IN718: HIGHER TEMPERATURE APPLICATION RANGE FOR...

15. - 17. 5. 2013, Brno, Czech Republic, EU

IN718: HIGHER TEMPERATURE APPLICATION RANGE FOR AN OLD SUPERALLOY

DI GIANFRANCESCO, P. LOMBARDI, D. VENDITTI

Centro Sviluppo Materiali SpA, Rome, Italy, EU

Abstract

The 718 is a very well know superalloy developed in the years ’60 worldwide used for aircraft engine,

petrochemical plants, oil and gas applications, with a very wide operating temperatures spectrum with an

upper limit close to 650°C.

The continuous trend in efficiency improving for the new fossil fuel fired plants for power generation needs

an increasing in temperature and pressure of the critical components: boiler and turbine. Therefore the

tubes, cast and forged components in the hottest part of the plant are hardly stressed and steel and alloys

with improved creep performances are required.

The steam turbine inlet temperature for the new generation plants is scheduled in the range of 720°C with a

pressure of 350MPa and the high and intermediate pressure rotor sections are going to be produced in

superalloys. In the frame of the Thermie Advanced (700°C) Pulverised Fuel Power European Project a trial

rotor disk has been forged by Società delle Fucine in Italy using a billet produced by AOD + double VAR

remelting at Acciaierie Foroni.

A new heat treatment was developed by Centro Sviluppo Materiali to increase the microstructural stability in

the range of 650-750°C and to improve the creep behaviour. Now the longest creep tests are approaching

the 100.000 hours and a creep data assessment show an increase of about 50°C respect to the standard

UNS718 material.

This work summarize the 12 years of work done that could now open the possibility of use the 718

superalloy up to 700°C for forged components in the new steam turbine for the Advanced Ultra Supercritical

Power Plants.

Keywords: IN718, forging, rotor, creep resistance, mechanical properties, microstructural evolution, heat

treatment.

1. INTRODUCTION

The driving forces improving the production of electric power have changed from growth of demand in the

60’s to considerations concerning environment and efficiency in the 90’s. In 1996 the European Commission

approved in the frame of the THERMIE programme the demonstration project "Advanced ("700°C") PF

Power Plant" proposed by a large group of material and components suppliers of the power industry, R&D

centres and leading utilities. The aim of the project is to develop a new generation of coal fired power plant

able to raise up to 375 bar/700°C steam conditions and to increase electrical efficiency up to 52-55% [1].

Consequently in the hottest part of the steam cycle it will be no longer possible to use steel components, but

new nickel-based superalloys are required; then it is necessary to optimise, test and qualify these materials.

In parallel, studies on the design of the large critical components have been performed to optimise the size

as function of production limits.

At the beginning of the program several existing superalloys have been taken into account for screening

tests in order to evaluate the most promising candidates for the component manufacture.


The Italian contribute to the program has been focused on the INCONEL 718 super alloy. It is well known by

literature [2] that the maximum operating temperature for IN718 treated in the standard conditions

(UNS7718) is about 650°C. Therefore a new heat treatment was developed to increase the high temperature

behaviours of the IN718 and his microstructural stability to target the AD700 goal and a prototype disk was

produced [3]. After the optimisation of the heat treatment, by mechanical and short term creep tests, a long

term creep program was launched in order evaluate the creep resistance of the new modified heat

treatment, as well as, microstructural analysis to investigate the alloy stability.

The aim of this paper is to present the results of this test programme that are discussed in relationship with

the microstructure evolution in the service conditions.

2. MATERIAL PRODUCTION

One ton billet produced by Electric Arc Furnace, followed by Argon-Oxygen Degassing refining and double

VAR (Vacuum Arc Remelting) remelted by Acciaierie Foroni, has been forged by Società delle Fucine using

a 12.600 ton press, following a procedure designed by model simulations in order to promote

recrystallisation. Figure 1 shows the billet before forging and the prototype forged disk having the following

dimension: diameter 850mm and thickness 350mm [3].

Fig. 1: IN718 billet before forging and the prototype disk after forging

Table 1 shows the chemical analysis of IN718 billet compared with that of the standard UNS7718; the trace

elements are the following: Pb <0.2, Bi <0.1, Se<1.0, Sn<10, Ag < 1.0 (values in ppm).

Table 1: Chemical analysis of Foroni IN718 billet (mass pct.)

