2004 ME Graduate Student Conference
April 17, 2004
STUDY OF OXIDATION AND IPAP NITRIDING CHARACTERISTICS OF
SUPERALLOY IN738LC
Mary Shanti Pampana
M.S. Candidate
Faculty Advisor: Aravamudhan Raman
Abstract
Machine components, which have to
withstand severe environment and high
temperatures, must be made of suitable material.
Inconel alloys have good oxidation and creep
resistance [1]. Power Generation industries use
Inconel alloys extensively in Gas turbines parts.
The physical metallurgy is sophisticated, yet the
relationship of properties to structures in these
alloys is certainly the best known of all materials
for use in temperature range of 650-1100°C
[1].This paper focuses on the oxidation and
Intensified
plasma
assisted
nitriding
characteristics of Inconel 738LC.
Oxidation
Oxidation kinetics, microstructural changes,
phases present in Superalloy IN738LC at
1000C, 1090C, 1140C and 1190C in air have
been investigated. It was observed that the
oxidation of IN738LC follows the parabolic rate
law and it losses weight at very high
temperatures like 1140C and 1190C after
certain interval of time. Volatilization of certain
oxides at higher temperatures can explain this
behavioral change of the alloy. The volatile
oxide forming elements present in the alloy are
Cr, Ta and Nb. Above 1140 C it is observed that
Cr2O3 gets converted to CrO3 which is volatile.
The XRD analysis of the oxidized
samples as given in the following figures showed
preferred orientation [2] of the matrix and the
precipitate of the metal below the oxide layer. At
1000C it was observed that the alloy below the
oxide is rich in solute having {131} orientation
and all the γ’ precipitate is dissolved in the
matrix forming a no precipitate region called the
precipitate depleted zone as shown in Fig 1.At
1090C it showed that fine γ’ precipitate is
formed. It is interesting to find coarse γ’
precipitate along with solute rich matrix at
1140C, while at 1190C the matrix changed its
orientation as that of γ’ precipitate .It was
indicated that the matrix and precipitate are
having {220} orientation. Using XPS, the
surface composition as a function of depth was
studied.
IPAP Nitriding
Many surface modification methods are
available in recent years for enhancing properties
like wear, fatigue and corrosion resistance [3].
Plasma vapor deposition and plasma assisted
chemical vapor deposition are thermo chemical
processes widely used in industrial applications.
Intensified plasma assisted nitriding process is
the one of the most advanced case hardening
technique and was used in the present study to
modify the surface morphology of IN738LC to
improve the reliability and performance in
service and in environmental conditions [3].
The Wear test of the nitrided samples
shows that the friction coefficient is
approximately 1.0.The microhardness tests on
the 3 nitrided samples showed that as the current
density was increased the hardness value of the
surface layer increased indicating the
improvement in mechanical properties. The
nitrogen content in sample 1, sample 2 and
sample 3 with current densities 0.75, 1.0 and
1.5mA/cm2 respectively was studied using XPS.
From the XRD analysis as shown in the
following figures, it was observed that the
nitrided samples have preferred orientation.
The nitrided sample 1 shows that the
fine precipitates were formed in solute rich
matrix, Nitrided sample 2 shows only the fine
precipitate and both matrix and the γ’ precipitate
have {220} orientation. The nitrided sample 3
showed that the precipitates grew and became
coarse due the higher current density.
Figures
1190'C
1200
intensity
1000
800
600
400
200
0
30
Fig.1 showing The oxide scale and precipitate
γ’ depleted region
35
40
45
50
55
60
65
70
75
80
85
90
95 100 105
Theta/2Theta
XRD pictures of As-receiced, oxided and nitided
samples
References:
1. Superalloys II, Chester T.Sims,
Norman S.S., W.S.Hagel.
2. Ercan Balikci, PhD Dissertation, p. 51,
LSU, Baton Rouge, LA, May 1998.
3. V.Singh, K.Marchev, C.V.Cooper,
E.I.Meletis, Surface and Coatings
Technology, Vol. 160,2002,p.249.