316LN SS Research Articles

Formation of Deformation Bands in Hot-Worked SS 316LN

The role of parameters such as strain, strain rate, temperature, and nitrogen content in the formation of deformation bands in SS 316LN during hot working is discussed. The study was carried out using two variants of SS 316LN. The effects of deformation parameters on band formation were studied by conducting deformation experiments under compression loading at temperatures of 1123 K to 1473 K and strain rates of 0.01 s−1, 1 s−1, and 10 s−1. It was observed that, for deformation at low temperature and high strain rate, there exists a strain beyond which the localized flow manifests as deformation bands. The intensity of these deformation bands depends on temperature. Their intensity reduces with increase in temperature due to the improved thermal conductivity of the material. High deformation rate and high nitrogen content tend to assist deformation band formation. On the basis of microstructural analysis, a mechanism for deformation band formation is proposed.

Effect of friction stir and activated-GTA welding processes on the 9Cr–1Mo steel to 316LN stainless steel dissimilar weld joints

ABSTRACTThe consequence of friction stir welding (FSW) and activated-gas tungsten arc welding (A-GTAW) processes on the evolution of microstructure and mechanical properties of 9Cr–1Mo (P9) steel to 316LN stainless steel dissimilar weld joint is investigated. The FSW specimen shows considerably higher tensile strength (∼652 MPa) compared to A-GTAW specimen (∼595 MPa) as well as its base metal of P9 (∼642 MPa) and 316LN (∼608 MPa) owing to the formation of tempered martensite and refined austenite in P9 and 316LN weld portion, respectively. The cross-weld tensile test revealed that the specimens failed in the base metal of 316LN SS for both FSW and A-GTAW process with ductile mode fracture. This study proves that FSW could be an alternate joining technique.

Long-term exposure of MgAl2O4 and Y2O3 thermal barrier coatings in molten sodium

Thermal barrier coatings (TBCs) of magnesium aluminate spinel (MgAl2O4) and yttria (Y2O3) were investigated for long term compatibility with molten sodium for applications in sacrificial core catcher of Future Fast Reactors (FFRs). Thermal barrier ceramic coatings were deposited over austenitic grade stainless steel SS 316LN (flat and cylindrical samples), with an intermediate NiCrAlY bondcoat, using Atmospheric Plasma Spray (APS) technique. The coated samples were exposed to high purity molten sodium at 400 °C under an inert argon atmosphere for 500 h and 1000 h. Sodium exposed samples were subjected to post-cleaning by vacuum distillation followed by chemical cleaning for effective removal of sodium from the specimens. Visual examinations revealed discoloration in the Na exposed samples. Microstructural characterization of Na exposed samples revealed densification in the ceramic topcoats. X-ray diffraction analysis on the topcoats did not reveal any phase change or formation of any corrosion product upon interaction with sodium. Tensile adhesion test on cylindrical samples, as per ASTM C633, revealed ~62% and ~33% drop in the bonding strength after 1000 h of sodium exposure for spinel and yttria coatings, respectively. The mechanical properties were correlated to the microstructural characteristics of the TBC system before and after Na exposure.

Evaluation of fracture resistance of AISI type 316LN stainless steel base and welded pipes with circumferential through-wall crack

Fracture resistance of 316LN SS base and welded straight pipes with circumferential through-wall cracks has been evaluated. The pipe J-R curves have been determined using existing experimental methods and analytical expressions available in literature. The J-R curves of 88.9 mm OD pipe and pipe welds have been compared with the J-R curves obtained from C(T) specimen tests. The results indicate that the C(T) specimen data is not suitable for fracture assessment of pipes due to difference in constraint level and the fracture resistance of the base pipe is inferior to welded pipe. Further, the limit load analyses based on expressions available in literature reveals that the initiation and crack propagation are observed prior to the maximum bending moment.

