservoir is highly diluted due to the significant difference in melting temperature between the substrate and the commonly used Ni-based alloy. The hardness, macrostructure, microstructure, and wear resistance effects of the defocusing distance were examined. The composition, microstructure, hardness, and wear resistance of phases have been examined with a pin-on-disk wear test, an energy dispersion spectroscopy (EDS), a scanning electron microscope (SEM), and X-ray diffraction (XRD). According to the findings, the height and diameter of the beads rose when the distance was increased from 17 mm to 37 mm. On the other hand, penetration and dilution decreased from 3.7 mm to 2.7 mm and from 9.7% to 3.1%, respectively. When the defocusing length is increased and the penetration profundity and dilution are limited, the density of energy per unit clad is accessible. The laser hard-faced surfaces contain microstructures of Ni-rich solid solution, boride, and carbide. These different factors cause higher hardness and resistance to wear.

Effect of Ce on creep damage behavior of 316LN austenitic stainless steel at 700 °C

The effect of rare earth element Ce on creep damage behavior of 316LN austenitic stainless steel (316LN) at 700 °C was systematically investigated under stress ranging from 125 MPa to 200 MPa. The results show that the addition of 0.032 wt% Ce remarkably improves the rupture life of 316LN steel under various stress. Ce addition in 316LN steel decreases the nucleation stress, stabilizes stress concentrations at particles, and notably raises the cavity nucleation rate. Moreover, Ce addition suppresses cavity growth during creep, thus the cavity growth rate significantly decreases and the creep rupture ductility increases under high stress over 175 MPa. However, the anomalous growth rate was engendered in some cavities by Ce addition in 316LN steel under low stress below 150 MPa. Consequently, Ce addition in 316LN steel reduces the creep rupture ductility and changes the rupture mode from trans-granular dimple fracture under high stress to mixed fracture under low stress.

Finite element analysis of butt joint of 316Ln steel thin plate welded by laser welding process

This study focuses on the FEM-based thermal analysis of laser welding for 1.5 mm thick austenitic stainless steel 316Ln, which is commonly used in high-temperature applications such as oil & gas industries and thermal power plants. The objective is to investigate the thermal behavior of the welding process and validate the FEM-based calculations with measured thermal cycles. The thermal cycles at a distance of 10 mm from the welding zone were measured using K-type thermocouples. To analyze the thermal behavior, a Finite Element Method (FEM) approach was employed using the SYSWELD software. The FEM-based calculated thermal distribution was then compared and validated against the measured thermal cycles. In order to accurately model the laser heat source distribution, two different heat source models were selected: a conical model and a combined double ellipsoidal plus conical model. The findings of this study provide valuable insights into the thermal characteristics of laser welding for austenitic stainless steel 316Ln. The validated FEM-based thermal analysis helps in understanding the temperature distribution and thermal effects during the welding process. Such knowledge is crucial for optimizing the welding parameters and ensuring the integrity and quality of the welded joints in high-temperature applications.

Prediction of characteristic points of single-peak flow curve through statistical technique and artificial neural network

ABSTRACT Artificial neural network (ANN) is an input–output modeling technique that has been successfully implemented in the field of material science to predict material behavior in terms of constitutive models. The material behavior considered here is the stress–strain behavior of the material at elevated temperatures that is characterized by a single-peak flow curve consisting of characteristic points viz. critical stress, peak stress and steady-state stress. The present work focuses on predicting these stress values along with the corresponding strain values through statistical regression analysis (SRA), ANN, and multi-layer complex neural network (MLCNN) with reasonable accuracy. The flow curves obtained from the axisymmetric compression test of 304LN austenitic stainless steel, performed at constant temperatures and constant strain rates, are used to train the models. The MLCNN has performed best while SRA has performed relatively worst with the current dataset due to rigorous and thorough computation. Both MLCNN and ANN are observed to perform better than SRA because of their non-parametric nature of handling data.

Measurements and predictions of diffusible hydrogen escape and absorption in catholically charged 316LN austenitic stainless steel

Hydrogen can have an impact on the service life of safety critical components, such as coolant pipes in nuclear reactors, where it may interact with other factors including irradiation. Hence, it is important to characterise such behaviour which in turn requires the capability to charge representative material specimens with hydrogen and to quantity the levels of hydrogen present. Hydrogen concentrations resulting from cathodic charging of 316LN stainless steel over short time periods (< 2 h) were estimated from hydrogen release rates obtained from potentiostatic discharge measurements and used to calibrate simulations based on Fick’s second law of diffusion in order to predict the hydrogen concentration after 24 h of charging. Leave-one-out cross-validation was used to establish confidence in results which were also validated using measurements from the melt extraction technique. The success of Fick’s second law for estimating escape rates showed that a majority of the absorbed hydrogen was diffusible rather than trapped. These results confirmed that the potentiostatic discharge technique can be used on materials with low diffusivity, and provide a new method through which hydrogen concentrations within a sample can be estimated after cathodic charging non-destructively without the need to remove samples from solution.

