Experimental Study on SCC Susceptibility of X60 Steel Using Full Pipe Sections in Near-Neutral pH Environment

2004 ◽  
Author(s):  
Bong Am Kim ◽  
Wenyue Zheng ◽  
G. Williams ◽  
M. Laronde ◽  
J. A. Gianetto ◽  
...  
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Loading Rate ◽  
Crack Depth ◽  
Metallographic Examination ◽  
Depth Measurement ◽  
Neutral Ph ◽  
Crack Tips ◽  
High Crack

Stress corrosion cracking (SCC) tests were performed using the pipe section buried in a clay type of soil with the pH adjusted to near-neutral range. Pipe specimens with various sizes of fatigue pre-cracks ahead of artifical notch tips on the outer surface were subjected to cyclic loading tests. Maximum level of hoop stress was 105% SMYS, and R-value (Ratio of minimum load to maximum load) was 0.5. Growth of cracks was observed from the fatigue crack tips. Fractographic and metallographic examination has confirmed the quasi-cleavage nature of the transgranular SCC that is typically observed in near-neutral pH SCC. Crack depth measurement using DCPD method revealed the relatively high crack growth rate up to 10−5 mm/s. Metallographic examinations showed the existence of many micro-cracks associated with MnS inclusions in the highly strained field ahead of the initial crack tips. The relatively high crack growth rate may be caused by MnS inclusions. The loading rate, dJ/dt, was calculated for each crack condition in order to correlate qualitatively the crack growth rate with the loading rate. J-integral was calculated through non-linear FEM analyses for semi-elliptical cracks based on the stress-strain relationships obtained from the tensile tests using the same X60 steel specimen. Linear relationship was then obtained between the crack growth rate and the loading rate, and therefore the possibility to predict crack growth rates for various loading condition in the field was demonstrated.

Application of Load Sequence to Control Crack Growth in Steel Pipelines Under Near Neutral pH SCC

2016 ◽  
Author(s):  
Olayinka Tehinse ◽  
Weixing Chen ◽  
Jenny Been ◽  
Karina Chevil ◽  
Sean Keane ◽  
...  
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Oil And Gas ◽  
Pressure Fluctuations ◽  
Operating Pressure ◽  
Variable Amplitude ◽  
Neutral Ph ◽  
Novel Approach ◽  
Load Sequence

Pipelines are designed to operate below a maximum operating pressure in service. However, there are pressure fluctuations during operation. The presence of pressure fluctuations creates a drive for crack growth in steel pipes. In order to prevent catastrophic failure of pipelines, there is need for better understanding of the contribution of pressure fluctuations to crack growth rate in steel pipelines. Analysis of pressure fluctuation data in oil and gas pipelines shows that there are different types of fluctuations in a pipe due to friction loss with distance from the pump or compressor station. All these fluctuation types show a form of variable amplitude loading classified in this research as underload, mean load and overload. Studies of some structural systems shows that underload can cause acceleration of crack growth while retardation of crack growth is observed after an overload. This research aims to apply pressure fluctuations to manage integrity of steel pipelines through a novel approach of load sequence involving underload and overload in near neutral pH environment. Clear knowledge of the effect of load interaction involving load sequence of underload and overload is vital to control crack growth in steel pipelines under near neutral pH environment. The result of crack growth rate under different load sequence on X65 steel indicate that increase in overload ratio of 2, 3 and 4 caused an increase in crack growth rate of 1.68E−3, 1.89E−3 and 2.31E−3 mm/block respectively. These results are compared with results from other tests under variable amplitude without load sequence. Analyses were carried out on the morphology of the crack tip and the fracture surface after the test.

Evaluation of Crack Growth Behavior of Alloy 625 Under Cyclic Loading in a Steam Environment at 750°C

2011 ◽  
Author(s):  
Yoichi Takeda ◽  
Hirofumi Sato ◽  
Shuhei Yamamoto ◽  
Takamichi Tokunaga ◽  
Akio Ohji
Keyword(s):  
Growth Rate ◽  
High Temperature ◽  
Cyclic Loading ◽  
Crack Growth Rate ◽  
Crack Length ◽  
Crack Growth ◽  
Loading Rate ◽  
Transition Period ◽  
Growth Behavior ◽  
Crack Growth Behavior

