High temperature deformation of Inconel 718

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Abstract

Several technological applications demand materials able to have good mechanical performance at relatively high temperatures (as high as 650 °C). This performance must be kept constant during long periods at these high temperatures. Superalloys, and particularly Ni–Cr–Fe alloys (Inconel series) appear to be candidates to accomplish such requirements. In these types of alloys, mechanical properties are achieved by precipitation of second phase particles after adequate thermal aging treatments. The present work is focused on studying the aging characteristics of Inconel 718. The study is complemented by the characterization of the hot forming behaviour of this material, and the effect of the particles on the deformation mechanism, and particularly on the softening mechanisms.

Introduction

Inconel alloys belong to the Ni–Cr based superalloys family which cover a wide range of compositions and mechanical properties. Ni and Cr provide resistance to corrosion, oxidation, carburizing and other damage mechanism acting at high temperature. Inconel alloys have good cryogenic properties, good fatigue and mechanical strength at moderate temperatures and relatively good creep behaviour. Usually, Inconel alloys are extra-alloyed with Al, Ti, Nb, Co, Cu and W to increase mechanical and corrosion resistance. Fe can also be present in amounts ranging 1–20%. These superalloys are indented for heat treatment recipients, turbines, aviation, nuclear power plants, and so on.

Inconel 718 is a relatively recent alloy as its industrial use started in 1965. It is a precipitation hardenable alloy, containing significant amounts of Fe, Nb and Mo. Minor contents of Al and Ti are also present. Inconel 718 combines good corrosion and high mechanical properties with and excellent weldabilitty. It is employed in gas turbines, rocket engines, turbine blades, and in extrusion dies and containers.

Ni and Cr contribute to the corrosion resistance of this material. They crystallize as a γ phase (face centred cubic). Nb is added to form hardening precipitates γ″ (a meta stable intermetallic compound Ni3Nb, centred tetragonal crystal). Ti and Al are added to precipitate in the form of intermetallic γ′ (Ni3(Ti, Al), simple cubic crystal). They have a lower hardening effect than γ″ particles. C is also added to precipitate in the form of MC carbides (M = Ti or Nb). In this case the C content must be low enough to allow Nb and Ti precipitation in the form of γ′ and γ″ particles. Mo is also frequent in Inconel 718 in order to increase the mechanical resistance by solid solution hardening. Finally, a β phase (intermetallic Ni3Nb), (sometimes called δ phase) can also appear. It is an equilibrium particle with orthorhombic structure. All theses particles can precipitate along the grain boundaries of the γ matrix increasing the intergranular flow resistance of the present alloy.

There are some discrepancies in the literature concerning the precipitation kinetics of these phases. Some authors [1], [2] affirm that γ′ and γ″ particles precipitate between 550 and 660 °C at large aging times, while others says that they can precipitate between 700 and 900 °C at short aging times as far as the relation (Ti + Al)/Nb = 0.66 (in at.%) is maintained. If this ratio goes up to 0.8, γ′ precipitation starts before than the γ″ one [3], [4]. A typical precipitation time temperature (PTT) diagram [5] for this alloy is shown in Fig. 1.

Hot working is the shaping process employed in materials with relatively low plasticity under cold conditions. This is particularly true in the case of Inconel 718. Forming at high temperature involves large strains at relatively low stresses, but at the same time heavy modifications on the microstructure occurs. The resulting grain size depends on some softening mechanisms, namely, dynamic recovery and recrystallization and static recrystallization. These mechanisms in turns depend on the initial microstructure, chemical composition and hot forming conditions (temperature, strain rate, etc). In the particular case of alloys presenting second phase precipitation, it is important to detect under which conditions deformation and precipitation takes place concurrently, that is, under which conditions dynamic precipitation occurs.

Bearing in mind the above introduction, the aim of the present work is to study the hot flow behaviour of a commercial Inconel 718, analysing under which conditions interaction with second phase particles must be expected.

Section snippets

Material and experimental method

The chemical composition of the Inconel 718 selected is listed in Table 1. Noticeable amounts of Nb, Ti and Al are present, as well as some Co.

This material was received in annealed state (1 h at 1010 °C), with a hardness HRB = 102. The as-received grain size was evaluated by conventional metallographic preparation, resulting in an average size of 85 μm.

Two parallel studies were carried out. The first oriented to check the validity of the PPT curve shown in Fig. 1. The second one to evaluate the hot

Results and discussion

The hardness measurements of the samples quenched after the soaking treatment are listed in Table 2.

Accordingly, precipitation has not yet started after 3 h at 1000 °C, while at 900 °C starts in any moment between 5 and 30 min. At 800 °C, precipitation starts almost immediately, and seems to be finished (constant hardness value) after 1 h. At 700 °C precipitation occurs more slowly than at higher temperatures. Finally at 600 °C no signs of precipitation were detected. These results are in perfect

Conclusions

The interval of temperatures at which precipitation of second phases takes place in the current Inconel 718 has been identified. Dynamic precipitation accelerates the precipitation kinetics and probably moves toward higher temperatures the static PTT curve. The hot flow behaviour can be reasonably well described by a hyperbolic sine type equation. The use of this equation allows one to detect the presence of precipitated particles. Additionally, it can be concluded that the deformation

Acknowledgments

Authors are grateful to the support given by Industrias Puigjaner. Additional financial support was received thorough project DPI2005-09324-C02-01.

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