ELEMENTS OF STEEL STRUCTURAL
5 MARK QUESTION
1.History of steelmaking
Steel was known in antiquity, and may have been produced by managing
, or iron
facilities, in which the bloom contained carbon.
The earliest known production of steel is a piece of ironware excavated from an
) and is about 4,000 years old.
Other ancient steel comes from
, dating back to 140
In the 4th century BC steel weapons like the
were produced in
was used by the
221 BC) had
while Chinese of the
AD) created steel by melting together wrought iron with cast iron, gaining an ultimate product of a
intermediate steel by the 1st century AD.
East Africa invented a type of
heat blast furnace which allowed them to forge carbon steel at
nearly 2,000 years
Wootz steel and Damascus steel
of the earliest production of high carbon steel in the
was found in
Samanalawewa area in
Wootz steel was produced in
by about 300 BC.
their original methods of forging steel, the Chinese had also adopted the production methods of
, an idea imported into
China from India by the 5th century AD.
In Sri Lanka, this
making method employed the unique use of a wind furnace, blown by the monsoon winds,
was capable of producing high
Also known as
, wootz is famous
durability and ability to hold an
. It was originally created from a number of different materials
. It was essentially a complicated alloy with iron as its main component.
Recent studies have suggested that
luded in its structure, which might explain
some of its legendary qualities, though given the technology available at that time, they were produced by
chance rather than by design.
tural wind was used where the soil containing iron was heated up with
the use of wood. The ancient Sinhalese managed to extract a ton of steel for every 2 tons of soil
, a remarkable feat at the time. One such furnace was found in Samanalawewa and archaeologists
were able to produce steel as the ancients did long ago.
, formed by slowly heating and cooling pure iron and carbon (typically in the form
charcoal) in a crucible, was produced in
by the 9th to 10th century AD.
In the 11th century, there
of the production of steel in
using two techniques: a "berganesque" method that
produced inferior, inhomogeneous steel and a precursor to the modern Bessemer process th
at used partial
decarbonization via repeated forging under a
Since the 17th cen
tury the first step in European steel production has been the smelting of iron ore into
pig iron in a
Originally using charcoal, modern methods use
, which has proven to
be a great deal cheaper.
Processes starting from bar iron
In these processes pig iron was "fined" in a
(wrought iron), which was
then used in steel
The production of steel by the
was described in a treatise published in Prague in
1574 and was in use in
from 1601. A similar process for
armour and files was
described in a book published in
in 1589. The process was introduced to England in about
It was produced by Sir
during the 1610s. The raw material for
this were bars of wrought iron. During t
he 17th century it was realised that the best steel came
from a region of
, north o
. This was still the usual raw material
in the 19th century, almost as long as the process was used.
2.Contemporary steel and uses
Modern steels are made with varying combina
tions of alloy metals to fulfill many purposes.
, composed simply of iron and ca
rbon, accounts for 90% of steel production.
High strength low
has small additions (usually < 2% by
weight) of other elements, typically 1.5% manganese, to
provide additional strength for a modest price increase.
Low alloy steel
is alloyed with other elements,
, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve
the hardenability of thick
Stainless steels and
surgical stainless steels
contain a minimum of
chromium, often combined with nickel, to resist
(rust). Some stainless steels, such as
stainless steels are
, while others, such as the
Some more modern steels include
, which are alloyed wi
th large amounts of tungsten
or other elements to maximize
. This also allo
ws the use of
and improves the alloy's temperature resistance.
Tool steel is generally used in axes, drills,
and other devices that need a sharp, long
lasting cutting edge. Other special
ten, which weather by acquiring a stable, rusted surface, and so can
be used un
Many other high
strength alloys exist, such as
, which is heat treated to contain both a
ferritic and martensitic microstructure for extra strength.
ed Plasticity (TRIP)
steel involves special alloying and heat treatments to stabilize amounts of
at room temperature
in normally austentite
alloy ferritic steels. By applying strain to the metal,
to martensite without the addition of heat.
alloyed with nickel and other elements, but unlike most steel contains almost no carbon at all. This creates
a very strong but still
Twinning Induced Plasticity (TWIP) steel uses a specific type of
strain to increase the effectiveness of work hardening on the alloy.
uses a combination of
over a dozen different elements in varying amounts to create a relatively low
cost metal for use in
weapons. Hadfield steel (after Sir
) or manganese steel contains 12
nganese which when abraded forms an incredibly hard skin which resists wearing. Examples
edges and cutting blades on the
jaws of life
Most of the more commonly
used steel alloys are categorized into various grades by standards
organizations. For example, the
Society of Automotive Engineers
has a serie
types of steel.
