Austenitic Stainless Steel Samples Research Articles

Effects of Nb addition on the microstructure and low-cycle fatigue properties of heat-resistant stainless steel

This study aims to improve the quality and durability of materials used in turbocharger housing, particularly to limit the degradation of automobile engine parts. As a result, niobium (Nb)-bearing heat-resistant cast austenitic stainless-steel samples have been explored. Two distinct steel samples, denoted as Alloy-A (with 0 wt.% Nb) and Alloy-B (with 1.5 wt.% Nb), were fabricated by centrifugal casting for investigating the effects of Nb content on the corresponding microstructure and low-cycle fatigue properties of these samples. The phases formed within these alloys were predicted by thermodynamic calculations using the Thermo-Calc software, with the results aligning well with the experimental results. The addition of Nb promoted the formation of niobium carbide, resulting in increased hardness and greater area fraction of the carbides present in the steel samples. Alloy-B displayed enhanced high-temperature strength over Alloy-A, while the low-cycle fatigue resistances of the two alloys were similar. Fatigue cracks predominantly initiated at the outer surface of the specimens, mainly in the carbide regions, due to the mismatch between the matrix and carbide hardness. This paper is the first to report the effect of Nb addition on the microstructure and low-cycle fatigue properties of heat-resistant cast austenitic stainless steel.

Microstructure and texture analysis of 304 austenitic stainless steel using Bragg edge transmission imaging

Bragg edge imaging using pulsed neutrons is a non-destructive technique for studying microstructure and texture of materials. It provides two-dimensional visualization of crystallographic information using a pixelated gas electron multiplier detector and a time-of-flight method. In this work, the properties of type 304 austenitic stainless steel samples were studied via Bragg edge imaging. The samples included hot-rolled, cold-rolled and heat-treated specimens, which were characterized to investigate texture, phase fraction and grain growth. The results showed that the crystallite size increased with increasing annealing temperature. The cold-rolled and annealed samples exhibited strong textures, while the hot-rolled sample showed no preferred orientation. The phase volume fraction of induced martensite in the cold-rolled sample was also obtained. Two-dimensional maps of microstructures and textures were obtained without destructive processes. The results were validated by electron backscatter diffraction and found to be consistent. This work provides valuable information for non-destructive characterization of bulk materials by performing Bragg edge imaging using the Hokkaido University compact accelerator neutron source.

Void swelling in additively manufactured 316L stainless steel with hafnium composition gradient under self-ion irradiation

Compositionally-graded austenitic 316L stainless steel (SS) samples with five different Hafnium (Hf) concentrations (up to 1 wt.%) were additively manufactured by directed energy deposition and then were irradiated using 5 MeV Fe2+ ions to 50 displacements per atom (dpa) at 500, 550, and 600 °C, respectively. A rastering beam was used to ensure homogenous irradiations for the large areas of ∼1.40 cm2. Composition-dependent void evolution was evaluated. Void size, void number density, and void swelling all tend to decrease with increasing Hf concentration at all three temperatures. When the nominal Hf concentration increases to 1 wt.% in the doped additively manufactured (AM) 316L SS, the void swelling is over an order of magnitude lower than that in pure AM 316L. Atom probe tomography results showed that about 0.14 wt.% Hf dissolved in the matrix at the 1 wt.%. Hf nominal concentration. The suppression of void swelling by Hf addition shows qualitative agreement with the numerical calculation based on the vacancy trapping mechanism, indicating that the oversized Hf reduces the steady state vacancy concentration, and hence the incubation period of swelling is extended, resulting in a dramatic difference in void swelling. Other factors including grain boundaries, dislocation and dislocation cells, Hf-rich particles, and delta ferrite are secondary to this mechanism. This work shows that additive manufacturing enabled microalloying is promising for developing void swelling resistant materials.

