Stable Austenitic Stainless Steel Research Articles

Bending fatigue behavior of metastable and stable austenitic stainless steels with different surface morphologies

AbstractThe surface morphology has a significant influence on the fatigue behavior of components. For austenitic stainless steels (ASSs), this issue is even more pronounced due to their metastability. Based on the complex deformation mechanisms of metastable ASSs, which include dislocation slip, deformation twinning, and deformation‐induced martensitic phase transformation, the metastable stainless steel AISI 347 was investigated in this study together with the stable AISI 904L as a reference material. Four‐point bending fatigue tests with load ratio R = 0.1 and testing frequency f = 10 Hz at ambient temperature were carried out on specimens with five technically relevant surface morphologies: mechanical polished, milled, microshot peened, laser shock‐peened, and ultrasonic modified. Systematic material characterizations were carried out to elucidate the crucial role of surface roughness and deformation‐induced α′‐martensite in the fatigue behavior of both metastable and stable materials. While surface roughness is a well‐known key factor in conventional fatigue cases, deformation‐induced martensite layers implemented by various surface modification methods were proven to improve the fatigue life in metastable austenitic steels, opening new perspectives to extend the lifetime of ASS components.

Experimental investigation of fracture mechanism in organically modified silica-based coating applied on austenitic stainless steel AISI 904L under monotonic and very high cycle fatigue loading

Surface morphology significantly influences the fatigue life of the metallic materials. Therefore, improving surface performance is important for economic and safety applications. Silica-based coatings are anticipated to protect the metallic materials under synergistic operating conditions, such as fatigue and corrosion. However, pure inorganic silica-based coatings are brittle and fail to protect metallic materials. The properties of the sol–gel coatings can be determined by optimizing synthesis factors, such as synthesis steps, precursors, solvents, and catalysts. Organically modified silica-based sol–gel coatings were synthesized using 3-glycidoxypropyl trimethoxysilane and 3-aminopropyltriethoxysilane as precursors, with ethanol as the solvent. The resulting coating was named EtOH. This coating was applied on the surface of stable austenitic stainless steel AISI 904L with the dip coating method. The fracture behavior of the coating during monotonic loading has been studied by interrupted tensile tests at different load steps and examined closely with scanning electron microscopy. On the other hand, very high cycle fatigue experiments were conducted using an ultrasonic fatigue testing system at load frequency of 20 kHz on coated specimens to investigate the fracture behavior of the coating during cyclic loading. Three different kinds of fracture due to stress relaxation in the coating and at its interface with substrate were observed in monotonic loading. The same kinds of stress relaxation were observed in cyclic loading with the difference being that the range of experienced elongation in cyclic tests was significantly smaller. Therefore, the fracture in cyclic loading were concentrated in the vicinity of macro crack. The application of organically modified coatings could suppress the persistent slip bands and result in increased fatigue life for coated specimens.

Determination of hydrogen compatibility for Cu-Al-Ni-Fe-Mn cast alloys prepared by varying Al and Ni contents

Hydrogen compatibility of Cu-Al-Ni-Fe-Mn cast alloys was investigated via slow strain rate tensile tests in high-pressure hydrogen gas. Four types of alloys were prepared by varying the Al and/or Ni contents. Larger Al and Ni contents resulted in more elongated microstructures with smaller grains and more precipitates, respectively. The tensile strength of the alloys increased with larger Al and/or Ni contents, whereas the ductility decreased only by adding a larger amount of Al. The maximum tensile strength was produced by adding larger amounts of both Al and Ni, which was fairly higher than those of stable austenitic stainless steels. Despite the difference in the microstructures and tensile strengths of the alloys, the hydrogen contents and hydrogen diffusivities showed no substantial difference. Additionally, their reduction in area measured after the tensile tests was not degraded by the high-pressure hydrogen gas, revealing an excellent resistance to hydrogen embrittlement.

