Intergranular Oxidation Research Articles

Influence of Heating Temperature on Edge Crack in Hot Rolling of 36%Ni-Fe Alloy

For the purpose of edge crack control in hot rolling of 36%Ni-Fe alloys, a high temperature tensile test and laboratory-scale hot rolling experiment were carried out. Intergranular oxidation has been considered to be one of the major factors in edge cracking. Edge cracks which initiate from intergranular oxidation grow to the inner side along coarse grain boundaries as the thickness reduction ratio increases. The depth of intergranular oxidation increases at higher reheating temperatures. However, recrystallization occurs at the crack tip, and this has the effect of suppressing crack growth. This suggests that promoting recrystallization during hot rolling by increasing the reheating temperature, rather than inhibition of intergranular oxidation as such, is effective for suppressing edge cracking.

Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism

Aeroengine manufacturers have to demonstrate that critical components such as turbine disks, made of DA Inconel 718, meet the certification requirements in term of fatigue crack growth. In order to be more representative of the in service loading conditions, crack growth under hold time conditions is studied. Modelling crack growth under these conditions is challenging due to the combined effect of fatigue, creep and environment. Under these conditions, established models are often conservative but the degree of conservatism can be reduced by introducing the crack growth threshold in models. Here, the emphasis is laid on the characterization of crack growth rates in the low ?K regime under hold time conditions and in particular, on the involved crack growth mechanism. Crack growth tests were carried out at high temperature (550 °C to 650 °C) under hold time conditions (up to 1200 s) in the low ?K regime using a K-decreasing procedure. Scanning electron microscopy was used to identify the fracture mode involved in the low ?K regime. EBSD analyses and BSE imaging were also carried out along the crack path for a more accurate identification of the fracture mode. A transition from intergranular to transgranular fracture was evidenced in the low ?K regime and slip bands have also been observed at the tip of an arrested crack at low ?K. Transgranular fracture and slip bands are usually observed under pure fatigue loading conditions. At low ?K, hold time cycles are believed to act as equivalent pure fatigue cycles. This change in the crack growth mechanism under hold time conditions at low ?K is discussed regarding results related to intergranular crack tip oxidation and its effect on the crack growth behaviour of Inconel 718 alloy. A concept based on an “effective oxygen partial pressure” at the crack tip is proposed to explain the transition from transgranular to intergranular fracture in the low ?K regime.

Advanced microstructural characterization of the intergranular oxidation of Alloy 600

Oxidation of solution-annealed Alloy 600 has been performed in a hydrogenated steam environment, which is considered to simulate PWR primary water exposure. FIB and analytical TEM characterization, coupled with detailed SEM analyses, provided unequivocal evidence of localized grain boundary migration and intergranular oxide penetrations. These localized migrations were associated with pronounced Cr and Fe depletions, highlighting the role of the grain boundaries for Cr diffusion for intergranular oxidation. These findings are discussed in relation to the early stages of intergranular oxidation, diffusion-induced grain boundary migration and subsequent SCC initiation.

Atom probe tomography of stress corrosion crack tips in SUS316 stainless steels

Novel atom probe tomography (APT) data of an intergranular stress corrosion crack tip has been acquired. Using APT for stress corrosion cracking research, very small, localized features and their distribution around the crack tip can be studied in 3D. This work details the development of a technique for the preparation of atom probe needles. Initial characterization via analytical transmission electron microscopy provides with a complementary analysis and accurately locates features that can be correlated with the reconstructed APT data. Ni enrichment and intergranular oxidation ahead of the crack tip have been studied with APT in 3D and with near-atomic resolution.

Multiscale model of metal alloy oxidation at grain boundaries.

