Stainless Steel - 42130 Metallic Materials
Margrét Eva Árnadóttir, s021707 k Morten Lyck Maegaard, s021812
March 22, 2004
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CONTENTS
Contents
1 What is stainless steel?
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2 Families of stainless steel
2.1 Austenitic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Austenitic - Mechanical properties . . . . . . . . . . . . . . . . . . . .
2.1.2 Austenitic - Applications . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Ferritic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Ferritic - Mechanical properties . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Ferritic - Applications . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Superferritics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Duplex (Austenitic-Ferritic) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Duplex (Austenitic-Ferritic) - Mechanical properties . . . . . . . . . .
2.3.2 Duplex (Austenitic-Ferritic) - Applications . . . . . . . . . . . . . . .
2.4 Martensitic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1 Martensitic - Mechanical properties . . . . . . . . . . . . . . . . . . .
2.4.2 Martensitic - Applications . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Precipitation-hardenable stainless steels . . . . . . . . . . . . . . . . . . . . .
2.5.1 PH - Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 PH - Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 Alloying elements and their influences on the physical properties of stainless
steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Corrosion properties
3.1 Types of corrosion . . . . . . .
3.1.1 Uniform corrosion . . .
3.1.2 Pitting corrosion . . . .
3.1.3 Crevice corrosion . . . .
3.1.4 Stress corrosion . . . . .
3.1.5 Intergranular corrosion .
3.2 Influence of alloying elements .
3.2.1 Chromium . . . . . . .
3.2.2 Molybdenum . . . . . .
3.2.3 Nickel . . . . . . . . . .
3.2.4 Carbon . . . . . . . . .
3.2.5 Sulfur & Phosphorus . .
3.2.6 Silicon & Manganese . .
3.2.7 Titanium . . . . . . . .
3.2.8 Nitrogen . . . . . . . . .
3.2.9 Copper . . . . . . . . .
3.3 Influence of surface conditions .
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4 Thermal properties
4.1 Metallurgical behavior during heating & welding
4.1.1 Austenite . . . . . . . . . . . . . . . . . .
4.1.2 Ferrite . . . . . . . . . . . . . . . . . . . .
4.1.3 Martensite . . . . . . . . . . . . . . . . .
4.1.4 Duplex . . . . . . . . . . . . . . . . . . .
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CONTENTS
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5 Concluding remarks
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6 References
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LIST OF FIGURES
List of Figures
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Compositional and property linkages in the stainless steel family of
Stress-strain curve . . . . . . . . . . . . . . . . . . . . . . . . . . .
World Austenitic and Ferritic Stainless Steel Production . . . . . .
Duplex stainless steel microstructure . . . . . . . . . . . . . . . . .
Duplex stainless steel microstructure, welded . . . . . . . . . . . .
Yield stress and creep rupture strength curve . . . . . . . . . . . .
Schaeffler constitution diagram for stainless steels . . . . . . . . . .
Phase diagram for chromium and iron . . . . . . . . . . . . . . . .
Phase diagram for nickel and iron . . . . . . . . . . . . . . . . . . .
Corrosion types according to pH . . . . . . . . . . . . . . . . . . .
Corrosion rate at different temperatures . . . . . . . . . . . . . . .
Corrosion rate at different chlorid concentrations . . . . . . . . . .
Pitting corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crevice corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stress corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Intergranular corrosion . . . . . . . . . . . . . . . . . . . . . . . . .
Influence of alloying elements and environments . . . . . . . . . . .
Table concerning thermal conductivity . . . . . . . . . . . . . . . .
Table of ordinary secondary precipitates . . . . . . . . . . . . . . .
alloys
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1 WHAT IS STAINLESS STEEL?
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What is stainless steel?
Actually, you might already know what stainless steel is, as it has a number of everyday
applications, including dinner sets, knives, drink shakers, etc. Let us try defining stainless
steel in a metallurgical context.
Stainless steel is a steel variant. It is steel with a minimum of 10.5 wt% chromium added.
The addition of chromium to the iron-carbon alloy produces an effective corrosion resistance
and increases the strength of the steel and its ability to be hardened.
This document will describe the various properties of stainless steel in order to present
a general overview for use in the science of metallic materials. The effect of alloying elements will also be dealt with.
Figure 1: Compositional and property linkages in the stainless steel family of alloys.
Source: "ASM Specialty handbook: Stainless steels", J.R. Davis.
2 FAMILIES OF STAINLESS STEEL
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Families of stainless steel
There are five families of stainless steel, of which four are based on the characteristic
microstructure of the alloys in the family. These four are: Austenitic, Ferritic, Duplex
(Austenitic-Ferritic) and Martensitic. The precipitation-hardenable alloys form the fifth
family and they are based on the type of heat treatment used instead of being based on
microstructure.