Static strain aging in cold rolled stable austenitic stainless steel

The effects of cold rolling reduction ratio, initial grain size, and aging condition (temperature and time) on static strain aging (SSA) behavior were systematically investigated in stable austenitic stainless steel. The aging treatment increased the yield strength (YS) and ultimate tensile strength (UTS), and the SSA effect was more pronounced at higher reduction ratio or smaller initial grain size. The SSA effect occurred in the temperature range of 400∼600 °C, where the accumulated dislocations remained in the form of wavy boundaries. Quantifying the dislocation density clearly showed that the SSA effect was directly related to the dislocation density. The volume shrinkage and increase in thermal expansion coefficient by aging treatment suggested the correlation between SSA behavior and atomic distribution changes. 3D atom probe tomography analysis clearly exhibited the clustering and segregation of solute C atoms during aging treatment. These results suggest that the SSA effect may be attributed to the interaction between solute C atoms and dislocations. Through grain refinement and optimal aging treatment at 500 °C for 1 h, the YS and UTS were significantly increased by 319 MPa and 230 MPa, respectively, to obtain a non-magnetic stainless steel with ultrahigh YS of up to 1600 MPa.

Modelling of hydrogen distribution at the fatigue crack tip of austenitic stainless steels in internal and external hydrogen tests

A hydrogen diffusion model was developed, which accounts realistically for the kinetics of hydrogen adsorption, absorption and their reverse processes at the gas-solid interface. Using the model, the hydrogen distribution at the fatigue crack tip of austenitic stainless steels (304, 316 L) in internal and external hydrogen tests were analyzed. Results indicate that for stable austenitic stainless steel, the hydrogen concentration at the fatigue crack tip is much higher in internal hydrogen test than that in external hydrogen test, which is attributed to the extremely low adsorption, desorption and diffusion rates of γ phase. For metastable austenitic stainless steel, hydrogen rapidly enters into strain-induced α′ martensite around the fatigue crack tip, forming an enrichment peak at the α′/γ interface in external hydrogen test. While significant hydrogen outgassing occurs in α′ phase in internal hydrogen test, leading to a much lower crack tip concentration compared with external hydrogen test.

Anisotropic effect of pre-strain on hydrogen embrittlement susceptibility in a stable austenitic stainless steel

Different pre-strain of uniaxial tensile deformation or cold rolling were applied to 0.02C–18Cr–11Ni-0.15 N austenitic stainless steel to investigate the effect of pre-strain stress modes on hydrogen embrittlement (HE). The susceptibility to HE was negligibly affected by tensile pre-strain, but it increased with the degree of pre-strain by cold rolling. This resulted from differences in strain distribution, twinning behavior, and changes in H diffusivity arising from the two deformation methods. Homogeneous strain distribution was observed after uniaxial tensile deformation, whereas the deformation was locally concentrated after cold rolling. This led to localized H adsorption to assist crack propagation. While twinning was the main deformation mechanism in the present steel, only the twins formed during cold rolling were vulnerable to stress localization owing to their crystallographic orientation. These twins acted as initiation sites for H-induced cracking and degraded the HE resistance. Finally, H diffusion was less active in tensile-strained samples, and high HE resistance was effectively maintained despite the increase of strength by pre-straining as a consequence.

Direct comparison of C and N effect on hydrogen embrittlement of stable 17Cr-10Ni-6Mn austenitic stainless steel

The effects of C and N on hydrogen embrittlement (HE) were investigated in 17Cr-10Ni-6Mn based stable austenitic stainless steels containing 0.2 wt% of either C or N. C and N increased tensile strength by solid solution hardening and high-density dislocation accumulation. C was more effective on further strengthening by active TWIP effect than N. The lattice expansion by C and N decreased hydrogen diffusivity and depth of hydrogen-affected zone, which increased the HE resistance. The HE resistance was further enhanced by C than N due to lower hydrogen diffusivity and crack severity and active cross slip by C.

Effect of copper addition on tensile behavior in Fe-Cr-Ni stable austenitic stainless steel

Effects of Cu on tensile properties and deformation mechanisms were investigated in the stable Fe-Cr-Ni austenitic stainless steel with different wt% of Cu. Addition of Cu slightly decreased the yield strength and negligibly affected the ultimate tensile strength, while it induced a considerably large reduction of fracture elongation. Deformation twins were generated during the tensile deformation, and fraction of deformation twins decreased with increasing Cu content. However, degree of twinning was similar, regardless of Cu content, at the same tensile strain, which indicated that development of twins was interfered by earlier fracture triggered by Cu addition and hardly contributed to different hardening behavior at the early stage of deformation. Cu significantly altered the motion of dislocations. The low-Cu-containing alloy mainly developed tangled dislocations and cellular structures, whereas the high-Cu-containing alloy did planar slip and high density of dislocation walls. Although the addition of Cu decreased the shear modulus and increased the stacking fault energy, dislocation substructure changed from tangled to planar slip with increasing Cu content. This indicated that slip planarity of high-Cu-containing alloys might be originated from the formation of short-range ordering or clustering, thereby leading to the local stress concentration and reduction of the elongation.

