THE "SECRETS" OF STAINLESS STEELS
by David P. Rowlands, BSc Eng (Witwatersrand), MIM, CEng.
Stainless Steel terminology, specifications, mechanical and physical properties,
thermal treatments, fabrication requirements and applications will be unfamiliar
when first encountered.
This article explains the basic principles
and the factors which govern the classification, properties and behaviour of different Stainless Steels.
The most common constituent elements
of Stainless Steel are
Iron (Fe)
Carbon (C)
Chromium (Cr)
Nickel (Ni)
Molybdenum (Mo) Titanium (Ti)
Only the chemical symbol will be used in
the text.
STAINLESS STEEL
Harry Brearly of Sheffield, England is generally attributed with the discovery of
Stainless Steel. While experimenting with
Steels for gun barrels in 1913 he produced
a steel containing ±13-14% Cr with a relatively high C content. This steel did not rust
when exposed to the atmosphere.
Advancements in welding and steel-making technology in the 1940's and 1960's
gave impetus to the development and
applications of Stainless Steels. Continuous developments are made to fulfill the
growing needs of industry sectors such as
the chemical, petro-chemical, mining,
power-generation, nuclear, and the food
processing industries.
Cr is not the only alloying element used by
the Family of Stainless Steels. Ni, Mo and
Copper (Cu) are alloying elements used to
enhance the passivity in more aggressive
conditions. Manganese (Mn), Silicon (Si),
Aluminium (Al), Nitrogen (N), Sulphur (S),
—•—
---X----•--•--
Selenium (Se) and Ti are alloying elements
which have a lesser effect on the corrosion/heat resistance, but modify the
mechanical and physical properties, fabrication processes, weldability, machinability etc.
Factors such as hot and cold work, the
thermal history (heat-treatment, welding)
and fabrication processes may also affect
the properties.
The primary property of Stainless Steels is
resistance to wet (aqueous) corrosion and
scaling resistance (oxidation at high temperatures - also called dry corrosion).
CORROSION - NORMAL ATMOSPHERE
CORROSION - MARINE ATMOSPHERE
SCALING AT HIGH TEMPERATURE
STAINLESS AND HEAT
RESISTANT STEELS
The principal effect of alloying Cr in Fe is
the increased resistance to both corrosion
and oxidation (scaling) at high temperatures. Refer Fig 1.
Passivity is a state in which a metal or alloy
exhibits very low chemical reactivity and is
inert in many corrosive media. Cr imparts
passivity to Stainless Steel.
In general terms Stainless Steels are Ironbased (Ferrous) materials containing
more than 11-12% Cr. This level of Cr
renders the steel passive by forming an
extremely thin (30-50 Angstrom [ie 3-5 x
10-7mm] thick), continuous and stable
Chromium Oxide film on the surface of the
Stainless Steel.
Brearly's discovery led to the development
of a Family of Stainless Steels.
5
10
15
20
% CHROMIUM (Cr) IN IRON (Fe)
Fig 1: Effect of Chromium (Cr) alloy additions to Corrosion and Scaling
Resistance of Iron (Fe).
Stainless Steel also exhibit many
secondary properties which make
them extremely versatile materials.
THE CLASSIFICATION OF
STAINLESS STEEL
Metals are crystalline solids. The atoms
are arranged in regular patterns
(crystal
structure)
which
are
repeated many million times within
any one grain of solidified metal. The
direction/orientation of the crystals
changes at the grain boundary.
In pure solid Fe this atomic
arrangement/
crystal
structure
changes at various tem peratures
and is stable over different ranges of
temperature. Fe is one of the few
metals which exhibits this change of
crys-tal structure. Steel is basically a
Fe-C alloy and Stainless Steel is
basically a Fe-Cr alloy. The same
changes in crystal struc-ture occur in
these alloys. The different crystal
structures are termed FERRITE and
AUSTENITE.
thermal treatment of such alloys. Different
types of steel (including the Family of
Stainless Steels) result, and the development
of a wide range of mechanical and and
physical properties is made possible.
Steel — Change in Crystal
Structure
Steel is essentially Fe alloyed with small
amounts of C.
