Effect of hydrogen on the stress corrosion cracking behavior of X80 pipeline steel in Ku'erle soil simulated solution

Hydrogen was a key factor resulting in stress corrosion cracking （SCC） of X80 pipeline steel in Ku＇erle soil simulated solution. In this article, the effect of hydrogen on the SCC susceptibility of X80 steel was investigated further by slow strain rate tensile test, the surface fractures were observed using scanning electron microscopy （SEM）, and the fracture mechanism of SCC was discussed. The results indicate that hydrogen increases the SCC susceptibility. The SEM micrographs of hydrogen precharged samples presents a brittle quasi-cleavage feature, and pits facilitate the transgranular crack initiation. In the electrochemical impedance spectroscopy （EIS） measurement, the decreased polarization resistance and the pitting resistance of samples with hydrogen indicate that hydrogen increases the dissolution rate and deteriorates the pitting corrosion resistance. The potentiodynamic polarization curves present that hydrogen also accelerates the dissolution rate of the crack tip.

Stress corrosion cracking of X80 pipeline steel exposed to high pH solutions with different concentrations of bicarbonate

Susceptibilities to stress corrosion cracking （SCC） of X80 pipeline steel in high pH solutions with various concentrations of HC03 at a passive potential of-0.2 V vs. SCE were investigated by slow strain rate tensile （SSRT） test. The SCC mechanism and the effect of HC03 were discussed with the aid of electrochemical techniques. It is indicated that X80 steel shows enhunced susceptibility to SCC with the concentration of HCO3 increasing from 0.15 to 1.00 mol/L, and the susceptibility can be evaluated in terms of current density at -0.2 V vs. SCE. The SCC behavior is controlled by the dissolution-based mechanism in these circumstances. Increasing the concentration of HCO3 not only increases the risk of rupture of passive films but also promotes the anodic dissolution of crack tips. Besides, little susceptibility to SCC is found in dilute solution containing 0.05 mol/L HCO3 for X80 steel. This can be attributed to the inhibited repassivation of passive films, manifesting as a more intensive dissolution in the non-crack tip areas than at the crack tips.

Influence factors on stress corrosion cracking of P 110 tubing steel under CO2 injection well annulus environment

Stress corrosion cracking(SCC) behavior of P110 tubing steel in simulated CO_2 injection well annulus environments was investigated through three-point bent tests, potentiodynamic polarization and EIS measurements. The results demonstrate that SCC of P110 tubing steel could occur in acidulous simulated environment, and the sensitivity of SCC increases with the decrease of pH, as well as increase of sulfide concentration and total environmental pressure. Both anodic dissolution and hydrogen embrittlement make contributions to the SCC. Adequate concentration of corrosion inhibitor can inhibit the occurrence of SCC on account of the inhibition of localized anodic dissolution and cathodic hydrogen evolution.

Stress Corrosion Cracking of Nitrogen-containing Stainless Steel 316LN in High Temperature Water Environments

Stress corrosion cracking （SCC） of stainless steels and Ni-based alloys in high temperature water coolant is one of the key problems affecting the safe operation of nuclear power plants （NPPs）. The nitrogen-added stainless steel is a kind of possible candidate materials for mitigating SCC since reducing the carbon content and adding nitrogen to offset the loss in strength caused by the decrease in carbon content can mitigate the problem of sensitization. However, the reports of SCC of nitrogen-added stainless steels in high temperature water are few available. The effects of applied potential and sensitization treatment on the SCC of a newly developed nitrogen-containing stainless steel （SS） 316LN in high temperature water doped with chloride at 250 ℃ were studied by using slow strain rate tests （SSRTs）. The SSRT results are compared with our data previously published for 316 SS without nitrogen and 304NG SS with nitrogen, and the possible mechanism affecting the SCC behaviors of the studied steels is also discussed based on SSRT and microstucture analysis results. The susceptibility to cracking of 316LN SS normally increases with increasing potential. The susceptibility to SCC of 316LN SS was less than that of 316 SS and 304NG SS. Sensitization treatment at 700℃ for 30 h showed little effect on the S CC of 316LN S S and significant effect on the S CC of 316 S S. The predominant cracking mode for the 316LN S S in both annealed state and the state after the sensitization treatment was transgranular. The presented conditions of mitigating stress corrosion cracking are some useful information for the safe use of 316LN SS in NPPs.