Development of two-dimensional temperature field solution method based on the stress–strain response of 316LN stainless steel at cryogenic temperatures

To better understand the tensile fracture behavior and temperature variation during the tension testing of the 316LN at cryogenic temperatures, a two-dimensional temperature field solution method is developed based on two-dimensional strain field measured by the digital image correlation technology combined with the Taylor Quinney coefficient model and the intrinsic model of strain-induced martensitic phase transformation. The method allows for visualization of the strain field and temperature variation of the 316LN austenitic stainless steel at cryogenic temperature. By numerical simulation, the localized temperature elevation of the 316LN is determined to be 13.3 K during the tensile test at 77 K, which is in good agreement with the experimental result of 10.2 K. This model reconstructed the two-dimensional temperature field variation throughout the entire tensile test, which is beneficial for facilitating the analysis of the mutual relation between the mechanical properties and microstructural evolution of the 316LN and other metastable austenitic stainless steels in cryogenic temperatures.

Sliding wear behaviour of salt bath nitrided 316LN austenitic stainless steel

The wear behaviour of AISI 316LN austenitic stainless steel has been studied by varying the duration of nitriding. AISI 316LN steel was nitrided using salt bath nitriding which consist of a mixture of 70:30 ratio of alkaline cynates and carbonates. Nitriding was carried out for two time durations namely 60 min and 100 min. Pin-on-Disc wear test were carried out for both the untreated and treated sample at room temperature. The wear parameters were changed to understand the wear mechanism of AISI 316LN steel along with the nitrided sample by varying the sliding distance for 250 m, 500 m and 1000 m. Due to the presence of different wear mechanism the rate of wear varies as a function of sliding distance. For untreated sample the wear mechanism was dominant with adhesion, abrasion and plastic deformation. An optical micrograph, X-ray diffraction analysis, wear morphology, hardness measurement and surface roughness were carried out. Due to the presence of compound layer as a result of nitriding AISI 316LN austenitic stainless steel, the mechanism of wear was restricted to abrasive wear. Untreated specimens have more significant wear loss when compared to treated specimen as work hardening and an increase in hardness of wear track resulted in material pull out. Stability of the compound layer is achieved as a result of longer duration of nitriding. Abrasive wear is resisted to a greater extend due the availability of compound layer. AISI 316LN steel which was nitrided for 100 min exhibited a more stable compound layer when compared to the same nitrided for 60 min.

Tensile properties variation along the thickness direction of hot rolled austenitic stainless steel

In the present work, single-pass hot rolling on 304LN austenitic stainless steel has been performed at initial sample temperatures of 900 °C, 1000 °C and 1100 °C. Tensile samples have been prepared from different thickness layers of the rolled samples to study the through-thickness tensile properties variation. Similarly, the microstructure of the rolled samples has been examined at the mid-thickness and subsurface through the electron backscattered diffraction (EBSD) technique. For samples rolled at an initial temperature of 900 °C and 1100 °C, a significant difference in ultimate tensile strength (UTS) has been observed for the surface. The sample rolled at an initial temperature of 1000 °C does not show much difference in UTS values along the thickness direction. However, a significant deviation in total elongation has been seen for the surface sample. The difference in the tensile behavior (plastic deformation) has been studied by considering the strain hardening exponent (n), strength coefficient (K) and strain hardening rate (θ). In the end, the through-thickness tensile properties are correlated to the developed microstructure and texture. Both microstructure and texture played an important role in the through-thickness tensile behavior.

