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Short communication

Microstructural changes and mechanical properties of Incoloy800 after 15 years service

R. Dehmolaeib, M. Shamaniana,⁎, A. Kermanpura

aDepartment of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, IranbDepartment of Materials Science and Engineering, Faculty of Engineering, Shahid Chamran University, Ahwaz, Iran

A R T I C L E D A T A A B S T R A C T

Article history:Received 25 June 2008Accepted 28 August 2008

Microstructure andmechanical properties of Incoloy 800 superalloy before (as-received) andafter 15 years service exposure were evaluated. The metallurgical variations such asformation and growth of the secondary precipitates, phase transformation of titaniumcarbide to Cr23C6+Ni16Ti6Si7 and decomposition of primary carbides were characterizedduring the long-term service of Alloy 800. It was shown that some of chromium and ironelements weremoved from solid solution to the carbides during the service exposure. It wasfound that due to the formation of precipitates during service, the strength and hardness ofAlloy 800 were improved, while the ductility and toughness were reduced.

© 2008 Elsevier Inc. All rights reserved.

Keywords:Incoloy 800Alloy 800AgingMicrostructureG phase (Ni16Ti6Si7)Mechanical properties

1. Introduction

Incoloy 800 is a solid solution-strengthened iron–nickel basesuperalloy which is extensively used in high temperatureenvironments, such as steam generator tubes, reformer tubes,pyrolysis tubes in the refinery and petrochemical industrials,nuclear power reaction tubes andgas turbines [1–5]. Evaluationof the microstructure and mechanical properties of Incoloy800 after long-term service exposure has been an importantmeans for estimating the life of equipment such as headersand superheater tubes. In addition, aging of alloys at hightemperature causes formation of secondary carbides and in-termetallic compounds in the matrix and along the grainboundaries. These precipitates improve the strength andhardness but reduce the toughness and ductility, resultingin a decrease of weldability [5,6]. Although many metallurgi-cal investigations have been carried out on Incoloy 800, less

attention has been paid on the microstructure of this alloyafter long-term service exposure.

In this paper, the microstructure and mechanical proper-ties of Incoloy 800 taken from the header of gas reformer tubesof the Isfahan oil refinery company, which have been inservice for 15 years in a hydrogenated environment at 815 °C,were characterized.

2. Experimental Procedures

The materials examined were10×20×40 mm specimens ofIncoloy 800 with composition of 39.5Fe, 31.1Ni, 17.9Cr, 0.09C,0.36Ti, 0.25Al, 1.0Mn and 0.7Si. The specimens were preparedfrom two tubes. The first was in the solution-annealed condition(as-received) and the second was taken from a tube which hadbeen in service for 15 years in a hydrogenated environment at

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⁎ Corresponding author. Tel./fax: +98 311 3915737.E-mail address: shamanian@cc.iut.ac.ir (M. Shamanian).

1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2008.08.012

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815 °C. The thickness and external diameters of both tubes were13.5 mm and 320 mm, respectively.

In order to perform metallographic examinations, thespecimensweremechanically polished and etched inMarble'sreagent (10 g CuSO4+50 cc HCl+50 cc H2O). The microstruc-tures of specimens were characterized by optical microscopy,as well as scanning electron microscopy (SEM) and transmis-sion electron microscopy (TEM), both equipped with energydispersive spectroscopy (EDS).

Tensile and charpy V-notch impact tests were carried outaccording to the standards ASTM E8 and ASTM E 23, respec-tively. Hardness tests were carried out with a Vickers Amsler,model 2RC-S. The applied load was 30 N for the Vickers scale.