American Society for Testing and Materials
has a separate set of standards, which
define alloys such as
, the most commonly used struc
tural steel in the United States.
Though not an alloy,
steel is a commonly used va
riety of steel which has been hot
for protection against rust.
Iron and steel are used
widely in the construction of roads, railways, other infrastructure, appliances, and
buildings. Most large modern structures, such as
supported by a steel skeleton.
Even those with a concrete structure will employ steel for reinforcing. In
addition, it sees widespread use in
. Despite growth in usage of
, it is
still the main material for car bodies. Steel is used in a variety of other
materials, such as
Other common applications include
such as bulldozers, office furniture,
in the form of personal vests
(better known as
rolled homogeneous armour
in this role). Steel was the metal of
choice for sculptor
and a frequen
t choice for
by many other modern sculptors.
3.Germicidal and antimicrobial applications
in brass ma
. Depending upon the type and concentration of
the medium they are in, brass kill
within a few minutes to eight hours of
properties of brass have bee
n observed for centuries and were confirmed in the
laboratory in 1983.
Subsequent experiments by research groups around the world reconfirmed the
antimicrobial efficacy of brass,
as well as copper and other copper alloys (see
Extensive structural membrane damage to
was noted after being exposed
U.S. Department of Defense
emedicine and Advanced Technologies Research Center
(TATRC) began to study the antimicrobial properties of copper alloys, including four brasses (C87610,
C69300, C26000, C46400) in a multi
site clinical hospital trial conducted at the
Kettering Cancer Center
(New York City), the
Medical University of South Carolina
, and the Ralph H.
Johnson VA Medical Center (South Carolina).
Commonly touched items, such as bed rails, over
bed tray tables, chair arms, nurse's call buttons, IV poles, etc. were retrofitted with antimicrobial
copper alloys in certain patient rooms (i.e., the “coppered” rooms) in
Intensive Care Unit
results disclosed in 2011 indicate that the coppered rooms demonstrated a 97% reduction in
versus the non
coppered rooms. This reduction is the same level achieved by
“terminal” cleaning regimens conducted after patients vacate their rooms. Furthermore, of critical
importance to health care profe
ssionals, the preliminary results indicated that patients in the coppered
ICU rooms had a 40.4% lower risk of contracting a
hospital acquired infection
versus patients in non
coppered ICU rooms.
The U.S. Department of Defense investigation contract, which is ongoing,
will also evaluate the effectiveness of copper alloy touch surfaces to prevent the transfer of
patients and the transfer of microbes from patients to touch surfaces, as well as the potential efficacy of
alloy based components to improve
indoor air quality
In the U.S., the
Environmental Protection Agency
regulates the registration of antimicrobial products.
After extensive antimicrobial testing
according to the Agency’s stringent test protocols, 355
, including many brasses, were found to kill more than 99.9% of
, and vancomycin
(VRE) within two hours of
Normal tarnishing wa
s found to not impair antimicrobial effectiveness.
Antimicrobial tests have also revealed significant reductions of MRSA as well as two strains of epidemic
1 and EMRSA
16) on brass (C24000 with 80% Cu) at room temperature (22 °C) within
ours. Complete kills of the pathogens were observed within
hours. These tests were performed
under wet exposure conditions. The kill timeframes, while impressive, are nevertheless longer than for
pure copper, where kill timeframes ranged between 45 t
o 90 minutes.
that mimics dry bacterial exposure to touch surfaces was developed because this test
d is thought to more closely replicate real world touch surface exposure conditions. In these
conditions, copper alloy surfaces were found to kill several million Colony Forming Units of
This observation, and the fact that kill timeframes shorten as the percentage of
copper in an alloy increases, is proof that copper
is the ingredient in brass and other copper alloys that
kills the microbes.
