1. 2 MATERIALS PERFORMANCE December 2009
Niobium
Stabilized
Alloys in Steam
Hydrocarbon
Reforming
Ramesh Singh, Gulf Interstate Engineering, Houston, Texas
This article explains the steam reforming process
and discusses the development of niobium-stabilized
microalloys. Also presented are the advantages of
these alloys over materials commonly used
for steam reformer tubes.
I
n the past, refining was done in ves-
sels made of Type 300 series stainless
steels (SS). Higher pressure and tem-
perature were addressed by use of
nickel and Ni-Cr-Fe solid solution alloys.
The processing of petroleum promised
innumerable possibilities. Increased ca-
pabilities and efficient processes were
designed and called “thermal cracking”
or “catalytic reforming.”
Catalytic Reforming
Catalytic reforming is designed to
upgrade the quality of petroleum byprod-
ucts. This process takes place in tubes that
are suspended in a furnace, where a
variety of reactions occur in the presence
of some catalytic agent. The tubes are fed
with a hydrocarbon and steam mixture.
The catalyst synthesizes ammonia (NH3
)
by chemically combining hydrogen and
nitrogen under pressure. The catalytic
reaction of the steam and hydrocarbons
mixture at an elevated temperature sup-
plies hydrogen for the reaction.
C H nH O nCO n Hn m + + +2 2( )m/2
(1)
(reforming reaction)
CO H O CO H+ +2 2 2 (2)
Since the number of moles of the
product exceeds the number of moles of
reactants, this is an endothermic reaction
and requires energy input from burning
natural gas or naphtha. The high tem-
perature causes axial stress in the body of
the tube, leading to longitudinal creep.
The pressure in the tube creates hoop
stress that develops circumferential creep.
Both kinds of stress reduce the life of the
reformer tube.
These actions create a need for special
material that has the best possible creep
strength and still meets the service condi-
tions in the furnace. Applicable tube mate-
rial must have the following properties:
• High temperature resistance under
internal pressure
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2. December 2009 MATERIALS PERFORMANCE 3
M AT E R I A L S S E L E C T I O N & D E S I G N
• High creep resistance
• Resistance to attack from furnace
contaminates
These properties aim to achieve high
creep rupture strength.
Important Aspects to
Consider in a Reformer
System
The following six aspects are consid-
ered in the design of an efficient reformer
system.
Reformer Tubes—
Material of Construction
Reformers are centrifugally cast Fe-
Cr-Ni tubes. A typical tube is machined
and has a wall thickness of 12 to 19 mm
and an internal diameter of 88.90 to
100.98 mm. The wall thickness is kept as
low as possible to reduce the weight and
the risk of developing longitudinal creep
stresses. Figure 1 illustrates the schematic
arrangement of a reforming plant. Figure
2 shows the tubes in an actual furnace.
Reformer material affects throughput
and the energy consumption. Conven-
tionally, alloys HK 40†
(UNS J94204), HP
35†
, HP 45†
, IN 519†
(UNS N08367), or
equivalent alloys have been used.
Physics of Grain Structure
Irrespective of the composition of the
material, the cast structure of these alloys
varies according to the cooling rates.
Within a controlled range, these alloy
castings may have the following common
grain structures (Figure 3):
• Columnar gain adjacent to skin of
the cast tubes
• Equiaxed structure
The austenitic cast materials tend to
have more columnar structure than ferrite
structure, often ranging from 90 to 100%
columnar structure when cooling is con-
trolled. This property of austenite is the
†
Trade name.
Schematic diagram of a reforming plant.
Rows of reformer tubes in a furnace.
FIGURE 1
FIGURE 2
key factor in the choice of conventional or
niobium-stabilized alloy variants.