3. Results

3.1. Microstructure

Fig. 1 shows the microstructure of Incoloy 800 in the solution-annealed condition. The optical image (Fig. 1a) shows the fullyaustenitic matrix containing several types of precipitates whichare found in theausteniticmatrixandalong thegrainboundaries.The SEMmicrograph shows the precipitate morphology (Fig. 1b).The chemical composition of these precipitates was obtained byEDS analysis. It was found that the cubic precipitates are rich intitanium and nitrogen, and may be identified as nitride orcarbonitride of titanium formed during solidification from themelt; the spherical precipitates are rich in titanium and carbon,and may be identified as titanium carbide (TiC). These results

Fig. 1 –Metallographyof Incoloy800: (a) light opticalmicrograph;(b) SEM image using backscattered electrons.

Fig. 2 –Microstructure of aged Incoloy 800: (a) light optical micrograph; (b) light optical micrograph with higher magnification;(c) SEM image; (d) SEM image of two-color precipitates.

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were obtained by other authors as well [7–9]. In addition, severalannealing twins can also be observed in Fig. 1a.

Fig. 2a shows that many metallurgical variations in theprimary structures and phases, as well as the formation ofnew compounds, can be identified when service-exposed. Inaddition, titanium nitride precipitates with cubic morphologyare observed in this condition because they can be stable nearthe melting temperature [1]. Fig. 2b shows other phases withdifferent morphologies in the matrix and along the grainboundaries. More studies by SEM (Fig. 2c) shows these phasesconsist of two different compounds [6]. Micrograph takenby SEM for two-color precipitates at higher magnification isshown in Fig. 2d. The chemical composition of these precipita-tionswere obtained by EDS analysis (Fig. 3).The EDS analysis ofthe precipitates on the grain boundaries (Fig. 3a) indicates thatthese precipitates are rich in chromium and carbon, and areprobably identifiable as M23C6 (M is chromium with a smallamount of iron). Fig. 3b shows that the dark regions of two-color precipitates are also rich in chromium and carbon, andcan also be identified as M23C6; however, the white regions are

rich in nickel, titanium and silicon (Fig. 3c), and are identifiedas G phase (Ni16Ti6Si7).

To show the fine secondary precipitates, TEM character-izationswere carried out. Fig. 4 shows the TEM image obtainedfor Incoloy 800 in the service-exposed condition. In Fig. 4a andb, the chromium carbide and G phase are indicated. Fig. 4c andd shows the EDS spectra for the chromium carbide (Cr23C6) andG phase precipitates, respectively.

3.2. Mechanical Properties

The results of mechanical tests for Incoloy 800 in both so-lution-annealed and service-exposed conditions are listed inTable 1. It is seen that during long-term service, ultimatetensile strength, yield strength and hardness are increased,while elongation and toughness are decreased. These changesin themechanical properties are due tometallurgical reactionsduring aging such as formation of the secondary carbides, andphase transformation of titaniumcarbide to Cr23C6+Ni16Ti6Si7.Therefore, the loss of ductility and toughness of Incoloy 800

Fig. 3 –EDS spectra of the precipitates in the aged Incoloy 800 basemetal: (a) precipitates on the grain boundaries; (b) dark part oftwo-color precipitates; (c) white part of two-color precipitates.

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after long-term service can be attributed to three factors: theformation and growth of secondary carbides in the matrix, Gphase transformation and decomposition of primary carbides.

4. Discussion

Cr Carbide (Cr23C6) precipitates are stable in the temperaturerange of 600–850 °C [7].Within this temperature range, chromiumatoms can diffuse to the austenite phase and grain boundariesmuch faster than titanium atoms. Due to high temperature(815 °C) and long-term service, significant chromium diffusionmay occur. On the other hand, the dissolved carbon of theaustenitic matrix at the service temperature has a tendency tomigrate to the grain boundaries [10]. Therefore, chromium candiffuse alongand throughgrainboundaries and reactwith carbonresulting in M23C6 [11,12].

