划伤690TT合金在高温含氧水中应力腐蚀裂纹萌生的研究

利用划伤技术研究了690TT合金在325 ℃高温含氧硼锂水中的裂纹萌生和生长情况。试样表面和截面显微分析的结果表明,划伤沟槽底部局部萌生了典型的沿晶应力腐蚀裂纹。由于应力集中,在慢速率拉伸阶段划伤沟槽底部产生了机械裂纹,而机械裂纹成为恒载过程中690TT合金沿晶应力腐蚀裂纹萌生和生长的先导。尖端非常接近晶界或者沿着晶界的机械裂纹可继续形成沿晶应力腐蚀裂纹。690TT合金在恒载荷条件下对应力腐蚀开裂仍有一定的敏感性。     

The effect of prior deformation on stress corrosion cracking growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water

The effect of prior deformation on stress corrosion cracking (SCC) growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water environment is studied. The prior deformation was introduced by welding procedure or by cold working. Values of Vickers hardness in the Alloy 600 weld heat-affected zone (HAZ) and in the cold worked (CW) Alloy 600 materials are higher than that in the base metal. The significantly hardened area in the HAZ is within a distance of about 2-3 mm away from the fusion line. Electron backscatter diffraction (EPSD) results show significant amounts of plastic strain in the Alloy 600 HAZ and in the cold worked Alloy 600 materials. Stress corrosion cracking growth rate tests were performed in a simulated pressurized water reactor primary water environment. Extensive intergranular stress corrosion cracking (IGSCC) was found in the Alloy 600 HAZ, 8% and 20% CW Alloy 600 specimens. The crack growth rate in the Alloy 600 HAZ is close to that in the 8% CW base metal, which is significantly lower than that in the 20% CW base metal, but much higher than that in the as-received base metal. Mixed intergranular and transgranular SCC was found in the 40% CW Alloy 600 specimen. The crack growth rate in the 40% CW Alloy 600 was lower than that in the 20% CW Alloy 600. The effect of hardening on crack growth rate can be related to the crack tip mechanics, the sub-microstructure (or subdivision of grain) after cross-rolling, and their interactions with the oxidation kinetics.     

The role of mechanical twinning on microcrack nucleation and crack propagation in a near-γ TiAl alloy

The role of mechanical twinning on microcrack nucleation and crack propagation in a near-γ TiAl alloy was investigated in 4-point bend specimens using selected area channeling patterns and electron channeling contrast imaging to obtain the true crystal orientation image crystal defects. Two types of bend specimens were used, a conventional specimen that was deformed to a surface strain of about 1.4%, but not fractured, and a bi-layer notched specimen with an aluminum backing that prevented catastrophic fracture so that crack arrest features could be examined. Most grains showed activity of 1–4 twinning systems, and their activity correlated closely with the Schmid factor. Grain boundary microcracks were most frequently found where twins interacted with the grain boundary, and crystallographic analysis indicated that the twinning shear caused these cracks to open. In the crack growth specimen, extensive twinning and dislocation activity occurred in the grain with the arrested crack tip, but with continued straining, these same twins caused grain boundary microcracks to form and link-up to form full intergranular cracks, resulting in renucleation of the primary failure crack.     

Understanding the effect of nitrogen in austenitic stainless steel on the intergranular stress corrosion crack growth rate in high temperature pure water

Understanding the effect of nitrogen content on the crack growth rate (CGR) due to intergranular stress corrosion cracking (IGSCC) in high temperature (288 °C) pure water, in non-sensitised and strain-hardened stainless steel (SS) type 304 LN was the focus of this study. Non-sensitised SS containing two different levels of nitrogen (0.08 and 0.16 wt.%) in the solution annealed condition was strain-hardened by cross-rolling at 200 °C (warm rolling). It has earlier been reported that SS with a higher nitrogen level in the warm rolled condition has a higher CGR in high temperature pure water. Tensile testing was carried out using both the SS in the warm rolled as well as in the solution annealed condition at 288 °C. Samples were prepared for transmission electron microscopy (TEM) from the warm rolled SS and from the tensile tested (at 288 °C) specimens. TEM studies indicated that twinning and shear band formation were the major modes of deformation due to rolling at 200 °C and these feature were observed to terminate at grain boundaries, leading to regions of higher strain and stresses at grain boundaries. Higher nitrogen SS has higher grain boundary strain and stresses making the grain boundary regions more susceptible to IGSCC, resulting in higher CGR values. At 288 °C dislocation entanglement and cross-slip were the predominant modes of deformation.     