2.1
Austenitic
Austenitic stainless steels are the most widely used type of stainless steel and count a large
number of alloys. Austenitic alloys have a face-centered cubic structure, like that of a high
temperature iron. The fcc structure of the alloys gives them excellent ductility, formability
and toughness.
Their composition is based on a balance between the alloying elements that either promote
ferrite formation or austenite formation. The main ferrite promoter in austenitic steels is
chromium, other ferrite promoting alloying elements that can be used are e.g. molybdenum,
titanium and aluminium. Nickel is the main austenitizing element, with carbon, nitrogen and
copper following. Manganese is also necessary, as it increases the solubility of the nitrogen
and prevents martensitic transformation.
Austenitic alloys cannot be hardened by heat treatment, only by cold working, and are
paramagnetic in the annealed condition.
2.1.1
Austenitic - Mechanical properties
Austenitic stainless steels can be divided into two categories: chromium-nickel alloys and
chromium-manganese-nitrogen alloys. The Cr-Mg-N alloys contain less nickel and need high
levels of nitrogen to maintain the austenitic structure. The Cr-Ni alloys have tensile yield
strengths from 200 to 275 MPa while the high-nitrogen alloys have yield strengths up to 500
MPa.
When austenitic steels are work hardened, they harden quite rapidly. Therefore is it possible
to obtain high strength in cold worked forms, e.g. drawn wires can have a tensile strength
of 1200 MPa.
The degree of hardening will depend on the alloying element content, increasing content
decreases the work-hardening rate. Those austenitic alloys that have low alloying element
content sometimes become magnetic when sufficiently cold worked or heavily deformed, as
they tend to transform to martensite.
2.1.2
Austenitic - Applications
Austenitic stainless steels possess extremely good cryogenic properties and good temperature
strength. Also, their non-magnetic nature enables their use in applications where magnetism
can cause problems. Such application can, for example, be security and detection devices.
Other applications includes housewares, containers, industrial piping and vessels.
2 FAMILIES OF STAINLESS STEEL
7
Figure 2: This image shows the difference in stress-strain for the different types of stainless
steel. Austenitic is the type that can tolerate the most strain.
Source: http://www.outokumpu.com/template/Page____5832.asp.
2.2
Ferritic
The ferritic stainless steels take their name after their body-centered cubic structure, which
is the structure of iron at room temperature. Ferritic steels contain between 11 wt% and 30
wt% Cr, but the most common ones are 11 wt% and 17 wt% chromium containing steels.
Ferritic steels are ferromagnetic and cannot be hardened by heat treatment. They are rarely
strengthened by cold work, as their strain-hardening rates are relatively low and cold work
lowers ductility quite a bit.
2.2.1
Ferritic - Mechanical properties
The strength and ability of the bcc structure to sustain plastic deformation are very temperature dependent. Strength increases rapidly and ductility drops sharply with a decreasing
temperature, as screw dislocations cannot cross slip as easily in the bcc structure.
Ferritic stainless steels are not as strong as most austenitic alloys, their annealed yield
strengths range from 275 to 350 MPa. They can have good ductility and formability, but
have a poor high-temperature strength (see figure 6).
This and their susceptibility to sensitization (see chapter 3.1.5), limit their fabricability and
the usable section size.
2.2.2
Ferritic - Applications
The low-chromium alloys, with 11 wt% Cr, are used mostly in structural applications, such
as in automotive exhaust systems. They can easily be fabricated at a low cost and have
fair corrosion and oxidation resistance. The 17 wt% Cr alloys are more used in housewares,
boilers, washing machines and indoor architecture. These alloys have poor toughness and
weldability, which makes them not as readily fabricated.
In general, the use of ferritic steels at ambient temperature is very limited, as they lose their
toughness and ductility at low temperatures.
2 FAMILIES OF STAINLESS STEEL
2.2.3
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Superferritics
The high-chromium alloys of ferritic stainless steel, those with chromium content between 19
wt% and 30 wt%, are often referred to as superferritics. They often contain either aluminium
or molybdenum and have a very low carbon content.
These alloys can obtain more strength than normal ferritics, a yield strength up to 515
MPa. Superferritics also offer exceptional resistance to localized corrosion and are therefore
often used in heat exchangers and piping systems for chloride-bearing aqueous solutions and
seawater.
Figure 3: World Austenitic and Ferritic Stainless Steel Production (1985 - 2005).
Source:http://www.dem.csiro.au/em/commodities/nickel/nickelmarket/.