Advanced ultra supercritical (A-USC) steam power generation, in which high-pressure steam is raised to beyond 700°C, is being studied internationally. The creep strength of Ni-based super alloys evaluated at these high temperatures in an air environment makes these materials promising candidates for the material to be used for the structural components of these generators. Since they are exposed to high temperature steam, it is important that the effect of the environment on the degradation of these materials is investigated. In this investigation, the crack growth rate under cyclic loading in a 750°C steam environment using a compact tension specimen was evaluated. Crack length monitoring using the direct current potential drop technique was applied to the growing crack in a high temperature environment in order to evaluate the time-dependent behavior of the crack growth. The dependence of the loading rate and amplitude in terms of the stress intensity factor was obtained. The crack growth rate increased with decreasing loading rate and increasing amplitude. Multiple loading patterns were applied to a single specimen during crack length monitoring. When the loading pattern was changed to a different pattern, in most of the cases, the crack growth rate started to change and then became stable aftera transition period. The influence of intermetallics and different phases on the crack growth behavior is discussed based on the oxidation rate of these phases.

scholarly journals EFFECT OF TEXTURE ON FATIGUE PROPERTIES OF AGE-HARDENED Al ALLOYS UNDER ULTRASONIC LOADING

2012 ◽  
Vol 06 ◽  
pp. 294-299
Author(s):  
HIRONORI MATSUSAKO ◽  
KOHJI KARIYA ◽  
NORIO KAWAGOISHI ◽  
QINGYUAN WANG ◽  
MASAHIRO GOTO
Keyword(s):  
Growth Rate ◽  
Crack Growth ◽  
Al Alloys ◽  
Fatigue Properties ◽  
Crack Growth Behavior ◽  
Loading Frequency ◽  
Slip Planes ◽  
Crack Tips ◽  
High Crack ◽  
The Difference

Effects of texture and loading frequency on the fatigue crack growth behavior of an extruded and a drawn Al alloys of 2017-T4 were investigated under ultrasonic loading frequency (20kHz) in the relative humidity of 25% and 85%, respectively. The extruded alloy has a marked texture of (111) orientation, but this specified orientation is not observed in the drawn alloy. Most of fatigue life was occupied by the growth life of small cracks in the both alloys regardless of humidity. In the low humidity, crack growth was retarded at about 0.3 mm in length in the both alloys. Although crack growth was accelerated by high humidity in the early growth process, there was no or little influence of humidity on the growth rate of cracks over about 0.3 mm in the both alloys. After the retardation of crack growth, fracture surfaces featured with many slip planes in the extruded alloy and many facets in the drawn one, respectively. The difference in growth mechanism between short cracks (<0.3 mm) and longer ones (>0.3 mm) was caused by the environment at crack tips due to high crack growth rate under ultrasonic loading, and that between the both alloys was related to the degree of texture.

The Behaviour of Shallow Cracks in a Pipeline Steel Operating in a Sour Environment

2008 ◽  
Author(s):  
C. M. Holtam ◽  
D. P. Baxter ◽  
I. A. Ashcroft ◽  
R. C. Thomson
Keyword(s):  
Experimental Data ◽  
Growth Rate ◽  
Stress Intensity ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Pipeline Steel ◽  
Crack Depth ◽  
Assessment Methodology ◽  
Material Behaviour ◽  
Ongoing Research

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behaviour. One area where both experimental data and a validated assessment methodology are lacking is the behaviour of shallow cracks. This paper describes the early results of an ongoing research programme aimed at addressing the shortfall in experimental data to characterise material behaviour in the shallow crack regime, with the long-term aim of improving the understanding and assessment of the early stages of environment assisted cracking (EAC). There is an industry need for a better understanding of material behaviour under these conditions, and for the development of a more robust assessment methodology. API 5L X65 pipeline steel parent material was tested in a sour environment with initial flaw sizes in the range 1–2 mm. Fatigue crack growth rate (FCGR) tests have been performed to investigate the influence of crack depth on crack growth rate (da/dN). Initial results suggest that crack growth rates for deep flaws can increase by a factor of 5–100 compared to air depending on the applied stress intensity range (ΔK). Shallow cracks have been shown to grow up to 130 times faster in a sour environment than in air and up to an order of magnitude faster than deep cracks in a sour environment at the same value of ΔK. Constant load tests have also been performed to investigate the influence of crack depth on the threshold stress intensity factor for stress corrosion cracking (KISCC). Preliminary results suggest that in this case there is no crack depth dependence in the range of flaw sizes tested. While further experimental work is required, the results obtained to date highlight the potential non-conservatism associated with extrapolating deep-crack data. Guidance is therefore provided on how to generate appropriate experimental data to ensure that subsequent fitness for service assessments are conservative.