Effects of Temperature and Pressure on Stress Corrosion Cracking Behavior of 310S Stainless Steel in Chloride Solution

310S is an austenitic stainless steel for high temperature applications, having strong resistance of oxidation, hydrogen embrittlement and corrosion. Stress corrosion cracking（SCC） is the main corrosion failure mode for 310S stainless steel. Past researched about SCC of 310S primarily focus on the corrosion mechanism and influence of temperature and corrosive media, but few studies concern the combined influence of temperature, pressure and chloride. on SCC of 310S stainless steel, prepared samples are investigated via For a better understanding of temperature and pressure＇s effects slow strain rate tensile test（SSRT） in different temperature and pressure in NACE A solution. The result shows that the SCC sensibility indexes of 310S stainless steel increase with the rise of temperature and reach maximum at 10MPa and 160~C, increasing by 22.3% compared with that at 10 MPa and 80 ℃. Instead, the sensibility decreases with the pressure up. Besides, the fractures begin to transform from the ductile fracture to the brittle fracture with the increase of temperature. 310S stainless steel has an obvious tendency of stress corrosion at 10MPa and 160℃ and the fracture surface exists cleavage steps, river patterns and some local secondary cracks, having obvious brittle fracture characteristics. The SCC cracks initiate from inclusions and tiny pits in the matrix and propagate into the matrix along the cross section gradually until rupture. In particular, the oxygen and chloride play an important role on the SCC of 310S stainless steel in NACE A solution. The chloride damages passivating film, causing pitting corrosion, concentrating in the cracks and accelerated SSC ultimately. The research reveals the combined influence of temperature, pressure and chloride on the SCC of 310S, which can be a guide to the application of 310S stainless steel in super-heater tube.

Hot deformation behaviors and flow stress model of GCr15 bearing steel

The hot deformation behaviors of GCr15 bearing steel were investigated by isothermal compression tests, performed on a Gleeble-3800 thermal-mechanical simulator at temperatures between 950 ℃ and 1 150 ℃ and strain rates between 0.1 and 10 s-1. The peak stress and peak strain as functions of processing parameters were obtained. The dependence of peak stress on strain rate and temperature obeys a hyperbolic sine equation with a Zener-Hollomon parameter. By regression analysis, in the temperature range of 950-1 150 ℃ and strain rate range of 0.1?10 s?1, the mean activation energy and the stress exponent were determined to be 351kJ/mol and 4.728, respectively. Meanwhile, models of flow stress and dynamic recrystallization (DRX) grain size were also established. The model predictions show good agreement with experimental results.

Multiaxial Fatigue Analyses of Stress Joints for Deepwater Steel Catenary Risers

In the present study, the dynamic and fatigue characteristics of two types of stress joints are investigated under ocean environmental condition. Connected with the riser and the platform, stress joint at the vessel hang-off position should be one of the main critical design challenges for a steel catenary riser （SCR） in deepwater. When the riser is under a high pressure and deepwater working condition, the stress state for the joint is more complex, and the fatigue damage is easy to occur at this position. Stress joint discussed in this paper includes two types： Tapered Stress Joint （TSJ） and Sleeved Stress Joint （SSJ）, and multiaxial fatigue analysis results are given for comparison. Global dynamic analysis for an SCR is performed first, and then the local boundary conditions obtained from the previous analysis are applied to the stress joint FE model for the later dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than the yield limit of material and the damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs： for TSJ, the place where the stress joint connects with the riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of the inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. In consideration of various factors, designers should choose the most suitable type and also geometric parameters.

Hot deformation behavior and flow stress model of F40MnV steel

Single hit compression tests were performed at 1 223-1 473 K and strain rate of 0.1-10 s-1 to study hot deformation behavior and flow stress model of F40MnV steel. The dependence of the peak stress, initial stress, saturation stress, steady state stress and peak stain on Zener-Hollomon parameter were obtained. The mathematical models of dynamic recrystallization fraction and grain size were also obtained. Based on the tested data, the flow stress model of F40MnV steel was established in dynamic recovery region and dynamic recrystallization region, respectively. The results show that the activation energy for dynamic recrystallization is 278.6 kJ/mol by regression analysis. The flow stress model of F40MnV steel is proved to approximate the tested data and suitable for numerical simulation of hot forging.

