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.     

A study on fatigue crack growth in the high cycle domain assuming sinusoidal thermal loading

The assessment of fatigue crack growth due to turbulent mixing of hot and cold coolants presents significant challenges, in particular to determine the thermal loading spectrum and the associated crack growth. The sinusoidal method is a simplified approach for addressing this problem, in which the entire spectrum is replaced by a sine-wave variation of the temperature at the inner pipe surface. The loading frequency is taken as that which gives the shortest crack initiation and growth life. Such estimates are intended to be conservative but not un-realistic. Several practical issues which arise with this approach have been studied using newly-developed analytical solutions for the temperature and stress fields in hollow cylinders, in particular the assumptions made concerning the crack orientation, dimensions and aspect ratio. The application of the proposed method is illustrated for the pipe geometry and loadings conditions reported for the Civaux 1 case where through wall thermal fatigue cracks developed in a short time, but the problem is relevant also for fast reactor components.     

Effect of vibration loading on the fatigue life of part-through notched pipe

A systematic experimental and analytical study has been carried out to investigate the effect of vibration loading on the fatigue life of the piping components. Three Point bend (TPB) specimens machined from the actual pipe have been used for the evaluation of Paris constants by carrying out the experiments under vibration + cyclic and cyclic loading as per the ASTM Standard E647. These constants have been used for the prediction of the fatigue life of the pipe having part-through notch of a/t = 0.25 and aspect ratio (2c/a) of 10. Predicted results have shown the reduction in fatigue life of the notched pipe subjected to vibration + cyclic loading by 50% compared to that of cyclic loading. Predicted results have been validated by carrying out the full-scale pipe (with part-through notch) tests. Notched pipes were subjected to loading conditions such that the initial stress-intensity factor remains same as that of TPB specimen. Experimental results of the full-scale pipe tests under vibration + cyclic loading has shown the reduction in fatigue life by 70% compared to that of cyclic loading. Fractographic examination of the fracture surface of the tested specimens subjected to vibration + cyclic loading have shown higher presence of brittle phases such as martensite (in the form of isolated planar facets) and secondary micro cracks. This could be the reason for the reduction of fatigue life in pipe subjected to vibration + cyclic loading.     

Creep fatigue crack behavior of two power plant steels

Crack initiation and crack growth under creep fatigue conditions were experimentally determined on a bainitic turbine rotor steel (30CrMoNiV4-11) and a martensitic pipe steel (X10CrMoVNb9-1). Side grooved compact tension (CT) specimens with 25 and 50 mm thickness as well as double edge notch tensile (DENT) specimens with 15 and 60 mm thickness have been tested in order to observe possible influences of geometry and thus to check the transferability of the specimen test results to the behavior of components.The creep fatigue crack test results can be described with the usual fracture mechanics parameters. A modified two-criteria-method can be used to estimate the crack initiation under creep fatigue conditions. The creep fatigue crack growth can be calculated from the accumulation of fatigue crack growth which is described by the Forman-law and creep crack growth which is described by the C^*-parameter.     

Crack propagation life prediction of a perforated plate under thermal fatigue

Superheater outlet headers of boilers are well known to be subjected to the cycling of high pressure and high thermal stress during plant operations. Thermal stresses during cyclic operation are generally severest on the inside surface of the ligaments between the stub-tube holes, where many examples of ligament cracking due to thermal fatigue have been found recently. A method to predict the crack propagation life of the ligaments of boiler headers under thermal fatigue has been required. Firstly in this paper, to model the crack propagation behavior of the ligament regions of boiler headers, a perforated plate of normalized and tempered 2 1/4Cr–1Mo steel was examined under out-of-phase thermal fatigue at a maximum temperature of 600°C in the air. Inelastic analysis of the perforated plate under thermal fatigue was carried out, and the nonlinear fracture mechanics parameters such as the J and C^* integral were obtained by the line integral for observed cracks. A simplified method was proposed for predicting these parameters under displacement-controlled conditions such as thermal fatigue. In this method, the change of the macroscopic stress–strain relation of the perforated plate with propagating cracks was combined with the reference stress concept under displacement-controlled conditions. The predicted fracture mechanics parameters from this method coincided well with those from the inelastic analysis. The prediction of the crack propagation life on the basis of the proposed method provided a good correspondence with the test results of the perforated plate under thermal fatigue.     

Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔK_th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔK_th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔK_th in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔK_th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔK_th in the hydrogen gas environment.     

