Stainless Steel Elbows Research Articles

Very low-cycle fatigue failure behaviours of pipe elbows under displacement-controlled cyclic loading

In this study, quasi-static very low-cycle fatigue tests were conducted on pipe elbows at room temperature to investigate the effects of the pipe thickness, internal pressure, and material on the failure behaviours under large-amplitude cyclic loads. Three types of elbow specimens comprising two different materials and two different pipe thicknesses were used in the tests. Elbow specimens were subjected to displacement-controlled, fully reversed, in-plane mode cyclic bending with and without internal pressure. All specimens failed due to through-wall cracking of which the location and orientation depend only on the pipe thickness regardless of the internal pressure and elbow material. For thin-walled elbows of Schedule 40, the axial crack at flank was developed, whereas for thick-walled elbows of Schedule 160, the circumferential crack at intrados. The number of cycles to failure decreased with increasing internal pressure in thin-walled elbows. However, the effect of internal pressure on the number of cycles to failure was negligible in thick-walled elbows. Regardless of internal pressure level, stainless steel elbows were more resistant to failure, compared with carbon steel elbows of identical dimensions. Finally, the strain-based evaluation model considering the void growth and shrinkage effect was applied to predict fatigue failure location and cycles. Two parameters in the model, the multi-axial fracture strain locus and void shrinkage ratio, were determined from monotonic tensile test data and the fatigue life curve of the material. The predicted crack initiation site and orientation were consistent with experimental finding. For all cases, the predicted failure cycles were less but overall close to experimental data.

Root cause analysis for cracked pipe in auxiliary system in nuclear power plant

Penetrated cracks were found in succession in 316LN stainless steel elbows in auxiliary system in a Chinese nuclear power plant after about 19 years’ service. In order to clarify the root cause of the cracking, the cracked elbow with nominal diameter of 12 in. was cut and its chemical composition, hardness, microstructure and residual strain were investigated. The results show that this elbow was heavily cold worked during push-bending and the degree of cold work was within 25∼40%. The manufacturer did not obey RCC-M M3317 to solution treat this elbow before delivery. The heavily cold-worked elbow suffered stress corrosion cracking in occluded primary water.

Ratcheting-fatigue behavior and life prediction of Z2CN18.10 austenitic stainless steel elbow

The ratcheting fatigue behavior of Z2CN18.10 austenitic stainless steel elbow under internal pressure and cyclic loading was studied. The evolution of axial and hoop ratcheting strains at four different positions, respectively at intrados (0°), 45°, crown (90°) and extrados (180°) on the outer surface of the elbow was discussed. It is shown that the hoop ratcheting strain of the elbow is always greater than the axial ratcheting strain. Under cyclic test with force control, the order of ultimate hoop ratcheting strain is intrados (0°)>crown (90°)> 45°> extrados (180°). Under cyclic test with displacement control, the order of ultimate hoop ratcheting strain is 45°> intrados (0°)> crown (90°)> extrados (180°). Under cyclic tests with both force and displacement control, the crown (90°) of the elbow leaked first, which is the critical point to evaluate ratcheting fatigue life. Afterward, the ratcheting fatigue life is predicted based on ASME code, and it is found that the prediction results tend to be non-conservative. Then, a modified Manson-Coffin equation considering ratcheting strain is proposed to predict fatigue life of elbow well.

Novel assessment of in-operations mapped stainless steel elbow pipe internal wall for detection of corrosion and scale deposition

Novel assessment of in-operations mapped stainless steel elbow pipe internal wall for detection of corrosion and scale deposition

Neural network method for the modeling of SS 316L elbow corrosion based on electric field mapping

Abstract Stainless steel is known for its superior corrosion resistance in industrial applications. In this work, corrosion modeling of stainless steel 316L is presented using artificial neural networks. The experimental setup consists of a loop containing stainless steel elbow with simulated seawater of known concentration continuously flowing at a specific flow rate, thus allowing to study the effect of flow dynamics and salt concentration on corrosion. Electric field mapping setup is used to collect the voltage and current information along with the temperature of the elbow section. In addition to modeling, characteristics of the observed scale deposits are also studied in-depth and briefly reported in this work.

