Ni Austenitic Stainless Steel Research Articles

Achieving ultra-high flow stress and hardness in a medium Mn stainless steel by the formation of strain-induced martensite and G-phase precipitates

This work demonstrates how the deformation of a medium Mn austenitic stainless steel and its subsequent thermal treatment results in the steel having an ultra-high flow stress, and with a microstructure containing strain-induced martensite (α′) and G-phase precipitates. A 14 wt.%Cr-8 wt.%Mn-1 wt.%Ni austenitic stainless steel was uniaxially compressed at room temperature and quasi-static strain rates to determine the evolution of strain-induced martensite with strain. Uniaxial compression resulted in a sigmoidal stress-strain curve (enhanced work-hardening) with the flow stress reaching ∼2.2 GPa, and the α′ fraction saturating at 0.8 true strain. Atom-probe tomography showed that aging at 400 °C for 72 h formed 2–3 nm sized G-phase precipitates containing Cu-rich clusters and having a nominal composition of Ni8Si2Mn20Cu17Cr8Fe44. Such a microstructure resulted in a steel with an ultra-high tensile strength and a compressive flow stress of around 2 GPa. Though the tensile ductility was low, there was sufficient flowability in compression.

Long-term creep behaviours and structural stabilities of austenitic heat-resistant stainless steels

ABSTRACT For heat resistant alloys, long-term structural stability at high temperatures is a critical issue for alloy design and applications. In this paper, the long-term creep behaviours and structural stabilities of six heat resistant high Ni alloys and austenitic stainless steels have been studied. The longest creep rupture life is up to 359 283 hours. High Ni and Cr alloys show a good combination of high creep and oxidation resistances. Precipitation of nano MX particles with a very low growth rate improves long-term creep resistance at high temperatures. Long-term stable multiple nanoprecipitates of MX, Cu-rich, Laves and M23C6 phases can greatly contribute to the creep strength. Low Ni austenitic stainless steels show comparatively low oxidation and creep resistances. It was first found that at 800°C, Cr2N could form in the low Ni steel with a long-term crept by the absorption of nitrogen from the air into the matrix.

Investigation of the high-temperature oxidation behavior of Fe-14Cr-9Mn-2.5Ni austenitic stainless steel in N2-21 vol%O2 environment

The high-temperature oxidation behavior of Fe-14Cr-9Mn-2.5Ni austenitic stainless steel in N2-21 vol％O2 environment at 900–1100 °C has been investigated. As the oxidation temperature rises, the scale expands from a thin bilayer to a thicker multilayer composed of an outer Fe-rich layer and an internal oxidation zone. The inner oxide consists of island Cr-rich spinel precipitates surrounded by remaining Ni-rich metallic. At 1000 °C, we observed an embedded Cr-rich dense layer that can slow down the oxidation rate. At a higher temperature (1100 °C), the outer oxide scale (Fe, Mn)2O3/(Fe, Mn)3O4/(Fe, Mn)O forms, and the oxidation rate significantly increases.

Corrosion characteristics and fuel cell performance of a cost‐effective high Mn–Low Ni austenitic stainless steel as an alternative to SS 316L bipolar plate

In this study, newly developed high manganese (Mn) and low nickel (Ni) austenitic stainless steels were investigated as an alternative to conventionally used SS 316L for bipolar plate applications in proton exchange membrane fuel cells. Systematic studies on the corrosion behavior were carried out in simulated hydrogen and oxygen environments, for both half- and fuel-cell conditions. The Mn-based SS revealed nobler corrosion potential and comparable passive current densities to that of SS 316L. The passive current density of Mn-based SS is well within the DoE 2020 target of <1 μA cm−2. Though MnSS1 steel has lean Ni content, the addition of Mn and N is beneficial for improving the corrosion performance, which is comparable to SS 316L. The recorded ICR values for Mn SS1 and SS 316L are 234.6 ± 20 and 155 ± 20 mΩ cm2 at a compaction force of 140 N cm−2, respectively. Both the steels do not to meet the DoE ICR target of 10 mΩ cm2, which requires conductive coating or improvement in oxide conductivity. The performances of the steels (both Mn-SS and 316L SS) with varying thickness were also investigated in a single fuel cell condition with serpentine flow field design as bipolar plates with varying thickness (10, 5 and 2 mm). A maximum power density of 370 mW cm−2 was achieved with the Mn-based metallic bipolar plates, whereas SS 316L showed 354 mW cm−2. By changing the composition of austenitic stainless steel, that is, using Mn SS1 instead of SS 316L the overall fuel cell cost decreases by three times.

