Cr Carbides Research Articles

Investigation of Intergranular Corrosion in 2nd stage gas turbine blades of an aircraft engine

Abstract Intergranular corrosion is one of the decisive tools for the life of turbine blade for its continuous use. The study was to ascertain the occurrence of Intergranular Corrosion (IGC) in the turbine blades of an aircraft. For this purpose 2nd stage turbine blades made up of Nickel Based Superalloy Udimet 500 were used to find out attack of IGC in the top & middle portion of blades as the same was detected during Eddy Current Testing. The turbine blades having different flying hours (700, 1700 & 2700) were selected for specimen while simulating actual conditions which the turbine blades encounter during flight & maintenance/overhaul. High temperature exposure (at 900 °C for 1min & 950 °C for 1min) and alkaline media exposure (at normal and aggressive condition) were introduced to find out the damaging results. It was found that Cr carbide is precipitated at GBs on exposure to 900 °C and an excessive Cr carbide precipitation at GBs on exposure to 950 °C due to Intergranular Corrosion. On exposure to alkaline media, pitting was observed within grains and at grain boundaries on the specimen of blades.

Synergistic effect of Cu/Cr co-doping on the wettability and mechanical properties of diamond-like carbon films

By choosing carbide-forming element Cr and non-carbide-forming element Cu as co-doped metal elements, we firstly fabricated Cu and Cr co-doped diamond-like carbon (Cu/Cr-DLC) films using a facile hybrid ion beam deposition system. The effect of Cu/Cr co-doping on wettability and mechanical properties of DLC films were focused. The resultant Cu/Cr-DLC films were shown to afford good hydrophobicity and superior mechanical performance. In particular, the film with Cu11.88%Cr6.57% (at.%) exhibited a relatively high water contact angle of 103.6°, lower residual stress of 0.89GPa and high hardness of 13.44GPa. The combined structural analysis demonstrated that the synergistic Cu/Cr co-doping resulted in the critical and significant relaxation of distorted CC bond length, which ultimately caused the reduction in residual stress. Due to the formation of hard Cr carbide nano-particles in amorphous carbon matrix, the films maintained the high value of hardness. In addition, noted that the interesting wetting variety from hydrophilic to hydrophobic state was attributed to the enhanced surface graphitization and emergence of copper oxidation phases, and the film topographical microstructure could also promote the hydrophobicity of film when the maximum height of roughness was in a certain range. These results provide an expected robust synthesis method to make DLC film a promising protective coating for long lasting hydrophobic application and related harsh fields.

Carbides in Co–Re–Cr-based high-temperature alloys

Co–Re-base alloys with very high melting point are being developed to supplement Ni-superalloys in future gas turbines in which much higher gas entry temperatures are expected. The microstructure of Co–17Re–23Cr–2.6C and Co–17Re–23Cr–1.2Ta–2.6C alloys has been investigated by scanning and transmission electron microscopy. These alloys are mainly strengthened by carbides of Cr and/or Ta, but it is not trivial to accurately quantify the composition of the carbides. The compositions of the various carbides in Co–Re–base alloys were quantified with near-atomic resolution using atom probe tomography. The Cr as well as Ta carbides exists in the alloys with different morphologies, ranging from extremely fine (nm scale) to large (μm scale) sizes. The composition and the crystal structure of the investigated phases were measured and reported. It is shown that both types of carbides are stable up to 1000 °C but the Ta carbides are more stable than the Cr carbides at 1200 °C after long-time ageing. They are also effective as strengthening precipitates.

Microstructure and corrosion behavior of hot-rolled GCr15 bearing steel

Microstructure, corrosion behavior and evolution of hot-rolled high-carbon–chromium bearing steel were investigated using scanning electron microscopy and energy dispersive spectrometer (EDS). The results show that corrosion initiates adjacent to the network carbide, which is the initial austenite grain boundary. With the further increase in corrosion time, corrosion fraction is increased and extended into the grains. Finally, the whole grain near the network carbide is etched off and the grain boundary is detached from the sample, which forms the corroded holes. Based on the EDS analyses, it is confirmed that this corrosion behavior is resulted from the depletion of Cr as solid solute at the grain boundary. The depletion of Cr is the result of the formation of Cr carbide near the grain boundary.

