Gaseous Hydrogen Embrittlement of Materials in Energy Technologies, 2012
The Effects of Deaerated Water on the Toughness of Nickel-Based Alloys
Busby/Environmental Degradation, 2012
ABSTRACT The fracture toughness of nickel based alloys can be degraded when exposed to low temper... more ABSTRACT The fracture toughness of nickel based alloys can be degraded when exposed to low temperature deaerated water. This paper uses elastic plastic fracture mechanics testing to explore some fundamental environmental and metallurgical factors that influence this form of environmentally assisted cracking. Elastic-plastic fracture toughness tests were performed using 0.6T compact tension samples of EN82H weld metal as a function of temperature, hydrogen content in the water, and hydrogen content in the weld metal. These results are used in conjunction with literature data and fundamental understanding to outline the environmental and material conditions that promote susceptibility to this form of hydrogen embrittlement.
This paper discusses the implications of the aqueous hydrogen level stress corrosion cracking (SC... more This paper discusses the implications of the aqueous hydrogen level stress corrosion cracking (SCC) functionality on modeling the temperature dependency (thermal activation energy) of nickel alloy SCC. Prior testing has identified a significant effect of hydrogen level on the SCC of nickel-based alloys in high temperature, high purity water. A maximum in susceptibility occurs near the Ni/NiO phase transition. This functionality has been fundamentality characterized by the extent that the alloy's electrochemical potential (EcP) deviates from the EcP of the Ni/NiO phase transition (∆EcP = EcP Ni/NiO -EcP). The implication of this understanding to the determination of SCC thermal activation energies is that thermal activation energy tests need to be conducted at a constant ∆EcP, not at a constant hydrogen level. Thermal activation energies are often biased high when determined from tests conducted at a constant hydrogen level. Recent testing and analyses show that the true (hydrogen level independent) thermal activation energy for Alloy 600 and X-750 is ~ 35 kcal/mol (146 kJ/mol). In order to better understand the dependence of the crack growth rate on the applied stress intensity factor, SCC growth rate tests were conducted on Alloy 600 as a function of the stress intensity factor and specimen size (0.6T to 2T compact tension specimens). Smaller specimens produced faster crack growth rates at high stress intensity factors (e.g., 66 MPa√m) relative to larger specimens tested at the same stress intensity factor. These results suggest that Alloy 600 stress intensity factor growth rate modeling could be biased by a data set largely populated with "small" specimens (1T CT and less) at high stress intensity factors.
Stress corrosion cracking (SCC) initiation tests were conducted on Alloy 600 and Alloy 600 weld m... more Stress corrosion cracking (SCC) initiation tests were conducted on Alloy 600 and Alloy 600 weld metal (EN82H) at elevated temperatures (315°C to 360°C). Tests were conducted with insitu monitored smooth tensile specimens under a constant load in both hydrogen dearated and aerated environments. The use of uniaxial tensile specimens is an improvement over conventional U-bend specimens in which the stress state is difficult to characterize due to stress relaxation. Three different loading methods were assessed: ring-loading, pressure-loading, and active-loading (i.e., electric actuator or dead weights). Three insitu monitors were investigated: DC-electric potential drop, linear variable differential transformers, and electrochemical noise. Electric potential drop was the only in-situ crack detection method that consistently detected the onset of SCC. While under equivalent initial conditions each loading method initiated SCC, reproducible results were only obtained with pressure and active loading. In ring loading, the extent of SCC initiation was much less relative to active or pressure loading which is likely due to differences in plastic strain. A load history dependency was observed in oxygenated-sulfate solutions. The thermal activation energy was measured as 140 kJ/mol in hydrogenated water. SCC initiation appears to have a dissolved hydrogen dependency similar to SCC crack growth. Preliminary results suggest that the mechanical parameter which controls SCC initiation is plastic strain and not stress.
