Microstructure influence on creep properties of heat-resistant austenitic alloys with high aluminum content
Introduction
Heat resistant austenitic stainless alloys are used in centrifugally cast tubes for cracking furnaces because of their excellent creep and oxidation resistances. A minimum temperature of 850 °C is needed for the cracking reaction to take place. This temperature is reached by heating the tubes between 950 °C and 1100 °C (metal temperature). An unwanted side-reaction of coking happens at the inner surface of the tube during service, it leads to a decrease in heat transfer (as well as an increase in the tube pressure). The tubes then have to be heated at even higher temperatures, typically up to 1100°C–1150 °C which corresponds to a temperature limit for the material. Moreover, when coking becomes critical, decoking process must be performed. Increase in temperature and decoking process both reduce the tube service life and are very costly.
Nowadays, industrials seek to develop new alloys with high Al content to improve environmental resistance, especially the coking resistance, by formation of a continuous alumina scale at the inner surface of the furnace tubes. This oxide is more stable than the Cr-oxide at high temperature [[1], [2], [3], [4], [5], [6], [7], [8]]. Nevertheless, Al is known to decrease the creep strength [6,9,10]. The understanding of the influence of Al addition on the microstructure and therefore on the creep properties is essential to find a middle ground between good oxidation properties and good creep resistance.
The influence of Al on the microstructure and the creep properties of austenitic alloys was widely studied in the temperature range of 550–900 °C. Fe–Ni–Cr–Al alloys (FCNA) with high Al level, between 4 and 5 wt %, are used as structural materials combining good mechanical properties and good oxidation resistance thanks to the precipitation of NiAl phase at 550–650 °C [11,12]. Alumina forming austenitic stainless steels (AFA) with Al-content between 2.5 and 4 wt % are used for structural purpose in fossil energy conversion and combustion system applications which require good creep and oxidation properties between 650 and 900 °C. Main hardening precipitates in these alloys are MC (M stands for metal like Nb and Ti) carbides and intermetallic compounds such as NiAl, Fe2(Mo, Nb)-Laves and Ni3Al phases [[13], [14], [15], [16], [17], [18], [19]]. Addition of high Al level can destabilize the single-phase austenitic matrix by promoting the formation of the weak body-centered cubic α-Fe phase resulting in a drop of creep properties [6,20].
For higher temperature applications (>950 °C), nickel base superalloys and austenitic alloys with high Al level were developed. Asteman et al. [8] investigated Ni-base alloys with 4 wt % Al (commercial Centralloy® HT-R (Ni25Cr4Al)) and 10 wt % Al (Ni25Cr10Al) for high temperatures applications (600–1200 °C). In Ni25Cr4Al alloy, secondary carbides and γ’ (Ni3Al) precipitation allow to obtain sufficient creep properties at high temperature. However, the γ’ particles are not stable at temperature above 1000 °C which is too low for the applications of our interest [8,21]. The authors [8] showed that the increase in Al content from 4 wt % to 10 wt % decreased the oxidation resistance because of the formation of a multiphase cast microstructure. Nevertheless, they did not investigate the influence of this multiphase structure on the creep properties. The heat resistant austenitic alloy Manaurite® XTM without Al allows to reach high temperature while keeping sufficient creep properties [22]. The aim is to design an alloy with Al, equivalent or more resistant in creep, i.e. with lower creep rate, no excessive elongation during service and longer creep service life than the Manaurite® XTM widely used in cracking furnaces.
The challenge is to propose alloys with Al-content higher than 3.5 wt % in order to increase the environmental resistance while keeping good creep properties. At cracking service temperature (950–1100 °C), the creep resistance of these austenitic alloys is mainly guaranteed by the control of secondary precipitation of MC (M stands for metal which is mainly Nb and Ti), and M23C6 (M is mainly Cr) carbides. M23C6 carbides are precipitated owing to transformation of the primary carbides M7C3 (M is mainly Cr) formed during solidification [[23], [24], [25], [26], [27], [28]]. However, the influence of high Al level (>3.5 wt %) on the microstructure and the creep properties of such alloys is not investigated yet.
This paper focusses on advanced Manaurite® XAl4 alloys [29], i.e. on Fe–Ni–Cr–Al alloys of high Ni content (45–55 wt %) with alloying addition levels of 25–32 wt % Cr, 3–5 wt % Al and 0.45 wt % C. The aim is to study the influence of Al additions on the microstructure and creep properties. Creep tests were performed, and microstructures were characterized after creep in order to make the link between the observed microstructures and the creep resistance. This work relies on the coupling of the CALPHAD (CALculation of PHAse Diagrams) method to forecast the aged microstructure especially the phase stability domains appearing at high temperature (intermetallic phases, hardening precipitates), creep tests and experimental characterization of as cast and ageing microstructures using SEM (Scanning Electron Microscopy) (imaging, EBSD (Electron Backscattered Diffraction) and EDS (Electron Dispersive Spectroscopy)), TEM (Transmission Electron Microscopy) (conventional TEM, STEM (Scanning Transmission Electron Microscopy) and STEM-EDS) and XRD (X-Ray Diffraction).
Section snippets
Materials
The tubes were produced by centrifugal casting. The average chemical compositions in weight percent of the FeNiCrAl alloys are provided in Table 1. The C content is measured by a combustion infrared detection technique (LECO) and the others element by SEM-EDS. Mn-content was not measured (NM) because of its low concentration (<0.1 wt %). Al-content decreases from alloy 4 to alloy 1: alloy 4 (4.8 wt %) – alloy 3 (4.5 wt %) – alloy 2 (4.1 wt %) – alloy 1 (3.5 wt %). Except Al content, the
As-cast microstructure
The centrifugation process and the fast cooling rate promote the formation of a large columnar zone as shown in the crystallographic orientation maps of alloy 2 (Fig. 2). Such large grains are beneficial for creep properties at high temperature for this kind of alloy. The average grain size in the transversal cross-section is higher than 400 μm for the five alloys.
The dendritic structure of these heat-resistant alloys is composed of an austenitic matrix with M7C3 Cr-rich carbides and (Nb;
Conclusions
Several FeNiCrAl alloys with 3.5–4.8 wt % of Al were investigated in this paper. Their as cast microstructure consists in M7C3 and MC carbides after solidification. During ageing, secondary precipitation of carbides occurs and as a consequence improves the creep properties. Depending on their chemical composition, these alloys can contain NiAl and α′ phases at high temperatures as predicted by the thermodynamic simulations performed with the nickel database of Thermo-Calc® (TCNI8) and validated
Funding
This work has been funded by the Agence Nationale de la Recherche (ANR), project IPERS, grant number (LAB COM – 15 LCV4 0003).
CRediT authorship contribution statement
A. Facco: Conceptualization, Validation, Investigation, Writing - original draft, Writing - review & editing. M. Couvrat: Conceptualization, Validation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. D. Magné: Investigation. M. Roussel: Conceptualization, Investigation. A. Guillet: Conceptualization, Supervision. C. Pareige: Conceptualization, Validation, Resources, Writing - review & editing, Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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