THE FAMILIES OF STAINLESS STEELS
Stainless steels are iron based alloys containing a minimum of about 12% chromium
(TAMMAN LAW, Fig 1) ; this forms a protective self-healing oxide film, which is the reason
why this group of steels have their characteristic "stainlessness" or corrosion resistance. The
ability of the oxide layer to heal itself means that the steel is corrosion resistant, no matter how
much of the surface is removed; this is not the case when carbon or low alloy steels are protected
from corrosion by metallic coatings such as zinc or cadmium or by organic coatings such as paint.
A fundamental point to be highlighted is the fact that carbon must be kept as low as possible.
Carbon, a useful interstitial element normally used to harden metal material by applying fast
cooling thermal treatments as like Tempering + Annealing, is extremely dangerous when preparing
inox steels because of it’s tendency to make carbures. In these Inox materials this process has to be
monitored because when trapped in a Carbure Cr does not act any more as passivation agent.
Although all stainless steels depend on the presence of chromium, other alloying elements are
often added to enhance their properties. The categorisation of stainless steels is unusual amongst
metals in that it is based upon the nature of their metallurgical structure - the terms
used denote the arrangement of the atoms which make up the grains of the steel, and which can be
observed when a polished section through a piece of the material is viewed at high magnification
through a microscope. Depending upon the exact chemical composition of the steel the
microstructure may be made up of the stable phases austenite or ferrite, a "duplex" mix of these
two, the phase martensite created when some steels are rapidly quenched from a high
temperature, or a structure hardened by precipitated micro-constituents.
Fig 1
Austenitic Stainless Steels
This group contain at least 16% chromium and 6% nickel (the basic grade 304 is sometimes
referred to as 18/8) and range through to the high alloy or "super austenitics" such as 904L and 6%
molybdenum grades. Additional elements can be added such as molybdenum, titanium or copper, to
modify or improve their properties, making them suitable for many critical applications
involving high temperature as well as corrosion resistance. This group of steels is also
suitable for cryogenic applications because the effect of the nickel content in making the steel
austenitic avoids the problems of brittleness at low temperatures, which is a characteristic of other
types of steel.
Ferritic Stainless Steels
These are plain chromium (11 to 18%) grades such as Grade 430 and 409. Their moderate
corrosion resistance and poor fabrication properties are improved in the higher alloyed grades such
as 434 and 444 and in the proprietary grade 3CR12.
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Martensitic Stainless Steels
Martensitic stainless steels are also based on the addition of chromium as the major alloying
element but with a higher carbon and generally lower chromium content (eg 12% in Grades 410
and 416) than the ferritic types; Grade 431 has a chromium content of about 16%, but
the microstructure is still martensite despite this high chromium level because this grade
also contains 2% nickel.
The different families of Stainless Steel may be appreciated in the Schaeffler diagram (Fig 2)
Fig 2
CHARACTERISTICS OF STAINLESS STEELS
The characteristics of the broad group of stainless steels can be viewed as compared to the more
familiar plain carbon "mild" steels.
As a
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generalisation the stainless steels have:
Higher work hardening rate
Higher ductility
Higher strength and hardness
Higher hot strength
Higher corrosion resistance
Higher cryogenic toughness
Lower magnetic response (austenitic only)
These properties apply particularly to the austenitic family and to varying degrees to other grades
and families.
These properties have implications for the likely fields of application for stainless steels, but also
influence the choice of fabrication methods and equipment.
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STANDARD CLASSIFICATIONS
There are many different varieties of stainless steel and the American Iron and Steel Institute
(AISI) in the past designated some as standard compositions, resulting in the commonly used three
digit numbering system. This role has now been taken over by the SAE and ASTM, who allocate
1-letter + 5-digit UNS numbers to new grades. The full range of these standard stainless steels
is contained in the Iron and Steel Society (ISS) "Steel Products Manual for Stainless Steels",
and in the SAE/ASTM handbook of Unified Numbering System.
Certain other grades do not have standard numbers, but are instead covered by other national or
international specifications, or by specifications for specialised products such as standards for
welding wire. (Fig 3, 4)
Precipitation Hardening Stainless Steels
These are chromium and nickel containing steels which can develop very high tensile strengths.
The most common grade in this group is "17-4 PH"; also known as Grade 630, with the
composition of 17% chromium, 4% nickel, 4% copper and 0.3% niobium. The great
advantage of these steels is that they can be supplied in the "solution treated" condition; in this
condition the steel is just machinable. Following machining, forming etc. the steel can be hardened
by a single, fairly low temperature "ageing" heat treatment which causes no distortion of the
component.
CORROSION
Although the main reasons why stainless steels are used is corrosion resistance, they do in fact
suffer from certain types of corrosion in some environments and care must be taken to select a
grade which will be suitable for the application. Corrosion can cause a variety of problems,
depending on the applications:
Corrosion of stainless steels can be categorised as:
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General Corrosion
Pitting Corrosion
Crevice Corrosion
Stress Corrosion Cracking
Intergranular Corrosion
General Corrosion
Corrosion whereby there is a general uniform removal of material, by dissolution, eg when
stainless steel is used in chemical plant for containing strong acids. Design in this instance is based
on published data to predict the life of the component.