C Ni Fe Cr Mo W Co Nb Ti Al B Zr Mn Si P S

Standard

UNS7718

<0.0

8

50

-

55

Bal

17

-

21

2.8

-

3.3

/

<1

4.8

-

5.5

0.7

-

1.2

0.2

-

0.8

<

0.006

/

<

0.35

<

0.35

/

<

0.015

Foroni

Billet

0.03

54

17.5

18.5

3

<0.1

<

0.1

5

0.9

0.5

0.004

<

0.0005

0.15

0.15

0.008

0.001

3. MECHANICAL BEHAVIOUR OF PROTOTYPE DISK

During the Phase 1 of the Thermie AD700 project [4] a new heat treatment has been developed by CSM [3],

in order to improve the high temperature properties of the IN718 and in particular the creep behaviour. After

the promising results obtained respect to the conventional heat treatment, the new one was applied on the


trial disk and the mechanical properties obtained have been compared with those obtained by the convention

heat treatment.

The standard IN718 super alloy is characterised by the precipitation of cuboidal ’ phase (Ni3Al,Ti) and ’’

phase (Ni3Nb), acicular and fine, as consequence of the amount of Nb in the matrix. Also MC carbides are

present. The ’’ phase, during exposure at high temperature, evolves into the phase, coarse and gets

acicular, producing a reduction of properties and ductility.

The heat treatment developed by CSM had the aim to stabilise ’ e ’’ phases: the solubilisation temperature

has been increased up to 1065°C, followed by air cooling and two aging stages. Also relatively high aging

temperatures have been employed (Table 2). Figure 2 shows a comparison of the obtained microstructures.

A largest amount of ’’ phase is present in the modified treated material. As summarised in table 2 the room

temperature properties of the treated material were in agreement with the standard requirements.

Table 2: Heat treatment conditions and mechanical properties (FC = Furnace cooling)

Solubilisation

(°C/h/Cooling )

First aging

(°C/h/Cooling )

Second aging

(°C/h/Cooling )

YS

(MPa)

UTS

(MPa)

Elong.

(%)

Impact

energy (J)

Grain size

(ASTM No.)

Standard 927-1010 720/8/FC 620/8/air >1035 >1240 >10

Foroni

Stand.

1000/2/water

718/8/FC

621/8/air

1135

1403

20.4

74

5

Foroni

Mod.

1065/1/air

760/8/FC

650/12/air

1120

1335

27.0

80

4

To evaluate the microstructural stability and the mechanical behaviour after standard and modified heat

treatment (HT), aging tests were carried out at 700 and 750°C up to 10200 h: the tensile strength slightly

decrease after aging (Figure 3). It is important to take into account that aging at 750°C for 3000 h, according

with Larson Miller Parameter (LMP), is equivalent to 60.000 hours at 700°C.

4. IN718 CREEP BEHAVIOUR WITH NEW HEAT TREATMENT

In order to evaluate the creep behaviour of the IN718 with modified heat treatment, a long term creep test

program was defined in the temperature range 650-750°C with smooth and notched specimens.

a b

Fig. 2: Microstructure after standard heat treatment: a) conventional HT; b) modified HT


0

200

400

600

800

1000

1200

1400

1600

20 21 22 23 24 25

PLM=(T+273)(20+logt)/1000

Str

ess

(M

Pa

)

UTS TT standard

YS TT standard

UTS TT mod

YS TT mod

Fig. 3: Effect of aging on mechanical properties

At present 2 specimens are still running over than 90.000 hours: figure 4 shows the isothermal curves of

smooth and notched specimens. The IN718 Mod creep behaviours are not sensitive to notch.

In figure 5, plotting the minimum creep rate vs applied stress at the different temperatures it is possible to

observe the variation of the Norton exponent that decrease from 11,5 at 650°C up to 1,4 at 750°C showing a

change in the creep mechanisms from dislocation to diffusive creep.

Fig. 4: Isothermal curves of smooth and notched specimens (yellow point are test still running)


y = 3E-36x11,567

R² = 0,9738y = 2E-12x3,0698

R² = 0,9871

y = 2E-07x1,3756

R² = 0,8832

1,E-06

1,E-05

1,E-04

1,E-03

1,E-02

50 500

Min

imu

m c

ree

p r

ate

(1

/h)

Stress (MPa)

650°C 700°C 750°C

Fig. 5: minimum creep rate vs applied stress

An assessment has been made by the ECCC (European Creep Collaborative Committee) procedures [5] in

2009 [6]. In figure 6 the current creep results of the smooth broken specimens are plotted and compared with

the data assessment. The value to obtain rupture at 700°C in 100.000, previously predicted in the range of

120+/-15 MPa, could be a little lower taking in account that the specimen still running at 700°C with 130MPa

is close to rupture having a current elongation of 12 and it is started the tertiary creep stage. A new

assessment will be carried out as soon as new rupture data will be available. In fact the current assessment

seems overestimate the 700°C performance and underestimate it at 750°C.