The different crystal structures will exist
over a range of both temperature and C
content. Change from one crystal structure
to another does not take place abruptly
(except for specific amounts of C), but
similarly occurs over a range of
temperature and C content during which a
mixture of two crystal structures exists.
Metallurgists use an EQUILIBRIUM DIAGRAM to show the range over which the
different crystal structures exist, and as a
guide to the manipulation of various compositions. To obtain an Equilibrium Diagram varying compositions are heated or
Stainless Steels are classified by the
inhe-rent crystal structures resulting
from both the chemical composition
and the thermal treatment viz Ferritic,
Austenitic and Mar-tensitic Stainless
Steels. Duplex and Precipitation
Hardening Stain less Steels will also
be explained. The Corrosion Resisting
Steel, 3CR12 is also referred to.
THE SECRETS OF STEEL
Pure Iron — Change in
Crystal Structure
Fig 2 shows the change in crystal
structure
(atomic
arrangement)
which occurs in pure Fe when
heated or cooled. Atoms have a high
energy in the liquid state and move in
a random manner.
This random motion ceases on
solidification at 1535°C. The atoms
form a geometric pattern with an
atom at each corner of a cube with
an additional atom in the centre of
the cube ie BODY-CENTRED CUBIC
STRUCTURE (BCC) known as
DELTA (8) IRON, which is magnetic.
On cooling to 1400°C the atomic
arrangement abruptly changes to an
atom in each corner of the cube and
an atom in the middle of each face of
the cube ie FACE-CENTRED CUBIC
STRUCTURE (FCC) known as
GAMMA (y) IRON, which is nonmagnetic. On further cooling to
910°C the atoms abruptly revert to a
BODY-CENTRED CUBIC (BCC)
structure which is nonmagnetic and is
known as BETA (/3) IRON.
At the Curie Temperature (±770°C)
the Fe once again becomes
magnetic, but there is no change in
the atomic arrangement. This is
termed ALPHA (a) IRON. These
changes in atomic arrangement (crystal
structure) of Fe are modified by both
alloying other elements with Fe and the
Fig 2: Change in the Crystal Structure (Atomic Arrangement) of pure Iron
(Fe) on heating or cooling.
cooled extremely slowly to allow the different atomic arrangements to attain equilibrium and to stabilise even at low temperatures when the atomic movement is relatively sluggish.
A section of the Fe-C Equilibrium Diagram
is shown in Fig 3. This diagram shows the
changes in the crystal structure of steel
and the ranges over which they exist.
• Slow cooling of a very low C(±0,05%C)
steel.
As the metal solidifies DELTA FERRITE
forms. The Fe atoms take on a BCC structure. The smaller C atoms move into the
spaces between the Fe atoms. C is therefore referred to as an interstitial element.
On further cooling the BCC Delta Ferrite
begins to change to FCC AUSTENITE over
a temperature range until a fully FCC crystal structure results. The Fe atoms take up
FCC positions with C in between. C has a
high solubility in the FCC Austenitic structure, which is shown by the size of the
Austenitic area in Fig 3.
On still further cooling the FCC Austenite
changes to BCC ALPHA FERRITE over a
temperature range. The Fe atoms form the
normal BCC crystal structure. Due to the
low solubility of C in Alpha Ferrite, the
small amount of excess C (because very
low 0,05% C) combines with Fe to form a
minute amount of Iron-Carbide compound
called CEMENTITE (Fe3C). The resultant
steel is soft (Dead Mild) and easily formed.
These are the Deep Drawing Grades.
• Slow cooling of a medium to high C
(eg 0,6% C) steel. BCC DELTA FERRITE
does not form. As the liquid solidifies over
a temperature range the Fe atoms assume
the FCC AUSTENITIC crystal structure and
the C atoms take up their interstitial positions.
On further cooling the Austenite begins to
change and some BCC ALPHA FERRITE
forms. At the end of transition (723°C) the
relatively larger amount of high C Austenite remaining changes to an equivalent
large amount of Cementite (Fe 3 C) in a
lamella mixed aggregate of Ferrite and
Cementite (called PEARLITE).
Fe3C is a hard brittle substance and
increases the strength and hardness of the
steel but decreases the ductility.
