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High temperature stainless steels
High temperature stainless steels within the steel and metals industry
High temperature stainless steels
The various process stages in the metallurgical industry,
right through from ore to the finished, rolled or forged
product, usually take place at high temperatures.
The production equipment used in these processes is
subjected to intense heat from hot gases or from red-hot
or molten materials, which makes heavy demands on
the construction materials used for that equipment.
The problem can be solved by selecting special alloys
for parts exposed to particularly difficult conditions.
CREEP RESISTANCE
The design stress for a material specifies the load to
which this material can be subjected at high tempera-
tures without failing or being significantly deformed
during service. From room temperature up to a certain
temperature (550 – 600°C for most austenitic steels),
the design stresses are based on the proof strength of
the material. Above that temperature, the more tempe-
rature dependent creep strength will determine the
design stress values.
As a rule, creep strength is expressed as the creep
rupture strength, i.e. the stress that causes rupture
after 10 000 or 100 000 hours (Rkm 10 000 and Rkm 100 000).
For components that are more sensitive to deformation,
the creep deformation strength, i.e. the stress resulting
in a strain of 1% after 10 000 or 100 000 hours (RA1/10 000
and RA1/100 000), should be used as a basis for design
calculations.
An often neglected mechanical property is the
ductility. In a creeping component, stress redistribution
due to creep can off-load the heaviest stressed parts,
provided the ductility is high enough. Moreover, the
resistance to low cycle fatigue (during start-ups and
shut-downs, or major service transients) is proportional
to the ductility.
MICROSTRUCTURAL STABILITY
Most high temperature alloys suffer from a common dis-
advantage when used at sufficiently high temperatures
– diffusion controlled microstructural changes, which
result in impaired properties. The most common type
of reaction is the precipitation of non-desirable phases.
Besides lowering the corrosion resistance by consuming
beneficial alloying elements (above all chromium), this
phenomenon leads to a reduced toughness/ductility of
the material – especially at room temperature.
The precipitates are often intermetallic phases such
as sigma, chi, and Laves phase, but carbides and nitrides
are also common.
At even higher temperatures, grain growth may
occur, possibly increasing the creep strength somewhat,
but simultaneously reducing the ductility substantially.
HIGH TEMPERATURE CORROSION
Oxidation
When a material is exposed to an oxidizing atmosphere
at high temperatures, an oxide layer is formed on the
surface. This layer will retard further oxidation. If the
temperature of the material increases, the oxide growth
rate will increase and the layer will finally crack and
spall off, thus losing its protective effect – the scaling
temperature has been reached. Although oxidation is
seldom the main cause of high temperature corrosion
failures, the oxidation performance is of primary
interest, because the properties of any formed oxide
layer will determine the resistance to other aggressive
elements in the environment.
The toughness and adherence of the oxide layer
also determine the erosion resistance of the alloy.
Water vapour
Most flue gases (except from coal combustion) have
an increased water vapour content. Its presence will
reduce the oxidation resistance of an alloy.
Carburization and nitridation
Carburization and nitridation are common heat treat-
ment processes in which the surface of the material is
intentionally enriched in carbon and/or nitrogen
to improve the hardness, the wear resistance, or the
fatigue strength of a component.
2
What properties are demanded of a high temperature alloy?
Depending on the operating conditions, the demandson high temperature alloys may be as follows:• High creep strength• Stability of internal microstructure• High resistance to oxidation and HT corrosion• Good resistance to erosion-corrosionThese properties are discussed in more detail in the following text.
Equipment manufacturers also make the followingdemands on the material they use:• Good formability and machinability• Good weldability• Good availability on the market
Table 1: Chemical compositions and designations of AvestaPolarit high temperature alloys
EN
1.49481.48781.48181.48281.48331.48351.48451.4854
High temperature stainless steels
3
Even if the furnace components in these processes are
constructed of more resistant materials, the cyclic
exposure to the carburizing/nitriding environment
will eventually lead to an excessive pick-up of carbon/
nitrogen. This will lead to problems such as embrittlement
due to precipitation of chromium rich carbides/nitrides
and impaired corrosion resistance because of the
simultaneous chromium depletion in the matrix.
These effects can also occur for other reasons, e.g.
carburization due to oil residues on heat-treated com-
ponents and nitridation due to overheating in nitrogen
containing gases or to cracking ammonia.
Attack by sulphur, halogens, and molten salts and metals
Sulphur attacks are often life limiting in many high
temperature applications. Due to kinetic factors, non-
equilibrium sulphides can form and grow under oxidizing
conditions. Even if initially formed sulphides are later
overgrown by oxide or dissolved, their earlier existence
has made the oxide layer less protective.
An old rule-of-thumb says that nickel-containing
alloys should be avoided in reducing sulphidizing envi-
ronments, since the formation of low melting point nickel-
sulphur compounds may lead to a rapid deterioration of
the alloy. In practice, however, the austenitic microstruc-
ture is required for good mechanical properties, and a
number of nickel containing alloys have shown excellent
performance in sulphur-bearing environments, since
their chromium contents were high enough to enable the
formation of a protective oxide layer.