Co-existence of γ'N phase and γN phase on nitrided austenitic Fe–Cr–Ni alloys- I. experiment

The formation of a metastable and nitrogen-supersaturated f.c.c. interstitial solid solution layer on Fe–Cr–Ni austenitic stainless steel at a moderate temperature around 650–720 K is not entirely understood. In the present work, three groups of austenitic Fe–Cr–Ni alloys, containing systematic variations of chromium, nickel and iron contents were nitrided by plasma-based low-energy ion implantation at 653 K for 4 h and investigated with light-optical microscopy (LOM), electron probe microanalysis (EPMA), (grazing incidence) X-ray diffraction (XRD) and transmission electron microscopy (TEM). Commercial AISI 304L was included for comparison. For the austenitic alloys with a Cr content below 12 wt.%, a duplex layer is observed in the nitrided case where the interface between the top layer, consisting of γ'-Fe4N like ordered γ'N, and the disordered γN (nitrogen enriched austenite) zone underneath is associated with a decrease in N content. For the alloys with a Cr-content over 12 wt.%, a featureless continuous zone is observed with LOM and a gradual decrease in nitrogen content is measured with EPMA. Nevertheless, a similar duplex structure of outer γ'-Fe4N like ordered γ'N and inner γN is confirmed by XRD and TEM for all nitrided alloys, irrespective of the Cr content. The results are discussed in terms of the short-range order (SRO) promoted by the Cr–N interaction and long-range order (LRO) caused by the Fe–N interaction.

Thermophysical properties of Alloy 709

Alloy 709 (UNS S31025) has shown enhanced performance, particularly in creep strength, over current generation high temperature structural materials, specifically Type 316 stainless steel and Grade 91. Accurate knowledge of the thermal properties of the material is important to design efficient systems and to allow calculation of thermally induced stresses and stress gradients. Thermal diffusivity, thermal conductivity, specific heat capacity, and coefficients of thermal expansion have been determined for Alloy 709 as a function of temperature in the range of 20–850°C. The results do not vary significantly for the three heats of Alloy 709 tested, and compare well with those reported for other high Cr, high Ni austenitic stainless steels. Deviations from monotonic behavior have been observed for the specific heat capacity and to a lesser degree for the thermal conductivity, while the thermal expansion and thermal diffusivity increase nearly linearly with increasing temperature.

Correlation between stacking fault energy and lattice parameter in nanocrystalline Fe–Cr–Ni austenitic stainless steels by atomistic simulation study

The correlation between stacking fault energy (SFE) and lattice parameter of Fe–Cr–Ni austenitic stainless steels was investigated. To obtain this relationship, four Fe–Cr–Ni alloy systems were considered. Molecular dynamics (MD) studies were carried out to calculate the intrinsic stacking fault energy and lattice parameter of Fe–Cr–Ni austenitic stainless steels as a function of composition. The embedded atom method (EAM) inter-atomic potential was used in the MD studies. The results show that as the Cr content increases, the SFE value significantly decreases while lattice parameter increases. Moreover, the results illustrates that the addition of nickel content hardly changes the SFE values, but on the contrary, decreases the lattice parameter. Almost linear dependence of SFE on lattice parameter could be established for Fe–Cr–Ni austenitic stainless steels.

Shape Memory Effect Induced by Stress-induced α′ Martensite in a Metastable Fe–Cr–Ni Austenitic Stainless Steel

It is not clear whether a shape memory effect (SME) can be realized by stress-induced α′ martensite in metastable austenitic stainless steels although the stress-induced ε martensite in these materials can result in the SME. To clarify this problem, the relationship between the shape recovery and the reverse transformation of the stress-induced ε and α′ martensite in a 304 stainless steel was investigated. The results show that the stress-induced α′ martensite can result in the SME when heating above 773 K. After deformation at 77 K and step heating or directly holding at 1073 K, two-stage shape recoveries below 440 K and above 773 K can be obtained due to the reverse transformation of the stress-induced ε martensite and α′ martensite, respectively. After deformation at room temperature, the α′ martensite produced can result in the SME only when directly holding at 1073 K. The intrusion of more dislocations before the formation of the α′ martensite at room temperature than at 77 K is the reason that the α′martensite induced at room temperature cannot result in the SME in the case of slow heating. The recovered strains resulting from the stress-induced ε and α′ martensite are proportional to the amounts of their reverse transformation, respectively.