* Rapid cooling of a steel containing
more than ±0,35% C causes different
results. Due to the fast cooling rate the C
atoms cannot attain their equilibrium
position, and hence jam the shift of Fe
atoms changing from FCC Austenite to
BCC Ferrite + Cementite. The Fe atoms
thus lock into a distorted, highly stressed
tetragonal crystal structure. This stressed,
hard and strong but brittle structure is
termed MAR-TENSITE
This effect of C increases rapidly up to
concentrations of ±0,65% C and then
more slowly at higher C levels. The tendency to form Martensite rapidly decreases at C levels lower than 0,35% C.
Lesser amounts of Martensite containing
less C do not stress the structure to the
same degree giving a softer and more ductile steel. At less than ±0.25% C the hardening effect of C is minimal, even with
extremely rapid cooling.
• The Fe-C Equilibrium Diagram also
illustrates other facets of steel.
-- Because steel changes its crystal structure it can be heat treated to develop a vast
range of properties. The crystal structure
is first changed by heating to within the
FCC Austenite range and then, depending
on the composition cooled at different predetermined rates to produce the desired
properties.
- Slow cooling will Anneal (Soften) the
steel.
- Fast cooling (Quenching) will Harden
the steel and produce high mechanical
properties.
Other alloy elements such as Cr, Mo, Ni,
Manganese (Mn), Tungsten (W) enhance
the response of steels which contain them
to heat treatment by quenching, and
higher properties can be developed. These
alloys shift the boundary lines of the phase
changes shown on the Fe-C Equilibrium
Diagram, but the changes are typified by
the Fe-C system.
-- The transformation of higher C content
and alloyed steels to Martensite is related
to the Weldability of steel. The thermal
cycle during welding is equivalent to a
heat treatment process on a confined
small area. The heat input during welding
raises the temperature into the FCC Austenite range and the heat extraction by the
surrounding cold steel is extremely rapid
(similar to water or brine quenching). If
Martensite is formed during the welding
thermal cycle the weld zone will be brittle
and have unacceptable properties for
most engineering applications.
-- Sub-Critical Stress Relieving relieves
stresses induced by the various fabrication
processes (eg cold forming and welding)
of carbon steel vessels. This process is carried out at a maximum temperature of
about 650°C which is high enough to give
the atoms sufficient mobility (albeit
relatively sluggish) to reorganise
themselves into new positions thus
relieving the stress, but is below the
critical temperature of 723°C at which
BCC begins to change to FCC.
-- For the heat treatment operations of
Annealing,
Quenching
and
Normalising the temperature is raised
to the lowest level necessary to attain
full transformation to the FCC Austenite
phase. A uniform fine grain size results
from the re-arrangement into the FCC
crystal
structure
at
such
lower
temperatures. This fine grain size is
retained in the transformation on subsequent cooling thereby enhancing the
mechanical properties of the steel.
-- If the temperature is raised to higher
levels within the FCC Austenite phase
some of the grain boundaries break
down and larger and coarser grains
result. A good degree of grain growth
therefore
occurs
at
the
high
temperatures (± 1250°C) required for
hot working (rolling or forging).
However the hot working processes
refine the coarse grains to a finer
uniform size. Any hot working
operation should finish at as low a
temperature as possible to ensure that
the grain refinement which has taken
place is not negated by the grain growth
due to a high residual temperature in the
steel.
-- Finally, temperatures at which melting
can begin must be avoided when
heating steel for hot work operations.
Melting
initiates
at
the
grain
boundaries, and if only minute
amounts of liquid metal is formed it
"lubricates" the grain boundaries. This
is termed over-heating or burning. If any
hot working is done under these
conditions the steel will disintegrate
along
these
"lubricated"
grain
boundaries.
THE SECRETS OF
STAINLESS STEEL
Changes in Crystal Structure
While steels are based on the alloying
of small amounts of C with Fe,
Stainless Steels are based on the
alloying of Cr with Fe. This necessitates
a different Equilibrium Diagram - the
Fe-Cr Equilibrium Diagram. Refer Fig 4.
BCC Ferrite and FCC Austenite do exist,
but the shapes and extents of the areas
are different to those of the Fe-C alloy
system (ie steel).
