In this study, the effects of internal pores on the tensile behavior of austenitic stainless steel 316L manufactured with laser powder bed fusion (L-PBF) additive manufacturing (AM) were ...investigated. Both fully-dense samples and samples with intentional internal pores of varying diameters were fabricated. For each sample with a pore, the internal pore was deliberately fabricated in the center of the cylindrical tensile sample during AM processing. By varying the diameter of the 180 μm-tall initial penny-shaped pores, from 150 to 4800 μm within 6 mm gauge diameter cylindrical samples, the impact of lack-of-fusion, commonly present in AM, as well as the impact of well-defined pores in general, on tensile mechanical properties was studied. To link the pore size and morphology to the mechanical properties, the sizes of the initial pores were evaluated using non-destructive Archimedes measurements, 2D X-ray radiography, 3D X-ray computed tomography, and destructive 2D optical microscopy. Samples with and without the single, penny-shaped pore were subjected to uniaxial tension to evaluate the defect size dependent mechanical properties. The intentional pore began to impact ultimate tensile strength when the pore diameter was 2400 μm, or 16% of the cross-sectional sample area. Elongation to failure was significantly affected when the pore diameter was 1800 μm or 9% of the cross-sectional sample area. This shows that 316L stainless steel manufactured by additive manufacturing is defect-tolerant under uniaxial tension loading.
•The usefulness of nitriding as pre-treatment for SOFC interconnects is tested.•In situ oxidation of samples was recorded with environmental SEM.•The electrical properties of the steels are not ...clearly improved.•Nitriding demonstrated to decrease chromium evaporation with porous coating.•No intergranular corrosion is observed in the samples.
Plasma nitriding was applied to ferritic stainless steel substrates to improve their performances as interconnects for solid oxide fuel cell devices. The samples underwent electrical conductivity test and SEM/EDS, TEM/EDS, environmental-SEM analyses. The first stages of corrosion were recorded in-situ with the e-SEM. Nitriding is effective in limiting the undesired chromium evaporation from the steel substrates and accelerates the corrosion kinetics, but its influence of the electrical conductivity is ambiguous. No intergranular corrosion is found in the steel substrate after long time operation. Nitriding helps commercially competitive porous coating to improve chromium retention properties of metal interconnects.
•Deformation behavior is studied over a wide range of strain rates covering quasi-static, medium, high, and very high regimes.•Enhanced flow stress increase is observed at strain rates above ...13000 s-1.•Dynamic strain aging is observed at temperatures between 673 and 873 K and strain rates below 10 s-1•Bell-shaped strain rate/temperature sensitivity curves due to dynamic strain aging are observed.•Description capacities of eight constitutive models in terms of strain hardening rate, strain rate/temperature sensitivities are analyzed.
Using the recently developed single shear specimen, deformation behavior of 304 austenitic stainless steel is studied systematically over a wide range of strain rates, covering quasi-static (0.001-0.1 s−1), medium (1-10 s−1), high (3000-6000 s−1) and very high (13000-39000 s−1) regimes, and temperatures between 77 and 1073 K were considered. The experimental flow stress curves show that: (1) the flow stress increases slightly at strain rates below 6000 s−1; concerning strain rates above 13000 s−1, however, an obvious flow stress upturn exists; (2) dynamic strain aging (DSA) occurs within the temperature range of 673 to 873 K and strain rates lower than 10 s−1; (3) both temperature sensitivity and strain rate sensitivity of the material are affected by DSA, and bell-shaped sensitivity curves form; (4) with increasing strain rates, peaks of the bell-shaped temperature sensitivity curves move to higher temperature regimes, and the evolution law is equally valid for the influence of temperature on the strain rate sensitivity curves. According to experiments, eight constitutive relations, including three phenomenological, four physical-based, and an artificial neural network, are established, including viscous drag effect at very high strain rates but excluding DSA phenomenon. Prediction capacities of the established constitutive models are checked by testing condition jump tests at randomly chosen strain rates and temperatures. Description capacities of the constitutive models in terms of strain hardening rate, temperature sensitivity and strain rate sensitivity are also analyzed.
