Using finite element and ultrasonic method to evaluate welding longitudinal residual stress through the thickness in austenitic stainless steel plates

This paper uses a 3D thermo-mechanical finite element analysis to evaluate welding residual stresses in austenitic stainless steel plates of AISI 304L. The finite element model has been verified by the hole drilling method. The validated finite element (FE) model is then compared with the ultrasonic stress measurement based on acoustoelasticity. This technique uses longitudinal critically refracted (L_CR) waves that travel parallel to the surface within an effective depth. The residual stresses through the thickness of plates are evaluated by four different series (1 MHz, 2 MHz, 4 MHz and 5 MHz) of transducers. By combining FE and L_CR method (known as FEL_CR method) a 3D distribution of residual stress for the entire of the welded plate is presented. To find the acoustoelastic constant of the heat affected zone (HAZ), a metallographic investigation is done to reproduce HAZ microstructure in a tensile test sample. It has been shown that the residual stresses through the thickness of stainless steel plates can be evaluated by FEL_CR method.     

Stress corrosion cracking of stainless steel pipes for Methyl-Methacrylate process plants

In a Methyl Methacrylate (MMA) plant, tree-like transgranular cracks were found near the weld of a pipe that had been used for transferring MMA material at 110 °C and 0.77 kg/cm². The pipe was made of ASTM A312 TP304 stainless steel.In this study, it was shown that the failure was due to the stress corrosion cracking (SCC) caused by the chloride that remained in the pipe. Corrosion pitting occurred on the inside surface of the pipe. The stress corrosion cracking started from the pits and grew out through the thickness. Concentrated chloride was found in the deposit stuck to the pipe in addition to the pre-process MMA materials. Many work-hardened grains were observed in the area of SCC, providing the evidence of high residual stress due to welding, which could serve as the driving force for the SCC. Recommendations are made for preventing further failure due to SCC in such cases.     

Analysis of material degradation and life assessment of 25Cr–38Ni–Mo–Ti wrought alloy steel (HPM) for cracking tubes in an ethylene plant

Radiant tubes made of wrought 25Cr–38Ni–Mo–Ti alloy steel (HPM) have been in-service for 76,500 h as cracking tubes in an ethylene plant and they are expected to provide reliable service for 100,000 h (11.4 years) or more. During service, the tube inner surfaces were operated at temperature in the range of 820–835 °C within which thermal cracking process occurred. These aged tubes were assessed to ensure continued safe operation. The assessment of material degradation was carried out using optical microscopy, scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) analysis, X-ray powder diffraction (XRD) analysis, Vickers microhardness measurement and stress rupture test to obtain stress–Larson–Miller parameter (LMP) curves for remaining life prediction. Results showed that microstructural degradation was observed at the inner surface of the radiant tubes marked by the damage of protective oxide film containing Cr₂O₃, Fe₂O₃ and SiO₂. Once this film was removed, carburization occurred and free C atoms involved during cracking of ethylene easily penetrated along austenitic grain boundaries. In addition, carbon diffusion into the tube metal seemed to promote precipitation of Cr₂₃C₆ at grain boundaries and within the grains resulting in a sharp increase in hardness. The outer surface of the radiant tubes, on the other hand, was exposed to higher temperature, typically 1040–1100 °C during operation and creep damage seemed to be the main cause of material degradation. Based on stress rupture test, the remaining life of the radiant tubes is expected to be 21,107 h (2.4 years) consistent with the design life. In the present investigation, factors affecting creep are discussed.     

Film condensation heat transfer characteristics of R134a on horizontal stainless steel integral-fin tubes at low heat transfer rate

Condensation heat transfer characteristics of R134a on the integral-fin tubes are experimentally investigated. The test tubes are made of stainless steel, and the root diameter of the tubes is 13.27 mm. The height of fin is 1.19 mm, and the densities of the integral fin are 19 fpi and 26 fpi. The present tests were conducted at the saturation temperatures of 20 °C and 30 °C. The condensation heat transfer coefficients of the tubes having 19 fpi and 26 fpi at the saturation temperature of 20 °C are higher than that of the plain tube by 4.4 and 3.1 times, respectively. When the temperature difference across the condensate film is less than 0.7 °C, the enhancement of the tube of 19 fpi is much larger than that of the tube of 26 fpi. The Honda and Nozu model shows the smallest mean deviation between the estimated values and experimental results among the existing models.     