PREDICTION OF FLOW STRESS OF HIGH-SPEED STEEL DURING HOT DEFORMATION BY USING BP ARTIFICIAL NEURAL NETWORK

The hot deformation behavior of TI (18W-4Cr-1V) high-speed steel was investigated by means of continuous compression tests performed on Gleeble 1500 thermomechan- ical simulator in a wide range of tempemtures (950℃-1150℃) with strain rotes of 0.001s-1-10s-1 and true strains of 0-0. 7. The flow stress at the above hot defor- mation conditions is predicted by using BP artificial neural network. The architecture of network includes there are three input parameters:strain rate,temperature T and true strain , and just one output parameter, the flow stress ,2 hidden layers are adopted, the first hidden layer includes 9 neurons and second 10 negroes. It has been verified that BP artificial neural network with 3-9-10-1 architecture can predict flow stress of high-speed steel during hot deformation very well. Compared with the prediction method of flow stress by using Zaped-Holloman parumeter and hyperbolic sine stress function, the prediction method by using BP artificial neurul network has higher efficiency and accuracy.

Pitting Corrosion of 316L Stainless Steel under Low Stress below Yield Strength

Pitting corrosion of 316L stainless steel （316L S S） under various stress was studied by potentiodynamic polarization, electrochemical impedance spectroscopy （EIS） and Mott-Schottky （M-S） analysis in 3.5% NaC1 solution. The results of polarization curves show that, with the increase of the stress, the pitting potentials and the passive current density markedly decrease firstly （180 MPa）, and then increase greatly （200 MPa）. The corresponding surface morphologies of the samples after the polarization test well correspond to the results. Mott-Schottky analysis proved the least C1- adsorbed to the surface of passive film with more positive flat potential, indicating that a moderate stress could increase the pitting corrosion resistance of 316L SS in 3.5% NaC1 solution.

STUDIES ON STRESS TRANSFERENCE MECHANISM OF STEEL FIBRE REINFORCED CONCRETE

The stress transfer mechanism of steel fibre reinforced concrete is studied. The solutions for the stress and displacement were regarded as the superposition of ' the elementary solutions' and ' the improved solutions'. The elementary solutions were found by using two-dimensional elastic theory and the improved solutions were found by using the Love displacement function method. The calculated results indicate that the solutions possess good convergence. By comparing the three-dimensional solutions with the shear-lag solutions, evident difference may be found.

Effects of laser heat treatment on the fracture morphologies of X80 pipeline steel welded joints by stress corrosion

The surfaces of X80 pipeline steel welded joints were processed with a CO2 laser, and the effects of laser heat treatment （LHT） on H2S stress corrosion in the National Association of Corrosion Engineers （NACE） solution were analyzed by a slow strain rate test. The fracture morphologies and chemical components of corrosive products before and after LHT were analyzed by scanning electron microscopy and energy-dispersive spectroscopy, respectively, and the mechanism of LHT on stress corrosion cracking was discussed. Results showed that the fracture for welded joints was brittle in its original state, while it was transformed to a ductile fracture after LHT. The tendencies of hydrogen-induced corrosion were reduced, and the stress corrosion sensitivity index decreased from 35.2% to 25.3%, indicating that the stress corrosion resistance of X80 pipeline steel welded joints has been improved by LHT.

Research on flow stress in ferrite deformation of a Ti-IF steel

The experiments of the ferrite warm deformation of ultra-low carbon （ULC） Ti-IF steel were carded out on a hot simulator and the influences of deformation temperature, strain, and strain rate on the flow stress were analyzed. New flow stress models suitable to ferrite warm forming of Ti-IF steel were given on the basis of analyzing the influence of deformation technology parameters on the flow stress.

Mechanisms of cast structure and stressed state formation in Hadfield steel

The paper describes the investigation of mechanisms of cast structure formation in Hadfield steel depending on the changes in the cooling rate of a casting in the following two temperature ranges: crystallization temperature(1,200-1,390 °С) and the temperature of excessive phase separation(560-790 °С). Changes in the cooling rate of the crystallization temperature range from 1.1 to 25.0 °С·s^(-1) result in the reduction of the average size of austenite grains from 266 to 131 μm. At the same time, the magnitude of developing shrinkage stresses changes from +195 to 0 MPa. When the cooling rate is higher than 16 °С·s^(-1), no shrinkage stresses are formed in the casting. Changes in the cooling rate of the casting in the temperature range of the excessive phase separation influence the number of phases, their morphology and chemical composition, the values of phase stresses, and the possibility of martensitic transformation. Changing in the cooling rate from 0.24 to 5.46 °С·s^(-1) results in the decrease of the amount of the excessive phase from 14.8% to 2.1%, which is composed of eutectic and carbides depending on the cooling rate, their quantitative ratio and morphology change. Such changes in the microstructure are reflected on the changes of value of developing phase stresses. When the cooling rate is 0.24 °С·s^(-1), it is +100 MPa, while the increase of the cooling rate to 1.4 °С·s^(-1) results in the decrease of tensile stresses to 0 MPa and their qualitative stresses change to compressive ones. Further increase of the cooling rate results in the increase of the value of compressive stresses. When the cooling rate is 5.5 °С·s^(-1), their value reaches-92 MPa. Martensite forming in the structure of Hadfield steel is possible if the cooling rate of the casting in the range of excessive phase separation is less than 0.25 °С·s^(-1).