High temperature intergranular oxidation and corrosion of metal alloys is one of the primary causes of materials degradation in nuclear systems. In order to gain insights into grain boundary oxidation processes, a mesoscale metal alloy oxidation model is established by combining quantum Density Functional Theory (DFT) and mesoscopic Poisson-Nernst-Planck/classical DFT with predictions focused on Ni alloyed with either Cr or Al. Analysis of species and fluxes at steady-state conditions indicates that the oxidation process involves vacancy-mediated transport of Ni and the minor alloying element to the oxidation front and the formation of stable metal oxides. The simulations further demonstrate that the mechanism of oxidation for Ni-5Cr and Ni-4Al is qualitatively different. Intergranular oxidation of Ni-5Cr involves the selective oxidation of the minor element and not matrix Ni, due to slower diffusion of Ni relative to Cr in the alloy and due to the significantly smaller energy gain upon the formation of nickel oxide compared to that of Cr2O3. This essentially one-component oxidation process results in continuous oxide formation and a monotonic Cr vacancy distribution ahead of the oxidation front, peaking at alloy/oxide interface. In contrast, Ni and Al are both oxidized in Ni-4Al forming a mixed spinel NiAl2O4. Different diffusivities of Ni and Al give rise to a complex elemental distribution in the vicinity of the oxidation front. Slower diffusing Ni accumulates in the oxide and metal within 3 nm of the interface, while Al penetrates deeper into the oxide phase. Ni and Al are both depleted from the region 3-10 nm ahead of the oxidation front creating voids. The oxide microstructure is also different. Cr2O3 has a plate-like structure with 1.2-1.7 nm wide pores running along the grain boundary, while NiAl2O4 has 1.5 nm wide pores in the direction parallel to the grain boundary and 0.6 nm pores in the perpendicular direction providing an additional pathway for oxygen diffusion through the oxide. The proposed theoretical methodology provides a framework for modeling metal alloy oxidation processes from first principles and on the experimentally relevant length scales.

Formation of hole-edge cracks in a combustor liner of an aero engine

A combustor liner from an aircraft engine, which is made of a wrought Ni-based superalloy GH3039, was investigated after service exposure of 1600 engine operating hours. A series of experiments, including macroscopic examination and microscopic examination, fractography observation and thermal exposure experiments, were conducted to investigate the formation mechanism of hole-edge cracks. The results show that microstructural degradation and thermal fatigue should be responsible for the formation of hole-edge cracks in complementary combustion zone (CCZ) of the combustor liner. CCZ is the highest temperature area instead of primary combustion zone (PCZ) during service, which results in significant microstructural degradation, e.g., the stripping of coating and intergranular oxidation, in the hole-edge in CCZ. The cracks initiate at the hole-edge due to coating microstructure degradations and intergranular Al2O3-type oxidation under the coating, and then propagate under the action of thermal fatigue, which results in final failure of the combustor liner.

Corrosion behavior and microstructural evolution of ASTM A182 Grade 92 steel in liquid sodium at 650 °C

ASTM A182 Grade 92 steel was exposed to liquid sodium at 650°C to investigate the effect of oxidation on microstructural evolution. Comparisons were made to the same material exposed to argon gas at 650°C. In the sodium-exposed specimens, intergranular oxidation and chromium depletion were observed at the near-surface. With the argon-exposed specimen, however, this behavior at the surface was not observed. At the near-surface in specimens exposed to 650°C liquid sodium, it was mostly M23C6 carbide that was observed, whereas a M2(W,Mo) phase was predominant in the bulk steel matrix in specimens exposed to both 650°C sodium and argon.

Optimization of processing conditions in plasma activated nitrogen–hydrocarbon carburizing

Problems with intergranular oxidation, energy efficiency and carbon footprint of the conventional endothermic atmosphere (CO–H2–N2) carburizing are forcing heat treating and manufacturing companies to move toward increasingly capital- and operating-cost expensive, low-pressure/vacuum carburizing methods. To offer a less-expensive alternative, a new activated carburizing method has recently been developed, bridging the endothermic atmosphere and vacuum processes, where a plasma-activated, oxygen-free, non-equilibrium nitrogen–hydrocarbons gas blend is utilized. The optimization of activated carburizing for industrial application presented here involved improvement of multi-dimensional case uniformity. As a part of this optimization, a computational fluid dynamics study was conducted for examination of gas flow field inside heat treating furnace and trays holding steel parts treated. To maximize carburizing rate and mitigate soot in non-equilibrium nitrogen–hydrocarbon atmospheres, effects of different gas mixture combinations were investigated. A mixture of N2–0.4% C3H8–1% CH4 mixture has been found to provide a well carburized case with minimized soot in the atmosphere and reduced coke deposition on the metal surface. The soot formation mechanisms for non-equilibrium carburizing atmospheres were also discussed.