A fatigue initiation parameter for gas pipe steel submitted to hydrogen absorption

Fatigue initiation resistance has been determined on API 5L X52 gas pipe steel. Tests have been performed on Roman Tile (RT) specimen and fatigue initiation was detected by acoustic emission. A comparison between specimens electrolytically charged with hydrogen and specimens without hydrogen absorption were made and it has been noted that fatigue initiation time is reduced of about 3 times when hydrogen embrittlement occurs. It has been proposed to use the concept of Notch Stress Intensity Factor as parameter to describe the fatigue initiation process. Due to the fact that hydrogen is localised in area with high hydrostatic pressure, definitions of local effective stress and distance have been modified when hydrogen is absorbed. This modification can be explained by existence of a ductile–brittle transition with hydrogen concentration. The fatigue initiation resistance curve allows that to determine a threshold for large number of cycles of fatigue non initiation. This parameter introduced in a Failure Assessment Diagram (FAD) provides supplementary information about defect nocivity in gas pipes: a non-critical defect can be detected as dormant or not dormant defect i.e., as non propagating defect.     

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.     

Influence of reference stress formulae on creep and creep-fatigue crack initiation and growth prediction in plate components

Creep and creep-fatigue crack growth in pre-cracked plates of 316L(N) austenitic stainless steel, containing a semi-elliptical surface defect and tested at 650 °C under combined axial and bending loading, are investigated. The results have been interpreted in terms of the creep fracture mechanics parameter C^∗ and compared with data obtained on standard compact tension (CT) specimens of the same material and batch. In making the assessments, the reference stress method has been used to determine C^∗. Several formulae exist for calculating the reference stress depending on whether it is based on a ‘global’ or a ‘local’ collapse mechanism and the assessment procedure adopted. When using this approach, it has been found that the most satisfactory comparison of crack growth rates with standard CT specimen data is obtained when the ‘global’ reference stress solution is used in conjunction with mean uniaxial creep properties. It has been found that the main effect of changing the fatigue cycle range from 0.1 to −1.0 is to cause an acceleration in the early stage of cracking.     

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.     

Temperature dependence of low cycle fatigue of 316(N) weld metals and 316L(N)/316(N) weld joints

Abstract

The low cycle fatigue behaviour of 316(N) weld metals and 316L(N)/316(N) weld joints have been investigated in the temperature range of 300–873 K, at a strain amplitude of ±0·6% and a strain rate 3 6 10^–3 s^–1, to study the influence of dynamic strain aging (DSA). The 316(N) weld metal exhibited better fatigue life than the weld joint, though the weld metal has shown higher cyclic stress response and higher plastic strain accumulation than the weld joint. Significant features observed in the temperature regime of 300–873 K include the maximum in fatigue life at 573 K and DSA in the range of 673–873 K. Occurrence of DSA has been manifested through drastic reduction in fatigue life in the range of 673–873 K, associated with anomalous stress response. Dominant DSA effects have been observed at about 773 K in the weld joint and at 823 K in the weld metal. However, the effect of DSA is found to be nominal beyond 823 K where the reduction in fatigue life is attributed to the combined effects of oxidation and DSA. Secondary crack density measurements (in the range of 300–873 K) in the weld joint specimens revealed the severity of the heat affected zone (HAZ) in inducing fatigue damage. Parameters have been identified to determine the temperature corresponding to dominant DSA effects.     

Influence of hydrogen pressure on fatigue properties of X80 pipeline steel

The low-cycle fatigue and fatigue crack growth (FCG) properties of X80 pipeline steel in hydrogen atmosphere were determined to investigate the variation of hydrogen pressure and its influence on fatigue life. The test environment was switched to a hydrogen atmosphere after 1000, 3000, or 5000 cycles of pre-fatigue testing in a nitrogen atmosphere. Notch tensile tests were conducted in nitrogen and hydrogen atmospheres after the specimens were pre-fatigued for 3000 or 5000 cycles. The results showed that the cycles to failure of X80 decreased exponentially with increasing hydrogen pressure. When the displacement amplitude (DA) values remained steady (below 3000 cycles), the X80 steels showed no noticeable deterioration in the fatigue properties with or without hydrogen. When the DA values increased (above 5000 cycles), cracks propagated slowly and fatigue properties were strongly reduced in the hydrogen atmosphere, but not in nitrogen. Hydrogen-accelerated crack growth dominates the reduction of fatigue life below 0.6 MPa of hydrogen pressure. Hydrogen-accelerated crack initiation plays a more important role than FCG in the reduction of fatigue life with increasing hydrogen pressure.     

