Estimating structural integrity,using the TPFC approach,of annealed type 304 stainless steel plate and pipes containing surface defects

Test results and an evaluation of the two-parameter fracture criterion (TPFC) are presented. The tests were conducted at room temperature with annealed Type 304 stainless steel flat-plate tensile specimens containing triangular-, ellipsoidal- or rectangular-shaped surface flaws, and pressurised pipe specimens with internal or external triangular-shaped surface defects. Generally accepted analytical techniques are not available for these and other very ductile materials used in many nuclear reactor components and an accurate assessment of the influence of defects on structural component integrity is needed.The TPFC approach was used in conjunction with the initial defect size and the loads required for the initiation of subcritical crack growth, for penetration through the wall thickness and for instability. Generally, the test results obtained from the flat specimens could be used to predict, from a conservative point of view, the behaviour of pipe specimens. Since $\text{K}{}_{\mathrm{<mtext>F</mtext>}}$ is thickness dependent, it is recommended that tests be conducted for the specific thickness of concern using specimens containing surface defects. The TPFC approach can provide an accurate means for predicting structural integrity.     

Fracture behavior of cracked Type 304 stainless steel pipes under tensile and thermal loadings

Because of frequent discoveries of stress corrosion cracking in BWR primary piping systems, it became an urgent problem to evaluate the possibility of unstable fracture of flawed stainless steel pipes. To understand the fundamental fracture property as well as the behavior under actual plant conditions, the authors performed the following four types of tests on the cracked Type 304 stainless steel plates or pipes.

1. 1. Plate fracture test under tensile loading.

2. 2. Pipe fracture test under tensile loading.

3. 3. Leak-before-break test under high-compliance loading.

4. 4. Thermal shock test.

These test revealed quite high resistance of the flawed stainless steel pipes against unstable fracture.     

Creep test of type 304 stainless steel tube containing a notch subjected to internal pressure

This paper summarises the results of experimental creep tests of type 304 stainless steel tube subjected to internal pressure at 650°C. The equipment used was especially developed for these tests.

The tubes without notches were tested at pressures of 9·32 and 7·36 MPa. Test results indicate that the rupture time of the tubes without notches is in good agreement with that of uniaxial specimens when the maximum stress is taken as the rupture criterion. The tubes containing axial and circumferential surface notches were tested at a pressure of 7·36 MPa. Test results indicate that the ductile fracture theory is applicable to the life prediction in the case of axial notches.

An electric potential method was very useful for monitoring the creep crack growth from the notch root. The relationship between the creep crack growth rate and the fracture mechanics parameter, σ_net or K₁, was investigated.     

Measurement and simulation of residual stress in type 304 weld overlay stainless steel pipe

Repair by welding overlay is a commonly used method mainly employed to rebuild piping systems suffering from intergranular stress corrosion cracking (IGSCC). It is desirable that the overlay welding technique, by attaching an overlay weld to the pipe and sustaining a heat sink of flowing water inside the pipe, induces a compressive residual stress at the inner surface of the welded pipe for prevention of IGSCC. A better understanding of the effect of a welding overlay repair on the residual stresses at the inner and outer surfaces of weld overlay is thus required. To obtain this understanding, it is necessary to investigate the distribution of residual stresses on the welded pipe.

In this study, the hole-drilling strain-gauge method was adopted to determine the residual stresses at the inner and outer surfaces of the weld overlay pipe. The incremental drilling technique was used on pipes with outside diameters of 267 mm. In addition, the Weld 3 code was applied to simulate the residual stress distribution for comparison and verification with the measured results.

The results obtained from the experimental and from the computational methods are in good agreement. The residual stress at the inner surface of the pipe is compressive with a magnitude approaching the yield stress of the material; that at the outer surface is tensile, also with a magnitude close to yield stress but smaller than the compressive stress. The experimental residual stress magnitude is generally greater than that from computation. This observation can be attributed to several factors including applied mechanics, temperature distribution, original residual stress, strain gauge location, mechanical grinding and the oxidation layer.     