The effect of thermal cycling in superplastic diffusion bonding of 2205 duplex stainless steel

In view of the requirement of large cold rolling deformation and bonding pressure in the conventional superplastic diffusion bonding of 2205 duplex stainless steel, a novel method of introducing thermal cycling into the process was proposed. During the thermal cycling process, due to the change of temperature, surface chemical activity of 2205 duplex stainless steel was improved, activity of atoms and grain boundaries were improved, and the recrystallized grains were refined. The shear bond strength of joint prepared in the mode of thermal cycling using specimens with the cold roll reduction of 60% was 15 MPa higher than that of conventional bonding using specimens with the cold roll reduction of 85%. Compared to the shear bond strength of 430 MPa under the specific pressure of 10 MPa after conventional bonding, shear bond strength of 623 MPa was obtained under the condition of T_max = 1000 °C, T_min = 900 °C, cycle number of heating and cooling N = 3, and specific pressure P = 5 MPa.     

Numerical model to predict deformation of corrugated austenitic stainless steel sheet under cryogenic temperatures for design of liquefied natural gas insulation system

Austenitic stainless steel exhibits nonlinear hardening behavior at low temperature and under various strain rate conditions caused by the phenomenon of transformation-induced plasticity (TRIP). In this study, a uniaxial tensile test for 304L austenitic stainless steel was performed below ambient temperature (−163, −140, −120, −50, and 20 °C) and at strain rates (10⁻⁴, 10⁻³, and 10⁻² s⁻¹) to identify nonlinear mechanical characteristics. In addition, a viscoplastic damage model was proposed and implemented in a user-defined material subroutine to provide a theoretical explanation of the nonlinear hardening features. The verification was conducted not only by a material-based comparative study involving experimental investigations, but also by a structural application to the corrugated steel membrane of a Mark-III-type cargo containment system for liquefied natural gas. In addition, an accumulated damage contour was represented to predict the failure location by using a continuum damage mechanics approach.     

Stainless steel foams made through powder metallurgy route using NH4HCO3 as space holder

Stainless steel (316) foams of varying porosities have been made through powder metallurgy route using NH₄HCO₃ as a space holder. Green compacts of stainless steel powder with NH₄HCO₃ were sintered at two different temperatures: 1100 °C and 1200 °C. At higher sintering temperatures, neighboring stainless steel powders fused together to form polycrystalline grain structure with iron–chromium intermetallic phases segregated along the grain boundaries. Whereas, the fusion of neighboring stainless steel powders was limited around the particle–particle contact only when the green compacts were sintered at 1100 °C, which resulted in a larger amount of microporosities in the cell wall. These foams exhibited strain hardening behavior in the plateau region under compressive loading. The yield stress and the flow stress (at lower strain levels) of foams, sintered at 1100 °C were higher. But, the reverse is true for the flow stress at higher strain levels. The exponents and the coefficients of the power law relationships varied with sintering temperature and strain levels.     

Investigation of stress corrosion cracks in a UNS S32750 superduplex stainless steel

Duplex and superduplex stainless steels are corrosion resistant alloys with many uses in chemical and petrochemical industries. It is generally accepted that these alloys present stress corrosion resistance superior to austenitic grades, but it does not mean that they are immune to this type of failure. Under severe conditions of temperature, stress, low pH, high chloride and H₂S contents superduplex steels may fail environmentally assisted cracking (EAC). In this work, superduplex UNS S32750 steel specimens were subjected to critical environmental conditions which produced stress corrosion cracks. In the first experiment the material was tested by slow strain rate tensile tests at 80 °C in a solution with 115,000 ppm of chloride, H₂S partial pressure of 6.75 psia, and pH = 3.0. In a second experiment the material was subjected to four bend beam test in a solution similar to experiment 1, but with a H₂S partial pressure of 30.0 psia. Finally, a third test was conducted in a bend plate of superduplex steel subjected to MgCl₂ saturated solution at 154 °C. The cracks produced in the three experiments showed quite different features, which were investigated by optical and scanning electron microscopy.     