•The environmental enhancement factor of 308 L weld metal reached a value of 50 when the frequency was decreased to 0.00001 Hz.•The 308 L weld metal showed excellent SCC resistance regardless of the ...thermal aging or water chemistry.•No preference of the γ/δ crack path and no elevated EAC either with thermal aging or in oxidizing water was attributed to the no sensitization of the γ/δ boundaries before and after thermal aging.
The environment assisted cracking (EAC) behavior, including stress corrosion cracking (SCC) and corrosion fatigue (CF), of a low carbon 308 L stainless steel weld metal was evaluated in high temperature water. The effects of thermal aging (400 °C for 10,000 h) on the crack growth rate (CGR) are discussed. Experimental results reveal an environmental acceleration effect on CGRs with decreasing loading frequency, and the environmental enhancement factor reached a value of 50 when the frequency was decreased to 0.00001 Hz. The 308 L weld metal showed excellent SCC resistance regardless of the thermal aging or water chemistry, which is related to the low carbon content and well-controlled duplex microstructure. After thermal aging, the ferrite/austenite (γ/δ) phase boundaries show negligible sensitization, consistent with the low carbon concentration. No preference of the γ/δ crack path and no elevated EAC either with thermal aging or in oxidizing water was attributed to the no sensitization of the γ/δ boundaries before and after thermal aging.
•The optimal DL-EPR test procedure is in 2 M H2SO4 + 1.25 M HCl + 0.001 M KSCN.•DOS is initially increases and then decreases, peaking at 10 h.•The sigma phase occurs at α/γ grain boundaries and ...inside the α phase.•The Cr2N forms at α/γ grain boundaries and inside the γ phase.•The DL-EPR tests are consistent with the etching pattern and LSC morphology.
The effects of thermal aging of hyper-duplex stainless steel (HDSS) 2707 at 900℃ for various aging times up to 26 h on the localized, selective corrosion were investigated by electrochemical methods. The results demonstrate that the optimum test condition is 2 M H2SO4 + 1.25 M HCl + 0 0.001 M KSCN with the scan rate of 1.5 mV·s−1 at 30℃. The degree of sensitization (DOS) of HDSS 2707 initially increases rapidly, because of particle precipitation (σ phase/Cr2N), and then decreases as a result of self-healing by back diffusion of Cr and Mo into the depleted areas, peaking at 10h.
•New multiaxial fatigue characterisation of 316 steel including high mean deformation and non-proportional load paths.•Compression induces a lower strain range thereby increasing the fatigue ...life.•Thorough and independent analysis of newly proposed critical plane method (Lei-Gan) indicates an acceptable performance.•The most accurate model for estimating the fatigue life of 316 steel is Liu II.•Fatemi-Socie yields the best estimations in terms of cracking angle.
In this work, the fatigue behaviour of 316 stainless steel is studied with different critical plane models. Seven cylindrical samples were used for the study, being subjected to different complex loading paths, generating combined stresses along the axial and transversal sample directions, these being: individual axial stress, individual hoop stress, alternating axial and hoop stress, a proportional combination of axial and hoop stress, and a non-proportional combination of L-shaped and square-shaped axial and hoop stress. The fatigue analysis is performed using five critical plane models; named Fatemi-Socie, Varvani-Farahani, Gan-Wu-Zhong, Liu I and Liu II. The models were assessed based on their fatigue life and crack angle prediction capacity. The Gan-Wu-Zhong recently proposed critical plane model was examined and provided acceptable results for the multiaxial loads tested on 316 steel. Nevertheless, Fatemi-Socie produced the most accurate results in terms of cracking orientation and Liu II gave the best fatigue life predictions.
