Intergranular Oxidation Research Articles

High-temperature oxidation behavior of additively manufactured IN625: Effect of microstructure and grain size

High-temperature oxidation of additively manufactured (AM) Ni-base alloy IN625 has been studied in air and Ar-5%H2-3%H2O at 900–1000 °C. AM material is found to oxidize faster than the conventionally manufactured (CM) IN625 due to severe intergranular oxidation observed in the former. The AM IN625 was heat treated at 1100–1250 °C and hot rolled at 980 °C in order to modify the AM microstructure, primarily grain size, and analyze its role in alloy oxidation behavior. Grain size is shown to affect overall oxidation kinetics but not the intergranular oxidation morphology.

Failure analysis of cracking in the welded joints of hydrogen reformer outlet pigtail tubes

Cracking failure of Incoloy 800H welded joints used for about eight months occurred in hydrogen reformer outlet pigtail tubes. Visual inspection, microscopic examination, chemical composition and microhardness testing were performed to find out the failure mechanism. The results indicated that microcracks were discovered in the fusion zone and heat affected zone (HAZ). The crack propagation mode was intergranular and irregular. The cracked regions were occupied with some oxides in the fusion zone. The Cr content at the grain boundaries was abnormal and intergranular oxidation was found in the HAZ. The microcracks in the outlet pigtail tubes were caused by stress relaxation, and oxidation at grain boundaries. The intergranular oxidation caused the embrittlement at the grain boundaries during long-term service. From the investigations, it was believed that the failure of welded joints in the outlet pigtail tubes originated from stress relaxation cracking during high temperature service.

Influence of Flow and Pressure of Carburising Mixture on Low-Pressure Carburising Process Efficiency

Low-pressure carburising (LPC) of steel is an industrially accepted method for improving the properties of a steel surface. LPC is environmentally friendly, does not cause intergranular oxidation and consumes less energy. Its effectiveness depends on the correct choice of process inputs. This paper aims to determine the effect of this type of carboniferous gas, pressure and flow rate on the efficiency of carbon transfer to the surface layer under low-pressure carburisation. A total of 40 disks of 16MnCr5 steel were carburised using pure acetylene or a mixture of acetylene, ethylene and hydrogen as a carboniferous gas, pressures of 2 or 6 hPa and two gas flow rates. The specimens were gravimetrically tested for the increase in the mass of carbon in the carburised layer. The results were analysed with U Mann–Whitney analysis and t-Student test. It was evidenced that carburising with pure acetylene resulted in a higher increase in carbon mass than carburising with the mixture (p < 0.05). Pressure and gas flow rates are important for carburising efficiency (p < 0.05).

High Temperature Oxidation of Additively Manufactured Structural Alloys

Metal alloys produced by additive manufacturing have specific microstructures and compositions. Their microstructure is generally out of equilibrium, textured, and inhomogeneities can be observed. They can have pores and nano-inclusions. The contents of volatile elements may deviate from the nominal composition. These alloys often experience significant internal stresses and present fairly rough surfaces depending on the process used. Hipping these alloys modifies their microstructure and defects. This literature review shows that open porosity can lead to much higher oxidation kinetics than dense materials, with very deep internal oxide penetrations. Nevertheless, when fabricated with optimized parameters, LBM- or EBM-alloys, such as TA6V, 718, or 316L, can behave as well as, or even better than, wrought alloys. The roughness of the surface is not necessarily problematic, but it can lead to local breakaway phenomena and premature spalling on some alloys. It has been verified that the nature of the grain boundaries strongly affects the intergranular oxidation. The effect of chemical segregations on the protective nature of the outer oxide layer has also been reported. Today, too few studies have been devoted to the effect of raw and pre-oxidized surface states after HIP on cyclic oxidation and on hot corrosion of these alloys.

The intergranular oxidation behavior of low-angle grain boundary of alloy 600 in simulated pressurized water reactor primary water

The preferential intergranular oxidation (PIO) behavior of low-angle grain boundaries (LAGBs) with misorientation angle θ ranging from 5.7 to 14.0° on alloy 600 were investigated after exposure to simulated pressurized water reactor primary water. Interestingly, all LAGBs are susceptible to PIO. When θ <8.8°, diffusion-induced grain boundary migration (DIGM) does not occur as Cr is immobile and the PIO depth varies slightly among those LAGBs. As θ increases above 8.8°, Cr starts to diffuse outwards and DIGM occurs. The depths of PIO and DIGM are positively correlated in this case. Also, the PIO depth is not related to the change of θ, but instead poses an inverse relationship with the atom packing density of GB planes.