The Cr atom takes up a place in the
crystal structure normally occupied by a
Fe atom ie the Cr atom substitutes for
an Fe atom. Cr suppresses the
formation of Austenite making the
Austenite (Gamma) phase field smaller,
and promotes the formation of a Ferritic
crystal structure making the Ferrite
phase field larger. Cr is therefore
termed
a
substitutional
Ferrite
stabiliser (or former).
A very significant feature of the FeCr
Equilibrium Diagram is the boundary between the Austenite and Ferrite fields,
known as THE GAMMA LOOP.
Stainless Steels contain more than 11-12%
Cr and are classified according to their
inherent crystal structure.
Referring to Fig 4 it is simple to see that
FERRITIC Stainless Steels containing 1418% Cr have a Ferritic crystal structure.
But AUSTENITIC Stainless Steels must
have a stable Austenitic crystal structure at
all temperatures, and MARTENSITIC
Stainless Steels require fast thermal transformation from Austenite to take place.
How is it possible to produce Austenitic
and Martensitic Stainless Steels when the
Austenitic (Gamma) phase field is shown
to be limited to Cr levels below 11 -12% Cr?
Ferritic Stainless Steels
The Ferritic Stainless Steels, which have a
Cr content of 14,5-27,0% Cr, have a BCC
Ferritic crystal structure which is retained
from room temperature to melting point.
This composition passes OUTSIDE the
Gamma Loop. Refer Fig 5
Ferritic Stainless Steels have a low C content which seldom exceeds 0,06% C, well
below the specified minimum.
Therefore the effect of C in moving the
Gamma Loop and expanding the FCC
Austenite phase field is limited and the
single phase Ferritic crystal structure is
not affected. The C also tends to form complex Fe-Cr Carbides which lock the C thus
further limiting its effect on shifting the
Gamma Loop. However due to the low C
content of Ferritic Stainless Steels, the
amount of Cr locked in these Carbides
does not have the opposing effect of decreasing the Cr level enough to affect its
ability to suppress the formation of Austenite. Most Ferritic Stainless Steels contain
a small quantity of finely dispersed Fe-Cr
Carbide precipitates.
FERRITIC Stainless Steels are
- Magnetic
- Non hardenable by thermal treatment as
the transformation from one crystal
structure to another cannot take place.
The normal air melted Ferritic Stainless
Steels suffer from high temperature
embrittlement and loss of corrosion resistance which result from short time exposures to high temperatures (1000°C and
higher). These detrimental effects are
related to the grain coarsening within the
single phase Ferritic crystal structure, and
to the levels of C and Nitrogen (N) in the
steel which form Cr Carbides and Nitrides.
The Heat Affected Zone (HAZ) adjacent to
a weld attains these temperatures,and
therefore suffers a loss of properties.
Ferritic Stainless Steels exhibit Low Temperature Brittleness. As the temperature
drops below room temperature they
change from being tough and ductile to
becoming exceedingly brittle at ±0°C. In
This effect of C is constrained by the
strong ability of Cr to maintain the BCC
Ferritic structure. Further, due to the high
affinity of Cr for C, Cr Carbides will form.
Therefore the C and Cr contents have to be
balanced, both to ensure the required
thermal transformation of crystal structure, and to avoid a reduction in the passivity resulting from an excessive amount
of Cr being extracted from the matrix and
locked up as Cr Carbides.
The Cr content of Martensitic Stainless
Steels is limited to relatively low levels of
12-18% Cr, the steels of lower Cr content
having lower C and vice versa,
the HAZ due to the effect of exposure to
high temperatures, this ductile to brittle
transition takes place at higher temperatures (40°-60°C and above).
These factors result in the inferior weldability of these steels which therefore
limits their use as welded components to
thin gauges.
To improve the properties associated with
the standard Ferritic Stainless Steels the
Super Ferritic Stainless Steels were
developed.
The significant features of the Super Ferritic Stainless are
- A higher Cr content (typically 18-25%
Cr) and Mo additions (typically 1 -4% Mo)
which improve the corrosion resistance.
- Low levels of both Nitrogen (N) and C
(less than 0,03% each) which prevent
the detrimental effects resulting from
the formation of Cr Nitrides and Cr Car
bides during welding.