Fatigue Crack Growth Study in P91 and 316LN Steels Using Acoustic Emission

Fatigue crack growth (FCG) behavior of ferritic steel P91 and austenitic stainless steel 316LN (SS 316LN) has been studied at ambient temperature. Acoustic emission (AE) signals generated during FCG tests were captured using 375 kHz resonant transducer. The FCG and AE results were compared for both steels in Paris and threshold regimes. The AE results indicated five and four substages of FCG for P91 steel and SS 316LN, respectively. Several parameters of AE signals such as rise time, peak amplitude and event duration were used to differentiate between various substages of the FCG. AE cumulative count was correlated with stress intensity factor range (ΔK) in Paris regime for both the steels. For SS 316LN, the exponent of the correlation of AE cumulative count with ΔK is in good agreement with Paris exponent. For P91 steel, higher value of the exponent of the AE cumulative count—stress intensity factor range correlation compared to Paris exponent is observed. Monitoring of AE continuously during fatigue crack growth of these two steels enables understanding the mechanisms of crack growth operative at different regions of the FCG curve.

Improved corrosion resistance of 316LN stainless steel performed by rotationally accelerated shot peening

Rotationally accelerated shot peening (RASP) technology was used to produce nanocrystallines and twins on the surface of 316LN stainless steel. Electrochemical corrosion experiments were carried out on the RASP-processed 316LN SS specimens in the 3.5 wt% NaCl solution. Improved passivity and better electronic properties of passive films were observed on the RASP-processed specimens. The experimental results showed that the structure with nanocrystallines and a high density of twins produced by the RASP process can provide more nucleation sites, adhesion work and homogeneous capillary force to improve the uniformity and compactness of the passive film, resulting in a better corrosion resistance.

An investigation of microstructural evolution in electron beam welded RAFM steel and 316LN SS dissimilar joint under creep loading conditions

Microstructural evolution in creep tested electron beam welded Reduced Activation Ferritic Martensitic (RAFM) steel and 316LN stainless steel dissimilar weld joints has been studied at 823 K under different stress levels. The rupture life of the weld joints spanned between 17,596 and 190 h with variation in applied stress from 160 to 220 MPa. Failure of the weld joints occurred in the RAFM steel side at all stress levels. Microstructural examination showed that the failure location shifted from the unaffected base metal region of the RAFM steel to the ‘soft region’ within the heat affected zone (HAZ) in the RAFM steel side with decrease in applied stress. The strength mismatch between various regions within the HAZ of RAFM steel and the microstructural instability of ‘soft region’ in the RAFM steel are the two important factors which dictate the failure location and rupture life of the joints.

Ductile damage parameters and far field J-integral for high hardening steel

PurposeThe purpose of this paper is to present a methodology to assess material damage parameters for ductile crack initiation and growth ahead of a crack/notch tip in high hardening steel like AISI type 316L(N) stainless steel.Design/methodology/approachDuctile damage parameter and far field J-integral have been obtained from standard FEM analysis for a crack/notch tip undergoing large plastic deformation and resulting in crack initiation/growth. In conjunction with experimental results, the damage variable for low strength and high hardening material has been derived in terms of continuum parameters: equivalent plastic strain (εeq) and stress triaxiality (φ). The material parameters for damage initiation and growth in 316LN SS have been evaluated from tensile and fracture tests. With these material tensile/fracture parameters as input, elastic-plastic eXtended Finite Element Method (X-FEM) simulations were carried out on compact tension (CT) specimen geometry under varying initial stress triaxiality conditions.FindingsThe material parameters for damage initiation and growth have been assessed and calibrated by comparing the X-FEM predicted load-displacement responses with the experimental results. It is observed that the deviations in the predicted load values from the experimental data are within 6 percent for specimens with a/W=0.39, 0.55, 0.64, while for a/W=0.72, it is 17 percent.Originality/valueThe present study is a part of developing methods to obtain calibrated material damage parameters for crack growth simulation of components made of AISI 316L(N) stainless steel. This steel is used for fast breeder reactor-based power plant being built at Kalpakkam, India.

Assessing Constitutive Models for Prediction of High-Temperature Flow Behavior with a Perspective of Alloy Development

The utility of different constitutive models describing high-temperature flow behavior has been evaluated from the perspective of alloy development. Strain compensated Arrhenius model, modified Johnson–Cook (MJC) model, model D8A and artificial neural network (ANN) have been used to describe flow behavior of different model alloys. These alloys are four grades of SS 316LN with different nitrogen contents ranging from 0.07 to 0.22%. Grades with 0.07%N and 0.22%N have been used to determine suitable material constants of the constitutive equations and also to train the ANN model. While the ANN model has been developed with chemical composition as a direct input, the MJC and D8A models have been amended to incorporate the effect of nitrogen content on flow behavior. The prediction capabilities of all models have been validated using the experimental data obtained from grades containing 0.11%N and 0.14%N. The comparative analysis demonstrates that ‘N-amended D8A’ and ‘N-amended MJC’ are preferable to the ANN model for predicting flow behavior of different grades of 316LN. The work provides detailed insights into the usual statistical error analysis technique and frames five additional criteria which must be considered when a model is analyzed from the perspective of alloy development.