On the role of intergranular nanocavities in long-term stress corrosion cracking of Alloy 690

The life extension of nuclear power plants beyond 60-years relies on the integrity of the steam generator tubes during the extended life. Alloy 690 steam generator tubes have been generally believed to be resistant to stress corrosion cracking (SCC) failures until the recent unexpected observation of intergranular cavities during long-term SCC testing. Here, a multi-technique characterisation approach has been used to study the behaviour of 30% cold worked samples exposed to long-term SCC testing at constant load. This has provided quantitative results on the effect of intergranular cavities and oxidation ahead of the crack tip. SCC mechanisms of creep cavity formation and associated oxidation through the cavity networks are discussed and the required stress to failure calculated, providing a quantitative measure of their weakening effect and a reliable way of assessing and predicting this important degradation problem.

The environmental degradation behavior of FeNiMnCr high entropy alloy in high temperature hydrogenated water

Co-free high entropy alloy (HEA) 28%Fe-27%Ni-27%Mn-18%Cr shows good irradiation resistance and could be a potential candidate for structural material in nuclear power industry. In this work, the environmental degradation behavior of this HEA was investigated in high temperature hydrogenated water. This HEA shows extensive intergranular cracking after constant extension rate tensile test, indicating that it is very susceptible to stress corrosion cracking (SCC) initiation. The microstructure analysis indicates that Mn and Cr diffuse outwards along the grain boundary during oxidation, inducing grain boundary migration. The Cr flux promotes the formation of Cr-enriched oxide layer above the grain boundary which prevents intergranular oxidation when the sample is unstressed. However, Mn is unstable in the oxide and tends to dissolve into the solution, making the oxide porous. The porous oxide fractures readily when stressed, inducing SCC initiation.

Effect of surface conditions and alloying elements on the early oxidation behaviour of two austenitic alloys in the pure steam environment

The early oxidation behaviour of 347 SS and Alloy 800H with various surface preparation methods, including grinding, mechanical polishing (M−polishing), and electropolishing (E−polishing), were investigated in 650 °C pure steam. For the ground specimens, a protective Cr-rich oxide layer was formed at the M/O interface and lowered the oxidation rate owing to the presence of a nano-crystalline subsurface layer which provided the fast Cr outward diffusion path. On the other hand, M− and E−polished specimens showed significant spallation for 347 SS, while larger weight gain for 800H. Low Cr in 347 SS resulted in the formation of Fe2O3 nodules due to easier Cr2O3 degradation for ground specimen and the absence of Cr-rich oxide at the internal oxidation zone (IOZ)/matrix interface for M− and E−polished specimens. Higher Cr in 800H contributed to duplex Cr2O3 for ground specimen and Cr-rich oxide at IOZ/matrix interface for M− and E−polished specimens. Higher Ni in 800H enhanced Cr outward diffusion and Cr activity at the M/O interface, which improved the oxidation resistance. Low Al content in Alloy 800H showed no positive effect on oxidation resistance due to the Al internal intergranular oxidation.

Long-term isothermal oxidation behavior of two industrial polycrystalline nickel base alloys in air at 700 °C – Evaluation of intergranular oxidation distribution and kinetic

Chromia forming Alloy 718 and AD730™ have been submitted to isothermal long-term air exposure up to 10 000 h at 700 °C. The resulting very low specific mass gain led to consider two alternatives to determine the oxidation kinetics. Measurements of the external layer thickness and of the intergranular oxide amount were carried out on SEM micrographs evidencing the intergranular oxidation contribute around 30% to the total specific mass change. AD730™ with a thicker external scale and a higher amount of intergranular oxide showed a twofold faster oxidation rate than Alloy718. The results demonstrated the need of even longer tests.

Durable Abrasive Tip Design for Single Crystal Turbine Blades

Abstract In order to create and maintain peak efficiency in turbine stages, it is useful to minimize hot air leakage over blade tips. For unshrouded blades, this means creating minimal clearance over the airfoil tip with rub-tolerate zero-gap materials systems. In this study, a refractory abrasive tip material (ABT) was developed and applied to a plain turbine blade tip to work in conjunction with an abradable coating on the tip-shoe. Process development efforts resulted in abrasive material that could be as much as 2 mm thick. Further, the abrasive media included imbedded grits that would remain in back-up until such time as they were exposed and were pressed into service. Application methods of both furnace brazing and induction brazing were explored as were pre- and post-consolidation net shaping. Evaluation testing included compatibility for the application process onto single-crystal blades. Performance testing included rub-rig testing and long-term oxidation testing. Service testing included commercial engine operation for 14,000 h followed by metallurgical reevaluation. Service performance was mechanically successful, although some material transfer from the tip-shoe was observed which decreased the abrasive nature of the tip. Metallurgically, some intergranular oxidation was observed, but the grits themselves were well retained and were sufficiently refractory to avoid microstructural or oxidation degradation. A production implementation of the coating by an induction heating process is shown.

