2 MATERIALS PERFORMANCE December 2009

Niobium Stabilized

Alloys in Steam Hydrocarbon

ReformingRamesh 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.

In 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 ReformingCatalytic 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

Singh.indd 2 10/26/09 8:36 AM

December 2009 MATERIALS PERFORMANCE 3

M A T 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

contaminatesThese 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 structureThe 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.

Singh.indd 3 10/26/09 8:36 AM

4 MATERIALS PERFORMANCE December 2009

M A T 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 31/ { – ( / ) }∆ ∆ ∆ (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 Nb4C3 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 ksNbC 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 (Nb4C3 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

Singh.indd 4 10/26/09 8:36 AM

December 2009 MATERIALS PERFORMANCE 5

M A T 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 CatalystThe 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, and magnesia α-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 ConditionsConventionally, 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 RatioThe 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 DesignThe 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-reformerInstalling a pre-reformer upstream of

the primary reformer is a common design practice, particularly in the naphtha-

TAbLE 1

Compositions of modified stabilized alloys

ElementsUNS 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

Singh.indd 5 10/26/09 8:36 AM

6 MATERIALS PERFORMANCE December 2009

M A T 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.

ConclusionsThis 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.

References1 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.

NACE Releases SP0199-2009

Revised Standard

NACE Members: Download this standard for free at www.nace.org/nacestore!

Can you afford $17.3 billion? Following technical specifi cations and practices that prevent corrosion in air pollution control equipment is critical to reducing this cost.NACE SP0199-2009, “Installation of Stainless Chromium-Nickel Steel and Nickel-Alloy Roll-Bonded and Explosion-Bonded Clad Plate in Air Pollution Control Equipment,” provides design, fabrication, and installation personnel with a basis for contract specifi cations when using these alloys, including:

• Material specifi cations• Weld joint design• Welding and installation practices• Inspection and repair of welds

List: $37NACE Member: $28 (for a printed copy of the standard)Item # 21087

Singh.indd 6 10/26/09 8:36 AM