Cr Austenitic Stainless Steel Research Articles

First-principles and cluster expansion study of the effect of magnetism on short-range order in Fe–Ni–Cr austenitic stainless steels

Short-range order (SRO), the regular and predictable arrangement of atoms over short distances, alters the mechanical properties of technologically relevant structural materials such as medium/high entropy alloys and austenitic stainless steels. In this study, we present a generalized spin cluster expansion (CE) model and show that magnetism is a primary factor influencing the level of SRO present in austenitic Fe–Ni–Cr alloys. The spin CE consists of a chemical cluster expansion combined with an Ising model for Fe–Ni–Cr austenitic alloys. It explicitly accounts for local magnetic exchange interactions, thereby capturing the effects of finite temperature magnetism on SRO. Model parameters are obtained by fitting to a first-principles data set comprising both chemically and magnetically diverse face-centered cubic configurations. The magnitude of the magnetic exchange interactions are found to be comparable to the chemical interactions. Compared to a conventional implicit magnetism CE built from only magnetic ground state configurations, the spin CE shows improved performance on several experimental benchmarks over a broad spectrum of compositions, particularly at higher temperatures due to the explicit treatment of magnetic disorder. We find that SRO is strongly influenced by alloy Cr content, since Cr atoms prefer to align antiferromagnetically with nearest neighbors but become magnetically frustrated with increasing Cr concentration. Using the spin CE, we predict that increasing the Cr concentration in typical austenitic stainless steels promotes the formation of SRO and increases order–disorder transition temperatures. This study underscores the significance of considering magnetic interactions explicitly when exploring the thermodynamic properties of complex transition metal alloys. It also highlights guidelines for customizing SRO through adjustments of alloy composition.

Analysis of Monkman-Grant relationships and damage tolerance factor for creep of 25Cr35NiNb micro-alloyed steel

Creep behavior of high Cr austenitic stainless steel was investigated at temperatures ranging from 650 to 1050 °C and stress levels in the range 47–120 MPa. Primary, secondary and tertiary creep strains were analyzed in terms of minimum creep rate, constant true stress and temperature. Minimum creep rates, strain to rupture and rupture time were analyzed in terms of Monkman-Grant and modified Monkman-Grant relation. Good correlation between minimum creep rate and rupture time using both Monkman-Grant relationships is observed for the creep data on the steel. Validity of the relationship between time to reach Monkman-Grant ductility tMGD and rupture life tr in terms of damage tolerance factor λ was established for the steel. Effect of temperature on microstructure and carbide content of the steel is presented in this paper. Microstructural studies reveal failure mechanisms by nucleation and growth of cavities around carbide particles at grain boundary regions. Based on the critical continuum damage mechanics approach, the ratio of tMGD to tr express by constant fCDM separated safe and unsafe regimes of creep behavior of the steel.

An Fe–Ni–Cr–H interatomic potential and predictions of hydrogen-affected stacking fault energies in austenitic stainless steels

While Fe–Ni–Cr austenitic stainless steels exhibit relatively good resistance to hydrogen embrittlement, they still suffer from significant degradation of ductility, fatigue and fracture properties in gaseous hydrogen environments. Experimental studies in the literature suggest that hydrogen reduces stacking fault energy in austenitic stainless steels. This phenomenon causes a large separation of partial dislocations and lower propensity for cross-slip. Whereas lower stacking fault energy does not correlate well with loss of ductility in the absence of hydrogen, lower stacking fault energy trends toward greater loss of ductility when hydrogen is present. Calculations of stacking fault energy are challenging for austenitic stainless steels. One main issue is that in alloys, stacking fault energy is not a single value but rather varies depending on local composition. Herein, we first report an Fe–Ni–Cr–H quaternary interatomic potential and then use this potential to perform time-averaged molecular dynamics simulations to calculate stacking fault energies for tens of thousands of realizations of local compositions for selected stainless steels alloys with and without internal hydrogen. From statistical analyses, our results suggest that hydrogen reduces stacking fault energy, which likely impacts deformation mechanisms of Fe–Ni–Cr austenitic stainless steels when exposed to hydrogen environments. We then perform validation MD simulation tests to show that hydrogen indeed statistically increases the stacking fault widths due to statistically reduced stacking fault energies.

