Chiang Mai J. Sci. 2009; 36(3) 287
Chiang Mai J. Sci. 2009; 36(3) : 287-295
Microstructural Restoration by HIP and Heat Treatment
Processes in Cast Nickel Based Superalloy, IN-738
Panyawat Wangyao [a], Gobboon Lothongkum [a], Viyaporn Krongtong [b],
Weerasak Homkrajai [c] and Nutthita Chuankrerkkul* [d]
[a] Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University,
Bangkok 10330, Thailand.
[b] National Metals and Materials Technology Center (MTEC), Pathum Thani, Thailand.
[c] Electricity Generating Authority of Thailand (EGAT), Nonthaburi, Thailand.
[d] Metallurgy and Materials Science Research Institute (MMRI), Chulalongkorn University,
Bangkok 10330, Thailand.
*Author for correspondence; e-mail: firstname.lastname@example.org
Received: 9 April 2009
Accepted: 7 May 2009
Inconel 738, as a cast nickel-based superalloy components for land-based gas turbine
blades, utilized at elevated temperatures, expresses microstructural stability with excellent
mechanical properties. In this study, the microstructural refurbishment by a hot isostatic pressing
(HIP) and different heat treatment conditions after long-term service of the alloy was
investigated. Although, the HIP temperature in the HIP process was high enough to act as
solutioning treatment, it could not completely dissolve coarse γ particles into the matrix,
resulting in the residue particles. However, the next step, solutioning treatment, could dissolve
these particles more in some extent depending on each solutioning temperature. The higher
solutioning temperature provided the finer γ particles and new γ precipitation after solutioning
that could take place during primary aging. Secondary aging led to the growth of coarse and/
or very fine γ particles. All heat treatment conditions after HIP process provided much more
uniform γ morphology comparing to as-received microstructure.
Keywords: hot isostatic pressing (HIP), rejuvenation, microstructural repair, and nickel-based
Nickel base superalloys are structural solid solution strengthening or carbide
materials with chemical composition and strengthening do. Microstructure, mainly
structure, which have been developed to be characteristics and morphology of γ particles,
utilized at high temperature applications. has significant effect on mechanical properties
The alloys, used for high-strength applications more than on physical properties [1-3]. The
at elevated temperatures such as materials microstructure and mechanical properties (for
for gas turbine blades, are strengthened both low and high temperatures) for turbine
significantly by γ precipitated particles in blades of fixed chemistry are vitally dependent
FCC matrix. The γ particles precipitated in γ with prior manufacturing processes such as
matrix provided the most strengthening of casting, mechanical and heat treatments.
superalloys even much higher than the additive However, the desired mechanical properties
288 Chiang Mai J. Sci. 2009; 36(3)
mainly derived from the end of the processing which is the most similar to initial one,
sequences, which provided the final micro- and should maximize the phase stability of
structure. One of these processes is heat precipitated strengthening gamma prime
treatment. Solution treatment in most cases is particles after long-term exposure. The aim
followed by a single or a double aging sequence of this research work is to determine the
to precipitate homogeneous distributions of most suitable and practicable repair-condition,
either cuboidal or spherical gamma prime which can give the proper microstructural
within the grains interior as well as discrete characteristics by rejuvenation method of hot
grain boundary carbides [4-6]. Full or partial isostatic pressing (HIP), followed by various
solution treatment temperatures, including heat treatments for long term exposed gas
aging treatments, have been developed and turbine blades, casting nickel-based superalloy
modified to optimize the completed precipita- grade IN-738 after 70,000-hour service
tion of gamma prime phase in matrix. operated by Electricity Generating of Thailand
Practically, the alloy’s standard reheat (EGAT).
treatment for cast superalloys does not always
work perfectly when applied to the long-term 2. MATERIALS AND METHODS
serviced microstructure to restore mechanical The cast nickel-based superalloy in this
properties as well as to the welded superalloy study was IN-738, with the composition
components or to HIPed superalloy parts to (mass%) of 15.84%Cr, 8.5%Co, 3.47%Ti,
rejuvenate and establish optimal microstruc- 3.46%Al, 2.48%W, 1.88%Mo, 1.69%Ta,
ture [1, 5-12]. For example, practical standard 0.92%Nb, 0.11%C, 0.07%Fe, 0.12%B,
heat treatment cycles used for the most 0.04%Zr and balance nickel. Rectangular
common industrial turbine rotating blade plates, having a dimension of 1 1 cm2, were
material, a cast polycrystalline IN-738 is in cut from the most severe degradation zone
the following step: 1) Solution treatment at of turbine blades. For the HIP condition,
1120 oC for 2-4 hr followed by a rapid gas specimens were HIPed at pressure of 100
quench (25 oC to 55 oC/minute) to below MPa for 5 hr at 1,200 oC. Next, the HIPed
650 oC and 2) Precipitation aging at 845 oC specimens were heat treated according to
for 20 hr followed by a rapid gas quench to heat treatment conditions, including solution
room temperature, which is similar to the one treatment and primary and secondary
of another cast superalloy, GTD-111. The precipitate aging treatments in vacuum furnace.