A study on the diffusion-induced grain boundary migration ahead of stress corrosion cracking crack tips through advanced characterization

Three austenitic alloys with different Ni content were stress corrosion cracking (SCC) tested in simulated pressurized water reactor (PWR) primary water at 320 and 360 °C. Diffusion-induced grain boundary migration (DIGM) associated with preferential intergranular oxidation (PIO) is observed ahead of all SCC crack tips. The occurrence of DIGM is revealed to be driven by PIO-induced diffusion of Cr, Fe, and Ni. The extent of DIGM is controlled by the lateral and in-depth elemental diffusion and PIO, with these processes interconnected. Temperature and alloy composition are revealed to affect the extent of DIGM by affecting these processes.

Effect of Intergranular Oxidation on Transition of Fatigue Crack Path of Alloy 617 in Oxidizing Environments at 750°C

Effect of Intergranular Oxidation on Transition of Fatigue Crack Path of Alloy 617 in Oxidizing Environments at 750°C

Effect of dissolved oxygen and hydrogen on the stress corrosion cracking behavior of alloy 600 in high temperature water

The stress corrosion cracking behavior of alloy 600 was studied in high temperature water at 288°C to 360°C. The effects of dissolved oxygen (DO) and dissolved hydrogen (DH) on crack growth rate (CGR) are discussed. Results show that the CGR of alloy 600 in hydrogenated water (at the Ni/NiO phase boundary) is about 2-200 times higher than in 2 ppm O2 oxygenated water at 325°C and 360°C, while the opposite behavior was observed at 288°C. Much more severe intergranular oxidation was observed at the crack tips after testing in DH water than in DO water at 325°C and 360°C. The good correlation between CGR and intergranular selective oxidation suggests that the peak CGRs in DH water might be attributed to the combined contribution of instability of the oxides and intergranular selective oxidation at the crack tip during the phase transition.

Using local GND density to study SCC initiation

Strain and geometric necessary dislocations (GNDs) have been mapped with nm-resolution around grain boundaries affected by stress corrosion cracking (SCC) or intergranular oxidation with the aim of clarifying which local conditions that trigger SCC initiation of Alloy 600 in primary water reactor (PWR) water environment. Regions studied included the cracked and uncracked portion of the same SCC-affected grain boundaries and a comparable grain boundary in the as-received condition. High-resolution “on-axis” Transmission Kikuchi Diffraction (TKD) was used to generate strain and GND density based on the cross-correlation image processing method to probe shifts of specific zone axis in the TKD patterns from all regions. All cracked boundaries analyzed had local GND densities higher than 1 × 1016 m-2. Similar grain boundaries, from as-received samples had GNDs of 5 × 1014 m-2, while an intermediate level was found in the oxidized but uncracked portion of the same GB. Results, together with a discussion on the advantages and limitations of the approach, will be presented.

Intergranular oxidation of Ni-base alloy 718 with a focus on additive manufacturing

The intergranular oxidation in air at 850 °C of alloy 718 produced by laser beam melting and electron beam melting was compared to that of the wrought alloy. Quantitative analyses revealed that the amount of grain boundary oxidation was similar for these alloys. However, the additively manufactured ones presented deeper and thicker oxides at grain boundaries, due to grain size heterogeneity and to a smaller number of special boundaries. Results show that intergranular oxidation kinetics follows Wagner’s theory on internal oxidation considering not only O diffusion at the intergranular oxide/metal interfaces but also Al and Ti diffusion in the bulk.