Pressure drop and heat transfer during two-phase flow vaporization of propane in horizontal smooth minichannels

This study examined the two-phase flow boiling pressure drop and heat transfer for propane, as a long term alternative refrigerant, in horizontal minichannels. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 5–20 kW m^?2, mass fluxes ranging from 50–400 kg m^?2 s^?1, saturation temperatures of 10, 5 and 0 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and lengths of 1000 mm and 2000 mm, respectively. The present study showed the effect of mass flux, heat flux, inner tube diameter and saturation temperature on pressure drop and heat transfer coefficient. The experimental results were compared against several existing pressure drop and heat transfer coefficient prediction methods. Because the study on evaporation with propane in minichannels was limited, new correlations of pressure drop and boiling heat transfer coefficient were developed in this present study.     

Evaluation of stress corrosion cracking and hydrogen embrittlement in an API grade steel

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of pipeline steels in contact with soil was investigated. Different soils were prepared in order to determine their physical, chemical and bacteriological characteristics. Slow strain rate testing was carried out by using aqueous extracts from soil samples and NS4 standard solution. Stress vs. strain curves of API 5L grade X46 steel were obtained at different electrode potentials (E_corr, 100 mV below E_corr and 300 mV below E_corr) with 9 × 10⁻⁶ s⁻¹ and 9 × 10⁻⁷ s⁻¹ strain rate. In addition, the hydrogen permeation tests were carried out in order to evaluate the susceptibility of hydrogen penetrates into theses steels. The results demonstrated the incidence of cracking and their dependence on the potential imposed. In that case, cracking occurred by stress corrosion cracking (SCC) and the hydrogen embrittlement (HE) had an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant reduction of ductility.     

Influence of temperature on the pitting corrosion behavior of AISI 316L in chloride–CO2 (sat.) solutions

The paper discusses the pitting corrosion behavior of AISI (American iron and steel institute) 316L stainless steel in aerated chloride solutions (0.1–2 M NaCl) at 25, 50 and 80 °C using potentiodynamic polarization technique. A comparison is made with CO₂-saturated chloride solutions. The results have revealed that pitting potential decreased in a logarithmic relationship with the chloride concentration, and decreased linearly with temperature. The influence of CO₂ on the chloride pitting of AISI 316L stainless steel is quite complex and found to be dependent on chloride concentration and test temperature. At 25 °C the presence of CO₂ appears to have insignificant effect on E_p irrespective of chloride concentration. As the temperature is raised to 50 or 80 °C the additions of CO₂ has caused marked negative shifts in pitting potential. The detrimental effect of CO₂ increases with NaCl concentration and temperature. The results indicate that pitting potential (E_p) is influenced by a synergy between chloride, CO₂ and temperature, and that this synergy depends on the chloride concentration and test temperature.     

Hot deformation behavior of a Nb-containing 316LN stainless steel

A Nb-containing 316LN stainless steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s^?1. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. The microstructural evolution was observed and the mechanism of flow instability was discussed. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate. On the contrary, the efficiency of power dissipation increased with them; Flow instability was manifested as cracking and flow localization; The hot deformation equation and the relationships between deformation condition and dynamic recrystallization grain size and fraction were obtained; For Nb-containing 316LN stainless steel, the favorite nucleation sites for dynamic recrystallization are in sequence of triple point, grain boundary, twin boundary and intragranular deformation band; The suggested processing window is given.     

Fabrication of in situ Al2O3 reinforced nanostructure 304 stainless steel matrix composite by self-propagating high temperature synthesis process

304 austenitic stainless steel reinforced by Al₂O₃ particles was prepared by microwave assisted self-propagating high temperature synthesis process using the Fe₂O₃Cr₂O₃NiOAlFe reaction system. Furthermore, effects of mechanical activation of the reactants and the addition of 21.2 wt.% extra Al to the chemical composition of the reactants on the chemical composition of the produced stainless steel was investigated. Atomic absorption spectroscopy analysis results indicated that by the addition of extra Al to the reactant mixture and using 30 minute mechanical activation, stainless steel containing 17.27 wt.% Cr and 7.73 wt.% Ni could be produced with its chemical composition very close to the chemical composition of 304 stainless steel. X-ray diffraction analysis showed that the stainless steel contains nanostructured austenite and ferrite phases. Also microstructural characterizations indicated that there is a uniform distribution of black particles in the steel matrix. Energy dispersive spectroscopy analysis showed that these particles are composed of Al and O elements while the matrix contains Fe, Cr and Ni elements. The presence of Al₂O₃ particles and nanostructure matrix improved the hardness and therefore the wear properties of the composite in comparison with the wrought 304 stainless steel plate.     