Fig. 6: 2012 creep data assessment with current smooth test running (yellow points)

The creep behaviours of IN718 mod superalloys have been compared with ORNL data from literature on

standard material [7]. The modified treated material shows the same creep resistance at 650°C, but a


relevant increase of the rupture time at the highest temperatures, especially at 750°C, as showed in figure 7.

The modified treated material shows at 100MPa the same rupture time of the standard material, but with an

increase of 46°C in term of service temperature (circle in the figure 7).

These differences can be also appreciated in figure 8 where the master curves of standard and modified

IN718 are compared.

Fig. 7: Comparison of isothermal curves of UNS718 and IN718 Mod

Fig. 8: Comparison of master curves of UNS718 and IN718 Mod

Furthermore the creep ductility showed by the IN718 with the modified heat treatment, appears interesting, in

terms of elongation and reduction of area to rupture, that increase with the temperature and the test time.

The figures 9 shown the elongation to rupture and the reduction of area versus time to rupture (tr), giving


evidence that the physiological evolution of ’’ in -phase doesn’t reduce ductility of the material with the

modified treatment. This behaviour was just observed in literature [8, 9] and was related to the effect of

particle to guide crack advance in a zigzag path resulted in absorbing strain energy and increasing plasticity.

Further analyses in order to confirm this mechanism are in progress.

Fig. 9: Creep Ductility of IN718 Mod: elongation and reduction of area vs rupture time (yellow points are test

still running)


5. MICROSTRUCTURAL EVOLUTION

In order to understand the reasons of these better behaviours the modified treated material TEM

investigation have been performed on both treated materials in aged conditions. Figures 10 shown a

comparison of the microstructure evolutions after 3000hours at 700°C and 750°C for the modified treated

material: at this magnification, on the specimen aged at 700°C only ’’ can be recognised, being ’ too finer

to be observed. At 750°C fast coarsening of both ’ and ’’ phases is well evident, as well as the formation of

the large plates, due to ’’ transformation. Figures 11 the microstructure after 3000 hours at 750°C for the

standard treated material shows a reduced amount of ’’phase, if compared with the microstructure of

material, in the same ageing conditions, when is treated with new heat treatment. (figures 10b and 11),

giving evidence of the effect of the new heat treatment to reduce the transformation kinetics of ’’ into

phase.

Quantitative analyses on aged specimens have been carried out to define the evolution of the main phases

[6]. The measured of ’, ’’ and sizes gave evidence that all the phases in the standard treated material

shown faster growing compared with the modified treatment. After 3,000h at 750°C, the measured

precipitate size distribution shows that ’ particles coarsening is rather similar both heat treatments, growing

up to 80nm; ’’ particles grow up close to micrometric dimension after 5.000-10,000 h and then they

decrease.

a) b)

Fig. 10: IN718 Mod treated material: TEM image on thin foil after 3.000 hours of aging: at 700°C; b) at 750°C

Fig. 11: IN718 standard treated material: TEM image on thin foil after 3.000 hours of aging at 750°C


This phenomenon can be related to the formation of phase which precipitates at the expense of ’’. By

means of the modified heat treatment, the nucleation and growth rates of phase are delayed.

Additional microstructural analyses have been carried out on crept specimens: the head of these specimens

have been analysed by light microscopy, SEM + EDS and by STEM + EDS analysis.

The figures 12 by light microscopy and SEM shown clearly the stages of evolution of the microstructure with

the transformation of the ’’ in to phase starting from the grain boundary. The phase nucleate at the grain

boundary and grow in to the grain (figure 12a) and a relevant amount is present in the specimens aged at

700 and 750°C (figure 12 b and c).

With investigations at higher magnification it was possible to identify the phases in the specimens aged at

700°C for 25.860 hours, shown in figure 13: the presence of Niobium reach round shape carbides and

acicular phase, as well as, between the -phase particles still the presence of some small’’ particles: that

means that the transformation ’’ to it is not jet completed in these ageing conditions.

At 750°C the transformation of ’’ in to -phase appears completed, as can be observed by figure 14,

showing that ’ it is still present [8] and practically no more ’’ is observable. The other small dark particles,

mainly observable at the grain boundaries, are Ti reach carbides (Figure 15).

The TEM analysis on thin foil on the specimen at 750°C after 33206 hours, figure 16, confirm that ’’

transformation has been completed, but there is still present a relevant amount of small ’ particles. Figure

17 shows a typical example of Orowan climb of a dislocation on a ’ particle confirming that ’ continues to

supply a strengthening effect.

