Behavior Of Austenitic Stainless Steel Research Articles

Metastability and fatigue behavior of austenitic stainless steels

This study presents the results of a detailed investigation of metastability and susceptibility to deformation induced α’-martensite formation of several austenitic steels (AISI 304, AISI 321, AISI 348 and two batches from AISI 347) in the solution-annealed state. Besides conventional characterization of metastability by calculating stacking-fault energy and threshold temperature (designated as MS and Md30), the present work introduced a new method for determining susceptibility to α’-martensite formation. The method was based on dynamically applied local plastic deformation and non-destructive micro-magnetic measurement of α’-martensite content. The parameter Iξ was established, which correlated very well with the grade of α’-martensite formation during cyclic loading. The cyclic deformation and phase transformation behavior of cyclically loaded specimens from different metastable austenitic steels were investigated in total-strain and stress controlled fatigue tests with load ratio R = -1 at ambient temperature. The influence of the strain rate on the cyclic deformation and phase transformation behavior was also examined. During the fatigue tests, besides stress-strain hysteresis and temperature measurement, in situ micro-magnetic measurements were performed. Using the compressive measured data, the influence of plastic induced self-heating of the specimen and the strain rate on α’-martensite formation was analyzed.

03.32: Analytical prediction of moment‐rotation behaviour of austenitic stainless steel bolted connections

ABSTRACTSteel framework is one of the most commonly used structural systems in modern construction and beam‐to‐column connections play a significant role in overall performance of the structure. Semi‐rigid behaviour of steel connections was investigated both experimentally and numerically over the last few decades. Stainless steel alloys are now increasingly used in construction industry owing to their superior material properties such as corrosion resistance, higher strength, enhanced ductility, significantly lower maintenance cost and attractive appearance. In this study, the performance of stainless steel top‐seat with or without double web angle (DWA) bolted connections are analytically investigated using a proposed four parameter power model assuming plastic hinges are formed in the connected angles. The proposed analytical model can predict the moment‐rotation behaviour using four key connection parameters such as initial stiffness, strain hardening stiffness, reference plastic rotation and shape parameter. These key parameters were expressed as functions of easily obtainable geometric and material properties. This model allows to predict complete moment‐rotation curves for stainless steel connections using the knowledge of easily available properties and configurations of the connecting elements. Analytically predicted moment‐rotation curves were compared with those obtained using 3D finite element models, and showed good agreement with actual connection behaviour.

Understanding the mechanical behaviour and the large strength/ductility differences between FCC and BCC AlxCoCrFeNi high entropy alloys

The deformation behaviour of three direct laser fabricated AlxCoCrFeNi high entropy alloys were examined. One alloy had a face-centered cubic (x = 0.3) structure, one had a two-phase structure comprising an ordered (B2) and disordered BCC phase (x = 0.85), and the third alloy contained a mixture of all three phases (x = 0.6). The deformation behaviour of all three alloys was examined by mechanical testing, scanning electron microscopy and transmission electron microscopy. The dislocation density of specimens was measured using x-ray diffraction. For the FCC alloy, it was found that the correlation between dislocation density, applied stress and compressive strain all benchmark closely with the behaviour of austenitic stainless steel. It is therefore found that this alloy does not show any unique deformation behaviours of note, and like austenitic steels, show a low yield point, moderately high work hardening rates and excellent plasticity. For the case of the alloy with a high aluminium concentration, which had a two-phase B2+BCC microstructure, an exceptionally high yield strength of 1400 MPa was observed. Nano-hardness examination of this alloy showed that the B2 phase had an exceptionally high hardness value. Since the alloy contained a large volume fraction of this hard intermetallic B2 phase, is was concluded that the B2 phase is responsible for the high strength of this alloy. It was shown that this alloy behaves like a composite, and its strength can be predicted closely using a simple rule of mixtures approach. The poor tensile ductility of BCC high entropy alloys has been suggested to be the result of the inability of the B2 phase to accommodate shape change, leading to interphase cracking that results in brittle fracture characteristics.