Pitting corrosion
Under certain conditions, particularly involving high concentrations of chlorides (such as
sodium chloride in sea water), moderately high temperatures and exacerbated by low pH (ie
acidic conditions), very localised corrosion can occur leading to perforation of pipes and
fittings etc. This is not related to published corrosion data as it is an extremely localised and severe
corrosion which can penetrate right through the cross section of the component. Grades high in
chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.
The Pitting Resistance Equivalent number (PRE) has been found to give a good indication of the
pitting resistance of stainless steels. The PRE can be calculated as:
PRE = %Cr + 3.3 x %Mo + 16 x %N
One reason why pitting corrosion is so serious is that once a pit is initiated there is a strong
tendency for it to continue to grow, even although the majority of the surrounding steel is still
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untouched. For many years 316 has been regarded as the “marine grade” of stainless steel. It must
be recognised however, that in the more aggressive marine environments 316 will not fully resist
pitting corrosion or tea staining It is also important that the surface is free of any contaminants.
Fig 3
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Fig 4
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Crevice corrosion
The corrosion resistance of a stainless steel is dependent on the presence of a protective oxide layer
on its surface, but it is possible under certain conditions for this oxide layer to break down, for
example in reducing acids, or in some types of combustion where the atmosphere is
reducing. Areas where the oxide layer can break down can also sometimes be the result of the way
components are designed, for example under gaskets, in sharp reentrant corners or associated
with incomplete weld penetration or overlapping surfaces. These can all form crevices which can
promote corrosion. To function as a corrosion site, a crevice has to be of sufficient width to permit
entry of the corrodent, but sufficiently narrow to ensure that the corrodent remains
stagnant. Accordingly crevice corrosion usually occurs in gaps a few micrometres wide, and is not
found in grooves or slots in which circulation of the corrodent is possible.
This problem can often be overcome by paying attention to the design of the component, in
particular to avoiding formation of crevices or at least keeping them as open as possible.
Crevice corrosion is a very similar mechanism to pitting corrosion; alloys resistant to one are
generally resistant to both. Crevice corrosion can be viewed as a more severe form of pitting
corrosion as it will occur at significantly lower temperatures than does pitting.
Chromium and Nickel
As seen during the lessons in class, Chromium is necessary to avoid weight reduction due to
corrosion problems. (Tamman) Indeed also Nickel is necessary to get Inox – Stainless steels.
This is due to a metallurgical problem. As may be appreciated in Fig 5 the presence of Cr reduces
the Gamma field available room.
In order to keep the zone borderlines to be used to apply thermal treatments Ni is added. Ni in fact
opens up, as seen in Fig 6, the Gamma space.
Fig 7 shows on the other hand that the same result could have been obtained by simply using
Carbon. As previously said the Carbon % must be kept as low as possible in order to reduce any
possible Carbure formation.
Fig 5
Fig 6
Fig 7
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OUT OF EQUILIBRIUM THERMAL TREATMENTS
Steel is usually defined as an alloy of iron and carbon with the carbon content between a few
hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5
wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels, stainless steels
(>11%) and heat resisting CrNi steels (>18%). Steels can exhibit a wide variety of properties
depending on composition as well as the phases and micro-constituents present, which in turn
depend on the heat treatment.
The Fe-C Phase Diagram
The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig 8).
Figure 8 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the
metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop,
especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of
more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or
metastable equilibrium) for different combinations of carbon concentration and temperature.
Fig 8
We distinguish at the low-carbon end ferrite (α-iron),which can at most dissolve 0.028% C, at
727°C (1341°F) and austenite -iron, which can dissolve 2.11 wt% C at 1148°C (2098°F). At the
carbon-rich side we find cementite (Fe3C). Of less interest, except for highly alloyed steels, is the δferrite existing at the highest temperatures. Between the single-phase fields are found regions with
mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At
the highest temperatures, the liquid phase field can be found and below this are the two phase fields
liquid + austenite, liquid + cementite, and liquid + δ-ferrite.
In heat treating of steels, the liquid phase is always avoided. Some important boundaries at singlephase fields have been given special names:
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A1, the so-called eutectoid temperature, which is the minimum temperature for austenite
A3, the lower-temperature boundary of the austenite region at low carbon contents, that is,
the γ/γ + α boundary
Acm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe3C boundary
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The carbon content at which the minimum austenite temperature is attained is called the eutectoid
carbon content (0.77 wt% C). The ferrite-cementite phase mixture of this composition formed
during cooling has a characteristic appearance and is called pearlite and can be treated as a
microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite
lamellae that degenerates into cementite particles dispersed with a ferrite matrix after extended
holding close to A1.The Fe-C diagram in Fig 8 is of experimental origin. The knowledge of the
thermodynamic principles and modern thermodynamic data now permits very accurate calculations
of this diagram. This is particularly useful when phase boundaries must be extrapolated and at low
temperatures where the experimental equilibria are extremely slow to develop. If alloying elements
are added to the iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries and the
eutectoid composition are changed. It suffices here to mention that all important alloying elements
decrease the eutectoid carbon content, the austenite-stabilizing elements manganese and nickel
decrease A, and the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten
increase A1(see lesson over Inox Steels)
Transformation Diagram
The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the
heat treatment of steels. The metastable phase martensite and the morphologically metastable
microconstituent bainite, which are of extreme importance to the properties of steels, can generally
form with comparatively rapid cooling to ambient temperature. That is when the diffusion of carbon
and alloying elements is suppressed or limited to a very short range.