Comparison of 2507 Duplex and 28 % Cr- Austenitic Stainless Steel Corrosion Behavior for High Pressure and High Temperature (HPHT) in Sour Service Condition with C-ring Experiment

The scarcity of oil and gas resources made High Pressure and High Temperature (HPHT) reservoir attractive to be developed. The sour service environment gives an additional factor in material selection for HPHT reservoir. Austenitic 28 Cr and super duplex stainless steel 2507 (SS 2507) are proposed to be a potential materials candidate for such conditions. C-ring tests were performed to investigate their corrosion behavior, specifically sulfide stress cracking (SSC) and sulfide stress cracking susceptibility. The C-ring tests were done under 2.55 % H2S (31.48 psia) and 50 % CO2 (617.25 psia). The testing was done in static environment conditions. Regardless of good SSC resistance for both materials, different pitting resistance is seen in both materials. The pitting resistance did not follow the general Pitting Resistance Equivalent Number (PREN), since SS 2507 super duplex (PREN > 40) has more pitting density than 28 Cr austenitic stainless steel (PREN < 40). SS 2507 super duplex pit shape tends to be larger but shallower than 28 Cr austenitic stainless steel. 28 Cr austenitic stainless steel has a smaller pit density, yet deeper and isolated.

A comparison of laser-metal inert gas hybrid welding and metal inert gas welding of high-nitrogen austenitic stainless steel

Abstract High-nitrogen austenitic stainless steel (HNASS) has many excellent properties, such as high strength and toughness, high strain-hardening ability, strong corrosion resistance and low magnetization compared with the traditional widely used Ni–Cr austenitic stainless steel. The high nitrogen content in HNASS leads to the welded joint in the risk of many problems, such as nitrogen escape and loss, nitride precipitation, formation of pores and thermal cracks, resulting in the poor welding performance. The HNASS joints without obvious defects were obtained by metal inert gas (MIG) welding and laser-MIG hybrid welding, separately. The microstructures and properties of welded joints were compared. The nitrogen content in fusion zones of the two joints was tested by an oxygen, nitrogen and hydrogen analyzer. The tensile strength and the impact absorption energy of laser-MIG hybrid welded joint were about 27% and 20% higher than those of MIG welded joint, respectively. It could be attributed to the stronger solid solution strengthening effect resulted by the high nitrogen content of laser-MIG hybrid welded joint. The finer grains and the reduction of ferrite and precipitation phase in the microstructure of laser-MIG hybrid welded joint contributed to the improvement of toughness.

Effect of Temperature of Spark Plasma Sintering on the Development of Oxide Compound in Fe-25wt%Ni-17wt%CrAustenitic Stainless Steel

Effect of Temperature of Spark Plasma Sintering on the Development of Oxide Compound in Fe-25wt%Ni-17wt%CrAustenitic Stainless Steel

Atomistic simulation of the obstacle strengths of radiation-induced defects in an Fe–Ni–Cr austenitic stainless steel

A molecular dynamics method is employed to simulate the interaction of an edge dislocation with a combined solute cluster-dislocation loop defect, as well as independent faulted interstitial dislocation loops and solute clusters in an Fe–12Ni–20Cr at a temperature of 300 K. The examined defects are typical radiation-induced defects in austenitic stainless steels used as core structural components in nuclear reactors, while the selected composition matches with commercial grade 304L alloy. The dislocation-defect interaction is examined, and the peak shear stress required to overcome the obstacle array, and its dependence on obstacle size, solute concentration, and orientation is determined. The peak shear stress is then converted to an effective obstacle strength (α). Results show that dislocation loops are stronger obstacles than solute clusters, and their strength is heavily dependent on orientation, while the strength of solute clusters is largely dependent on the solute concentration. The total strength of the combined solute cluster-dislocation loop defect is largely contributed by the dislocation loop, though a fraction of the cluster strength is added as well.

Atomic resolution characterization of strengthening nanoparticles in a new high-temperature-capable 43Fe-25Ni-22.5Cr austenitic stainless steel

Advanced scanning transmission electron microscopy (STEM) was used to study two distinct populations of nanoparticles associated with the extraordinary strengthening of the highly-alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700 °C. Fully coherent and homogeneously dispersed Cu-rich nanoparticles precipitate rapidly as a result of thermal exposure, along with nanometer-sized incoherent NbC carbides that nucleate on dislocations during the cyclic loading at high temperature. The atomic structure of nanoparticles was investigated by probe-corrected high-angle annular dark-field STEM imaging. Compositional analysis of the nanoparticles was conducted using high spatial resolution energy dispersive X-ray spectroscopy combined with electron energy-loss spectroscopy. Experimental observations were validated by image simulations of the Moiré-like contrast exhibited by NbC carbides. The important role of both nanoparticle populations for the overall cyclic response is discussed. As a result of pinning effects and associated obstacles, dislocation motion is significantly retarded preventing formation of substructures with lower stored internal energy. With recovery heavily suppressed, forest dislocation strengthening supported by precipitation and solid solution hardening, leads to the remarkable increase of cyclic strength at elevated temperatures.

