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I am attaching the write up on our calorised pipes for your information as under:
Calorising is a metallurgical process for treating the Process quality is monitored by testing coupons ( of
surface of steels, stainless steels and alloys, with the same grade of metal) that are run in the retort with
aluminium that provides protection against elevated- the production materials. The nature of the process,
temperature, scaling and corrosion. the air-tight retorts and the controlled atmospheres
ensure uniform alloy protection over the entire surface
Adding aluminium to carbon and stainless steel is of the Calorising materials. After the process is
commonly known to improve corrosion resistance. A completed, the coupons are removed from the retort,
side effect of the process, however, is unfavorable sectioned and examined in a laboratory for quality and
changes in the mechanical properties of the base steels. depth of diffusion. Special standards and processes
may be selected by the client.
Calorising solves this problem. Calorising diffuses
aluminium into the steel surface to form an alloy with The end result of the Calorising process is a true alloy
excellent heat and corrosion-resistance properties, with the base steels. The process is not a coating and
retaining the base steel's inherent strength & rigidity, there is no mechanical interface with the substrate.
but does not change the high-temperature mechanical The protective diffusion zone cannot be removed
properties of the base steels. The protection provided except by a machining operation.
by the Calorising diffusion zone remains effective at all
temperatures up to the boiling point of the base metal. Calorising is used to enable engineered material to
better resist high temperature sulfidation, oxidation,
During the Calorising process, the steel is chemically carburization, scaling and hydrogen permeation. All
cleaned, treated with flux at 700C and then again types of wrought and cast steels can be Calorised:
treated with another flux at 7300C. After this, the plain carbon and low alloy grades, ferritic and
steel dipped in molten aluminum alloy at 7700C. austenitic steels, high-temperature values of the process
This results in an evenly aluminium coated steel. Then determine the specification of steel to be Calorised.
it is positioned in retort. The retort is sealed and placed
in an atmosphere-controlled furnace. The Advantages of an Calorised surface include:
aluminium diffuses into the surface of the steel at * Elimination of problems inherent in coating
elevated temperatures and forms an alloy with the processes due to the difference in thermal
substrate. retort and excess powder is removed.
expansion between coating and substrate;
After furnace cooling, the steel is taken out of the * Ease with which fabricated shapes, internals
retort. Straightening, trimming, beveling and and tube internal diameters can be treated; line
other secondary operations are the performed of sight is not required.
if required.
Technical Benefits
• High corrosion resistance.
• Case depth up to .015" for tubing.
• Operates in continues temperatures up to 1750 degrees F.
• Retains the base steel's inherent strength and rigidity, depending on base material.
• Does not change the high-temperature mechanical properties of the base steel.
• Utilizes Indra Udyog Technologies' patented diffusion alloying process.
Common Applications
• Sulfuric acid/ acid gas removal : Resists sulfidation from H2S, SO3; increases tube life up to 20 years.
• Refining: Charge heaters, delayed cokers, sulfur recovery, heat exchangers.
• Petrochemical: Reformers, ammonia, heat exchangers.
• Reformers – DRI: Secondary reformers, waste heat boilers.
• Boilers : PR boiler (pulp and paper), waterwall panels, traditional boilers.
When Stainless Steel Fails
Calorised steel performs well in high-temperature The nickel sulfide eutectic has a melting temperature
environments where even stainless steel fails. of 6450C. However, at temperatures as low as
5500C, sulfur will begin to penetrate the nickel alloy,
Stainless steel are often considered the final answer causing rapid embrittlement.
to every kind of corrosion problem. But there are
those situations in which stainless steel, because of Once the eutectic forms, the nickel is preferentially
its metallurgical or chemical properties, is almost melted our of the alloy, leading in many cases to
certain to fail (e.g. catastrophic sulfidation of high- catastrophic corrosion and failure.
nickel stainless materials).
Calorising stainless steel will passivate the surface of
Austenitic grades of stainless steel are used because the material, tying the nickel into an iron-aluminum
of their excellent resistance to high-temperature alloy and preventing the formation of the eutectic.
oxidation. Generally, the higher the temperature to
which the metal will be exposed, the higher the nickel In an experimental plan in which a technique
content of the alloy must be. for direct reduction of iron ore was being studied, one
For example, a type 201 stainless (16/18% nickel) of the components was a 1" IPS schedule 160 pipe
can withstand a continues operating temperature of made of a high nickel proprietary alloy. During a
15500 F while a type 310 stainless (19/22% nickel) 17-hour period in which this piece was exposed to
can withstand continuous service temperatures as high-temperature sulfur bearing gases, the pipe wall
high as 21000 F. was completely penetrated in a number of places.
Alloys with greater nickel contents can withstand even The replacement piece was Calorised and the pipe
higher working temperatures. There is a limit, remained sound and unaffected by many subsequent
however, to the maximum amount of nickel that can exposures to the same reagents at the same
be used in an alloy exposed to high-temperatures temperatures
sulfidation.
