Wrought And Cast Heat Resistant Stainless Steels And Nickel Alloys For The Refining And Petrochemical Industries
By D. J. Tillack and J. E. Guthrie
Nickel Institute Technical Series #10071
A wide variety of iron and nickel-based materials are used for pressure vessels, piping, fittings, valves
and other equipment in refineries and petrochemical plants. The most common of these is plain carbon steel.
Although it is often used in applications up to 900-950F (482-516C), most of its use is limited to 600-650F
(316-343C) due to loss of strength and susceptibility to oxidation and other forms of corrosion at higher
temperature. Ferritic alloys, with additions consisting primarily of chromium (0.5-9%) and molybdenum
(0.5-1%), are most commonly used at temperatures up to 1200F (650C). Their comparative cost, higher strength,
oxidation and sulphidation resistance and, in some cases, their resistance to certain non-corrosive but
debilitating environments (eg. hydrogen) result in their being the material of choice. However, these low
alloys have inadequate corrosion resistance to many other elevated temperature environments for which more
highly alloyed Ni-Cr-Fe alloys are required. Hydrogen-hydrogen sulphide and ammonia are two common examples
of such environments.
For applications for which carbon or low alloy steels are not suitable, the most common choice of material is
from within the 18 Cr-8Ni austenitic group of stainless steels. These alloys and the 18Cr-12Ni steels are
favoured for their corrosion resistance in many environments and their oxidation resistance at temperatures
up to 1500F (816C). Above 1200F (650C) their falling strength becomes a consideration and more heat resistant
alloys must often be used.
There are many diverse applications for the stainless and heat resistant alloys throughout the range of
temperature encountered in the refining and petrochemical industries. Some of the more important of those
applications at temperatures below 1200F (650C) will be discussed because of their significance to the
industries involved and to the use of these materials. However, the emphasis of this document is on
applications of heat resistant alloys above 1200F (650C). Since there is almost no use of the chromium-iron
or duplex stainless above 600F (316C), these alloys are not covered. A large percentage of the applications
for stainless and heat resistant materials above 1200F (650C) are in connection with fired heaters. However,
piping components, manifolds, valves, cyclones etc. also are constructed of these materials and are
discussed. Applications for both wrought and cast versions of the heat resistant alloys are addressed. With
rare exception, the latter are used commonly above 1500F (816C) and almost exclusively for applications above
1850F (1010C) up to 2100F (1150C).
Selection Criteria
Table I is a listing of the wrought stainless and heat resistant alloys most commonly used for refining and
petrochemical plant equipment. Table II lists some of the newer wrought alloys and Table III the most
commonly used cast alloys. Nominal compositions are included. The lists are not intended to be all inclusive
but represent the largest volume of such alloys used.
The selection criteria used by the materials engineer in choosing from this group of materials includes a
list of qualities that are either desirable or necessary. Unfortunately, the optimum properties associated
with each selection criteria seldomly all occur in a single material, especially when the operating
conditions become aggressive. Thus, compromises must frequently be made to realize the best performance of
the material selected. The principle selection criteria applied to materials for refining and petrochemical
plant equipment include, but are not necessarily limited to, the following:
- Mechanical properties;
- Corrosion resistance;
- Stability of properties;
- Fabricability;
- Availability;
- Cost.
Mechanical Properties
Most refining and petrochemical process equipment is designed and fabricated to the requirements of the
American Society of Mechanical Engineers (ASME) or equivalent pressure vessel and piping codes of other
countries. These codes include only approved materials and establish the basis for and the setting of
allowable stresses. Thus, the mechanical properties of a material is usually the first criteria that the
materials engineer applies in the selection process. This is especially important for applications at
temperatures in the creep range where a minor difference in design/operating temperature can significantly
affect the load carrying ability of the material. However, other applications may require that the component
be able to support only itself. Obviously, in this case, criteria other than strength will take
priority.
Tensile strength is the criteria used to select carbon and alloy steels for ASME Code applications below the
creep temperature range. Some other codes rely on yield strength. Within the creep range, however, a limiting
creep rate (eg. 1%/100,000hours) or a stress rupture life (eg. 100,000 hours) is usually the governing
criteria. Figure 1 represents one form in which stress rupture data are presented for use in material
selection and component design. A more traditional format involves a log-log plot of stress vs. rupture time
at different temperatures.
FIGURE 1: One manufacturers format for presenting creep rupture data for a centrifugally cast heat resistant
HP-type alloy. (LMP=Larson-Miller Parameter). Source: Manoir Industries.
Because applications for heat resistant alloys often involve frequent thermal cycling, their thermal fatigue
resistance is an important mechanical property to consider in the selection process. This property is a
function of composition but it is also affected by section thickness and geometry. An example of this
relationship involves 180 degree return bends in ethylene cracking service. Some fitting designs include a
heavy outside wall to absorb erosion from coke particles. These designs can suffer thermal fatigue as a
result of the cyclic nature of regular operations, decoking, and startup and shutdowns. An example of the
cracking that can result is shown in Figure 2. Thinner, uniform walled tubes and similar components
experiencing the same magnitude and frequency of cycles did not develop this problem. Fortunately,
through-wall cracking such as this occurs infrequently. It usually is arrested about midwall as the probable
result of the crack acting as a hinge.
FIGURE 2: HP-type statically cast return bend from the shield section of an ethylene cracking furnace after 4
years service. Through-wall cracks on heavy backwall are due to thermal fatigue.
TABLE I: Established heat resistant alloys for refining/petrochemical applications, Nominal
Composition.
TABLE II: Newer heat resistant wrought alloys for refining/petrochemical applications, weight
%
TABLE III: Nominal composition and mean stress to rupture in 100,000 hours for those heat resistant cast
alloys most commonly used in refining and petrochemical applications.
Corrosion
Resistance
The importance of this selection criteria is a close second to mechanical properties in terms of
optimizing performance. Without adequate corrosion resistance (or corrosion allowance), the component will
fall short of the minimum design life desired. In the refining and petrochemical industries, this is
typically set at ten years or more. The additional cost usually associated with choosing increased corrosion
resistance during the selection process is invariably less than that due to product contamination or lost
production and high maintenance costs due to premature failure.