reason why the standard heat treatment does Detail of the experimental heat-treatment is
not often work well is that the γ solution shown in Table 1. Heat treated plates were
temperature for the alloy range from 1175 oC cross sectioned in order to observe micro-
to 1,190 oC . Furthermore, the heat treatment structure comparing to those of parallel
condition resulted only in a partial homogeni- grinded and polished surface of turbine
zation by exclusive dissolution of dendritic blades. All sectioned samples were polished
core g’ particles [1-8]. Therefore, the alloy was using standard metallographic techniques
recommended to be solutioning treated at and were subsequently etched in marble
higher temperatures, such as at 1,200 oC, to etchant, i.e. 10 g CuSO4, 50 ml HCl, and
fully restore the microstructure after long-term 50 ml H 2 O. The microstructures were
service. observed by scanning electron microscope
The proper re-heat treatment condition with secondary electron mode and image
should provide the optimum microstructure, analyzer.
Chiang Mai J. Sci. 2009; 36(3) 289
Table 1. Heat treatment conditions applied to long-term exposed IN-738.
Primary precipitate Secondary precipitate
No. Solution annealing
1 -------------------- -------------------- 845 oC / 20 hr (AC)
2 -------------------- 925 oC / 1 hr (AC) 845 oC / 20 hr (AC)
3 -------------------- 1055 oC / 1 hr (AC) 845 oC / 20 hr (AC)
4* 1125 oC / 2 hr (AC) -------------------- 845 oC / 20 hr (AC)
5 1125 oC / 2 hr (AC) 925 oC / 1 hr (AC) 845 oC / 20 hr (AC)
6 1125 oC / 2 hr (AC) 1055 oC / 1 hr (AC) 845 oC / 20 hr (AC)
7 1175 oC / 2 hr (AC) -------------------- 845 oC / 20 hr (AC)
8 1175 oC / 2 hr (AC) 925 oC / 1 hr (AC) 845 oC / 20 hr (AC)
9 1175 oC / 2 hr (AC) 1055 oC / 1 hr (AC) 845 oC / 20 hr (AC)
10 1205 oC / 2 hr (AC) -------------------- 845 oC / 20 hr (AC)
11 1205 oC / 2 hr (AC) 925 oC / 1 hr (AC) 845 oC / 20 hr (AC)
12 1205 oC / 2 hr (AC) 1055 oC / 1 hr (AC) 845 oC / 20 hr (AC)
* Standard heat treatment condition
3. RESULTS AND DISCUSSION have spheroidized and secondary gamma
3.1 The microstructure of as-received prime coarsened in the airfoil samples.
SEM micrographs of the transverse 3.2 HIPed microstructure
sections at about mid blade height of the SEM investigation in specimens found
airfoil is shown in Figure 1. The microstructure that the microvoids were very rarely to be
of as-cast alloy generally consists of non- detected. This fact is very important to
uniform precipitation of ordered L1 2 γ consider about the advantage of HIP process
intermetallic phase within dendrite core and for refurbishment of superalloy components.
in the interdendritic region. Coalescence of HIP process resulted in a reduction of internal
the primary and secondary gamma prime voids and/or microcracks due to the alloy
particles, as a result of long-term service, yielding in compression under the action of
seems to occur causing larger and rounded the applied hot pressure [13-15]. Figure 2
particles. The degree of degradation, as illustrates the etched microstructure of HIPed
measured by the gamma prime particle size, specimen with the uniform dispersion of
increases with exposed time and service some smaller coarse gamma prime particles,
temperature. In this study, however, the which previously were partially dissolved into
average coarse gamma prime particle size was the matrix during HIP at high temperature.
approximately 1 mm. The airfoil microstructure It should be noted that the previous coarse
shows significant degradation in service gamma prime particles could not be
comparing to the microstructure of the root completely solutioned at 1,200 oC for 5 hr.
section. The primary gamma prime particles
290 Chiang Mai J. Sci. 2009; 36(3)
Figure 1. As-received microstructure after long-term service showing the
coalescence of γ’ particles.
Figure 2. HIPed microstructure showing partially dissolved gamma prime particles.