Anisotropy in mechanical property and hydrogen embrittlement resistance of selective laser melted 304 austenitic stainless steel

Selective laser melting (SLM), one of the 3D printing technologies can be used to produce a variety of complex parts for hydrogen service. In this study, 304 austenitic stainless steel (ASS) samples are manufactured by SLM technology with building directions of 0°, 45° and 90°. The results show that the 3D-0° sample with equiaxed grain has lower strength and ductility, but better HE resistance; and the strength and plasticity of 3D-45° sample with mainly inclined grains are higher, but the HE resistance is a little worse; while 3D-90° sample dominated by columnar grains cannot be used in the structural parts due to the poor mechanical properties.

Investigation of the interaction of liquid tin-lithium alloy with austenitic stainless steel at high temperatures

This paper describes research work to determine the corrosion compatibility of material of the capillary-porous structure (CPS) matrix with a liquid tin-lithium (Sn-Li) alloy at high temperatures. The studies were carried out with the Sn-Li alloy containing 73 at.% of tin and 27 at.% of lithium and samples of 12Cr18Ni10Ti grade austenitic stainless-steel. This steel was proposed as one of the options as a candidate material for the Sn-Li CPS matrix manufacturing. Experiments on the interaction of a liquid Sn-Li alloy with stainless-steel at high temperatures were carried out on the TiGrA experimental facility based on the Mettler Toledo TGA/DSC 3+ thermogravimetric analyzer. The temperature gap in corrosion experiments ranged from 600 °C to 1000 °C, the interaction time for each temperature level was about 10 h. In the course of the work, experiments were carried out to study the compatibility of a Sn-Li alloy in the liquid phase with stainless-steel at temperatures of 600 °C, 800 °C and 1000 °C. Post-experimental studies of stainless-steel samples were carried out using microstructural and energy-dispersive analysis. Based on the results obtained, it was determined that when stainless-steel interacts with an Sn-Li alloy at high temperatures, complex physical-chemical processes occur, such as: selective dissolution of steel components by a liquid alloy; permeation of the liquid alloy into stainless-steel; mass transfer of dissolved metals from a solid metal to a liquid one.

The high strength and hydrogen embrittlement resistance of selective laser melted 304 austenitic stainless steel

3D printing technology can be used to produce a variety of complex parts for hydrogen service. In this study, 304 austenitic stainless steel (ASS) sample was manufactured by selective laser melting (SLM), one of the 3D printing technologies, then its high strength and hydrogen embrittlement resistance were discovered in comparison to traditional solution-treated 304 ASS. The unique high-density dislocation cellular structure of SLM sample remarkably restrained the nucleation of α'-martensite and slowed down the diffusion of hydrogen. Moreover, the possibility of hydrogen induced intergranular fracture was significantly reduced because of the alleviated hydrogen segregation at the grain boundary.

On the origin of grain refinement and twin boundaries in as-fabricated austenitic stainless steels produced by laser powder bed fusion

316 L austenitic stainless steel samples have been fabricated by laser powder bed fusion using two powder batches of slightly different compositions but keeping the same processing parameters. Compared to the first powder batch, the second powder batch led to a finer grain structure, a more random texture, and an extremely high density of Σ3 twin boundaries. A review of the literature shows that these two kinds of microstructures had been observed in previous works, but separately and not connected to changes in composition. Several hypotheses are reviewed to explain these microstructure differences. We conclude that local liquid ordering, also described as icosahedral short range order (ISRO) can have an effect on the microstructure formation during solidification. ISRO in the liquid is thought to be stronger in the second powder batch. Finding ways to enhance the influence of the ISRO in the liquid on the microstructure formation can be seen as a promising strategy to design alloys for additive manufacturing.