Microstructure and corrosion resistance of radial friction welded supermartensitic stainless steels

In this work, supermartensitic stainless steel pipes were radial friction (RF) welded and their corrosion behaviours were studied based on potentiodynamic polarisation and double-loop electrochemical potentiokinetic reactivation tests. Measurements were performed on samples taken from the base metal (BM), weld interface and consumable ring (CR) of the RF weldment. The corrosion properties are discussed in terms of their resulting metallurgical microstructure. The precipitation of Cr carbides that takes place during the tempering treatment induces a substantial Cr depletion value (Ir/Ia = 54.22%). On the other hand, CR and weld interface regions, which had their microstructure transformed and their Cr carbide precipitates redissolved by the RF welding thermomechanical cycle, present a low level of Cr depletion (Ir/Ia < 1%). The AC microstructure, which is composed of a mixture of virgin martensite and stable retained austenite, presents an increase in pitting corrosion resistance compared to the tempered structure of the BM region. It was also observed that the δ-ferrite decreases the pitting resistance of the weld interface region.

Prospects for Martensitic 12 % Cr Steels for Advanced Steam Power Plants

Creep strength improvements of martensitic 9 % Cr steels have been obtained by controlled additions of V, Nb, N and B to the steels, which resulted in precipitation hardening by fine stable nitrides based on V and Nb as well as in stabilization of Cr carbides against coarsening. The best steels P92 and FB2 are now used in power plants up to 600–620 °C steam temperature. For higher steam temperatures up to 650 °C steels with 11–12 % Cr are needed for better resistance against steam oxidation. However, fine V and Nb based nitrides may transform to coarse Z-phase [Cr(V,Nb)N] nitrides in steels with such Cr contents, leading to a strength loss in long-term creep. The paper presents the status of development of new stable 12 Cr steels based on the concepts of either strengthening by fine Z-phase nitrides or on elimination of nitrides from the steels.

The role of intergranular chromium carbides on intergranular oxidation of nickel based alloys in pressurized water reactors primary water

Alloy 600 is used in pressurized water reactors (PWRs) but is susceptible to primary water stress corrosion cracking (PWSCC). Intergranular chromium carbides have been found beneficial to reduce PWSCC. Focussed ion beam coupled with scanning electron microscopy (FIB/SEM) 3D tomography has been used to reconstruct the morphology of grain boundary oxide penetrations and their interaction with intergranular Cr carbides in Alloy 600 subjected to a PWR environment. In presence of intergranular Cr carbides, the intergranular oxide penetrations are less deep but larger than without carbide. However, the intergranular oxide volumes normalized by the grain boundary length for both samples are similar, which suggest that intergranular oxidation growth rate is not affected by carbides. Analytical transmission electron microscopy (TEM) shows that the intergranular oxide consists mainly in a spinel-type oxide containing nickel and chromium, except in the vicinity of Cr carbides where Cr2O3 was evidenced. The formation of chromium oxide may explain the lower intergranular oxide depth observed in grain boundaries containing Cr carbides.

Corrosion kinetics of 316L stainless steel bipolar plate with chromiumcarbide coating in simulated PEMFC cathodic environment

Stainless steel with chromium carbide coating is an ideal candidate for bipolar plates. However, the coating still cannot resist the corrosion of a proton exchange membrane fuel cell (PEMFC) environment. In this work, the corrosion kinetics of 316L stainless steel with chromium carbide is investigated in simulated PEMFC cathodic environment by combining electrochemical tests with morphology and microstructure analysis. SEM results reveal that the steel’s surface is completely coated by Cr and chromium carbide but there are pinholes in the coating. After the coated 316L stainless steel is polarized, the diffraction peak of Fe oxide is found. EIS results indicate that the capacitive resistance and the reaction resistance first slowly decrease (2–32h) and then increase. The potentiostatic transient curve declines sharply within 2000s and then decreases slightly. The pinholes, which exist in the coating, result in pitting corrosion. The corrosion kinetics of the coated 316L stainless steel are modeled and accords the following equation: i0=7.6341t−0.5, with the corrosion rate controlled by ion migration in the pinholes.