Effect of strain amplitude on the low-cycle fatigue behavior of a new Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy

The low-cycle fatigue (LCF) properties and post-fatigue microstructure of a Fe–15Mn–10Cr–8Ni–4Si austenitic alloy were investigated under an axial strain control mode with total strain amplitudes, Δεt/2, ranging from 2.5×10−3 to 2×10−2. The fatigue resistance of the alloy was described by Coffin–Manson’s and Basquin’s relationships, and the corresponding fatigue parameters were evaluated. In addition, the Masing behavior, which is associated with a constant deformation mode during fatigue, was revealed at the examined strain amplitudes. Microstructural observations of the fatigue fractured samples showed that the strain induced ε-martensitic transformation accompanied by a planar slip of the Shockley partial dislocations in the austenite is the main deformation mode controlling the fatigue behavior of the studied alloy at Δεt/2<2×10−2. However, at Δεt/2=2×10−2, the formation of a cell structure was found in the austenite in addition to ε-martensitic transformation. The LCF resistance of the alloy was compared with conventional Cr–Ni austenitic stainless steels, ferrous base TRIP and TWIP steels and low yield point damping steels. It was found that at the studied strain amplitudes the alloy possessed a higher LCF resistance compared to conventional Fe-base alloys and steels. Remarkably, the fatigue ductility coefficient, εf′, of the studied alloy is 1.3–6 times higher than that of the stainless steels because of a cyclic deformation-induced ε-martensitic transformation. The results showed that the ε-martensitic transformation that occurred in the studied alloy during LCF is the main reason for the improved LCF resistance.

Reducing Geometrical, Physical, and Chemical Constraints in Surface Integrity of High-Performance Stainless Steel Components by Surface Modification

High-performance manufacturing is difficult to perform using conventional materials removal processes since a surface integrity demand for high-performance components is strongly restricted by intrinsic interactions between the geometrical feature of components and the physical and chemical characteristics of the base material. Surface modification techniques based on known processing loads, including mechanical, thermomechanical, and thermochemical loads, are utilized for manufacturing the Fe–Cr–Ni austenitic stainless steel components. The geometrical feature and the physical and chemical characteristics as well as the controllable interactions between them are identified in the surface integrity of the surface-modified components by creating new surface layers coupled with base material. The effective surface states control, including surface morphology, microhardness, and residual stress, leads to surface integrity improvement by reducing geometrical, physical, and chemical constraints from base materials, otherwise unobtainable merely using conventional materials removal manufacturing. The fatigue life of the surface-modified components is significantly increased due to the improved surface integrity. It is proposed that high surface integrity possesses a pivotal role between the functional properties of components and their geometrical feature and materials characteristics for the high-performance manufacturing.

Low-nickel austenitic stainless steel as an alternative to 316L bipolar plate for proton exchange membrane fuel cells

Suitability of low Ni austenitic stainless steel (201 SS) as an effective alternative to 316L stainless steel bipolar plates for proton exchange membrane fuel cells (PEMFC) was investigated. Polarization studies showed 201 SS behaved similar to 316L in the cathodic environment of fuel cell, but performed poorly in the anodic environment. Electrochemical impedance spectroscopy (EIS) studies revealed that in the anodic environment the oxide thickness and resistance of 201 SS is lower than 316L, whereas in the cathodic environment both steels showed similar behavior. Mott–Schottky analysis revealed that the defect concentration in oxide layer is higher for 201 SS at the anodic environment, whereas the oxide stability is better than 316L at the cathodic environment. The interfacial contact resistance (ICR) for both the steels was similar.