Dependence of hydrogen embrittlement on hydrogen in the surface layer in type 304 stainless steel

Hydrogen embrittlement (HE) together with the hydrogen transport behavior in hydrogen-charged type 304 stainless steel was investigated by combined tension and outgassing experiments. The hydrogen release rate and HE of hydrogen-charged 304 specimens increase with the hydrogen pressure for hydrogen-charging (or hydrogen content) and almost no HE is observed below the hydrogen content of 8.5 mass ppm. Baking at 433 K for 48 h can eliminate HE of the hydrogen-charged 304 specimen, while removing the surface layer will restore HE, which indicates that hydrogen in the surface layer plays the primary role in HE. Scanning electron microscopy (SEM) and scanning tunnel microscopy (STM) observations show that particles attributed to the strain-induced α′ martensite formation break away from the matrix and the small holes form during deformation on the specimen surface. With increasing strain, the connection among small holes along {111} slip planes of austenite will cause crack initiation on the surface, and then the hydrogen induced crack propagates from the surface to interior.     

Fracture behaviour of stainless steel pipes containing circumferential cracks at room temperature and 280°C

The paper presents the experimental results of a research programme on fracture behaviour of austenitic stainless steel and TIG welds in pipes containing circumferential through-wall cracks at room temperature and 280°C. Pipes were loaded in pure bending using a four-point bend test method. The diameter of the pipes under investigation was 168 mm and 324 mm, with a thickness varying from 10 to 17 mm.

As opposed to the behaviour of carbon steel pipes, it is found that the Net Section Collapse (NSC) criterion predicts the moment of instability. Crack mouth opening displacements (COD) and collapse moments calculated using the GE-EPRI engineering approach show a rather high scatter with respect to experimental results.     

Effect of plasma nitriding on behavior of austenitic stainless steel 304L bipolar plate in proton exchange membrane fuel cell

A dense and supersaturated nitrogen layer with higher conductivity is obtained on the surface of austenitic stainless steel 304L by the low temperature plasma nitriding. The effect of plasma nitriding on the corrosion behavior and interfacial contact resistance (ICR) for the austenitic stainless steel 304L was investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environment using electrochemical and electric resistance measurements. The experiment results show that the stable passive film is formed after the potentiostatic polarization at the specified anodic or cathodic potentials under PEMFC operation condition, and the plasma nitriding improves slightly the corrosion resistance and decreases markedly the ICR of 304L. The ICR of the plasma nitrided 304L increases after the potentiostatic polarizations for 4 h, and lower than 100 mΩ cm² at the compaction force of 150 N cm⁻².     

Numerical modeling of non-Fourier heat transfer and fluid flow during plasma arc welding of AISI 304 stainless steel

ABSTRACT

This paper is an attempt to study the evolution of temperature profiles and weld pool geometry during plasma arc welding (PAW) by solving the transient Navier–Stokes and Energy equations. The analysis for an AISI 304 stainless steel rectangular plate was carried out using a flexible written program in Fortran. Due to the low accuracy of the Fourier heat transfer equation for short times and large dimensions, a non-Fourier form of heat transfer equation was used. Gaussian heat source is considered as the heat source model. The fluid flow in the molten pool is of interest because it can change the temperature distribution in and around the molten zone. The governing equations for fluid flow were solved by the finite-volume method in which the SIMPLE method was utilized for pressure–velocity coupling. The effects of heat conduction, fluid flow, and force actions at the weld pool were considered. Thermo-physical properties such as thermal conductivity, specific heat, and dynamic viscosity vary as a function of temperature. There are two mechanisms involved which actively cause heat transfer to the surroundings: radiation and convection heat transfer. The numerical results are compared to the experimental data. The results corroborate that the weld pool thickness in the cross section of PAW and the time taken by molten metal to reach the end of thick metal are in good agreement with the experimental measurements. Finally, the results obtained from the assumed Fourier heat transfer are compared for the same study.     