Orientation dependence of the substructure characteristics in a Ni-30Fe austenitic model alloy deformed in hot plane strain compression

The present work examines in detail the substructure and texture characteristics in a Ni-30Fe austenitic model alloy subjected to deformation by plane strain compression (PSC) at temperatures between 700 and 900 °C to strains between 0.2 and 1 using a strain rate of 1 s⁻¹. The flow curves display characteristics typical of limited dynamic recrystallization. The deformed matrix texture is similar to that expected for rolling/PSC deformation of face-centred cubic metals while the comparatively weaker dynamic recrystallization texture is dominated by the Cube component. The non-Cube deformed matrix grains contain “organized”, self-screening arrays of microbands aligned along the slip planes with high Schmid factors. By contrast, the Cube deformed matrix grains exhibit more “random” cell substructure with a low density of superimposed larger-angle dislocation walls. These walls are related to {1 1 1} slip planes at low strains, but tend to follow the rigid body rotation toward the compression plane at large strains.     

Effect of microstructure on stress corrosion cracking of alloy 600 and alloy 690 in 40% NaOH

Stress corrosion cracking (SCC) behaviors of Alloy 600, Alloy 690 and the Ni-10Cr-10Fe alloy have been studied using a C-ring in 40% NaOH solution at 315°C. The current density of Alloy 690 in polarization curves was higher at 200 mV above corrosion potential than that of Alloy 600. SCC resistance increased with Cr content for the chromium carbide free alloys, probably due to facilitation of SCC crack tip blunting with an increase in Cr content. Both thermally treated Alloy 600 and sensitized Alloy 600 have a comparable amount of intergranular carbide. But the former is more resistant to SCC than the latter, which might be attributed to the presence of the slight Cr depletion around the grain boundary in the former one. Sensitized Alloy 600 showed higher SCC resistance than the solution annealed one due to intergranular carbide in sensitized Alloy 600. This implies that the beneficial effect of intergranular carbide overrides the harmful effects of Cr depletion for sensitized Alloy 600. SCC resistance of Alloy 600 increased with grain size. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOFS),et al.     

Zum Einfluß der mechanischen Belastung auf die interkristalline Spannungsrißkorrosion von unlegiertem Stahl in Carbonat/Bicarbonat-Lösung

Effect of mechanical stress on intergranular stress corrosion cracking of unalloyed steel in carbonate/bicarbonate solution Intergranular stress corrosion cracking of a carbon steel in 0.5 M NaHCO₃ + 0.5 M Na₂CO₃ solution at 70°C was examined by normal, interrupted and cyclic CERT testing. Within the critical potential range, the lengths of intergranular cracks increase with increasing strain rate, and decrease when the material is cold worked. The fracture time indicates that the crack velocity increases with increasing strain rate. This is also observed with tensile specimens subjected to interrupted and cyclic strain rates. Detailed information on crack propagation and threshold stress levels can be obtained by interrupted and cyclic CERT tests. Intergranular cracks arise if surface region of the material undergoes plastic deformation in excess of a definite amount. In the case of a 60°-notch specimen the plastic zone must be greater than 0.3 mm. For cyclically strained specimens the crack velocity can markedly decrease with time or can become even zero as a consequence of strain hardening. From this one can conclude that stress corrosion cracking can only occur if stress levels are very high.     

On the relationship between grain boundary connectivity, coincident site lattice boundaries, and intergranular stress corrosion cracking

The present work reports on refinements proposed to existing models for predicting the distribution of intergranular stress corrosion cracking based on the frequency of “special”, crack-resistant, low-Σ CSL grain boundaries in the microstructure. The accuracy of these models in predicting the extent of intergranular cracking in low-stacking fault FCC materials ranging from Alloy 600 to Admiralty Brass is improved by factors ranging from 2 to more than 6 by discounting isolated, “neutral-twins” from the measured total special boundary fraction. A stochastic model is presented for easily and accurately determining the proportion of neutral-twins, which do not disrupt connectivity in the network of random boundaries based on the relative frequency of twin-variant boundaries (i.e. Σ9s and Σ27s) in the microstructure. Improvements in the predicted susceptibility to intergranular attack achieved from the refinements described herein will enhance current abilities to identify components at high risk of failure by IGSCC based on grain boundary structure.     