Development of materials model based on microstructure evolution for formability analysis of low-Ni austenitic stainless steel

Low Ni austenitic stainless steels exhibit outstanding mechanical properties due to the occurrence of two deformation mechanisms: dislocations glide (slip) and phase transformation (austenite fcc phase to strain induced a martensite (bcc)). Both mechanisms have strong dependence on strain and strain paths. The local misorientation development due to slip (grain average misorientation (GAM)) is more in biaxial strain-path (BS) followed by plane strain (PS) and uniaxial strain-path (US); whereas martensite volume fraction followed an opposite trend (US > PS > BS) as compared to GAM. In the present study a constitutive material model is developed based on phase transformation and GAM which has been implemented to describe the strain hardening behaviour of material for various strains and strain paths. Variation of normal anisotropy (r̄) with stain and strain paths was also considered. Finally the constitutive model and instantaneous r̄ were incorporated in Finite Element (FE) based simulation to improve the strain path predictability for this material.

An EBSD investigation on deformation-induced shear bands in a low nickel austenitic stainless steel under controlled shock-loading conditions

Adiabatic shear band is an important materials phenomenon often observed in metals when processed at high strain rates. The mechanical responses and microstructure evolution in it attract strong interests from the scientists of materials science and engineering. We report the results of the microstructure characteristic of a 200 series Fe–Cr–Ni austenitic stainless steel with low nickel contents deformed at high strain rates (about 5.8×105s−1) by a split Hopkinson pressure bar. The hat shaped specimens are used to induce the formation of the adiabatic shear bands under shock-loading tests. The microstructure and microtexture of the shear band in a 200 series Fe–Cr–Ni austenitic stainless steel are investigated by means of optical micrograph electron backscatter diffraction. The shear bands can be generated at about 78μs after the true flow stress reaches the value about 923MPa. The grains in the boundary of the shear band are elongated along the shear direction, and the core of the shear band consists of ultrafine equiaxed grains with diameter 0.1–0.3μm and low dislocation density. According to the orientation distribution, the microtexture peaked at (45°, 65°, 0°) in the matrix slightly shifts towards the recrystallization microtexture (60°, 60°, 0°) in the shear band center, and the grain boundaries in the shear band are geometrical necessary boundaries with high-angles. Calculations of temperature rise about 943K suggest that the temperature in the shear band is above the recrystallization point. Finally, the grain refinement in an adiabatic shear band in the 200 series Fe–Cr–Ni austenitic stainless steel is described as a consequence of the rotational dynamic recrystallization.

Corrosion behaviour of a Low Ni austenitic stainless steel in carbonated chloride-polluted alkali-activated fly ash mortar

This study concerns the corrosion behaviour of a low nickel and AISI 304 austenitic stainless steels embedded in alkaline activated fly ash mortars. They were carbonated and exposed for 650days to increasing amounts of chloride ions to simulate a highly corrosive environment. The corrosion process was monitored by applying the electrochemical impedance spectroscopy (EIS) technique and, at the end of the 650day exposure, by recording polarization curves on selected steel bars. After slab breakage, observations for detection of pitting attack and analysis of the total and free chloride contents in the mortars were carried out. These tests and some results obtained in simulated mortar pore solutions indicated that the fly ash mortars offered the rebars a higher protection against pitting attack than ordinary Portland cement mortar. The tests also evidenced the very good performances of the less expensive low nickel stainless steel.

Hot Ductility and Microstructure in Slab Shell of Low Ni Austenitic Stainless Steel

Hot ductility in slab shell of two types of low Ni austenitic stainless steels was investigated via tensile test. Results showed that, reduction of area (RA) in Cr17Mn6Ni4Cu2N decreased gradually from surface to inside. However, RA in Cr15Mn9Cu2NiN shell surface was low, and increased from surface to inside. Analysis suggested that, the difference of RA between the two steels due to the different solidification process. In the shell of former steel, δ ferrite solidifies as the primary phase, then transforms into austenite. The space between primary arms increases from surface to inside, leading to the decrease of hot ductility. In the latter steel, δ ferrite does not solidify entirely as the primary phase because of the high cooling rate, but austenite solidifies directly from the retained liquid between δ ferrite dendrites. The change of solidification mode is most obviously in slab surface, which decreases its hot ductility.