Austenitic stainless steels of high strength and ductility

Abstract High strength and high ductility together are prime requirements for modern structural metallic materials. Important industrial examples are steels for car components such as body sheets, tubes and forgings. The present paper traces two lines of alloy development, stable austenitic stainless steels and intentionally metastable austenitic stainless steels. Although both groups have different advantages and disadvantages, each one exceeds all other presently available structural metallic materials in terms of strength to density ratio at equal ductility.

Age-Hardening Behavior in High-Nitrogen Stable Austenitic Stainless Steel

Age-Hardening Behavior in High-Nitrogen Stable Austenitic Stainless Steel

C-Dislocation Interaction During Static Strain Aging in Cold-Rolled Stable Austenitic Stainless Steel

C-Dislocation Interaction During Static Strain Aging in Cold-Rolled Stable Austenitic Stainless Steel

Age-hardening mechanisms of heterogeneous-nanostructured SUS316LN stainless steel fabricated by heavy cold rolling

A stable SUS316LN austenitic stainless steel was heavily cold-rolled to 92% in reduction to have complicated heterogeneous nanostructure composed of “eye”-shaped twin domains, shear bands and conventional low-angle lamellae. The average boundary spacings in the twin domains and the low-angle lamellae were approximately 25 nm and 40 nm, respectively. The average width of the shear bands was about 40 nm. While the tensile strength along transverse direction was notably high to be 1.9 GPa, it was further raised up to 2.2 GPa by peak aging at 748 K. Nevertheless, any hardening mechanisms as spinodal decomposition, formation of G.P. zone, clustering, etc. could not be detected. The 3D-atom probe tomography analyses revealed segregation of the solute elements of Mo, Si and so on at twin and low-angle lamellar boundaries. A finite element calculation using the multiscale crystal plasticity simulation system indicated rise of strength due to impediment of dislocation motion by the grain-boundary segregation. The combined mechanisms of nano-lamellar structure and grain-boundary segregation, therefore, caused extremely high strengthening.

Влияние электролитического насыщения водородом на механизмы деформации аустенитной нержавеющей стали 01Х17Н13М3 при холодной прокатке

In this work, we investigated the effect of the current density during electrolytic hydrogen-charging before deformation on the mechanisms of plastic deformation of stable austenitic stainless steel Fe-17Cr-13Ni-3Mo-0.01C during rolling. Using transmission electron microscopy and backscattered electron diffraction, it has been shown that plastic deformation by rolling at room temperature with 25 % reduction causes the formation of a high density of crystal lattice defects in the initially coarse-grained steel samples. The main deformation mechanism during rolling is dislocation slip, which is accompanied by mechanical twinning as an additional mechanism contributing the structure fragmentation. Regardless of the mode of preliminary hydrogen-charging, it promotes more active development of twinning. The higher the current density during hydrogen pre-charging, the higher the density of twin boundaries and dislocation density in the steel structure during rolling. After hydrogen-charging at a current density of 200 mA / cm2, the γ → ε transformation is also realized upon deformation. At the same time, an increase in the current density upon hydrogen pre-charging from 10 to 200 mA / cm2 does not contribute to a significant increase in the concentration of hydrogen adsorbed by the material (0.0017 – 0.0018 mass.%), which, together with microstructural studies, indicates a different distribution of hydrogen in the samples immediately before deformation and the important role of the hydrogen concentration gradient on the deformation mechanisms of the steel during subsequent rolling. With a decrease in the deformation temperature (due to the cooling of the samples before rolling to the temperature of liquid nitrogen), hydrogen-charging also causes an increase in the linear density of twin boundaries and the formation of ε-martensite, but these effects are more significant compared to room-temperature deformation. The data obtained indicate that at similar values of the concentration of adsorbed hydrogen in the structure of steels, the saturation mode affects the deformation mechanism and phase transformations in austenitic steel.