Enhanced Oxygen Diffusion Within the Internal Oxidation Zone of Alloy 617 in Controlled Impurity Helium Environments from 1023 K to 1123 K (750 °C to 850 °C)

Alloy 617 was exposed to He-CO-CO2 environments with \( P_{\text{CO}} /P_{{{\text{CO}}_{2} }} \) of either 9 or 1320 at temperatures from 1023 K to 1123 K (750 °C to 850 °C) to determine the oxygen diffusion coefficients within the internal oxidation zone of the alloy. The oxygen diffusion coefficients determined based on both intergranular and transgranular oxidation rates were several orders of magnitude greater than those reported in pure nickel and in nickel-based binary alloys, indicating that the rapid internal aluminum oxidation of Alloy 617 was primarily due to enhanced oxygen diffusion along the incoherent Al2O3-alloy interfaces. The range of activation energy values determined for oxygen diffusion associated with the intergranular aluminum oxidation was from 149.6 to 154.7 kJ/mol, and that associated with the transgranular aluminum oxidation was from 244.7 to 283.5 kJ/mol.

Modeling selective intergranular oxidation of binary alloys.

Intergranular attack of alloys under hydrothermal conditions is a complex problem that depends on metal and oxygen transport kinetics via solid-state and channel-like pathways to an advancing oxidation front. Experiments reveal very different rates of intergranular attack and minor element depletion distances ahead of the oxidation front for nickel-based binary alloys depending on the minor element. For example, a significant Cr depletion up to 9 μm ahead of grain boundary crack tips was documented for Ni-5Cr binary alloy, in contrast to relatively moderate Al depletion for Ni-5Al (∼100 s of nm). We present a mathematical kinetics model that adapts Wagner's model for thick film growth to intergranular attack of binary alloys. The transport coefficients of elements O, Ni, Cr, and Al in bulk alloys and along grain boundaries were estimated from the literature. For planar surface oxidation, a critical concentration of the minor element can be determined from the model where the oxide of minor element becomes dominant over the major element. This generic model for simple grain boundary oxidation can predict oxidation penetration velocities and minor element depletion distances ahead of the advancing front that are comparable to experimental data. The significant distance of depletion of Cr in Ni-5Cr in contrast to the localized Al depletion in Ni-5Al can be explained by the model due to the combination of the relatively faster diffusion of Cr along the grain boundary and slower diffusion in bulk grains, relative to Al.

ICONE23-2021 INTERGRANULAR OXIDATION WITHIN CREVICE OF AUSTENITIC STAINLESS STEEL IN HIGH TEMPERATURE WATER

Intergranular oxidation (corrosion) occurred within crevice of austenitic low-carbon stainless steel (solution treated, almost no applied stress) after immersion in hightemperature water (288°C, 8.5MPa, dissolved oxygen conc. 32ppm, electrical conductivity: 1.2±0.2 μS・cm^<-1> (measured value at 25℃)) for 500h. The intergranular oxidation occurred at specific position within the crevice that is relatively distant from the crevice mouth with relatively low crevice gap. Both the grain boundary and grain matrix were oxidized. In the oxidized area, Fe and Ni were depleted and Cr was enriched compared to the matrix. Maximum penetration depth of the oxidation was approximately 50μm after 500h. In order to understand potential-pH condition within the crevice, surface oxide layer was microscopically and thermodynamically investigated. Thermodynamic properties of the surface oxides near the intergranular oxidized area indicated lowered pH of approximately 3.2 to 3.4. In-situ measurement of local solution electrical conductivity was carried out using small electrodes (dia. 800μm) imbedded into the crevice former plate. The solution pH was estimated using theoretically calculated pH vs. electrical conductivity relationship. In the area where the intergranular oxidation occurred, the solution electrical conductivity was nearly 100 times higher than that of bulk water and which indicated lowered pH of approximately 3.5. The above results suggested that, in the high temperature and relatively high purity water, acidification occurs within crevice of stainless steels and such aggressive corrosion condition result in the intergranular oxidation.