Tensile Properties Variation Across the Dissimilar Metal Weld Joint Between Modified 9Cr–1Mo Ferritic Steel and 316LN Stainless Steel at RT and 550 °C

In liquid metal cooled fast breeder reactors (LMFBR), modified 9Cr–1Mo ferritic steel (P91 or Grade 91) is a preferred material for constructing steam generators due to its creep strength and stress corrosion cracking resistance. The austenitic stainless steels (SS 316LN and SS 304LN) are widely used for primary and secondary piping systems because of its oxidation resistance and excellent creep strength. So, the dissimilar metal weld joint (DMWJ) between P91 and SS 316LN is inevitable. Nickel-based consumables (Alloy 82 and Alloy 182) are preferred to join these materials. The DMWJ will experience the temperature up to 550 °C. For accurate integrity assessment, the mechanical properties of individual regions are to be evaluated at room temperature (RT) and 550 °C. Hence, the present investigation is focused on evaluating the mechanical properties of various regions of DMWJ at RT and 550 °C. From this investigation, it is understood that the tensile properties are heterogeneous across the DMWJ at RT and 550 °C. The high-temperature (550 °C) tensile properties are significantly lower with respect to the RT properties. The development of complex microstructures at the interfaces will alter the mechanical properties across the DMWJ.

Experimental Evaluation of Mechanical and Tribological Behaviours of Gas Nitride treated AISI 316LN Austenitic Stainless Steel

Austenitic stainless steel is limited to utilize directly in numerous industries on account of its low surface hardness. Hence, in this work, an effort has taken to enhance the surface hardness of austenitic stainless steel type AISI 316LN Stainless Steel (316LN SS) by Gas Nitride (GN) technique. The GN was done on 316LN SS at 520°C for 42 hours. The GN was done in two phases to limit the white nitride layer. The GN treatment enhances the surface hardness from 210 HV25g (untreated 316LN SS sample) to 1230 HV25g (GN treated sample). The GN treatment produces a thick and uniform nitride layer on the 316LN SS. The tribolgical test conclude that the GN layer on 316LN SS eliminates adhesion bonds at the contact points and show smooth surface even after 1000m sliding distance. Consequently, the GN treatment was not just reduce the friction and also formed an excellent wear resistance GN layer on the 316LN SS substrate.

Low Temperature Embrittlement studies on Stainless Steel 304 LN TIG Welds

Austenitic stainless steel 304LN (SS 304LN) is widely used in nuclear industry. TIG welding is selected here to weld the stainless steel plates because it offers the possibility of producing high quality and porosity free welds. However, weld metal experiences hot cracks during welding of austenitic stainless steels. Due to this, austenitic stainless steel welds are designed to have 3-8% delta ferrite in the weld metal to avoid hot cracks during solidification. With the presence of delta ferrite, when these materials are exposed to elevated temperature (300 - 500°C) the weld transforms into embrittlement phases such as iron rich α and chromium rich α, G phase and M23C6. These embrittlement phases deteriorate the mechanical properties of the weld. In this study, an attempt is made to study the low temperature embrittlement on SS 304LN TIG Welds. The low temperature embrittlement behaviour on aged samples at (300, 350, 400°C) was evaluated for 10000 hours by mechanical property testing and micro structural examination. Mechanical testing involves Hardness test and Impact test. Delta ferrite measurement was done to see any transformation of delta ferrite. Fracture surfaces were characterized using SEM. Increase in hardness with increase in aging temperature and decrease in delta ferrite content at 350°C and 400 °C clearly indicate that aging temperature affects kinetics of transformation of delta ferrite. Scanning Electron Microscopy did not show any transformation of delta ferrite. It was found that aging treatment has a strong influence on impact toughness. Decreases in impact toughness value especially at sub – zero temperatures indicate kinetics of transformation of delta ferrite is affected by aging temperature during low temperature aging.

