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What is Stainless Steel ?

Stainless steel is essentially low carbon steel that contains chromium at 10% or more by weight. It is this addition of chromium that gives the steel its unique stainless, corrosion resisting properties.

The Chromium content of the steel allows the formation of a rough, adherent, invisible, corrosion-resisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen.

There are more than 60 grades of stainless steel. However, the entire group can be divided into five classes. Each is identified by the alloying elements which affect their microstructure and for which each is named.

Development
photo of Harry Brearley

The inventor of stainless steel, Harry Brearley, was born in Sheffield, England in 1871. His father was a steel melter, and after a childhood of considerable hardship, he left school at the age of twelve to get a job washing bottles in a chemical laboratory. By ears of private study and night school he became an expert in the analysis of steel and its production. Having already established his reputation for solving metallurgical problems, Brearley was given the opportunity in 1908 to set up the Brown Firth Laboratories, which was financed by the two leading Sheffield steel companies of the day. This was a highly innovative idea for its time; research for its own sake on the problems of steel making.

In 1912 Brealey was asked to help in the problems being encountered by a small arms manufacturer, whereby the internal diameter of rifle barrels was eroding away too quickly because of the action of heating and discharge gases. Brearley was therefore looking for steel with better resistance to erosion, not corrosion. As a line of investigation he decided to experiment with steels containing chromium, as these were known to have a higher melting point than ordinary steels. Chromium steels were already at that time being used for valves in aero engines. Iron has an atomic weight of 56, chromium 52, so chromium steel valves are lighter than their carbon steel counterparts, another reason why they were adopted so quickly by the emerging aircraft industry.

Using first the crucible process, and then more successfully an electric furnace, a number of different melts of 6 to 15 % chromium with varying carbon contents were made. The first true stainless steel was melted on the 13th August 1913. It contained 0.24% carbon and 12.8% chromium. Brearley at this time was still trying to find more wear-resistant steel, and in order to examine the grain structure of the steel he needed to etch (attack with acid) samples before examining them under the microscope. The etching re-agents he used were based on nitric acid, and he found that this new steel strongly resisted chemical attack. He then exposed samples to vinegar and other food acids such as lemon juice and found the same result.

At the time table cutlery was silver or nickel plated. Cutting knives were of carbon steel that had to thoroughly washed and dried after use, and even then rust stains would have to be rubbed off using carborundum stones. Brearley immediately saw how this new steel could revolutionise the cutlery industry, then one of the biggest employers in Sheffield, but he had great difficulty convincing his more conservative employers. On his own initiative, he had knives made at a local cutler's, R.F. Mosley. To begin with, Brearley referred to his invention as "rustless steel". It was Ernest Stuart, the cutlery manager of Mosley's who first referred to the new knives as "stainless" after in experiments he had failed to stain them vinegar. "Corrosion resisting" steel would be really the better term, as ordinary stainless steels do suffer corrosion in the long term in hostile environments.

Other claims have been made for the first invention of stainless steel, based upon published experimental papers that indicated the passive layer corrosion resistance of chromium steel or patented steels with a 9% chromium content intended for engineering purposes. Brearley's contribution was that having come to a conclusion by purely empirical means he immediately seized on the practical uses of the new material.

Within a year of Brearley's discovery, Krupp in Germany was experimenting by adding nickel to the melt. Brearley's steel could only be supplied in the hardened and tempered condition; the Krupp steel was more resistant to acids, was softer and more ductile and therefore easier to work. There is no doubt that but for Brearley's chance discovery the metallurgists at Krupp would have soon made the discovery themselves. From these two inventions, just before the First World War, were to develop the "400" series of martensitic and "300" series of austenitic stainless steels.

The First World War largely put a halt to the development of stainless steel, but in the early 1920s a whole variety of chromium and nickel combinations were tried including 20/6, 17/7 and 15/11. Brearley fell out with his employers regarding the patent rights to his invention of stainless steel, and he left to join another Sheffield company, Brown Bayleys. His successor at the Brown Firth Laboratories was Dr W.H. Hatfield, who is credited with the invention in 1924 of 18/8 stainless steel (18% chromium, 8% nickel) that, with various additions, still dominates the melting of stainless steel today. Dr Hatfield also invented 18/8 stainless with titanium added, now known as 321.