Effects of hydrogen on tensile properties and fracture surface morphologies of Type 316L stainless steel

The effects of hydrogen on the tensile properties and fracture surface morphologies of Type 316L stainless steel were investigated using virgin and prestrained specimens. Hydrogen gas exposure at 10 MPa and 250 °C for 192 h resulted in its uniform distribution in the specimens. Such internal hydrogen degraded the tensile ductility of the specimens. Cup–cone fracture occurred in the non-, Ar-, and H-exposed specimens. The fracture surfaces were covered with large and small dimples. The H-exposed specimens exhibited larger small-dimple areas than the non- and Ar-exposed ones. The diameter of the large dimples decreased with increasing small-dimple area. Three-dimensional analysis of the dimples showed that the small-dimple regions were void sheets produced by local shear strain. Hydrogen accelerated nucleation of voids and formation of the void sheets by enhancing localization of shear deformation, thereby reducing the average size of the dimples.     

Increase in the local yield stress near surface of austenitic stainless steel due to invasion by hydrogen

In order to determine the effect of hydrogen on the local yield stress near the surface of austenitic stainless steel, an indentation test combined with inverse problem analysis was employed. For austenitic stainless steel, the indentation test is an effective method since the hydrogen is distributed near to the surface because of its high solubility and low diffusion coefficient. Although uniaxial tensile tests can also provide useful data, greater variations in the mechanical properties due to the presence of hydrogen can be detected through indentation tests. In this study, Secondary Ion Mass Spectrometry (SIMS) was used to measure hydrogen depth profiles in order to establish the relationships between the hydrogen absorption depth and the effects due to hydrogen evaluated using the indentation test. The results showed that the yield stress doubled due to hydrogen absorption and then reverted to its initial state due to hydrogen desorption at room temperature. Also, hardening due to the presence of hydrogen, which was determined using an indentation test, was found to be dependent on the relationship between the plastic deformation depth and the hydrogen absorption depth.     

Monitoring of stress corrosion cracking of sensitised 304H stainless steel in nuclear applications by electrochemical methods and acoustic emission

Abstract

A hybrid monitoring technique for stress corrosion cracking (SCC) has been developed that employs simultaneously localised corrosion monitoring, electrochemical noise and acoustic emission (AE) techniques. The application of the hybrid technique for detection of SCC initiation and propagation in sensitised 304H stainless steel in dilute tetrathionate solutions at ambient temperature is reported. Initial result shows that SCC initiation and its early stage propagation can be detected by the localised corrosion monitoring and electrochemical noise methods. The dimensions of the crack can be estimated from the charge values derived from the detected transients. The locations of AE events determined using two sensors are in good agreement with the locations of cracks observed in the specimen. The AE technique is sensitive to rapid crack propagation, but does not appear to be sensitive to SCC initiation and early stage propagation for the present material environment load combination. It is postulated that AE is sensitive to SCC propagation involving a relatively large volume of plastic deformation. On the basis of test results and on information from the literature, it is suggested that in this material environment system SCC cracks initiate via slow anodic dissolution at the chromium depleted grain boundaries. Subsequently, elemental sulphur adsorbed on the surface around the crack tip catalyses the entry of hydrogen atoms produced by the hydrogen reduction reaction into the steel matrix ahead of the crack tip; this hydrogen accumulates gradually over a relatively long period of time and preferentially at carbide/matrix interfaces, eventually causing hydrogen induced brittle fracture along grain boundaries.     

