Pierre Steinmetz

State of the Sub-surface Microstructure of Carbides strengthened cast Superalloys after High Temperature Oxidation - Use of Thermodynamic Modelling for a better Understanding

Several cast strengthened superalloys, Ni base and Co base, were exposed to high temperature oxidation for long times and metallographically examined. Different phenomena occurred in the sub-surface microstructure, depending on both alloy and temperature. Thermodynamic modelling was used to know what it happened for carbon during oxidation, then to explain the observed microstructural changes. It appears that carbon atoms either quit the alloy probably after its oxidation into gases, or on the contrary go deeper into the bulk where they promote the precipitation of new carbides by solid state transformation. Thereafter, thermodynamic modelling allowed to know the new local refractoriness of the zones affected by oxidation, then to appreciate the new mechanical properties in the sub-surface.

Chromium Deposition on Cobalt-Based Alloys by Pack-Cementation and Behaviour of the Coated Alloys in High Temperature Oxidation

Two TaC-strengthened cobalt-based alloys, with compositions Co-10Cr-0.25C-4.4Ta and Co-10Cr-0.5C-8.7Ta (wt.%) underwent Cr-deposition by pack-cementation, followed by heat-treatment to allow Cr diffusing deeper in their sub-surface. Thereafter they were tested for 50 hours at 1200°C in a thermobalance. These alloys were successfully enriched in chromium, with a maximal content on surface of about 30wt.% and a depth of enrichment of several hundreds of micrometers. This allowed a chromia-forming behavior of the cemented alloys despite the 10wt.%Cr of the bulk. This was especially true for the {C,Ta}-richest alloy the oxidation kinetic of which was wholly parabolic and analogous to a Ni-30Cr alloy known for its good behavior in oxidation at 1200°C. The parabolic constants and chromia-volatilization constants were at the same levels of the Ni-30Cr ones. However, the chromium enrichment step, especially the diffusion heat-treatment following the cementation, causes fragmentation and surface fraction reduction of the tantalum carbides which strengthen the alloy.

Oxidation and Nitridation of Pure Chromium at Elevated Temperature in Synthetic Air – Effect of Silicon Addition

The oxidation and nitridation of pure chromium and of chromium alloys containing 0.5 to 5at% silicon was investigated at 1300°C in several atmospheres. Global methods like thermogravimetry were associated to thickness measurements and microstructural characterisation to evaluate oxidation mechanisms. The contribution of nitridation to weight gain during high temperature exposure is discussed, examining nitride volume fraction. Experiments demonstrate that the presence of nitrogen in the substrate is always consecutive to a breakdown of the oxide layer and does not result from diffusion through the Cr2O3 layer. Silicon, when present in chromium solid solution, slows down the oxidation kinetics and limits the progression of nitrogen in the chromium lattice and at grain boundaries. The absence of the Cr2N layer beneath the metal/oxide interface does not benefit to the adherence of the oxide scale. Oxidation of silicon in chromium leads to the formation of discontinuous particles of SiO2 at the metal/oxide interface, at the grain boundaries and dispersed in the bulk in the alloy sub-surface.

Development of Burner Rig Corrosion Tests for Gas Turbine Alloys: Study of Protective Metallic Coatings and Inhibitors for Hot Salt Corrosion

The efficiency of all types of gas turbine engines (aircraft, terrestrial and marine) is proportional to firing or turbine inlet temperature. In most cases, the use of heavy or contaminated fuels (by Na, S, V) in such combustion equipment is the life-limiting factor for the metallic parts constituting the hot gas path. Since several years, a burner rig was developed in the laboratory to evaluate the corrosiveness of the ash deposit formed on the gas turbine alloys in dynamic conditions. It has been designed to easily adapt the ash deposit rate and the level of the contaminants. The first part of the present paper is devoted to the evaluation of the resistances of several types of metallic coatings against sodium-induced hot corrosion. These coatings were deposited on IN738 alloy and have been treated during 1000h of test. For this, a NaCl solution was injected in the hot gas flux to obtain a deposit rate of 1 mg/cm 2 /100h at 850°C. Different types of aluminide coatings obtained by pack cementation were tested and compared to a NiCrAlY coating produced by LPPS process. Among all these coatings, the Pt-modified alumides show the best anti-corrosion behaviour against the Na-induced hot corrosion. In the second part, the burner rig has been used to test additives introduced into crude oil burned in gas turbines. Indeed, the usual and low-cost defence against (Na,V)-induced hot corrosion is the addition to the fuel of some metals as corrosion inhibitors. To simulate this corrosion and its inhibition in the burner rig, an aqueous solution containing adjusted concentrations of sodium, vanadium with or without inhibitor was injected into the combustion gas just after the burner. A Mg/V ratio equal to 3 was tested to evaluate the efficiencies of the inhibition and the deposition rate in the presence of magnesium.