- Additions of small amounts of Ni which
improve the resistance to high tempera
ture embrittlement.
However, the weldability, even though
improved, is still a constraining factor
limiting the general use of Super Ferritic
Stainless Steels as welded components to
a maximum thickness of ±5mm.
Martensitic Stainless Steels
It would appear impossible to have a steel
with more than 11-12% Cr to make it
"Stainless", and to be able to attain the
necessary thermal transformation by
rapidly cooling the'FCC Austenite to produce the "jammed up", distorted and
therefore hard Martensitic crystal structure.
C, a powerful interstitial Austenite
stabiliser is used as the "alloying element"
to shift the Gamma Loop to higher Cr contents thereby expanding the FCC Austenitic phase field. Refer Fig 6.
MARTENSITIC Stainless Steels are
- Magnetic
- Of moderate corrosion resistance due
to being alloys of Fe-Cr-C with signifi
cant amounts of C and a relatively low
maximum Cr content, some of which is
tied up as Cr Carbides thus not con
tributing to the passivity of the steel.
- Of such an alloy content that when
heated they pass through the Gamma
Loop to FCC Austenite. Subsequent
moderate to fast cooling produces the
hard Martensitic atomic crystal struc
ture. They are hardenable by heat treat
ment.
Martensite has high strength and hardness but is brittle and of low ductility and
toughness. It must therefore be subjected
to a further heating cycle, Tempering, at
temperatures below that at which the
Austenite transformation occurs. The
Tempering temperature is varied to obtain
the required combinations of strength,
hardness, ductility and toughness.
Heat Treatment of Martensitic Stainless
Steels maximises their corrosion resistance.
- Of very poor weldability. The heat
input and subsequent cooling of the
HAZ is equivalent to heat treatment carried out on a confined area. Hard brittle
Martensite forms in the HAZ. Special
precautions are employed to avoid this,
d must be tempered. The properties
associated with welded Martensitic
Stainless Steels are
In some grades of Austenitic Stainless
Steels small amounts of Nitrogen (N) are
added. It is an effective interstitial Austenite stabiliser and complements the Ni
in increasing and stabilising the Austenitic
crystal structure.
to high temperatures.
The Ni content of the 300 series Austenitic
Stainless Steel is adjusted to cater for the
various chemical compositions of the
different Austenitic Stainless Steel
grades, eg
But the crystal structure "wants" to
change and is therefore termed METASTABLE. These changes occur during cold
working when movement occurs along
planes, termed slip panes, within the
grains. Constraints are eased and
there is enough energy for a crystal
structure change to take place at
extremely small localised areas along
th eslip planes.
•
17 % Cr needs a minimum of 7% Ni
to stabilise the Austenite but 26% Cr,
as in some heat resisting grades,
needs 20% ni to ensure a stable
Austenitic crystal structure.
•
Mo is added to improve the corrosion
resistance. It is a substitutional
Ferritic stabiliser and the Ni content
has to be increased in these grades
to counteract this effect.
•
C,
an
interstitial
Austenite
stabiliser is reduced to low level in
the “L” grade Austenitic Stainless
Steel. The Ni content therefore has to
be increased to overcome the lower
tendency to form and stabilise the
Austenitic crystal structure.
Austenitic Stainless Steel
The Cr content of Austenitic Stainless
Steels exceeds 16% Cr. Referring to Fig 4
they should therefore appear to have a Ferritic crystal structure.
Refer to Fig 7.
Alloying elements which shift the Gamma
Loop have to be used, both to expand the
Austenitic crystal structure into the Ferritic
regions of higher Cr content, and to retain
it at the lower temperatures. Ni (in the 300
series) is the most commonly used alloying
element, but Manganese (Mn) can also
be used to replace some of the Ni (in the
200 series).
These elements are substitutional Austenite stabilisers/formers which take the
place of a Fe atom in the crystal structure.
They are large atoms and diffuse slowly in
Fe and therefore stabilise the Austenitic
crystal structure down to temperatures
below that at which the atoms have sufficient mobility for a crystal structure
change to occur.
AUSTENITIC Stainless Steels therefore
- Are non-magnetic.
- Have an extremely stable crystal struc
ture.
- Have excellent weldability.
