A mechanism for hydrogen-induced intergranular stress corrosion cracking in alloy 600

Intergranular stress corrosion cracking (IGSCC) of Alloy 600 in high-temperature, deaerated water or steam has been termed “hydrogen-induced IGSCC.” We believe these cracks are initiated by the nucleation of a high density of bubbles on the grain boundary under the combined action of the applied stress and high pressure methane formed from carbon in solution reacting with hydrogen injected by corrosion. The bubbles then grow together by grain boundary diffusion to give local failure. This is supported by transmission electron microscope (TEM) observations of two-stage replicas, which show the subsurface formation of closely spaced (0.2μm) bubbles along boundaries, and their growth into fine cracks before they open to communicate with the corroding atmosphere. The kinetics are examined and shown to be in quantitative agreement with several experimental observations. This mechanism involves no grain boundary dissolution of the metal, the only role of corrosion being the injection of hydrogen at a high fugacity. It predicts an activation energy roughly equal to that for grain boundary dψusion of nickel in Alloy 600.     

A mechanism for hydrogen-induced intergranular stress corrosion cracking in alloy 600

Intergranular stress corrosion cracking (IGSCC) of Alloy 600 in high-temperature, deaerated water or steam has been termed “hydrogen-induced IGSCC.” We believe these cracks are initiated by the nucleation of a high density of bubbles on the grain boundary under the combined action of the applied stress and high pressure methane formed from carbon in solution reacting with hydrogen injected by corrosion. The bubbles then grow together by grain boundary diffusion to give local failure. This is supported by transmission electron microscope (TEM) observations of two-stage replicas, which show the subsurface formation of closely spaced (0.2μm) bubbles along boundaries, and their growth into fine cracks before they open to communicate with the corroding atmosphere. The kinetics are examined and shown to be in quantitative agreement with several experimental observations. This mechanism involves no grain boundary dissolution of the metal, the only role of corrosion being the injection of hydrogen at a high fugacity. It predicts an activation energy roughly equal to that for grain boundary dψusion of nickel in Alloy 600.     

Solute segregation and hydrogen-induced intergranular fracture in an alloy steel

Hydrogen-induced intergranular fracture of laboratory heats of a 3.5 Ni-1.7 Cr steel doped with P, Sn, or Sb and having a yield strength of 840 MPa and a prior austenite grain size of 120 μm has been compared with that of an undoped steel at a hydrogen pressure of 0.17 MPa (1.68 atm). The intergranular concentrations of the impurities were controlled by varying the time of aging at 480 °C. Cracking of the undoped steel tested in hydrogen occurred along martensitic lath boundaries at high stresses. However, the susceptibility of the doped steels to hydrogen-induced intergranular cracking increased precipitously with impurity concentration. The susceptibility was measured in terms of the threshold stress intensity Kthfor the first detectable crack extension in precracked specimens and in terms of the threshold stress σ_th for microcrack formation in notched specimens. A comparison between the intergranular strength in hydrogen and in air revealed that absorption of hydrogen produced a profound intergranular weakening when the grain boundaries contained even a small amount of a segregated embrittling element. The relative embrittling potencies of P, Sn, and Sb in hydrogen gas were the same as in air. The combined effects of hydrogen and the impurities in reducing intergranular cohesion are discussed in terms of a newly proposed dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack.     

Test temperature and strain rate effects on the properties of a tungsten heavy alloy

Liquid phase sintered tungsten heavy alloy specimens with a 90W-7Ni-3Fe composition were tested for temperature and strain rate effects on mechanical behavior. Both fracture stress and strain were measured for samples tested at 20, 300, or 600 °C, with crosshead speeds ranging from 0.004 to 400 mmJs in an argon atmosphere. Fracture surface examinations showed a dramatic increase in tungsten cleavage as the ductility increased. The effect of an increasing strain rate is a slight strength increase with a concomitant ductility decrease. Alternatively, higher test temperatures degrade strength with a nonsystematic effect on ductility; maximum ductility occurs at 300 °C and a slow strain rate. Surface oxidation at 600 °C greatly degrades ductility. The results are mathematically modeled using classic strain rate dependent equations.     