Environmental Effect of Crack Growth Rate of Pipeline Steel in Near-Neutral pH Soil Environments

2004 ◽  
Author(s):  
Weixing Chen ◽  
Robert Sutherby
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Soil Chemistry ◽  
Pipeline Steel ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
General Corrosion ◽  
Neutral Ph

The laboratory work reported here was initiated to determine whether different soils can be shown to give rise to different growth rate for a given pipeline steel. Two soil synthetic environments with different near neutral pH value were designed based on various soil chemistries collected near the pipeline in the field where near-neutral pH SCC was found. The crack growth behavior in both the environments were determined using compact tension specimen. The crack growth rate was in situ monitored by the potential drop system. It was found that soil chemistry has a profound effect on crack growth rate. Although it is insensitive to the soil chemistry and cyclic frequency, the crack growth rate in the high ΔK regime has been significantly enhanced in comparison with that in air. In the low ΔK regime, the growth rate is shown to have minor dependence on ΔK value but strong dependence on the testing environments. The observed crack growth behavior in different ΔK regimes and environments was related to the crack tip sharpness and crack crevice wideness as a result of corrosion and room temperature creep deformation. Soil solutions with low general corrosion rate are associated with a blunt crack tip and wide crack crevice, which would result in lower stress intensity at the crack tip and weaker crack closure effect, respectively. Similarly, a loading wave allowing shorter creep time on a given volume of material at the crack tip at high loading stress tends to produce a sharper crack tip and narrow crack crevice. These two factors have opposite effect on crack growth rate, and the observed crack growth rate reflects the combined effect of these two opposite factors.

Retarding Crack Growth by Static Pressure Hold for Pipeline Steel Exposed to a Near-Neutral pH Environment

2016 ◽  
Author(s):  
Mengshan Yu ◽  
Weixing Chen ◽  
Karina Chevil ◽  
Greg Van Boven ◽  
Jenny Been
Keyword(s):  
Growth Rate ◽  
Cyclic Loading ◽  
Crack Growth Rate ◽  
Crack Initiation ◽  
Crack Growth ◽  
Growth Curve ◽  
Pipeline Steel ◽  
Early Stage ◽  
Pipe Surface ◽  
Neutral Ph

From extensive investigations for over 30 years since the discovery of near-neutral pH stress corrosion cracking (NNpHSCC), the physical processes of crack initiation and growth have been determined, despite that some details in various aspects of crack initiation and growth are still to be understood. The growth curve is a function of crack growth by direct dissolution of steels at localized areas on pipe surface during initiation or at the tip of a crack during early stage of crack growth (the dissolution growth curve), and by a process involving the interaction of fatigue and hydrogen embrittlement (corrosion fatigue, the hydrogen enhanced fatigue growth curve) in Stage II after crack initiation and early stage of crack growth. For the latter case, recent research shows that crack growth rate can be substantially enhanced by variable amplitude cyclic loading. One of the most severe scenarios of cyclic loading in terms of crack growth rate is the underload type of pressure fluctuations that is often found within 30 km downstream of a compressor station. This investigation is aimed to evaluate pressure scenarios that could reduce or retard crack growth during pipeline operation. Specifically, the effect of pressure holds was investigated. Different periods of static hold were performed to an X65 pipeline steel exposed to a near-neutral pH solution. It was found that a static hold at the maximum load for one hour immediately after a large depressurization-repressurization cycle (underload cycle) yielded the lowest crack growth rate, which was about one third of that of constant amplitude fatigue without the static hold. Static holds for a period shorter or longer than one hour have yielded higher crack growth rates. This observation can be applied to field pipelines during operations to retard crack propagation.

Corrosion Fatigue of Simulated C-Mn Steel HAZs in Sour Produced Fluids

2008 ◽  
Author(s):  
R. I. Hammond ◽  
D. P. Baxter
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Fatigue Crack Growth Rate ◽  
Stress Test ◽  
Crack Depth ◽  
Rate Data ◽  
Line Pipe