Effects of blade structures on the dissolution and gas-liquid mass transfer performance of cup-shaped blade mixers

BackgroundA mixer with the advantages of excellent mixing performance and easy manufacturing is widely needed in industrial applications. In this work, the solid-liquid and gas-liquid mass transfer characteristics of a cup-shaped blade mixer, which is obtained by cutting a 90° stainless-steel elbow, are investigated. MethodsThe effects of blade structure, blade angle, blade height, baffle presence, surface tension, and gas holding on mass transfer characteristics were investigated by the conductivity probe and the dissolved oxygen dynamic method. Significant findingsResults show that the solid-liquid and gas-liquid mass transfer performance is enhanced as the inlet area of the cup-shaped blade is increased and the optimum blade angles are 22.5° and 15° respectively. The solid-liquid mass transfer performance can be improved when the baffles are removed. The gas-liquid mass transfer performance first deteriorates but then improves as the surfactant concentration is increased. Moreover, the mass transfer performance of the cup-shaped blade mixer is superior to that of the 45° -pitched blade and Rushton turbine. Meanwhile, the correlations of the solid-liquid and gas-liquid mass transfer coefficients are obtained, which would provide insight for the industrial design, optimization, and scale-up of cup-shaped blade mixers for multiphase systems.

MODELING OF STEEL ELBOW MANUFACTURING PROCESSES VIA FINITE ELEMENT METHOD AND OPTIMIZATION OF PRODUCTION PARAMETERS

With the advancement of today's technologies, parts of pipe and elbow elements, natural gas pipelines, petrochemicals, machinery manufacturing industry, water distribution and flow systems, sewage systems, foundation piles, shipbuilding industry, nurse containers, aviation, space and defense industry, pipe networks, refineries , steel structures, tube and pipe engineering, etc. widely used in areas. In this study, extrusion method, which is one of the cold forming methods of elbow pipes, has been investigated. When the literature is examined, the production conditions of steel pipes and the problems that occur in production have been examined by different people and suggestions have been made. In this study, the production process of AISI 316L (EN 1.4404) quality stainless steel elbow pipes has been investigated. The mold design model created with the Solidworks program was transferred to the non-linear finite element analysis software ANSYS program for simulations. The process was modeled and simulated in the transient structural analysis section of the ANSYS program. In the process of transforming the pipe into an elbow product, we have learned about the stress distributions and obtaining the total deformation changes on the elbow. The parameters of the production process have been optimized by comparing the work done in the simulation and experimental data. In this way, cost-effective production is aimed by minimizing production costs. The data was extracted by simulating the production environment and conditions.

Ratcheting assessment of corroded elbow pipes subjected to internal pressure and cyclic bending moment

The present study aimed to study ratcheting strains of corroded stainless steel 304LN elbow pipes subjected to internal pressure and cyclic bending moment. To this aim, spherical and cubical shapes corrosion are applied at two depths of 1 mm and 2 mm in the critical points of elbow pipe such as symmetry sites at intrados, extrados, and crown positions. Then, a Duplex 2205 stainless steel elbow pipe is considered as an alternative to studying the impact of the pipe materials, due to its high corrosion resistance and strength, toughness, and most importantly, the high fatigue strength and other mechanical properties than stainless steel 304LN. In order to perform numerical analyzes, the hardening coefficients of the materials were calculated. The results highlight a significant relationship between the destructive effects of corrosion and the depth and shape of corrosion, so that as corrosion increases, the resulting destructive effects increases as well, also, the ratcheting strains in cubic corrosions have a higher growth rate than spherical corrosions. In addition, the growth rate of the ratcheting strains in the hoop direction is much higher across the studied sample than the axial direction. The highest growth rate of hoop strain was observed at crown and the highest growth rate of axial strains occurred at intrados position. Altogether, Duplex 2205 material has a better performance than SS 304LN.

Effect of distance between two elbows in series on erosion for gas dominated conditions

Solid particle erosion is a potential problem in pipelines for the oil and gas processing industries. Solid particle erosion usually occurs in piping components or equipment when the direction of the flow changes, such as in elbows and tees. The distance between two elbows effects on the behavior of solid particle erosion on the second elbow has been investigated experimentally and computationally. Firstly, paint removal experiments were carried out to investigate the erosion pattern in a standard 3-inch (76.2 mm) elbow with a bend radius to pipe diameter (r/D) ratio equal to 1.5 for a gas-sand condition with different particle sizes. The investigated distances between the two elbows are 2D and 12D. Secondly, the erosion magnitudes in a standard 3-inch (76.2 mm) stainless steel elbow were measured using a state-of-the-art ultrasonic technique. These results have been compared with the previous experimental data in a 4-inch (101.6 mm) elbows to investigate the pipe size effect. Thirdly, a commercial CFD code was used to simulate the experimental conditions and compare erosion predictions with the experimental results. Different turbulence and particle rebound models were applied in the CFD simulations to examine their influence on the predicted erosion on the elbows. Finally, the effect of distance between the two elbows on the particle trajectories were investigated using the CFD approach.