Molten salts and slags can attack an existing protective
oxide film. The extent of the attack will depend on the
composition of both the alloy and the melt. Halogens
(such as chlorine and fluorine) or their salts (halides)
may also cause serious damage.
Molten metal corrosion is rarely occurring, but when it
does, it can be very detrimental. Two types of attack can
appear – dissolution of the solid metal (or one or another
alloying element) in the melt, or penetration of the melt
into the grain boundaries of the solid metal, causing
rapid brittle cracking.
Erosion-corrosion
Particle impact on and/or abrasion of an oxide layer can
remove it, or at least make it less protective. A ductile
and adherent oxide layer is therefore beneficial.
Chemical composition, %, typical values
COMPOSITION AND STANDARDS
ASTM
304 H321 HS30415
–309SS30815310SS35315
49484878153 MA48284833253 MA4845353 MA
C
0.050.050.050.040.060.090.050.05
N Cr
18.317.518.52022.5212525
Ni
8.79.59.51212.5112035
Si
0.50.51.32.00.51.71.01.5
Others
–TiCe––
Ce–
Ce
BS
304S51304S51
––
309S16–
310S24–
DIN
1.49481.48781.48911.48281.48331.48931.4845
–
NF
Z6 CN 18-09Z6 CNT 18-10
–Z17 CNS 20-12Z15 CN 23-13
–Z8 CN 25-20
–
SS
233323372372
––
23682361
–
AvestaPolaritdesignation
––
0.15––
0.17–
0.15
National steel designations, superseded by EN
153 MA, 253 MA, and 353 MA are patented grades with trademarks used by AvestaPolarit. 253 MA and 353 MA are registered.
AvestaPolarit high temperaturestainless steels
Besides the common HT alloys presented below (i.e.,
4948, 4878, 4828, 4833, and 4845), there are three propri-
etary AvestaPolarit alloys: 153 MA, 253 MA, and 353 MA .
These three alloys are based on the same concept:
• Improved oxidation (and thus also HT corrosion)
resistance by an increased silicon content and
addition of very small quantities of rare earth metals
(micro-alloying=> MA).
• Enhanced creep strength due to increased contents of
nitrogen (and carbon for 253 MA). In many cases, the
properties of these steels have proved to be equiva-
lent or even superior to those of grades with higher
contents of alloying elements. Materials selection
will be determined by the application and operating
conditions in each individual case. 153 MA is normally
intended for use at somewhat lower service tem-
peratures than the other two grades. The chemical
compositions of the AvestaPolarit high temperature
steels are shown in the table below.
TENSILE AND CREEP STRENGTH PROPERTIES
Most strength values are tabulated in the AvestaPolarit
data sheet “High Temperature Stainless Steel”.
Therefore, the strength and its variation with tempera-
ture are only shown graphically here.
Diagram 1 shows clearly that 153 MA and 253 MA
have higher proof strength values at room temperature
as well as at elevated temperatures. This is a result of
the higher nitrogen contents in these two alloys.
353 MA has a similarly high room temperature
strength. At present, there are no specified proof strength
values at higher temperatures.
Diagram 2 shows the 100 000 hours creep strength
as a function of temperature for all our HT steels. The
higher creep strength of the MA alloys is, also in this
case, a result of the higher nitrogen content.
A more obvious way to illustrate the difference
between various steels is to use relative, instead of
absolute values:
For each alloy and temperature, the relative strength
has been calculated by dividing the stress value that
gives rupture after 100 000 hours with the correspond-
ing value for 253 MA.(E.g., at 800°C, 4828, 4833,
and 4845 are only half as strong as 253 MA, i.e., twice
the material thickness is required for “normal”
dimensioning.)
The analogous curves for the other creep strength
parameters (i.e. Rkm 10 000, RA1/10 000, and RA1/100 000) show
similar behaviour.
MICROSTRUCTURAL STABILITY
Upon service exposure at elevated temperatures,
most alloys become more or less embrittled.
4948, and especially 4878 are little affected, while the
loss in toughness is substantial for 4828, 4833,
and 4845, due to an extensive precipitation of the
intermetallic sigma phase.
In 253 MA and 353 MA, much less sigma is formed.
Instead, carbide and nitride precipitation will take
place during service, which will result in a loss in RT
impact toughness. In fact, it may be as low as for a
sigma phase embrittled alloy, and in addition, the
4
High temperature stainless steels
Diagram 2: 100 000 hours creep rupture strength.
Diagram 3: Relative 100 000 hours creep rupture strengthDiagram 1: Elevated temperature proof
Diagram 4: Charpy V toughness after 200 hours’ ageing
Maximum service tem-perature (°C) in dry air
toughness reduction will be more rapid since the
precipitation of carbides/nitrides is faster than that of
sigma phase. However, service experience indicates
that the ductility is superior at lower deformation rates.