The mechanisms of antimicrobial action by copper and its alloys, including brass, is a subject of inten
and ongoing investigation.
It is believed that the mechanisms are multifaceted and include the
hrough the outer membrane of bacteria;
balance disturbances; 3) Binding to
that do not require or util
ize copper; 4)
Research is being cond
ucted at this time to determine whether brass, copper, and other copper alloys can
help to reduce cross contamination in public facilities and reduce the incidence of
(hospital acquired infections) in healthcare facilities.
20 MARK QUESTION
1.Beneficial properties of copper for electrical wire and cable
Nearly all electrical devices rely on copper wiring because of its multitude of inherent b
properties. The most useful beneficial properties for electrical applications are summarized here.
is a measure of how wel
l a material transports an
This is an
essential property in electrical
wiring systems. Copper has the highest electrical conductivity rating of all
(electrical conductivity of copper = 101% IACS (International Annealed Copper
Standard); electrical resi
stivity of copper = 16.78 nΩ•m at 20 °C).
Free Electronic (OFE)
achieves a minimum of 101% IACS.
The solid state theory of metals
helps to explain the unusually high electrical conductivity of copper.
In a copper
, the outermost 4s energy zone, or
, is only half filled, so
are able to carry
. When an
is applied to a copper wire, the
conduction of electrons accelerates towards the
end, thereby creating a current. These
electrons encounter resistance to their passage by colliding with impurity atoms, vacancies, lattice ions,
and imperfections. The
average distance travelled between collisions, defined as the “
mean free path
inversely proportional to the resistivity of the metal. What is unique about copper is its lo
ng mean free
path (approximately 100 atomic spacings at room temperature). This mean free path increases rapidly as
copper is chilled.
Because of its superior conducti
copper became the international standard to which all other
electrical conductors are compared. In 1913, the
International Electrotechnical Commission
conductivity of copper in its International Annealed Copper Standard (IACS) to 100%. Today, copper
The main grade of copper used for electrical applications, such as building wire,
tough pitch (ETP) copper
designation C100140). This
copper is at leas
t 99.90% pure and has an electrical conductivity of at least 101% IACS. ETP copper
contains a small percentage of
(0.02 to 0.04%). If high conductivity copper needs to
or used in a reducing atmosphere, then
free high conductivity
(CW008A or ASTM designation C10100) may be used.
Several electrically conductive metals are lighter than copper, but since they
require larger cross sections
to carry the same current, they are unacceptable when limited space is a major requirement.
Aluminium has 61% of the conductivity of copper.
The cross sectional area of an aluminium conductor
must be 56% larger than
copper for the same current carrying capability.
The need to increase the
restricts its use in several applications,
such as in small motors and
automobiles. In some applications such as aerial
electric power transmission
cables, thickness is an
advantage and copper is rarely used. Though more expensive, some power companies use aluminum in
high amperage / voltage scenario
s because it is less susceptible to "brown" like copper will if over
hot (be ruined, non
conductive, need replacing). Aluminum routing also needs careful heat dissipation
because it melts at far lower temperatures.
, is the only metal with a higher electrical conductivity than copper. The electrical
conductivity of silver is 106% of that of annealed copper on the IACS scale, and the electrical resist
of silver = 15.9 nΩ•m at 20°C.
The high cost of silver combined with its
its use to special applications, such as joint plating and sliding contact surfaces.
measures the force required to pull an object such as rope, wire, or a structural beam to
the point where it breaks. The tensile strength of a material is the maximum amount of tensile stress it c
take before breaking.
