High Temperature Stainless Steels Feb07 91981742 (2)

High temperature stainless steels

High temperature stainless steels within the steel and metals industry


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


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

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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


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


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


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


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


• 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


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


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.

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13


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.


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


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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High Temperature Stainless Steels Feb07 91981742 (2)

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