Influence of heat input on the evolution of δ-ferrite grain morphology of SS308L fabricated using WAAM-CMT

Wire-arc additive manufacturing (WAAM) is acquiring impetus for fabricating medium-complexity and large metallic components owing to its high material deposition with faster building rates. Austenitic stainless steels are commonly used among the numerous materials processed by WAAM and microstructural study of single walled components has been widely researched in the past. In this paper, manual cold metal transfer (CMT) welding is used to produce WAAM samples of SS308L austenitic stainless steel with SS304 as a substrate. Detailed microstructural studies with grain morphology of δ-ferrite based on heat input at various layers fabricated using WAAM-CMT are mainly explored. Results showed that the concentration of γ-austenite drops as heat input increases, but δ-ferrite and micro-inclusions increase. Within δ-ferrite, lathy ferrite morphology evolves into faceted morphology while vermicular evolves into a globular (spheroidized) ferrite morphology as heat input increases. The grain morphology pattern repeats itself for each layer as moving along the building direction, resulting in a repetitive heterogenous grain morphology with nearly identical chemical compositions.

Profile evaluation of the ion irradiation-induced swelling in austenitic stainless steel with varying nickel content

Comparative studies of porosity and calculation of the swelling profile in samples of austenitic stainless steel with a nickel content of 10 and 20 wt.%. The samples were irradiated at the Tandem-3M accelerator to the same doses of 300 dpa with Ni ions with an ion energy of 11.5 MeV at a temperature of 550°C with preliminary implantation of He. To calculate the swelling profile, digitally processed images were obtained by scanning transmission electron microscopy (STEM). In addition, comparative studies of the phase composition and radiation-induced segregations at grain boundaries, pore/matrix interfacial boundaries, and on the surface of phase precipitates were carried out on irradiated samples with varying nickel contents.

Torsion of a rectangular bar: Complex phase distribution in 304L steel revealed by neutron tomography

Metastable austenitic stainless steel (304L) samples with a rectangular cross-section were plastically deformed in torsion during which they experienced multiaxial stresses that led to a complex martensitic phase distribution owing to the transformation induced plasticity effect. A three-dimensional characterization of the phase distributions in these cm-sized samples was carried out by wavelength-selective neutron tomography. It was found that quantitatively correct results are obtained as long as the samples do not exhibit any considerable preferential grain orientation. Optical microscopy, electron backscatter diffraction, and finite element modeling were used to verify and explain the results obtained by neutron tomography. Altogether, neutron tomography was shown to extend the range of microstructure characterization methods towards the meso- and macroscale.

Low-pressure hollow cathode plasma source carburizing of AISI 304L austenitic stainless steel at low temperature

A carburizing treatment of AISI 304L austenitic stainless steel was performed at a low temperature of 450 °C using the low-pressure hollow cathode plasma source technique. The hollow cathode plasma was produced in a cathodic cage made of stainless steel screen inlaid with a stainless steel tubes array. A pulsed high voltage power, with a peak voltage of 800 V and a pulse frequency of 30 kHz, was applied at a low gas pressure of 50 Pa. The austenitic stainless steel samples were carburized with a direct current bias of −200 V under the CH4/H2 ratio changing in a range from 0.11 to 9, for a carburizing time of 4 h. Under the lower CH4/H2 ratio of 0.11, a diamond-like carbon film of approximately 260 nm thick formed on the stainless steel surface. As the CH4/H2 ratio increased to 0.67 and 1, both carburizing cases possessed a similar thickness of about 11 μm, with the surface carbon concentration being 6 to 6.5 at.%. A layer of carbon supersaturated face-centered-cubic (γC) phase formed, together with carbon-rich (Fe,Cr)5C2 carbide precipitation. Under the highest CH4/H2 ratio of 9, a single γC phase layer was obtained in a carburizing case of similar thickness, with a high surface carbon concentration of 7.5 at.%. The maximum surface hardness value of HV0.25 N 7.1 GPa occurred on the single γC phase layer. No pitting corrosion occurred on the single γC phase in the polarization curve. The higher passive film resistance and the lower donor and acceptor concentrations and flat band potential indicated that a more compact passive film is grown on the γC phase layer. The (Fe,Cr)5C2 carbide precipitates in the γC phase matrix degraded the passivity of the γC phase layer due to the reduced compactness of the passive film. A carbon transport competition mechanism between carbon adsorption onto the surface and carbon inward diffusion that was mainly dependent on the heavier CHn+ ion bombardment, was proposed to explain the transition with increasing CH4/H2 ratio from carbon film deposition, to a carburizing case composed of ae single γC phase, with both increased hardness and higher corrosion resistance.