Stacking fault energies in austenitic stainless steels

We measure the stacking fault energy of a set of 20 at% Cr-austenitic stainless steels by means of transmission electron microscopy using the weak beam dark field imaging technique and the isolated dislocations method. The measurements are analyzed together with first principles calculations. The results show that experiment and theory agree very well for the investigated concentration range of Mn (0–8%) and Ni (11–30%). The calculations show that simultaneous relaxation of atomic and spin degrees of freedom is important in order to find the global energy minimum for these materials. Our results clearly show the great potential of the weak beam dark field technique to obtain accurate measurements of the stacking fault energy of austenitic steels and that the reliable predictability of first principles calculations can be used to design new steels with optimized mechanical properties.

High Temperature Oxidation of 22wt.% Cr Austenitic Alloys upon Exposure to Hot Corrosion Environment with Na2so4

High temperature corrosion studies in today’s austenitic steels are of great interest to develop novel corrosion- and creep-resistant austenitic steels for advanced zero-emission power generation plants. These efficient power plants can offer advantageous considerations for the global need for energy and mitigation of climate change. However the aggressive environmental conditions poised challenges to current austenitic steels for long-term applications. To date, long-term application of austenitic steels has been restricted to service temperatures lower than 635 °C while future boilers are targeting temperatures of 700 – 750 °C. Therefore, extensive microstructural evolution studies of Fe-Cr-Ni steels exposed to high temperature conditions will contribute useful information to help retard degradation or enhance corrosion resistance properties while in search of novel compositions in austenitic steels.Protective Cr-rich oxide films formed on austenitic stainless steels play an effective role to lower high temperature corrosion. These oxide scales of low porosity, good adherence, low growth rate, and high mechanical and thermodynamic stability can provide good corrosion resistance properties. Hence, in the formation of a protective Cr-rich oxide scale as a barrier to further corrosion, diffusional transport of Cr within the alloy to the oxide scale is critical.The conditions of the exposure environments of the austenitic steels highly affect the growth and stability of these protective Cr-rich oxide scales. For example, the formation of volatile Cr species can lead to a depletion of Cr in the oxide scales and form Fe-rich oxide films, which inadequately function as a barrier to corrosion. Since the rate of diffusional transport of Cr in the oxide scale is compensated, accelerated localized attacks within the grains occur. In addition, the presence of H2O and CO2 in the exposure environment results in the formation of Fe-rich oxide scales and accelerated oxidation rates of most technical steels.Sulphurous corrosive components (SO2 and sulphate-based salt components) in commercial high temperature applications, for e.g. combustion, accelerate the corrosion rate, resulting in adverse modification of degradation properties in Fe-Cr-Ni steels. This deposit-induced intensified oxidation process is known as hot corrosion. In addition, Na and S exist as fuel impurities in power plants and lead to deposits of Na2SO4. These molten deposits can cause sulphate-deposit induced accelerated oxidation processes, which destroys the reaction-product oxide scales. Propagation modes of hot corrosion determined by these mechanisms are also dependent on the exposure temperatures and durations, and compositions of exposure gas, oxides and deposits. Hence, the influence of different exposure temperatures and durations on the mechanisms of the oxidation behavior of Fe-Cr-Ni steels studied in this work will help predict the behavior of austenitic steels in hot corrosion environments.In this work, the isothermal high temperature oxidation behaviors of a 22 wt.% Cr austenitic stainless steels, Sanicro25 (42Fe22Cr25NiWCuNbN), subjected to various exposure temperatures have been investigated. The exposure environment consists of O2+H2O+CO2+SO2+Na2SO4. The alloys were subjected to respective exposure temperatures of 600, 700 and 750 °C with different exposure durations of 300 h and 1000 h. They were also compared to results observed in Sanicro25 alloys which were subjected to similar environmental conditions at 700 °C but for a shorter exposure duration time of 168 h [1].Detailed microstructural investigations within the individual grains of the formed oxide, the oxide-metal interface and in the vicinity of the oxide-steel interface using XRD, SEM, FIB/SEM, TEM and EDX have been performed. These results yield insights to the microscopic mechanisms governing the oxidation behavior of steels with deposits of Na2SO4when subjected to aggressive high temperature environments related to oxy-fuel combustion.