2.3
Duplex (Austenitic-Ferritic)
Duplex stainless steels are Cr-Ni-Mo alloys that contain a mixture of fcc austenite and bcc
ferrite. The exact amount of each phase is a function of composition and heat treatment,
most alloys have about equal amounts of each. The main alloying elements are chromium and
nickel but others (such as nitrogen, copper, silicon, molybdenum) can be added to control
structural balance and to induce corrosion-resistance characteristics.
Duplex alloys solidify as essentially 100 wt% ferrite. Austenite nucleates start to grow at
high temperatures and by diffusing alloying elements it is possible to induce a transformation
of ferrite into austenite. These alloying elements are used to obtain the requested balance
between ferrite and austenite, nitrogen is most important in determining the relative ease of
achieving a near-equilibrium balance.
2.3.1
Duplex (Austenitic-Ferritic) - Mechanical properties
Duplex alloys are magnetic and their structure results in improved stress-corrosion cracking
resistance and improved toughness and ductility, compared to ferritic stainless steels.
They have tensile yield strengths ranging from 550 to 690 MPa in the annealed condition.
This is approximately twice the strength of each phase alone (if we consider the Cr-Ni
austenitic alloys). Its toughness is also far superior to that of heat treated and hardened
alloys. However, the high alloy content and the presence of ferrite make the duplex steels
susceptible to embrittlement and loss of mechanical properties through prolonged exposure
to elevated temperatures.
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2 FAMILIES OF STAINLESS STEEL
Figure 4: Duplex stainless steel microstructure, in a normal state.
Source: http://www.trinex.ca/
products-duplex.html.
2.3.2
Figure 5: Duplex stainless steel microstructure, after being welded.
Source:http://www.mee−inc.com/
imagegallery.html.
Duplex (Austenitic-Ferritic) - Applications
Duplex alloys are used in intermediate temperatures where resistance to acids and aqueous
chlorides is required, for example in petrochemical, paper, pulp and shipbuilding industries.
They are not suitable for cryogenic applications.
As the duplex stainless steel has a high yield strength, it opens up the possibility of producing
it in thin plates and yet keep its pressure-containing and load-bearing capabilities.
2.4
Martensitic
Martensitic stainless steels are alloys of chromium and carbon that have a body-centered
tetragonal, bct, structure in the hardened condition. They are ferromagnetic and are both
strong and hard with moderate corrosion resistance. The chromium content ranges normally
from 10.5 wt% to 18 wt%.
At room temperature, the equilibrium microstructure is a mixture of ferrite and carbides,
but at high temperatures its structure is almost entirely austenitic. With cooling, the microstructure becomes martensitic very easily.
2.4.1
Martensitic - Mechanical properties
Carbon content almost entirely determines the hardness of martensite, adding only 0.1 wt%
C to martensite will result in a hardness value of about 35 HRC. Around 0.5 wt% C the
hardness value increases to over 60 HRC and doesn’t change much at higher levels of carbon
content. In production, one must be aware that by increasing the carbon content in the
steel, ductility and toughness is decreased and corrosion occurs more easily.
In the annealed condition, martensitic stainless steels have a tensile yield strength of about
275 MPa, the heat treatment of the alloy results in added strength, however their creep
strength is far superior to those of the other types of stainless steel (see figure 6).
Molybdenum and nickel can be added to improve corrosion and toughness properties. Nickel
also helps to maintain the desired microstructure. However, since high amounts of these elements can result in a not fully martensitic microstructure, their addition has been restricted.
2 FAMILIES OF STAINLESS STEEL
2.4.2
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Martensitic - Applications
Martensitic stainless steels are used when an application calls for good tensile strength, creep
and fatigue strength properties, in combination with moderate corrosion resistance and heat
resistance up to about 920K. These applications can, for example, be: jet engines, gas and
steam turbines, gears and surgical instruments.
Figure 6: Yield strength and creep rupture strength curve with regard to temperature.
Source: http://www.outokumpu.com/template/Page____5832.asp.
2.5
Precipitation-hardenable stainless steels
These are chromium-nickel grades containing precipitation-hardenable (PH) elements such
as copper, aluminium and titanium. They can either be austenitic or martensitic in their
annealed condition, the austenitics can be transformed to martensite through conditioning
heat treatments. By precipitation hardening of the martensitic structure, the steel is able
to obtain high strengths. The PH alloys can be divided into three groups: martensitic,
semi-austenitic and austenitic, based on their martenisite start and finish temperatures and
resultant behaviour upon cooling.
2.5.1
PH - Mechanical properties
PH alloys can attain high yield strengths from 500 to 1420 MPa, tensile strengths from 850
to 1500 MPa and elongations from 1% to 25%. Cold working prior to aging can result in an
even higher strength. These PH alloys, in general, have good ductility and toughness with
a moderate-to-good corrosion resistance.