Influence of some Test Parameters on Automated Ball Indentation Test Results

Automated Ball Indentation (ABI) technique has the potential to evaluate tensile and fracture properties of materials by testing a small volume of material. ABI testing involves various parameters such as surface finish of test specimen, maximum depth of indentation, number of loading-unloading cycles and material of indenter. As ABI test is yet to be standardized, it is necessary to assess and understand the consequences of the various test parameters. In this investigation, the influence of the above test parameters on the tensile properties of 316LN SS has been investigated at 298 K. It was found that by increasing the maximum depth of indentation, number of loading-unloading cycles and surface finish of test specimen at the cost of time and efforts, ABI results were not altered significantly. The selection of indenter material has to be done judiciously, according to test temperature.

A study on the effect of weld groove designs on residual stresses in SS 304LN thick multipass pipe welds

Present research describes the effect of weld groove designs on residual stresses of thick SS 304LN pipe welds which are commonly utilized for applications such as boiling water reactor system, oil and steam piping. The faying surfaces of the pipe joints were machined to generate conventional and narrow groove geometries for welding. Circumferential multipass pipe welding was carried out without applying preheat and post weld heat treatment. Blind hole drilling technique was employed for measuring hoop and axial residual stresses in narrow and conventional grooved SS 304LN pipe welds considering plasticity and stress concentration effects of hole drilling process. The result exhibited 20 to 30 percent reduction in residual stresses in narrow grooved pipe welds. A finite element model was developed for estimating the pass by pass peak temperature distributions in the pipe welds. Sequentially coupled thermomechanical elastic-plastic analysis of both conventional and narrow groove pipe welds indicated close agreement between the predicted and experimentally determined hoop and axial residual stresses.

Creep damage and life assessment of thick-walled spherical reactor using Larson–Miller parameter

Creep damage and remnant life assessment of a thick-walled spherical reactor made of 316LN austenitic stainless steel (316LN SS) have been investigated. The Robinson's linear damage accumulation rule has been used to obtain damage and remnant life assessment in which time to rupture is determined by Larson-Miller Parameter (LMP). Due to high temperature, creep is the most significant damage mechanism exhausting the lifetime of the reactor. The material properties, except Poisson's ratio, are assumed to depend on the temperature. An analytical solution employed to calculate the stress rates followed by an iterative method using initial thermoelastic stresses at zero time to obtain effective stress histories and then using LMP to calculate the damage and remnant life assessment. It is concluded that the temperature gradient has a significant effect on the effective stress histories so that effective stresses are decreasing with time in a uniform temperature field while they are increasing in the presence of a thermal gradient.

Aging precipitation behaviors of Nb-contained 316LN SS

Precipitation behaviors of Nb-contained 316LN austenitic stainless steels during isothermally aging at 650, 750 and 800 °C up to 1000 h have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrolytic extraction experiment. The experimental results show that, long-term thermal aging process introduced signification amount of precipitates both at grain boundaries and on dislocations inside austenitic matrix to Nb-contained 316LN SS. The quantitative calculation of precipitates volume fraction confirmed the sensitive aging temperature was approximate 750 °C. The precipitation behavior of minor phases during Nb-contained 316LN SS different aging treatment was summarized. In Nb-contained 316LN SS aging process, Z-CrNbN phase was the first and predominant nitride phase. Nb element addition into 316LN SS, promoted the formation of σ-FeCrMo and η-Fe2Mo phase, restrained the formation of carbide Cr23C6.

Studies on Dissolution Behavior of the Surface Layer of Sodium-Exposed SS 316LN in Decontaminating Formulation Using PEMHS

The Prototype Fast Breeder Reactor (PFBR) is nearing completion at Kalpakkam, India. Sodium is the heat transfer medium for PFBR, and austenitic steel SS 316LN is the material of construction for the sodium circuits of the reactor. During reactor service, the inner surfaces of the sodium circuit pipelines undergo corrosion by interacting with liquid sodium, forming ferritic layers. Radioactive nuclides formed by the activation of corrosion products are deposited on the ferritic surface, resulting in a radioactive burden on maintenance personnel. Chemical decontamination is generally carried out by dissolving the surface ferritic layer on the inside surface of the sodium circuit. In this context, a study of the dissolution behavior of the ferritic layer on SS 316LN samples formed by exposure to liquid sodium at 823 K was carried out by monitoring the H2 released during the chemical interaction with decontamination formulation. The decontamination chemical formulation was a mixture of sulfuric acid and phosphoric acid. This paper discusses the sample preparation, formation of the ferritic layer, and studies carried out on its dissolution behavior in decontamination formulation by monitoring the hydrogen released during the reaction using a proton exchange membrane–based hydrogen sensor.