The carbon/nitrogen solubilities in the MA alloys
increase with increasing temperature, and above a
certain temperature, the post-service toughness will
be sufficiently high. This temperature is 850°C for
253 MA and 1000°C for 353 MA. These alloys can of
course be applied at lower temperatures if the loss in
RT impact toughness is born in mind when main-
tenance and repair work is performed. 153 MA was
developed as a leaner alloyed variant of 253 MA for
applications where high demands are made on tough-
ness. 153 MA will have a sufficiently high toughness
after service at all temperatures.
HIGH TEMPERATURE CORROSION
Oxidation
The oxidation resistance of all HT grades rely on the
formation of a protective oxide layer, rich in chromium,
aluminium, and/or silicon. Additional alloying elements
may improve the properties further. Diagram 5 shows
that, in spite of its lower chromium content, 253 MA
shows better oxidation resistance than 4845 under cyclic
conditions.
Obviously, the REM addition and increased Si
content of 253 MA have improved the adherence of the
oxide so that the alloy can retain a thicker oxide layer
before it starts to spall due to thermal shock. Short-term
tests, as in Diagram 5, are a rapid method of ranking
alloys. However, one must bear in mind that this
ranking can change with increasing time, cf. Diagram 6.
Historically, the oxidation resistance of an alloy
has been specified as the “scaling temperature”, i.e.
the temperature, at which the oxidation rate becomes
unacceptably high. Since this temperature is of little
technical importance, we have abandoned the “Scaling
temperature” concept, for “Maximum recommended
service temperature”, which is based on service
experience together with long- and short-time,
isothermal and cyclic laboratory tests, see Table 2.
Water vapour
The presence of water vapour in the environment will
make any formed oxide layer more porous and hence
less protective. The reduction in maximum service
temperature can be 50 – 150°C, depending on steam
content.
Carburization/nitridation
The resistance of high temperature alloys to carburiza-
tion/nitridation increases primarily with increasing
nickel content but also with increasing contents of
silicon and chromium. 353 MA is therefore the best of
the MA grades, but 253 MA has also performed well
under certain conditions in carburizing/nitriding
environments, despite of its lower alloy content.
Experience has shown that it takes only traces of
5
High temperature stainless steels
Diagram 5: Cyclic oxidation at 1150 °C. The specimens werecooled down to room temperature every two hours
Diagram 6: Long-term oxidation at 1100°C. The specimens werecooled down to room temperature once a week for weighing.
1.49481.48781.48181.48281.48331.48351.48451.4854
304H321HS30415
–309SS30815310SS35315
49484878153 MA48284833253 MA4845353 MA
800800
100010001000110011001150
AvestaPolarit designation EN ASTM
Table 2: Recommended maximum service temperatures
oxygen in the furnace gas (e.g. in the form of carbon
dioxide or steam) to produce a thin and tough oxide
layer on 253 MA, which provides good protection
against pick-up of both carbon and nitrogen. However,
under reducing conditions, when such a scale cannot
form, 353 MA and 4845 are better alternatives.
Sulphur attack
While high nickel content is beneficial to the resistance
of the material to carburization and nitridation, it can
be a disadvantage in a sulphur-rich environment. In
oxidizing gases, where sulphur occurs in the form of
sulphur dioxide, attack is delayed only as long as the
material is protected by a thin, continuous oxide film.
However, if the oxide grows in thickness and begins to
crack, the gas will be able to penetrate through to the
base material and continue the attack.
Due to their firmly adhering protective oxides,
153 MA, 253 MA, and 353 MA are better suited for such
environments than materials with similar or higher
nickel contents. Nevertheless, the maximum service
temperature is lower than in air. In reducing sulphurous
atmospheres, the oxide layer is rapidly dissolved and the
bare metal is exposed to attack. Under such cicumstances,
nickel-free (or at least low Ni) alloys should be used.
Molten salts and metals
Certain heat treatment steps are carried out in molten
salt or metal pots. The corrosion problems often occur
at the melt-air-interface, but can be managed.
Attacks from e.g. molten flue gas deposits or
accidentally contaminating metals/alloys can be much
more damaging.
Erosion-corrosion
Replacing salt/metal pots with fluidized bed furnaces
will put other demands on the construction material
from being corrosion resistant to being able to with-
stand the abrasive wear.
Another type of erosion-corrosion occurs in flue gas
channels, where particles are often entrapped in the
rapidly moving combustion gas stream.
In both these types of erosion, the MA grades have
shown excellent resistance due to the thin adherent
oxide layer formed on them, see e.g. Diagram 7.
FORMING, MACHINING, AND WELDING
The workability of 153 MA, 253 MA, and 353 MA is
similar to that of ordinary austenitic stainless steels.