Surface oxidation and crystallization of FeNi-Based soft magnetic nanocrystalline and amorphous nanocomposite alloys

In this work, we present characterization of surface oxidation and crystallization behavior of Metal Amorphous Nanocomposite (MANC) (Fe 70 Ni 30 ) 80 Nb 4 Si 2 B 14 alloys via complementary x-ray diffraction, electron microscopy, and x-ray photoelectron spectroscopy techniques. An adherent amorphous oxide of roughly 15–20 nm in thickness is observed on the wheel side of the ribbon, enriched in Fe relative to Ni and with significant content of Si and Nb as would be expected based upon a comparison of the Gibbs free energy of formation of the stable oxide phases. The amorphous character of the oxide is likely to result from the significant Si-content coupled with the amorphous and nanocrystalline character of the underlying metallic layer. Higher density of FeNi-enriched nanocrystals is also observed within ∼100 nm of the wheel-side surface, presumably due to reduction in local glass former concentration. A rough, crystalline copper oxide layer is also present and forms through contact between the copper wheel and the solidifying melt during the rapid solidification process. On the air side of the ribbon, a crystalline oxide layer of ∼15–20 nm is formed as well as intergranular oxidation that penetrates along the grain boundaries between highly textured surface crystallites of ∼50–100 nm in size. The crystalline nature of the oxide on these surfaces is asserted to result from the lack of significant Si-content and the pre-existing surface crystallites that form during the rapid solidification process which promote a crystalline oxide surface layer. These results shed new insights into the details of surface oxidation and can help to inform processing and alloy composition adjustments which seek to tailor surface properties.

Effect of Mo in Combination with Nb on Austenite Grain Size Control in Vacuum Carburizing Steels

Vacuum carburizing with high-pressure gas quenching is increasingly employed to reduce near-surface intergranular oxidation and reduce quenching distortion. Vacuum carburizing can be conducted at higher operating temperatures, as high as 1100 °C, to reduce the processing times and increase furnace productivity. However, processing at elevated temperatures may result in excessive austenite grain coarsening, leading to the degradation of fatigue performance. Microalloying to form small carbo-nitride precipitates is one effective method to limit austenite grain growth during carburizing. In this study, the effects of microalloying a carburizing steel with molybdenum (Mo) and niobium (Nb) on microstructural grain refinement in the core have been investigated. Additions of Nb alone are found to provide some control of abnormal austenite grain growth. Additions of Mo in combination with Nb provide enhanced resistance to austenite grain growth, especially at high carburizing temperatures up to 1050 °C. The enhanced control is attributed to solute and precipitation effects.

Oxidation behaviour of solution-annealed and thermally-treated Alloy 690 in low pressure H2-Steam

Oxidation experiments were performed on Alloy 690 in a low pressure H2-steam environment at 480 °C to investigate the effects of the microstructure and thermal treatment on the “precursors events” associated with Primary Water Stress Corrosion Cracking. Advanced analytical electron microscopy characterisation revealed that the Alloy 690 did not exhibit preferential intergranular oxidation but rather only diffusion-induced grain boundary migration promoted by the formation of an external Cr2O3 oxide layer for both material conditions (solution-annealed vs. thermally-treated). These data demonstrate that for Alloy 690 the diffusion-induced grain boundary migration is a necessary but not sufficient condition for preferential intergranular oxidation.

The damage mechanism of Alloy 690TT against Alloy 600 caused by fretting wear in room temperature pure water

The damage mechanism of Alloy 690TT against Alloy 600 induced by fretting wear in room temperature pure water was investigated. Detail characterization was performed for the microstructure evolution and the cracks by using transmission electron microscopy. The results indicated that the fretting wear mechanism was a combination of abrasive wear, delamination and severe plastic deformation down to nanostructures. Intergranular oxidation along the nanograin boundaries and internal oxidation in the subsurface caused the crack initiation and propagation. The dislocation interactions during the severe plastic deformation and the interaction between oxidation and amorphization were responsible for the formation of the nano-voids. Amorphization was caused by the joint action of further plastic deformation and oxidation of the nanostructures, resulting in the acceleration of the crack propagation.

Correlation between Grain Boundary Migration and Stress Corrosion Cracking of Alloy 600 in Hydrogenated Steam

Solution-annealed Alloy 600 samples were tested under active load conditions in a superheated low pressure H2-steam environment over a range of oxidising potentials relevant to Primary Water Stress Corrosion Cracking (PWSCC). Increased SCC susceptibility was observed for potentials the more reducing than the Ni/NiO transition where a deeper Preferential Intergranular Oxidation (PIO) occurred. Advanced microstructural analysis clearly showed that SCC initiated along the grain boundaries that exhibited Diffusion-Induced Grain Boundary Migration (DIGM), as well as enrichments in Al and Ti. Rather than the result of a single dominant mechanism, the initiation of SCC in Ni-base alloys involves the synergistic interactions between DIGM and the formation of Al- and Ti-enriched oxides, promoting PIO that can fracture with applied stress.