Copper’s higher tensile strength (200
annealed) compared to aluminium is another reason
why copper is used extensively in the buildi
ng industry. Copper’s high strength resists stretching, neck
down, creep, nicks and breaks, and thereby also prevents failures and service interruptions.
installations and machinery using non
copper wiring, nicks and scratches due to vibration
and flexing can deteriorate into large breaks in the wiring and lead to failure and long
For example, when long runs of aluminium are pulled through conduit and cable trays,
they can stretch and neck
down. These effects reduce current carrying capacity, wastes energy, and can
cause overheating. Because o
f copper’s higher tensile strength, these problems are minimized in copper
is a material's ability to
. This is often char
acterized by the material's
ability to be stretched into a
. Ductility is especially important in
e materials that
crack or break under stress cannot be hammered, rolled, or drawn (drawing is a process that uses tensile
forces to stretch metal).
Copper has a higher ductility than alternate metal conductors with the exception of gold and silver, both
xpensive precious metals reserved for highly specialized wiring applications.
Because of copper’s
high ductility, it is easy to draw down to diameters with very close
Strength and ductility combination
Usually, the stronger a metal is, the less pliable it is. This is not the case with copper. A unique
combination of high
strength and high ductility makes copper ideal for wiring systems. At junction boxes
and at terminations, for example, copper can be bent, twisted, and pulled without stretching or breaking
is the gradual deformation of a material from constant expansions and contractions under “load, no
load” conditions. This process has adverse effects on electrical systems: terminations can become loose,
ing connections to heat up or create dangerous arcing.
Copper does not creep or loosen at its connections. For other metal conductors that creep, extra
maintenance is required to check terminals periodically and ensure that screws remain tightened to
nt arcing and overheating. These extra measures can be avoided with the use of copper wire.
is the unwanted breakdown and weakening of a material due to chemical reactions.
resists corrosion from moisture, humidity, industrial
pollution, and other atmospheric influences.
However, any corrosion oxides, chlorides, and sulfides that do form on copper are conductive. Therefore,
copper connections and terminations will not overheat from corrosion. Aluminium corrosion products, on
e other hand, are resistive and therefore can cause unwanted heat. To prevent corrosion and protect
joints, special surface preparations or oxide
inhibiting pastes are applied to aluminium. Copper
connections do not require these preparations and their ass
ociated additional costs.
tough pitch (ETP) copp
, which is used in building wire, is a noble metal
. It is
not subject to galvanic corrosion when connected to other, less noble metals and alloys
Coefficient of thermal expansion
Metals and other solid materials expand upon heating and contract upon cooling.
occurrence in electrical systems. Copper has a low coefficient of
for an electrical
conducting material. Aluminium, an alternate common
conductor, expands nearly one third more than
copper under increasing temperatures. This higher degree of expansion, along with aluminium’s lower
ductility can cause electrical problems when bolted connections are improperly installed. By using proper
ware, such as spring pressure connections and cupped or split washers at the joint, it may be possible
to create aluminium joints that compare in quality to copper joints.
is the ability of a material to conduct heat.
In electrical systems, high thermal
conductivity is important for dissipating waste heat, particularly at terminations and connections. It is also
important for reducing energy consumption due to the generation of waste heat.
Copper has a
60% better thermal conductivity rating than aluminium,
so it is better able to reduce
thermal hot spots in electrical wiring systems.
is a process whe
reby two or more metals are joined together by a heating process.
sirable property in electrical systems. Some electrical codes require soldered joints.
readily soldered to make durable connections when necessary.
Ease of i
The inherent strength, hardness, and flexibility of copper building wire make it very easy to work with.
Copper wiring can be installed simply and easily with no special tools, washers, pigtails, or joint
compounds. Its flexibility makes it eas
y to join, while its hardness helps keep connections securely in
place. It has good strength for pulling wire through tight places (“pull
through”), including conduits. It
can be bent or twisted easily without breaking. It can be stripped and terminated du
ring installation or
service with far less danger of nicks or breaks. And it can be connected without the use of special lugs
and fittings. The combination of all of these factors makes it easy for electricians to install copper wire.
2.Types of copper wi
re and cable
Solid vs. stranded
strand copper wire, also called solid wire or solid
core wire, consists of one piece of copper metal
wire surrounded by an insulator.
strand copper conductors are typically used as magnet wire in
motors and transformers. They are relatively rigid, do not bend easily, and are typically installed in
permanent, infrequently handled, and low flex applications. Single strand copp
er wires also provide
mechanical ruggedness and good protection against the environment.