They have good formability in cold condition, although
they work-harden in the same way as other austenitic
stainless steels. However, since these grades have high
nitrogen contents, they also have higher mechanical
strength and require higher deformation forces during
cold working. Hot forming should be carried out
in the temperature range 1150 – 900°C (the minimum
temperature for 353 MA is 980°C).
Since 153 MA, 253 MA, and 353 MA are harder than
conventional austenitic steels, their machinability is also
affected. Their tendency towards work hardening during
cold deformation must also be taken into account in
machining. See “Machining Guidelines” for each alloy.
These grades have good weldability. Suitable
welding methods are shielded metal arc welding, inert
gas welding with pure argon, or submerged arc welding
(the latter not for 353 MA).
The best results are achieved by using AvestaPolarit
253 MA filler metal for both l53 MA and 253 MA.
If a somewhat poorer oxidation resistance, creep
strength, and microstructural stability are acceptable,
AvestaPolarit 309 filler metal can also be used.
A new SMAW electrode, 253 MA-NF, has been
developed for applications where embrittlement is
unacceptable. More detailed welding instructions are
given in a special AvestaPolarit Welding brochure
entitled “How to weld 253 MA”. Also for 353 MA, there
is a filler metal with a matching composition and a
special welding instructions brochure.
It is generally not necessary to perform heat treat-
ment after forming or welding since the material will
be exposed to high temperatures when in operation.
In some cases, heat treatment may be required to
relieve material stresses (e.g. fan impellers).
6
High temperature stainless steels
Diagram 7: Erosion test results
AVAILABLE PRODUCT FORMS
Sheet and plate products are manufactured by
AvestaPolarit, welded pipes and tubes, fittings, wire
and filler metals are manufactured by AvestaPolarit
subsidiaries, seamless tubes by AB Sandvik Steel, and
castings are produced by licensees.
The entire AvestaPolarit range of high temperature
steels, both standard and micro-alloyed, is outlined
on the last page of this brochure.
The application of heat-resistant alloys, principally for
the various process stages in the production and pro-
cessing of iron and steels, is described below. In many
cases, these examples will also apply to the production
of non-ferrous metals, such as copper, aluminium, etc.
Special attention will be given in the descriptions to
the application of the micro-alloyed high temperature
(X53 MA) steels developed by AvestaPolarit.
PELLET SINTERING PLANTS
Sintering is used for converting fine-grained ore
concentrate into larger pieces, which are better suited
for the blast furnace process.
This can be carried out in batches in tiltable pallets
(Fig. 1) or continuously on a conveyor type of sintering
furnace. The furnace and conveyor belt both have a
cast grid base with intervening gaps for the extraction
of combustion gases. The charge, which consists of a
mixture of ore concentrate, limestone, and coal dust, is
ignited in both cases from above by means of a burner
Hot rolled plate and sheet
Cold rolled plate, sheet and strip
Bar; Wire
Welded pipe and tube
Seamless tube
Fittings
Fabricated products of sheet and plate
Welding consumables
Castings
153 MA 253 MA 353 MA
• • •
• • ••
• •• •
• •
• •• ••
placed over the surface of the charge. A wind box is
connected below the grid, and the vacuum it creates
causes the combustion front to move down through
the charge.
In the sintering process, high temperature materials
are used principally in the form of castings for the grids
and sheet for the wind boxes and burners in the firing
hoods. The grids are subjected to relatively rapid
temperature variations from the charging of cold ore
concentrate mixture up to the ignition and discharge of
the sintered material. The most common material
problems in this application are the deformations caused
by high and fluctuating temperatures.
Since both the ore and the coal dust contain sulphur,
153 MA and 253 MA are more suitable than alloys with
higher nickel contents. By employing castings with
higher carbon contents and special cast microstructures,
a creep strength, which is higher than that of rolled
material, is assured. Cast grids of 253 MA have been
dimensionally stable over a long service time, without
the gas permeability being affected or the grids becoming
jammed or distorted.
BLAST FURNACE PLANTS
In blast furnaces, high temperature materials are
typically used for the recuperators in which the blast
air is preheated by the furnace gas (Fig. 2), the charging
mechanism for pulverized stock charged into the
furnace, the circulation fans, gas piping, etc. The coking
ovens used for producing the blast furnace coke are
also equipped with recuperators for recovering heat
from the hot gases. High temperature alloys may also
be necessary for the discharge doors and collecting
grids of the coking ovens, which are subjected to high
temperatures and abrupt temperature changes in
conjunction with water-cooling.
The temperature in the recuperators may vary from
1150°C at the hot-gas inlet end, down to the ambient
outdoor temperature at the combustion air intake.
Since both the coking oven gas and the blast furnace
7
High temperature stainless steels
Applications in the steel andmetals industries
Figure 1
ignitioncool dust
ore concen-trate finest
recycledbedding sinter
grid
suction
combustionfront
gas contain sulphur, ferritic chromium steels have
commonly been used, which has led to problems of
creep deformation in the hottest zones. As both 153 MA
and 253 MA have much higher creep strength than
ferritic steels, they are better suited for this application.