Cyclic deformation behavior of austenitic stainless steels in the very high cycle fatigue regime—Experimental results and mechanism-based simulations

Abstract

Evaluation of the Effect of Molybdenum on the Pitting Corrosion Behavior of Austenitic Stainless Steels Using Electrochemical Noise Technique

Electrochemical noise (EN) studies were conducted on three austenitic stainless steels (SSs) with different molybdenum contents, 0.02 wt% Mo (Type 304LN SS), 2.53 wt% Mo (Type 316LN SS), and 3.58 wt% Mo (Type 317LN SS), in 0.01 M FeCl3 solution at the corrosion potential (Ecorr) and at a sampling frequency of 1 Hz. The EN data were analyzed using shot noise and wavelet analysis techniques. Current transient analysis showed that the total number of current transients, as well as transients with high current amplitude, decreased with increase in Mo content indicating increased resistance to pitting corrosion. Shot noise analysis revealed higher normalized characteristic charge (q)N at low frequency in Type 304LN SS as compared to Type 316LN SS and Type 317LN SS, implying increase in pitting corrosion resulting from the absence of Mo in this steel. Pit current decreased substantially with increase in Mo content. These results were supported by the standard deviation of partial signal (SDPS) values generated ...

Flow Softening Index for Assessment of Dynamic Recrystallization in an Austenitic Stainless Steel

The present study proposes a novel technique to assess dynamic recrystallization (DRX) and related microstructural phenomena during hot deformation of austenite. A ‘Flow Softening Index (FSI)’ has been identified on the basis of investigations on elevated temperature deformation behaviour of austenitic stainless steel. This index corresponds to dominant microstructural phenomena at different deformation conditions. For this investigation, experimental results obtained from isothermal, constant true strain rate compression tests in a temperature range of 1173 (900)-1473 K (1200 °C) and strain rate range of 0.01-100 s−1 have been used. Resultant microstructures have been quantified using average grain size and grain size distributions. The dominant microstructural phenomena have been identified at different conditions using electron backscatter diffraction. Low FSI values are associated with the grain growth, intermediate values with DRX, and high values with the work-hardening and flow localisation phenomena. FSI also quantitatively indexes the average grain size and grain size distributions at different temperature-strain rate combinations. Analysis of the specific deformation conditions, particularly where 3.4 < FSI < 3.5, indicates a common thermo-mechanical origin of flow localisation and DRX. The potential technological implications thereof are discussed and a semi-empirical model of microstructural evolution is developed for the studied material.

Nitriding Process Characterization of Cold Worked AISI 304 and 316 Austenitic Stainless Steels

The nitriding behavior of austenitic stainless steels (AISI 304 and 316) was studied by different cold work degree (0% (after heat treated), 10%, 20%, 30%, and 40%) before nitride processing. The microstructure, layer thickness, hardness, and chemical microcomposition were evaluated employing optical microscopy, Vickers hardness, and scanning electron microscopy techniques (WDS microanalysis). The initial cold work (previous plastic deformations) in both AISI 304 and 306 austenitic stainless steels does not show special influence in all applied nitriding kinetics (in layer thicknesses). The nitriding processes have formed two layers, one external layer formed by expanded austenite with high nitrogen content, followed by another thinner layer just below formed by expanded austenite with a high presence of carbon (back diffusion). An enhanced diffusion can be observed on AISI 304 steel comparing with AISI 316 steel (a nitrided layer thicker can be noticed in the AISI 304 steel). The mechanical strength of both steels after nitriding processes reveals significant hardness values, almost 1100 HV, on the nitrided layers.