A stranded copper wire refers to a group of copper wires that are braided or twisted together. A stranded
copper wire is more flexible and easier to install than a sin
strand copper wire of the same cross
section. Stranding also improves wire longevity for applications with moderate to high vibration. A
section of a stranded conductor gives it essentially the same resistance characteristics as a
strand conductor, but with added flexibility.
A copper cable consists of two or more copper wires running side by side and bonded, twisted or braided
to form a single assembly.
Electrical cables may be made more flexible by stranding the wires.
Copper wires in a cable may be bare or they may be plated to reduce
with a thin layer of
another metal, most often
. Plating may lengthen wire life and
Cables can be made with one or two differen
t types of wire. For example, all
copper cables are used in a
wide range of applications, including
. Combination conductor
cables, such as copper and
, are used when increased strength with high conductivity is required over
(e.g., several hundred
meter spans), such as for telephone cables or for thin hookups,
Some cables are designed to be multi
functional, such as those installed in residences to carry power,
telephone, video, and control/communications signal
They are usually made from copper.
carrying cables radiate an
. Cables also pick up energy from any existing
electromagnetic fields that are around it. These effects are often undesirable, in the first case amounting to
unwanted transmission of energy which may adversely affect nearby equipment o
r other parts of the same
piece of equipment; and in the second case, unwanted pickup of noise which may mask the desired signal
being carried by the cable, or, if the cable is carrying power supply or control voltages, cause equipment
Three principal cable designs (shielding, twisted
pair geometry, and coaxial geometry) help to minimize
electromagnetic pickup and transmission.
ables are encased in foil or wire mesh. The wires inside the shielding are mostly decoupled
. Simple shielding is not too effective against low
resulting, for example, in a magnetic "hum" from a nearby power transformer.
Twisted pair cables
cabling is a type of wiring in which two conductors (the forward and return conductors of a
single circuit) are twisted together to cancel out electromagnetic interference (EMI) from external sources
and reduce signal loss
This is why twisted pairs have been used in telephone communications for
y decades. For further information regarding the application of twisted pair cables in communication
frequency magnetic transmission and pickup.
They consist of two or more
wires that are wrapped
and separated by a
lation material. The term, coaxial,
was coined because the center conductor and the outer conductor, or shield, form concentric
induced by a magnetic field between the shield and the core conductor to consist of
two nearly equal magnitudes which cancel out each other.
The center conductor of a coaxial cable may
be a single strand or it may be stranded.
Common conductor materials used in coaxial cables include copper, tinned or silver plated
. Less freque
ntly, aluminium is used as an alternate
The outer conductor is typically made from a woven copper wire mesh braid shield
layer, or less frequently, al
uminium foil. This layer also gives the wire protection from
The cables are insulated with a flexible, tubular insulating layer made
fluorinated ethylene propylene
The advantage of coaxial design is that the electric and magnetic fields are confined to the
little leakage outside the shield. Con
versely, electric and magnetic fields outside the cable are largely kept
from causing interference to signals inside the cable. This property makes coaxial cable a good choice for
carrying weak signals that cannot tolerate interference from the environment
or for higher power signals
that must not be allowed to radiate or couple into adjacent structures or circuits.
tough pitch (ETP) copper
, a high
purity copper that contains
represents the bulk of
applications because of its high
ETP copper is used for
Common applications include building wire, motor windi
are used to resist
when extensive amounts
eded, and for applications requiring higher
When hydrogen embrittlement is a concern and low electrical resistivity is not
may be added to copper.
For certain applications, copper alloy conductors are preferred instead of pure copper, especially when
higher strengths or improved
resistance properties are required. An example of a
copper alloy conducto
wire, which is used for
However, relative to pure copper, the hi
gher strength and corrosion resistance benefits that
are offered by copper alloys are offset by their lower electrical conductivities. Design engineers weigh the
advantages and disadvantages of the various types of copper and copper alloy conductors when
etermining which type to specify for a specific electrical application.