They also have better resistance to the effects of sulphur-
rich gases than equivalent high temperature steels and
nickel-base alloys.
AvestaPolarit 253 MA has also been successfully
used in expansion bellows (Fig. 3) for cyclically heated
components. Expansion bellows for recuperator
installations used to be made of 4878 or 4948, but a
change to 253 MA, increased the service life of the
bellows from 3–6 months to several years.
Figure 2
Figure 3
STEEL MELTING, SMELTERS, AND CONTINUOUS
CASTING PLANTS
When steels and other metals are melted and refined in
arc furnaces and converters, components such as fume
extraction hoods, flue gas ducts, dampers, hatches,
bridges, and the preheaters for ladles and scrap are
subjected to high thermal stresses. This applies
particularly to equipment, which cannot be protected
by water cooling or refractory lining. Depending on
the maximum service temperature, 153 MA, 253 MA,
or even 353 MA may be used in these applications to
avoid serious deformation and frequent repairs. 253 MA
and 353 MA have also been used successfully in chutes
for feeding e.g. scrap into the arc furnace or other
alloying additions into the converter.
ROLLING MILLS
Before rolling or forging, ingots, slabs, and billets are
usually heated in box-type or continuous reheat furnaces.
Gas or oil burners or electric resistance elements are
used for heating. In such furnaces, the components
subjected to high temperature stresses are principally
the rollers, slide-rails, or walking beams used for
moving the material through the furnace. The frame-
work and edge reinforcements for the charging and
discharging doors are also subjected to high tempera-
tures. Due to its high creep strength, AvestaPolarit
253 MA has proved to be an excellent material for such
components. Numerous installations at rolling mills in
several countries have yielded very favourable results.
Lately, there has been a transition from “common”
burners to oxy-fuel burners, where the combustion air
is replaced by oxygen. In addition to all the benefits,
there is one draw-back – the flue gas water vapour
content will increase substantially (10 – 40%), which
will increase the demands on oxidation resistance of
the construction materials.
HEAT TREATMENT FURNACES
Steelworks, metal works, and special hardening shops
carry out heat treatment to give various products the
required properties. Many different types of furnace
with different atmospheres and temperature cycles are
used for this purpose. If heat treatment requires a
controlled furnace atmosphere – an inert gas, an active
gas, or vacuum – a gas-tight inner casing is used in the
furnace. This is known as a muffle or retort and is made
of a high temperature steel or a nickel base alloy.
8
High temperature stainless steels
Figure 5Figure 4
muffle(inner cover)
intermediatepartition
base
fan
diffuser
The retort is actually a pressure vessel and is thus
intended for higher gas pressures than a muffle.
The most important furnace types and the material
problems commonly occurring are discussed below.
Bell-type furnaces
A bell-type furnace consists of a vertical cylindrical or
rectangular shell, with a domed end welded to the top.
The shell or “bell” has a refractory insulation and is
placed over the muffle, which encloses the material to
be heat-treated in a controlled furnace atmosphere.
Heating is carried out by gas or oil burners, by electric
resistance elements, or by radiant tubes between the
bell and the muffle. A fan at the bottom of the muffle
circulates the hot gas inside it to ensure a uniform
temperature throughout the furnace. The material to
be heat-treated may be coils of strip, wire rods, bars,
or small parts. The material is placed on a base above
a grid known as the diffuser, which helps to distribute
the circulating gas in the muffle Fig. 4.
The problems usually arising are that the muffle is
distorted adjacent to the burner zones due to non-
uniform temperature, or that the entire bottom part of
the muffle deforms due to creep. The base, the diffuser,
and the fan impeller may also distort because of the
high temperatures and mechanical stresses.
The material selected for the muffle will depend on
the maximum service temperature and the atmosphere
in the furnace. AvestaPolarit 153 MA and 253 MA are
suitable alternatives to conventional high temperature
steels, such as AvestaPolarit 4833 (309S), 4845 (310S), or
4828 (W.-Nr. 1.4828), due to their better creep resistance.
Service experience shows that furnace components
made of these alloys are easier to repair and require
less maintenance. 253 MA should be employed for
temperatures above 850°C. If there is a risk of carburiza-
tion and/or nitridation (and 253 MA has proved
inadequate), more highly alloyed nickel alloys such as
353 MA will be necessary.
Pit furnaces
A pit furnace is, in principle, an inverted bell-type
furnace, which is recessed into the floor. The material
problems and their solutions are therefore similar to
those associated with bell-type furnaces.
Box-type furnaces
The box-type furnace is charged horizontally through
a door and is provided with a gas-tight muffle if used
for heat treatment in a controlled atmosphere. If electric
heating is employed, the heating elements in the bottom
are protected by a hearth made of high temperature
material (Fig. 5).
In box-type furnaces, heat-resistant materials are
also used for fans to ensure uniform temperatures and
for pier protection caps. The most common material
problem is that the muffle and hearth become distorted
due to high temperatures and temperature differences.
