Stainless Steel Essay, Research Paper
?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
?m/m/?C ?m/m/?C ?m/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) ?/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 transportation costs are reduced. After the materials arrive at the construction site, the pre-fabricated pieces are erected and pre-drilled holes are used for secure connections.
With the wide array of stainless steel applications, an understanding of the available attachment methods is more appropriate than an understanding of the installation process of each.
Welding is a common method for attaching two metal sheets at a joint. Arc welding melts the adjacent sheets and allows the steel to intermingle and form a bond as strong as the steel itself. Conversely, soldering uses a second type of metal to form the connection. While requiring less heat and less skilled labor, the filler metal is often weaker than the steel and its lower yield strength must be taken into consideration.
As discussed earlier, welding temperatures can cause warping in stainless steel and is not always the best option. Dozens of other methods exist for a designer to choose among. Rivets are popular when high strength is needed, but the necessary equipment is quite heavy and difficult to use. Rivets are usually limited to structural applications. More delicate work utilizes various machine screws, bolts, or large capscrews. Pre-drilled holes make these attachment methods simple to use. Special epoxy adhesives can also be employed, as well as a joint-and-seam system. In the joint-and-seam method, the end of the first piece of sheet metal is bent or curved. The second sheet is shaped to the opposite bend and fitted together. Solder can be added for additional strength.
As stated earlier, applications for stainless steel span from its beginnings in cutlery and cookware to surgical scalpels. Stainless is the preferred material in many health-care applications because it is non-porous and does not harbor bacteria (7) Due to its durability, harsh chemical disinfectants can be used without damage to the surface. Possibilities extend from the familiar kitchen sinks to works of art, as show below.
Courtesy RPArt (11)
Architects often employ stainless steel as structural adornment, not only for the durability of the finish, but its aesthetic qualities as well. In fact Petronas Twin Towers, the tallest building in the world, is one such structure. Notice a majority of the fa?ade is stainless steel.