2 FAMILIES OF STAINLESS STEEL
2.5.2
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PH - Applications
Most applications for PH are in aerospace and high-technology industries because of their
high strengths.
Figure 7: Schaeffler constitution diagram for stainless steels. The Schaeffler diagram can be
used to determine the type of microstructure that can be expected when different compositions are mixed together in a weld.
Source: "ASM Specialty handbook: Stainless steels", J.R. Davis.
In the following chapters there will be further discussions regarding properties, corrosive
and thermal properties, of the different types of stainless steel.
2.6
Alloying elements and their influences on the physical properties of stainless steel
In order to control microstructure and properties, various elements can be added to the basic
iron-chromium systems. Here is a brief overview:
- Nickel is mostly alloyed to improve the formability and ductility of stainless steel. It also
improves strength properties, toughness and the steel’s ability to be hardened.
- Manganese reduces sulfur brittleness, increases hardness and improves the strength and
toughness of the steel.
- Molybdenum improves grain refinement, increases strength for high temperature applications, increases creep resistance and the steels ability to be hardened.
- Silicon increases strength, reduces ductility and increases the ability to be hardened.
- Sulfur improves machinability, reduces ductility and weldability.
2 FAMILIES OF STAINLESS STEEL
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Some mechanical properties are common for the various families, these are for example
density and the modulus of elasticity.
Density values don’t vary much for various grades of stainless steel, they range from 7.5 to
g
8.0 cm
3 . This means that stainless steel have a density almost three times that of aluminium
alloys.
As with density values, the values for the modulus of elasticity (Young’s modulus) vary little.
The values are of the same order for all grades, from 193 to 204 GPa, or nearly twice that
of copper alloys and nearly three times that of aluminium, which makes it fairly rigid.
Figure 8: Phase diagram for chromium and iron. For stainless steel it is only relevant
for chromium content over 10.5 wt%. The microstructure is determined by the quenching
temperature and chromium content.
Source: http://web.met.kth.se/dct/pd/element/Cr-Fe.html.
Figure 9: Phase diagram for nickel and iron. For stainless steel it is relevant for nickel
content between 0 wt% and 30 wt%. Within this interval, the microstructure is mainly fcc.
Source: http://web.met.kth.se/dct/pd/element/Fe-Ni.html.
3 CORROSION PROPERTIES
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Corrosion properties
Judging from the name, stainless steels are supposed to be stainless, that is without corrosion
at all. Unfortunately this is not the case. Although their resistance to corrosion may be far
better than is the case for conventional iron-carbon metals, corrosion does occur in various
forms.
As already mentioned, chromium is the main alloying element in stainless steels, as it facilitates the creation of a thin film of chromium oxide, a few nanometers thick, on the steel
surface. The film acts as a protective layer between the steel and the surrounding environments, as long as these are not too harsh. In addition the chromium oxide ensures a
shiny and hygienic surface. As the steel surface faces its service conditions, minor scratches
and holes are likely to be produced in the chromium oxide layer, but due to electrochemical
reactions, the film is continously maintained, preventing corrosion initiation. This ability of
chromium is called passivation.
A number of factors determine the toughness of the conditions in which the stainless steel
can withstand corrosion. The most significant factors are the alloying elements and the surface topography. Before we dig into these important subjects, let’s first look at the various
types of corrosion.
3.1
Types of corrosion
Dispite their generally good corrosion resistance, stainless steels are far from immune to
corrosion. The list below contains the five overall corrosion types, encompassing corrosive
behavior of stainless steels in aqueous environments.
• Uniform corrosion
• Pitting corrosion
• Crevice corrosion
• Stress corrosion
• Intergranular corrosion
The corrosion types to be aware of are strongly determined by the acidity of the service
environment. At very acidulous conditions, pH below 2-2.5, the ability of chromium to passivate is poor and thus there is a risk of especially uniform corrosion, but also risks of stress
corrosion and intergranular corrosion must be kept in mind. In the wide area of pH between
2.5 and 14 where the main part of stainless steels are applied, pitting and crevice corrosion
are the main risks, but stress corrosion might also occur here. Finally, for pH above 14,
problems regarding the passivation of chromium occur, due to the formation of oxianions
(carbonates, phosphates, etc.) and the corrosion they imply. For this reason uniform corrosion is the predominant corrosion type, and at high temperatures stress corrosion also
becomes significant. Figure 10 illustrates these considerations.
3.1.1
Uniform corrosion
This type of corrosion occurs only at extreme values of pH and is thus rather rare. As its
name implies, the corrosion is not confined to certain areas of the metal, but covers the entire
exposed surface. No stainless steels can completely resist uniform corrosion, so the goal is
a minimum effect. The corrosion rate is, as for many corrosion types, very temperature
3 CORROSION PROPERTIES
14
Figure 10: The figure displays the dependancy of corrosion type on environment acidity.