Some of the major application markets for copper wire and cable are summarized below.
3.Types of stainless steel
Stainless steel does not
or stain with water as ordinary steel does, but despite the name it is
not fully stain
proof, most notably under low oxygen, high salinity, or poor circulati
is also called
when the alloy type and grade are not detailed,
particularly in the aviation industry. There are different
grades and surface finishes of stainless steel to
suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and
resistance to corrosion are required.
Stainless steel differs from carbon steel by the amount of c
hromium present. Unprotected carbon
readily when exposed to air and moisture. This
film (the rust) is
accelerates corrosion by forming more iron oxide, and due to the dissimilar size of the iron and iron oxide
molecules (iron oxide is larger) these tend to flake and fall away. Stainless steels contain sufficient
chromium to form a passive film
of chromium oxide, which prevents further surface corrosion and blocks
corrosion from spreading into the metal's internal structure, and due to the similar size of the steel and
oxide molecules they bond very strongly and remain attached to the surface
ere are different types of stainless steels: when
is added, for instance, the austenite structure of
iron is stabilized. This crystal structure makes such steels virtually non
temperatures. For greater
and strength, more
is added. With proper
razor blade steels
are used for such things as razor blades, cutlery, and tools.
Significant quantities of
have been used in many stainless steel compositions. Manganese
preserves an austenitic structure in the steel, similar to nickel, but at a lower
Stainless steels are also c
lassified by their
, or 300 series, stainless steels have a
n austenitic crystalline structure, which is a
crystal structure. Austenite steels make up over 70% of total stainless steel production.
ain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel
and/or manganese to retain an austenitic structure at all temperatures from the
region to the
ting point of the alloy. The most widely used austenite steel is the
grade or A2 stainless
not to be confused with A2 grade steel, also named
, a steel). The second most
common austenite steel is the
grade, also called marine grade stainless, used primarily for its
increased resistance to corrosion. A typical composition of 18% chromium and 10% nickel,
commonly known as
, is often used in
are also available.
stainless steels, such as alloy
and 254SMO, exhibit great resistance to
chloride pitting and crevice corrosion because of high
content (>6%) and nitrogen
additions, and the higher nickel content ensures better resistance to stress
versus the 300 series. The higher alloy content of superaustenitic steels makes them more
expensive. Other steels can offer similar performance at lower cost a
nd are preferred in certain
applications, for example ASTM A387 is used in pressure vessels but is a low alloy carbon steel
with a chromium content of 0.5% to 9%.
versions, for example
L or 304L, are
used to avoid corrosion problems caused by welding. Grade 316LVM is preferred
is required (such as body implants and piercings).
The "L" means that
the carbon content of the alloy is bel
ow 0.03%, which reduces the
(precipitation of chromium carbides at grain boundaries) caused by the high temperatures
involved in welding.
stainless steels generally have better engineering properties than austenitic grades, but
have reduced corrosion resistance, because of the lower chromium and nickel conten
t. They are
also usually less expensive. They contain between 10.5% and 27% chromium and very little
nickel, if any, but some types can contain lead. Most compositions include molybdenum; some,
aluminium or titanium. Common ferritic grades include 18Cr
2Ni. These alloys can be degraded by the presence of
chromium, an intermetallic
phase which can precipitate upon welding.
stainless steels are not as corrosion
resistant as the other two classes but are
extremely strong and tough, as well as highly
, and can be hardene
d by heat
treatment. Martensitic stainless steel contains chromium (12
14%), molybdenum (0.2
nickel (less than 2%), and carbon (about 0.1
1%) (giving it more hardness but making the
material a bit more brittle). It is quenched and magnetic.
stainless steels have corrosion resistance comparable to
austenitic varieties, but can be
to even higher stren
gths than the other
martensitic grades. The most common,
, uses about 17% chromium and 4% nickel. The
Joint Strike Fighter
is the first aircraft to use a precipitation
Carpenter Custom 465
in its airframe.
stainless steels have a mixed microstructure of austenite and ferrite, the aim usually being
to produce a 50/50 mix, although in commercial allo
ys the ratio may be 40/60. Duplex stainless
steels have roughly twice the strength compared to austenitic stainless steels and also improved
resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion
cracking. They are
characterized by high chromium (19
32%) and molybdenum (up to 5%) and
lower nickel contents than austenitic stainless steels.
