Creep crack growth behaviour of Type 316H steels and proposed modifications to standard testing and analysis methods

Tests have been performed on Type 316H stainless-steel compact tension specimens from four ex-service components and creep crack growth rates from these tests have been characterised using C^*$C^{*}$. Several modifications to standard creep crack growth testing and analysis methods have been proposed, including an improved approach for determining whether widespread creep conditions have been developed in the specimens. The observed behaviour has then been compared with existing creep crack growth rate data for this material. A change in cracking mode from ductile to brittle intergranular fracture was observed with increasing test duration. In addition, creep crack growth rates for several of the longest-term tests lie above an extrapolation of existing data from shorter-term tests. Models based on ductility exhaustion have been used to derive new equations for predicting creep crack growth rates in Type 316H steel at temperatures of 525 and 550 °C.     

Effect of sulfide on corrosion behavior of stainless steel 316SS and Hastelloy C276 in sub/supercritical water

Stainless steel 316SS and Hastelloy C276, as the representatives of iron-based and nickel-based alloy, respectively, were employed to explore the corrosion properties under reducing subcritical and supercritical water containing sulfide. Experiments were executed at a pressure of 25 MPa, temperatures of 350 °C–520 °C, and sulfur concentrations of 1000 and 5000 ppm for 80 h. An isothermal equilibrium phase diagram involving the oxidation/sulfidation products of Fe, Cr, and Ni, was established by theoretical calculation in supercritical water system at 520 °C, in order to predict the corresponding products under various conditions and assist the discussion on corrosion mechanism. The results show that whether in subcritical water or in supercritical water, 316SS always exhibited better corrosion resistance relative to C276. In subcritical water at 350 °C, a portion of corrosion film peeled off from 316SS specimen, while numerous pores or cracks appeared on the surface of scale for C276. Under supercritical water at 520 °C, a compact scale grown on 316SS sample surface was composed of Fe₃O₄, FeCr₂O₄, and FeS. For C276, a duplex-layer scale formed on alloy surface. However, due to the higher content of Ni in C276, Ni-sulfide channels through the inner layer were developed, accelerating the sulfidation corrosion of alloys. Overall, the high-temperature alloys with high Cr content and low Ni/Cr ratio can be considered as the candidate material of equipment in supercritical water gasification of sulfur-containing coal.     

Numerical simulation of tearing–fatigue interactions in 316l(N) austenitic stainless steel

The loading history of engineering components can influence the behaviour of defects in service. This paper presents, the results of a numerical study aimed at using the Gurson ductile damage model, calibrated against J R-curve data, to simulate load-history effects on ductile tearing behaviour in austenitic materials. The work has demonstrated that ductile crack growth resistance is influenced by sub-critical crack growth by an intervening mechanism such as fatigue. Fatigue crack growth under a positive R-ratio leads to increase in subsequent tearing resistance through three main mechanisms: (i) re-sharpening of the crack tip; (ii) crack extension through the fracture process zone; and (iii) cyclic loading effects on void development. The ratio of minimum to maximum stress during fatigue loading (R-ratio) has been shown to influence subsequent tearing resistance, with an R-ratio of 0.2 generally leading to a greater enhancement in tearing resistance than an R-ratio of 0.1. This behaviour is due to the influence of R-ratio on void development ahead of the fatigue crack tip. Finally, relevant experimental data compare favourably with the predicted J R-curves.     

Additively manufactured 316L stainless steel: An efficient electrocatalyst

In the quest of finding an economical, yet efficient material, the idea of fabricating 316L stainless steel using additive manufacturing technology was explored to produce material with refined sub-granular structure. The surface of the stainless steel was further chemically treated with an etching solution to expose the grain boundaries. The grain boundary enriched surface resulted in more active sites for the oxygen evolution reaction (OER) in additively manufactured treated (AM-T) 316L stainless steel. AM-T sample manifests enhanced catalytic activity for OER with an overpotential of 310 mV to draw a 10 mA/cm² current density, along with a lower Tafel slope of 42 mV/dec compared to AM and wrought samples. These features were validated from the increased double-layer capacitance of AM-T and approximately 1.5 times larger electrochemically effective surface area of AM-T due to etching treatment compared to the wrought sample. Furthermore, AM-T also possesses stable activity retention for 100 h at a current density of 10 mA/cm².     