Bauschinger effect in haynes 230 alloy: Influence of strain rate and temperature

Quasistatic and dynamic Bauschinger behavior in HAYNES 230 alloy is examined. At low strain rate (10⁻³/s), theas- received 230 alloy does not show a drop in flow stress,i.e., noBauschinger effect is displayed. At high strain rate (10³/s), a drop in flow stress of 240 MPa was observed upon stress reversal. In contrast, theprecipitation- strengthened condition exhibited a Bauschinger effect in both low and high strain rate stress-reversal experiments. The magnitude of the Bauschinger effect was found to increase with increasing strain rate, forward strain, and decreasing temperature. The substructure evolution accompanying the forward loading cycles was investigated by transmission electron microscopy and is related to the back stresses that developed. The increased Bauschinger stress drop observed at high strain rate and/or low temperature was correlated to an increased degree of planar slip under these conditions. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10⁻⁵ s⁻¹ to 3×10⁻³ s⁻¹. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10⁻⁵ s⁻¹ and 3×10⁻³ s⁻¹ and beyond 923 K for 3×10⁻⁴ s⁻¹ strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.     

Analysis of hydrogen-induced subcritical intergranular crack growth of iron and nickel

Hydrogen-induced subcritical crack growth of iron and nickel alloys has been shown to occur in both gaseous and aqueous environments and is a concern in a variety of applications. Identification of the mechanisms by which hydrogen induces crack growth in these materials remains an unsettled issue, with hydrogen-enhanced shear and hydrogen-induced decohesion the primary mechanisms under consideration. The purpose of this paper is to report on subcritical crack growth tests of iron and nickel conducted at catholic potentials under conditions in which the crack growth mode was predominantly intergranular. Subcritical crack growth tests were conducted using compact tension samples with thicknesses of 2, 5, and 10 mm at cathodic potentials ranging from −0.6 to −1.25 V for iron and −0.3 to −1.0 V for nickel in 1 N H₂SO₄. Crack growth thresholds and crack velocities were determined as a function of sample thickness and test potential. Plastic zone sizes for iron and nickel were determined using electron channeling patterns, microhardness, and transmission electron microscopy. Comparisons between plastic zone size and crack growth behaviour were made. The hydrogen-induced intergranular subcritical crack growth behavior of iron and nickel were found to exhibit several similarities including K_th and stage I crack growth kinetics. Nickel exhibited stage II behavior with a limiting velocity of about 10⁻⁴ mm/s; iron did not. This limiting velocity was consistent with the rate limiting process being hydrogen diffusion in nickel.     

Equilibrium and growth characteristics of hydrogen-induced intergranular cracking in phosphorus-doped and high purity steels

By means of fracture mechanics analyses, acoustic emission techniques, and fracture surface analyses by scanning Auger microscopy and X-rays, it was determined how segregated phosphorus, yield strength and grain size affect the equilibrium and growth characteristics of hydrogen-induced intergranular cracking in high strength steels. The effect of yield strength on the threshold stress intensity was found to be greater than those of phosphorus segregation and grain size. The intergranular phosphorus segregation greatly accelerated the growth rate of hydrogen-induced intergranular cracking and caused a large number of acoustic signals to be emitted during the crack growth. The crack growth rate increased in a steel with segregated phosphorus and slightly decreased in high purity steels, where only occasional acoustic emissions were measured during the cracking process, by increasing grain size. Fracture surface analyses indicated more featureless intergranular fracture facets and higher levels of residual strain in the lattice adjacent to the fracture surface in the phosphorus-doped steel than in the high purity steels. These results suggest that while in steels with segregated impurities the macrocrack tends to grow by discretely rapid formation of intergranular microcracking which gives rise to dislocation emissions at the growing crack tip, in high purity steels slow growth of intergranular microcracking proceeds which is accompanied by either the absence or substantial mitigation of dislocation generation at the crack tip.     

Effect of the strain rate and test temperature on the mechanical properties of a Ti-Ni-Fe shape memory alloy

The test temperature (from ?196 to +50°C) and the strain rate (from 10^?4 to 10³ s^?1) are found to affect the character of deformation of a shape memory alloy TN1K based on titanium nickelide and alloyed with iron. The shapes of the tensile and compressive curves are shown to depend on the position of the test temperature with respect to the characteristic phase-transition temperatures. The mechanical properties are extremal in the temperature ranges corresponding to the R phase region. As the strain rate increases in the quasi-static range, the strength characteristics of the material increase and the plastic characteristics decrease. As the strain rate increases in the quasi-static range, the yield strength changes analogously; in this case, a yield drop appears in the compressive and tensile stress-strain diagrams. The data obtained are used to optimize the technology of the thermomechanical joints of pipelines and construction elements.     