- Are non-hardenable by heat treatment,
but hardenable by cold work.
Ni diffuses slowly even at high temperatures. Very little grain growth and embrittlement occur during lengthy exposure
The substitutional Austenitic stabilising
elements prevent a crystal structure
change on cooling. Therefore hardening
by thermal treatment cannot take place.
Tiny patches of “Martensite” are
formed. This “Martensite” is of low C,
tough, and of varying crystal structure
(either BCC or Hexagonal Close
Packed [HCP]). It is different from the
Martensite formed in Martensitic
Stainless Steels and is therefore often
referred to as “Quasi-Martensite”.
The normal Austenite to Martensite
volume expansion takes place and
these small "Martensite" islands act as
keys along the slip planes. Further
movement is thus inhibited making the
Austenitic
Stainless
Steels
harder,
stronger and resistant to further distortion
by cold work.
It has been work hardened and extremely
high strength levels can be developed by
this mechanism (over 2200 MPa in cold
drawn wire).
The lean alloy or lean composition grades
of Austenitic Stainless Steels have a minimum amount of Austenite stabiliser alloying elements to make them fully Austenitic. These grades (eg Grade 301) work-harden rapidly. Grades specifically intended
for cold working operations (eg Grade 305
for Deep Drawing) contain an over-sufficiency of Ni. The 200 series, which use
Manganese (Mn) to partly replace Ni as the
Austenite stabiliser, work-harden more
rapidly than the 300 series. Due to the
"Martensite" produced during cold work,
work-hardened
Austenitic
Stainless
Steels will exhibit a slight degree of
magnetism which depends on the
amount of cold work and the composition
of the steel.
Solution Annealing can remove the work
hardened condition. The steel is heated to
high temperatures (±1050°C). The "Martensite" precipitates dissolve and are
taken back into an equilibrium solution of
a fully recrystallized Austenitic structure.
Duplex Stainless Steels
Duplex Stainless Steels are two-phase
having a dispersion of FCC Austenite in a
matrix of BCC Ferrite.
This is because they contain an insufficient amount of the Austenite stabilising
element (Ni).
Duplex Stainless Steels are relatively new
members within the Family of Stainless
Steels. They are generally available as
proprietary alloys.
Duplex Stainless Steels have improved
corrosion resistance while maintaining the
excellent mechanical and fabricational
properties of the Austenitic Stainless
Steels.
The higher Cr content, and in most alloys
an addition of Mo, results in better passivity and therefore greater resistance to Pitting Corrosion.
The occurrance of Stress Corrosion
Cracking (SCC) may be considered as
limited to the Austenitic crystal structure.
The propagation of SCC which may initiate
within the Austenite fraction is arrested by
the Ferrite fraction of the Duplex structure.
The Ferrite:Austenite ratio of Duplex
Stainless Steels depends on the composition, ie the amounts of Ferrite formers (eg
Cr, Mo) and the Austenite formers (eg Ni,
Nitrogen [N]). This ratio varies in the different alloys from Ferrite:Austenite of
±70:30 to ±50:50.
The size and distribution of the Ferrite and
Austenite phases in the Duplex structure is
dependent on both the thermo-mechanical (hot working) cycles and the heat treatment. This relationship is also important in
developing the mechanical and physical
properties of Duplex Stainless Steels,
specifically the higher Tensile and Yield
strengths of Duplex when compared to
Austenitic or Ferritic Stainless Steels.
In the initial stages of their development,
the weldability of Duplex Stainless Steels
was a constraint due to the formation and
retention of Ferrite in the HAZ. This limited
their use as welded components to sheet
and thinner plate thicknesses. Technological developments and improvements
have changed this position.
The weldability of the majority of Duplex
Stainless Steels in thick section may now
be classified as good. The alloying element Nitrogen (N), a powerful interstitial
Austenite stabiliser, has contributed most
to eliminating the detrimental effects of retained Ferrite in the HAZ, It promotes the
formation of a higher fraction of Austenite
within the crystal structure and assists the
reformation of Austenite within the HAZ.
Good ductility, toughness and corrosion
resistance equivalent to those of the
parent metal result within the HAZ.