Most of the standard grades still in use today were invented in the period 1913 to 1935, in Britain, Germany, America and France. Once these standard grades became accepted, the emphasis changed to finding cheaper, mass-production methods, and popularising the use of stainless steel as a concept. This tended to stifle the development of new grades. However, after the Second World War, new grades with a better weight-to-strength ratio were required for jet aircraft, which led to the development of the precipitation hardening grades such as 17:4 PH. From the 1970s onwards the duplex stainless steels began to be developed. These have far greater corrosion resistance and strength than the grades developed in the 1920s and are really the future for the increasing use of stainless steel.

Basic Branches
Stainless Steel Characteristics and Usage by Steel Type
AUSTENITE
CLASSIFICATION CHARACTERISTICS USAGE
301 Has lower Cr, Ni content than 304 steel. Its tensile strength increases with cold drawing. It is non-magnetic and acquires magnetism after cold drawing. Trains
Aircraft
Belt conveyours
Vehicles
Bolts
Spring
301L 302 steel is created by lowering the C content in 301 steel and by improving grain boundary corrosion resistance of the welding part. Strength deterioration due to reduced C content is reinforced by adding N. Train frames
Buliding exterior material
303 Good free-cutting property by adding S and excellent anti-quenchability. Shafts for electric appliances
OA products
Bolts and nuts
304 Most widely used steel type. Good corrosion resistance, thermal resistance, low temperature strength and mechanical properties. Good drawability such as Deep Drawing, Bending and does not harden during heat treatment. (non-magnetic, usable temperature:-196~800℃) Hollow and flat ware
Sinks
Interior piping
Hot-water boilers
Bath tubs
Boilers
Automobile parts(Wiper, Muffer, Molding)
Medical instruments
Building materials
Facilities in chemical
Food and dairy industries
Vessel parts
304L 304L steel is low carbon 304 steel. In normal conditions, it has similar corrosion resistance. Excellent resistance to inter-granular corrosion after welding and stress relieving. Has corrosion resistant properties without heat treatment and is generally used at temperatures under 400℃ (non-magnetic, usable temperature:-196~800℃) Machinery and tools used inthe chemical, coal, and petroleum industry that require high inter-granular corrosion resistance
Buliding materials, heat resistant part and parts that are difficult to implement heat treatment on
304Cu By adding Cu, it has good pliability and good drawability. Useful for deep drawing products which requires a hygienic environment Thermos bottles
Kitchen sinks
Pots
Group food serving facilities
Door knobs
Products requiring spinning of drawing
304N1 304 steel is made by lowering the S and Mn Content in 304 steel and adding N to prevent ductility reduction. Strength is improved and thickness reduced. Structural use
Street lights
Water tanks
Water pipes
304H,M,S Steel types formed for wire rods by adjusting the C content in 304 steel. Strength increases with cold drawing. H : Wire ropes, Hooks, CD Bars
M : Mesh, Bolts, Nuts, CD Bars
S : Mesh
304HA,HC,HD,HN Steel types formed for wire rods by adjusting the C content in 304. Good cold headability and free-cutting property. HA : Shaft
HC : Medium and large size bolts and nuts
HD : CD Bars
HN : Nails
305 High Ni content. Non-magnetic and suitable for deep drawing use due to good cold formality. Dinnerware
Electrical parts
316 Excellent corrosion resistance, pitting corrosion resistance and high temperature strength by adding Mo. Useful in severe/harsh conditions. Excellent drawing hardening (non-magnetic). Sea water equipment
Equipment for chemicals
Ppaper, dye, acetic acid, fertilizer, photo and food industry
Construction in coastral areas
Ropes
CD Bars
Nuts and bolts
316L Low carbon steel type. Has the normal properties of 316 plus excellent inter-granular corrosion resistance. Made with 316 steel
Excellent inter-granular corrosion resistance
Mesh
316S Has 316 steel properties and suitable for ultra fine wire. Mesh
321 By adding Ti, prevents intra-granular corrosion, suitable under 430℃~900℃ Airplane exhaust pipes
Boilers
Heat exchangers
FERRITE
CLASSIFICATION CHARACTERISTICS USAGE
409L By adding Ti, has good weldability and drawability. Used for automobile exhaust pipes
Heat exchangers
Container for which post-heat treatment is not applied.
410L Excellent welding bendability by lowering the C content in 410 steel; high temperature oxidation resistance(magnetic). Machine frames, Engine exhaust pipes
Boiler combustors
Burners
430 Steel type that represents Ferrite; low thermal expansion rate and excellent drawing and oxidation resistance. Heat resistant tools
Burners
Household electric appliances
Sink covers
Buliding materials
Nuts and bolts
CD Bars
Mesh
430J1L Adding Mo, Ti and Nb results in good corrosion resistance, weldability and high temperature oxidation resistance. Washing machine tanks
Automobile exhaust pipes
Electric appliance parts
Three fold bottom pots
436L Adding Mo, Ti and Nb results in excellent corrosion resistance, drawability and weldability. Automobile exhaust pipes
Water supply facilities
444 Due to inherent characteristics of 400 series stainless steel, drawability and weldability fall below those of 304. However, drawability is improved by using extremely minute [C+N] and the ferrite-only phase is maintained by adding Nb, a ferrite stabilizing element. As a results, this new type of steel for water tanks has an improved corrosion resistance and softness as well as higher resistance to stress, corrosion, and cracking. Furthermore, the heat treatment before and after welding can be omitted. Water tanks
Drainage solar energy water heater
Electric water heater
MARTENSITE
CLASSIFICATION CHARACTERISTICS USAGE
410 Steel type that represents Martensite, goof strength but not suitable for use in harsh/severe corrosion conditions. Has excellent drawability and hardens with heat treatment(magnetic). General knkfe blades
Machine parts
Petroleum refining apparatus
Bolts & Nuts
Pump shafts
Spoons & Forks
420J1 In quenching condition, has high strength and good corrosion resistance(magnetic). General knife (dinner-k nife)
Turbine blades
420J2 Compared to 420J1, higher post-quenching strength(magnetic). Heat resistant tools
Burners
Household electric appliances
Sink covers
Buliding materials
Nuts and bolts
CD Bars
Mesh
AVAILABLE SIZE
Cold Rolled Base
THICKNESS : 0.23㎜ ~ 2.30㎜
WIDTH : 600㎜ ~ 1650㎜ (UNDER 800㎜ WIDTH, TO BE NEGOTIATED)