The distortion is accentuated at points where the muffle
is secured or at the bottom, due to the cooling effect of
the supports. Other problems include failure of welded
joints and carburization and/or nitridation from the
9
High temperature stainless steels
• Bell-type furnaces
• Pit furnaces
• Box-type furnaces
• Molten salt/lead pots
• Continuous furnaces
• Furnaces with fluidized beds
Figure 6
furnace atmosphere, which may lead to serious oxida-
tion attacks or embrittlement. The materials used and
alternative solutions employed are the same as those
described above for bell-type furnaces.
Molten salt/metal furnaces
Salt bath furnaces are frequently used for liquid
carburization and/or nitridation (case hardening),
but also for “neutral” heat treatments, due to the
excellent heat transfer and energy efficiency.
For the case hardening salt pots, a high nickel alloy
should be beneficial. For the neutral salt mixtures of
KCl, NaCl, and BaCl2, the main problems are attacks
from salt vapours and from contaminations in the
salt bath.
The most common molten metal application is
patenting of wire in molten lead (or bismuth) baths.
The lead itself is not extremely aggressive unless the
construction material has a too high nickel content.
The main problem is instead attacks from lead oxide
at the metal/air surface, which should be covered with
pulverized coal.
Furnaces with fluidized beds
In more recent generations of furnaces, based on heat
transfer by the fluidized bed principle, 253 MA has
proved to be suitable as a structural material for the
furnace walls. In this context, the resistance to erosion
caused by the pulverous bed material is important.
This type of furnace may, for example, be used as a
replacement for molten lead or salt bath furnaces for
heat treatment of steel wire.
Continuous furnaces
In a continuous furnace, heat treatment of the material
takes place as the material is continuously fed through
the furnace. A common type is the straight tunnel
furnace used for the annealing, hardening, or tempering
of rolled strip, wire, machine components, or other
separate work pieces (see Fig. 6). These furnaces can
also be equipped with a gas-tight muffle (Fig. 7) made
of high temperature material, if the annealing process
demands a controlled furnace atmosphere. The feed of
the charge through the furnace are carried out by
means of e.g. walking beams, rollers, chains, and trolleys.
Another conveying device is the conveyor belt,
on which the heat-treated material is pulled through
the furnace. It is usually made of wire mesh, slats, or
possibly a solid strip of heat-resistant material.
The conveyor belts must have good resistance to the
furnace environment, so that it does not corrode or
become embrittled. A more common problem is that
the conveyor belts become elongated after a certain
service time and must be shortened. The creep strength
(and ductility) of the materials used for such conveyor
belts is thus crucial. 253 MA has yielded better results
than materials such as 4845 (310S) and materials with
even higher contents of alloying elements. At lower
temperatures, 153 MA is a suitable alternative to type
4833 (309S). Heat-resistant materials are also used for
driving gears and deflector rolls.
FURNACE COMPONENTS AND ACCESSORIES
In addition to the furnace structure itself discussed
above, furnace components and accessories that are
common to a number of furnace types, also require
high temperature materials. These components are
e.g. radiant tubes, electric resistance elements, fans,
heat exchangers, anchor bolts for insulating mats, trays,
baskets, and fixtures, and thermocouple sheathing.
Radiant tubes
If oil or gas burners are used, the combustion gases
must be kept away from the charge. Therefore, radiant
tubes are used for heat transfer to the furnace. The hot
gas flows through the tubes, which are thus heated and
emit radiant heat from the outer surfaces. The tubes
may be straight, U-shaped, or W-shaped, and are made
of high temperature material, either in cast or in
welded form (Fig. 8).
10
High temperature stainless steels
Figure 7
Figure 10
In the past, most radiant tubes were cast. Relatively
thin-walled tubes in straight lengths can be produced
by centrifugal casting. However, all-welded tubes are
becoming increasingly common. Welded tubes offer the
following advantages compared to cast tubes:
• easier to manufacture to suit the requirements of
the users, due to the availability of high temperature
materials in the form of plate, sheet, and strip
• lower weight and more efficient heat transfer due
to thinner material
• reduced sensitivity to thermal fatigue
• easier to reinforce in exposed areas and easier to
repair by welding
• reduced likelihood of deposits and less risk of high
temperature corrosion, due to smoother surfaces.
The most common material problems are deformation
and embrittlement due to carburization and/or
nitridation and overheating, caused e.g. by misaligned
internal burners.
Welded tubes of 253 MA have successfully replaced
centrifugally cast radiation tubes in continuous heat
treatment furnaces with a nitrogen/hydrogen gas
atmosphere. In these cases it has been possible to reduce
the wall thickness from 8–10 to 3–4 mm. 353 MA
may be a suitable alternative for more aggressive gas
environments.
Electrical resistance material
The materials used for electrical resistance elements are
usually ferritic chromium-aluminium steels or nickel-
base alloys. The former can withstand high temperatures,
but become brittle after some service time. They also
have a low creep strength and thus deform readily.