During high temperature-long time service of Incoloy 800,the diffusion of chromium, silicon and nickel from the auste-niticmatrix occurs towards titaniumcarbides (TiC),where it can

react with carbon in the TiC and can form some Cr carbide(Cr23C6) or mixed (Ti,Cr)C. On the other hand, silicon and nickeldiffuse from the austenitic matrix towards titanium carbideand can form G phases (Ni16Ti6Si7), with some of the Ti in thetitanium carbide. Therefore, during service, TiC particles werefully transformed to G phase and Cr23C6 in Alloy 800 as aconsequence of the instability of TiC at this temperature (about815 °C). [Note that titanium carbide (TiC) is a high temperatureprecipitate, which is formed at temperatures between 1000 and1200 °C at very short times [13]].

Titanium carbide (TiC) is a good source for the formation ofthe other carbides and intermetallic phases [11]. Decomposi-tion reaction of this carbide is as follows:

ðTi;MoÞC þ ðNi;Cr;Al;TiÞ→Cr21Mo2C6 þ Ni3ðAl;TiÞ

and reaction of G phase formation is as follows:

TiC þ ðNi;Cr; Si;TiÞ→Cr23C6 þ Ni16Ti6Si7ðGphaseÞ

Phase transformations similar to G phase transformation havealso been reported in other alloy systems [6]. Long-term service

Table 1 –Mechanical properties of Incoloy 800

Tensile strength (MPa) Yield strength (MPa) Elongation (%) Hardness (V) Toughness (J)

Incoloy 800 521 212 42 138 175Aged Incoloy 800 569 280 28.2 154 33.47

Fig. 4 –TEM image of aged Incoloy 800, showing details of the secondary precipitates: (a) chromium carbide and (b) G phase. EDSspectra (with TEM) of the above precipitates (c) chromium carbide and (d) G phase.

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of Incoloy 800 causes the important metallurgical variations inthemicrostructure, suchas formationof secondaryprecipitate (Crcarbide) and two-color precipitates (Cr23C6+Ni16Ti6Si7). Thesevariations can increase the material embrittlement causing lowductility and poor weldability (see Table 1). Other investigationshave indicated similar results for heat resistant steels [6,14–16].

5. Conclusions

• The formation of secondary carbides, transformation ofprimary carbides and formation of G phase (Ni16Ti6Si7) arethe main microstructural changes taking place during thelong-term service of Incoloy 800 at 815 °C.

• Increasing the strength and hardness and decreasing theductility and toughness during the service of Incoloy 800 canbe attributed to these microstructural variations.

R E F E R E N C E S

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[5] Coppola R, Fiorentin SRe. Study of γ'-precipitation kinetics inalloy 800 at 575 °C by small angle neutron scattering. NuclInstrum Methods Phys Res 1987;B22:564–72.

[6] Haro S, Lopez D, Velasco A, Viramontes R. Microstructuralfactors that determine the weldability of a high Cr–high Si HK40 alloy. Mater Chem Phys 2000;66:90–6.

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[10] Paul AR, Kaimal KNG. Migration of carbon in Incoloy 800.Metals Chem Phy 1998;53:189–94.

[11] Lewis MH, Hatersley B. Precipitation of M23C6 in austeniticsteels. Acta Metall 1965;13:1159–68.

[12] Tavassoli AA, Colombe G. Effect of minor alloying elementvariation on the properties of alloy 800. Metall Trans A1977;18:1577–80.

[13] KouS.Weldingmetallurgy. JohnWiley&Sons, Inc; 2003. p. 436–7.[14] AlmeidaLH,RibeiroAF, LeMay I.Microstructural characterization

of modified 25Cr–35Ni centrifugally cast steel furnace tubes.Mater Charact 2003;49:219.229.

[15] Dehmolaei R, Shamanian M, Kermanpur A. Improvingweldability of aged 25Cr–35Ni heat resistant steel/alloy 800dissimilar welds. Sci Technol Weld Join 2007;12:586–92.

[16] Dehmolaei R, Shamanian M, Kermanpur A. Effect of solutionannealing on weldability of aged alloy 800/25Cr–35Ni steeldissimilar welds. Sci Technol Weld Join 2008;13:516–23.

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