A novel approach for nondestructive depth-resolved analysis of residual stress and grain interaction in the near-surface zone applied to an austenitic stainless steel sample subjected to mechanical polishing

The choice of the grain interaction model is a critical element of residual stress analysis using diffraction methods. For the near-surface region of a mechanically polished austenitic steel, it is shown that the application of the widely used Eshelby-Kröner model does not lead to a satisfactory agreement with experimental observations. Therefore, a new grain interaction model called 'tunable free-surface' is proposed, allowing for the determination of the in-depth evolution of the elastic interaction between grains. It has a strong physical justification and is adjusted to experimental data using three complementary verification methods. It is shown that a significant relaxation of the intergranular stresses perpendicular to the sample surface occurs in the subsurface layer having a thickness comparable with the average size of the grain. Using the new type of X-ray Stress Factors, the in-depth evolution (up to the depth of 45 μm) of residual stresses and of the strain-free lattice parameter is determined.

Dynamic modulus, internal friction, and transient creep at high temperature in austenitic stainless steel

Dynamic shear modulus and internal friction of a coarse-grained austenitic stainless-steel sample (AISI 304) were measured by mechanical spectroscopy technique at elevated temperatures to 1200 ℃ and oscillation periods ranging from 1 s to 1000 s. The results were fitted to an extended Burgers model, showing a new thermally activated dissipation peak with relaxation strength 0.075 ± 0.005 superimposed on the high-temperature background. Across the temperature interval 1000–750 ℃, the dissipation peak traverses the 1–1000 s observational window with an activation energy of 210 ± 14 kJ/mol. Complementary time-domain information acquired by microcreep testing within the same temperature range reveals the extent to which the inelastic strain is recoverable (i.e. anelastic). A transient creep map was constructed to quantitatively analyze the contribution of different deformation mechanisms. The combined analysis involving Burgers model and a transient creep map highlights the likely role of stress-induced migration of dislocations in both the anelastic relaxation and the viscous deformation. This study thus provides a more robust understanding of the mechanical behavior of austenitic stainless steel at high temperatures and low strain amplitudes with implications, for example, for its use in jacketing other materials for mechanical testing under conditions of high temperature and pressure.

The Effect of Reversion of Strain-Induced Martensite (SIM) and Grain Refinement on the Intergranular Corrosion of High Mn Steel

The surface phase constituent of high-manganese austenitic stainless steel after cold work (15%, 30%, and 50%) and thermal aging at 900°C for 30 min and 6 h, was characterized using x-ray diffraction. The microstructural analysis was conducted using an optical microscope, a scanning electron microscope, and the electron backscatter diffraction technique. The double-loop electrochemical potentiokinetic reactivation test was used to measure the intergranular corrosion resistance (degrees of sensitization). The results showed that fine-recovered grains of austenite and strain-induced martensite together formed the surface of high-manganese steel after cold work. Because of the formation of strain-induced martensite in the cold-worked samples, their intergranular corrosion was much higher than that of the as-received sample. Also, the degree of sensitization of 15% cold work was higher (i.e., more intergranular corrosion) compared to the degrees of sensitization of 30% and 50% cold work. On the contrary, the intergranular corrosion of high-manganese austenitic stainless steel sample subjected to cold work was eliminated during thermal aging at 900°C for 6 h because of the reversion of strain-induced martensite and fine-reverted austenite grains. Owing to this grain refinement of austenite, faster diffusion rate of Cr at higher temperature and cold work helped healing of Cr-depletion zone in a shorter time. In other words, because the results showed that on 50% cold work and thermal aging at 900°C for 6 h, the high-manganese austenitic stainless steel does not become susceptible to intergranular corrosion. Hence, it could be beneficial to investigate the intergranular corrosion of high-manganese austenitic stainless steel.