Figure of merit for the quality of ZrO2 coatings on stainless steel and nickel-based alloy surfaces

A figure of merit (FOM) has been developed to define the quality of ceramic (e.g., ZrO₂) coatings on metal and alloy [Type 304SS and Alloy 600] surfaces. Zirconia (ZrO₂) coatings were developed as a means of protecting the metal/alloy surfaces from stress corrosion cracking (SCC) in boiling water reactor (BWR) primary heat transport circuits, by inhibiting the cathodic reaction (reduction of oxygen and hydrogen peroxide) on the surface external to the crack. The distribution of pores in the coating plays an important role in corrosion prevention, such that the lower the porosity of the coating, the better the protection afforded to the system against SCC. Since the reactors operate at high temperature (e.g., at 288 °C under full power conditions), the temperature dependence of the FOM was investigated. The figure of merit (FOM) was developed by measuring impedance data over a wide range of frequency (10 mHz-5 kHz) at temperatures of 25, 100, 200, and 288 °C in hydrogenated, buffered solutions, with the hydrogen electrode reaction (HER) being used as a “fast” redox couple. An “equivalent circuit” analog was first developed from the bare surface impedance data and this analog was then employed in a second step to model the pore bottom in defining the pore distribution on the coated surface. A lognormal distribution (LND) of the pores was assumed and the parameters of the LND were determined using a constrained optimization technique to fit the model to the experimental data for the coated surface at different temperatures. The results suggest that, as temperature increases, the coating becomes more porous, making the substrate more susceptible to corrosion cracking. At 288 °C, 87% of the SS and 85% of the Ni-alloy surfaces become porous with the pore radius varying from 0.0001 cm to 0.01 cm.     

Kinetics and driving forces of abnormal grain growth in thin Cu films

The abnormal growth of individual (1 0 0) oriented grains is monitored by the in situ electron backscatter diffraction technique for more than 24 h at three different annealing temperatures (90 °C, 104 °C and 118 °C) in 1-5 μm thick Cu films on polyimide substrates. The (1 0 0) grain growth velocity increases with higher film thickness and annealing temperature, as suggested by an earlier model by Thompson and Carel. As a result, the final (1 0 0) texture fraction becomes more dominant for higher annealing temperatures and larger film thicknesses. The Thompson-Carel model, however, predicts that the (1 1 1) grains will preferably grow at temperatures up to 118 °C. Our calculations of the driving forces revealed that in addition to minimization of the strain energy (due to the thermal mismatch between film and substrate) and of the surface energy, the energy stored in the dislocations plays a decisive role in grain growth. Our observations can be understood by the notion that initially available (1 0 0) grain nuclei start to grow very rapidly, due to dislocation annihilation, and thus “overrun” the (1 1 1) grains in size.     

690合金中晶界网络分布的控制及其对晶间腐蚀性能的影响

利用形变及热处理工艺提高了690合金的低Σ重位点阵（Coincidence Site Lattice, CSL）晶界比例,通过电子背散射衍射（EBSD）技术表征了由不同类型晶界构成的网络特征,结果表明通过晶界工程处理,能够形成以大尺寸“互有Σ3n取向关系晶粒的团簇”显微组织为特征的晶界网络分布,这种显微组织是再结晶过程中多重孪晶充分发展的结果。通过晶间腐蚀浸泡实验表明通过晶界工程处理的样品抗晶间腐蚀性能较未经过晶界工程处理的样品明显提高。腐蚀后样品的显微形貌表明大尺寸“互有Σ3 n 取向关系晶粒的团簇”能够阻止晶间腐蚀向样品内部扩展,并且能够保护下层的显微组织。     

In situ micro-cantilever tests to study fracture properties of NiAl single crystals

