Cyclic oxidation of Haynes 230 alloy

The cyclic oxidation of Haynes 230 alloy (Ni-Cr-W-Mo alloy) was investigated in air at three different temperatures, 871, 982 and 1093 °C. Studies indicated that during cyclic oxidation, protective scales formed which were predominantly Cr₂O₃, with Kirkendall voids formed both at the scale/alloy interface and grain boundaries. Intergranular oxides were observed at temperatures above 982 °C while internal oxide particles were found above 1093 °C. Both intergranular and internal oxides were identified as aluminium oxide. A 50 m chromium-depleted zone developed after 70 h exposure at 1093 °C and was accompanied by disastrous scale spalling. The lowest chromium concentration within the depleted zone was 14 wt% which still provided a sufficient supply of chromium for development of a continuous Cr₂O₃ rich scale. Decarburization was observed at the higher temperature of 1093 °C, and a carbide-free zone developed. Also, it was found that Haynes 230 is subject to a sensitization process. At the lower exposure temperature of 871 °C, large amounts of chromium carbide formed preferentially at the grain boundaries. While at the surface region chromium carbide precipitation occurred at the twin boundaries.     

Susceptibility to Hot Cracking and Weldment Heat Treatment of Haynes 230 Superalloy

This study investigates the susceptibility of hot cracking and weldment heat treatment of Haynes 230 superalloy. The Varestriant test was conducted to evaluate this susceptibility. Welding was performed by gas tungsten arc welding （GTAW） and plasma arc welding （PAW） with stress relief heat treatment and solid solution heat treatment. A tensile test is then performed to measure the changes in the mechanical properties of the heattreated material. The results indicate that the number of thermal cycles does not affect the susceptibility of Haynes 230 superalloy to hot cracking. However, it does increase the strain. In weldment of heat treatment, stress relief annealing increases the yield strength and tensile strength of the welded parts. The section of the tensile specimens shows fibrous fractures on the welded parts, regardless of whether they are heat-treated.     

A study on the hot corrosion behavior of Ti-6Al-4V alloy

The titanium alloys are potential materials for high temperature applications in turbine components due to their very high temperature strength and lightweight properties. However, hot corrosion is a life-limiting factor when Ti alloys are exposed to different chemical environments at high temperature. In the present paper, hot corrosion behavior of Ti-6Al-4V (Ti-31) alloy in different salt environments viz. air, Na₂SO₄-60% V₂O₅ and Na₂SO₄-50% NaCl at 750 °C was studied. The parabolic rate constants were calculated for different environments from the thermo-gravimetric data obtained for the samples and they show that corrosion rate is minimum in air when compared to chemical environment. The scale formed on the samples upon hot corrosion was characterized by using X-ray diffraction (XRD), SEM, and EDAX analysis to understand the degradation mechanisms.     

Study of hot corrosion of alloy 800 using linear polarisation resistance and weight loss measurement

Abstract

A study of the hot corrosion of alloy 800 at 700 and 900°C in 80Na₂SO₄+20V₂O₅ (wt-%) has been carried out. The techniques included linear polarisation resistance (LPR) and weight loss measurement during 10 days of experiments, supported by X-ray measurements and electron microscopy. The degradation of the alloy was discussed in terms of the dissolution of the external Cr₂O₃ layer with subsequent oxidation and sulfidation of the alloy. Both LPR and weight loss results showed that the corrosion rate is slightly higher at 900 than at 700°C. At the beginning of the experiments, the corrosion rates obtained using the two techniques increase at both temperatures, but decrease later on until they reach steady values. The measured corrosion rates were within the same order of magnitude for both techniques, but the rates obtained with the electrochemical technique were lower than those obtained using weight loss at 700°C, while at 900°C, the opposite was observed. This is because at 700°C the step that controls the corrosion process is the diffusion of species in the alloy or in the melt, whereas at 900°C, the step that controls the reaction is either charge transfer or there is a mixed control. It is evident that electrochemical techniques are powerful tools for monitoring on line hot corrosion processes when the corrosion rate is controlled by an electrochemical reaction, but may induce some errors when the molten salt changes from an ionic to an electronic conductor.     