The properties of duplex stainless steels are achieved with an overall lower alloy content than
c grades, making their use cost
effective for many applications.
Duplex grades are characterized into groups based on their alloy content and corrosion resistance.
refers to grades such as
S32101 (LDX 2101), S32304, and S32003.
is 22% chromium with UNS S31803/S32205 known as 2205 being the most
is by definition a duplex stainless steel with a pitting c
(PRE) > 40, where PRE =
%Cr + 3.3x(%Mo + 0.5x%W) + 16x%N. Usually super duplex
grades have 25% chromium or more and some common examples are S32760 (
0 (2507) and S32550 (Ferralium), although not all Ferralium grades are super duplex
refers to duplex grades with a PRE > 48 and at the moment only UNS S32707
and S33207 are available on the market.
Stainless steel grades
There are a nu
mber of different systems for
grading stainless and other steels
. The article on US
details a large number of grades with their properties.
Stainless steel in 3D printing
providers have developed proprietary stainless steel
blends for use in rapid
prototyping. Currently available grades do not vary significantly in their properties.
, also called
where the main interstitial
American Iron and Steel Institute
(AISI) defines carbon steel as the follo
wing: "Steel is
considered to be carbon steel when no minimum content is specified or required
, or any
other element to be added to obtain a desired alloying effect; when the specified minimum for copper
does not exceed 1.04 percent; or when the maximum content specified for any of the following elements
does not exceed th
e percentages noted:
The term "carbon steel" may also be used in reference to steel which is not
; in this use
carbon steel may include alloy steels.
As the carbon content rises, steel has the ability to become
this also makes it less
dless of the heat treatment, a higher carbon content
. In carbon steels, the higher carbon content lowers the melting point
4.Types of carbon steel
Carbon steel is
broken down in to four classes based on carbon content:
Mild and low carbon steel
Mild steel is the most common form of steel because its price is relatively low while it provides material
properties that are acceptable for many applications. Low carbon s
teel contains approximately 0.05
and mild steel contains 0.16
ing it malleable and ductile, but it
cannot be hardened by heat treatment. Mild steel has a relatively low tensile strength, but it is cheap and
malleable; surface hardness can be increased through
It is often used when large quantities of steel are needed, for example as
. The density of
mild steel is approximately 7.85
GPa (30,000,000 psi).
Low carbon steels suffer from
where the material has two
. The first yield
point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield
point. If a low carbon steel is only stressed to some point between th
e upper and lower yield point then
the surface may develop
Low carbon steels contain l
ess carbon that other steels and are
easier to cold
form, making them easier to handle.
Higher carbon steels
Carbon steels which can successfully undergo heat
treatment have a ca
rbon content in the range of 0.30
1.70% by weight. Trace impurities of various other
can have a significant effect on the quality
of the resulting steel. Trace amo
in particular make the steel
. Low alloy carbon
steel, such as
grade, contains about 0.05% sulfur and melts around 1426
1538 °C (2599
is often added to improve the
of low carbon steels. These additions
turn the material into a
low alloy steel
by some definitions, but
's definition of carbon steel allows up
to 1.65% manganese b
Medium carbon steel
0.59% carbon content.
Balances ductility and strength and has good wear
resistance; used for large parts, forging and automot
High carbon steel
0.99% carbon content.
Very strong, used for
springs and high
high carbon steel
2.0% carbon content.