Source: "Korrosionsbestandigt rustfrit stål. Hvordan?", E. Rislund.
dependant. This is illustrated in figure 11.
The most corrosive strong acids causing uniform corrosion are sulphuric and phosphoric
acids, strong hydrochloric acid and chloride. Figure 12 exemplifies this crucial fact.
Finally, this corrosion type is primarily confined to underwater applications.
Figure 11: Corrosion rate in contaminated phosphoric acid at different temperatures for a
number of stainless steel alloys.
Source: The Sandvik Group, http://www.smt.sandvik.com/sandvik/0140/internet/se01598.nsf/
cdatas/A1B9D152AF4F522C41256632002AD818.
3 CORROSION PROPERTIES
15
Figure 12: Corrosion rate in contaminated phosphoric acid at different chloride concentrations at 100◦ C.
Source: The Sandvik Group, http://www.smt.sandvik.com/sandvik/0140/internet/se01598.nsf/
cdatas/A1B9D152AF4F522C41256632002AD818.
3.1.2
Pitting corrosion
Together with crevice corrosion, pitting corrosion is the most common corrosion type. The
corrosion arises from minor holes in the chromium oxide layer and is a very localized and
fast reaction. Hence, it often results in complete penetration or fracture of the material. An
example is seen in figure 13.
As for other corrosion types, increasing temperature or chloride concentration increases the
probability of pitting corrosion. In connection hereto, the Critical Pitting Temperature,
CPT, designates the temperature at which pitting corrosion is initiated under specific circumstances.
Pitting corrosion can take place above, as well as below, the water surface, however, above
water the effects are mostly cosmetic.
3.1.3
Crevice corrosion
The characteristics of this type of corrosion are a lot like those of pitting corrosion, as to
the corrosion mechanism and the hazardous environments. The difference is that crevice
corrosion is confined to cracks and flaws in the material, whereas pitting corrosion occurs on
the "free surfaces". Hence the crevice geometry is of great importance. The deeper and narrower the crevice, the greater risk of still water conditions and hence concentration build-up,
resulting in crevice corrosion.
The passivation ability of stainless steel is responsible for the mechanism of crevice corrosion.
A weak liberation of metal ions, called the passivation current, is caused by the passivation
effect. These metal ions both react acidically and attract chloride, establishing a very acidulous and chlorine-containing environment in the crevice where fluid exchange is low. This
causes crevice corrosion as seen in figure 14.
3 CORROSION PROPERTIES
16
Figure 13: An example of pitting corrosion. viewed through a SEM.
Source: http://www.atclabs.com/Photos.htm.
Figure 14: Stainless steel bolt (bottom) inappropriately used in seawater, has experienced
crevice corrosion. Here viewed after five years of exposure.
Source: http://www.corrosion-doctors.org/Localized/Crevice.htm.
Differences in the ability of stainless steels to resist crevice corrosion is often expressed
by the Critical Crevice corrosion Temperature, CCT, which normally lies 20 − 25 ◦ C below
the CPT temperature. Hence, the risk of crevice corrosion is always greater than the risk of
pitting corrosion.
Crevice corrosion only occurs in underwater conditions, due to the need of vast volumes of
bulk-electrolyte and chloride sources.
3.1.4
Stress corrosion
The most localized and destructive type of corrosion is stress corrosion. Accordingly, complete penetration of for instance a stainless steel tube, can happen within days or even just
a few hours. As the name implies, stress in the material is among the decisive factors when
stress corrosion occur. Stresses need not be externally applied, also residual stresses in the
material are of importance, and their magnitudes are crucial even below the tensile strength
of the material. Also crystal structure, environmental conditions and especially temperature
determine the rate at which this corrosion-form progresses.
3 CORROSION PROPERTIES
17
Figure 15: This figure displays stress corrosion cracking in a stainless steel heat exchanger.
Source: http://www.atclabs.com/Photos/300%20series%20SS%20SCC.jpg.
Stress corrosion is the result of alternating activation and passivation of the protective
oxide film, rather than complete activation as is the case when crevice and pitting corrosion
develop. This indecisiveness of the material results in deep cracks almost splitting the steel
apart.
The most stress corrosion promoting environments contain high temperature chlorides and
extreme pH values, both above and below water. Seawater water could be one of these
special environments, since it contains 2% chloride.