Nickel-base alloys are less prone to embrittlement and
deformation, but are more expensive.
253 MA may be used for heating elements for
furnaces operating at moderate temperatures, i.e. in the
range between 800 and 1050°C, due to its high creep
strength and lower risk of embrittlement. This material
has been tested in the form of resistance wire as well as
corrugated strip elements (Fig. 9) and has yielded good
results. Experience has shown that 253 MA used as
resistance material may have a service life of up to
twice that of ferritic materials.
Fans
Fans used for circulating or extracting hot gases are
subjected to very difficult conditions due to the stresses
caused by the centrifugal force, and the effect of hot,
aggressive gases containing abrasive dust.
A fan impeller must not become so brittle that it fails,
neither must it deform nor accumulate thick deposits,
since it could then become unbalanced. So, the choice
of material must be based on a thorough assessment of
the operating conditions.
AvestaPolarit 153 MA and 253 MA are suitable for
the fans used in bell-type furnaces (Fig.4), due to their
combination of high resistance to oxidation and high
creep strength. When used for fans, for which abrasive
dust has given rise to problems, 253 MA has also proved
to be more resistant to erosion than e.g. 4845 (310S).
Heat exchangers
Recuperators for heat recovery from blast furnace
gases have been mentioned earlier. Tubular heat
exchangers (Fig. 10) and plate heat exchangers are
also used for improving the efficiency of (p)reheating
and heat treatment furnaces.
Material selection will depend on the temperature
and gas environment. Heat-resistant materials are
also used for tube spacers and supports.
11
High temperature stainless steels
Figure 8 Figure 9
Figure 13
Figure 12
Figure 11
Anchor bolts and fasteners
Modern heat treatment furnaces are often insulated
with highly effective fibre mats instead of refractory
bricks or ceramic compounds. These insulating mats
are secured to the inside of the furnace wall by means
of special bolts with lock washers. The bolts are welded
to the inside of the shell at suitable intervals. The mat is
then pressed over the bolts and is held in position by
the lock washers (Fig. 11). Fasteners of high temperature
materials are also used for securing electrical resistance
elements, radiant tubes, refractory linings, ceramic
compounds (Fig. 12), etc.
The fasteners and anchor bolts employed for this
purpose may be made of bar, wire rod, or plate, and
must have a high creep strength and a good resistance
to oxidation to perform their task satisfactorily.
AvestaPolarit 253 MA has proven to be a good
alternative to both nickel-base alloys and other high
temperature materials.
Trays, baskets, and fixtures
Small machine components that require heat treatment
are often loaded into baskets or onto trays, which are
then charged into the furnace (Fig .13). The materials
used for these baskets and trays must be capable of
withstanding the temperature cycles and furnace atmos-
pheres when used repeatedly over a long service time.
Alloys with high nickel contents are often used for
this purpose, so AvestaPolarit 353 MA may be a suitable
alternative. In spite of its lower content of alloying
elements, 253 MA has produced good results for trays
and baskets thanks to its very high creep strength.
Thermocouple sheathing
Thermocouples used for recording and controlling the
furnace temperatures must be protected from attack by
the furnace gases if they are to provide correct tempera-
ture readings. These thermocouple sheathings must be
thin-walled to ensure fast temperature response and
must also be capable of withstanding the temperatures
and gases in their environment. Sheathings made of
253 MA have yielded good results in this application as
well as for use in gas analysers.
12
High temperature stainless steels
13
High temperature stainless steels
The various examples of applications of AvestaPolarit
micro-alloyed high temperature steels in the steel and
metals industries can be summarized as follows:
Pellet sintering plants
Grids, wind boxes, burners, fans, etc.
Blast furnace plants
Charging pipes for pulverized coal (and ore pellets),
circulation fans, piping, expansion bellows,
recuperators for blast furnace gas, and heat exposed
parts of coking ovens.
Steel melting, smelters, and continuous casting plants
Extraction hoods, flue gas ducts, feed chutes, dampers,
doors, bridges, and preheaters for scrap and ladles.
Rolling mills (heating furnaces)
Furnace rollers, slide-rails, walking beams, framework,
edge reinforcements for doors, etc.
Heat treatment furnaces and furnace accessories
Muffles, retorts, fans, heat exchangers, tube spacers
and supports, furnace hearths, pier protection caps,
conveyor belts, radiant tubes, electric heating elements,
anchor bolts and fasteners for refractory materials,
fixtures for brazing work, trays and baskets, thermo-
couple sheathing, tubes in gas analysers, etc.
Summary of the areas of application
Steel melting, smelters, and continuous casting plants
Rolling mills (heating furnaces)
Heat treatment furnaces and furnace
accessories
Blast furnace plantsPellet sintering plants
EN
1.49481.48781.48181.48281.48331.48351.48451.4854
AvestaPolarit and its subsidiaries offer a wide range of stainless steel grades and products.For high temperature applications, AvestaPolarit can provide both micro-alloyed stainlesshigh temperature steels as well as standard steels of the chromium-nickel type.