Dynamic strain aging-mediated temperature dependence of ratcheting behavior in a 316LN austenitic stainless steel

Tensile tests and ratcheting tests were performed on a 316LN austenitic stainless steel at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, to study the influence of dynamic strain aging (DSA) on deformation behavior and microstructural evolution. The DSA temperature range in present work was determined to be ≥ 400 °C in both uniaxial tests and ratcheting tests. Especially in the ratcheting tests, a sudden drop in ratcheting strain accumulation as well as in steady ratcheting strain rate was evident when the temperature raising from 300 ° to 400 °C. While within the DSA temperature region, the ratcheting behavior was insensitive to the temperature. Microstructural examinations revealed dislocation cells and tangles in the sample ratcheted at 300 °C (free of DSA), while complex dislocation structure in the samples ratcheted at ≥ 400 °C (with DSA) that was consisted mainly by high-density dislocation walls, planar slip bands, and Lomer-Cottrell locks. This clearly indicates a change in predominant ratcheting deformation mechanism from cross-slip at DSA-free temperature region to planar-slip at DSA region. It was further demonstrated that, within the temperature range from 400 °C to 600 °C, the DSA-induced hardening effect balanced with the thermally-activated softening effect and this balance led to temperature-insensitive ratcheting behavior as experimentally observed. Finally, the DSA-affected dislocation structure was quantitatively evaluated in the ratcheted samples, with an aim in correlating to the ratcheting behavior.

Microstructure evolution of austenitic stainless steels under high-cycle-fatigue loading at deep cryogenic temperature

In this work, the microstructure evolution and magnetic properties of the 316LN austenitic stainless steels under high-cycle-fatigue loading at liquid helium temperature (4.2 K) were assessed by using the X-ray diffractometer (XRD), electron microscopy and magnetic measurements. The microstructure was composed of a single face-centered cubic (FCC) phase, and the high local misorientation was found near the grain boundaries. Observations through the high-resolution transmission electron microscopy reveals that the deformation was dominated by the dislocation and stacking faults. The magnetic susceptibility varies slightly after cyclic loading test at 4.2 K.

Impact of Rare Earth Addition on Creep Rupture Behavior of 316LN Austenitic Stainless Steel at 700°C

Effect of rare earth (RE) on creep rupture behavior of 316LN austenitic stainless steel (316LN steel) was investigated after crept at 700°C under the stress in the range from 125MPa to 200MPa, by the optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show RE addition in 316LN steel increased the creep rupture ductility at high stress, but reduced the creep rupture ductility at low stress. Under 200MPa, RE addition increased the creep rupture strain of 316LN steel from 0.558 to 0.787 but the creep rupture strain after crept under 150MPa was decreased from 0.875 to 0.566. The fracture mode of 316LN steel was also apparently impacted by the RE addition. The typical ductile fracture feature of homogeneous dense dimples was obviously observed in NRE steel after crept rupture under all stresses. While in 32RE steel, small amount of intergranular fracture fractographs under low stress appeared instead of partial dimples under high stress. Moreover, it is noted that RE addition in 316LN steel promotes to precipitate a great number of fine Laves particles within grains. These Laves particles strengthening the matrix resulted in the strain concentration on grain boundaries, which might sensitively induce crack initiation on grain boundaries during long-term creep under the low stress.

Effect of Microstructure on Mechanical Properties of 316 LN Austenitic Stainless Steel

The microstructure development of 316 LN austenitic stainless steel (316 LNSS) during the aging process is investigated in this article. The thermal aging processes were conducted at 750 °C with different periods ranging from 50 to 500 h. The metallographic results show that the coherent and incoherent twins were present in the original 316 LNSS grains, but dwindled as the aging period increased. After 50 h of aging, many fine, dispersed particles precipitated from the matrix, which were identified as M23C6 by energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM). Additionally, the impact toughness and Brinell hardness (HBW) changed during the aging, which was closely related to the effects of dispersion strengthening and solution strengthening. A negatively linear relationship between Brinell hardness and Charpy impact energy was established, which could be utilized to predict the degree of thermal embrittlement.

Research on the pitting behavior of high nitrogen austenitic stainless steel 316LN in sodium chloride solution by using modified potentiostatic pulse test

• The modified potentiostatic pulse test is used to study the pitting behavior of 316LN. • The pitting size and number of 316LN are regulated. • The Si-enriched inclusions are initial sites of metastable pit on 316LN surface. • The pitting resistance of 316LN is enhanced after the potentiostatic pulse test. The pitting behavior of high nitrogen austenitic stainless steel 316LN was studied by using the modified potentiostatic pulse test (PPT). The results show that the duration of the high potential (E h ) and the low potential (E l ) significantly affect the pitting size and number of 316LN. However, the pitting behavior of 316LN only changes during the first five pulse cycles, because the metastable pitting initiates at the limited inclusions. The optimal PPT condition is regarded as E h = 0.5 V SCE for 1 s, E 1 = 0.2 V SCE for 2 s and ten pulse cycles. Metastable pits initiate at the susceptible sites of inclusions which are identified as SiO 2 and SiC. The metastable pits will further grow or restrain with the increasing time of E h or E l , respectively. After PPT modification, the pitting resistance of 316LN is enhanced due to the removal of the Si-enriched inclusions from the 316LN surface. Meanwhile, the fewer current transitions, the lower passivation current density and the higher impedance all suggest that the protectiveness of the oxidation film formed on 316LN surface is improved after PPT.