Active screen plasma nitriding of 316 stainless steel for the application of bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) has attracted considerable interest because of its superb performance, and many researches are focused on the development of high-performance, long-life bipolar plates. Stainless steel bipolar plates offer many advantages over the conventional graphite bipolar plates, such as low material and fabrication cost, excellent mechanical behaviour and ease of mass production. However, the insufficient corrosion resistance and relatively high interfacial contact resistance (ICR) become the major obstacles to the widespread use of stainless steel bipolar plates. In this work, active screen plasma nitriding (ASPN), a novel plasma nitriding technique, was used to modify the surface of 316 austenitic stainless steel. A variety of analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), glow discharge optical emission spectrometer (GDOES), were employed to characterize the nitrided samples. The results reveal that a nitrogen supersaturated S-phase layer has been successfully produced on the surface of all nitrided 316 stainless steel samples. The interfacial contact resistance (ICR) value can be decreased dramatically after ASPN treatment and the corrosion resistance can also been improved. In addition, better corrosion resistance can be achieved by active screen plasma nitriding with a stainless steel screen than with a carbon steel screen. This technique could be used to improve the performance and lifespan of bipolar plates for fuel cells.     

Anticorrosion properties of Ta-coated 316L stainless steel as bipolar plate material in proton exchange membrane fuel cells

In order to reduce the cost, volume and weight of the bipolar plates used in the proton exchange membrane fuel cells (PEMFC), more attention is being paid to metallic materials, among which 316L stainless steel (SS316L) is quite attractive. In this study, metallic Ta is deposited on SS316L using physical vapor deposition (PVD) to enhance the corrosion resistance of the bipolar plates. Simulative working environment of PEMFC is applied for testing the corrosion property of uncoated and Ta-coated SS316L. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods (potentiodynamic and potentiostatic polarization) are also used for analyzing characteristics of uncoated and Ta-coated SS316L. Results show that, Ta-coated SS316L has significantly better anticorrosion property than that of uncoated SS316L, with corrosion current densities of uncoated SS316L being 44.61 μA cm⁻² versus 9.25 μA cm⁻² for Ta-coated SS316L, a decrease of about 5 times. Moreover, corrosion current densities of Ta-coated SS316L in both simulative anode (purged with H₂) and cathode (purged with air) conditions are smaller than those of uncoated SS316L.     

Carbon-coated stainless steel as PEFC bipolar plate material

Stainless steel is quite attractive as bipolar plate material for polymer electrolyte fuel cells (PEFCs). Passive film on stainless steel protects the bulk of it from corrosion. However, passive film is composed of mixed metal oxides and causes a decrease in the interfacial contact resistance (ICR) between the bipolar plate and gas diffusion layer. Low ICR and high corrosion resistance are both required. In order to impart low ICR to stainless steel (SUS304), carbon-coating was prepared by using plasma-assisted chemical vapor deposition. Carbon-coated SUS304 was characterized by Raman spectroscopy and atomic force microscopy. Anodic polarization behavior under PEFC operating conditions (H₂SO₄ solution bubbled with H₂ (anode)/O₂ (cathode) containing 2 ppm HF at 80 °C) was examined. Based on the results of the ICR evaluated before and after anodic polarization, the potential for using carbon-coated SUS304 as bipolar plate material for PEFC was discussed.     

Conductive amorphous carbon-coated 316L stainless steel as bipolar plates in polymer electrolyte membrane fuel cells

Amorphous carbon (a-C) film about 3 μm in thickness is coated on 316L stainless steel by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The AFM and Raman results reveal that the a-C coating is dense and compact with a small size of graphitic crystallite and large number of disordered band. Interfacial contact resistance (ICR) results show that the surface conductivity of the bare SS316L is significantly increased by the a-C coating, with values of 8.3–5.2 mΩ cm² under 120–210 N/cm². The corrosion potential (E_corr) shifts from about −0.3 V vs SCE to about 0.2 V vs SCE in both the simulated anode and cathode environments. The passivation current density is reduced from 11.26 to 3.56 μA/cm² with the aid of the a-C coating in the simulated cathode environment. The a-C coated SS316L is cathodically protected in the simulated anode environment thereby exhibiting a stable and lower current density compared to the uncoated one in the simulated anode environment as demonstrated by the potentiostatic results.     

Corrosion resistance of chromized 316L stainless steel for PEMFC bipolar plates

Corrosion resistance of the chromized 316L stainless steel was studied in a proton exchange membrane fuel cell (PEMFC) operating condition. Cr-rich surface layer was formed by pack cementation technique and electrochemical properties of the chromized surface were examined by potentiodynamic and potentiostatic tests. Results showed that the Cr-rich layers underneath the free surface passivated the surface and protect the surface from corrosion in 0.5 M H₂SO₄ solution at 80 °C. However, the Cr-rich layers showed columnar grains with voids when the stainless steel was pack cemented for an extended period of time, resulting in drastic degradation of corrosion resistance. The optimum condition for the best corrosion resistance in the PEMFC operating condition was obtained without sacrificing the interfacial contact resistance.     