Failure analysis of a fluidisation nozzle in biomass boiler and the long-term high temperatures oxidation behaviour of 304 stainless steel

The long-term (> 10 years) oxidation behaviour of stainless steels (SS) at high temperatures was previously unknown. The behaviour was studied through a case study of failure analysis. A fluidisation nozzle made of 304 austenitic SS. After over 100,000 h of service at temperatures of 790–820 °C in a biomass boiler, the nozzle fractured. Failure analysis pinpoints that the nozzle wall temperature fluctuation caused the oxide scale cracking, which intensified the oxidation. The brittle fracture was due to fully oxidization.The long-term oxidation behaviours are distinct from the short-term oxidation behaviours. XRD analysis indicates that the scale was mainly Fe^+ 2Cr₂O₄ and (Fe_0.6 Cr_0.4)₂O₃. ESEM/EDS analysis indicates internal oxidation and sulfidation along the grain boundaries. The different diffusion rates of the Fe, Cr and Ni atoms formed a Ni-rich (48%) layer underneath the scale, and a Cr-rich (35%) core in the remained SS. A schematic is proposed to describe the diffusion mechanism of the internal oxidization and sulfidation behaviour.     

Validity of a new fracture mechanics technique for the determination of the threshold stress intensity factor for stress corrosion cracking (KIscc) and crack growth rate of engineering materials

A novel fracture mechanics technique has been employed for the determination of crack growth rate and threshold stress intensity factor for stress corrosion cracking (K_Iscc) using small circumferential notch tensile (CNT) specimens. The technique was applied successfully for testing SCC susceptibility of heat treated 4340 steel in sodium chloride (NaCl) solutions at room temperature. Crack growth rates have been determined at different stress intensity factors (K_I), and the K_Iscc has been determined to be 15 MPa m^1/2. The obtained K_Iscc value using CNT specimen was found to be very close to K_Iscc values obtained by others using standard specimens.     

Assessment on the use of common correlations to predict the mass-flow rate of carbon dioxide through capillary tubes in transcritical cycles

Capillary tubes are extensively used in small refrigeration and air-conditioning systems with synthetic refrigerants and hydrocarbons. For CO₂ transcritical applications, it has been shown that the capillary tube demonstrates an intrinsic capability of adjusting the upper pressure close to the optimal value in response to changes of gas-cooler heat sink temperature. The CO₂ flow rate through four capillary tubes of various lengths, diameters and materials was measured in a test rig. Each capillary tube was tested with inlet pressure varying from 7.5 MPa to 11 MPa and inlet temperature from 20 °C to 40 °C. Outlet pressure varied from 1.5 MPa to 3 MPa. The experimental results were validated against different numerical and approximate analytical solutions of the capillary tube equations. These models give good predictions only if the friction factor of the capillary tube is calculated accounting for its dependence on the tube roughness.     

Dependence of dynamic strain ageing on strain amplitudes during the low-cycle fatigue of TP347H austenitic stainless steel at 550 °C

Low-cycle fatigue (LCF) tests are carried out on TP347H stainless steel at a strain rate of 8 × 10⁻³ s⁻¹ with total strain amplitudes (Δε_t/2) of ±0.4% and ±1.0%, at room temperature (RT) and 550 °C. It is found that the stress responses and dislocation structures under cyclic loading strongly depend on the value of strain amplitude at 550 °C. Compared with those at the same strain amplitude at RT, the material shows a rapid strain softening, and finally attains a stabilized state at Δε_t/2 = ±0.4% and 550 °C, but the one presents an anomalous behavior, i.e., first a rapid hardening to the maximum stress, followed by a reducing softening at Δε_t/2 = ±1.0% and 550 °C. More cells resulting from dislocation cross-slip and planar structures due to dynamic strain ageing (DSA) restricting cross-slip develop at low strain amplitude of ±0.4% at the first cycle. However, there are more complicated dislocation structures, such as cells, elongated cells, walls/channels and planar structures at Δε_t/2 = ±1.0%. The observations of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) exclude the effects of martensitic transformation, creep, oxidation, and precipitations on these stress responses and microstructure evolutions, which result from DSA appearing at 550 °C.