Influence of Changes in Pressure and Temperature of Supercritical Water on the Susceptibility to Stress Corrosion Cracking of 316L Austenitic Stainless Steel

The supercritical water reactor (SCWR) is one of the Generation IV designs. The SCWR is characterized by its high efficiency, low waste production, and simple design. Despite the suitable properties of supercritical water as a coolant, its physicochemical properties change sharply with pressure and temperature in the supercritical region. For this reason, there are many doubts about how changes in these variables affect the behavior of the materials to general corrosion or to specific types of corrosion such as stress corrosion cracking (SCC). Austenitic stainless steels are candidate materials to build the SCWR due to their optimum behavior in the light water reactors (LWRs). Nevertheless, their behavior under the SCWR conditions is not well known. First, the objective of this work was to study the SCC behavior of austenitic stainless steel 316 type L in deaerated supercritical water at 400°C/25 MPa and 30 MPa and 500°C/25 MPa to determine how variations in pressure and temperature influence its behavior with regard to SCC and to make progress in the understanding of mechanisms involved in SCC processes in this environment. Second, the oxide layer formed at 400°C/30 MPa/&lt;10 ppb O2 was analyzed to gain some insight into these processes.

Resurgence of texture in cyclically deformed austenite

Many studies monitoring the low cycle fatigue behaviour of austenitic stainless steels at various loading amplitudes and environments are reported in the open published domain literatures since last semi centennial. In those literatures, the cyclic plasticity behaviour is mainly interpreted and discussed in terms of microstructural characterizations, solid state phase transformation behaviour and related dislocation characteristics of these alloys. There is lack of literatures' data available on the orientation density distribution after strain controlled low cycle fatigue deformation of metastable austenite at various loading amplitudes under ambient temperature, which has been primarily aimed at in the current investigation. The propensity of maximum pole density distribution of deformed polycrystalline austenite grains has been found to be decreased consistently with increasing strain amplitude under strain controlled low cycle fatigue deformation at various domains of pole angles defined for all stereograms of upper hemisphere and hence variation in cyclic plastic response of the alloy. The continuous resurgence of crystallographic textures of polygonal austenite happens with the orderly variation of loading amplitudes during cyclic plasticity. Present research will specifically help to understand the related dislocation characteristics and mechanisms of martensitic phase transformations owing to fatigue damage accumulation and fracture micro mechanisms.

A Modified Mechanical Threshold Stress Constitutive Model for Austenitic Stainless Steels

This paper presents a modified mechanical threshold stress (m-MTS) constitutive model. The m-MTS model incorporates variable athermal and dynamic strain aging (DSA) Components to accurately predict the flow stress behavior of austenitic stainless steels (ASS)-316 and 304. Under strain rate variations between 0.01-0.0001 s−1, uniaxial tensile tests were conducted at temperatures ranging from 50-650 °C to evaluate the material constants of constitutive models. The test results revealed the high dependence of flow stress on strain, strain rate and temperature. In addition, it was observed that DSA occurred at elevated temperatures and very low strain rates, causing an increase in flow stress. While the original MTS model is capable of predicting the flow stress behavior for ASS, statistical parameters point out the inefficiency of the model when compared to other models such as Johnson Cook model, modified Zerilli-Armstrong (m-ZA) model, and modified Arrhenius-type equations (m-Arr). Therefore, in order to accurately model both the DSA and non-DSA regimes, the original MTS model was modified by incorporating variable athermal and DSA components. The suitability of the m-MTS model was assessed by comparing the statistical parameters. It was observed that the m-MTS model was highly accurate for the DSA regime when compared to the existing models. However, models like m-ZA and m-Arr showed better results for the non-DSA regime.

Elucidating the Effect of Alloying Elements on the Behavior of Austenitic Stainless Steels at Elevated Temperatures

The effect of carbon and molybdenum on elevated temperature behavior of austenitic stainless steels was studied. It was revealed that carbon does not alter the overall grain coarsening behavior but molybdenum significantly retards the growth of grains toward higher temperatures and slower kinetics and effectively increases the grain growth activation energy due to an interaction energy between Mo and grain boundaries. These observations were based on especial activation energy plots, which facilitate the interpretation of results.