Cracking Failure Analysis on 316L Stainless Steel Elbow

The 316L stainless steel elbow used in the pulse soot blower cracked during service, and the cracking occurred at the maximum bending radius of the elbow. There is no obvious plastic deformation at crack and there is much rust deposit on the inner elbow wall of cracked elbow. In order to avoid similar accidents, the methods such as chemical composition analysis, microstructure analysis and crack pattern scanning are adopted to analyze crack reason. The results show that stress corrosion is the main reason to cause crack of 316L stainless steel elbow.

Synergistic Erosion-Corrosion Behavior of AISI 2205 Duplex Stainless Steel Elbows in Potash Brine-Sand Slurry and the Associated Microstructural Changes

Slurry erosion-corrosion damage to materials used in mineral processing industries occurs due to the combined effects of electrochemical and mechanical processes. In this study, AISI 2205 duplex stainless steel elbows were tested in a flow loop using slurries consisting 10, 20, 30, 40 and 50 wt.% sand particles and saturated potash brine. Erosion-corrosion tests were conducted at flow velocities of 2.5 and 4.0 m/s. The results of the gravimetric analysis revealed that erosion-corrosion rates increased with increasing particle concentration at both velocities. The elbows exhibited negative synergy for all operating conditions, which is attributed to the presence of passive oxide film on the surface. Microstructural examination of the eroded surfaces showed that materials removal occurred by cutting and plastic deformation. Electron backscattered diffraction analysis suggests that plastic deformation, which occurred approximately 4 µm below the eroded surface, led to the transformation of metastable austenite phase to martensite.

Stress Corrosion Cracking of a 304 Stainless Steel Elbow

This research was focused on the failure analysis of a cracked SS304 stainless steel elbow from a chemical plant. The service life of the elbow was 2 years and 8 months. The elbow was welded with two straight pipes at two ends. Cracks were located mainly at positions close to the weld joint and the arc where very strong tensile residual stress existed. Corrosion pits were distributed on the inner surface of the elbow. Cracks were initiated from the corrosion pit bottoms. Branching was observed from the cracks. Cl and S elements were detected in corrosion products from the corrosion pits and the cracks by EDS. SEM results reviewed the intergranular and transgranular brittle nature of the fracture. It is concluded that stress corrosion cracking was the main reason for the cracking of the elbow.

Low-cycle fatigue behavior of stainless-steel elbows having local wall thinning

Low-cycle fatigue behavior of stainless-steel elbows having local wall thinning

Buckling characteristics of externally pressurised toroidal shell

ABSTRACT This study investigates the buckling characteristics of toroidal shells with a closed circular cross-section and loaded by static external pressure. With the thickness imperfection considered as the initial geometrical imperfection, the nonlinear buckling numerical calculation of the toroidal shells is discussed in detail. The finite element model was used for the toroidal shell and validated accordingly. The numerical results of mean thickness, variable thickness, and mode imperfection, which was introduced to simulate the real thickness imperfection without the initial thickness imperfection being specified, were compared with the experimental results. In addition, four laboratory-scale models were manufactured and tested. Each of models was assembled by welding four 90° 304 stainless steel elbows. The geometry, toroidal shell thickness, buckling load, and final collapsed mode were measured for each toroidal shell; the material properties of the corresponding sheets were also measured. The results of the theoretical, numerical, and experimental investigations are compared in this paper.

Study on the surface forming mechanism of the solid–liquid two-phase grinding fluid polishing pipe based on large eddy simulation

To study the surface forming mechanism of the solid–liquid two-phase abrasive flow processing, the large eddy simulation method was used. Taking a 90° stainless steel elbow as the research object, the action mechanism of the dynamic pressure, wall shear force, flow state of the abrasive flow at different cross-sections, formation of the vortex, and trajectory of the abrasive flow on the inner surface of the elbow with abrasive flow composed of solid-phase silicon carbide and liquid-phase hydraulic oil are discussed. This study explores the distribution characteristics of the flow pattern of solid–liquid two-phase abrasive flow. Moreover, the wear and erosion of the abrasive grain and the wall of the workpiece are discussed. The mechanism of surface formation of solid–liquid two-phase abrasive flow is revealed.