Plastic deformation of SS 316LN:Thermo-mechanical and microstructural aspects

Abstract The successful manufacturing of SS 316LN components for strategic sectors needs a thorough understanding of deformation behavior including mechanical response, relevant microstructural evolution and associated instabilities in both cold and hot working domains. This work gives a holistic view of these aspects using experimental results obtained from plastic deformation of the SS 316LN under compressive and shear loading. The results indicate that any temperature can be chosen for the deformation if the strain rates are below 1s -1 . At higher strain rates, the steel becomes insensitive to the rate of deformation. However, high strain rates are preferred when the deformation temperature is above 1273K. The work also presents views on large deformation up to 200% strain in a single pass.

Evaluation of nitrogen containing reducing agents for the corrosion control of materials relevant to nuclear reactors

Materials undergo enhanced corrosion in the presence of oxidants in aqueous media. Usually, hydrogen gas or water soluble reducing agents are used for inhibiting corrosion. In the present study, the feasibility of using alternate reducing agents such as hydrazine, aqueous ammonia, and hydroxylamine that can stay in the liquid phase was investigated. A comparative study of corrosion behavior of the structural materials of the nuclear reactor viz. carbon steel (CS), stainless steel (SS-304 LN), monel-400 and incoloy-800 in the oxidizing and reducing conditions was also made. In nuclear industry, the presence of radiation field adds to the corrosion problems. The radiolysis products of water such as oxygen and hydrogen peroxide create an oxidizing environment that enhances the corrosion. Electrochemical studies at 90 °C showed that the reducing agents investigated were efficient in controlling corrosion processes in the presence of oxygen and hydrogen peroxide. Evaluation of thermal stability of hydrazine and its effect on corrosion potential of SS-304 LN were also investigated in the temperature range of 200–280 °C. The results showed that the thermal decomposition of hydrazine followed a first order kinetics. Besides, a change in electrochemical corrosion potential (ECP) was observed from −0.4 V (Vs SHE) to −0.67 V (Vs SHE) on addition of 5 ppm of hydrazine at 240 °C. Investigations were also made to understand the distribution behavior of hydrogen peroxide and hydrazine in water-steam phases and it was found that both the phases showed identical behavior.

Studies on Creep Deformation and Rupture Behavior of 316LN SS Multi-Pass Weld Joints Fabricated with Two Different Electrode Sizes

Effect of electrode size on creep deformation and rupture behavior has been assessed by carrying out creep tests at 923 K (650 °C) over the stress range 140 to 225 MPa on 316LN stainless steel weld joints fabricated employing 2.5 and 4 mm diameter electrodes. The multi-pass welding technique not only changes the morphology of delta ferrite from vermicular to globular in the previous weld bead region near to the weld bead interface, but also subjects the region to thermo-mechanical heat treatment to generate appreciable strength gradient. Electron backscatter diffraction analysis revealed significant localized strain gradients in regions adjoining the weld pass interface for the joint fabricated with large electrode size. Larger electrode diameter joint exhibited higher creep rupture strength than the smaller diameter electrode joint. However, both the joints had lower creep rupture strength than the base metal. Failure in the joints was associated with microstructural instability in the fusion zone, and the vermicular delta ferrite zone was more prone to creep cavitation. Larger electrode diameter joint was found to be more resistant to failure caused by creep cavitation than the smaller diameter electrode joint. This has been attributed to the larger strength gradient between the beads and significant separation between the cavity prone vermicular delta ferrite zones which hindered the cavity growth. Close proximity of cavitated zones in smaller electrode joint facilitated their faster coalescence leading to more reduction in creep rupture strength. Failure location in the joints was found to depend on the electrode size and applied stress. The change in failure location has been assessed on performing finite element analysis of stress distribution across the joint on incorporating tensile and creep strengths of different constituents of joints, estimated by ball indentation and impression creep testing techniques.