Chiang Mai J. Sci. 2009; 36(3) 291
3.3 Heat treated microstructure after HIP Figure 3b shows the HIPed microstructure
process followed with both primary aging at 925 oC
According to the previous work , / 1 hr and secondary aging at 845 oC for
repeating the standard heat treatment sequence 20 hr. It consists of homogeneous distribution
does not always work well. The structure and of precipitated gamma prime particles with
properties were not fully recovered by this slightly coarser size in the matrix comparing
refurbishment treatment applied to these than those of heat treated microstructure of
IN-738 blades. It is reported that the program No.1. The average diameter size
microstructure was only partially recovered of these coarse precipitated particles is
by such simple re-heat treatment. However, approximately 0.18 μm. However, it should
most reheat treatments after HIP process in be noted that the volume fraction of
long-term serviced turbine blade IN-738, precipitated phases is also slightly higher
all provide the various microstructural than that of microstructure in program No.1.
restoration characteristics with more It can be noted that the primary aging had an
homogeneous structure comparing to the effect on the obtained microstructure
initial one, as shown in Figure 3. This was due characteristics. During primary aging at
to the fact that the HIP temperature was high 925 oC , the gamma prime phase could early
enough to dissolve previous coarse gamma reprecipitate throughout the matrix and
prime particles acting as partial solutioning continue to grow under secondary aging
process. It was found that the heat treated period, resulted in slightly coarser size and
microstructure according to program No. 1, higher volume fraction of γ particles, see
which followed by only secondary aging, Figures 4 and 5.
consists of the uniform distribution of gamma Figure 3c shows the HIPed microstructure
prime particles precipitating in the matrix as followed with primary aging at 1,055 οC / 1
mixing between cubic and round shape, hr and then secondary aging at 845 οC for 20
see Figure 3a. It should be noted that the hr. The microstructure consists of gamma
microstructure contains only single size of prime particles in mixing round-cubic shape
precipitated gamma prime particles having the precipitating uniformly throughout the matrix.
average diameter size about 0.15 μm. In this The average size of diameter of precipitated
structure, it could be counted that the heat gamma prime particles is about 0.17 μm with
during HIP process worked as a partial slightly less volume fraction than those of
solution treatment at 1,200 oC for 5 hr and conditions 1 and 2. The primary aging at
then followed by aging at 845 oC for 20 hr. 1,055 οC/1 hr could have an influence on the
The microstructure after HIP process morphology of γ particles, which is different
was partially prepared for the next step, from the lower temperature aging (at 925 οC ).
precipitation aging, which previous coarse During this primary aging with too high
gamma prime particles were partially temperature, gamma prime phase could
dissolved into the matrix in some degree precipitate into the matrix but in lower
resulting in smaller size of gamma prime rate and/or amount comparing to the micro-
particles, see Figure 2. When precipitation structure after aging at lower temperature
(secondary) aging was applied, uniform (925 οC) in the alloy. Therefore, the obtained
gamma prime particles precipitated again volume fraction of gamma prime phase is
from existed particles with higher volume slightly less than the microstructure of condition
fraction and diameter size. No.2. After secondary aging at 845 οC for
292 Chiang Mai J. Sci. 2009; 36(3)
Figure 3. Microstructures of specimens after HIP and heat treatment: a) Condition No. 1, b)
Condition No. 2, c) Condition No. 3, d) Condition No. 4, e) Condition No. 5, f) Condition
No. 6, g) Condition No. 7, h) Condition No. 8, i) Condition No. 9, j) Condition No. 10, k)
Condition No. 11 and l) Condition No. 12.
20 hr, the size and amount of gamma prime characteristics. The solution treatment at the
particles precipitated increasingly and temperature provided morphology with
uniformly. bimodal γ phase precipitation, as shown in
The solution treatment at 1,125 οC/2 hr Figures 3d-3f of conditions 4-6. The average
has a great effect on microstructure diameter size of γ particles were about 0.37
Chiang Mai J. Sci. 2009; 36(3) 293
prime particle diameter (Micron)
Average size of gamma
Heat treatment condition
Figure 4. The relationship between heat treatment condition and average size of γ particle
Volume fraction of gamma prime
Heat treatment condition
Figure 5. The relationship between heat treatment condition and volume fraction of γ particles.
and 0.07 μm for coarse and very fine particles, to the coarser size and more cubic shape.
respectively. This solution treatment also Hence, after final aging at 845 oC / 20 hr with
resulted in more cubic shape of coarse γ or without primary aging, these γ particles
particles. The solutioning at 1,125 oC / 2 hr could continuously grow up and/or repreci-
led to more dissolution into matrix of pitate into the matrix with coarser size and
previous or residual γ particles after HIP slightly higher volume fraction of total γ
process. However, this solutioning temperature precipitated phase comparing to those final
with short heating time also resulted the rapid microstructures according to conditions No.