Fatigue crack growth in Haynes 230 single crystals: an analysis with digital image correlation

Fatigue crack growth was investigated in Haynes 230, a nickel‐based superalloy. Anisotropic stress intensity factors were calculated with a least squares algorithm using the displacements obtained from digital image correlation. Crack opening/sliding levels were measured by analysing the relative displacement of crack flanks. Reversed crack tip plastic zones were calculated adopting an anisotropic yield criterion. The strains measured in the reversed plastic zone by digital image correlation showed a dependence on crystallographic orientation. Finally, a finite element model was adopted to examine plasticity around the crack tip. Results were compared with the experimentally observed strains.     

Low cycle fatigue behaviour of HAYNES 230 alloy

Abstract

HAYNES® 230® alloy is a commercial solid–solution strengthened Ni-base superalloy that has found wide spread use in the hot sections of gas turbines. Recently, the alloy has been considered for other applications, e.g. in concentrated solar power plants. One common key performance criterion for all these applications is the ability of the material to withstand large numbers of thermal cycles due to start-ups, operatings, and shut-downs. Such thermal cycles may trigger low-cycle fatigue (LCF) due to differential thermal expansion and contraction. This paper reports isothermal LCF performance data of 230 alloy standard mill-annealed plate product for a wide range of temperatures 427–982°C and strain ranges (up to 1·5%), relevant to many of the aforementioned applications. Low-cycle fatigue lives are analysed in terms of the contributions of cyclic elastic and plastic strain components to fatigue damage. The fit parameters are discussed in the context of the alloy’s tensile properties.     

Creep deformation and monotonic stress-strain behavior of Haynes alloy 556 at elevated temperatures

Creep and monotonic stress-strain behaviors of Haynes alloy 556 are studied and characterized at elevated temperatures using experimental results and analytical models. The -Projection and Garofalo creep models describe the variations of the creep curve shape at different temperatures and stress levels reasonably well. The -Projection method, however, results in accurate prediction of the rupture lives, while Garofalo model overestimates them. Both models provide fair predictions of minimum creep rate variations with stress at a given temperature. An incremental time method, which combines the time independent stress-strain data from standard tensile test with the creep data, is used to predict the stress-strain curve in slow-strain-rate tensile tests at elevated temperatures.     

The corrosion performance of die-cast magnesium alloy MRI230D in 3.5% NaCl solution saturated with Mg(OH)2

The environmental behavior of die-cast magnesium alloy MRI230D designated for high-temperature applications was evaluated in comparison with regular AZ91D alloy. The microstructure examination was carried out using SEM, TEM, and X-ray diffraction analysis; the corrosion performance in 3.5% NaCl solution was evaluated by immersion test, salt spray testing, potentiodynamic polarization analysis, and stress corrosion behavior by Slow Strain Rate Testing (SSRT). Although the general corrosion resistance of MRI230D was slightly improved compared to that of AZ91D alloy its stress corrosion resistance was relatively reduced. The variations in the environmental behavior of the two alloys were mainly due to the differences in their chemical composition and microstructure after die casting. In particular, the differences were related to the reduced Al content in MRI230D and the addition of Ca to this alloy, which consequently affected its relative microstructure and electrochemical characteristics.     