Niobium-stabilized alloys have austen-
itic structure; hence, on solidification,
they develop large columnar grains. The
foundry practice to cool the outer surface
of the mold to arrest austenite structure
forms the nuclei and helps in the forma-
tion of the columnar grain by increasing
the cooling rate in the direction of heat
travel. At solidification, the isotherm
moves nearly equal to the growth rate of
the macrostructure. The columnar
growth is obtained in dendrite as well as
austenitic alloys. The dendrite or eutectic
fronts grow at the speed Vs. This speed
is directly related to that of the isotherm
(Vm). This is but only one factor in the
promotion of columnar structure. The
other variables are temperature-related.
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3. 4 MATERIALS PERFORMANCE December 2009
M AT E R I A L S S E L E C T I O N & D E S I G N Niobium Stabilized Alloys in Steam Hydrovarbon Reforming
• Δ Tc = Temperature at which co-
lumnar grains starts to form
• Δ Tn = Temperature at which nu-
clei are formed
• Δ Te = Temperature at which
equiaxed solidification starts
Under ideal conditions, the 100%
columnar growth can be predicted using
Equation (3):
G A No Tn Tc Tc> ⋅ 1 3 3
1/
{ – ( / ) }∆ ∆ ∆
(3)
where No = density of grain at Tn and
A = constant.
A higher percentage of columnar
structure is one of the important reasons
for selecting a centrifugal cast material
over wrought material for steam hydro-
carbon reformer tubes. The other factors
that would impact the formation of co-
lumnar structure are the superheat tem-
perature, mold rotation speed, and den-
sity of the fluid material.
The Carbon/Niobium Ratio
Carbon as an austenite former and
niobium as ferrite former influence the
creep rupture properties of austenitic al-
loys. Their influence on creep properties
is both positive and negative, depending
on the specific combination of the two
elements in the alloy. Several combina-
tions of these two elements have been
tried. The results were compared to de-
termine how they affect the creep rupture
properties of an austenitic alloy.
At a given level of carbon, increasing
the niobium content increases the maxi-
mum creep rupture life. This increase in
creep rupture strength is limited, how-
ever, by the stoichiometric composition
ratio of carbon and niobium—highest
creep strength is obtained at this ratio.
The creep strength of these alloys is basi-
cally the result of the precipitation of
Nb4
C3
from solution. The undissolved
niobium carbide (NbC) tends to control
the rupture strength.
Observation of the microstructure
suggests that the dislocations of undis-
solved NbC cause low creep ductility.
Such dislocations nucleate a dense pre-
cipitation around the particles, which
cause added strengthening. But a very
large amount of undissolved NbC causes
reduction in the rupture life. This is as-
cribed to the formation of eutectic during
solidification, causing the undissolved
particles to enlarge, thus increasing the
numbers of dislocation centers and
reducing the localized precipitation
strengthening. Statical analysis has de-
termined that at any given solution treat-
ment temperature, the solubility product
is given as:
[ ][ ]Nb C = ks
(4)
The amount of niobium present in the
undissolved NbC, NbNbC
, is equated to
7.75CNbC
. The Nb or C at a solution treat-
ment temperature can be expressed as
total NbT
or CT
. A quadratic equation can
be derived using the solubility product:
7 75 7 7
0
. ( ) –{ . }
–
C Nb C
Nb C ks
NbC T T NbC
T T
5 C + +
=
(5)
The equation will give the amount of
carbon in undissolved NbC. Using a
solubility relationship, the amount of
undissolved niobium can be determined.
It is suggested that at 700 °C, NbC avail-
able for precipitation and NbCu
(undis-
solved) were significant at 0.1% and the
following relations were derived:
(6)1. Log rupture life(e ± =
± +
18 98
2 44 2 20 7 6
. )
. ( . ) . 66 0 47 1 24NbC NbCuppt ( . . )± +
2. %elongation ( ± =
±
18 98
30 06 48 34 2
. )
. – . (NbCppt 44 22. )
(7)
3. %reduction in area 73.16 ( ± =30 66
73 16 9
. )
. – 99 03 24 22. ( . )NbCppt ±
(8)
NbCppt
= NbC available for precipita-
tion during testing.