High temperature-high strain rate fracture of inconel 600

The hot fracture of Inconel 600 has been studied over the temperature range from 800° to 2000°F using a hot torsion tester that is capable of superimposing either axial tensile or compressive stresses on the torsional shearing stresses. Microscopic studies of fracture initiation have been made over the entire temperature region. From 800° to 1200°F fracture initiates at inclusions and propagates by transgranular shear. In the temperature region of minimum ductility, 1300° to 1500°F, fracture initiates at grain boundaries and propagates readily in an intergranular manner. At 1600°F and above, fracture initiates easily at grain boundaries, but because recrys-tallization intervenes crack propagation is difficult and strain to fracture is high. Microcracks initiate at the peak in the torque-twist curve. The higher the temperature the smaller is the strain at which fracture initiates. Correlations have been found between the stress state and the shearing strain at crack initiation and total fracture strain. These correlations show the strong influence of a compressive normal stress on retarding crack initiation and resisting crack propagation.     

The initiation of intergranular failure in inconel-600

The nature of slip interaction with the grain boundaries was examined on the polished surface of Inconel-600 pulled in tension at 370°C and at the initial strain rate of 1.65 × 10^?6 sec^?1. It was shown that the grain boundaries could not always accommodate slip taking place at the boundary regions. This led to the separation of grains at the boundary. Depending on the detailed morphology of the carbides, these intergranular cracks could form at axial strains as low as 10 pet. Since the cracking takes place in the inert atmosphere, it is concluded that the grain boundaries are inherently embrittled by the formation of carbides and/or by solute segregation. There was no evidence of grain boundary sliding, but when carbides were absent highly localized slip in the boundary region gave the appearance of grain boundary offsets. The implications of these observations to stress corrosion cracking are discussed.     

Dynamic strain aging and hydrogen-induced softening in alpha titanium

Compression tests were carried out on samples of commercial-purity titanium charged with up to 4.7 at. pct hydrogen. Strain rates of 10⁻³ to 1 s⁻¹ were employed and testing was limited to the α phase field at temperatures of 773 to 973 K. The dependences of the flow stress on strain, strain rate, and temperature were determined. A plateau or bulge appeared in the temperature and strain-rate dependences of the flow stress, and the work-hardening rate also showed peaks. Serrations were observed on some of the stress-strain curves. All these features indicated that dynamic strain aging (DSA) was occurring. Analysis of the results (together with data from other authors) indicates that there are three ranges of DSA behavior in this material within the experimentally investigated temperature range; these appear to be associated with the diffusion of iron, carbon, and oxygen, respectively. Alloying with hydrogen decreases the magnitude of the DSA attributable to these elements and displaces the phenomenon to higher temperatures and/or to lower strain rates. The dependence on strain rate and temperature of the relative softening attributable to hydrogen addition was determined. The results indicate that hydrogen-induced softening is related to the occurrence of DSA in this temperature range. Possible explanations for this relationship are discussed. O.N. SENKOV, on leave from the Institute of Solid State Physics, Russian Academy of Sciences, Moscow Region 142432, Russia     

Grain-boundary chemistry and intergranular corrosion in alloy 825

Alloy 825, a former candidate material for radioactive high-level waste containers, was investigated to assess its thermal stability and the time-temperature conditions for sensitization. Alloy specimens with a carbon content of 0.01 wt pct in the mill-annealed (MA) and solution-annealed (SA) conditions were studied after thermal exposure to temperatures ranging from 600 °C to 800 °C for periods of up to 1000 hours. Sensitization was evaluated by using corrosion tests that were correlated to grainboundary chemistry analyses. Sensitized microstructures were found to contain M₂₃C₆-type carbides and a chromium-depleted region in the vicinity of the grain boundaries. Thermal aging at 700 °C for 100 hours resulted in the highest sensitization. While heat treatment at 640 °C showed a progressive development of sensitization with time, healing was found to occur after aging at 800 °C for 100 hours. The degree of sensitization, quantified by an equivalent chromium-depleted-zone size, correlates well with the corrosion rate in the nitric acid test. Thermodynamic models were used to calculate the interfacial chromium concentration, chromium depletion profile, and the depleted-zone width. Comparisons between experimental measurements and model calculations indicate that reliable prediction depends on the selection of key model parameters.     