The microstructural investigations by SEM and TEM will continue as soon as the creep specimens still

running will be broken in order to consolidate the knowledge in the evolution of the phases on the In718 with

this modified treatment.

a

b


c

Fig. 12: microstructures of aged specimens by light microscopy (right) and SEM (left): a) 650°C – 25.355h;

b) 700°C – 25.860h; c) 750°C – 33.206h

Fig. 13: SEM images of the crept specimen at 700°C after 2.5860 hours: a) MC Nb rich particles; b)

phase; c) ’’ phase

Fig. 14: SEM images of the crept specimen at 750°C after 33206 hours: a) MC Nb rich particles; b) phase;

c)’ phase

a

b

a

b

c

c


Fig. 15: SEM images of the crept specimen at 750°C for 33.206hours: Ti reaches carbides (black particles)

6. DISCUSSION AND CONCLUSIONS

The manufacture of a trial rotor disk for possible application at 700°C in steam turbine has been provided.

The trial disk has been successful forged at Società delle Fucine using an AOD double VAR billet produced

by Foroni Steels.

A new heat treatment to improve the creep resistance has been developed.

The capability of the new heat treatment to increase the creep behaviours of IN718 at temperature over then

650°C has been confirmed from the results of the long term creep tests. The assessment of the creep data

could be able to guarantee 120 +/- 15 MPa at 700°C, but in the next future a new one will be realised after

the rupture of the specimens still running.

Fig. 16: TEM images of the crept specimen after 33.206 hours at 750°C: a) -phase; b)’-phase

a

b


Fig. 17: Orowan climb of a dislocation on a ’ particle in the specimen after 33.206 hours at 750°C

Comparing the behaviour of the standard IN718 at 704°C, the new treatment at the same stress (100MPa),

but at T = 750°C, it is able to guarantee the same time to rupture: that means that an improvement of 46°C

in the service temperature has been obtained.

Other relevant results are that the new heat treatment it is able to guarantee a very good ductility of creep

specimens after high temperature and long time tests, without embrittlement phenomena.

The reasons of these improved behaviours for the new heat treatment have been explained, on the base of

metallographic investigations, as follow:

The new heat treatment promote the’ and’’ precipitation, without presence of -phase after heat

treatment,

The presence of ’’ after heat treatment delay the nucleation of -phase, as well as, the transformation

of’’ in to -phase,

The ’ is more stable and it is still present, without relevant growing phenomena, after long term at

750°C.

Further microstructural investigation has been planned and will carry out on the two creep specimens still

running, when broken.

On the basis of results obtained the Inconel 718 with modified heat treatment can be included in the list of

candidate materials for the manufacture of forged components, such as rotor disks or bolts, for the new

generation of advanced Ultra Super Critical power plant that will be operate with 700°C steam temperature.

ACKNOWLEDGEMENTS

A part of this activities have been financially supported by European Community in the frame of “AD

700 (PF) Power Plant “ Project.

The author’s thanks Società delle Fucine and Foroni steel for the production activities and support

for the tests and analysis performed.

REFERENCES

[1] R.Kehlhofer, “Power Engineering, status and trends”, Materials for power advanced engineering”, 1998

[2] Guo, Han, You, “Creep Crack Growth behaviour of alloy 718”, Superalloy 718, 625, 706 and Various Derivates”,

Ed. Loria, 1998

[3] A. Di Gianfrancesco, O. Tassa, L. Cipolla, A. Finali, M. Calderini, R. Montani: Mechanical and microstructural

qualification of a prototype IN718 forged disk; IFM2003, Kobe October 2003

400nm


[4] www.ad700.dk/

[5] Creep data validation and assessment procedures: ECCC Recommendations Volumes 2005

[6] L. Cipolla, M. Calderini, A. Di Gianfrancesco, L. Foroni, R. Montani, O.Tassa, D. Venditti: Mechanical and

microstructural qualification of a prototype IN718 forged disk: ECCC Conference, Zurich CH 22-24 April 2009

[7] R.W. Hayes, “Creep deformation of Inconel 718 in the 650°C to 760°C temperature regime”, Superalloy 718, 625,

706 and Various Derivates, Ed. Loria,

[8] Z. Yun et al., Superalloys 718, 625, 706 and various derivatives; TMS, ed. Loria, 1997, p. 229.

[9] J. Dong, X. Xie, Z. Xu, S. Zhang, M. Chen, J. Radavich: TEM study on microstructure behaviour of Alloy 718 after

long term exposure at high temperature: Superalloys 718, 625, 706 and various derivatives; TMS, ed. Loria, 1994,

p. 649.

www.ad700.dk/

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