Oxidation of nickel-base welds 182 and 82 in simulated primary water of pressurised water reactors

To date, nickel-base welds 182 and 82 used in pressurised water reactors have shown a higher susceptibility to stress corrosion cracking (SCC) during laboratory tests than in power plants. However, the increasing number of cracks reported in American, Swedish and Japanese nuclear power plants on Alloys 182/82 enlightens the need for a predictive initiation model of SCC. Initiation of SCC involves several variables interacting together such as material microstructure and composition, primary water chemistry and loading history. Building such a model requires studying each contribution of individual phenomenon, in order to have a better understanding of the mechanisms involved at a local scale in crack initiation. Before SCC initiation, some grain boundaries exposed to primary water present intergranular oxidation reaching several hundreds of nanometres. The oxide penetration kinetics could be modified by several parameters such as local metallurgy, dissolved hydrogen content of the primary water and mechanical history before oxidation. The grain boundaries affected by oxidation during a tensile test demonstrate a lower resistance to failure compared to ‘healthy’ grain boundaries. The main objective of this study is to determine key local parameters controlling grain boundary oxidation in order to calibrate an oxidation model which will be integrated to SCC initiation modelling.

Effect of grain size on the oxidation of Fe–13Cr–5Ni alloy at 973 K in Ar–21 vol%O2

Effect of grain refinement from 370 to 34μm on oxidation of transition Fe–13Cr–5Ni alloy at 973K in Ar–21vol%O2 was investigated. The measured weight gains were fitted taking into account a mixed kinetic model with an initial linear and subsequent parabolic growth behaviour. Different scale microstructural evolution was discussed. Contribution of grain refinement to the oxidation is predominantly due to its effect on the inward oxidation process. Decreasing grain size promoted intergranular oxidation, accelerated formation of the Cr-containing inner scale, contributed to the faster transition to the steady state oxidation and correspondingly slightly improved the oxidation resistance.

The effect of weld chemistry on the oxidation of Alloy 82 dissimilar metal welds

Alloy 82 dissimilar weld joints between carbon steel and Alloy 600 were exposed to a simulated primary water environment consisting of hydrogenated steam at 480°C and 1bar. Dilution due to mixing from the parent materials to the weld was significant, particularly in the root where there was Fe enrichment (35at.%) and Cr depletion (10at.%). The heterogeneous composition of the weld from root to crown resulted in differences with respect to surface versus internal oxidation. A Fe-rich surface oxide formed on the weld root which may help to prevent embrittlement and SCC by internal intergranular oxidation.

Grain boundary depletion and migration during selective oxidation of Cr in a Ni–5Cr binary alloy exposed to high-temperature hydrogenated water

High-resolution microscopy of a high-purity Ni–5Cr alloy exposed to 360 °C hydrogenated water reveals intergranular selective oxidation of Cr accompanied by local Cr depletion and diffusion-induced grain boundary migration (DIGM). The corrosion-product oxide consists of a porous, interconnected network of Cr2O3 platelets, with no further O ingress into the metal ahead. Extensive grain boundary depletion of Cr (to

Internal oxidation of Alloy 600 exposed to hydrogenated steam and the beneficial effects of thermal treatment

Alloy 600 in the solution annealed (SA) and thermally treated (TT) conditions was exposed to a hydrogenated steam environment considered to simulate primary water in pressurized water reactors. The likely susceptibility to primary water stress corrosion cracking (PWSCC) was evaluated using the internal oxidation model. A FIB was used to extract cross-sections from Alloy 600 samples and elemental maps were generated; internal oxidation was observed intragranularly in all cases, resulting in expulsion of metallic nickel to the surface. Intergranular oxidation and embrittlement was observed in SA samples, while a thick and dense Cr-rich oxide was formed intergranularly on TT samples.