Unlike mechanical properties, there are no codes governing corrosion properties. For some applications or
services, recommended practices have been published by the American Petroleum Institute (API), NACE
International, etc. Data upon which to base material selection are available inexpensive literature and
manufacturers' publications. Very reliable data are available from the literature for high temperature
corrosion in air and low sulphur flue gases and for some other common refinery and petrochemical
environments. However, small variations in the composition of a process stream or in operating conditions can
cause very different corrosion rates. Therefore, the most reliable basis for material selection is operating
experience from similar plants and environments or from pilot plant evaluations.
Stability Of Properties
The properties of materials used at elevated temperatures may degrade from a variety of causes. The
consequences of this degradation depends on the process and the expectations of the material. For example,
consider a material in which the intermetallic phase, sigma, can form.
FIGURE 3: Schaeffler Diagram showing the phases present in various stainless steels as a function of
their composition.
Sigma Phase
In ferritic stainless steels this phase is composed of iron and chromium alone. In austenitic
stainless alloys, it is much more complex and will include nickel, manganese, silicon, niobium, etc. in
addition to iron and chromium. Sigma phase forms in ferritic and austenitic stainless steels from ferrite, or
metastable austenite, during exposure at 1100-1700F (593-927C). It causes loss of ductility and toughness at
temperatures under 250-300F (120-150C) but has little effect on properties in the temperature range where it
forms unless the material has been put into service with considerable residual cold work. In this case, creep
strength can be adversely affected. Otherwise, as long as the component continuously operates at the elevated
temperature, there will be little consequence. However, care must be taken to avoid impact or suddenly
applied high stress when the unit cycles to the lower temperature range. Cracking could occur if the
component were impacted or stressed during maintenance work. Formation of the chi-phase during exposure in
the above elevated temperature range also causes low temperature embrittlement. Both of these phases can be
redissolved by holding the material at 1850-1950F (1010-1066C) for1-4 hours depending on thickness and amount
of sigma that has formed.
Over time, sigma phase formation is unavoidable in many of the commercial alloys used within the temperature
range where it forms. Fortunately, few failures have been directly attributed to it. However, if a component
is to be exposed in the critical temperature range and subsequently subjected to extensive cyclic conditions
or to shock loadings, an immune or more stable material should be used. Increased resistance or immunity is
achieved by selecting a composition that is balanced with respect to austenite vs. ferrite-forming elements
so that no free ferrite is present. This can be determined using the Scuffler diagram or the more recently
developed DeLong or Welding Research Council-92 diagrams (Figure 3). Alloy 800 is not immune but is less
susceptible to sigma formation than the 300 series of stainless steel.
Sensitization
Another form of elevated temperature degradation of austenitic stainless steels is sensitization.
This is caused by the precipitation of chromium carbides preferentially at grain boundaries (Figure 4). The
immediately adjacent chromium-depleted zone is susceptible to accelerated corrosion in some aqueous
corrodents. Sensitization can occur during fabrication from the heat of welding, improper heat treatment or
through service exposure in the temperature range of 900-1500F (480-815C). Sensitization has little or no
effect on mechanical properties but can lead to severe intergranular corrosion in aggressive aqueous
environments such as polythionic acid. Polythionic acid can form during downtime on equipment that has been
even mildly corroded by hydrogen sulphide at elevated temperature. The iron sulphide corrosion product
combines with air and moisture to form the acid and induces intergranular corrosion and cracking.
To minimize the chance of sensitization during fabrication, carbide-forming stabilizers are added. The most
common are titanium (Type 321) and niobium (Type 347). As long as their lower strengths are taken into
account, another alternative is to use low carbon grades (Types 304L, 316L) with carbon <0.03%. To
minimize the effects of frequent or continuous exposure within the susceptible temperature range, a thermal
stabilization treatment of Type 347 at 1600-1650F (870-900C) for 4 hours is recommended. Type 321 does not
respond acceptably to this treatment. Use of the low carbon grades would be better still on the basis of
stability. However, their lower strength and/or code limitations may preclude this alternative.
The higher carbon content of heat resistant alloys and the presence of other elements cause these alloys to
"age" during exposure to elevated temperatures. Aging results from formation of secondary carbides and other
precipitates. This usually results in higher strength but also causes loss of ductility at ambient
temperature leading to potential fabrication problems. This is more of a problem with cast than wrought heat
resistant alloys because of the typically higher original carbon content.
Recovery from all of the above forms of degradation is possible by solution annealing the material at
temperatures appropriate for the alloy grade followed by rapid cooling. For the 300 series stainless steels,
annealing can be done at 1950F(1066C) while the high carbon heat resistant alloys may require treatment as
high as 2150F(1177C). Recovery is not permanent. Re-exposure to the causative conditions will result In
re-degradation.
FIGURE 4: An example of grain boundary sensitization in a Type 304 stainless steel exposed to operating
temperatures of 1250-1350F (677-732C)
Fabricability
There are many outstanding materials with highly desirable mechanical properties and corrosion resistance
which are seldom used because they cannot be fabricated. There are few refinery or petrochemical plant
applications where welding or bending or some other forming operation is not required to construct a useable
piece of equipment. Also, there are some materials which have excellent properties that can be fabricated as
produced but, because of "aging", cannot be modified or repaired after exposure to operating conditions.
Therefore, materials must be selected based on their maintainability as well as their original fabricability.
In general, the wrought heat resistant alloys have greater fabricability than the cast materials. This is in
part due to the dendritic structure of the latter and to the addition of alloying elements in concentrations
that cannot be tolerated by the wrought materials during the severe production operations to which they are
subjected (rolling, forging, etc.). The cast alloys typically can tolerate significantly higher
concentrations of carbon, silicon, tungsten, molybdenum, etc., which are added to enhance mechanical
properties, corrosion resistance, or both. But, these elements also can adversely affect the original,
as-produced fabricability and make maintainability, particularly weldability, difficult, if not
impossible.