Duplex Stainless Steels which contain
Nitrogen (N) to attain the higher balanced
Austenitic ratio within the crystal structure
are often referred to as 2nd Generation
Duplex Stainless Steels.
DUPLEX Stainless Steels are
- Magnetic.
- Non-hardenable by heat treatment.
- Of good mechanical and physical pro
perties (generally similar or superior to
Austenitic and Ferritic Stainless Steels).
- Of excellent corrosion resistance ( gen
erally equivalent or superior to Austeni
tic Stainless Steels).
Precipitation-hardening
Stainless Steels
Austenitic Stainless Steels are not heat
treatable and have, in the forms utilized in
fabrication, low strength but excellent corrosion resistant and fabricational properties.
treated to develop high strength but this
limits their ability to be fabricated (especially their weldability). Their corrosion
resistance is only fair to moderate.
The Precipitation-hardening Stainless
Steels were developed to overcome these
limitations.
There are three types of Precipitation-hardening Stainless Steels:.
- Martensitic
- Semi-Austenitic
- Austenitic
Note: These terms should not be confused with the same terms used to designate the standard classifications of Stainless Steels.
An exact balance of the chemical composition is critical to ensure the development of the phases and precipitates required to achieve the desired properties.
Several elements, eg Aluminium (Al), Copper (Cu), Ti and Mo, are used, either alone
or in combination, to obtain the precipitation-hardening reactions.
Precipitation-hardening Stainless Steels
are mostly available as proprietary alloys.
Precipitation-hardening Stainless Steels
have simitar or superior mechanical properties to the Martensitic Stainless Steels
and have a corrosion resistance approaching that of Grade 304 Austenitic Stainless
Steel.
The thermo-mechanical (hot working) and
the complex heat treatments and welding
procedures which are necessary to develop and retain the properties of these
Stainless Steels must be allowed for in any
considered application of these steels.
The precipitation-hardening heat treatment is a time-temperature relationship
dependent on the type of alloy.
In the Martensitic and Semi-Austenitic
types the atoms of the precipitating phase
collect in clusters which are continuous
and coherant with the matrix phase. They
are not visible by ordinary optical means
as no actual precipitation has yet occurred. This is termed pre-precipitation and
maximum strengthening occurs. If the precipitation process is continued the clusters of atoms grow and precipitate out as
intermetallic compounds forming a grain
boundary between the precipitate and the
matrix phase. This reduces the strain and
coherence is lost. Therefore the strength
drops and the material becomes overaged,
In the Austenitic types the precipitates are
allowed to form second phase intermetallic compounds. This increases the
strength in the Austenite matrix but not to
the same extent as in the Martensitic and
Semi-Austenitic types of Precipitation-hardening Stainless Steels.
- MARTENSITIC TYPES
The chemical composition is balanced so
that a Martensitic crystal structure results
when cooled to ambient temperature after
solution treatment These are also railed
Typical alloys include : 17-4PH, 13-8, Stainless W, 15-5PH, PH13-8Mo. Custom 450
and 455.
Typical Properties which can be attained
are: 0,2% Proof Stress 1200-1600 MPa,
Tensile Strength 1300-1690 MPa, Hardness 42-49 HRC.
These are the most used of the Precipitation-hardening Stainless Steels, utilized as
bar, rod, wire, heavy forgings, sheet and
thinner plate.
-- SEMI-AUSTENITIC TYPES
These are essentially Austenitic in the
solution annealed condition, which is converted to a Martensitic structure by various
heat treatments.
Typical alloys include : 17-7PH, PH 14-8
Mo, Ph 15-7 Mo, AM 350 and 355.
Typical properties which can be attained
are: 0,2% Proof Stress 1250-1793 MPa,
Tensile Strength 1500-1825 MPa, Hardness 45-50 HRC.
These are the next most used of the Precipitation-hardening Stainless Steels,
utilized mostly as sheet and strip.
-- AUSTENITIC TYPES
These steels have a stable Austenitic crystal structure.
17-1OP, A286,
Typical alloys include
HNM.
Typical properties which can be attained
are: 0,2% Proof Stress 675 MPa, Tensile
Strength 975-1025 MPa, Hardness 32-34
HRC.
These are the least used of the Precipitation-hardening Stainless Steels. However
they have the advantage of being able to
be used at both higher and lower temperatures than the other types.
