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Stainless steel is the name given to a group of corrosion resistant and high temperature steels. Their remarkable resistance to corrosion is due to a chromium-rich oxide film which forms on the surface. When ordinary carbon steel is exposed to rain water, for example, it corrodes forming a brown iron oxide, commonly called rust, on the surface. This is not protective and eventually the entire piece of steel will be converted to rust.

But for stainless steel, it is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain ‘less’ than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of the chrome-containing oxide, called the passive firm. The sizes of the chromium atoms and their oxides are similar. So they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion.

This is what we call stainless steel and there are several different types, and different grades.

Types of Stainless Steel
The three main types of stainless steels are austenitic,ferritic,and martensitic.These types of steels identified by their microstructure or predominant crystal phase.

1. Austenitic + MORE

The majority of stainless steels contain nickel(Ni),which is added for a number reasons but particularly to change the crystal structure from ferrite to austenite.Austenitic stainless steels are ductile,tough and most importantly,easy to form and weld.These steels are not magnetic in annealed condition. Austenitic steels are not hardenable by heat treatment.

The most familiar stainless steel is probably Type 304.Type 304 surgical stainless steel is an austenitic steel containing 18-20% chromium and 8-10% nickel, it is widely used in those equipment and mechanical which production requirements is good performance(corrosion resistance and formability).The lower carbon version,Type 304L is always preferred in more corrosive environments where welding is involved.Lower carbon content makes the heat affected zone which is near the weld to minimize carbide precipitation may lead to stainless steel have intergranular corrosion in certain environments(weld erosion).

When 2-3% molybdenum is added to Type 304 or 304L, we create Type 316 or 316L stainless steel.Molybdenum is added to increase their corrosion resistance,particularly in marine and acidic environments.It increases an alloy pitting and crevice corrosion resistance.These corrosion forms are caused by the common and highly aggressive chloride ion,which is present in sea salt and table salt.They are also known as the acid resistant ,better corrison resistance in some acids such as sulphuric acid.Range of applications is wide,from building fadaces in aggressive atmospheres to piping onboard chemical tankers.

2. Martensitic + MORE

Martensitic stainless steels are not as corrosion resistant as the other two ferritic and austenitic.Chromium iron alloys with 12% to 17% chromium. And carefully controlled carbon content,hardenable by quenching and tempering.It has magnetic properties. Commonly used in knives,razor blades.Martensitic grades are strong and hard,but are brittle and difficult to form and weld.

3. Ferritic + MORE

Ferritic stainless steels have ferrite(body centered cubic crystal) as their main phase.The basic composition of stainless steel is iron and chromium.The ferritic stainless steels are magnetic like ordinary steel.Ferritic steel is less ductile than austenitic steel and is not hardenable by heat treatment.They contain between 12% and 27% chromium and very little nickel, with 12% chromium used mostly in structural applications and 17% in housewearss,boilers,washing machines,cooking utensils and indoor architecture.

Surface Finish

No.1 Silver White, Matt
Hot Rolled, annealed and descaled.Generally used in industrial applications,where heat or corrosion resistance is required
Surface smoothness or gloss is not particularly important.
No.2D Silver
Dull, smooth finish produces by cold rolling,annealing and pickling.Cold rolled after heat treatment and pickling,and sometimes facial hair roller in the final of a light rolling of a matt surface finish
Suited for deep drawing as well as general use
No.2B Luster is stronger than No.2D
Bright,smooth cold rolled finish obtained by skill pass rolling No.2D.
General timber.
No.3 Coarse Grinding
Be No.2D and No.2B materials,intermediate polished finish obtained with rather coarse abrasives (#100 to #120 mesh)
Building Materials, Kitchen utensils
No.4 Middle ground
Be No.2D and No.2B materials, a standard polished finish produced with finer abrasives (#100 to #120 mesh)
Restaurant and kitchen dairy processing equipment.
No.240 Fine grinding
Be No.2D and No.2B materials,polish finish about #240 tube abrasive belt grinding.
Restaurant and household kitchen equipment.
No.320 Fine grinding
Be No.2D and No.2B materials,polish finish about #320 tube abrasive belt grinding.
Restaurant and household kitchen equipment.
No.400 Luster close to BA
High luster finish produced by polishing No.2B first with #400 buffing iron and then with buffing cloth.
General use materials. Kitchen appliances.
No.7 Close to the mirror grinding
With #600 rotating polishing wheel to grind.
Art use, decorative.
No.8 mirror grinding
Mirror with polished wheels to grind.
Reflective mirrors, decorative.
BA Bright as a mirror
Highly reflective finish produced by coil rolling,bright annealing and temper rolling.
Lustrous surface specially required, kitchen utensils.
HL Fat grinding grain
Special polished finish having continues lines produced with abrasives of appropriate grain size.
Mainly used in architectural, building materials.

Usage Of Stainless Steel

Martensitic Stainless Steels 410 1.4006
The most common grade of economical 13 Cr steels. In the annealed condition, this grade may be drawn or formed. Used for machine structures, home appliances, etc.
420 1.4028
High carbon content results in high abrasion resistance,greater hardness may be obtained by heat treatment. Cutlery, gages, hospital, odontological and surgical instruments,mining and steel plants, knives chians, etc.
Ferritic Stainless Steels 430 1.4016
The most common and widely used of the ferritic grades.This steel may be drawn or formed and has high corrosion resistance. Domestic utensils (tableware, skins and cutlery), kitchen equipment (stoves, refrigerators, microware ovens), etc.
434 1.4113
Designed to resist atmospheric corrosion especially for use as automotive trim, used for wheel covers, hub caps, grilles, mouldings, etc.
439 1.4510
Exhaust syatem (tubes, mufflers), washing machines, microware ovens, etc.
Austenitic Stainless Steels 301 1.4310
This grade can be cold worked to very high strength levels, shows obvious work-hardening phenomenon in the deformation. Used for structural applications, in equipment for the food industry, aviation, knives and blades, skins and bowls, etc.
302 1.4300
302 stainless steel is substantially a variant of 304 stainless steel but has higher carbon content, cold roll can make it higher intensity. Suited for oil, chemical, paper and food processing machinery, dairy installations, buildings, kitchen equipment, etc.
304 1.4301
Widely used in those equipment and mechanical which production requirements is good performance(corrosion resistance and formability). Civil constructions and architectural use, equipment for the food industry, oil industry, general an deep drawing.
304L 1.4307
It is a low carbon 304 stainless steel variant, for use where carbide precipitation after welding is undesirable. Civil constructions and architectural use, equipment for the food industry, oil industry, general an deep drawing, tubes and tanks, etc.
321 1.4541
Suited for use in a 500°C to 850°C temperature range in which carbides tend to precipitate. Heat resistant for the power generating industry, welded components, food industry, tubes and tanks in general, etc.
316 1.4401
High resistant to such reducing acids as sulphuric, sulphurous, acetic and hydrochloric acids, also high creep strength at elevated temperatures. Used in applications in the paper, texitile and chemical industries, manufacturing of tubes and pressure veseels, alcohol distilleries.
316L 1.4404
A special 316 grade intended for use where carbide precipitation after welding is undesirable. Civil engineering and architectural uses, equipment for industries such as aeronautic, railroad, shipyards, chemical and petrochemical, etc.
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