Availability
Materials engineers and purchasing agents become frustrated in trying to obtain materials that have
a limited number of producers or a limited production volume. Such frustration can be particularly high when
a small amount of material is needed to finish a job or replace a failed piece. Prior to the original
specification of a material, consideration should be given to its future availability for repairs or
replacement in the form or forms that it will be used. In those cases where it might not be available,
alternative replacement materials should be identified. This will be extremely helpful 5 or 10 years later
when the optimum original material may not be available to the maintenance engineer.
Cost
Economics enter into every business decision. But, the important criteria should not be the initial cost of a
material, but its life-cycle cost or cost effectiveness. It usually is much more cost effective to specify a
material that will provide an extended life, particularly in areas that are difficult to repair or in
components that would cause major shut-downs in case of failure. In these situations, the original cost of
the material can be insignificant compared to the loss of production caused by the use of a lower cost, but
less effective, material. Unfortunately, competitive bidding and corporate bottom lines frequently create
barriers that inhibit realization of long equipment life. The enlightened company will recognize the value of
the life-cycle cost approach on long-term financial health and not embrace only the low initial cost
option.
Principal Forms Of High Temperature Corrosion
Although some examples of application at lower temperatures are discussed, as stated earlier, the
focus of this publication is on alloy applications at temperatures above 1200F (650C). Table IV lists the
most common forms of corrosion encountered in this temperature range. Often, more than one of these forms are
acting to degrade the metal. For example, a furnace tube could be corroding on the outside from the oxidation
mechanism while simultaneously being carburized on the inside. This possibility makes the materials
engineer's choice even more complex.
TABLE IV: Principle forms of high temperature corrosion
- Oxidation;
- Sulphidation;
- Carburization;
- Metal dusting;
- Nitridation;
- Halogen corrosion;
- Fuel ash deposits.
Oxidation
This is the most commonly encountered form of high temperature corrosion. However, oxidation is not
always detrimental. In fact, most corrosion and heat resistant alloys rely on the formation of an oxide film
to provide corrosion resistance. Chromium oxide is the most common of such films. As temperature is
increased, the rate of oxidation also increases and becomes deleterious. Increased chromium content is the
most common way of improving oxidation resistance. The effect of chromium on the rate of oxidation of several
alloys in air is shown in Figure5 (1) The rate of oxidation for these same alloys would be somewhat lower in
a typical low sulphur flue gas. (2)
Other elements including aluminum, silicon and some of the rare earths are often added to enhance oxidation
resistance. Because of resulting instabilities, fabrication difficulties or for other reasons, there are
limits to the amount of these elements that can be added. Few alloys contain more than 30% chromium. Silicon
is usually limited to no more than 2% and aluminum to less than 4% in wrought alloys. Yttrium, cerium and the
other rare earths are usually added only as a fraction of a percent.
Increasing the nickel content of the austenitic stainless steels up to about 30%, with relatively constant
chromium content, dramatically reinforces the effect of chromium on oxidation resistance as shown in Figure
6.(3) Some continued improvement occurs at higher concentrations but at a much diminished rate. Higher nickel
content causes the oxide to be more resistant to spalling and increases the metallurgical stability of the
composition.
High temperature oxidation rates can vary significantly for quite similar centrifugally cast, modified HP
heat resistant alloys as shown in Figure 7. These alloys are commonly used in ethylene cracking and reformer
furnaces. The comparative oxidation rates shown were extrapolated from cyclic tests conducted in air over a
period of 500 hours at 1960F (1070C) and 2100F (1150C). The rates shown are unlikely to be sustained in
longer, continuous exposures but do demonstrate that differences in performance can be expected between
alloys of similar composition produced by different suppliers. This difference is most notable at the higher
temperature. Of particular note are the paired Alloys 967/973 and 969/970 at 2100F(1150C). Alloy 972,
25Cr-32Ni, was included for comparison with the higher alloys. Excessive oxidation rates are rarely a cause
of reformer or ethylene furnace tube failures but can be indicative of coking and carburization
tendencies.
FIGURE 5: Oxidation resistance of steels in air after 1000 hours at temperatures of 1100-1700F (590-930C).
Ref. 1
FIGURE 6: Cyclic oxidation resistance of stainless steels and nicekl-base alloys in air at 1800F
(980C). Ref. 3.
FIGURE 7: Annual oxidation rates for centrifugally cast heat resistant alloys. Rates are extrapolated from
500 hour cyclic exposure in air.
FIGURE 8: Corrosion rates for carbon steel, Cr-Mo alloys and stainless steel in a hydrogen-free,
hydrogen sulphide environment.
FIGURE 9: Shows significantly higher corrosion rates for some of the same materails in Figure
8 when hydrogen is present in a H2/H2S environment.
Sulphidation
Organic sulphur compounds such as mercaptans, polysulphides, and thiophenes, as well as elemental sulphur,
contaminate practically all crude oils in various concentrations. These contaminants can cause heavy
corrosion at lower temperatures but are especially aggressive in refining operations above 500-550F
(260-288C). This is a result of their partial conversion to corrosive hydrogen sulphide during atmospheric
distillation. Usually this form of sulphur corrosion can be handled adequately with the 5 to 9% Cr-Mo alloys
unless the crude also contains naphthenic acids. In that case, Type 316 or Type 317 stainless steel is
required. Sulphur corrosion rates are shown in Figure 8.
Elemental sulphur and any remaining unconverted sulphur compounds are converted to hydrogen sulphide through
hydrogenation processes and subsequently removed by amine or similar recycle treatment processes. Hydrogen
sulphide in the presence of hydrogen becomes extremely corrosive above 500-550F (260-288C) and the austenitic
grades must be used to avoid excessive scaling or potential catastrophic sulphidation. Excessive scaling not
only shortens equipment life but also adversely affects operations by fouling downstream catalyst beds and
exchanger bundles. Hydrogen - hydrogen sulphide corrosion rates are shown in Figure 9.
