Stainless Steel Essay

This essay has a total of 4039 words and 24 pages.

Stainless Steel



“Metal is the material of our time. It enables architecture to become sculpture; it also
expresses technological possibility as well as the time-honored characteristics of quality
and permanence.”

Frank O. Gehry (4)
This quotation is a categorical description of stainless steel. Superiority in mechanical
properties, distinct characteristics, and variety of options make stainless steel a
preeminent choice in material selection. Designers, architects, artisans and others alike
have all found applications for this elemental composite. The discovery, as well as its
discoverer, is as equally interesting.

Having left school at the age of twelve, Harry Brearley took up the job of a bottle washer
in a Sheffield, England chemical laboratory. Through years of self-teaching, he quickly
earned a reputation as an expert in the analysis of metallurgical problems. He later
became known as the inventor of stainless steel.

In 1912, Brown Firth Laboratories sought to develop a steel for the production of rifles
in which the thin diameter of the barrel would not erode away. This erosion was in
reaction to the extreme heat and discharged gasses created in firing. Brearley was
quickly employed and began to experiment with Chromium mixtures to reduce the atomic
weight of steel so as to allow for a denser barrel. To determine the resistance to wear,
he needed to etch samples of the new alloy with an acid to examine the grain structure.
Finding a strong resistance to even nitric acid, Brearley realized the significance of his
new mixture. Independent from his employers, he had cutlery knives formed from his
“rustless steel”. A friend and local chef found that the new knives not only were
rustproof, but also remained unstained by vinegar and dubbed them “stainless steel”. (6)
Since then, experiments with the amounts of Chromium added have expanded to numerous
grades and applications.


The process of creating steels, stainless or any of its other forms begins with the basic
material of Iron ore. Iron ore is a naturally occurring resource, comprised of iron,
oxygen, sulfur and silicon. The iron is refined in a blast furnace, using coke (a porous
form of carbon), limestone and heated air as a catalyst. In the refining process, the
carbon in the coke absorbs the oxygen from the raw ore while the limestone separates the
silicates. The ending product is an iron-iron carbide, also known as raw steel.(5)

Three distinct forms of iron are discussed in the manufacturing of steel products. These
are ferrite, austenite and cementite and are essentially iron crystals formed during the
cooling process. Cooled the quickest, cementite has the largest grain crystals and is the
hardest, followed by austenite and the softest, ferrite. Manipulating the cooling process
allows the creation of varying amounts of the different crystal types; this results in a
wide array of different steel products and the properties unique to each.

There are five main classifications of steel. Closely related, the first three are low
carbon, medium carbon, and high carbon. Low carbon steel, contains less the 0.2% carbon
by weight. This steel is available in rolled sheets and is often “stamped” into a desired
shape, such as car bodies. The next class is medium carbon steel, with 0.4% carbon
content by weight. This form of steel has an equal composition of ferrite (soft) and
cementite (hard) iron. A popular manifestation of this type of steel is pearlite, know
for its iridescent finish. The austenite is formed in pearlite during a more rapid
cooling process, making it a sub-category of medium steel. High carbon steel, the third
classification, contains nearly 0.8% carbon. While extremely durable, it is also
difficult to form and is often used it railroad rails and spikes.

Manipulating the cooling process of medium carbon steel creates the fourth category of
steel, marensite. The creation process is similar to that of pearlite, except that the
austenite is cooled too rapidly to allow it to fully dissolve and become pearlite. The
large crystal structure lends marensite extreme hardness, but this also means it is
brittle and has very little impact resistance. This disadvantage can be addressed by
locally tempering the metal. Applying a heat source, such as a torch, and immersing in
cool water repeatedly is often necessary for steel tools. A chisel tip or leading edge
needs to be hard, but the remainder needs to retain impact resistance. Tempering has been
a staple for blacksmiths for centuries, and allows marensite to be the predominate steel
in producing metalworking tools.

The final category of steel is the alloy steels. By adding other elements, steel can be
manufactured to contain virtually any desired properties and results in the creation of
countless patented subcategories. Alloy steel must contain at maximum 1.65% manganese,
0.6% silicon, and 0.6% copper by weight to be considered within this classification,
though other elements may be added. U.S. Steel Corporation has recently developed
Cor-Ten, comprised additionally of Cr-Si-Cu-Ni-P molecules to offer a highly durable and
corrosive resistant structural steel. (1) Further manipulation of the elemental compounds
result in a product custom designed for any environmental conditions, be it salt water,
highly acidic atmospheres or cryogenic temperatures.