With nickel contents of 25% or more, the nickel will
preferentially combine with sulfur to create a low-
melting temperature nickel sulfide eutectic.
Reducing Hydrogen Permeation
Calorising the surface of a steel substrate is very Example # 3: A bayonet- type heat exchanger in
effective in reducing hydrogen permeation, especially which a heating medium was losing sensible heat
when both the inner and outer surfaces of the steel are through the annulus between the inner tube and outer
processed. Evidence suggests that Calorised ferritic tube. It was postulated that hydrogen diffusion
and austenitic steel reduce hydrogen permeation by through the wall of the inner tube the annulus was
three orders of magnitude compared to untreated providing a means for conducting heat from the inner
alloys of the same composition. tube. Thereby reducing the temperature of the
Example # 1: Hydrogen permeation is also related to heating medium and causing the failure.
other types of degradation, such as stress corrosion
cracking and hydrogen blistering. In "Sulfide Stress The metallurgy of the inner tube was a low-chromium
cracking Studies on Aluminized Steel," Dr.L.H. ferritic steel, API 5-A grade N-80. A section of this
Gundiler (New Mexico Institute of Technology) states: tube was Calorised inside and out and tested for
"since AI has a very low hydrogen permeability hydrogen permeation by the designer, a major
coating with AI will act as a hydrogen barrier and is industrial firm in the
expected to decrease SSC susceptibility". Dr. the double Calorised surface reduced total hydrogen
Gundiler's studies were carried out on dip aluminized permeation by almost three orders of magnitude. the
substrates, which develop a thin intermetallic layer. entire heat exchanger (over two miles of pipe) was
When the surface aluminium is lost, the intermetalic then Calorised. The process worked under this new
layer remains effective. His report further states (p. configuration to its theoretical design efficiency.
63): "Both aluminum layer and the iron-aluminum
intermetallic layers have hydrogen permeability rates
several order of magnitude less than that of iron.
Consequently, aluminizing produces an excellent barrier
for hydrogen diffusion and prevent (hydrogen induced)
sulfuric stress cracking (SSC) failure of high strength
steels."
Example # 2 : Calorising reduces the permeation of
deuterium through an austenitic or martensitic steel
substrate. One study (K.C. Forcey, D.K. Ross, J.C.B.
simpson, and D.S. Evans, Journal of Nuclear
materials, 160 (1988) 117) found that the permeation
rate of tritium through aluminized material was nearly
four order of magnitude lower than through untreated
steel. The authors inferred from this data that
permeation of trirum through aluminized martensitic
and austenitic steel coupons would also be greatly
reduce, particularly at the operating temperature
range to 6000C (11000F).
Sour Gas Plant Waste heat Boiler
The Shell Caroline gas plant, one of the largest sour All carbon steel tubes were Calorised to eliminate
natural gas processing facilities in the world, high-temperature sulphidation. Although the hot
successfully used Calorised tubes to eliminate high- gases pass through the inside of the tube, both
temperature sulphidation. surfaces were Calorised. Care was taken to eliminate
any aluminum alloy in the area of the tube, both
The Caroline plant, which began operation in 1992, is surfaces were Calorised . Care was taken to eliminate
designed to process 8,533,000m3/ day of raw gas any aluminum alloy in the area of the tube that was
and gas equivalent of liquid hydrocarbons. The welded into the tube sheets during assembly.
overall sulfur recovery for the facility is 99.8% of the
total sulfur recovery production is expected to be In accordance with standard practice, the tubes were
approximately 4,000tonne/day. Gas is Calorised, then rerounded by passing them through a
contaminated with H2S (approximately 35%). roller straightener to bring them to ASTM A450
tolerances. The tubes were trimmed to final length,
During operation, the total raw gas and liquid ends dressed, and protective plugs inserted in the
hydrocarbons from the field are processed in the inlet ends. The tubes were then trucked to
feed facilities. The overhead sour gas from the feed assembly.
fractionator, after being stripped of heavy
hydrocarbons, is sent to the gas sweetening facilities NOTE: The heat of the Calorising process may cause
where the acid gases are removed by physical and a slight change in tube length. For this reason tubes
chemical absorption. The acid gas is sent to the or pipes with a finish length of longer than about 16'
claus sulphur recovery unit. (4,900mm) are typically ordered 1% over length and
then trim cut to final length after Calorising.
In the Claus process, H2S in the acid gas stream is
partially oxidized in the reaction furnace. This Calorised tubing is specified in a number of gas plants
reaction is highly exothermic. The Caroline plant is to protect the tubes in the waste heat boilers and
designed around a predicted exit gas temperature condensers. A wide variety of metallurgies and
between 22000F and 23000F(12040C / 12600C). The configurations are Calorised for use in many types of
exit gas temperature of other plants is between gas-processing plants.