Softening mechanisms in ultrasonic treatment of deformed austenitic stainless steel

The effect of ultrasonic treatment on the microstructural evolution and the related softening process in tensile pre-deformed 316 stainless steel was studied by means of electron backscatter diffraction method, optical microscopy, and microhardness measurement. It was observed that different levels of ultrasonic energy induced complex microstructural changes in the treated samples. A large decrease in twin boundaries was observed, which is an indication of the de-twinning process under ultrasonic treatment. A new mechanism for the de-twinning process under oscillatory stress of ultrasonic vibration was proposed. It was shown that de-twinning under ultrasonic treatment led to dislocation production from twin boundaries. Inverse pole figures investigation revealed strong grain rotation following ultrasonic treatment in tensile pre-deformed samples. Subgrain formation in the ultrasonic treated austenitic stainless steel samples indicated that considerable ultrasonic energy was induced by the ultrasonic vibration, which provided the activation energy needed for dislocation climb and cross-slip. The ultrasonic induced subgrain formation, dislocation annihilation, and de-twinning, which resulted in a decrease of the microhardness in the samples, can be considered as possible mechanisms for the acoustic softening in the austenitic stainless steels.

Study on the Grain Boundary Misorientation Change and Texture Evolution in SUS304 Austenitic Stainless Steel at Low Cold‐Rolling Reductions

To study the grain boundary misorientation change and texture evolution in austenitic stainless steel at low cold‐rolling reductions, SUS304 austenitic stainless steel samples with a cold‐rolling reduction of 5%, 8%, 10%, 15%, 20%, and 23% are prepared. The electron backscatter diffraction analysis reveals that with the increase in the cold‐rolling reduction, the proportions of low‐angle grain boundary (LAGB) and high‐angle grain boundary (HAGB) gradually increase and decrease, respectively. The decrease in HAGB reduces the toughness of the material. With the increase in cold‐rolling reduction, the strain area inside the material increases, which enhances the toughness and strength of the steel plate. Austenite textures at different cold‐rolling reductions are composed of Brass, Goss, S, and shear texture {011}<122>. The intensity of the Brass‐type texture increases with the increasing cold‐rolling reduction. The martensite texture mainly consists of R‐Cube, Brass, shear texture {111}<110>, {111}<112>, and {332}<113>. When the cold‐rolling reduction is less than 15%, the Brass texture appears in the α′‐martensite phase. In addition, the intensity of the R‐Cube texture increases with the increasing cold‐rolling reduction. The martensite phase texture is affected by the austenite phase texture, and its formation can be attributed to the Kurdjumov–Sachs orientation relationship.

Corrosion Resistance of Open Die Forged Austenitic Stainless Steel Samples Prepared with Different Surfaces

The use of corrosion-resistant metal materials in highly aggressive environments contributes to the preservation of the environment because it reduces the use of protective agents and coatings. Most metal objects are produced by some metal-forming process. It is well-known that plastic deformation affects the corrosion resistance of different metal materials in different ways. As a rule, austenitic stainless steels show a positive impact of plastic deformation on corrosion resistance, especially when hot deformed with protective surface oxide layers. However, most research carried out on these metals involves a carefully prepared surface which is either finely ground or polished. This paper investigates the corrosion resistance of cold-formed (open die forged) austenitic stainless steel in three different surface states for three different degrees of deformation. In doing so, we simulate possible damage to the treated surface and evaluate the stability of the material with respect to corrosion. Good corrosion resistance is shown for all three stages of deformation and for all three surface states, with some differences in the obtained results. Although the polished surface shows the highest corrosion resistance, as expected, the other two surfaces also demonstrate good results when exposed to aggressive environments. All of the results were statistically processed and presented. The results demonstrate the high usability of such materials in corrosion-aggressive environments with minimal danger of corrosion and minimal need to include additional surface protection agents, even against possible surface damage.