Molecular dynamics simulations of cascades in strained carbide inclusions embedded in α-iron

The effect of strain on the amount of point defects created in Fe and Cr carbide inclusions embedded in ferrite has been investigated. The spherical carbide inclusions consisted of either Fe3C or Cr23C6. Recoil energies from 100 eV to 3 keV and strains from −0.15 (compressive) to 0.01 (tensile) were used. The overall tendency is that the number of point defects — such as antisites, vacancy and interstitials — inside the carbide is lowered when the strain grows more negative (compressive). Outside the carbides, the number of defects is markedly higher for strongly compressive strains than for e.g. zero strain, especially at high energies.

Impact of Corrosion Test Container Material in Molten Fluorides

The effects of crucible material choice on alloy corrosion rates in immersion tests in molten LiF–NaF–KF (46.5–11.5-42 mol. %) salt held at 850 °C for 500 hrs are described. Four crucible materials were studied. Molten salt exposures of Incoloy-800H in graphite, Ni, Incoloy-800H, and pyrolytic boron nitride (PyBN) crucibles all led to weight-loss in the Incoloy-800H coupons. Alloy weight loss was ∼30 times higher in the graphite and Ni crucibles in comparison to the Incoloy-800H and PyBN crucibles. It is hypothesized galvanic coupling between the alloy coupons and crucible materials contributed to the higher corrosion rates. Alloy salt immersion in graphite and Ni crucibles had similar weight-loss hypothesized to occur due to the rate limiting out diffusion of Cr in the alloys to the surface where it reacts with and dissolves into the molten salt, followed by the reduction of Cr from solution at the molten salt and graphite/Ni interfaces. Both the graphite and the Ni crucibles provided sinks for the Cr, in the formation of a Ni–Cr alloy in the case of the Ni crucible, and Cr carbide in the case of the graphite crucible.

Effect of Specimen Thickness on Microstructural Changes During Oxidation of the NiCrW Alloy 230 at 950–1050°C

An accurate procedure for predicting oxidation-induced damage and lifetime limits is crucial for the reliable operation of high-temperature metallic components in practical applications. In order to develop a predictive oxidation lifetime model for Ni–Cr alloys, specimens of wrought NiCrW alloy 230 with different thicknesses were cyclically oxidized in air at 950–1050°C for up to 3000 h. After prolonged exposure, two types of carbides as well as a Cr-rich nitride (π-phase) precipitated in the γ-Ni matrix. The oxidation-induced loss of Cr from the alloy resulted in the formation of subscale zones, which were free of the Cr-rich carbide and nitride but also of the Ni-W rich M6C. The width of the M6C-free zone was smaller than that free of the Cr-rich precipitates. Thermodynamic and diffusion calculations of the observed time- and temperature-dependent Cr depletion processes identified that back diffusion of C occurred which resulted in an increased volume fraction of M23C6 in the specimen core. With increasing time and temperature, the amount of π-phase in the specimen core increased. The subscale depletion of the initially present Cr-nitrides and the formation of Cr-nitrides in the specimen center is believed to be related to a mechanism which is qualitatively similar to that described for the Cr carbide enrichment. However, with increasing time and decreasing specimen thickness, N uptake from the atmosphere becomes apparent. As a result, the precipitates present in the specimen center eventually consisted almost exclusively of nitrides.

DFT study of the effects of interstitial impurities on the resistance of Cr-doped γ-Fe(111) surface dissolution corrosion.