These findings led to several experi-
ments to develop alloys with niobium in
stoichiometric composition with carbon
(Nb4
C3
composition). UNS N08367 is
one such alloy. The trend to use micro­
alloys has proven most useful to the in-
dustry. Other similar alloys have been
developed to achieve higher creep resis-
tance. The typical composition of modi-
fied stabilized alloys HP Nb†
(UNS
†
Trade name.
(a) (b)
Grain structure of reformer tubes. (a) Fine columnar grains at the tube skin with
large well-oriented grains in the rest of the pipe wall. (b) Complete equiaxed
structure in ferritic steel.
Equiaxed structure of varying grain sizes
caused by machine vibration during
casting.
FIGURE 3 FIGURE 4
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4. December 2009 MATERIALS PERFORMANCE 5
M AT E R I A L S S E L E C T I O N & D E S I G N
N08367) and the previously mentioned
UNS N08367 are given in Table 1 and
are identified as UNS N08367(CH) and
UNS N08367(I), respectively.
These materials have highly stable
carbide, increased creep strength, higher
durability, and oxidation resistance com-
pared to the conventional materials. The
advantages of using these microalloys are:
• Possibility of operation at higher
temperature and pressure
• Reduced reformer wall thickness
• Increased quantity of catalyst pack-
ing in the same space—this aspect
has been utilized advantageously for
increasing the capacity and reduc-
ing the energy consumption of exist-
ing reformers
Reforming Catalyst
The selection of the correct catalyst is
essential for efficient operation. The fol-
lowing factors affect the performance of
a catalyst.
• Chemical composition of the cata-
lyst—typically, metallic nickel dis-
persed over some support material
is used. Commonly used support
materials are α-alumina, calcium
aluminates,andmagnesiaα-alumina
spinel—a crystal system with oxide
anions arranged in a cubic close-
packed lattice and cations occupying
some or all octahedral and tetra­
hedral sites in the lattice.
• Calcium aluminates are generally
used for naphtha reforming. Mag-
nesium aluminate, as support mate-
rial, has the advantage of higher
surface area. High-temperature
calcined, magnesium oxide (MgO)-
free material is required to prevent
hydrolyzing at temperatures below
572 °F (300 °C).
• Geometry of the catalyst—the main
mechanism of heat transfer from the
inner tube wall to the gas is through
convection. Hence, the efficiency
depends on the gas distribution over
the catalyst bed. The catalyst with a
better geometrical shape results in
lower temperature of the tube skin.
Operating Conditions
Conventionally, the reformers were
operated at pressures of ~3 MPa, because
the reformer tube material could not
withstand higher pressures. The use of
microalloy reformer tubes allows for rela-
tively higher operating pressure of up to
4 MPa. The reformer reaction yields an
increase in volume of the gases; this allows
a significant saving in compression en-
ergy. Another advantage of increasing the
reformer pressure is that it allows higher
heat of condensation of the recovered
surplus steam. The elevation of reformer
pressure, however, tends to shift the equi-
librium toward the left, requiring addi-
tional firing to bring back process equi-
librium.
Steam-to-Carbon Ratio
The steam-to-carbon (S/C) ratio is an
important parameter. The maintenance
of this ratio is key to preventing excessive
deposition of carbon on the catalyst, shift-
ing conversion of carbon monoxide (CO),
and reducing carburization damage to the
tube material. Early designs were based
on a S/C ratio of 4.0 to 4.5. With the
development of superior catalysts, which
are active at a lower S/C ratio, it is pos-
sible to maintain a ratio of 2.7 to 3.0. The
lower ratio has the advantages of:
• Dropping pressure in the front end
of an NH3
plant
• Reducing mass flow inside the re-
former tube, leading to reduction of
firing for the endothermic reaction
In an NH3
plant, these advantages
create an overall reduction in energy
consumption by ~0.2 Gcal/MT of NH3
.
Furnace Design
The furnaces are either top or side
fired. In the top-fired furnace, the process
and flow gases have co-current flow.