Corrosion characteristics of 316L as transpiring wall material in supercritical water oxidation of sewage sludge

Systematical corrosion tests of austenitic stainless steel 316L exposed to sewage sludge SCWO (supercritical water oxidation) were conducted in a batch stirred reactor with hydrogen peroxide as oxidant. Experiment conditions such as temperature, oxidation coefficient, pH value, corrosion medium, were chosen mainly keeping in mind the place and environment of reactions (i.e. surrounding transpiring wall). The exposed samples were ultimately analyzed by weight measurement, scanning electron microscopy in conjunction with energy dispersive spectroscopy, and X-ray diffraction analysis. The results show that severe pitting corrosion occurred as the sample was exposed to complicated environments, and different oxides including Fe₃O₄, FeCr₂O₄ and MoO₃ were found on the sample surface. The corrosion rate at all test conditions (360–450 °C pH = 5.2–10.05, oxidation coefficient of 0–2.0, sewage sludge or its SCWO reactor effluent) was in the range of 0.12–0.66 mm/y, and it increased as temperature and OC increased at supercritical conditions. Moreover, potential corrosion mechanism of 316L in sewage sludge SCWO is proposed, and influences of operating parameters on 316L corrosion properties are summarized. 316L and reactor effluent could be considered as transpiring wall material and transpiring water in sewage sludge SCWO with transpiring wall reactor, respectively.     

Evaluation of hydrogen permeation through standalone thermally sprayed coatings of AISI 316L stainless steel

This research evaluates hydrogen permeation and its diffusion characteristics through standalone thermally sprayed coatings of AISI 316L stainless steel. The effects of various charging currents and other parameters on hydrogen diffusion coefficient were scrutinized using electrochemical hydrogen permeation tests. Hydrogen permeation through the thermally sprayed coatings displayed anomalous behavior such that a maximum pinnacle was observed in the permeation curves, attributed to heavily trapped hydrogen atoms in the delayed surface cracks. Therefore, new diffusion parameters were defined for modeling of the anomalous permeation curves. The fitted diffusion parameters were consistently identified, and hence, the model perfectly explained experimental data. The results showed that the increase in charging current caused fast activation and development of surface cracks. The measured diffusion coefficient of hydrogen in the stainless steel thermally sprayed coating was relatively high because the microstructure of the coating contained some ferritic phases and dense dendritic structure, which configure fast diffusion paths.     

The dependence of fatigue crack growth on hydrogen in warm-rolled 316 austenitic stainless steel

The fatigue crack growth rate of warm-rolled AISI 316 austenitic stainless steel was investigated by controlling rolling strain and temperature in argon and hydrogen gas atmospheres. The fatigue crack growth rates of warm-rolled 316 specimens tested in hydrogen decreased with increasing rolling temperature, especially 400 °C. By controlling the deformation temperature and strain, the influences of microstructure (including dislocation structure, deformation twins and α′ martensite) and its evolution on hydrogen-induced degradation of mechanical properties were separately discussed. Deformation twins deceased and dislocations became more uniform with the increase in rolling temperature, inhibiting the formation of dynamic α′ martensite during the crack propagation. In the cold-rolled 316 specimens, deformation twins accelerated hydrogen-induced crack growth due to the α′ martensitic transformation at the crack tip. In the warm-rolled specimens, the formation of α′ martensite around the crack tip was completely inhibited, which greatly reduced the fatigue crack growth rate in hydrogen atmosphere.     

Influence of fluoride ions on corrosion performance of 316L stainless steel as bipolar plate material in simulated PEMFC anode environments

Corrosion performance of 316L stainless steel as a bipolar plate material in proton exchange membrane fuel cell (PEMFC) is studied under different simulated PEMFC anode conditions. Solutions of 1 × 10⁻⁵ M H₂SO₄ with a wide range of different F⁻ concentrations at 70 °C bubbled with hydrogen gas are used to simulate the PEMFC anode environments. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to study the corrosion behavior. Scanning electron microscope (SEM) and atomic force microscope (AFM) are used to examine the surface morphology of the specimen after it is potentiostatic polarized in simulated PEMFC anode environments. X-ray photoelectron spectroscopy (XPS) analysis is used to identify the compositions and the depth profile of the passive film formed on the 316L stainless steel surface after it is polarized in simulated PEMFC anode environments. Mott–Schottky measurements are used to characterize the semiconductor passive films. The results of potentiostatic analyses show that corrosion currents increase with F⁻ concentrations. SEM examinations show that no localized corrosion occurs on the surface of 316L stainless steel and AFM measurement results indicate that the surface topography of 316L stainless steel becomes slightly rougher after polarized in solutions with higher concentration of F⁻. From the results of XPS analysis and Mott–Schottky measurements, it is determined that the passive film formed on 316L stainless steel is a single layer n-type semiconductor.     