Hot Rolled Base
THICKNESS : 1.40㎜ ~ 4.50㎜
WIDTH : 50㎜ ~ 1060㎜ (OVER 1070㎜ WIDTH, TO BE NEGOTIATED)
Stainless Steel Surface Treatment
SURFACE TREATMENT CODE METHOD AND APPLICATION
No.1 A product that has undergone hot rolling treatment and pickling treatment. It is used in cold rolling materials, industrial tanks, and industrial chemical devices.
No.2D A non-glossy cold-rolled product that has undergone heat treatment and pickling treatment after hot rolling. Used in petrochemical plants, automobile parts, building materials, and pipes.
No.2B Skin-pass(temper-rolling) performed on No.2D. Better gloss and surface flatness compared to No.2B. A typical surface class with improved mechanical properties. It is applicable for almost all uses.
No.3 Polished with 100~120 mesh abraser. Avariety of uses including building exterior and interior materials, exterior casing for various kinds of electronic products, and kitchen installations that require an attractive gloss.
No.4 Polished with 150~180 mesh abraser. Compared to No.3, this product has a finer abrasion condition. Its surface is an attractive ailvery-white and is used for bathtubs, building exterior and interior materials, food industry installations and others.
BA A product that has undergone cold rolling, bright annealing and skin-pass(temper-rolling). Its surface is highly reflective. BA is used in home appliances, small-sized mirrors, kitchenware, building materials and others that require reflective properties.
HL A product with continuous abrassive pattern acquired by polishing with No4 abraser with appropriate grain density. It is widely used for building exterior and interior materials. Also used in sashes, door, and building panels.
DULL A product produced by reducing gloss from No.2B. Ruggedness is created on the surface with a roll that has fine surface ruggedness. It is widely used in automobile exterior and interior and buildings that require reduced light reflection.
Hot Rolling
Cold Rolling
Long Products
Section & Handling
STEEL TYPE SELECTION
Stainless steel must be selected according to usage and environment in order to extend the life cycle and maintain the fine exterior.
STEEL ENVIRONMENT SUBURB CITY INDUSTRIAL AREA COASTRAL AREA
I L M H I L M H I L M H I L M H
High Corrosion Resistant STS
STS316
STS304
STS430
Examples(Initial Letter)
I (Indoor Environment) : Indoor
L(Low Grade Environment) : Lightly corrosive environment(low temperature, high humidity)
M(Medium Grade Environment) : General environment
H(High Grade Environment) : Severely corrosive environment(high temperature, high humidity)

Examples(Mark)
: over reactive
: appropriate
: usable if cleaned ofter
: inappropriate
STORING AND TRANSPORTING

Stainless steel has good corrosion resistance because the surface has a passivity layer and needs to be handled with care to protest and maintain this film.