The most recognized type of alloy steel is stainless steel. Stainless steel is produced
by the addition of 10-20% of both nickel and chromium in late stage refining. Due to the
multitude of manipulative factors of the steel making procedure, including cooling process
and elemental make-up, innumerable varieties of stainless steel are available. Three
different systems are used in the nomenclature of these products. (12) The first system
utilizes the metallurgical structure-referring to the dominate iron present-ferrite,
austenite, cementite or duplex (equal ferrite and austenite). The second system, Grade,
is the most commonly used. This method creates a table listing the various stainless
steels in order of elemental content by weight. For example, Grade 404 would have a
higher content of carbon, manganese, silicon and copper (among others) than would Grade
403. The third system, Unified Numbering System (UNS) is similar to the grade method, but
allows for the inclusion of products between the grade system. Grade 404 in UNS is s40400
and grade 403 is s40300, which allows for subtle variations between the two. The most
common stainless steels used are those in the austenitic classification, particularly
grades 304 and 316.

Stainless steel is a readily available material in most of its grades. Sizes produced
depend on the grade requested. Sizes do vary between manufactures and can often be
fabricated to custom sizes. For sheets, specifications are made regarding thickness,
width and length. (See next page for sheet sizes commonly available in various UNS
grades) (10)







Hot Rolled Plate Product Program

Hot Rolled Quarto Plate Size Range For Standard
Grades
MM 1000 13751500 2000 2032 25002600 2800 3000 31003200
5.00 1 1 1 1 1 1
6.00 1 1 1 1 1 1 1
7.00 1 1 1 1 1 1 1 1 1 1 1
8.00 1 1 1 1 1 1 1 1 1 1 1
9.00 1 1 1 1 1 1 1 1 1 1 1
10.00 1 1 1 1 1 1 1 1 1 1 1
50.00 1 1 1 1 1 1 1 1 1 1 1
100.00 1 1 1 1 1 1 1 1 1 1 1
105.00 1 1 1 1 1 1 1 1 1 1 1
Surface Finish:
1 = Hot Rolled, Annealed and Pickled
Note: Grades, sizes and finishes outside this standard
program, as well as plate, edge prepared for welding can
be delivered by agreement.
Courtesy Avesta Sheffield Steel (10)

Rolls of stainless steel are available in various thicknesses and commonly widths of
1000mm, 1250mm, and 1500mm. Custom sizes are also available through many steel
manufactures.


Thermal expansion is the most important structural factor to consider when working with
stainless steel. Since the conductivity (ability to transfer heat evenly over the entire
metal object) is low, welding during installation causes huge localized temperature
increases, which leads to surface distortion or warping, as well as a weakening of the
area. Compensating for this tendency involves placing copper or aluminum bars around the
welded area to transfer the heat away from the surface, and basic measures such as using
the minimum welding amperage required for a consistent weld. (7)

Furthermore, architectural applications with long runs of stainless steel, such as roof
tops, uneven heating over the surface can result in disproportionate expansion and
buckling. Expansion joints every 7-12 meters at least 6mm thick are recommended to avoid
any potential structural failure. A similar method must also be used in piping system to
prevent rupture. Flexible joints or ball joints are employed at the end of stainless
steel pipes, since gaps in the run of the pipe are not practical.

For exact thermal expansion rates of various grades, see table below.
Typical physical properties - Annealed condition
Mean Coefficient of Thermal
Thermal Expansion (b) Conductivity Grade Elastic
or UNS Density Modulus
type No. (kg/m3) (a) 0-100°C 0-315°C 0-538°C At At
GPa 100°C 500°C
µm/m/°C µm/m/°C µm/m/°C W/m.K W/m.K
302 S30200 8000 193 17.2 17.8 18.4 16.2 21.5
302B S30215 8000 193 16.2 18.0 19.4 15.9 21.6
303 S30300 8000 193 17.2 17.8 18.4 16.2 21.5
304 S30400 8000 193 17.2 17.8 18.4 16.2 21.5
304L S30403 8000 193 17.2 17.8 18.4 16.3 21.5
304N S30451 8000 196 17.2 17.8 18.4 16.3 21.5
314 S31400 7800 200 - 15.1 - 17.5 20.9
316 S31600 8000 193 15.9 16.2 17.5 16.2 21.5
316N S31651 8000 196 15.9 16.2 17.5 14.4 -
317 S31700 8000 193 15.9 16.2 17.5 16.2 21.5
317L S31703 8000 200 16.5 - 18.1 14.4 -
321 S32100 8000 193 16.6 17.2 18.6 16.1 22.2
409 S40900 7800 200 11.7 12.0 12.4 24.9 -
410 S41000 7800 200 9.9 11.4 11.6 24.9 28.7
416 S41600 7800 200 9.9 11.0 11.6 24.9 28.7
430 S43000 7800 200 10.4 11.0 11.4 26.1 26.3
430F S43020 7800 200 10.4 11.0 11.4 26.1 26.3
431 S43100 7800 200 10.2 12.1 - 20.2 -
434 S43400 7800 200 10.4 11.0 11.4 - 26.3
631 S17700 7800 204 11.0 11.6 - 16.4 21.8
(a) 1GPa = 1000MPa
(b) µ/m/°C = x 10-6/°C
(c) 1% flow in 10,000 hours at 540°C
Courtesy ASSDA (7)


