low-Ni Austenitic Stainless Steel Research Articles

Strengthening mechanism and martensite transformation behavior in grain-refined low-Ni austenitic stainless steel

Low-Ni (less than 3 wt%) austenitic stainless steels with different C contents were designed and heat treated to utilize both the grain refinement and carbide strengthening, and their effects were systematically investigated to elucidate the mechanism of tensile property improvement. Grain refinement to 2–3 μm with homogeneously distributed Cr23C6 carbides resulted in excellent combination of yield strength (YS) and tensile elongation (YS × Tensile elongation of nearly 30,000 MPa %) without yield point elongation. The strengthening due to grain refinement was complemented by precipitation hardening. Grain refinement inhibited deformation-induced martensite transformation (DIMT) to decrease the strain hardening rate, while fine Cr23C6 carbides uniformly distributed within austenite grains promoted DIMT and increased strain hardening rate by depleting Cr and C concentrations. Two competing effects of grain refinement and Cr23C6 carbide formation on DIMT resulted in the increase of strain hardening rate as the latter was more dominant on austenite stability. Strengthening mechanisms were clearly explained by quantifying the contributions from solid solution, grain refinement and precipitation hardening. Stacking fault energy and driving force for γ → α’ varied with C contents and annealing treatment, indicating that the optimum combination of C addition and thermomechanical treatment can yield the high performance low-Ni austenitic stainless steels.

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.

Dynamic recrystallization and constitutive equation of 15Cr-10Mn-Ni-N steel under hot deformation

Hot working as one of the indispensable processes for all metal materials can effectively refine the grain to improve the material properties. However, the study on the hot-deformation process of low-Ni austenitic stainless steel is scarce. In this study, we conducted single-pass compression tests to explore the dynamic recrystallization process of a low-Ni austenitic stainless steel with the chemical compositions of 15Cr-10Mn-Ni-N during deformation at various temperatures and strain rates. According to the experimental data, the true stress-strain curves are drawn and constitutive equation is established to predict the variations of stress and strain, accompanied with high accuracy. The microstructural characterizations show that the increase of deformation temperature leads to the occurrence of recrystallization in spite of the enhanced softening effect. In particular, the increase of strain rate promotes the refinement of grain size, and the average grain size significantly decreases from 200 µm to 20 µm under the compression condition of 1100 ℃ and 10 s−1. The decreasing grain size can compensate the sacrificing strength due to the reduction of Ni addition. The peak stress of 15Cr-10Mn-Ni-N austenitic stainless steel is 186 MPa at 1000 °C and 0.1 s−1, which is basically similar with the conventional austenitic stainless steel. Evidently, the austenitic stainless steel in this study has a cost-effective advantage because of its low Ni addition, i.e., we have the advantage of reducing material costs while maintaining the identical mechanical properties. Here the recrystallization process of 15Cr-10Mn-Ni-N austenitic stainless steel provides a basis for the selection of thermal processing parameters and a general understanding of the microstructure evolution.

Effect of Heat Input on the Microstructural, Mechanical, and Corrosion Properties of Dissimilar Weldment of Conventional Austenitic Stainless Steel and Low-Nickel Stainless Steel

In the present investigation, an attempt has been made to investigate the replacement compatibility of conventional austenitic stainless steel (316L) with low-Ni austenitic stainless steel (201) by employing their dissimilar welding using gas tungsten arc welding technique with varying heat input. The effects of heat input on the microstructural, mechanical, and corrosion properties were studied. The result depicts the balanced austenite/ferrite content in the fusion zone for both the heat inputs. The low heat input process, which results in a faster cooling rate, demonstrates higher tensile strength and microhardness. Similarly, the pitting corrosion resistance of the fusion zone demonstrates better properties on the low heat input process attributed to the lesser dendritic length and lesser interdendritic arm spacing.

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.

Prediction of strain path and forming limit curve of AHSS by incorporating microstructure evolution

Austenitic stainless steel is known as second-generation advanced high strength steel (AHSS) due to its very high strength and elongation. The stability of austenite phase under deformation at room temperature depends on alloying elements such as Ni and Mn. Low-Ni austenitic stainless steel undergoes phase transformation, austenite to martensite, during plastic deformation. The phase transformation and crystallographic texture development are strain and strain path dependent. Consequently, material properties especially work hardening exponent (n) and normal anisotropy ($$ \overline{r} $$) are altered during plastic deformation. To understand the effect of dynamic mechanical properties on the formability of low-Ni austenitic stainless steels, limiting dome height (LDH) tests were performed. Strain path diagrams (SPD) and forming limit diagrams (FLD) were plotted for this material. Finite element (FE) analyses were carried out with constant and varying (with strains and strain paths) n and $$ \overline{r} $$ values. The work hardening exponent was calculated for uniaxial strain path and correlated with in-grain misorientation and martensite phase fraction at different strains. This relationship is further extended to estimate the work hardening parameter for other strain paths. Strain path–dependent instantaneous $$ \overline{r} $$ values were determined by using bulk texture analysis. The strain path–dependent n and $$ \overline{r} $$ values were incorporated during FE simulation which remarkably improved the SPD and FLD predictability of low-Ni austenitic stainless steel.