Distribution of Welding Residual Stress of Mixed Steel U-Rib-Stiffened Plates

To analyze the effects of width and thickness of each composition element of mixed steel U-rib-stiffened plates on the welding residual stress distribution, the distribution of the U-rib and the plate residual stress was calculated using a simplified calculation method. The method involved welding the mixed steel U-rib-stiffened plates for a structure with different sizes and different strength ratios of U-rib to plate. Based on a welding residual stress numerical simulation method validated by the blind hole method test, the distribution law of the mixed steel U-rib stiffened plate was studied. The results showed that the change of plate width has little impact on the welding residual stress and that the ratio of the thicknesses of the plate to U-rib stiffeners, the thickness of the plate, and the thickness of the U-rib has a great influence on the distribution of the welding residual stress. The thickness of plate and steel strength also greatly influenced the distribution width of the residual tensile stress. While analyzing the compression capacity of U-rib-stiffened plates, the simplified distribution of welding residual stress was used.

Threshold Stress Intensity of Hydrogen-Induced Cracking and Stress Corrosion Cracking of High Strength Steel

The threshold stress intensity of stress corrosion cracking(SCC) for 40 CrMo steel in 3.5%NaCl solution decreased exponentially with the increase of yield strength.The threshold stress intensity of hydrogen-induced cracking during dynamical charging for 40 CrMo steel decreased linearly with the logarithm of the concentration of diffusible hydrogen.This equation was also applicable to SCC of high strength steel in aqueous solution.The critical hydrogen enrichment concentration necessary for SCC of high strength steel in water decreased exponentially with the increase of yield strength.Based on the results,the relationship between K_(ISCC) and σ_(ys) could be deduced.

Effect of anions on stress corrosion cracking behaviors of ultra-high strength steel 23Co14Ni12Cr3Mo

The effects of chloride,sulfate and carbonate anions on stress corrosion behaviors of ultra-high strength steel 23Co14Ni12Cr3Mo were studied by stress corrosion cracking(SCC)test method using double cantilever beam(DCB)specimens.The SCC morphology was observed by using scanning electron microscopy(SEM)and the composition of corrosion products was analyzed by using energy dispersive spectrometer(EDS).The results show that the crack propagates to bifurcation in NaCl and Na2SO4 solution,while the crack in Na2CO3 solution propagates along the load direction.The SCC rate in NaCl solution is the highest,while lower in Na2SO4 solution and little in Na2CO3 solution.From the SEM morphologies,quasi-cleavage fracture was observed in NaCl and Na2SO4 solutions,but intergranular features in Na2CO3 solution.The mechanism of anion effect on SCC of steel 23Co14Ni12Cr3Mo was studied by using full immersion test and electrochemical measurements.

Possibility of Inducing Compressive Residual Stresses in Welded Joints of SS400 Steels

Since the welded constructions produce easily stress corrosion cracking (SCC) or fatigue disruption in corrosive medium or under ripple load, two methods inducing compressive stress on structural surfaces by anti-welding-heating treatment (AWHT) and explosion treatment (ET) are presented. The results show that they are good ways to resisting SCC on the welded SS400 steel or other components.

Effects of Strain Rate on Stress Corrosion of S355 Steel in 3.5% NaCl Solutions

The stress corrosion of S355 steel in 3.5% NaCl solution under the different strain rates was analyzed with the slow strain rate test（SSRT）, the stress corrosion cracking（SCC） behaviors of S355 steel under the different strain rates in the solution were investigated, and the fracture morphologies and compositions of corrosion products under the different strain rates were analyzed with scanning electron microscopy（SEM） and energy dispersive spectrometerry（EDS）, respectively. The experimental results show that the SCC sensitivity index is the highest when the strain rate is 2×10-6, and the medium corrosion is the main reason resulting in the highest SCC sensitivity index. The SCC sensitivity index is the least when the strain rate is 5×10-6, and the stress is the main reason resulting in the stress corrosion. The SCC sensitivity index is the middle when the strain rate is 9×10-6, the interaction of stress and medium is the stress corrosion fracture mechanism.