growth of some previous γ particles leading 1-3. The addition of primary aging (at both
294 Chiang Mai J. Sci. 2009; 36(3)
925 oC and 1,055 oC for 1 hr) had a slight 845 oC / 20 hr, the γ particles could uniformly
effect on precipitation of very fine γ particles reprecipitate in single size and round shape
but also provided great effect on volume throughout the matrix. It should be pointed
fraction of coarse γ particles. These coarse out that no γ particle in big size was observed,
cubic γ precipitated particles with primary Figure 3j. However, when inserting primary
aging could lead to the less volume fraction aging process between solutioning and final
of very fine γ particles during final aging. aging (or secondary aging), this could lead to
Figures 3g-3i of conditions 7-9 show the more precipitation of γ particles in bigger
effect of higher solutioning temperature of size during primary and secondary agings, as
1,175 oC for 2 hr on final microstructures. shown in Figures 3k and 3l. The higher
Comparing to microstructures with the lower temperature (1,055 oC ) of primary aging in
solutioning temperature (at 1,125 oC) of condition No. 12 resulted in more coarsening
conditions No. 4-6, it was found that this of γ particles than that of the lower primary
higher temperature of solution treatment aging temperature of 925 oC in condition
provided the smaller average size of diameter No. 11.
of precipitated γ particles, see Figure 4. This
higher solutioning temperature could better 4.1 CONCLUSIONS
dissolve the previous γ particles into the 4.1 The most proper solutioning tempera-
matrix comparing to that of lower one. This ture in this experimental program that should
could lead to the more homogeneous be used after HIP process is 1,125 oC, which
reprecipitation during primary and secondary could provide the final microstructures with
aging comparing to those with lower solutioning highest volume fractions as well as proper
temperature or without solutioning. The shape and size of coarse γ particles.
primary aging at 925 oC / 1 hr did not provide 4.2 In most cases, the addition of primary
significant effect on microstructure but only aging could aid more uniform distribution
resulted in the precipitation of very fine γ of both coarse and very fine γ particles as
particles in higher volume fraction as well as well as the increase of volume fractions, in
the less volume fraction of coarse γ particles comparison with those without primary aging.
comparing to microstructures without primary However, when applying the primary aging
aging. However, when primary aging at higher after too high solutioning temperatures
temperature of 1,055 oC for 1 hr was applied, (of 1,125 oC, 1,175 oC and 1,205 oC), the com-
the γ precipitation could take place rapidly, bination with the aging with highest
resulting in higher volume fraction and temperature would lead to fast and uniform
coarser size of coarse γ particles after final precipitation of coarse γ particles, conditions
aging comparing to those of conditions No. 6, 9 and 12.
No. 7 and 8. 4.3 The most proper heat treatment
The solutioning at the highest tested conditions after HIP process for the alloy
temperature of 1,205 oC / 2 hr of conditions should be condition No. 6 due to its highest
10-12 could cause very high dissolution of volume fractions of γ particles. The total
previous or residual coarse γ particles after volume fraction of γ’ particles was nearly 60%
HIP process into the matrix. The obtained and coarse γ particles precipitated uniformly
microstructures after this solutioning were and densely. However, mechanical testing,
expected to be very highly homogeneous. especially at elevated temperatures, such as
Therefore, during next step long term aging at creep, fatigue and thermal fatigue should
Chiang Mai J. Sci. 2009; 36(3) 295
be performed in further works in order Gamma Prime Particle Coarsening
to investigate the relationship between Behavior at Elevated Temperatures in
the rejuvenated microstructures and the Cast Nickel Base Superalloy, GTD-111
mechanical properties. EA, High Temp. Mater. Processes, 2008; 27:
5.ACKNOWLEDGEMENTS  Wangyao P., Chuankrerkkul N., Polsilapa
This research work was financially S., Sopon P. and Pornprasertsuk R.,
supported by the postdoctoral research grant Gamma Prime Phase Stability after
Long-Term Thermal Exposure in Cast
(No. MGR 4880088) from The Thailand
Nickel Based Superalloy, IN-738, Chiang
Research Fund (TRF) and The Commission
Mai J. Sci., Accepted 2009; 36(3): 287-295.
of Higher Education of Thailand. Special
 Kim M.T., Chang S.Y. and Won J.B.,
thank is also extended to Electricity Generating
Effect of HIP process on the micro-
Authority of Thailand (EGAT), Nonthaburi,
structural evolution of a nickel-based
Thailand for material support and technical superalloy, Mat. Sci. Eng., A, 2006; 441:
 Kim M.T., Kim D.S. and Oh O.Y., Effect
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