Influence of Ni and N on generalized stacking-fault energies in Fe–Cr–Ni alloy: A first principle study

The alloying effects of Ni and N were examined in terms of the generalized stacking-fault (SF) energy (GSFE). GSFE is associated with the basal plane of Fe–Cr–Ni austenitic stainless steel. The GSFE profiles were obtained using first-principle calculations. The results show that Ni increases the intrinsic SF energy γ isf and the unstable SF energy γ us. N does the opposite. However, a γ isf/ γ us ratio close to zero accompanies the addition of Ni or N. This ratio implies that deformation by partial dislocation is preferred. Overall, the effect of N on Fe–Cr–Ni alloys is more evident than that of Ni.

Hydrogen uptake in austenitic stainless steels by exposure to gaseous hydrogen and its effect on tensile deformation

Hydrogen solubility and diffusivity of Fe–Cr–Ni austenitic stainless steels were measured through exposure to gaseous hydrogen at a pressure of 10 MPa over the temperature range 110–235 °C. The hydrogen solubility depended on the alloy compositions, whereas the diffusion coefficients were nearly identical at a given temperature. Hydrogen uptake in the stable austenitic steels by exposure to high-pressure gaseous hydrogen led to some loss of ductility, while their fracture surfaces showed evidence of plastic deformation. This was attributed to the enhanced inhomogeneity of plastic deformation in the presence of hydrogen and the increased stress for plastic instability with increasing hydrogen concentration.

Fatigue behavior of ultrafine-grained and coarse-grained Cr–Ni austenitic stainless steels

High-cycle bending fatigue behavior of an ultrafine-grained Cr–Ni Type 301LN austenitic stainless steel, obtained by reversion annealing, was investigated and compared to that of the conventional coarse-grained counterpart. The fatigue limit was significantly increased from 350 MPa to 630 MPa, reaching 59% of the tensile strength, a behavior attributed to grain refinement. Fatigue cycling resulted in hardness increments that were very different between these structures; in the coarse-grained steel 47%, but only 6–10% in the ultrafine-grained steel. The fatigue damage was observed to occur by grain boundary cracking in the ultrafine-grained steel, while mostly by slip band formation and crack propagation along these slip bands and grain boundaries in the coarse-grained counterpart. Dislocation structures observed reflected these pronounced differences in the cyclic behavior.

EBSD study of solidification characteristics of austenitic stainless steel weld pool

This present paper studied the solidification characteristics of a high Fe–Cr–Ni austenitic stainless steel weld pool using an EBSD system. The results revealed that the crystallography of the weld metal grains was affected by both the heat affected zone (HAZ) base metal grains at the fusion boundary and the thermal condition in weld pool. It exhibited an orientation relationship of <100> // <100> and {100} // {100} between the HAZ base metal and weld metal along the fusion boundary, which was resulted from the epitaxial growth in the weld pool. Additionally, a high fibre texture <100> toward the weld centreline was determined by the effect of the competitive mechanism at the beginning of the weld solidification.

X-ray diffraction of high nitrogen face centred cubic phase formed on nitrogen modified austenitic stainless steel

High nitrogen face centred cubic phase formed on the nitrogen modified Fe–Cr–Ni austenitic stainless steel, which is named by γN phase, has a combined wear and corrosion resistance. X-ray diffraction (XRD) patterns of the γN phase on the stainless steel depict a set of broad and asymmetry diffraction peaks with a peak shift to low Bragg angles and a decrease in intensity, even disappearing of some peaks of high index planes, from each austenite peak for the γ matrix. The peak shift of the XRD patterns was first explained by Warren's XRD theory by Blawert et al. (Surf. Coat. Technol., 2001, 136, 181). In this paper, a systematic study of XRD on the peak shift, peak asymmetry, peak broadening and peak intensity of the γN phase has been carried out, based on a fault induced scattering geometry in diffraction to imperfect crystals by Warren's theory and Wagner's method. Both the higher deformation faults density α in a range of 0·02–0·25 and the lower twin faults density β of 0·01–0·1 were successfully used to describe the line profiles in the XRD patterns of the γN phase formed on plasma source ion nitrided 1Cr18Ni9Ti (18-8 type) austenitic stainless steel. A novel stacking faults factor Sαβ dependent on α and β described, in good agreement, the peak intensity of the γN phase. The calculated XRD patterns of the γN phase using the imperfect crystals model of a face centred cubic phase with a higher deformation faults density and a lower twin faults density were associated with the experimentally structural characteristics.