Exfoliation corrosion of extruded Mg-Li-Ca alloy

Exfoliation on as-extruded Mg-1 Li-1 Ca magnesium alloy was investigated after an immersion in 3.5 wt%NaCl aqueous solution for 90, 120 and 150 days through optical microscope, digital camera, scanning electron microscope, electrochemical workstation, scanning Kalvin probe, X-ray diffraction and Fourier transform infrared spectroscope. The results demonstrated that exfoliation corrosion occurred on extruded Mg-1 Li-1 Ca alloy due to elongated microstructure parallel to surface, and delamination of lamellar structure resulted from galvanic effect and wedge effect. Skin layer with fine grains exhibited better corrosion resistance, whereas the interior with coarse grains and the intermetallic compound,Mg_2 Ca particles existing in a fibrous structure, dispersed along grain boundaries and extrusion direction in a line. Furthermore, galvanic effect between Mg_2 Ca particles and their neighboring a-Mg matrix facilitated dissolution of Mg_2 Ca particles and a-Mg matrix; wedge effect was caused by formation of corrosion products. Exfoliation corrosion of extruded Mg-Li-Ca alloys might be a synergic effect of pitting corrosion,filiform corrosion, intergranular corrosion and stress corrosion. Finally, exfoliation corrosion mechanism was proposed.     

Electrochemical evaluation of hot corrosion resistance of metallic materials

A fast, effective method for the evaluation of hot corrosion resistance of metallic materials is presented. This method is based on the extension to high temperature field of electrochemical techniques for the study of corrosion in aqueous environments. Its results are consistent with those obtainable by thermogravimetric techniques or by bench testing.An application example is given.     

Interaction of hot corrosion and creep in Alloy 617

In the present work, Alloy 617 was subjected to creep in the absence and presence of hot corrosion (i.e. with and without a deposit of sodium salts) under different load magnitudes at 850 °C. Additionally, a sample of the alloy was exposed to hot corrosion without applying creep. The results revealed that the creep behavior of Alloy 617 was substantially affected by hot corrosion which accelerated creep and led to a considerable reduction in the fracture ductility and creep life of the alloy. Microscopic examinations demonstrated that the strain-to-rupture decreased due mainly to the damage accumulation during tertiary creep. Various mechanisms of hot-corrosion/creep interaction were discussed that highlighted the destructive role of M₂₃C₆ precipitates in the interaction. It was explained that hot corrosion contributed to the reduction of grain-boundary cohesive-strength followed by extensive cavitation at transvers grain boundaries.     

The effect of SO3 on vanadate-induced hot corrosion

Unrefined fuel oils may contain considerable amounts ofboth sulphur and vanadium. Despite this, exposure to combustion gases seldom yields deposits which consist ofboth vanadates and sulphates. Calculations for the sodium sulphate/vanadate system show that this is due to thermodynamics ofthe system. Sodium sulphate cannot exist in equilibrium with fused vanadate unless the sulphur trioxide pressure in the ambient atmosphere is high or the vanadium to sodium ratio in the deposits is less than one.

Thermogravitmetric studies which delineate the conditions for simultaneous sulphate- and vanadate-induced corrosion at 650 to 8OO°C have been performed. NiCrAlY coatings with K-sodium vanadyl vanadate deposits and the deposit alone have been exposed to oxygen containing 4% sulphur dioxide at 650 to 8OO°C. The results generally confirm the calculations from available thermodynamic data, but the solubility ofsulphur oxide in fused sodium vanadate is higher than expected from the literature values. The ' corrosion mechanism changes from initial vanadate-induced to essentially sulphate-induced hot corrosion when the sodium trioxide pressure is high enough that sodium sulphate may be formed.     

Effect of the cooling rate in the corrosion behavior of a hot worked Ti-6Al-4V extra-low interstitial alloy

This work discusses the effect of the cooling rate during a forging process on the microstructure and corrosion behavior of a Ti–6Al–4V extra-low interstitial (ELI) alloy, which is commonly used as biomaterial. The samples were hot forged at two different temperatures, both of them within the dual phase field (α + β) and a constant strain rate of 4 × 10⁻³ s⁻¹ was employed during the tests. The samples were cooled in three different cooling media (water, air and clay) and the microstructure was analyzed using scanning electron microscopy (SEM). The corrosion resistance was determined by cyclic polarization tests in Ringer’s solution at 37 °C. Comparison between the results obtained for forged and commercial samples allowed to establish some correlations between cooling rate, microstructure and corrosion resistance. It was found that the clay as a cooling medium is a good candidate to obtain a proper microstructure and properties for biomedical applications, eliminating the requirement of subsequent heat treatment and reducing costs.