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.     

Corrosion fatigue behaviour of 317LN austenitic stainless steel in phosphoric acid

The corrosion fatigue crack-growth behaviour of AISI 317LN stainless steel was evaluated in air and in 85% phosphoric acid at 20 °C. Austenitic stainless steels with high molybdenum content have high corrosion resistance and good mechanical properties. However, this increase in the molybdenum content and other elements such as nitrogen can also modify the microstructure. This leads to a modification of its mechanical properties. The corrosion fatigue crack-growth rate was higher in phosphoric acid immersion than in air. Austenitic stainless steels with a fully austenitic microstructure were more ductile, tough, and behave better against corrosion fatigue. The higher resistance to corrosion fatigue was directly associated to its higher resistance to corrosion.     

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.     

Creep crack growth testing of P91 and P22 pipe bends

Laboratory component tests play an important role in the development of life assessment procedures for high temperature crack initiation and growth. Thus, the working programme of the project BE 1702 HIDA, which addressed the validation, expansion and harmonisation of existing procedures for high temperature defect assessment, included a comprehensive experimental programme with feature tests of components as its core. Because of their relevance for the high temperature industry, P91 and P22 were included in HIDA among five materials. This paper presents laboratory creep crack growth tests of P91 and P22 pipe bends, discusses the test experience and draws some conclusions for laboratory component tests in general. The components were prepared with spark-eroded notches at the outer surface. The test temperature was 625°C for P91 and 565°C for P22.     

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.     

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.     

An investigation into nickel implanted 316L stainless steel as a bipolar plate for PEM fuel cell

A nickel-rich layer about 100 μm in thickness with improved conductivity was formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The effect of ion implantation on the corrosion behavior of SS316L was investigated in 0.5 M H₂SO₄ with 2 ppm HF solution at 80 °C by potentiodynamic test. In order to investigate the chemical stability of the ion implanted SS316L, the potentiostatic test was conducted in an accelerated cathode environment and the solutions after the potentiostatic test were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES). The results of potentiodynamic test show that the corrosion potential of SS316L is shifted toward the positive direction from −0.3 V versus SCE to −0.05 V versus SCE in anode environment and the passivation current density at 0.6 V is reduced from 11.26 to 7.00 μA cm⁻² in the cathode environment with an ion implantation dose of 3 × 10¹⁷ ions cm⁻². The potentiostatic test results indicate that the nickel implanted SS316L has higher chemical stability in the accelerated cathode environment than the bare SS316L, due to the increased amount of metallic Ni in the passive layer. The ICP results are in agreement with the electrochemical test results that the bare SS316L has the highest dissolution rate in both cathode and anode environments and the Ni implantation markedly reduces the dissolution rate. A significant improvement of interfacial contact resistance (ICR) is achieved for the SS316L implanted with nickel as compared to the bare SS316L, which is attributed to the reduction in passive layer thickness caused by the nickel implantation. The ICR values for implanted specimens increase with increasing dose.     

Assessment of resistance to fatigue crack growth of natural gas line pipe steels carrying gas mixed with hydrogen

Hydrogen embrittlement of line pipe steels in the natural gas transmission and distribution network is investigated. The objective is to assess whether the existing network can be used to safely transport a mixture of hydrogen and natural gas. The surveyed literature indicates that the hydrogen-induced acceleration of fatigue crack growth induced by natural gas pressure fluctuations can be the most probable type of failure. We analyzed the fatigue crack growth in line pipe steels containing a long axial crack in the inner diameter (ID) surface by accounting for random cyclic loading due to random and realistic pressure fluctuations, crack closure, and accurate calculation of the stress intensity factor. Using the available experimental data for the crack growth rate vs. stress intensity factor range in the presence of hydrogen, we simulated crack growth over a period of 100 years. The results show that under typical pressure fluctuations in the natural gas network, cracks with depths less than 40% of the wall thickness will never reach depths equal to 75% of the wall thickness. This is a conservative estimate that results from i) the nature of the geometry of the initial flaw in the ID surface that we used in the analysis, ii) the fact that the existing experimental data for the effect of hydrogen on the Paris law are for pressures that are orders of magnitude larger than the partial pressures intended for the hydrogen gas in the mixture, and iii) the experimental data are for fatigue crack growth in pure hydrogen gas without impurities normally present in natural gas, such as oxygen or methane, that can inhibit hydrogen uptake.