Any of the 18 Cr-8Ni, stainless steel grades can be used to control sulphidation. However, it is best to use
the stabilized grades mentioned earlier. Some sensitization is unavoidable if exposure in the sensitizing
temperature range is continuous or long term. Stainless equipment subjected to such exposure and to
sulphidation corrosion should be treated with a 2% soda ash solution or an ammonia solution immediately upon
shutdown to avoid the formation of polythionic acid which can cause severe intergranular corrosion and stress
cracking.
Vessels for high pressure hydrotreating and other heavy crude fraction upgrading processes (eg.
hydrocracking) are usually constructed of one of the Cr-Mo alloys. To control sulphidation, they are
internally clad with one of the 300 series stainlesses by roll or explosion bonding or by weld overlay. In
contrast, piping, exchangers, valves, etc. exposed to high temperature hydrogen-hydrogen sulphide
environments are usually constructed of solid 300 series stainless alloys. In some designs Alloy 800H has
been used for piping and headers. In others, centrifugally cast HF-modified piping has been used.
High nickel alloys are rarely used in refinery or petrochemical plants in hydrogen-hydrogen sulphide
environments because of their susceptibility to the formation of deleterious nickel sulphide. They are
particularly susceptible to this problem in reducing environments. Although high chromium helps, as a general
rule, the higher the nickel the more susceptible the material.
Vapour diffusion aluminum coatings (AlonizingTM) have been used with carbon, Cr-Mo and stainless steels to
help control sulphidation and reduce scaling. For the most part, this has been restricted to smaller
components. Aluminum metal spray coatings have also been used but not widely nor very successfully.
Multiple Environments
A dilemma occurs in designing equipment that requires resistance for variable times of exposure to
multiple environments such as oxidizing and sulphidizing conditions. If the predominant amount of time
involves exposure to oxidation - with minimal but occasional exposure to sulphidation - it may be prudent to
design with a high-nickel, high-chromium alloy, such as some of the newer alloys listed in Table II (HR-120,
HR-160, Alloy 602CA or Alloy 45TM). If the environment is purely sulphidizing, a low-nickel, high-iron,
high-chromium alloy would be the best choice. Sulphidation resistance for highly reducing conditions can be
enhanced by high aluminum or silicon contents and by keeping the chromium above 15% with a low Ni/Fe
ratio.
Carburization
Carburization can occur when metals are exposed to carbon monoxide, methane, ethane or other
hydrocarbons at elevated temperatures. Carbon from the environment combines primarily with chromium but also
with any other carbide formers (Nb, W, Mo, Ti, etc.) present in the alloy. As a result, the carbides may be
quite complex. They form within the grains and along grain boundaries. The carbides are strong and hard but
very brittle. The overall effect is to drastically reduce ductility at temperatures up to 900-1000F
(482-538C). By tying up the chromium, carburization also reduces oxidation resistance. It also adversely
affects creep strength and because of the volume increase associated with the carbon uptake and carbide
formation, it imposes additional stress that contributes to mechanical failure. These stresses are evidenced
often by the localized bulging of tubes that are carburized. An example of carburization is shown in Figure
10.
FIGURE 10: A typical example of a heavily carburized HP-Nb type tube with oxidation occurring at the
chromium-depleted ID
FIGURE 11: Effect of nickel on the resistance of Cr-Ni alloys to carburization. Ref.
4.
Carburization is not a common occurrence in most refining operations because of the relatively low tube
temperatures of most refinery fired heaters. However, it can and does occur in those higher temperature
process heaters (eg. cokers) where its control in 9Cr-Mo tubes has been somewhat successful through the use
of an aluminum vapour diffusion coating (AlonizingTM). Carburization can also occur during an upset which
results in the exposed material being heated to unacceptably high temperature. Where it is expected to occur
within the range of normal operating conditions, the broader approach has been to use Type 304H for
temperatures up to about 1500F (816C). There is no advantage in using either of the stabilized grades since
any unreacted titanium or niobium from the original melt would be quickly tied up. Type 310 or Alloy 800H may
be used for temperatures up to about 1850F (1010C). For the most part, refinery application of the latter two
alloys for this purpose is confined to hydrogen reformer furnaces. Unfortunately, the 300 series stainless
alloys, including Type 310, are subject to sigma phase embrittlement in the temperature range where they have
useful carburization resistance. Alloy 800H would be a better choice.
The most effective element in controlling carburization is nickel in combination with chromium. As shown in
Figure 11 (4), absorbed carbon is plotted against nickel content with carbon absorption shown to decrease
with increased nickel content. Silicon is also shown to have a strong effect. Aluminum in excess of 3.5-4%
also is beneficial. Unfortunately, the presence of much more than 2% Si adversely affects the rupture
strength and weldability of both wrought and cast heat resistant alloys. Aluminum, in concentrations higher
than 2-2.5%, has an adverse effect on ductility and fabricability - properties that are essential for piping,
tubing, pressure vessels, etc.
Coatings and surface enrichment using silicon, aluminum, chromium, and combinations thereof, have been tried
to control carburization of heat resistant alloys. Unfortunately, none of these have been successful for the
long term. Vapour diffused aluminum enrichment showed promise and performed well at lower temperatures but
broke down after relatively short times at temperatures above 1850-1900F (1010-1040C).
Carburization is far more common in the petrochemical industry than in refining. The most common occurrence
is in the radiant and shield sections of ethylene cracking furnaces. Carburization is a serious problem in
these furnaces because of the high tube metal temperatures - up to 2100F (1149C) - and high carbon potential
associated with the ethane, propane, naphtha and other hydrocarbon feedstock which are cracked. However, it
also occurs, albeit less frequently and with lessor severity, in reforming operations and in other processes
handling hydrocarbon streams or certain ratios of CO/CO2/H2 gas mixtures at high temperatures.
A survey(5 ) published in 1981 indicated that carburization was the most frequent cause of ethylene furnace
tube replacement. Most furnaces of that era were tubed with centrifugally cast HK-40, wrought Type 310 or
Alloy 800/800H. Since the mid-1980s, more resistant, centrifugally cast, modified HP alloys (Table III) have
been more commonly used but carburization appears to continue to be the primary cause of tube replacement.