Comparative Study of Microstructure-sensitive Fatigue Crack Propagation in Coarse- and Fine-grained Microstructures between Stable and Metastable Austenitic Stainless Steels Using Miniature Specimen

Comparative Study of Microstructure-sensitive Fatigue Crack Propagation in Coarse- and Fine-grained Microstructures between Stable and Metastable Austenitic Stainless Steels Using Miniature Specimen

Quantitative Evaluation of Solute Hydrogen Effect on Dislocation Density in a Low-carbon Stable Austenitic Stainless Steel

Quantitative Evaluation of Solute Hydrogen Effect on Dislocation Density in a Low-carbon Stable Austenitic Stainless Steel

Age-hardening Behavior in High-nitrogen Stable Austenitic Stainless Steel

Age-hardening Behavior in High-nitrogen Stable Austenitic Stainless Steel

Correction to: Effect of Carbon and Nitrogen on the Hydrogen Embrittlement of 15Cr-15Mn-4Ni-Based Stable Austenitic Stainless Steels

Correction to: Effect of Carbon and Nitrogen on the Hydrogen Embrittlement of 15Cr-15Mn-4Ni-Based Stable Austenitic Stainless Steels

Amorphization under fracture surface in hydrogen-charged and low- temperature tensile-tested austenitic stainless steel

ABSTRACT The microstructure just below the fracture surface in hydrogen-charged stable austenitic SUS 316L stainless steel, which was subjected to a low strain rate tensile test at −70°C, was studied by a combination of the focused-ion-beam method and transmission electron microscopy. An amorphous region with a chemical composition almost identical to that of the polycrystalline region was found under the lath-like structure on the fracture surface, although no deterioration of tensile properties by hydrogen appeared. In the amorphous region, band-like regions with wavy contrasts were observed, which were often accompanied by cracks at the boundaries. The presence of the amorphous region with band-like regions implies that amorphization occurred due to high-density vacancies accompanied by agglomerations of excess vacancies in the hydrogen-charged SUS 316L stainless steel that was tensile-tested at low temperatures.

Lean duplex TRIP steel: Role of ferrite in the texture development, plastic anisotropy, martensitic transformation kinetics, and stress partitioning

Constitutive micromechanical behavior of transformation induced plasticity (TRIP) stainless steel (SS) alloys was investigated using high-energy synchrotron x-ray diffraction (S-XRD) and in-situ neutron diffraction techniques. First, four different steel alloys were designed and produced: (1) a metastable austenitic TRIP SS, (2) a stable austenitic SS, which is a stable counterpart of the TRIP SS, (3) a lean duplex TRIP SS with ferrite and metastable austenite phases, and (4) a lean duplex stable SS, which is a stable counterpart of duplex TRIP SS. Then, effects of chemical composition, microstructure, and texture on the plastic anisotropy, martensitic transformation kinetics, and residual stress concentration during a tensile deformation were investigated. The results show that the plastic anisotropy, governed by the initial microstructure and texture, has insignificant effect on macroscopic tensile properties and martensitic phase transformation kinetics despite different R-values observed along different loading directions. On the other hand, the interplay between stress partitioning among constituent phases and martensitic phase transformation plays a critical role in the micromechanics of plastic deformation, and, consequently, determines the distributions of in-situ martensite fraction and residual stresses. The phase stress partitioning in the TRIP alloy clearly shows that a large tensile residual stress of 1.8 GPa can concentrate on the martensite phase with 30% plastic strain. In contrast, the introduction of the tensile load-sharing ferrite phase in the cost-effective lean duplex TRIP alloy significantly reduces the tensile residual stress concentration in the martensite phase, which could improve the formability of high-strength TRIP steels.

Hydrogen, as an alloying element, enables a greater strength-ductility balance in an Fe-Cr-Ni-based, stable austenitic stainless steel

After pre-charging at the gaseous phase with a concentration of ~7,000 at. ppm, solute hydrogen was discovered to have an abnormal effect on both the strength and ductility enhancement of a commercially-available, Fe-24Cr-19Ni-based, stable austenitic stainless steel that had been subjected to tensile testing at various strain-rates. Specifically, the impact of hydrogen on material strength was accompanied by amplified yield and flow stresses, as well as tensile strength, while the improvement in ductility featured extended uniform elongation and strain-to-fracture, both of which became more pronounced as hydrogen concentration intensified. The product between tensile strength and uniform elongation served as indicators of the strength-ductility balance, at which hydrogen maximally optimized the indicator at the particular intermediate strain-rate. The yield/flow stress augmentations were interpreted in terms of solid-solution strengthening, whereas the enhanced ductility was primarily ascribed to the facilitation of mechanical twinning, whereby dynamic hydrogen-dislocation interaction exerted a critical influence as was indirectly revealed by supplemental stress-relaxation experiments.