Combined effects of prior plastic deformation and sensitization on hydrogen embrittlement of 304 austenitic stainless steel

Austenite stainless steels (ASSs) may suffer from both cold deformation and sensitization prior to hydrogen exposure. There is scant data in literature on the combined effect of prior deformation and sensitization on the hydrogen embrittlement (HE) of ASSs. The present study investigated the combined effects of tensile plastic prestrain (PS) and 650 °C sensitization (ST) on the HE of 304 steel by hydrogen pre-charging and tensile testing. The results are explained by terms of pre-existing α′ martensite content. PS higher than 10% can enhance HE significantly by inducing severe α′ transformation prior to hydrogen exposure. Prior ST also enhances HE, but submitting the prestrained and α′-containing 304 steel to short-time ST can diminish the enhancement of HE by prestraining, as ST can cause the reversion of α′ to austenite, reducing pre-existing α′ content. It is inadvisable to make 304 steel be sensitized/welded firstly and deformed subsequently, even if the ST time is short such as what happens during welding, because this treating sequence can induce more α′ than prestraining alone, enhancing HE more significantly. Apparent hydrogen diffusivity can be related quantitatively to pre-existing α′ content, proving directly that α′ platelets can act as diffusion “highways” in ASSs. It is indicated that pre-existing α′ can enhance subsequently the HE of ASSs is because it can lead to a large amount of hydrogen entering the ASSs during hydrogen exposure by acting as diffusion “highways”. HE is enhanced by increasing hydrogen amount rather than by pre-existing α′ itself.     

Surface and corrosion properties of modified passive layer on 304 stainless steel as bipolar plates for PEMFCs

In this study, the relation between surface chemistry and corrosion properties of modified 304 stainless steels (304SS) was investigated. 304SS samples were submitted to plasma nitriding performed at two different temperatures: 420 °C (low) and 520 °C (high). Then, a thermo reactive deposition (TRD) was used in a mixture of ferro niobium, alumina and ammonium chloride. Finally, a pickling treatment was performed to access high corrosion resistant surfaces. Surface treated samples, both before and after pickling, were investigated by using Scanning Electron Microscopy (SEM) and Potentiodynamic (PD) techniques. X-ray Photoelectron Spectroscopy (XPS) was performed for the analysis of the surface layers of the samples after pickling.The surface layers were mostly comprised of iron and chromium oxides and hydroxides. XPS results proved the presence of a surface layer mostly constituted by iron oxides and oxyhydroxides for both the treated samples that resulted more homogenous for the sample nitrided at low temperature. Chromium was detected by XPS in the predominant form of oxide on the surface of the sample nitrided at low temperature.     

316L奥氏体不锈钢厚板焊接工艺及接头性能研究

根据316L奥氏体不锈钢的焊接性,制定了316L厚板的焊接工艺,采用钨极氩弧焊和焊条电弧焊的组合焊接工艺对316L厚板进行了焊接,并对焊接接头进行了力学性能、金相检测和晶间腐蚀试验。结果表明,所获得的焊接接头无缺陷,力学性能合格,焊缝组织为奥氏体和少量δ铁素体,焊接接头无晶间腐蚀倾向。焊接工艺合理,满足316L奥氏体不锈钢厚板焊接生产要求。     

Influence of temperature on corrosion behavior,wettability, and surface conductivity of 304 stainless steel in simulated cathode environment of proton exchange membrane fuel cells

The effects of temperature on corrosion behavior, wettability, and surface conductivity of 304 stainless steel (SS304) in simulated cathode environment of proton exchange membrane fuel cells (PEMFC) are investigated systematically using electrochemical tests and surface analyses. The results indicate that although the corrosion resistance of SS304 is decreased with the rising of solution temperature, the current density of SS304 at the working potential in the simulated PEMFC cathode environment can still meet the 2025 U.S. Department of Energy (DOE) technical target (i_corr < 1 μA cm^?2). Meanwhile, the surface wettability and ICR of SS304 samples after potentiostatic polarization show a continuous increase with the rise of the simulated solution temperature. The surface conductivity of SS304 both before and after polarization cannot reach the 2025 DOE technical target (<0.01 Ω cm²) and needs to be improved by surface modification.     