This is probably due to more severe operating conditions - mainly higher temperatures. Some operators are
beginning to use the higher 35Cr-45Ni cast alloy, with various additions, to combat these conditions. For
short residence time furnaces, tubed with small(<1.5 in. ID) tubes which cannot be cast, wrought alloys
including HK4M and HPM, Alloy 803 and Alloy 800H (Tables I, II) are being used. Although these alloys do not
appear to have carburization resistance equivalent to the modified-HP or higher Cr-Ni alloys, longer term,
in-service experience is required upon which to base comparisons. Other wrought alloys (eg. 85H and HR-160,
both with high silicon) are used successfully to combat carburization of trays, retorts and other components
used in carburizing treatments. However, their limited fabricability precludes broad use in the refining or
petrochemical industry.
The rate of carburization of ethylene cracking tubes of a given alloy is process driven. As mentioned
earlier, temperature and carbon potential are the primary factors affecting its rate. Increasing steam
dilution will reduce the rate. The type of feed is also a factor with lighter feeds generally being more
aggressive than heavier feeds because of their higher carbon potential. Some operators pre-sulphide their
coils while others use feedstock with crack able sulphur present or added. Ostensibly, this reduces the
catalytic characteristics of the tube surface and reduces coke formation. This, in turn, reduces the
frequency of decoking which many believe to be a major cause of carburization.
The form and severity of decoking operations appear to play important roles in the rate of carburization.
High temperature decoking with low quantities of steam are thought to accelerate carburization. Likewise,
steam/air decoking appears to be more deleterious than steam only. (6) Appropriate metallurgy can be used to
reduce but rarely totally eliminate carburization. The most important characteristic of a successful alloy is
its ability to form and maintain a stable, protective oxide film. Chromium oxide is considered to be such a
film. However, it is not sufficiently stable at the higher operating temperatures and low oxygen partial
pressures. Alumina or silica are much better. Unfortunately, the addition of aluminum or silicon to the heat
resistant alloys in quantities to develop full protection involves tradeoffs in strength, aged ductility,
and/or weldability that are often unacceptable. Viable alloys are generally restricted to about 2% of either
element. This is helpful but not a total solution.
Carburization is
Very Non-uniform
One insidious aspect of carburization of ethylene furnace tubes and some other equipment is its
unpredictability and its non-uniform nature. Models have been developed, based on operating experience, that
can be used with some modicum of success to predict the general rate of carburization. Unfortunately, none of
the known models are completely accurate or acceptable. The primary reason is the non-uniform manner in which
carburization occurs. This non-uniformity is demonstrated in Figures 12 and 13 which show eleven
representative tube sections removed from a furnace after several years operation under identical conditions.
The transverse sections were removed from the same location in each tube and from the same end of the
longitudinal sections shown in Figure 13.
These examples show that not only can the degree of carburization vary dramatically around the circumference
of tubes but also over very short distances along the length of the tube. Thus, temperature alone is not a
determining factor. Surface condition was identified as a major factor many years ago but again, is not a
singularly effective factor since these tubes were initially identical and saw the same service conditions
for many years. This non-uniformity also presents a problem in interpreting the results of measurements of
the degree of carburization.
FIGURE 12: Transverse sections of HP-Mo type tube sections removed from identical positions in an
ethylene cracking furnace after several years service. Note the varying degrees of carburization from tube to
tube around the circumference.
FIGURE 13: Longitudinal sections of the same tubes as in figure 12 show great variation in
the degree of carburization within very short distances and from tube to tube.
Carburization causes the normally non-magnetic wrought and cast heat resistant alloys to become magnetic. The
resulting magnetic permeability has been used for many years to assess the degree of carburization that has
occurred. Measuring equipment used ranges from the hand held magnet to the more technologically
sophisticated, multi-frequency eddy current instruments in use today. While the latter can be helpful in
determining not only the degree of carburization but its pattern as well, care must be exercised in
interpreting the results because of the variability discussed earlier. There are other benefits from such
surveys of radiant section coils. Carburization patterns can reveal uneven firing patterns that might
otherwise have gone undetected. They can also be helpful in implementing selective metallurgical alternatives
for tube alloys. In multi-tube coils, for example, the upstream tubes could be made of the lower cost, lower
nickel modified HP alloys and outlet tubes of the higher cost, but more resistant, 35Cr-45Ni alloys.
Metal Dusting
Metal dusting is considered to be a form of carburization experienced in some refining and petrochemical
processes. While some of its characteristics are similar, it differs in many ways from classical
carburization. It can cause extremely rapid loss of metal which normal carburization does not and the depth
of carburization in advance of the metal loss is usually quite shallow, but intense. Most often it is
associated with CO-rich CO/CO2/H2 gas streams often associated with reforming, synthesis gas, partial
oxidation or other processes.
It typically occurs in the temperature range 900-1600F (482-871C) with peak reaction rate at 1300-1350F
(704-732C). The reaction rate can be very rapid and the results catastrophic. It causes damage usually taking
the form of rounded pits with a dusty surface. It is rather unpredictable and most of the stainless and heat
resistant alloys can be attacked. Gas diffusion aluminum coatings have been used effectively and, when they
can be added, steam, sulphur or ammonia also can be used for its control. The most positive cure, however, is
to adjust the gas composition by reducing the CO partial pressure.
Nitridation
Similar to carburization, nitridation occurs when chromium and other elements combine with nitrogen
to form embrittling nitrides. Nitridation usually occurs when carbon, low alloy and stainless steels are
exposed to an ammonia-bearing environment at elevated temperatures. As in the case of carburization, higher
nickel alloys are very resistant to nitridation attack due to the low solubility of nitrogen in nickel. Alloy
600, with 72% nickel, is often used in the heat treating industry and occasionally in refining and
petrochemical applications involving ammonia at temperatures above 650F (343C). Economics and lower strength,
compared with Alloy 800H and cast modified HP, have minimized its applications in the refinery and
petrochemical industries. These alloys are not immune to attack but are more cost effective.