Influences of Si, W, and Cu on the Localized and Stress Corrosion of 18 % Cr Austenitic Stainless Steel

Effects of Si, W and Cu on the localized and stress corrosion of 18 % Cr austenitic stainless steel were investigated using experimetal strategy of full factorial design. The resistance to localized and stress corrosion of the alloy was improved with increasing the contents of Si, W and Cu in the range of 0.5 ~ 1.5 % for Si, 0.1 ~ 0.5 % for W and 0.2 ~ 2.0 % for Cu. In addition, we found an interaction effect between Si and Cu on the reistance to localized and stress corrosion of the alloy.

Development of high nitrogen, low nickel, 18%Cr austenitic stainless steels

Two high nitrogen stainless steels are studied through metallographic, mechanical and corrosionistic tests and the results are compared with those shown by a standard AISI 304. These high nitrogen steels show a significantly higher mechanical strength than usual AISI 304 while their corrosion resistance lie among that of standard austenitic and that of standard ferritic stainless steels.

Low-activation Mn–Cr austenitic stainless steel with further reduced content of long-lived radioactive elements

Low-activation austenitic stainless steel based on Mn–Cr non-magnetic steels has been developed. The alloying elements of long-life activation, such as Ni, Mo and Co, were eliminated and substituted with Mn along with an addition of N. A Mn–Cr austenitic stainless steel, 24.5Mn–13.5Cr–0.02C–0.2N, has been developed successfully. Examined material properties, including mechanical, thermal and magnetic properties, as well as weldability and characteristics of corrosion resistance, are presented. It was found that the alloy has excellent material properties virtually equivalent to those of 316SS. In this study, the applicability of the Schaeffler, DeLong and Hull constitution diagrams for the stainless steels with low Ni and high Mn contents was also examined. The boundary conditions distinguishing the single austenite phase from the others have been identified for the Mn–Cr steels.

Mechanical and Corrosion Properties of High Strength 18% Cr Austenitic Stainless Steel Weldment for Boiler.

The properties of weld metal of 18Cr heat resistant austenitic steel containing high copper, niobium and nitrogen were investigated to obtain the matching filler metal for an austenitic stainless steel of 18Cr-9Ni-3Cu-Nb-N steel. This 18Cu-9Ni-3Cu-Nb-N steel tube has been registered as KA-SUS304HJ1TB in the Japanese official standard for fired power plant materials. This steel tube has a high temperature strength 1.4 times as high as the conventional 18Cr steel ASME TP347H. Welding material applicable to this steel is necessary in the fabrication. However, the weld metal consisting of the same chemistries as the base metal had a lower strength than that of the base metal. In this work, the effect of chemistries of molybdenum and nitrogen on the strength of weld metal was investigated. The addition of molybdenum up to 1 mass% was effective for improving the strength without a deterioration of the corrosion resistance. The nitrogen was also effective for improving the strength, but the nitrogen can be decreased by vaporizing during tungsten inert gas arc welding. For improving the solubility of nitrogen in the molten pool, the addition of manganese was effective. From these findings, the chemistry of welding material was determined as 18Cr-16Ni-3Cu-1Mo-3Mn-0.6Nb-0.2N and the tube weldment with that welding material was evaluated in terms of mechanical properties including creep rupture property. It was confirmed that the weldment had comparable properties to the base metal.

Creep rupture properties of oxidised 20% Cr austenitic stainless steels

Under operational conditions in advanced gas cooled reactors (AGRs), fuel cladding alloys are exposed to an oxidising environment for prolonged periods approaching 850°C. It is essential, from safety considerations, that the mechanical integrity of this thin section material is not threatened by any potential oxidation effects. This aspect is addressed in the present work. Sheet specimens of 20Cr–25Ni niobium stabilised steel and nitrided 20Cr–25Ni–Ti stainless steel, both used as fuel cladding materials in AGRs, have been oxidised in simulated reactor gas [CO2– (1–2% CO)] for periods up to 7·0×106 s (1900 h) at 850°C, including intermediate thermal cycles to room temperature. These exposures have been carried out in thermobalances which provided a full gravimetric analysis for each specimen. The oxidised specimens have been creep tested subsequently at 750°C, under conditions of constant stress, together with non-oxidised and thermally aged specimens, which formed the ‘baseline’ mechanical property data. After testing, the specimens have been examined using optical metallography and transmission electron microscopy to determine the microstructural changes caused by thermal aging and by oxidation. The paper compares and contrasts the response in mechanical behaviour to oxidation effects for both of the fuel cladding alloys. The creep rupture properties are affected differently for the two materials, but it is shown that this can be interpreted in terms of the observed changes in microstructure and the nature of the oxidation attack.MST/1127