Numerical simulation to study the effect of repair width on residual stresses of a stainless steel clad plate

Clad plates are widely used in the construction of corrosion resistant equipment. During the repair of clad plates, residual stresses are generated and influence the structure integrity. This paper uses the finite element method (FEM) to predict the residual stresses in a repair weld of a stainless steel clad plate. The effect of repair width on residual stresses has also been investigated by numerical simulation. Due to the material mismatching between clad metal and base metal, a discontinuous stress distribution has been generated across the interface between clad and base metals. The peak residual stress occurs in the heat affected zone (HAZ) of the base metal, because the yield strength of the base metal is larger than that of the clad metal. With an increase in repair width, the residual stresses are decreased. When the repair width is increased to 24 mm, the residual stresses in the weld have been decreased greatly and the peak residual stresses have been reduced to less than the yield strength. Therefore, the recommended repair width should not be less than 24 mm, which provides a reference for optimizing repair welding technology for this stainless steel clad pate.     

Effects of radial gap and penetration depth on vibration fatigue behaviour of 304L stainless steel piping with socket weld

Abstract

Fatigue related failures of small bore piping caused by vibration of socket welds often occur at nuclear power plants. The purpose of the present study is to evaluate the integrity of the socket weld in nuclear piping under the condition of vibration testing. The specimens were bolted to the shaker table and shaken simultaneously near their resonant frequencies to produce the desired stress. The test results show that the failures at higher stress tend to originate at the toe, while that for the case of lower stress failures tends to occur at the root. The effects of penetration depth and radial gap were also investigated by finite element method. Results show that the higher penetration depth and radial gap can decline the stress distribution in the weld root, which is beneficial to fatigue life. Further, the desired radial gaps can significantly reduce the von Mises stresses, which decrease from 150·4 to 82·7 MPa with the width of radial gap from 0·1 to 0·4 mm.     

The shape of a surface crack in a plate based on a given stress intensity factor distribution

A new method is developed for the evaluation of a crack shape based on a given stress intensity factor (SIF) distribution for a surface crack under Mode-I loading conditions. The SIF distribution along the crack front is investigated using a direct simulation technique, in which the effect of crack closure on fatigue crack growth is considered. Then a SIF distribution function is chosen based on the numerical results. Crack shape (and the SIF) is achieved based on the given SIF distribution function using a numerical iterative procedure. Empirical SIF equations for surface cracks in plates subjected to tension and pure bending fatigue load are determined by systematic curve fitting of the numerical results. The depth ratio and the aspect ratio are considered in the ranges of 0.1–0.9 and 0.2–1.2, respectively. The aspect-ratio variation of surface cracks under fatigue loading is predicted. The application of the new method to predict the shape of a surface crack in plates subjected to tension and pure bending and comparisons of the results obtained with the predictions of the empirical equations proposed by Newman and Raju are presented.     

An investigation of heat transfer and fluid flow on laser micro-welding upon the thin stainless steel sheet (SUS304) using computational fluid dynamics (CFD)

A transient three-dimensional model is numerically developed using the method of computational fluid dynamics (CFD) to characterize some thermal phenomena and characterization of heat transfer and fluid flow in laser micro-welding by considering the heat source and the material interaction leads to rapid heating, melting and thermal cycles in the heating zone. The application of developed thermal models has demonstrated that the laser parameters, such as laser power, scanning velocity and spot diameter, have considerable effects on the peak temperature and resulted weld pool. The heat source model is consisted of surface heat source and adaptive volumetric heat source that could be well represented the real laser welding as the heat penetrates into the material. In the computation of melt dynamics, mass conservation, momentum and energy equations have been considered to compute the effects of melt flow and the thermo-fluid energy heat transfer. The simulation results have been compared with two sets of experimental research to predict the weld bead geometry and solidification pattern, which laser welds are made on thin stainless steel sheet (SUS304). The shape comparison describes those parameters relevant to any changes in the temperatures and melt dynamics are of great importance on the heat distribution and formation of weld pool during laser micro-welding process. The fair agreement between simulated and experimental results, demonstrates the reliability of the computed model.