Enhancement Mechanism of Cerium in 316LN Austenitic Stainless Steel During Creep at 700 °C

Enhancement Mechanism of Cerium in 316LN Austenitic Stainless Steel During Creep at 700 °C

Numerical and Analytical Studies of Low Cycle Fatigue Behavior of 316 LN Austenitic Stainless Steel

Abstract Mechanical components are frequently subjected to severe cyclic pressure and/or temperature loadings. Therefore, numerical and analytical low cycle fatigue methods become widely used in the field of engineering to estimate the design fatigue lives. The primary aim of this work is to evaluate the accuracy of the most commonly used numerical and analytical low cycle fatigue life methods for specimens made of 316 LN austenitic stainless steel and subjected to fully reversed uniaxial tension–compression loading, in the room temperature condition. It was found that both maximum shear strain and Brown–Miller criterions result in a very conservative estimation for uniaxially loaded specimens. However, maximum shear strain criteria provide better results compared to the Brown–Miller criteria. The total strain energy density approach was also used, and both the Masing and non-Masing analysis were adopted in this study. It is found that the Masing model provides conservative fatigue lives, and non-Masing model results in a more realistic fatigue life prediction for 316 LN stainless steel for both low and high strain amplitudes. The fatigue design curves obtained from the commonly used analytical low cycle fatigue equations were reexamined for 316 LN SS. The obtained design curves from Langer model and its modified versions are nonconservative for this type of material. Consequently, the authors suggest new optimized parameters to fit the given test data. The obtained curve using the currently suggested parameters is in better agreement with the experimental data for 316 LN SS.

Ratcheting-induced twinning/de-twinning behaviors in a 316LN austenitic stainless steel

Although the evolution of dislocation substructure during ratcheting deformation has been previously studied in 316LN austenitic steel, studies on ratcheting-induced twinning and de-twinning behaviors are very scarce. In this paper, microstructural evolutions, especially the twinning/de-twinning behaviors, were systematically investigated in a 316LN austenitic steel under ratcheting cycling. Different peak stresses (320, 400, 450, and 500 MPa) were deliberately designed to demonstrate a potential correlation between the twinning/de-twinning and the dislocation pattern. The experimental results showed that, at the low peak stress of 320 MPa, annealing twins were quite stable and the dislocation pattern mainly consisted of planar dislocation arrangement. Raising the peak stress to 400 MPa, most of the annealing twins were gradually de-twinned and the dislocation pattern displayed a different feature prevailed by dislocation veins, walls and cells. As the peak stress reaches 450 MPa and in particular up to 500 MPa, the annealing twins get quickly de-twinned, while stacking faults and deformation twins were induced besides the dislocations. The critical stress to trigger deformation twins in the 316LN austenitic steel was evaluated to be around 500 MPa, which is in broad agreement with the experimental results. This indicates that the nucleation of deformation twins under ratcheting cycles should be controlled by the peak stress rather than the accumulation strain. An additional cycling hardening stage was observed in the ratcheting cycling response under the peak stress of 450 and 500 MPa, which should be closely related to the formation of stacking faults and deformation twins. Moreover, the stress-dependent de-twinning of annealing twins were discussed in terms of dislocation interaction with annealing twins.

Thermomechanical and isothermal fatigue crack propagation testing method for 316LN austenitic stainless steel

AbstractInvestigating fatigue crack growth behavior of materials is crucial to the life design and safety evaluation of engineering structures. During the practical service condition, the temperature keeps changing, which cannot be studied by the traditional fatigue crack growth test method at constant temperature. In this study, we focus on constructing a new thermomechanical fatigue (TMF) crack propagation testing method, developing a thermal deformation compensation method, and proposing an equivalent compliance method suitable for variable temperature conditions based on ASTM E647 standard to measure the crack length. Furthermore, the load‐ and strain‐controlled TMF tests under different phase angles are carried out to prove the validity of the new method.