3.1.5
Intergranular corrosion
The last corrosion type to be described is intergranular corrosion, which is also the least
occuring. Normally, intergranular corrosion arises from inappropriate heat treatment by, for
instance, welding. The heat treatment facilitates the formation of chromium carbides at the
grain boundaries which "drains" the surroundings of chromium, leaving chromium deficient
areas less corrosion resistant. The process is called sensitization. Eventually some grains are
surrounded by corrosion and simply fall out of the material, see figure 16.
High chloride concentrations, low pH and high temperature are the crucial factors causing
intergranular corrosion. Only at underwater conditions is the corrosion significant. As
indicated above, intergranular corrosion is a severe problem in welding, where it is called
weld decay.
3.2
Influence of alloying elements
A certain stainless steel alloy is only suitable for application in certain environments, owing
to its alloying elements. The many different alloying elements strengthen or weaken the steel
in each their way and normally the strengthening (or weakening) increases with alloy element
content. Thus, it is of absolute importance to know the effects of each alloying element. In
the following the effects of many alloying elements are described.
3 CORROSION PROPERTIES
18
Figure 16: Grain boundary corrosion and intergranular cracking from excessive solution
annealing temperatures in a 316L stainless steel microstructure.
Source: http://www.hghouston.com/x/24.html.
3.2.1
Chromium
Stainless steel by definition contains at least 10.5 wt% chromium. Chromium is a ferrite
promoter and is used because it passivates effectively. The steel resistance of pitting and
crevice corrosion increases with increasing chromium content, but the effect is little with
regards to uniform and stress corrosion.
3.2.2
Molybdenum
This element is normally added in concentrations ranging from 1.5 wt% to 7.5 wt%. Like
chromium, molybdenum is a ferrite promoter and passivates extremely effectively. The
resistance of pitting and crevice corrosion increases with increasing molybdenum content.
Molybdenum is generally better than chromium, especially under acidulous and oxygen
deficient conditions. Furthermore, it reduces stress and uniform corrosion.
3.2.3
Nickel
Is present in stainless steels from 0 wt% to 30 wt%. The main reason for alloying with
nickel is that it is an austenite promoter and the austenite content therefore depends on the
amount of nickel in the steel. Nickel increases resistance towards stress and uniform corrosion
especially. Has no effect on the initiation of pitting and crevice corrosion, but makes these
processes go slower once started.
3.2.4
Carbon
Copper is an austenite promoter, but its content is normally kept as low as possible. Carbon
increases risk of intergranular corrosion by binding chromium in the grain boundaries, leaving
the surrounding areas chromium depleted.
3 CORROSION PROPERTIES
3.2.5
19
Sulfur & Phosphorus
Like carbon, sulfur and phosphorus are harmful elements even in minor quantities. The
effect is most obvious for sulfur as it forms manganese sulfides, which act as initialisation
spots for pitting corrosion. This is especially apparent in the low corrosion resistance of
sulfur alloyed free machining steel.
3.2.6
Silicon & Manganese
These elements are austenite promoters without any particular harmful or beneficial effects.
Manganese must normally be held low to minimise the amount of manganese sulfides, but
very low manganese content renders the steel harder to hot roll.
3.2.7
Titanium
Counteracts carbon by binding it in titanium carbides and hence lowers the risk of intergranular corrosion. Is traditionally used as an alternative to reducing the carbon content.
Titanium is a moderate ferrite promoter.
3.2.8
Nitrogen
Is normally added in small amounts, between 0.1 wt% and 0.3 wt%. Compared to chromium
and molybdenum, nitrogen increases the passivation effect drastically and thus decreases
the risk of pitting and crevice corrosion significantly. Nitrogen is a austenite promoter and
therefore acts as both passivator and nickel replacement. The problem with nitrogen alloying
is primarily that the process is very hard to handle at the steel mill.
3.2.9
Copper
An austenite promoter. Increases resistance towards uniform corrosion in strong acids.
Because many alloying elements have the same effect, but with different magnitudes, it
is nessesary to find a way to compare different alloys with regards to their resistance to
corrosion. Scientists have therefore empirically deduced the following formula which gives a
relative expression of the ability of stainless steel to resist pitting corrosion. The formula is
called the PREN formula1 .
P REN = wt%Cr + 3.3 · wt%M o + 16 · wt%N
Though increasing content of alloying element normally enhances the steel, there are
limits. The steel mill must still be able to obtain the desired microstructure as well as it
has to stay workable in various processes and weldable. The alloying elements are normally
more expensive than iron and the steel gets more difficult to work as the alloying element
concentration goes up; the steel price therefore rises with alloying element content. Figure
17 sums up the effects of the different alloying elements discussed above as well as the effects
of various environments.
1 PRE for "Pitting Resistance Equivalent" and the N is used for nitrogen containing steels.
Source: "Korrosionsbestandigt rustfrit stål. Hvordan?", E. Rislund.