Steels that can be tempered to great hardness. Used for special
purposes like (non
purpose) knives, axles or
. Most steels with more than 1.2% ca
content are made using
. Note that steel with a carbon content above 2.0% is
The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility,
hardness, yield strength, or impact resistance. Note that the electrical and thermal conductivity are slightly
altered. As with most strengthening techniques for steel,
is unaffected. Steel has a higher
solid solubility for carbon in the
phase; therefore all heat treatments, except spheroidizing and
process annealing, start by heating to an austenitic phase. The rate at which the steel is cooled through
reaction affects the rate at which carbon diffuses out of austenite. Generally speaking,
cooling swiftly will give a finer
critical temperature is reached) and cooling
slowly will give a coarser pearlite. Cooling a hypoeutectoid (less than 0.77 wt% C) steel results in a
pearlitic structure with
at the grain boundaries. If it is hypereutectoid (more than 0.77 wt% C)
steel then the structure is full pearlite with small grains of
scattered throughout. The relative
amounts of constituents are found using the
. Here is a list of the types of heat treatme
: Spheroidite forms when carbon steel is heated to approximately 700
°C for over
30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this
is a diffusion
controlled process. The resul
t is a structure of rods or spheres of cementite within
primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The
purpose is to soften higher carbon steels and allow more formability. This is the softest and most
tile form of steel. The image to the right shows where spheroidizing usually occurs.
: Carbon steel is heated to approximately 40
°C above Ac
for 1 hour; this
assures all the
might still exist if the carbon
content is greater than the eutectoid). The steel must then be cooled slowly, in the real
m of 38°C
(68.4°F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still
inside. This results in a coarse pearlitic structure, which means the "bands" of
are thick. Fully
annealed steel is soft and
, with no internal stresses, which is often necessary for cost
forming. Only spheroidized steel is softer and m
: A process used to relieve stress in a cold
worked carbon steel with less than
wt% C. The steel is usually heated up to 550
°C for 1 hour
, but sometimes temperatures as
high as 700
°C. The image rightward shows the area where process annealing occurs.
: It is a process in which hypoeutectoid steel is heated above the upper
critical temperature and this temperature is mai
ntained for a time and then the temperature is brought
down below lower critical temperature and is again maintained. Then finally it is cooled at room
temperature. This method rids any temperature gradient.
: Carbon steel is heated to approxima
°C above Ac
for 1 hour; this
assures the steel completely transforms to austenite. The steel is then air
cooled, which is a cooling
rate of approximately 38
°C (100.4 °F) per minute. This results in a fine pearlitic structure, and a
iform structure. Normalized steel has a higher strength than annealed steel; it has a relatively
high strength and ductility.
: Carbon steel with at least 0.4
wt% C is heated to normalizing temperatures and then
rapidly cooled (quenched) in water, brine, or oil to the critical temperature. The critical temperature is
dependent on the carbon content, b
ut as a general rule is lower as the carbon content increases. This
results in a martensitic structure; a form of steel that possesses a super
saturated carbon content in a
centered cubic (BCC) crystalline structure, properly termed body
(BCT), with much internal stress. Thus quenched steel is extremely hard but
, usually too brittle
for practical purposes. These internal stresses cause stress cracks o
n the surface. Quenched steel is
approximately three to four (with more carbon) fold harder than normalized steel.
: Martempering is not actually a tempering procedure, hence the
term "marquenching". It is a form of isothermal heat treatment applied after an initial quench of
typically in a molten salt bath at a temperature rig
ht above the "martensite start temperature". At this
temperature, residual stresses within the material are relieved and some bainite may be formed from
the retained austenite which did not have time to transform into anything else. In industry, this is a
process used to control the ductility and hardness of a material. With longer marquenching, the
ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and
outer temperatures equalize. Then the steel is coole
d at a moderate speed to keep the temperature
gradient minimal. Not only does this process reduce internal stresses and stress cracks, but it also
increases the impact resistance.
Quench and tempering
: This is the most common heat treatment encountered, because the final
properties can be precisely determined by the temperature and time of the tempering. Tempering
involves reheating quenched steel to a temperature below the
temperature then coo
elevated temperature allows very small amounts of spheroidite to form, which restores ductility, but
reduces hardness. Actual temperatures and times are carefully chosen for each composition.
: The austempering process is the same as martempering, except the steel is held in
the molten salt bath through the bainite transformation te
mperatures, and then moderately cooled.
The resulting bainite steel has a greater ductility, higher impact resistance, and less distortion. The
disadvantage of austempering is it can only be used on a few steels, and it requires a special salt bath