Ductility and fatigue properties of low nickel content type 316L austenitic stainless steel after gaseous thermal pre-charging with hydrogen

The susceptibility of low nickel content type 316L austenitic stainless steel to hydrogen was quantified using low strain rate tensile tests and strain-controlled low-cycle fatigue life measurements. Both tests were performed under air condition after charging with high-pressure 10-MPa hydrogen gas at 300 °C for eight days. No significant influence of hydrogen was recognized in 0.2% proof stress, but the strain at fracture and reduction area was decreased significantly in both hydrogen pre-charged and in gaseous hydrogen conditions compared to companion tests conducted in air. The decrease of fatigue life in the high strain amplitude region was related to a significant decrease in the plastic component while the effect of hydrogen on the elastic component was negligible. Highly localized deformation and a pronounced martensite transformation occurred near the site of the fracture surface in the high strain amplitude regime, resulting in the early formation of abundant micro-surface cracks in this regime of the hydrogen pre-charged samples.     

Influences of oxygen on corrosion characteristics of TiO2/316L stainless steel in supercritical water

Supercritical water gasification (SCWG) is a promising technology for converting organic wastes to hydrogen. Less amount of oxygen is beneficial for increasing hydrogen generation rate. However, the corrosion rate of reactor material would be accelerated. TiO₂ coating with a thickness of 0.1 mm was prepared on the surface of 316L stainless steel (SS316L) to improve its corrosion resistance in supercritical water (SCW). The corrosion performances of TiO₂/SS316L were tested in a bath SCW reactor at 400 °C, 25 MPa. The influences of oxygen concentration (0–1000 mg/L) on surface morphologies and corrosion depths were studied thoroughly. Results indicated that the surface of TiO₂/SS316L exhibited cracks and pores after exposed in SCW. And the average corrosion rates accelerated at higher oxygen concentrations. The interface between the coating and medium was relatively smooth and there was no obvious change in the thickness of the coating with oxygen concentration of 0 and 500 mg/L. While for that with 1000 mg/L oxygen, the surface of TiO₂/SS316L exhibited reticulate crack. The cross section showed a serrate structure, and only 0.08 mm thick of the coating was remained. In addition, the corrosion mechanism of coating was discussed.     

Thermal creep properties of alloy D9 stainless steel and 316 stainless steel fuel clad tubes

Uniaxial thermal creep rupture properties of 20% cold worked alloy D9 stainless steel (alloy D9 SS) fuel clad tubes for fast breeder reactors have been evaluated at 973 K in the stress range 125–250 MPa. The rupture lives were in the range 90–8100 h. The results are compared with the properties of 20% cold worked type 316 stainless steel (316 SS) clad tubes. Alloy D9 SS were found to have higher creep rupture strengths, lower creep rates and lower rupture ductility than 316 SS. The deformation and damage processes were related through Monkman Grant relationship and modified Monkman Grant relationship. The creep damage tolerance parameter indicates that creep fracture takes place by intergranular cavitation. Precipitation of titanium carbides in the matrix and chromium carbides on the grain boundaries, dislocation substructure and twins were observed in transmission electron microscopic investigations of alloy D9 SS. The improvement in strength is attributed to the precipitation of fine titanium carbides in the matrix which prevents the recovery and recrystallisation of the cold worked microstructure.     

The energy benefit of stainless steel recycling

The energy used to produce austenitic stainless steel was quantified throughout its entire life cycle for three scenarios: (1) current global operations, (2) 100% recycling, and (3) use of only virgin materials. Data are representative of global average operations in the early 2000s. The primary energy requirements to produce 1 metric ton of austenitic stainless steel (with assumed metals concentrations of 18% Cr, 8% Ni, and 74% Fe) is (1) 53 GJ, (2) 26 GJ, and (3) 79 GJ for each scenario, with CO₂ releases totaling (1) 3.6 metric tons CO₂, (2) 1.6 metric tons CO₂, and (3) 5.3 metric tons CO₂. Thus, the production of 17 million metric tons of austenitic stainless steel in 2004 used approximately 9.0×10¹⁷ J of primary energy and released 61 million metric tons of CO₂. Current recycling operations reduce energy use by 33% (4.4×10¹⁷ J) and CO₂ emissions by 32% (29 million tons). If austenitic stainless steel were to be produced solely from scrap, which is currently not possible on a global level due to limited availability, energy use would be 67% less than virgin-based production and CO₂ emissions would be cut by 70%. The calculation of the total energy is most sensitive to the amount and type of scrap fed into the electric arc furnace, the unit energy of the electric arc furnace, the unit energy of ferrochromium production, and the form of primary nickel.     