Corrosion Behavior of Austenitic Stainless Steel in Supercritical CO2 Containing O2 and H2O

The corrosion behavior of 347H stainless steel was investigated in supercritical CO2 (sCO2) containing H2O and O2 simulating heat exchanger conditions that would exist in direct sCO2 power cycles. Thermodynamic properties of oxygenated sCO2 aqueous systems related to the corrosion phenomena was determined using NIST developed software REFPROP. The exposure tests of the corrosion samples were performed at a pressure of 80 bar and two temperatures, 50 °C and 248 °C up to 1500 hours. The samples were exposed to oxygenated H2O-containing CO2, and oxygenated sCO2-containing H2O. The corrosion rate of the alloys was determined by mass change measurements. The surface microstructure and composition of the corrosion films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD results for the corrosion products on the sample surfaces formed in the oxygenated CO2-containg H2O and water-containing CO2 and O2 at 248°C revealed the presence of Fe2O3 and Fe3O4,-rich phase, respectively. The XPS results revealed that the samples had thicker films after exposure to 0.01 O2/ 0.04 H2O ratio in CO2 at 248°C than at 50°C. The outer film layer contained Fe and O and the inner layer Cr and O at 248°C. Mostly Cr and O were found at 50°C.

Fatigue crack growth characteristics of austenitic stainless steel for cold‐stretched pressure vessels at cryogenic temperatures

Cold‐stretched pressure vessels made from austenitic stainless steel are widely used for storage and transportation of liquefied gases. Compared with conventional pressure vessels, cold‐stretched vessels have lots of advantages such as having thin walls, light weight, and low energy consumption. However, pressure vessels can be subjected to alternating loads at cryogenic temperatures, so it is important to investigate the fatigue characteristics of austenitic stainless steel for cold‐stretched vessels. This study presents the static and fatigue behavior of austenitic stainless steel 304 sampled from cold‐stretched pressure vessels in accordance with the American Society of Mechanical Engineers code. To compare the mechanical properties and fatigue behavior of austenitic stainless steel 304 in as‐received and cold‐stretched conditions (9 % strain level), tensile and fatigue crack growth tests were performed at room and cryogenic temperatures respectively. Fractography of the fractured specimens was carried out using a scanning electron microscope.

Cyclic deformation behavior of austenitic Cr–Ni-steels in the VHCF regime: Part II – Microstructure-sensitive simulation

In Part I – Experimental study, the cyclic deformation behavior of two austenitic stainless steel grades (AISI 304, AISI 316 L) were experimentally investigated at low stress amplitudes in the very high cycle fatigue (VHCF) regime. The observations indicate that during VHCF the metastable austenitic stainless steel (304 grade) performs a pronounced localization of plastic deformation in shear bands followed by a deformation-induced martensitic phase transformation. The 316 grade undergoes only a very limited local plastic deformation in shear bands with almost no phase transformation. Consequently, both materials exhibit distinctly different cyclic softening and hardening characteristics during VHCF. In order to provide a more detailed knowledge about the individual deformation mechanisms and their effect on the cyclic softening and hardening behavior the experimental study is extended by microstructure-sensitive modeling and simulation. Two-dimensional (2-D) microstructures consisting of several grains are represented using the boundary element method and plastic deformation within the microstructure is considered by a mechanism-based approach. Specific mechanisms of cyclic plastic deformation in shear bands and deformation-induced martensitic phase transformation – as documented by experimental results and based on well-known model approaches – are defined and implemented into the simulation. The fatigue behavior at low stress amplitudes observed in experiments can be well represented in simulations so that the underlying model helps to understand the cyclic deformation behavior of austenitic stainless steels at low stress amplitudes in the regime of VHCF strength. In a comparative study based on the resonant behavior the effect of certain deformation mechanisms on the global cyclic softening and hardening characteristics is pointed out for both materials.