14000F and 23000F(7600C and 12600C). with most in
the range of 17000F/ 22000F (9270C/ 12040C). The
Caroline plant contains two sulphur recovery units,
each having two reaction furnaces and waste heat
boilers operating in parallel. Each boiler contains
1010 tubes, 2 ¼" (57.1mm) diameter, approximately
12 meters in length, and has a generating capacity of
approximately 100t/hr at 3650 kPa.
Performance Data on Calorised Pipes
A comparison between ordinary steel pipes, expensive imported Alloy Steel Pipes and Stainless Steel Pipes under different corrosive conditions is as following :
Effect of SO2 Gas
TIME HOURS EXPOSURE
(a) Effect of H2S Gas SO2 GAS
103 gm
Wt. Change
(b) Comparison between Stainless Steel, and Calorised Stainless Steel under Co. CI & CO2 Depth of Carbon Penetration x 10-1 inch
HOURS EXPOSURE
(c) Comparison of Lancing Pipe of Mild Steel, Stainless Steel and Calorised Mild Steel.
OXIDATION AIR : *
METAL TEMPERATURE 0F
• Temperature at which scaling is negligible (less than 10 mg / sq. cm.)
FREQUENTLY ASKED QUESTIONS
Is Calorising a coating? Can Calorising be performed on the outside
Calorising is a diffusion is an alloying process, NOT a coating diameter and inside diameter?
Calorising diffuses aluminum molecules directly into the
substrate of the steel. Coatings such as hard chrome have Calorising can be performed on any or all surface of
an interface with the host material, which means they can tubes or other parts. Areas that are not to be Calorised
separate. can be "masked off' to protect them from diffusion
alloying. This is quite common for boiler tubes being
What is Calorising? fabricated into tube sheets.
During the Calorising process, the steel is chemically Can Calorising be applied to other products?
cleaned, treated with flux at 700C and then again
treated with another flux act 7300C. After this, the The Calorising process can be applied to a wide range
steel dipped in molten aluminium alloy at 7700C. of materials used in many different industries where
This result in a evenly aluminium coated steel. Then carburization, sulfidation, oxidation and corrosive
it is positioned in retort. The retort is sealed and placed environments exist.
in an atmosphere-controlled furnace. The
aluminum diffuses into the surface of the steel at What happens to the mechanical properties of
elevated temperatures and forms an alloy with the the substrate material?
substrate.
The mechanical properties vary from material to
After furnace cooling, the steel is taken out of the material after they have been processed. Test data
retort. Straightening, trimming, beveling and supporting the mechanical properties of some materials
other secondary operations are the performed after processing is available. Some customers do
if required. however request tensile and yields for large size
projects. This is usually provided at the customer's
What are the turn around times for Indra Udyog cost.
products?
What is the maximum operating temperature of
Processing time depends on the volume of tubes ordered. the Calorised surface?
Processing other parts generally takes 2-5 weeks,
depending on the size and material of the part and The maximum continuous operating temperature is
Indra Udyog production schedule. Rush jobs can 1750 degree F. or 955 degrees C.
sometimes be completed more quickly. Cool down duration
is subject to the mass of the retorting and required Can the Calorised surface be machined?
mechanical properties. Yes, the Calorised surface can be machined; but the
thickness of the case depth must be considered.
What is the diffusion zone thickness of case Machining of the surface of a processed component will
depth of Calorising? reduce the case depth of the diffused zone.
The normal thickness for the alloy formed is about Can I weld to the Calorised surface?
0.076mm for austenitic grades of steel, cobalt-based
alloys, and super alloys; .127mm for ferritic alloys: Yes, Calorised pipe welding procedures are available
and about 0.178mm minimum for carbon steels. from Indra Udyog.
The case depth can be substantially thicker when required on
many materials, as high as 0.300mm. Can the Calorised surface be repaired if the
diffused surface is damaged?
What is the largest part that can be processed?
No, but the part may be processed again to re-establish
Parts up to 450mm diameter and length up to 8 mtr a Calorised surface.
can be processed by Indra Udyog. The part to be
processed must be able to fit into one of our furnaces.
What is the largest tube that can be processed?
Indra Udyog can process pipe/tubing up to 8 mtr in overall
length and from very small to very large diameter. Our
normal tube diameters are limited to 150mm but custom
fixturing can allow for processing larger diameters.
What materials can be Calorised?
Plain carbon steels with no, or few alloys are the best
material for Calorising. Indra Udyog can also treat
low alloy steel, stainless steels, iron-, nickel, and cobalt
-based superalloys, as well as copper-based alloys.
Indra Udyog
Manufacturer of Lancing Pipes, Calorised Pipes, Bright Bars & Fabricators
W-9/W-15, MIDC Industrial Estate, Taloja, Navi Mumbai – 410208, INDIA.
Tel - 27410277/27402645 –Fax - 27402645
E Mail: info@indraudyog.com,
Web Site: www.indraudyog.com
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