Enhancement of corrosion protection of low nickel austenitic stainless steel by electroactive polyimide-CuO composites coating in chloride environment

Purpose The purpose of this study is to improve the anticorrosion performance of low nickel stainless steel (AISI 201) in 3.5% NaCl by electroactive polyimide/copper oxide (EPI/CuO) composites coating. Design/methodology/approach Electroactive polyimide/copper oxide (EPI/CuO) composites were prepared by oxidative coupling polymerization followed by thermal imidization method. Findings The functional and structural properties of composites were characterized by X-ray diffraction, Fourier transmission infra-red and ultra violet-visible spectroscopy and the surface topography was characterized by field emission scanning electron microscope analysis and anticorrosion performance in 3.5 Wt.% NaCl was evaluated by electrochemical techniques. The obtained results of electrochemical techniques measurement indicated that the composites coated samples give better corrosion protection against attacking electrolyte. Originality/value The ever-increasing price of nickel (Ni) is driving the industries to use low-Ni austenitic stainless steels (ASSs). However, it exhibits relatively poor corrosion resistance as compared with conventional Cr-Ni ASSs. Nonetheless, its corrosion resistance can be enhanced by polymeric (electroactive polyimide [EPI]) coating. CuO particles exhibit the hydrophobic properties and can be used as inorganic filler to incorporate in EPI to further enhance the corrosion protection. The present research paper is beneficial for industries to use low-cost AISI 201, enhance its corrosion resistance and replace the use of costly conventional Cr-Ni ASSs.

Enhanced corrosion resistance of Cr-Mn ASS by low temperature salt bath nitriding technique for the replacement of convectional Cr-Ni ASS

Purpose The purpose of this paper is to enhance the corrosion resistance of Cr-Mn austenitic stainless steel (ASS) via low temperature salt bath nitriding and to replace the convectional Cr-Ni ASS with newly developed enhanced corrosion resistive Cr-Mn ASS. Design/methodology/approach The low temperature salt bath nitriding was performed on Cr-Mn ASS at 450°C for 3 h in potassium nitrate salt bath. Findings The present paper compares the corrosion resistance of salt bath nitrided Cr-Mn ASS with convectional Cr-Ni ASSs (316 L and 304 L ASSs) in 3.5 per cent NaCl by electrochemical techniques. The electrochemical impedance spectroscopy result shows the increase in film resistance and potentiodynamic polarization results show the enhanced corrosion resistance of nitrided Cr-Mn ASS, which is almost equivalent to that of 316 L and 304 L ASSs. This is attributed to the formation of nitrogen supersaturated dense nitride layer. The present results therefore suggest that the nitrided Cr-Mn ASS may replace costly convectional Cr-Ni ASSs for commercial and industrial applications. Originality/value Ever-increasing price of nickel (Ni) is driving the industries to use Ni-free or low-Ni austenitic stainless steels (ASSs). But its corrosion resistance is relatively poor as compared to conventional Cr-Ni ASSs. However, its corrosion resistance can be improved by nitriding. The low temperature salt bath nitriding of Cr-Mn ASS and its electrochemical behavior in 3.5 per cent NaCl has not been studied. The present research paper is beneficial for industries to use low cost Cr-Mn, enhance its corrosion resistance and replace the use of costly conventional Cr-Ni ASSs.

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.

Effect of Residual Stress and Strain-Induced α′-Martensite on Delayed Cracking of Metastable Austenitic Stainless Steels

The role of residual stresses and strain-induced α′-martensite in delayed cracking of metastable austenitic stainless steels was studied by means of Swift cup tests, measurement of residual stresses by X-ray diffraction and ring slitting, and α′-martensite content determination. Low-Ni, high-Mn austenitic stainless steels, e.g., AISI 201, were compared with Fe-Cr-Ni austenitic stainless steels. The presence of α′-martensite seemed to be a necessary prerequisite for delayed cracking to occur in austenitic stainless steels with typical internal hydrogen concentrations (<5 ppm). Stable low-Ni austenitic stainless steel was not prone to delayed cracking. The low-Ni metastable grades showed more severe cracking at lower degree of deformation and lower volume fraction of α′-martensite than that of the metastable 300-series grades. The limiting α′-martensite content, below which delayed cracking did not occur, decreased along with the nickel content of the material. The strain-induced martensitic transformation substantially increased the magnitude of residual stresses in deep-drawn cups. One explanation for high sensitivity of the low-Ni grades to delayed cracking after deep drawing is their higher residual stresses compared to that of the Fe-Cr-Ni grades. Alloying elements of the stainless steels, nickel, and carbon in particular, influence the sensitivity to delayed cracking through their effect on the properties of the α′-martensite.

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.

SIMS study on the surface chemistry of stainless steel AISI 304 cylindrical tensile test samples showing hydrogen embrittlement

The local surface chemistry of a low-Ni austenitic stainless steel AISI type 304 used for tensile testing in hydrogen atmosphere is characterized by secondary ion mass spectrometry (SIMS). A chemical map on cylindrical austenitic stainless steel samples can be obtained even after a tensile test. In an effort to obtain the proper chemical element distribution on the samples, the influence of contamination and sample orientation is discussed. An iron oxide on top of a chromium oxide layer is present and Si segregation at grain boundaries is detected. Oxides are judged to reduce the initial hydrogen attack but to be of minor importance for crack propagation during the embrittlement process.

General Corrosion Behavior of Low Ni Austenitic Stainless Steels Cathodically Modified with Interstitial Elements

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Solidification mode and residual ferrite in low-Ni austenitic stainless steels

The solidification modes of two new classes of austenitic stainless steels with a low content of Ni are shown. Their chemical composition is similar to that of the standard AISI 304 and AISI 316, except for the content of nickel, manganese and nitrogen. It is found that standard formulas for predicting the residual ferrite can be fairly well used in the prediction of the solidification mode while they do not work in predicting the residual ferrite content. In particular, it is found that ferrite is the first phase to solidify for values of the equivalent ratio (calculated according to the formulas developed by Hammar and Svensson) greater than 1.50, otherwise austenite is the first phase to solidify. A new set of equations for predicting the residual δ-ferrite in these new classes of materials is determined via multivariable linear regression. The influence of the steel solidification mode on the material structural transformations during heat treatment is also shown.