Studies have shown that grain boundary chromium carbides improve the stress corrosion cracking (S... more Studies have shown that grain boundary chromium carbides improve the stress corrosion cracking (SCC) resistance of nickel based alloys exposed to high temperature, high purity water. However, thermal cycles from welding can significantly alter the microstructure of the base material near the fusion line. In particular, the heat of welding can solutionize grain boundary carbides and produce locally high residual
Strain Rate Dependent Environment Assisted Cracking of α/β-Ti Alloys in Chloride Solution
ASTM International eBooks, Apr 18, 2008
SCC Initiation Testing of Alloy 600 in High Temperature Water
Springer eBooks, 2011
Stress corrosion cracking (SCC) initiation tests have been conducted on Alloy 600 at temperatures... more Stress corrosion cracking (SCC) initiation tests have been conducted on Alloy 600 at temperatures from 304 to 367°C. Tests were conducted with in-situ monitored smooth tensile specimens under a constant load in hydrogenated environments. A reversing direct current electric potential drop (EPD) system was used for all of the tests to detect SCC initiation. Tests were conducted to examine the effects of stress (and strain), coolant hydrogen, and temperature on SCC initiation time. The thermal activation energy of SCC initiation was measured as 103 ± 18 kJ/mol in hydrogenated water, which is similar to the thermal activation energy for SCC growth. Results suggest that the fundamental mechanical parameter which controls SCC initiation is plastic strain not stress. SCC initiation was shown to have a different sensitivity than SCC growth to dissolved hydrogen level. Specifically, SCC initiation time appears to be relatively insensitive to hydrogen level in the nickel stability region.
Strain Rate Dependent Environment Assisted Cracking of α/β-Ti Alloys in Chloride Solution
Environmentally Assisted Cracking: Predictive Methods for Risk Assessment and Evaluation of Materials, Equipment, and Structures
Microstructural and strain rate effects on the environment-assisted cracking of alpha/beta-titanium alloys in aqueous chloride
The objectives of this research are to determine the effect of microstructure and crack tip strai... more The objectives of this research are to determine the effect of microstructure and crack tip strain rate on environment assisted cracking (EAC) of Ti-6Al-2Sn-2Zr-2Mo-2Cr-Si, and to understand crack tip damage in terms of hydrogen/microstructure/plasticity interaction. Beta-extrusion of Ti-6-22-22, in the alpha/beta solution treated and aged (STA) condition, is susceptible to EAC under active loading in NaCl. KJTH is as low
The goal of the work is to provide stress corrosion cracking (SCC) initiation data for Alloy 600 ... more The goal of the work is to provide stress corrosion cracking (SCC) initiation data for Alloy 600 that is not compromised by (1) specimens that suffer from stress relaxation, (2) specimens which have an unknown stress state, (3) specimens which are tested at unknown positions electrochemically relative to the Ni/NiO phase transition, and (4) testing which relies on the period
This investigation combines microstructural characterization, fracture mechanics analyses, atomis... more This investigation combines microstructural characterization, fracture mechanics analyses, atomistic modeling, and experimental crack growth rate data to better elucidate the mechanism of stress corrosion cracking of nickel-based alloys exposed to high temperature, high purity deaerated water. Additionally, this paper develops a mechanistically based equation that is suggested to be generally applicable to SCC of Alloy 600-type alloys exposed to high purity water. Results show that stress corrosion crack tips are truly intergranular, sharp (~5-10 nm crack tip openings), and are well described by moving crack fracture mechanics. These findings, combined with the clear dependency of the crack growth rate on the electrochemical potential and the constancy of the apparent activation energy (see Morton's paper in these proceedings) suggest that the stress corrosion crack growth rate in high purity water is governed by the supply rate of an embrittling species to the crack tip process zone and by the tearing resistance of the material immediately in front of the crack tip (i.e. the local J-R curve). Consideration of both hydrogen and oxygen embrittlement show that both mechanisms are feasible, although there is somewhat more support for a hydrogen mechanism. An example of the crack growth rate model and data fitting procedures are given for Alloy 600 heat affected zone (HAZ) material. Results show that the fitting procedure can have a large effect on model parameters and subsequent extrapolations. For the data considered, nonlinear curve fitting in real space (vice log space) resulted in the most accurate fit. The Alloy 600 HAZ modeling shows that the apparent activation energy for crack growth is lower than is typically reported (91.2 kJ/mol ± 27.4| 95% kJ/mol vice ~130 kJ/mol), the crack growth rate is weakly dependent on the applied stress intensity factor (SCCGR ∝ K 1 ), and the effect of electrochemical potential is significant (~3.6X near Ni/NiO).
Computer modeling the fatigue crack growth rate behavior of metals in corrosive environments
The emphasis of the second phase of this research project has been to develop a single computer p... more The emphasis of the second phase of this research project has been to develop a single computer program that would establish the foundation to incorporate deleterious environmental effects on fatigue crack propagation laws into NASA FLAGRO. The program was the result of team research projects conducted by three undergraduates in the Department of Mechanical and Aerospace Engineering at University of
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Papers by Edward Richey