Inconel600合金高温微动磨损特性

采用SRV-Ⅳ高精度微动磨损试验机研究核电材料Inconel600合金的高温微动磨损行为和机制.温度升高有利于黏着区的形成,抑制微滑区的产生,促使摩擦系数和磨损量逐渐减小.摩擦氧化主要发生在环状滑动区,中心黏着区相对很少.高温下氧元素分布较室温下的更加聚集.中心黏着区表面氧含量较低,表层大量存在Ni、Cr和Fe的单质.磨痕表面氧化物由NiO、Cr₂O₃和Fe₃O₄组成.室温和高温条件下磨痕表面中心黏着区和环状滑动区交界处产生了微裂纹,高温下裂纹萌生在微滑区,与室温下相比,高温下裂纹萌生的数量更少,长度更短.     

The effect of strain on hydrogen-induced dislocation morphologies in single crystal iron

The details of the dislocation morphologies in iron single crystal during stage I plastic straining with and without the presence of a concurrent supply of hydrogen have been studied. Hydrogen was shown to greatly enhance the tendency of strain localization especially near hydrogen-containing spherical inclusions. Three different strongly operating slip systems have been identified in the localized dislocation tangled structure, in contrast to the surrounding matrix or the region around the same particles in hydrogen free specimens, where one primary slip system predominated. It was suggested that hydrogen not only enhances the mobility of primary screw dislocations, but also affects the local stress and strain associated with particles, to promote dislocation generation on those slip systems associated with twin formation. If there is no competing slip system, the twinning-sense dislocations catalyzed by hydrogen can lead to the formation of crystallographic identifiable twins. If, on the other hand, a strong anti-twinning primary slip operates, as in the present study, the twinning process is suppressed and the associated dislocations interact locally with the primary system to promote a dislocation tangled structure. Hydrogen-induced microcrack formation was found to have an orientation similar to the strain localization band, suggesting that a direct correspondence exists between the presence of hydrogen-induced strain localization bands and the initiation of hydrogen-induced cracks.     

The effects of temperature and strain rate on the strength of beryllium sheet

The stress-strain behavior and the deformation dynamics of compression rolled and hot upset beryllium sheet prepared from SR powder were investigated in tension over the range 25° to 355°C. The true stress-true strain curves were approximated by the relationσ =σ(0) +hε ^1/2. Both σ(0) andh decreased with temperature. σ(0) was higher at all temperatures for the compression rolled sheet, whereash was the same for the two materials. The difference in σ(0) was in the athermal component of the stress. The effects of temperature and strain rate on the flow-stress of the polycrystalline sheet agreed with those for prism slip in single crystals. Thermal activation analysis of the deformation dynamics yielded values of 123 to 158 b³ for the activation volume, 1.5×10⁷ sec^?1 for the preexponential factor and 1.8 ev (0.18 μb³) for the activation energy,Ho.     

Formation kinetics and rupture strain of Ni-Cr-Fe alloy corrosion films formed in high-temperature water

Nickel alloys such as Alloy 600 undergo stress corrosion cracking (SCC) in pure water at temperatures between about 260 °C and the critical point. Increasing the level of Cr in Ni-Fe-Cr alloys increases SCC resistance in aerated and deaerated water. The mechanism for Cr influence is not understood. The effect of Cr composition on the in-situ oxide rupture strain and corrosion kinetics of Ni-9Fe-Cr alloys was determined experimentally, to evaluate whether the rupture-dissolution model for SCC can account for the effect of Cr on SCC. The alloy corrosion rate and corrosion product oxide microstructure and mechanical properties are strongly influenced by Cr composition. As Cr concentration increases from 5 to 30 pct, oxide rupture strains measured in pressurized water at 288 °C increase from about 8×10⁻⁴ to 2×10⁻³ mm/mm. Corrosion kinetics are parabolic; the corrosion rate first increases and then decreases as Cr increases from 5 to 39 pct. These observations are qualitatively consistent with a rupture-dissolution SCC mechanism. However, parametric modeling of the SCC growth process, applying available creep, oxide rupture strain, and corrosion kinetics data, indicates that the rupture-dissolution mechanism accounts for only a fraction of the effect of Cr on SCC resistance.     

Influence of the temperature and strain rate on the deformability of low-alloy carbon steel

The influence of the temperature, strain, and strain rate on the deformability of low-alloy carbon steel is studied experimentally. Samples are tested on the STD 812 torsional plastometer at Czestochowa Technological University. The hardening of low-alloy carbon steel at different strain rates and temperatures is plotted on the basis of the results for its resistance to deformation. For all the curves, the rate of hardening is high in the initial section, with no relaxation processes. The influence of the strain rate and temperature on the maximum resistance to deformation is quantitatively determined; this is important for practical purposes. The resistance to deformation declines on account of relaxation. The influence of the strain rate and temperature on the mean hardening rate in the range 0 < ε_u < ε_u*is also studied.