Bainite is a eutectoid decomposition that is a mixture of ferrite and cementite.
Martensite, the hardest constituent, forms during severe quenches
from supersaturated austenite by a shear transformation. (Fig 9) Its
hardness increases monotonically with carbon content up to about
0.7 wt%. If these unstable metastable products are subsequently
heated to a moderately elevated temperature, they decompose to
more stable distributions of ferrite and carbide. The reheating
process is sometimes known as tempering or annealing. The
transformation of an ambient temperature structure like ferritepearlite or tempered martensite to the elevated-temperature
structure of austenite or austenite-carbide is also of importance in
the heat treatment of steel. One can conveniently describe what is
happening during transformation with transformation diagrams.
These include time-temperature-transformation (TTT) diagrams,
describing the decomposition of austenite (Fig 10).
Martensite is therefore a non-equilibrated phase at room
temperature and is not possible to be found in the standard
thermodynamic Fe-C diagram
Fig 9
Martensite can be obtained if:
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Interstitial Carbon is present in the metallic phase
Metal is heated above its critical T, that is a function of the C%
Metal is then cooled down in a out-of-equilibrium way,
in order to avoid any interstitial migration and keep the C trapped in the Alfa – lattice
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One should indeed remember that Gamma Lattice has a
different Carbon solubility if compared to the Alfa one.
By taking in account of the migration processes ruled by
the I and II Fick’s laws the Martensite phase forms when
the cooling phase is so fast to block any interstitial atom
movement.
The resulting lattice is a tethraedrical one and it appears the
more distorted the faster the cooling rate.
Fig 10
Fig 11 shows a simple TTT curve.
Cooling rate must run on the left of the transformation
curves that drive metals to more stable, and less hard,
structures, as like Bainite, Troostite, Brownite and so on.
So, in this case, it is evident that the only possible cooling
rate is an extremely severe path, indicated by the red line.
In this case the consequences can be dramatic:
 The material faces a very serious thermal-induced
tensile state.
 The obtained metal is overstressed and needs a
further heat treatment to reduce the overall tension
state. This additional heating treatment is visible as
the blue line of Fig 11. It is called “annealing” and
it is performed to relax the final metal structure.
Industrial reasons suggest to avoid this kind of severe
treatment:
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It is extremely expensive
It provides an overstressed material
Fig 11
Usually it is much better to move to a higher class material instead of applying these treatments
It is important to bear in mind that the higher the C% the lower the temperatures at which
martensite starts (Ms) and finishes (Mf) its conversion. At the same time the transformations curves
shift to the right. So for a high C% metal or for an alloy, as like an inox steel, the overall space
available for a cooling path increases. But one has to remember that also Ms and Mf move down to
lower temperatures. These implies that to obtain a complete conversion one has to achieve final T
that sometimes are located well below 0 C. Again, industrial reasons do not support this way if any
other choice is available.
As a final remark it is important to remember that it is possible to obtain a different, and usually
higher, surface hardness values by applying a forced enrichment of interstitial atoms (C or N).
These treatments are performed by exposing the metal devices to liquid or gases able to provide C
or N to the metal structure and are performed at medium high temperatures (around 500 C) for a
total exposition between the 24 and 48 hours. After these applications the metal device shows a
surface composition quite different if compared to the bulk one.
By following the thermal treatments requirements it is therefore clear that to obtain Martensite one
has to bear in mind the fact that the transformation curve for the core structure, where a lower C% is
present, will be more severe than what is needed for the metal surface, where a higher C% exists.
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To fix this problem is therefore necessary to adopt a double-thermal –treatment process.
bulk
annealing
surface
Start transf.
End transf.
Fig 12
Fig 12 shows the different C % obtained over a bar section. The Fig 5 diagram illustrates the two
different cooling curves that have to be applied in sequence. As a fist step it is applied the more
severe curve finalised to temper the bulk material. This process at the same time will bring the
surface on a high disturbed phase. The whole material will experience a great volume expansion
and a huge tensile state will appear over the material surface. This tensile state is extremely
dangerous for any further application. A tensile state in fact put the surface in the condition to risk
to be broken for excess of tension. As a second step is therefore applied the tempering treatment for
the surface, with a smoother curve that fits with its higher C% . This second applications fulfils to
two contemporary tasks. It hardens the surface but at the same time it acts as annealing treatment
for the bulk phase. This fact greatly reduces the overall volume expansion, driving the surface to a
final tangential compression state that help material to resist to any injury and risk to crack opening
events. The final material therefore shows a great tenacity (tempered and annealed bulk phase) with
a hard and stiff surface (tempered phase).
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