3 CORROSION PROPERTIES
20
Figure 17: The risk of various corrosion types as a function of increasing alloy content in
the steel and increasing temperature, chlorine content, corrosion potential and pH. "A" designates that the alloying element in question is a austenite promoter, while "F" designates
a ferrite promoter. "Uparrow" designates that the risk of initiation of the corrosion type
in question increases with increasing alloy content or environment parameter; "two or three
uparrows", that the risk increases tremendously; "downarrow" that the corrosion risk decreases with increasing parameter; "two or three downarrows", that the corrosion risc falls
drastically. "Up- and downarrow" designates that the effect can go both ways and finally
"zero", that there is no effect in particular of the parameter in question.
Source: "Korrosionsbestandigt rustfrit stål. Hvordan?", E. Rislund.
3.3
Influence of surface conditions
A smooth and even floor is easier to keep clean than a rough and irregular one. The same
is valid for a metal surface. The more rugged the surface, the harder it is to clean and the
greater is the risk of local pollutive deposits accompanied by corrosion. Hence, it is essential
for, for instance, applications in the food industry to have equipment with very even surface
topography in order to maintain a hygienic production facility.
During steel manufacturing, iron contamination from processing tools is a corrosion problem.
Iron on stainless steel parts will rust, covering the steel surface. During service, a local and
potentially corrosive environment will be established at this surface irregularity, increasing
risk of steel corrosion.
Finally, when stainless steel at atmospheric conditions is heated beyond 200 − 300 ◦ C, the
3 CORROSION PROPERTIES
21
surface reacts with oxygen and forms a layer of iron/chromium oxides. As some chromium
has now diffused to the surface, a chromium depleted region susceptible to corrosion remains
beneath the surface and the overall corrosion resistance of the stainless steel is reduced.
There are a couple of different standards for surface treaments, including the American
ASTM system and the German DIN system.
4 THERMAL PROPERTIES
4
22
Thermal properties
The low thermal conductivity of stainless steels in general is by far its most interesting
thermal property. For instance, if you have a look at your pots and pans in your kitchen,
you will notice that many of the handles are made of stainless steel to make you able to hold
them while cooking. The low thermal conductivity of stainless steels is also utilized in more
extreme applications such as racer car engines and engines for use in air- and spacecraft.
Using conventional steels, the heat from the engines would set the entire craft on fire, but
with stainless steels the heat stays inside the engine, leaving the surroundings virtually
unaffected by the enormous heat present. Table 18 displays the uniqueness of the stainless
steels with regards to thermal conductivity.
Figure 18: The table compares the coefficient of thermal conductivity of different metals
including two types of stainless steel and the "Inconel" steel, which is extremely expensive
and hard to work with.
Source: http://www.burnsstainless.com/TechArticles/Stainless_article/
stainless_article.html.
4.1
Metallurgical behavior during heating & welding
It is of crucial importance to be aware of the metallurgical changes imposed by welding and
heat treating metals in general. Hence, this is also the case for stainless steels.
We must consider the steel from the beginning of its life-cycle, that is, starting in the steel
mill. Here it is important to produce a homogenous material with uniform concentration of
the alloying elements throughout the entire volume, because even very small concentration
deviations can lead to corrosion. Frequently, the solubility of the alloying element is greater
in the melt than in the hardened material that has just solidified. This results in concentration differences, the phenomenon called segregation of the alloying elements. Segregation
increases corrosion susceptibility as parts of the steel become depleted of corrosion preventing alloying elements. The effects of segregation are reduced by obtaining a fine grained
microstructure.
The area affected by heat during welding is called the heat affected zone or HAZ. In this
zone the material will almost inevitably experience grain size growth, which is undesirable as
it reduces mechanical and sometimes also corrosion properties. Using electron beam welding
or lazer welding minimizes the HAZ and hence the unwanted consequences.
Due to solubility differences of the various alloying elements, a number of the popular stainless steel types exhibit limited structure and phase stability. Heating during welding thus
makes it possible for unwanted secondary phases to precipitate in the HAZ. The secondary
phases often contain higher concentrations of the alloying elements, for instance chromium
4 THERMAL PROPERTIES
23
or molybdenum, leaving the immediate surroundings depleted of alloying elements and hence
more susceptible to corrosion. An example of this behavior was described as sensitization in
the section about corrosion properties. Table 19 shows the relationship between alloy types
and precipitated secondary phases.
Figure 19: This table shows ordinary secondary precipitates for different types of stainless
steels.
Source: "Korrosionsbestandigt rustfrit stål. Hvordan?", E. Rislund.
