High-Performance Bolting Technology for Offshore Oil and Natural Gas Operations (2018) / Chapter Skim
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Appendix K: Threaded Fastener Failure Modes
Appendix K: Threaded Fastener Failure Modes
Pages 226-242
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From page 226... ...
The transition temperature is most commonly determined through Charpy testing, although a notched fracture test in which the sample is loaded in tension can be used. The latter type of test reports a fracture toughness value for crack propagation (KIC)
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From page 227... ...
It can also result from a process referred to as overheating which causes precipitation of the sulfides on the grain boundaries.1,2 In the latter case the ductile fracture mode is intergranular. Brittle fracture occurs with much lower energy absorption.
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From page 228... ...
Hydrogen embrittlement has several characteristics that should be noted.6,7, 8,9,10,11 • The crack growth is time dependent. A crack can propagate in a stable fashion, once it reaches a critical threshold stress intensity, and continue until it reaches a length such that, for a given applied stress, rapid fracture takes over.
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From page 229... ...
This parameter is subject to modifications by the details of the plastic and fracture process zones; hydrogen trapping and concentration dependent diffusion are critical factors. • The threshold stress intensity for cracking is reduced and the crack growth rate is increased by the diffusible hydrogen concentration to a different degree for a given material.
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From page 230... ...
1/2) .13 It should be noted that similar behavior14 occurs in quenched and tempered reactor steels.15 The root cause of SCC of steels in seawater has been shown to be diffusible hydrogen concentration within the alloy regardless of source.16 The stress intensity dependence of subcritical crack growth rate produced in a tempered martensitic 12 pen O systems have continual hydrogen production while closed systems does not.
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From page 231... ...
steel exposed in three separate environments that produce atomic hydrogen at the crack tip are shown in Figure K.2.17 Here H2S and H2 gas are more severe than 3.5 percent NaCl and an overall toughness decreases to the range of 10-20 MPa (m) 1/2 is observed at applied stress intensities above these thresholds, a stage I region is typically followed by a stage II region of crack growth rate that strongly depends on the environment and potential and, in turn, the hydrogen concentration at the crack tip.
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From page 232... ...
700 735 in Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys (R.W. Staehle, ed.)
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From page 233... ...
It is notable that while at very high strength levels, maraging steels are also susceptible to stress corrosion cracking in seawater, they are less susceptible than quenched and tempered 4340 steel at the same strength level due to differences in the microstructure.18,19 Vacuum melted, quenched, and tempered steels with low amounts of trace elements compare even more favorably with maraging steels in resisting stress corrosion at high-strength levels. For example, modern clean UHSS such as Aermet 100TM do not crack from prior austenite grain boundaries but instead exhibit cracking at lath interfaces.20 Unfortunately the drop in toughness with hydrogen content even with a carefully controlled microstructure is still significant.
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From page 234... ...
The cause of HE is the establishment of a high CH,diff at the crack tip governed by chemical and electrochemical factors, as well as crack tip metallurgy, where CH,diff is the diffusible hydrogen content or 24 .F. Brown, "Stress Corrosion Cracking of High Strength Steels," p.
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From page 235... ...
b) KTH versus Eapplied for UNS K92580 showing scanning electron images of the fracture surfaces for applied potentials of −1.1 VSCE and −0.5 VSCE, respectively.
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From page 236... ...
Under cathodic polarization, the crack becomes increasingly alkaline relative to the bulk as a result of proton discharge and water reduction. In addition, the crack tip potential is shifted to more positive potentials due to ohmic voltage drop.
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From page 237... ...
It is readily shown that acidification and crack tip hydrogen uptake occurs in anodically polarized high strength steels. When normalizing to hydrogen content such as CH,diff, it can be seen that in cases of both anodic and cathodic polarization cracking is similar.
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From page 238... ...
For instance, cathodic protection with zinc is usually more severe than with aluminum, and corrosion protection coatings such as zinc or cadmium or other heavy metals not only can promote hydrogen uptake at crack tips but they enable co-deposition of hydrogen during deposition. In studies on propagation of stress corrosion cracks in 18 percent nickel mar aging steel Peterson and his associates found that when the steels was polarized to a potential of −0.77 V by coupling with a cadmium anode, the stress required for crack propagation in seawater was raised to a value close to that for crack propa gation in air.
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From page 239... ...
phases. General HE Susceptibility Nickel-chromium-iron alloys containing more than about 40 percent nickel are not susceptible to stress corrosion cracking by concentrated chloride solutions such as magnesium chloride.
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From page 240... ...
Similar data have been developed for solution heat treated and aged Monel K-500.41 The trend is the same but the details differ and must be understood to evaluate suitability in a given exposure environment. It should be noted that the crack growth rate slows considerably as the applied potential becomes less cathodic.
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From page 241... ...
The CH versus Eapp data was used to predict Kth and could also be adjusted to take into account the IR drop and alkalinity of cathodic crack tips leading to the family of curves relating hydrogen concentration to potential for various recess depths labeled as 0, 10, 5, and 100 mm in a rescaled recess with a 1 mm gap as shown in Figure K.7. Kth can then be predicted as a function of crack depth and applied potential in seawater for aged Alloy 718.
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From page 242... ...
Other high strength nickel base alloys are expected to display the same trends and is regulated by the details of the crack or occlude site geometry. This thinking can be applied to cracks and threaded geometries with confined spaces.
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