Comparative Corrosion Behavior of Austenitic Stainless Steels in Carbonate Melt at 650 °C Under Controlled CO2-O2 Environment

Concentrated solar power (CSP) has recently attracted much attention as clean energy mainly for base load applications. High temperature CSP technology has high efficiency and it can attract wide commercial applications. Corrosion of metallic materials is a key problem for high temperature CSP systems. Careful selection of molten medium and metallic material can slow down the corrosion processes and hence extend the overall life span of CSP plants. High temperature oils and nitrate-based salts are commonly used for commercial CSP plants. These materials are not stable at temperature higher than 600 °C. However, alkali metal carbonates are stable beyond 800 °C and have working temperature of ~ 400-800 °C. These carbonates are less expensive and more environment friendly than the state-of-art high temperature oils [1]. Employing high corrosion resistance of cheaper alloys, like stainless steels, could significantly promote the popularity of CSP plants. In this study, corrosion behavior of austenitic stainless steels (SS) is observed in ternary alkali carbonate melt at 650 °C under defined CO2-O2 gas environment. Weight loss, electrochemical characterization and morphological and microstructural examination were carried out to understand the corrosion behavior of Type 304 SS, Type 316L SS and Type 310S SS in 43Li2CO3-31.5Na2CO3-25K2CO3 (mol%) melt at 650 °C under flowing 0.98 atm CO2-0.02 atm O2 mixed gas. Weight loss observed due to 24 hours corrosion is 7.7 mg cm-2 for Type 304 SS, 6.5 mg cm-2 for Type 316L SS and 1 mg cm-2 for Type 310S SS. Potentiodynamic polarization showed that corrosion of SS is a cathodic-controlled process. Corrosion potential of Type 310S SS is nobler than that of Type 316L and Type 304 SS. Reduced weight loss of Type 310S SS due to corrosion should be due to high potential of anode reaction. Cathodic limiting current is not significantly different for all the SS; however, it is slightly higher for Type 304 SS than that of Type 316L and should be responsible for slightly higher weight loss of Type 304 SS. Anodic polarization behavior well agrees with the weight loss observed for all the examined SS. SEM and EDS analysis showed that a very thin Cr-rich uniform corrosion layer is formed on Type 310S; however, thicker corrosion layer formed on Type 304 is composed of a heterogeneous mixture of Cr- and Fe-rich phases. Corrosion layer formed on Type 316L is Cr-rich but locally contains Fe-rich phases. Corrosion resistance of SS in carbonate melt in decreasing order is as: Type 310S>Type 316L>Type 304. This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). Reference K. Vignarooban, X. Xu, A. Arvay, K. Hsu, A. M. Kannan, Applied Energy, 146 (2015) 383-396

Sensitizing Effects of Ti Alloying Contents on Damage Behavior of Austenitic Stainless Steel Exposed to Ultrasonic Vibratory Cavitation

The cavitation damage characteristics of austenitic stainless steel with different concentrations of Ti were investigated. The microstructure of the alloys was observed with optical microscope to identify its correlation with cavitation resistance. Hardness of the alloys was measured to examine its contribution to cavitation damage. It was found that the microstructure played a more significant role in cavitation damage behavior of austenitic stainless steel with Ti than the hardness. The findings in this study revealed that Ti addition in austenitic stainless steel may present either a beneficial or detrimental effect on cavitation damage behavior, depending on the microstructural characteristics. In particular, Ti content of 1.0% represented the most deteriorated cavitation characteristics due to the formation of relatively coarse precipitates. Therefore, control of Ti concentration is essential for marine application of austenitic stainless steel.

Corrosion Behavior of Austenitic Stainless Steel in Supercritical CO2 Containing O2 and H2o