Storing
When Storing, It is important to avoid exposure to moisture, dust, oil, lubricant, in order to prevent rust and reduced/lowered corrosion resistance when welding. When exposed to moisture, the pace of corrosion accelerates when the protective film is not applied. Store in a dry and well ventilated place preferably in the original package covering. Materials with protective film should be kept away from direct sunlight and the film should be inspected regularly. If the film shows signs of deterioration (film lifespan is approx. 3 months), it should be replaced immediately. When packaging materials including paper get wet, they should be removed immediately to prevent surface corrosion.

Transporting
When transporting, rubber or wood support / underpinning should be used to avoid surface scraching. Special stainless equipment should be used and to avoid smudges from fingerprints, gloves should be worn.

FORMING AND CONSTRUCTION
Cut and Press
For punching and shearing, more pressure is required and when cutting the space between the dice and blade should be precise to prevent hardening of the pads and work Plasma and Laser cuts are recommended; if gas or arc cutting is unavoidable, grinding and heat treatment must be applied as necessary.

Bending Process
Bending of sheets is (thickness of 2mm or less) possible for 180 flat working. However, to reduce cracking of bent areas it is recommended to apply the same inner radius as the sheet thickness. Bending of plates (thickness of 200mm and over) in rolling direction should be twice the thickness of the plate; bending in a tight angle to rolling direction should be 4 times the curvature radius. When bending, to prevent cracking of the welded areas, surface grinding is recommended.

Drawing
When deep drawing, friction and surface damage is likely to occur. Therefore, lubricants must be pressure resistant and heat resistant and after drawing, molding grease on the surface must be removed thoroughly.

Welding
Grease, moisture, and paint should be removed to enable welding. The Welding bar must be selected for each steel. When tack welding, pitch must be shorter thatn carbon steel and when removing slag, a stainless brush must be used. After welding, grinding treatment and cleaning process are needed to prevent local corrosion and diminished hardness.

Notice When Building
Stainless steel is shipped with a protective film to prevent scratching and contamination, but over time problems related to adhesive residues and heat treatment can occur. When removing film after molding, surface must be cleaned. Tools should be stainless steel tools and when used on other sheets, the tool must be cleaned. Make sure tiles and cleaning detergents do not come in contact with the stainless steel surface. When stainless steel comes in contact with such materials, it must be cleaned immediately. At construction sites, stainless steel should not be exposed to cement and dust. After constrauction, the stainless steel must be cleaned with detergent and water. Chemically colored, etched, painted and coated materials are susceptible to discoloration and surface scratches and repair work is difficult.

How to Clean the Surface
To mainain the characteristics of stainless steel and to prevent rust, regular cleansing and maintenance is necessary. Avoid surface scratching and using detergents with bleaching agents. Use scrubbers and polishing tools and rinse with clean water.
CLEANING INTERVALS ACCORDING TO ENVIRONMENT AND AREA
Environment Suburban Area City, Industrial and Coastal Area
Area Structure Normal Environment Corrosive Environment
(high humidity, air pollution)
Exposed to moisture No residues of
contamination
and sediments
1~2 times / year 2~3 times / year 3~4 times / year
Residues 2~3 times / year 3~4 times / year 4~5 times / year
Not exposed to moisture No residues of
contamination
and sediments
1~2 times / year 3~4 times / year 4~5 times / year
Residues 2~3 times / year 4~5 times / year 5~6 times / year
Surface Conditions & Cleaning
SURFACE CONDITION HOT TO CLEAN
Dust and other stains Use soap, soft detergent in warm water.
Labels and protective film Use detergent in warm water, and use alcohol or organic solvent for adhesives.
Fat, oil, grease contamination / stains Use soft cotton/material or paper, then use neutral detergent or ammonia solution/solvent or use special detergent chemicals.
Bleachin agent and other acidic adhesive Rinse with water immediately, soak in ammonia bicarbonate soda solution and clean with warm water or use detergent.
Carbon organic and inorganic substances, carbon adhesion Soak in neutral detergent or ammonia solution/solvent before cleaning, use detergent with polishing material.
Fingerprints Use organic solution (ether or benzine) on cotton and rinse with water.
Rainbow pattern Usually caused by over use of detergent or oil. Use warm water and neutral detergent.
Discoloration after welding Clean with 10% of nitric acid solution or non-oxalic acid, neutralize in ammonia or bicarbonate solvent or water. Use special detergent products.
Rust from surface contamination clean with oxalic acid, sulfur acid, nitric acid(10%) or polishing detergent. Use special detergent products.
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