Strengthening mechanism and hydrogen-induced crack resistance of AISI 316L stainless steel subjected to laser peening at different power densities

Microstructural response of AISI 316L stainless steel to laser peening (LP) was examined by means of optical microscopy (OM) and transmission electron microscopy (TEM) in order to analyze the effects of LP on hydrogen-induced cracking (HIC) resistance. Depth profiles of near-surface microhardness and surface compressive residual stress (CRS) of LP treated specimens were presented respectively. Slow strain rate tensile tests were performed on the hydrogen-charged samples and their corresponding stress-strain curves as well as fracture morphologies were finally investigated in detail. The results demonstrated that LP induced a grain refinement effect on the treated surface while a maximum refining rate of 56.18% was achieved at the laser power density of 10 GW/cm². The near-surface microhardness also exhibited an attenuation trend with the increasing depth. The surface CRS positively correlated with power density before it reached a threshold value. A special U-shaped dislocation tangle band was observed in the LP treated specimen which served as hydrogen trapping sites. The LP treated samples exhibited better toughness after hydrogen charging from both macro mechanical properties and micro fracture morphologies. LP-induced grain refinement and CRS are believed to be the main contributing factors towards inhibiting the diffusion of hydrogen atoms which ultimately leads to the reduction of the hydrogen embrittlement sensitivity of AISI 316L stainless steel.     

Experimental-numerical study on ballistic impact behavior of 316L austenitic stainless steel plates against blunt and ogival projectiles

316L austenitic stainless steel (ASS) is the standard reference material in fabricating pressurized hydrogen storage tanks due to its low hydrogen embrittlement sensitivity and excellent corrosion resistance. Ballistic performance of such tanks is of course a concern of safety. In this study, ballistic impact behavior of 316L ASS was studied against blunt and ogival nose shaped projectiles within impact velocity range of 160.3–324.1 m/s. Ballistic impact behavior of 316L ASS is sensitive to nose shapes of projectiles. For targets against blunt projectile, shear plugging with ejected plugs is observed, and target deflection is limited; for targets against ogival projectile, failure mode is ductile hole enlargement, small bulge and some fragments are observed on front and rear sides of targets, respectively. Ballistic limit velocities (BLVs) for two projectiles are respectively 180.9 m/s and 333.5 m/s, indicating better energy absorption against ogival projectile. Numerical simulations of ballistic impact tests were carried out using either the Lode independent MJC or the Lode dependent modified Mohr-Coulomb (MMC) fracture criterion. Numerical prediction by the latter is more accurate than the former as ballistic impact tests are dominated by stress state where Lode parameter is strong enough to cause a big difference between MMC and MJC criteria, and fracture behavior is accurately predicted by the latter but overestimated by the former.     

Three-dimensional interconnected MoS2 nanosheets on industrial 316L stainless steel mesh as an efficient hydrogen evolution electrode

The development of inexpensive and competent electrocatalysts for high-efficiency hydrogen evolution reaction (HER) has been greatly significant to realize hydrogen production in large scale. In this paper, we selected the inexpensive and commercially accessible stainless steel as the conductive substrate for loading MoS₂ as a cathode for efficient HER under alkaline condition. Interconnected MoS₂ nanosheets were grown uniformly on 316-type stainless steel meshes with different mesh numbers via a facile hydrothermal way. And the optimized MoS₂/stainless steel electrocatalysts exhibited superior electrocatalytic performance for HER with a low overpotential of 160 mV at 10 mA cm⁻² and a small Tafel slope of 61 mV dec⁻¹ in 1 M KOH. Systematic study of the electrochemical properties was performed on the MoS₂/stainless steel electrocatalysts in comparison with the commonly used carbon cloth to better comprehend the origin of the superior HER performance as well as stability. By collaborative optimization of MoS₂ nanosheets and the cheap stainless steel substrate, the interconnected MoS₂ nanosheets on stainless steel provide an alternative strategy for the development of efficient and robust HER catalysts in strong alkaline environment.