Using density-functional calculations, we studied the interaction between interstitial impurities (N, C) and γ-Fe(111) surfaces doped, or not, with Cr, as well as the effect of Cr doping on the dissolution corrosion resistance of the γ-Fe(111) surface. The elementary processes studied afforded microscopic insights into the formation of a Cr-depleted zone, a phenomenon that leads to local corrosion of the stainless steel surface. The aim of this work was to study, at the atomic scale, the effects of N and C on the segregation behavior of Cr and the synergetic effect between co-doped atoms on the resistance to dissolution corrosion of austenitic stainless steel surfaces. The results showed that interstitial impurities prefer to be trapped at near-surface sites, which can impact the segregation behavior of Cr such that it shifts from the surface to the subsurface. Electrode potential calculations and density of states analysis demonstrated that doping with Cr or inserting interstitial impurities into the solid solution can improve the surface corrosion resistance of an fcc Fe substrate, but detrimental effects on the surface corrosion resistance are induced by interactions between Cr and interstitial impurity atoms in co-doped surfaces. The formation of near-surface Cr carbides and nitrides (speculated to be Cr2N and Cr23C6 due to the results obtained for particular co-doped surfaces) was also noted. The results of our theoretical calculations explain some of the experimental results observed at the atomic scale.

Metal dusting, carburization and diamond deposition on Fe–Cr alloys in CH4–H2 plasma atmospheres

Fe–Cr model alloys (with 20–80wt.% Cr) were exposed to a CH4–H2 mixture in a microwave plasma enhanced chemical vapor deposition (CVD) atmosphere. Severe metal dusting and carburization are observed on the low-Cr alloys, meanwhile, diamond cannot nucleate until a graphite intermediate layer has formed on the substrate, which takes a long incubation time. Increasing Cr concentrations in the Fe–Cr alloys promotes the formation of a Cr carbide buffer layer, which inhibits metal dusting and the formation of graphite interlayer. Consequently, diamond nucleation and growth are greatly enhanced, and continuous diamond films with enhanced adhesion are deposited on the Fe–80Cr alloys.

Interface behavior and mechanical properties of 316L stainless steel filling friction stir welded joints

In the present work, the feasibility to repair keyhole left at 316L stainless steel friction stir welding/friction stir processing (FSW/FSP) seam by filling friction stir welding (FFSW) using consumable tools was investigated. Interface behavior and mechanical properties of 316L stainless steel FFSW joints were investigated. The results showed that significant microstructural refinement occurred around the interface of refilled keyhole due to extreme levels of plastic deformation and thermal exposure. No σ phase but few Cr carbides were formed at the refilled joint interface, which would not result in obvious corrosion resistance degradation of 316L stainless steel. Void defects formed at the bottom of the refilled keyhole and the FFSW joint fractured at the interface during the tensile test.

Comparative tests of plasma-surfaced nickel layers with chromium and titanium carbides

This article presents the results of structural tests of composite coatings on a NiSi2B alloy matrix containing carbides of Ti and Cr, which have been plasma surfaced from a powder mixture onto a low-alloyed steel substrate. The content and distribution of the strengthening phase in the matrix have been determined. It has been shown that the different nature of the interaction between Ti and Cr carbides and the liquid Ni alloy significantly affects the formation of composite surfacing layers as well as the distribution and content of strengthening phase particles in the matrix.

Механiчнi та триботехнiчнi властивостi детонацiйних покриттiв iз композицiйного матерiалу на основi карбiду титану–хрому

Механiчнi та триботехнiчнi властивостi детонацiйних покриттiв iз композицiйного матерiалу на основi карбiду титану–хрому

Joining of Inconel-625 alloy through microwave hybrid heating and its characterization

Joining of bulk metals using microwave energy is being explored as a new processing method in the era of high technology applications. The present work investigates the results of on-going project in which joining of Inconel-625 alloy has been effectively carried out through microwave hybrid heating. Characterization of the microwave developed joints is done through SEM, XRD, UTM and Vicker's microhardness tester. The microstructural analysis through scanning electron microscope exhibits a fully fused weld interface free from interfacial cracks. XRD study reveals the formation of carbides of Ni, Cr and Mo in the joint interface. The average Vicker's microhardness in the joint region observed was 360±20Hv. Assessment of tensile strength shows an ultimate tensile strength of 328MPa with 9.04% elongation. Further the fractured joints are subjected to fractography study which possibly reveals a mixed mode fracture.