Ratcheting behavior of pressurized-bending elbow pipe after thermal aging

Z2CND18.12N austenitic stainless steel elbow pipes were studied experimentally under reversed in-plane load of 20 kN and constant pressure of 17.5 MPa after thermal aged 1000 h and 2000 h respectively at the same thermal aging temperature of 500 centigrade degrees (oC). The ratcheting behaviors of pressurized elbow pipes were analyzed compared with original elbow pipe. The results indicate that the ratcheting behavior of pressurized elbow pipe is prominently affected by thermal aging. The uniaxial tensile properties of Z2CND18.12N austenitic stainless steel were also studied under the same condition of thermal aging. It is shown that yield stress of material decreases obviously with the increase of thermal aging period. Chen-Jiao-Kim (CJK) kinematic hardening model was used to evaluate the ratcheting behaviors of pressurized elbow pipes under different thermal aging periods. Simulation results are perfectly agreement with experimental data. Ratcheting shakedown boundaries of thermal aged elbow pipes were determined by CJK model.

Failure of AISI 304H stainless steel elbows in a heat exchanger

Abstract This work investigates the effects of grain boundary oxidation on the failure of AISI 304H stainless steel elbows in a heat exchanger in a power plant after 8 years of service at 400 °C. Optical and electron microscopes, elemental mapping, electron dispersive spectroscopy (EDS), inductively-coupled plasma atomic energy spectroscopy (ICP-AES) and hardness tester were used to understand the failure mechanism. Elemental mapping of the samples operating for 8 years showed high concentrations of oxygen and carbon at the grain boundaries. Results showed that cracks were propagated along the grain boundaries due to the brittleness of the oxide layer at the boundaries. Results confirmed the intergranular and brittle nature of the fracture. Microhardness profilometry showed that oxidized/carburized grain boundaries are significantly harder than matrix inside grains.

Experimental and Numerical Studies on Inelastic Dynamic Behavior of Stainless Steel Elbow Under Harmonic Base Excitation

In the present work, inelastic dynamic behavior of a pressurized stainless steel elbow is studied under harmonic base excitation with the emphasis on strain accumulation known as ratcheting. Initially, sine sweep test is carried out on the long radius stainless steel (SS 304L) elbow to evaluate the free vibration characteristics. Then, incremental harmonic base excitation with the first resonant frequency is applied to the elbow till failure and the resulting response is studied. The tested elbow is analyzed using a simplified method and the simulated ratcheting strain is compared with experimental results. The effect of thickness variation in the elbow on strain accumulation is also studied. Levels of base excitation corresponding to different failure criteria are evaluated and the details are provided in the paper.

Thermal Aging Effect on the Ratcheting Behavior of Pressurized Elbow Pipe

In this study, the ratcheting behaviors of pressurized Z2CN18.10 austenitic stainless steel elbow pipe influenced by the thermal aging process were experimentally investigated in controlled constant internal pressure and reversed in-plane bending after different thermal aging periods (1000 h and 2000 h) at thermal aging temperature of 500 °C. It is shown that the ratcheting behavior of pressured elbow pipe is highly affected by the thermal aging process. The evaluation of ratcheting behavior of pressured elbow pipe was performed using Chen–Jiao–Kim (CJK) kinematic hardening model as a user subroutine of ANSYS. The relationships of yield stress σs and multiaxial parameter χ with thermal aging time were proposed. Ratcheting shakedown boundary of aged elbow pipe was evaluated by CJK model with thermal aging time.

Failure analysis of elbow pipe cracking in a residual heat-removal system of a nuclear power plant

Abstract Failure happened to a 316LN stainless steel elbow pipe of the nuclear power plant (NPP) located at south China. Detailed analysis has been made to figure out the mode and root cause. Non-destructive tests, chemical composition analysis, mechanical property tests, scanning electronic microscopy, energy spectrum and micro-structure analyses, and cold-work elongation analysis have been conducted. Results reveal lots of cold-work slip bands exist in the microstructure. Deformation coefficient is about 32% in the crack regime. The hardness is 390 HV0.1, and cracks present brittle characteristics and propagate along the grain boundary. Furthermore, the fracture possesses crystal characteristics. Thus, indicate failure of elbow pipe occurs due to inter-granular stress corrosion cracking (IGSCC) that penetrates the whole pipe wall, and the root cause of elbow pipe failure is the high-cold-work condition.