Although the nitrogen molecule is quite stable, sufficient dissociation does occur at combustion temperatures
to cause nitridation of 800H or HP-type furnace tubes at metal temperatures of 1850F (1010C) and
higher.
Halogen Corrosion
Halogens, especially dissociated chlorine (chloride ions), frequently contaminate refinery and petrochemical
feed streams and can lead to serious corrosion problems. Most common of these is chloride stress corrosion
cracking of the austenitic stainless steels that can occur at temperatures above 120F (50C) in aqueous
solutions containing quite low concentrations of chloride. The halogen salts are also very corrosive. Ferric
chloride, for example, is added to acid solutions as a standard test to evaluate the pitting resistance of
alloys. Chlorides and fluorides contribute to high temperature corrosion as well by interfering with the
formation of protective oxides or breaking them down if already formed.
In refining operations, chlorides most commonly enter the process operations as salt water or brine. Most of
this material is removed in desalters. The residual is hydrolysed during atmospheric distillation and is
absorbed in the overhead condensate. If not neutralized, this condensate can be very corrosive. Occasionally,
organic chlorides find their way into crude feed. These are not removed in the desalters but are generally
removed in the distillation process. Chlorides can get into the downstream processes from slop rerun, cooling
water leaks (from a salt water system) or from salt water contamination during shipment of semi-finished
product. Fluoride contamination is usually the result of blending streams from an alkylation operation.
This downstream contamination not only can affect refining equipment but also impact on petrochemical
facilities which take feed from these sources. Petrochemical facilities are more subject to feed
contamination during shipment or storage. The latter is often done in salt domes which can be a source of
chlorides.
The high temperature halogen attack mechanism is similar to that for oxidation and sulphidation except scales
do not generally form because of the volatility of the corrosion products.(7) Resistance to high temperature
halide attack increases with increasing levels of both nickel and chromium. The high nickel alloys are
significantly more resistant than the stainless steels to chlorine but not fluorine which is more soluble in
nickel. The stainless steels are more resistant than the lower alloyed steels. Minimizing Mo and W helps the
resistance of alloys to oxidizing halogen corrosion, while aluminum improves the resistance of nickel-base
alloys. (8 )
Fuel Ash Deposits
Some refinery heaters, boilers, etc. are fired with "dirty" fuels. This requires the use of special alloys
that are not only heat and oxidation resistant but can also resist corrosion by fuel ash deposits containing
vanadium, sodium and/or sulphur present in the fuel used. The melting point of one of these mixed compound
deposits (Na2SO4-V2O5) can be as low as 1166F (630C) at which point corrosion can be catastrophic. The
50Ni-50Cr-Nb alloy is about the only alloy that can be used for hangers, tubesheets, supports, etc. when
their operating temperatures approach or exceed this temperature. "Dirty" fuels are not used in reformers or
ethylene furnaces because of the catastrophic corrosion that would occur at their high operating
temperatures. And, use of the 50Ni-50Cr alloy for all of the high temperature components would neither be
practical nor cost effective for the latter applications.
Refinery/petrochemical Applications Of Wrought And Cast Heat Resistant Alloys
As stated earlier, the Ni-Cr-Fe alloys are extensively used in refining and petrochemical plant equipment for
both liquid and gaseous low temperature corrosion resistance and for heat resistant applications. In refining
operations, most equipment that operates below 500-600F (260-316C) is constructed of carbon steel or Cr-Mo-Fe
alloys. Exceptions are the alkylation processes where highly alloyed materials are required to handle streams
containing some sulphuric or hydrofluoric acid. Other examples would be the use of Types 316 and 317 for
handling crude fractions with a high naphthenic acid content and other higher alloys for flue gas
desulphurization processes. Since petrochemical plant environments generally are more diverse and often more
corrosive, there is more extensive use of the Ni-Cr-Fe and nickel-based alloys in this temperature
range.
FIGURE 14: Fried heaters typical of those found in the refining and petrochemical
industries.
At temperatures from 600-1000F (316-538C), there is increased use of the Ni-Cr-Fe alloys in both industries.
But the increased usage in refining is probably proportionately greater because of the need for resistance to
high temperature sulphidation. Much of the equipment used in hydrodesulphurisation or in the upgrading of
heavier fractions must be made of solid or clad 300 series stainless steels. It is for equipment in services
above 1200F (650C) where the stainless and heat resistant alloys are used most extensively in both the
refining and petrochemical industries.
The single most important use of both wrought and cast heat resistant Ni-Cr-Fe alloys in refining and
petrochemical applications is in fired heaters, such as are shown in Figure 14, where they are used for
tubes, hangers, supports, tubesheets, etc. Examples of these components can be seen in Figures 15-17. Most
fired heaters for refining processes operate at tube metal temperatures under 1200F(650C). Consequently, they
can be constructed of carbon or low alloy steels. However, for aggressive environments which can cause
sulphidation, carburization or the other forms of corrosion, even at lower temperature, the stainless or heat
resistant grades must be used as discussed earlier. Coker heaters and those in desulphurization service are
two examples that would require the higher alloys - at least in the radiant section.
At temperatures above 1200F(650C), the higher carbon content stainlesses (Types 304H, 316H, 321H, 347H, 309H)
or more highly alloyed heat resistant grades (ie. Type 310, Alloy 800H) must be used for their oxidation
resistance and strength. Above 1500F(816C), the Ni-Cr-Fe and nickel-based alloys required. With the exception
of catalytic steam hydroformers for hydrogen production, there are no fired heaters in the refining industry
that operate above 1500F (816C).