Investigation of LP Turbine Blade Failure in a Low Bypass Turbofan Engine

Failure of low pressure turbine rotor blade in a low bypass turbofan engine is analyzed to determine its root cause. Forensic and metallurgical investigations are carried out on the blade failed. The failure has originated from the leading edge and has propagated towards the trailing edge. Intergranular features and high oxidation on the fractured surface are the cause of failure which is probably due to creep-stress rupture. This failure has caused extensive damages in low pressure turbine module and also in downstream modules as a secondary effect. Remedial measures are also suggested to prevent such failures.

Directly correlated transmission electron microscopy and atom probe tomography of grain boundary oxidation in a Ni–Al binary alloy exposed to high-temperature water

Intergranular oxidation of a Ni–4Al alloy exposed to hydrogenated, high-temperature water was characterized using directly correlated transmission electron microscopy and atom probe tomography. These combined analyses revealed that discrete, well-separated oxides (NiAl2O4) precipitated along grain boundaries in the metal. Aluminum was depleted from the grain boundary between oxides and also selectively from one side of the grain boundary. The discrete oxide morphology, disconnected from the continuous surface oxidation, suggests intergranular solid-state internal oxidation of Al.

Influence of Heat on Non-Ferrous Objects: Metallurgical Analysis in Fire Scene Investigations

NFPA 921 is a widely accepted standard to perform fire and arson analysis: it states that heat damage on items and components must be recognized to perform a more careful analysis of fire spread and to better define area and point of origin. With reference to metallic materials involved in a fire scene, NFPA 921 only indicate two useful parameters for temperature estimation, i.e. melting temperature and surface oxidation. However, the melting point of metals and alloys may define a wide range of temperatures (for example iron-based alloys normally melt between 1500 and 1600°C); in addition surface oxidation colors can only inform in a qualitative way. To better estimate the temperature range experienced by various items, much more information can be inferred from in-depth metallurgical analyses. Intergranular oxidation, recrystallization, second phase precipitations and incipient melting were some of the metallurgical features easily observable: if the investigator knows the approximate temperature of these microstructure modifications, a better estimate can be done of the “effective fire temperature” that the material might have reached. To this aim some components that can be found in fire a scene (aluminium airshaft, brass tap and copper gas pipe) were exposed to simulated fire conditions of known time durations and peak temperatures. The collected items were then examined by metallurgical techniques, identifying reference temperatures. The obtained results can give useful information to understand the fire scene.

Intergranular crack tip oxidation in a Ni-base superalloy

High-temperature intergranular crack tip oxidation under a single 600 s long sustained tensile load at 700°C was studied for the Ni-base superalloy Allvac 718Plus. High-resolution analytical techniques showed oxidation to take place at and immediately ahead of the tip of an open crack, forming a closed but layered oxide structure about the prior (now oxidized) grain boundary. Near the prior grain boundary the oxide is Ni-rich, with a Co-enriched layer furthest away from the metal and a Fe-enriched region below this. A Cr-rich oxide is present below the outer Ni-rich oxides throughout the crack, also in the direction of crack growth. This is believed to have a hindering effect on oxidation ahead of the crack. Ni3(Nb,Al) γ′ precipitates close to the grain boundaries were found to oxidize and form regions of near-stoichiometric NiO within the oxide layers. Remaining constituents of γ′ (e.g. Al and Nb) were found to be enriched in the surrounding oxidized matrix and also to produce thin oxide layers near the interface between the unoxidized metal and the Cr-rich oxide. The formation of the crack tip oxides is discussed with regard to thermodynamics, kinetics and the influence of applied mechanical load.