Optimal treatment of bone fractures with minimal complications requires implant alloys that combine high strength with high ductility. Today, TiAl6V4 titanium and 316L steel are the most applied ...alloys in bone surgery, whereas both share advantages and disadvantages. The nickel-free, high-nitrogen austenitic steel X13CrMnMoN18-14-3 (1.4452, brand name: P2000) exhibits high strength in combination with superior ductility. In order to compare suitable alloys for bone implants, we investigated titanium, 316L steel, CoCrMo and P2000 for their biocompatibility and hemocompatibility (according to DIN ISO 10993-5 and 10993-4), cell metabolism, mineralization of osteoblasts, electrochemical and mechanical properties. P2000 exhibited good biocompatibility of fibroblasts and osteoblasts without impairment in vitality or changing of cell morphology. Furthermore, investigation of the osteoblasts function by ALP activity and protein levels of the key transcription factor RUNX2 revealed 2x increased ALP activity and more than 4x increased RUNX2 protein levels for P2000 compared to titanium or 316 steel, respectively. Additionally, analyses of osteoblast biomineralization by Alizarin Red S staining exhibited more than 6x increased significant mineralization of osteoblasts grown on P2000 as compared to titanium. Further, P2000 showed no hemolytic effect and no significant influence on hemocompatibility. Nanoindentation hardness tests of Titanium and 316L specimens exposed an indentation hardness (HIT) of about 4 GPa, whereas CoCrMo and P2000 revealed HIT of 7.5 and 5.6 GPa, respectively. Moreover, an improved corrosion resistance of P2000 compared to 316L steel was observed. In summary, we could demonstrate that the nickel-free high-nitrogen steel P2000 appears to be a promising alternative candidate for applications in bone surgery. As to nearly all aspects like biocompatibility and hemocompatibility, cell metabolism, mineralization of osteoblasts and mechanical properties, P2000 was similar to or revealed advantages against titanium, 316L or CoCrMo.
The effect of applied stress on the crevice corrosion of 304 stainless steel in 3.5 wt% solution is investigated. The delayed and immediate crevice corrosion of 304 stainless steel could occur under ...various applied stresses and different polarized potential. Both these two types of crevice corrosion are developed from the evolution of metastable pits. The lifetime of metastable pits is prolonged with the increased stress and the secondary metastable pit is observed under 400 MPa applied stress. This contributes to increased probability of metastable pits transiting to stable pits and then promotes crevice corrosion development with the increased stress.
•Crevice corrosion of 304 stainless steel is developed from metastable pits.•The concentration of defects in passive film increases with the increased stress.•The applied stress could drastically extend the lifetime of metastable pits.•Secondary metastable pit can be induced by applied plastic stress.•Array reference electrodes can monitor evolution of metastable pit inside crevice.
•321 stainless steel and Inconel 738 was diffusion bonded using 54Ag-40Cu-5.0Zn-1.0Ni powder-mixture as interlayer.•Effect of holding pressure on metallurgical and mechanical properties of the joint ...was investigated.•The increase in pressure improves the interatomic diffusion and facilitates the formation of a more uniform and wider diffusion affected zone.•An increase in the holding pressure increases the shear strength of the diffusion brazed joints.
The application of 54Ag-40Cu-5.0Zn-1.0Ni powder-mixture interlayer in the diffusion-brazing of Inconel 738 and AISI 321 stainless steel was investigated under a holding pressure variance (1 and 2 MPa). The resultant microstructure, shear strength, and fracture of the brazed joints were investigated. An increase in the holding pressure facilitates better atomic diffusion, uniform thermally-induced reaction layer, and improves the shear strength of the brazed joints. The interfacial reaction layers are made of Cu-rich and Ag-rich phases irrespective of the difference in the holding pressure. Thus, a higher bonding/holding pressure is recommended for the diffusion brazing of Inconel and stainless steels when the brazing medium is a powder mixture.
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•Strain-induced reactivation of a corrosion pit was observed, in-situ.•Pit stability products and 3D pit growth kinetics were estimated.•Applied strain resulted in diffusivity ...parameter (Deff ΔC) of 4.5×10−8molcm−1s−1.•Lacy metal cover fractures were observed.
The reactivation of a corrosion pit under the synergetic effect of strain and electro-chemical polarisation has been observed in a type 304L stainless steel using X-ray computed tomography. The pit reactivation process was associated with the formation of a new pit, directly adjacent to a pre-existing pit. Pit growth kinetics were estimated, revealing an increase of the diffusivity parameter (Deff ΔC) from 3.0×10−8molcm−1s−1 to 4.5×10−8molcm−1s−1 with the application of strain, indicating higher metal dissolution rates. Applied strain resulted in fractured lacy metal covers, and its effect on pit growth kinetics is discussed.
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