Microwave Boriding to Improve the Corrosion Resistance of AISI 304L Austenitic Stainless Steel

In this study, the corrosion behaviors of AISI 304L austenitic stainless steel samples were subjected to pack-boriding at 850, 900 and 950 °C process temperatures for 2, 4 and 6 hours with microwave hybrid heating, and examined. Boride layers were characterized by optical microscope and XRD. As a result of XRD analyses, the presence of FeB, Fe‎‎‎‎‎2B, Cr2B and Ni2B compounds in the boride layers were determined formed on the sample surfaces. As an alternative to conventional heating, AISI 304L austenitic stainless steel samples subjected to pack-boriding with microwave hybrid heating, as a result of the corrosion tests carried out during the 3rd, 7th and 10th days in 2% V/V (for volume per volume) HNO3 acid solution, the corrosion resistance of the AISI 304L austenitic stainless steel samples as loss in mass increased with the increase in the temperature and duration of the boriding process and the corrosion resistance increased 95 times compared to the untreated AISI 304L stainless steel samples.

Features of the kinetics of cyclic elastoplastic deformation diagrams at dwells in cycles and superimposition of variable stresses on them

The goal of the study is determination of the regularities of changes in cyclic strains and related deformation diagrams attributed to the existence of time dwells in the loading modes and imposition of additional variable stresses on them. Analysis of the obtained experimental data on the kinetics of cyclic elastoplastic deformation diagrams and their parameters revealed that in contrast to regular cyclic loading (equal in stresses), additional deformations of static and dynamic creep are developed. The results of the studys are especially relevant for assessing the cyclic strength of unique extremely loaded objects of technology, including nuclear power equipment, units of aviation and space systems, etc. The experiments were carried out on the samples of austenitic stainless steel under low-cycle loading and high temperatures of testing. Static and dynamic creep deformations arising under those loading conditions promote an increase in the range of cyclic plastic strain in each loading cycle and also stimulate an increase in the range of elastoplastic strain due to active cyclic deformation. At the same time the existence of dwells on extrema of stresses in cycles without imposition of additional variable stresses on them most strongly affects the growth of plastic strain ranges in cycles. Imposition of additional variable stresses on dwells also results in the development of creep strains, but their growth turns out to be somewhat less than in the presence of dwells without stresses imposed. The diagrams of cyclic deformation obtained in the experiments are approximated by power dependences, their kinetics being described in terms of the number of loading cycles using corresponding temperature-time functions. At the same time, it is shown that increase in the cyclic plastic deformation for cycles with dwells and imposition of additional variable stresses on them decreases low cycle fatigue life compared to regular loading without dwells at the same stress amplitudes, moreover, the higher the values of static and dynamic creep, the greater decrease in low-cycle fatigue life. This conclusion results from experimental data and analysis of conditions of damage accumulation for the considered forms of the loading cycle using the deformation criterion of reaching the limit state leading to fracture.

Crack Detection during Laser Metal Deposition by Infrared Monochrome Pyrometer.

Laser metal deposition (LMD) is an advanced technology of additive manufacturing which involves sophisticated processes. However, it is associated with high risks of failure due to the possible generation of cracks and bubbles. If not identified in time, such defects can cause substantial losses. In this paper, real-time monitoring of LMD samples and online detection of cracks by an infrared monochrome pyrometer (IMP) could mitigate this risk. An experimental platform for crack detection in LMD samples was developed, and the identification of four simulated cracks in a 316L austenitic stainless-steel LMD sample was conducted. Data at temperatures higher than 150 °C were collected by an IMP, and the results indicated that crack depth is an important factor affecting the peak temperature. Based on this factor, the locations of cracks in LMD-316L austenitic stainless-steel samples can be determined. The proposed technique can provide real-time detection of cracks through layers of cladding during large-scale manufacturing, which suggests its relevance for optimizing the technological process and parameters, as well as reducing the possibility of cracks in the LMD process.