In situ micro-cantilever tests were carried out to determine the anisotropic fracture toughness of NiAl single crystals. Notched micro-cantilever beams with a beam length of 8 μm, 1.5 μm thickness and 1.8 μm width were milled in so-called “hard” and “soft” orientations of NiAl using a focused ion beam. These cantilevers were loaded in situ with the help of a cantilever-based nanoindenter mounted inside a scanning electron microscope. A fracture toughness of 3.52 ± 0.29 MPa m^1/2 was obtained for the “soft” orientation and 5.12 ± 0.50 MPa m^1/2 for the “hard” orientation, which is in good agreement with literature values on the fracture toughness of macroscopic NiAl specimens. Furthermore, nanoindentations were performed for studying the size effects occurring at small length scales for both orientations. The applicability of the small sample geometries for testing the fracture toughness is finally discussed in terms of size effects in the flow stress of the material due to dislocation nucleation and strain gradients at the crack tip.     

The effects of film formation and mechanical factors on the initiation of stress corrosion cracking of unalloyed steels in carbonate solutions

Investigations into the intergranular stress corrosion cracking (SCC) of unalloyed steel St 36 in carbonate-bicarbonate solutions have shown that SCC susceptibility is limited to the potential range of magnetite formation for both constant load and constant strain rate tests. Experiments to determine the creep behaviour of the steel in the corrosive solution show that creep rates corresponding to the critical strain rates in the constant strain rate test occur during the crack initiation state after loading of the tensile specimen with a constant crack initiating load. These experimental findings establish, therefore, that critical crack initiating strain rates of the unalloyed steel are always present during the initial stage for SCC regardless of the test method used. The cause of crack initiation at grain boundaries can be attributed to the fact that the magnetite layers feature faults and grooves in the oxide along the grain boundaries in the entire critical potential range. These oxide grooves, which are also observed on unstressed steel, cause a notch-like localized corrosion attack. The depth and crack tip radius of the resultant grain boundary notches appear to be sufficient to initiate propagating cracks under the single action of predominately pure stresses without macroscopic deformation of the steel. This does not necessarily mean, however, that a purely stress induced corrosion process is involved, for it is not out of question that the concentration of stress in the base of grain boundary notches is sufficient to cause localized deformation processes required for crack initiation.     

不同晶粒尺寸5083铝合金晶间腐蚀和应力腐蚀开裂的差异性

通过冷轧和退火获得具有不同晶粒尺寸(8.7～79.2μm)的5083铝合金板.研究其微观结构、晶间腐蚀(IGC)、应力腐蚀开裂(SCC)和裂纹扩展行为.结果表明,粗晶粒样品表现出更好的抗IGC性能,其腐蚀深度为15μm.慢应变速率测试结果表明,细晶粒样品表现出更好的抗SCC性能,敏感性指数ISSRT为11.2％.此外,...     

Effect of ageing heat treatments on the microstructure and intergranular corrosion of powder metallurgy duplex stainless steels

The influence of ageing heat treatments (675 and 875 °C for 1.5 to 48 h) on the microstructure and intergranular corrosion resistance of sintered in nitrogen duplex stainless steels was investigated. The materials were obtained by sintering mixtures of austenitic AISI 316L and ferritic AISI 430L powders. Corrosion behaviour was evaluated by using electrochemical techniques. The beneficial effect of nitrogen on corrosion behaviour of solution annealed samples was established. During ageing, secondary phases were precipitated and the intergranular and transgranular corrosion resistance significantly decreased though repassivation was observed in specimens aged at 875 °C for times up to 8 h.     

Correlation between corrosion resistance properties and thermal cycles experienced by gas tungsten arc welding and laser beam welding Alloy 690 butt weldments

This study investigates the correlation between the thermal cycles experienced by Alloy 690 weldments fabricated using gas tungsten arc welding (GTAW) and laser beam welding (LBW) processes, and their corresponding corrosion resistance properties. The corrosion resistance of the weldments is evaluated using a U-bend stress corrosion test in which the specimens are immersed in a boiling, acid solution for 240 h. The experimental results reveal that the LBW inputs significantly less heat to the weldment than the GTAW, and therefore yields a far faster cooling rate. Moreover, the corrosion tests show that in the GTAW specimen, intergranular corrosion (IGC) occurs in both the fusion zone (FZ) and the heat affected zone (HAZ). By contrast, the LBW specimen shows no obvious signs of IGC.     