Fluid Catalytic Cracking Units (FCCU)
Other than steam reforming, this is the only other common refining process that has some critical components
operating at temperatures in excess of 1200F(649C). Regenerator cyclones are made of Types 304/304H and
321/321H while Type 304H is used for the high temperature small diameter piping around both the regenerator
and reactor. The cast versions of these alloys have been used for high temperature FCCU valves but these are
generally below 1200F(649C). Expansion bellows in the regenerator catalyst standpipe have been problem areas,
and Alloy 800 has been used with some success in this application. (9) Alloy 617LCF, a recently introduced
modification of the popular aerospace alloy, has been suggested for this application because of its superior
low cycle fatigue and creep properties.
FIGURE 15: Assembled coil of centrifugally cast tubes joined by statically cast return bends -- both
in HP-Nb alloy. For use in a "conventional" ethylene cracking furnace. Photo courtesy of Paralloy
Inc.
Catalytic Steam Reformers for Hydrogen/Ammonia/Methanol Production
Catalytic steam reforming is a widely used process for the production of hydrogen for use in refinery
hydrogenation processes and for the production of ammonia and methanol in the petrochemical industry. Much
use is made of heat resistant alloys in the primary reformer furnace which is a key component in this
process. They are also widely used for the other components from the primary reformer through the waste heat
boiler. These components typically operate at temperatures of 1300-1850F(704-1010C) and at high pressures. In
most designs, vessels and large piping are refractory-lined to conserve heat and to take advantage of higher
metal strength at lower temperatures. To prevent erosion, these linings are frequently shrouded with
relatively thin sheets of Type 304 - up to 1500F(816C) - and Type 310 or Alloy 800 at higher
temperatures.
FIGURE 16: Assembled coils of small diameter tubes centrifugally cast in HP-Nb for a short resistance
time ethylene cracking furnace. The bent sections are usually made from a lower carbon version of the same
alloy. Photo courtesy of Paralloy Inc.
Materials for unlined components are required to have good high temperature strength, good ductility and good
thermal fatigue properties. Alloy 800H has been used successfully for catalyst tubes, pigtails, header
manifolds, high temperature transfer piping between the primary and secondary reformers and for secondary
reformer internal parts. However, cast catalyst tubes are more commonly used than Alloy 800 in reformer
furnaces. These have been most frequently made of centrifugally cast HK-40, HP-Nb or IN-519. Tubesheets,
fittings and other components are made of these alloys in the form of static castings. In general, HK-40 is
being phased out in favour of HP-Nb because of the latter's higher strength and better oxidation resistance.
With its higher strength, the tube wall of HP-Nb can be thinner thereby increasing the catalyst capacity of
the same size tube. More important, thermal stress is reduced thereby helping to improve tube life. And,
thermal efficiency is improved. Ammonia reformer furnaces are said to be the largest single application for
the HP-Nb alloy.(10)
FIGURE 17: Statically cast tube sheets (bottom) and fittings (inserts) of HP-Nb alloy.. Photo
courtesy of Pose-Marre, Gmbh.
Centrifugally cast 20Cr-32Ni-Nb alloy is a common alternative to 800H for outlet header manifolds. Some of
the secondary reformer internals may be exposed to temperatures as high as 2100F(1149C). Strength is less of
a factor for these components than oxidation, carburization and nitridation resistance. The waste heat boiler
typically takes 1800F(982C) effluent from the secondary reformer to generate steam at up to
1500psi(10.3Mpa).(11) Alloy 600 is frequently used for cladding tubesheets and for ferrules on the hot inlet
end where the tubesheet is typically protected by a refractory layer. Throughout the process, Type 304H is
used for components at the lower end of the process temperature range.
Ethylene Cracking
In ethylene production, the hydrocarbon feeds (ethane, propane, naphtha, gas oils, etc.) are thermally
cracked in the presence of steam at low pressure and process temperatures of 1450-1550F(788-843C). The
radiant section of some of these cracking furnaces operate at end-of-run tube metal temperatures up to
2100F(1149C). This is the practical upper limit for most of the fabricable, heat resistant alloys.
FIGURE 18: 100,000 hour rupture strength of HP-high carbon vs the same alloy with micro-alloying
additions.
The shield section, the lower convection section, the outlet transfer line and the quench unit of the
ethylene cracking furnace operate at significantly lower temperatures but also must be made of heat resistant
alloys. For the tubes and other components, Alloys 800 and 253MA have been used successfully up to
1850-1900F(1010-1038C). However, radiant and shield tubes more often use the centrifugally cast modified HP
family of alloys - even at the lower temperatures. The most commonly used of the alloys of this family is the
HP grade with niobium. Others that are used are alloyed with other carbide forming elements and solid
solution strengtheners such as Mo and W. There are also grades, referred to as "microalloyed", which have
small amounts of Ti, Zr, and rare earth elements added. The volume of this family of alloys in this
application is about two-thirds that for reformers. Recently, some operators have begun to use the higher
nickel-chromium (35Cr-45Ni) alloys for their higher temperature operations.
FIGURE 19: A section of wrought Inco Alloys 803 alloy with internal, spiralled fins. Tubes also are
produced with straight fins by Inco and Sumitomo for newer, short residence time ethylene cracking
furnaces.
Many of the modified HP alloys are considered proprietary by their developers. However, they are commonly
copied with only minor variations. An example is the recent proliferation of the "micro-alloy" grade which
was actively marketed by only one supplier in 1986-87. Most major suppliers now offer a similar product. The
micro-alloy additions enhance the stress-rupture properties, as shown in Figure 18,(12) presumably through
the development of finer, more dispersed carbides. The rare earth addition also appears to improve oxidation
and carburization resistance. The effectiveness of micro-alloying is very dependent on the melting and
pouring practice. Total deoxidation of the melt and good mixing are required for consistent and improved
performance.
The majority of the cast alloys have a carbon content of 0.4-0.5% but there have been low carbon (0.10-0.18%)
variants produced for use as sweep bends in some of the short residence time furnaces that use small diameter
tubes (~1.5 - 2.5 in. (38-65mm) ID) to improve ethylene yield. Other short residence time furnaces use even
smaller diameter tubes that cannot be cast so must be made of Alloy 800H, Alloy 803, HK4M, HPM or other
wrought alloy. Two other candidate wrought alloys include AC66 and HR-120.