Elaboration and Characterization of the Properties of Refractory Cr Base Alloys

For energy production and also for the glass industry, finding new refractory alloys which could permit to increase the process temperatures to 1200°C or more is a permanent challenge. Chromium base alloys can be good candidates, considering the melting point of Cr itself, and also its low corrosion rate in molten glass. Two families of alloys have been studied for this purpose, Cr-Mo-W and Cr-Ta-X alloys (X= Mo, Si..). A finer selection of compositions has been done, to optimize their chemical and mechanical properties. Kinetics of HT oxidation by air, of corrosion by molten glass and also creep properties of several alloys have been measured up to 1250°C. The results obtained with the best alloys (Cr-Ta base) give positive indications as regards the possibility of their industrial use.

Refractoriness, Microstructures and High Temperature Oxidation of TaC-Reinforced Iron-Based Alloys Protected by Chromium-Rich Coatings

In some cases chromium, necessary for the resistance to hot corrosion, unfortunately tends to lower the solidus temperature of alloys destined to high temperature applications. However a decrease of its content in the bulk can be allowed by a Cr-enrichment of the alloys sub-surfaces. To examine this possibility in the case of refractory TaC-strengthened iron-based alloys, three {Fe(bal.) – 10Cr – 0.2 or 0.4C – 3 or 6Ta} – alloys were elaborated by foundry in inert atmosphere and subjected to Cr deposit and inwards diffusion by a pack-cementation process. Bulk and subsurface microstructures examinations and concentrations profiles were performed after each step of elaboration of the future coated alloys as well as after a 50 hours / 1000°C oxidation test in air. A 500μm-deep carbide-free sub-surface with a chromium content twice the bulk one was obtained for the 0.2C-3Ta and 0.4C-6Ta alloys (less deep for the 0.4C-3Ta alloy) and allowed the alloys resisting high temperature oxidation.

Study of the Microstructure and Oxidation Behavior of Chromium Base Alloys Strengthened by NiAl Precipitates

In this study, chromium based alloys containing a hardening NiAl phase are investigated from 1100°C to 1300°C in air. The influence of the NiAl content and of the Al/Ni ratio on microstructure modification and on oxidation behaviour are characterized by metallography and thermogravimetry. Increasing the Al/Ni ratio leads to a higher Al content in the chromium solid solution but does not modify the amount of primary NiAl. At high temperature, and for NiAl content exceeding 16 at%, a duplex oxide layer forms at the surface of the alloys exposed in air, alumina in the inner part, and chromia in the outer part. The oxidation behavior is discussed according to oxidation profile and to the thermodynamic predominance diagram of the involved species.

Revisiting the Inhibition of Vanadium-Induced Hot Corrosion in Gas Turbines

Since the 70’s, nothing substantially new has been published in the Gas Turbine Community about the hot corrosion by vanadium and its inhibition, after the “inhibition orthodoxy” based on the formation of magnesium vanadate, was established. However, the experience acquired since the late 80’s with Heavy Duty Gas Turbines burning ash-forming fuels in Southern China, shows that the combustion of very contaminated fuels does not entail corrosion nor abundant ash-deposit on gas turbines buckets. Analyses of deposits collected from gas turbines fired with these crude oils showed that the ash-deposit contains a large amount of nickel.These new facts led to revisit the role played by nickel and envisage its possible inhibiting action against the vanadium-induced hot corrosion. A thorough review of the literature on the vanadium-induced corrosion have been carried out, and the study of the nickel effects with respect to magnesium effects on the ash deposit have been performedResults show that nickel presents an interesting way to substitute magnesium for the inhibition of vanadium-induced hot corrosion. The advantages of nickel with respect to magnesium are to be efficient at a low Ni/V ratio, to produce less abundant, less adherent ash and to act, to some extent, as a self-cleaning agent for the blades of the turbine.© 2001 ASME