If dimensional strength is a deciding factor in material selection, steels are oft the
chosen option. Austenetic grades of stainless steel can be hardened though cold-rolling.
(4) This involves rolling newly formed sheet metal between cold drums to reduce the
thickness required to achieve the desired strength. The creation of the duplex grades
offers yet another option in strength-to-weigh materials. “Yield” strength is a
measurement of pounds of pressure per square inch (psi), and indicates the mass that can
be supported without any damage to the metal. Annealing during the manufacturing phase
can increase the weight a grade of steel can support. This process involves heating metal
to a specific temperature, holding that temperature for an extended period of time, and
then slowly cooling. Below is a table showing yield strength for popular grades of
stainless steel, as well as “ultimate strength”, or the maximum supportable mass before
rupture.

Grade Yield Strength Ultimate Strength

304 30,000psi 70,000psi
316 25,000psi 70,000psi
201 38,000psi 95,000psi
401 32,000psi 60,000psi
430 35,000psi 60,000psi
(4)

Since several processes are used to accomplish strength ratings, the manufacturer or
distributor should be consulted to create the selected grade with an appropriate psi
capacity.

Resistance to pollution and moisture corrosion is another appealing characteristic of
stainless steel. Rust, an oxide formed from a chemical reaction between Carbon and
Oxygen, is a familiar sight on steel; in stainless steel, Chromium in the steel mixture
reacts with oxygen present in the atmosphere, forming an oxide barrier. (7) The Chromium
essentially “rusts” first. This thin molecular covering actually prevents oxygen from
coming in contact with the Carbon in the steel. This invisible barrier also prevents
other corrosive agents, such as acid rain, from corroding the metal. Note that the oxide
barrier can be compromised by chloride solutions. The chlorine in tap water and washing
detergents is responsible for “pitting” in stainless steel cookery. Pitting, small
indentations in the surface, is a sign that the chromium-oxide barrier has been dissolved
in a localized area, but is purely superficial. (7)

Any damage to the oxide barrier is quickly self-repaired. With the now exposed metal,
Chromium again reacts with oxygen and forms a new protective coating. Contaminates, such
as oil, grease, or particles of standard steel on the metal surface can sometimes
interfere with this process. Pickling is an acid cleaning process that removes this
debris, but the chemicals involved include strong hydrofluoric acids and must be used with
great care. Reforming of the chromium-oxide usually takes 24 hours, but can be returned
instantaneously by the application of nitric acid to serve as a catalyst.

Since the Great Chicago Fire of 1871, fire resistance has been an important factor in
material selection. The wood frames of the closely spaced structures ignited and spread
the flames further until the city was in ashes. The only buildings left were those
constructed of steel. Steel will melt, but can not ignite, which prevents steel from
conducting any flames to nearby structures. Low conductivity of steel also slows the
transfer of heat to adjoining levels in a structure and can actually contain a blaze
within its walls.

Fire does still pose a threat to steel and stainless steel none the less. The yield
strength of most steel is greatly compromised at 500 0 C. (2) However, measures can be
taken to increase both the temperature and amount of time endured before structural
failure. Critical temperature can be extended to 600 C by using only 50% of the load
bearing strength, for example.

Increases in steel temperature can be slowed by several methods. Spraying with substances
such as vermiculite or mineral fiber plaster, while messy, can be applied at the
installation site to any steel requiring additional fire protection. In the 1970’s,
asbestos materials were used for this purpose, until it was determined to be a
cancer-causing agent.

Steel columns can be wrapped in intumescent materials as an alternative to spraying.
Intumescent coverings, while normally invisible, swell in a fire and absorb the heat,
preventing the steel from reaching destabilizing temperatures.

Though steel members can be damage in a fire, they can often be repaired or replaced
without damaging the overall building stability. Additionally, steel will not expel any
toxic fumes in a fire of any severity. (7)


Due to the vast types and applications of stainless steel, numerous installation methods
are available for designers. Stainless is often cut or formed to the required size and
shape. (5) In thin sheets, the desired form can be stamped or bent to the desired shape,
reducing the amount of installation attachments required. Sinks are commonly formed of
grade 316 stainless steel in this method and can be easily installed by a plumber. In the
case of structural steel applications, beams are cut to length during fabrication; this
reduces cutting and waste at the job site. Since only the required amount is shipped, the
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