Wrought tubes can be made in single lengths greater than 40 feet(12m) while the smallest cast tube is
restricted to about 9 feet(2.8m). Thus, significant fabricating costs and sources of potential defects are
eliminated by using wrought tubes. On the downside, these alloys do not appear capable of performing
adequately at temperatures as high as the high carbon, highly alloyed cast tubes.
An interesting development is the internally finned tube pictured in Figure 19. Tubes in this configuration
in Alloy 800H, Alloy 803, HK4M, and HPM are being used in ethylene service for radiant section cracking
tubes. A typical 2 in.(51mm) outside diameter tube with a 0.236 (6mm) wall provides a 17% increase in ID
surface area with addition of the fins. This increase in surface area increases thermal efficiency and allows
shorter residence time.
The lower convection section tubes of ethylene cracking furnaces are usually Type 304H while the tubesheets
that support them are static castings of HK-40 or, more likely, HP-Nb. Fittings such as elbows, return bends,
etc. for use within the radiant and shield sections are static cast versions of the same alloy as the tubes.
Outlet manifolds and other non-tubular components outside the firebox may be made in static castings of the
low carbon alloys for easier fabricability or, if the temperature is low enough, the20Cr-32Ni-Nb alloy may be
used. Tubulars up to the quench point or the transfer exchanger are usually centrifugally cast in HP-Nb (high
or low carbon) or 20Cr-32Ni-Nb.
Styrene Production
Styrene is one of many important derivatives of ethylene. Much of the equipment used in its
manufacture operates at temperatures around 1200F(650C). This includes reactor reheaters and the piping and
bellows connecting the reactor and the reheater. This equipment is usually fabricated of Alloy 800H. Up to
one million pounds of Alloy 800H are used per year in the construction of styrene plants.
Failure of some Alloy 800 styrene plant components have been attributed to excessive retained stress, usually
resulting from less-than-optimum fabrication practice. Elimination of this problem has been achieved by
stress-relief annealing the fabricated parts prior to placing the unit in service.
Summary
Wrought and cast heat-resistant alloys are required in numerous refinery and petrochemical
applications because of the combination of aggressive environments and strength requirements. High levels of
nickel and chromium provide alloys with the capability of fulfilling these requirements on an economical
basis. The alloys used for reformer tubing have been going through a transformation from HK-40 to HP-Nb for
tubes and, more recently to the micro-alloyed grades. HK-40 was used almost exclusively for 20-25 years.
There has been a similar evolution in the alloys used in ethylene furnaces which started earlier. The once
commonly used wrought, heat resistant alloys were displaced almost completely by HK-40 which in turn has been
displaced by HP-Nb. This evolution is continuing as reflected by the increasing use of the 35Cr-45Ni alloys.
Some of the newer wrought alloys that have been introduced are now displacing the cast alloys in some niche
applications (eg. short residence time ethylene cracking furnaces).
References
1. "The Making, Shaping and Treating of Steel", United States Steel Corporation, 8th ed., p.1136,
1964.
2. ASM Metals Handbook, 8th ed., Volume 1, p. 598.
3. H. L. Eiselstein, E. N. Skinner, ASTM STP No. 165, p.162, 1954.
4. C. M. Schillmoller, "HP-Modified Furnace Tubes for Steam Reformers and Steam crackers", Nickel Institute
Technical Series, No. 10,058.
5. G. E. Moller and C. W. Warren, "Survey of Tube Experience in Ethylene and Olefins Pyrolysis Furnaces",
NACE T-5B-6 Task Group Report, April, 1981.
6. D. J. Hall, M. Kamal Hossain, R. F. Atkinson,"Carburization Behaviour of HK-40 Steel in Furnaces Used for
Ethylene Production", High Temperatures-High Pressures, Vol. 14, pp. 527-39, 1982.
7. P. Elliott, "Practical Guide to High-Temperature Alloys", Nickel Institute Technical Series No. 10,056,
1990.
8. G. Y. Lai, "High Temperature Corrosion of Engineering Alloys", p. 114, ASM International.
9. R. A. White, E. F. Ehmke, "Materials Selection for Refineries and Associated Facilities", NACE, p.
34.
10. C. M. Schillmoller, "HP-Modified Furnace Tube Market Survey, Nickel Institute Technical Series, No.
10,059.
11. "Stainless Steels in Ammonia Production", Committee of Stainless Steel Producers, AISI, Nov., 1978.
12. "Results of MPC's Evaluation of Stress-Rupture Data for HP-Modified and Microalloyed Alloys - Proposed
Allowable Stresses", Materials Property Council Technical Advisory Committee, Nov. 1991.
Recommended Reading
"The Role of Stainless Steels in Petroleum Refining", American Iron and Steel Institute, 1977 (Available from
the Nickel Institute as reprint No. 9021.). R. A. White and E. F. Emke
"Materials Selection for Refineries and Associated Facilities", NACE International, 1991. George Y. Lai
"High Temperature Corrosion of Engineering Alloys", ASM International, 1990. J. Gutzeit, R. D. Merrick, and
L. R. Scharfstein
"Corrosion in Petroleum Refining and Petrochemical Operations", ASM Metals Handbook, 9th ed., Vol. 13, 1987.
G. L. Swales
"High Temperature Corrosion Problems in the Petroleum Refining and Petrochemical Industries"
Presented at the International Conference on the Behaviour of High Temperature Alloys in Aggressive
Environments, Petten, Netherlands, 1979. C. M. Schillmoller
"Solving High Temperature Problems in Oil Refineries and Petrochemical Plants", Nickel Institute Technical
Series No 10,001. Reprinted from Chemical Engineering, January 6, 1986.
Acknowledgment
D. J. Tillack and J. E. Guthrie are consultants in the United States to the Nickel Institute The
material presented in this publication has been prepared for the general information of the reader and should
not be used or relied on for specific applications without first securing competent advice. The Nickel
Institute, its members, staff and consultants do not represent or warrant its suitability for any general or
specific use and assume no liability or responsibility of any kind in connection with the information
herein.