Creep rupture properties of oxidised 20% Cr austenitic stainless steels

Under operational conditions in advanced gas cooled reactors (AGRs), fuel cladding alloys are exposed to an oxidising environment for prolonged periods approaching 850°C. It is essential, from safety considerations, that the mechanical integrity of this thin section material is not threatened by any potential oxidation effects. This aspect is addressed in the present work. Sheet specimens of 20Cr–25Ni niobium stabilised steel and nitrided 20Cr–25Ni–Ti stainless steel, both used as fuel cladding materials in AGRs, have been oxidised in simulated reactor gas [CO2– (1–2% CO)] for periods up to 7·0×106 s (1900 h) at 850°C, including intermediate thermal cycles to room temperature. These exposures have been carried out in thermobalances which provided a full gravimetric analysis for each specimen. The oxidised specimens have been creep tested subsequently at 750°C, under conditions of constant stress, together with non-oxidised and thermally aged specimens, which formed the ‘baseline’ mechanical property data. After testing, the specimens have been examined using optical metallography and transmission electron microscopy to determine the microstructural changes caused by thermal aging and by oxidation. The paper compares and contrasts the response in mechanical behaviour to oxidation effects for both of the fuel cladding alloys. The creep rupture properties are affected differently for the two materials, but it is shown that this can be interpreted in terms of the observed changes in microstructure and the nature of the oxidation attack.MST/1127

Frictional and structural characterization of ion-nitrided low and high chromium steels

Low Cr steels AISI 4140, AISI 4340, and high Cr austenitic stainless steels AISI 304. AISI 316 were ion nitrided in a dc glow discharge plasma consisting of a 75% H2–25% N2 mixture. Surface compound layer phases were identified, and compound layer microhardness and diffusion zone microhardness profiles were established. Distinct differences in surface compound layer hardness and diffusion zone profiles were determined between the low and high Cr alloy steels. The high Cr stainless steels after ion nitriding displayed a hard compound layer and an abrupt diffusion zone. The compound layers of the high Cr stainless steels had a columnar structure which accounts for brittleness when layers are exposed to contact stresses. The ion nitrided surfaces of high and low Cr steels displayed a low coefficient of friction with respect to the untreated surfaces when examined in a pin and disk tribotester.

Damage structural behaviour by electron irradiation of MnCr austenitic stainless steels

A simulation experiment by electron irradiation using a HVEM was carried out at a dose rate of about 2 × 10 −3 dpa s −1 at a temperature of 723 K on high manganese-chromium austenitic steels with various solute concentrations, i.e., 19 wt% Mn, 16 wt% Cr, 0.1 wt% C and 0.03−5.9 wt% Ni. Voids formed in all specimens and their distribution was different depending on Ni solute concentration. For the specimen with higher Ni concentration, void formation tended to be retarded, and the incubation irradiation dose for void nucleation became longer in comparison with the specimen with lower Ni concentration. The resultant void swellings at 13 dpa were 1.3% and 0.3% for the specimens with lowest and highest Ni contents, respectively. In addition to their void swelling observations we also observed during irradiation the formation of new phases with strain contrast in the vicinity of the grain boundaries. The results of the compositional analysis of these regions including the new phases showed the segregation of Si and Ni, and the depletion of Mn and Cr. From these results it is suggested that compositional change of solute due to segregation and/or depletion induces an austenite phase instability.

Temperature as a Crevice Corrosion Criterion

Abstract Using temperature as a crevice corrosion criterion, a number of commercial and experimental Mo-bearing 18% Cr austenitic stainless steels have been evaluated for crevice corrosion resistance. These data have been correlated with the effect of Mo content on the rate of active corrosion in the very aggressive acid solution expected in the crevice itself. Alloyed Mo has been found to play the dominant role in reducing the relative rate of crevice corrosion and in defining the go/no-go crevice corrosion behavior in terms of the crevice corrosion temperature (CCT) in oxidizing acid chloride solutions. For 18% Cr austenitic stainless steels the empirical relationship has been found.

Effect of Alloying Additions on the Pitting Resistance of 18% Cr Austenitic Stainless Steel

Abstract Using temperature as a pitting criterion, the effects of Mo and other supplementary alloying additions on the pit initiation resistance of wholly austenitic stainless steel containing nominally 18% Cr have been evaluated. Quantitative results for Mo, Cu, Ni, Mn, N, and Si alloy additions are presented and empirical relationships are given which describe the pitting resistance of several families of alloys. Both experimental and commercial alloys have been evaluated.