The healing behavior of protective oxide scales on heat-resistant steels after cracking under tensile strain

The healing of cracks in protective oxide scales on four heat-resistant steels has been investigated at 800°C in constant-extension-rate tests, in metallographie and in SEM-examinations. The experimental results suggest the existence of a critical strain rate below which crack healing leads to an instant repair of the cracked scale thuspreventing increased internal corrosion. The existence of this critical strain rate can be explained by model considerations, the results of which are in agreement with the results from the experiments. On the basis of scale cracking data from a previous paper and healing data in the present paper scale cracking/healing maps have been developed characterizing the protective capabilities of the oxide scales under tensile strain.     

A new understanding of intergranular stress corrosion cracking resistance of pipeline steel through grain boundary character and crystallographic texture studies

The roles of grain boundary character and crystallographic texture on the intergranular stress corrosion cracking (IGSCC) of API X-65 pipeline steel has been studied using scanning electron microscope (SEM) based electron backscattered diffraction (EBSD) and X-ray texture measurements. It has been found that low angle and special coincident site lattice (CSL) boundaries, mainly Σ11 and Σ13b and, possibly Σ5, are crack-resistant while the CSL boundaries beyond Σ13b and the random high angle boundaries are prone to cracking. However, several cracks were found to have been arrested even when the random high angle grain boundaries were available for them, both at the crack-tips and areas immediately ahead of them, to continue propagating. Texture studies in the vicinities of these crack-arrest regions, as well as in the cracked areas, provided a new understanding of crystallographic orientation-dependent IGSCC resistance: the boundaries of {1 1 0}‖rolling plane (RP) and {1 1 1}‖RP textured grains, mainly associated with 〈1 1 0〉 and 〈1 1 1〉 rotation axes, respectively, were crack-resistant due to their low energy configurations, while the cracked boundaries were mainly linked to the {1 0 0}‖RP textured grains.     

Transgranular SCC mechanism of thermally treated alloy 600 in alkaline water containing lead

This work aims to understand a SCC failure mode of thermally treated steam generator tubing materials in high temperature water containing lead. The effect of lead contents on the anodic polarization curves of alloy 600 (UNS NO6600) and alloy 690 (UNS NO6900) has been studied in a solution of pH 10 at 200 °C and 315 °C. Lead increased the active peaks of alloy 600 and alloy 690 in mild alkaline water at high temperatures. A reduction of PbO to a metallic lead in alloy 690 is easier than that of alloy 600. When lead was added into the solution, a relative ratio of Cr from among the main metallic elements (Cr, Fe, and Ni) of alloy 600 and alloy 690 decreased in the outer corrosion film. Alloy 690 TT showed a transgranular stress corrosion cracking (TGSCC) in a 10 M NaOH solution with 5000 ppm of lead. Intergranular stress corrosion racking (IGSCC) was observed in the 100 ppm lead condition, and some TGSCC was detected on the fracture surface of the alloy 600 MA cracked in the 10,000 ppm lead solution. IGSCC seemed to be retarded by a crack blunting around the grain boundaries, and the TG cracking mode of the thermally treated alloy 600 and 690 seemed to be related to a crack blunting at the grain boundary carbide and a film dissolution by lead in an alkaline solution.     

Plastic and elastic strains in short and long cracks in Alloy 600 studied by polychromatic X-ray microdiffraction and electron backscatter diffraction

The microscopic strain distributions were studied for stress corrosion cracks produced electrochemically in C-rings of Alloy 600 (0.65 Ni, 0.16 Cr, 0.08 Fe). The strain data were obtained using polychromatic X-ray microdiffraction (PXM) and (in part) by electron back-scatter diffraction (EBSD). PXM was used to measure plastic and elastic strain distributions around the tip of a short crack, along with the changes to the direction and shape of the diffraction spots (ellipticity). For a sample with a short (30 μm) crack, the misorientation map showed a well-defined region of plastic deformation along the grain boundary in advance of the crack tip, extending to the next triple point. For the large crack sample, plastic and elastic stains as well as crystalline order could be measured in high detail with respect to the crack path. However, no correlation between these could be obtained, except for a notable degradation of crystalline order near the crack mouth. A comparable EBSD misorientation map shows strong correlation between misorientation and the crack edges; this may in part reflect the role of sharp edges in the more surface-sensitive approach.