The corrosion behavior of 347H stainless steel was investigated in supercritical CO2 (sCO2) containing H2O and O2 to simulate heat exchanger conditions that would exist in direct sCO2 power cycles. To understand the thermodynamic properties of oxygenated sCO2 aqueous systems related to the corrosion phenomena, thermodynamic modeling was performed using REFPROP software. Samples were exposed to oxygenated water-containing CO2, and oxygenated CO2-containing H2O. The exposure tests were carried out at a pressure of 80 bar and two temperatures, 50 °C and 248 °C. The environmental conditions were simulated by filling each 1100 ml autoclave with 400 ml of deionized water, bringing each autoclave to its respective test temperature, and then pressurizing it with a CO2:O2mixture having a molar ratio of 95:1. The corrosion rate of each sample was determined by mass loss measurements. The surface morphology and the composition of the corrosion product layers were analyzed using surface analysis techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS) for oxygen and chromium, and X-ray photoelectron spectroscopy (XPS). Weight gain results reveal that the samples exposed to higher temperatures experience a significant increase in oxide growth compared to the lower temperature counterparts. XRD results confirmed this result in that corrosion products present on the samples exposed to sCO2 containing H2O and O2 and H2O containing sCO2 and O2 at 50 °C were too thin for conventional Bragg-Brentano XRD to detect anything aside from the base material. However, the samples exposed to the environments at 248 °C had sufficiently thick corrosion layers for this method. The XRD results for the corrosion products on the sample surfaces formed in the oxygenated CO2-containing H2O and water-containing CO2 environments at 248 °C revealed the presence of primarily hematite (Fe2O3) and magnetite (Fe3O4), respectively. These results indicate that the corrosion degradation mechanism for 347H in oxygenated H2O containing sCO2 is different from the mechanism in oxygenated sCO2 containing H2O at 248 °C.

Oxidation of alloys for energy applications in supercritical CO2 and H2O

To facilitate development of supercritical CO2 (sCO2) power plants, a comparison of the oxidation behavior of austenitic stainless steels and Ni-base alloys in sH2O and sCO2 were made. Experiments were conducted at 730°C/207bar (sCO2) and 726°C/208bar (sH2O). Ni-base alloys in sCO2 did not exhibit much change with pressure. Ni-base alloys in sH2O had an increase in corrosion rate and the log of the parabolic rate constant was proportional to pressure. Fine-grain austenitic stainless steels in sCO2 and sH2O were both less protective with pressure as the dense protective chromia scale was replaced with faster growing Fe-oxide rich scales.

Corrosion Behavior of Austenitic Stainless Steel in Hydrochloric Acid Solution and Flow

The corrosion behaviors of austenitic stainless steels, 304 and 316L as a contrast material, are investigated in hydrochloric acid corrosion under different conditions including immersion corrosion, single-phase flow and two-phase flow, respectively. The corrosion mass loss test and electrochemical test were carried out to evaluate the influence of medium temperature and concentration. The specimen surface morphologies and chemical compositions were obtained using scanning electron microscope to analyze the corrosion mechanism. The results show that the static corrosion rates of 304 and 316L steels and erosion corrosion rates of 304 steel increase obviously with the medium temperature increasing, but, the effect of hydrochloric acid concentration on their corrosion rate is less than that of the temperature. The corrosion current density values of 304 stainless steel increase and its passivation region of the polarization curve becomes narrowing and disappeared as the medium temperature increase. Uniform corrosion is the main way found on the surfaces of the stainless steels at room temperature in the hydrochloric acid solution. However, the selective corrosion occurs on their surface with the medium temperature increasing, and the higher temperature and concentration of corrosion medium, the more serious selective corrosion.

Effect of nitrate on corrosion of austenitic stainless steel in boiling nitric acid solution containing chromium ions

ABSTRACTThe oxidation behavior of chromium and the corrosion behavior of austenitic stainless steel in boiling nitric acid solution containing highly concentrated nitrates were investigated using UV-visible spectroscopic measurements, Raman spectral measurements, immersion tests, and potentiodynamic polarization measurements. The oxidation rate measurement of chromium from Cr(III) to Cr(VI) was performed by 1 M boiling nitric acid solution containing each highly concentrated nitrates: Al(NO3)3, Nd(NO3)3, Ca(NO3)2, Mg(NO3)2, and NaNO3 as a simulant of uranium nitrate in uranium concentrator in reprocessing plants. As a result, the rate of chromium oxidation was different depending on the added nitrates even at the same nitric acid concentration. In addition, the oxidation rate of chromium was increased with increasing the calculated partial pressure of nitric acid in consideration of the hydration of cation of nitrates. Furthermore, the corrosion rate of type 310 stainless steel was accelerated by the solution having a high chromium oxidation rate containing nitrates. These results indicated that the acceleration of the corrosion rate in the solutions depending on the oxidation rate of chromium, and the rate is affected by the salt-effect of nitrates.