3CR12 Corrosion Resisting
Steel
This is a proprietary alloy developed by
Middelburg Steel and Alloys which was
commercially launched in 1980.
It has a Cr content of 11-12% Cr and is
therefore included in the Family of Stainless Steels, and classified as a Ferritic
Stainless Steel. However, because of its
minimum Cr content it is not normally
referred to as a Stainless Steel but rather
as a Corrosion Resisting Steel.
Refer to Fig 4. The Cr content of 3CR12
places it at the critical boundary of the
Gamma Loop, ie small variations within its
chemical composition which are either
Austenite or Ferrite Stabilisers (Formers)
could render the crystal structure either
Austenitic or Ferritic at high temperatures.
The composition is therefore controlled
during manufacture to ensure that a critical balance between Austenite and Ferrite
exist at high temperatures employed for
hot working.
Both C and Nitrogen (N) are strong Austenite formers and am both limited to low
levels. If the levels of these elements
approach the specified maximum (0,03%
each) they need to be constrained by a
further alloy addition of the stabilizing element Ti. Stable Ti Carbides and Nitrides
are formed which minimize the effect of C
and Nitrogen (N). Ti being a Ferrite former
has to be counterbalanced by the addition
of Ni which is an Austenite former.
Technical improvements have been
accomplished whereby both C and Nitrogen (N) are controlled to levels well below
the maximum specified. Therefore the
necessity to use Ti to constrain these elements and Ni to balance the crystal structure has fallen away, but it is still an option
that may be utilized.
The final crystal structure is dependent on
the thermal history.
With the improvements noted above and
employing a controlled slow cooling rate
from the final hot rolling temperature, the
Austenitic fraction transforms at elevated
temperatures and a predominantly fine
grained Ferritic structure is developed.
Fast cooling rates would transform the
Austenite fraction to "Martensite", with the
Ferrite remaining. The Ferrite/"Martensite" balance is directly related to the Ferrite/Austenite which existed at the high
temperatures.
The "Martensite" formed in 3CR12 is not
the same as that in Martensitic Stainless
Steels. It has a very low C content and is
therefore not highly stressed. The atoms
are arranged imperfectly with vacant
atomic sites (ie highly dislocated or of
high dislocation density). This LATH
MARTENSITE is relatively tough and ductile.
Annealing of fast cooled material is necessary to develop the properties of strength,
toughness and ductility which are required for general engineering materials. The
annealing temperature is below that at
which the "Martensite" would invert to
Austenite. This annealing is therefore,
more correctly, a tempering operation.
Hot forming operations must be carried
out at a sub-critical temperature range
(600°-700°C) to prevent any inversion to an
Austenitic crystal structure.
Indiscriminate heating of 3CR12, either for
fabrication purposes or in operation, can
seriously affect the properties of the material.
The weldability of 3CR12 as compared to
the standard plain Cr Ferritic Stainless
Steels has been greatly improved. This is
due to the superior properties of the HA2
which result from
- The very low levels of both C and Nitro
gen (N) prevent the detrimental effects
due to the formation of Cr Carbides and
Cr Nitrides.
- The two-phase crystal structure which
develops at high temperatures inhibits
grain growth in the HAZ, thus limiting
the
embrittling
effects.
—
The
"Martensite" which results on cooling from
the Austenite fraction formed at high
temperatures is a low C "Martensite" of
such a nature as to be relatively tough
and ductile.
CONCLUSION
The main factors which govern the internal
crystal (micro) structures which give rise
to the different classifications of Stainless
Steel have been covered. The crystal structure also governs the various mechanical,
physical and fabrication properties of
Stainless Steel which renders them an
extremely versatile group of materials.
Some aspects have been greatly simplified
to illustrate the basic principles. Figs 5, 6
and 7 are not exact but merely depict the
changes which take place in the crystal
structure.
ACKNOWLEDGMENT: THE ASSISTANCE,
CONTRIBUTION AND SUPPORT OF MIDDELBURG STEEL AND ALLOYS (PTY) LTD IN THE
PRODUCTION OF THIS PAMPHLET IS
HEREBY GRATEFULLY ACKNOWLEDGED.