These furnaces have high heat flux and
are generally preferred for high capaci-
ties. The burners are limited, however,
and positioned at one level, thus limiting
heat adjustment.
The side-fired furnace has multiple
burners located at different levels. This
allows for flexibility to use different burn-
ers to achieve uniform heating, which
provides uniform tube skin temperature
and better heat control. The limitation of
this design is capacity, which is overcome
by providing multiple chambers.
The efficiency in dispersion of heat is
also improved by replacing most con-
ventional firebricks and lining the fur-
nace walls with ceramic fiber refectories.
This helps keep the outside temperature
<572 °F.
Installation of Pre-reformer
Installing a pre-reformer upstream of
the primary reformer is a common design
practice, particularly in the naphtha-
table 1
Compositions of modified stabilized alloys
Elements
UNS N08367(H)
% Composition
UNS N08367(I)
% Composition
Carbon 0.45 0.25-0.35
Chromium 25 24
Nickel 35 24
Niobium 1.25-1.5 1.5
Titanium 0.1-0.3 Nil
Silicon 1.00 1.00
Manganese (max.) 1.00 1.00
Iron Balance Balance
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5. 6 MATERIALS PERFORMANCE December 2009
M AT E R I A L S S E L E C T I O N & D E S I G N Niobium Stabilized Alloys in Steam Hydrovarbon Reforming
based NH3
plants. The pre-reformer
process breaks down naphtha into
methane (NH4
), CO, and hydrogen at
~932 °F (500 °C). This allows for the
primary reformer to function purely as a
gas reformer. Other advantages of pre-
reformer are:
• It allows flexibility in feedstock, in-
cluding liquefied petroleum gas,
naphtha with a higher boiling point,
and kerosene.
• The primary reformer can act as a
pure natural gas reformer.
• It acts as a sulfur guard to the cata-
lyst in the primary reformer.
• It extends the life of the primary
reformer catalyst.
Pre-reformers also reduce the S/C in
the primary reformers. They have been
designed and installed in many fertilizer
plants. This change has generated sub-
stantial energy saving of up to 0.4 Gcal/
MT of NH3
.
Conclusions
This work led to understanding the
advantages of niobium-stabilized micro-
alloys over conventional materials for
steam reformer tubes. These alloys have
highly stabilized carbide and high
strength, durability, and oxidation resis-
tance. Tubes manufactured from these
alloys offer the possibility of operation at
high temperatures and pressures, reduced
wall thickness, and increased quantity of
catalyst in the same space.
References
1 R. Singh, “Metallurgy and Weldability
of Steam Hydrocarbons Reforming
Equipment,” thesis paper TWI-2000.
2 S.R. Keown, F.B Pickering, “Effect of
Niobium Carbide on the Creep Rupture
Properties of Austenitic Steels,” ASM
Handbook, Service Conditions and Require-
ments in the Chemical Industry (Materials
Park, OH: ASM), pp. 138-143.
3 B.M. Patchett, R.W. Skwarok, “Welding
and Metallurgy of 20Cr-32Ni-Nb and
HP 45 Castings,” Proc. of Conference
by the Metallurgical Society of CIM,
1998.
4 A. Chitty, D. Duval, “Creep Rupture
Properties of Tubes for High Tempera-
ture Steam Power Plants,” Proc. of Joint
International Conference on Creep, The
Institute of Mechanical Engineers, Lon-
don, 1963.
RAMESH SINGH is a Senior Principal Engineer at Gulf
Interstate Engineering, 16010 Barkers Point Ln.,
Houston, TX 77079, e-mail: rsingh@gie.com. He
specializes in materials, welding, and corrosion. He
has an M.S. degree in engineering management
from California Coast University and gained his
basic metallurgical education from the Air Force
Technical Institute, India. He is a registered
engineer by the British Engineering Council and a
member of The Welding Institute. An eight-year
member of NACE International, Singh has served as
secretary and vice chair of the NACE Houston
Section. He is the author of several journal articles
and paper presentations.
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Singh.indd 6 10/26/09 8:36 AM