Finally, the heating of the HAZ leads to residual stresses in the material. They arise due
to differences in the microstructure of the HAZ which is accompanied by differences in the
thermal expansions of the material. The stresses increase the risk of stress corrosion and can
only be removed by annealing heat treatment.
Now follows a short description of the metallurgical properties with regards to heating and
welding of austenite, ferrite, duplex and martensite microstructures in stainless steel, respectively.
4 THERMAL PROPERTIES
4.1.1
24
Austenite
The group of austenitic stainless steels is very wide and hence, so are the problems regarding
heating and welding. These steels retain their austenitic structure at all temperatures and
therefore cannot be strengthened by heat treatment. They have the lowest coefficient of
thermal conductivity of all stainless steel types, which means that HAZ is smaller than for
other stainless steels, but also that cooling of the HAZ is slow. The susceptibility to grain
boundary growth is much less for the austenitic steels than for ferritic steels, due to a high
amount of toughness.
Precipitation of carbides resulting in sensitization may be a problem, if the carbon content
is over 0.03 wt% without the steel being stabilised by titanium or niobium.
4.1.2
Ferrite
These stainless steels generally exhibit lower corrosion resistance and is much harder to weld
compared to austenitic stainless steels, because of risk of grain growth and embrittlement.
Highly alloyed ferrite steels with high chromium and carbon content are less suitable for
welding due to risk of sensitization leading to intergranular corrosion. These types also have
higher risk of secondary phase precipitation, as depicted in figure 19.
The biggest problem for ferritic stainless steels is grain growth producing drastic decreasing
toughness in the HAZ.
4.1.3
Martensite
Martensitic stainless steel needs special procedures for welding taking into account the risk
of hydrogen cracks and carbide formation. Especially sulfur alloyed free machining steels are
known as being very difficult to weld.
Heating of the surface to over 800 − 900◦ C can lead to hardening and hardening cracks.
4.1.4
Duplex
During welding almost all the material in the HAZ transform to ferrite. Therefore the
resulting austenite content depends on quenching speed and the amount of austenite forming
alloying elements. Ferrite content must be between 30-70 wt% to avoid serious changes in
corrosion resistance.
Duplex stainless steels are normally not applied in the temperature range 350 − 900 ◦ C, due
to risk of imbrittlement. In addition, it is generally recommended not to use welded duplex
steel constructions provided that service temperatures exceed 300◦ C, as the heat supplied
during welding can cause minor segregations and imbalance in the desired ferrite/austenite
ratio.
5 CONCLUDING REMARKS
5
25
Concluding remarks
When all elements of this document have been taken into consideration, the following
overview of the excellent qualities of stainless steel can be put forth.
• Variety of types
• Corrosion resistance
• Low thermal conductivity
• Strength-to-weight advantage
• Impact resistance
• Long lasting
• Hygienic
• Aesthetic appearance
Stainless steel is a solid choice of material for various purposes. Other materials, such
as titanium, have properties superior to stainless steel, but here the price factor steps in.
Stainless steel is considerably cheaper than titanium and shares many of its qualities. The
combination of superior properties and cost efficiency is probably why stainless steel has
achieved such a big marketshare.
6 REFERENCES
6
26
References
1. American Society for Metals Metals Handbook : Properties and Selection: Stainless
Steels, Tool Materials and Special-Purpose Metals„ 9. ed., Vol. 3, American Society
for Metals, 1980
2. Callister Jr., W. D. Materials Science and Engineering - An Introduction, 6. ed., Wiley,
2003
3. Davis, J.R. ASM Specialty handbook: Stainless steels, ASM International, 1994
4. Peckner, D. a.o. Handbook of Stainless Steel, McGraw-Hill, 1977
5. Rislund, E. Korrosionsbestandigt rustfrit stål. Hvordan?, 1. ed., Industriens Forlag,
1996
6. http://www.outokumpu.com/template/Page____5832.asp
7. http://www.dem.csiro.au/em/commodities/nickel/nickelmarket/
8. http://www.trinex.ca/products-duplex.html
9. http://www.mee−inc.com/imagegallery.html
10. http://web.met.kth.se/dct/pd/element/Cr-Fe.html
11. http://web.met.kth.se/dct/pd/element/Fe-Ni.html
12. http://www.mas.dti.gov.uk/browse.jsp?classification=fact
13. http://www.burnsstainless.com/TechArticles/Stainless_article/stainless_article.html
14. http://www.smt.sandvik.com/sandvik/0140/internet/se01598.nsf/cdatas/
A1B9D152AF4F522C41256632002AD818
15. http://www.atclabs.com/Photos.htm
16. http://www.corrosion-doctors.org/Localized/Crevice.htm
17. http://www.atclabs.com/Photos/300%20series%20SS%20SCC.jpg
18. http://www.hghouston.com/x/24.html