Chemical composition, %, typical values
STEEL GRADES
ASTM
304 H321 HS30415
309SS30815310SS35315
49484878153 MA48284833253 MA4845353 MA
C
0.050.050.050.040.060.090.050.05
N Cr
18.317.518.52022.5212525
Ni
8.79.59.51212.5112035
Si
0.50.51.32.00.51.71.01.5
–TiCe––
Ce–
Ce
BS
304S51321S51
––
309S16–
304S24–
DIN
1.49481.48781.48911.48281.48331.48931.4845
–
NF
Z6 CN 18-09Z6 CNT 18-10
–Z17 CNS 20-12Z15 CN 23-13
–Z8 CN 25-20
–
SS
233323372372
––
23682361
–
AvestaPolarit
––
0.15––
0.17–
0.15
National steel designations, superseded by EN
153 MA, 253 MA, and 353 MA are patented grades with trademarks used by AvestaPolarit. 253 MA and 353 MA are registered.
High temperature stainless steels
What can AvestaPolarit offer the steel and metals industries?
Hot-rolled plateWidths: 1000–3000 mm Thicknesses: 5–86 mmSteel grades: 153 MA, 253 MA, 353 MA, 4878, 4833, 4845
Cold-rolled sheet and stripWidths: 5–790 mm Thicknesses: 0.15–1.6 mmSteel grades: 153 MA, 253 MA, 4828, 4833, 4845
Widths: 50–1350 mm Thicknesses: 0.4–4 mmSteel grades: 153 MA, 253 MA, 353 MA, 4878, 4833, 4845, 4828
Widths: 1350–2000 mm Thicknesses: 1.5–6.35 mmSteel grades: 153 MA, 253 MA, 353 MA, 4878, 4828, 4833, 4845
BarSections: round, rectangular, flat, angle and other profilesSteel grades: 253 MA,4878, 4845
Drawn wireDiameters: 0.8–5 mmSteel grade: 253 MA
Welded pipe and tube, fittingsDiameters: 6–1600 mm Wall thicknesses: 1–25 mmSteel grades: 153 MA, 253 MA, 353 MA 4828, 4845, 4878
Manufactured products from plate and sheetTo purchaser's specifications
Welding consumablesManual welding electrodes:Steel grades: 253 MA, 353 MA, 409, 310, P10 (nickel-base)
Welding wire for automatic welding:MIG, TIG,Submerged arc Steel grades: 253 MA, 353 MA, 309L, P7, P10
CastingsFrom licensees.
More detailed information concerning each product isavailable in special AvestaPolarit data sheets which canbe obtained from your nearest AvestaPolarit office ordownloaded from our website: www.avestapolarit.com
ADVICE
Advice in matters concerning AvestaPolarit materials as well as references to previous
deliveries can be obtained from the Application Department at the Avesta Research Centre
or from your local AvestaPolarit representative.
Advice and assistance provided without charge are given with the best knowledge
and in good faith, but without any responsibility.
14
Others
PRODUCTS
The steel should be judged under dark or dimly lit conditions – not indirect sunlight. The colour scale should be viewed in normal diffuse daylight – not sunlight or lamplight.
Colour-temperature scale for glowing steel
15
High temperature stainless steels
1200°C
1100°C
1050°C
980°C
930°C
870°C
810°C
760°C
700°C
650°C
600°C
Technical Application Department:AvestaPolarit ABAvesta Research CentreSE-774 80 AvestaTel: +46 (0)226-810 00Fax: +46 (0)226-810 77E-mail: [email protected]
www.avestapolarit.com
An Outokumpu Group company
AvestaPolarit is one of the world's leading stainless steel producers. The Group combines cost-efficient production with a global sales and distributionnetwork and offers customers one of the broadest product ranges on the market.AvestaPolarit's focus is exclusively on stainless steel, a fast-growing industry sector.Ever since the Group's formation in January 2001, AvestaPolarit's vision has been to become “Best in stainless”. Today, AvestaPolarit is an integral part of theOutokumpu metals and technology group, in which the stainless steel business is a core area.
Information given in this publication is subject to alteration without notice.Care has been taken to ensure that the contents of this publication are accurate butAvestaPolarit and its subsidiary companies do not accept responsibility for errors or for information which is found to be misleading. Suggestions for or descriptions of the end use or application of products or methods of working are for informationonly and the company and its subsidiaries accept no liability in respect thereof.Before using a product supplied or manufactured by the company, it is the respon-sibility of the customer to ensure the suitability of the product for its intended use.If further assistance is required, the company, which has extensive research facilities,will often be able to help.
The cover picture shows 253MA radiant U-tubes mounted horizontally in aheat treatment furnace (courtesy by Rolled Alloys, Inc)
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