Efeito do Tratamento Térmico de Solubilização e Estabilização Convencional e Alternativo na Microestrutura de uma Junta Soldada com Aço Inox 347

Este trabalho estuda a microestrutura formada na zona termicamente afetada (ZTA) e na zona fundida (ZF) de uma junta soldada pelo processo TIG automático no passe de raiz, processo plasma nos passes de enchimento e soldagem SAW para os passes de acabamento em junta de topo com chanfro “V” simples, usando o aço inoxidável austenítico AISI 347. Foram estudadas as condições: como soldada; após tratamento térmico convencional de solubilização a 1060 °C e estabilização a 900 °C; e após tratamento térmico alternativo, onde a peça foi solubilizada a 1060 °C e mantida no forno que foi resfriado até 900 °C para a estabilização. Os resultados mostram que, na condição como soldado houve crescimento de grão na ZTA e a ZF apresentou 13% de ferrita �� com diferentes morfologias, tendo sido observada a precipitação de carbetos de Cr e de Nb. Após o tratamento térmico convencional de solubilização e estabilização, foi observada uma diminuição significativa do teor de ferrita �� na ZTA e na ZF, tendo sido observados carbetos de Nb e Cr, distribuídos de forma mais dispersa e com tamanhos menores, que contribuem para minimizar a possibilidade de ocorrer o fenômeno da sensitização. Após tratamento térmico alternativo de solubilização e estabilização, a microestrutura da ZTA e ZF foi similar à obtida no tratamento convencional, sendo que o teor de ferrita δ na ZF foi ainda mais reduzido, de 4,5%. Foi concluído que o tratamento térmico alternativo de solubilização e estabilização foi tão eficaz quanto o tratamento térmico convencional para a adequação da microestrutura da junta soldada, com uma significativa vantagem do tratamento térmico proposto em reduzir o tempo e o custo desta operação.

Simple bond-order-type interatomic potential for an intermixed Fe-Cr-C system of metallic and covalent bondings in heat-resistant ferritic steels

It is known that M23C6(M = Cr/Fe) behavior in heat-resistant ferritic steels affects the strength of the material at high temperature. The ability to garner direct information regarding the atomic motion using classical molecular dynamics simulations is useful for investigating the M23C6 behavior in heat-resistant ferritic steels. For such classical molecular dynamics calculations, a suitable interatomic potential is needed. To satisfy this requirement, an empirical bond-order-type interatomic potential for Fe-Cr-C systems was developed because the three main elements to simulate the M23C6 behavior in heat-resistant ferritic steels are Fe, Cr, and C. The angular-dependent term, which applies only in non-metallic systems, was determined based on the similarity between a Finnis-Sinclair-type embedded-atom-method interatomic potential and a Tersoff-type bond-order potential. The potential parameters were determined such that the material properties of Fe-Cr-C systems were reproduced. These properties include the energy and lattice constants of 89 crystal structures; the elastic constants of four realistic precipitates; the bulk moduli of B1, B2, and B3 crystals; the surface energies of B1 and B2 crystals; and the defect-formation energies and atomic configurations of 66 Fe-Cr-C complexes. Most of these material properties were found to be reproduced by our proposed empirical bond-order potentials. The formation energies and lattice constants of randomly mixed Fe-Cr alloys calculated using the interatomic potentials were comparable to those obtained through experiments and first-principles calculations. Furthermore, the energies and structures of interfaces between Cr carbide and α-Fe as predicted through first-principles calculations were well reproduced using these interatomic potentials.

The Effect of Precipitated Phases on Creep Behavior

The effect of precipitated phases on the creep in 300s type austenitic stainless steel used in auto-tailpipe was studied. Flat tensile samples with marker scratches on the surface were strained at slow rate at 400°C. It show that the sensitized material decreased creep rate and needed more stress to generate deformation of grain boundary, suggesting Cr carbides decreased the grain boundary sliding. Thus, the resistance to SCC crack propagation could be improved. Characterizations of the tensile samples were carried out using a SEM in order to understand whether second phases can affect the creep behavior and grain boundary moving.