Steel Catenary Risers are subjected to both fatigue loading (from waves and tides) and corrosive environments (internal and external). The financial benefit of using C-Mn steels for SCRs is significant, therefore there is a need to establish the limitations of the material. These limitations are likely to be controlled by the HAZ, as HAZ microstructures are typically more susceptible than parent steel to cracking during exposure to a sour environment, especially if they have high hardness. Samples of API 5L X65 line pipe steel were heat treated to provide materials exhibiting two microstructures comparable to those seen in girth weld HAZs with two levels of hardness. Fatigue tests were then performed in a sour environment to determine the influence of microstructure, frequency and crack depth on the observed crack growth rate. Tests were performed using a ‘frequency scanning’ technique which involved maintaining a constant ΔK by continually monitoring and shedding the applied load range as crack length increased during the test. Frequency was varied in the range 10–0.01Hz. As there was a concern that the effect of frequency might be masked by an effect of crack depth, a second series of tests was also carried out on each microstructure to explicitly investigate the effect of this latter variable. A stress ratio of 0.5 was used to ensure a relatively high mean stress, to simulate the presence of a tensile residual stress. Test data suggested that harder material exhibited a higher fatigue crack growth rate in the sour environment, and that frequency had only a small effect over the range examined. A second series of tests suggested that crack growth rate was independent of crack depth for crack depths greater than 6mm, although there was some evidence that for shallower flaws the crack growth rate may be higher. It is suggested that when performing fracture mechanics calculations using fatigue crack growth rate data, it is important to ensure that the latter are associated with tests performed on material exhibiting a comparable microstructure to the component being modelled. For relatively shallow flaws it may be the case that crack growth rates in a sour environment are higher (for a given ΔK) than for longer flaws. It should therefore be recognised that in certain applications, defect assessments which use crack growth rate data derived from tests using deeply-cracked specimens may result in non-conservative life predictions.

Analysis of Mixed-Mode Fatigue Crack Growth under Stress Redistribution at Crack Tips

2011 ◽  
Vol 148-149 ◽  
pp. 552-555
Author(s):  
Wen Feng Tu ◽  
Zeng Liang Gao ◽  
Zhao Ji Hu
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Mixed Mode ◽  
Growth Parameters ◽  
Stress Redistribution ◽  
Loading Direction ◽  
Crack Tips

The experiments of mixed-mode I-II fatigue crack growth under stress redistribution at crack tips were conducted with compact specimens made of 16MnR steel. This test consisted of the 1st, the 2nd and 3rd step. When the crack reached a certain length in 1st step, the loading direction was switched to a certain angle. Finally, the loading direction was returned to the original orientation. The stress redistribution of mixed-mode fatigue was formed by altering the loading direction at propagating crack tips. A finite element analysis was also conducted to obtain the and values, and then mixed-mode crack growth parameters were adopted to evaluate the crack growth rate and crack paths. The crack growth paths had a tendency perpendicular and across to the loading axis. Right after the loading direction was changed, the crack growth rate was decreased rapidly, and crack growth paths under the stress redistribution zone (SRZ) were affected.

The Behavior of Shallow Cracks in a Pipeline Steel Operating in a Sour Environment

2009 ◽  
Vol 131 (3) ◽  
Author(s):  
C. M. Holtam ◽  
D. P. Baxter ◽  
I. A. Ashcroft ◽  
R. C. Thomson
Keyword(s):  
Experimental Data ◽  
Growth Rate ◽  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Pipeline Steel ◽  
Crack Depth ◽  
Material Behavior ◽  
Assessment Methodology

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behavior. One area where both experimental data and a validated assessment methodology are lacking is the behavior of shallow cracks. This paper describes the early results of an ongoing research program aimed at addressing the shortfall in experimental data to characterize material behavior in the shallow-crack regime, with the long-term aim of improving the understanding and assessment of the early stages of environment assisted cracking. There is an industry need for a better understanding of material behavior under these conditions and for the development of a more robust assessment methodology. API 5L X65 pipeline steel parent material was tested in a sour environment with initial flaw sizes in the range 1–2 mm. Fatigue crack growth rate tests have been performed to investigate the influence of crack depth on crack growth rate (da/dN). Initial results suggest that crack growth rates for deep flaws can increase by a factor of 5–100 compared with air depending on the applied stress intensity factor range (ΔK). Shallow cracks have been shown to grow up to 130 times faster in a sour environment than in air and up to an order of magnitude faster than deep cracks in a sour environment at the same value of ΔK. Constant load tests have also been performed to investigate the influence of crack depth on the threshold stress intensity factor for stress corrosion cracking (KISCC). Preliminary results suggest that in this case there is no crack depth dependence in the range of flaw sizes tested. While further experimental work is required, the results obtained to date highlight the potential nonconservatism associated with extrapolating deep-crack data. Guidance is therefore provided on how to generate appropriate experimental data to ensure that subsequent fitness for service assessments are conservative.

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