Stainless steels

Stainless Steels - An introduction

Stainless steels are defined as high- steels that have superior corrosion resistance than other steels because they contain large amounts of chromium. They contain between 4 and 30 percent chromium, however most contain around 10%. Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it stains less), but it is not stain-proof. Carbon steel rusts when exposed to air and moisture. This happens since the iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amount of chromium present so that a passive film of chromium oxide forms which prevents further surface corrosion and blocks corrosion spreading in the metal's internal structure.

There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. They can be divided into three basic groups based on their crystalline structure: austenitic, ferritic, and martensitic. Another group of stainless steels known as precipitation-hardened steels are a combination of austenitic and martensitic steels.

The following table lists the typical properties of steels at room temperature (25°C). The wide ranges of ultimate tensile strength, yield strength, and hardness are largely due to different conditions.

Stainless Steels - History

The "discovery" of stainless steel occurred in the 1900 to 1915 time period. However, as with many discoveries, it was the accumulated efforts of several individuals that actually began in 1821. That year a Frenchman named Berthier, noted that iron-chormium alloys were resistant against attack by some acids and suggested their use in cutlery. Others studied the effects of chromium in an iron matrix, but using a low percentage of chromium. To be stainless steel, the chromium content needs to be at least 10.5%. In 1872, Messrs. Woods and Clark applied for a British patent for what they identified as an acid and weather resistant alloy containing 30 to 35% chromium and 1.5 to 2% tungsten. Then, in 1875, another Frenchman named Brustlein recognized the importance of carbon levels in addition to chromium. However, metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be practical.

In the late 1890s, of developed an aluminothermic () process for producing carbon-free chromium. In the years 1904–1911 several researchers, particularly of France, prepared alloys that would today be considered stainless steel. built the 366-ton sailing yacht Germania featuring a chrome-nickel steel hull in Germany in 1908. In 1911, reported on the relationship between the chromium content and corrosion resistance. On , , engineers Benno Strauss and Eduard Maurer patented stainless steel.

Similar developments were taking place contemporaneously in the United States, where Christian Dantsizen and Frederick Becket were industrializing stainless.

In the troubled years immediately before , arms manufacturing increased significantly in the UK, but practical problems were encountered due to erosion (excessive wear) of the internal surfaces of gun barrels. Harry Brearley began to research new steels which could better resist the erosion caused by high temperatures (rather than corrosion, as is often mentioned in this regard). He began to examine the addition of to steel, which was known to raise the material’s melting point, as compared to the standard steels. In 1908 Brearley was given the opportunity to set up the Brown Firth Laboratories, which was financed by the two leading Sheffield steel companies of the day.

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. At this time Brearley was still trying to find a 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 made of carbon steel, which 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 but he had great difficulty convincing his more conservative employers. On his own initiative, he than had knives made at a local cutler's, R.F. Mosley.

Brearley initially called the new alloy "rustless steel"; the more "stainless steel" was suggested by Ernest Stuart of , a local cutlery manufacturer. He came up with this name after he had failed to stain the new knives with vinegar, and eventually prevailed. He was subsequently awarded the 's in 1920.

An announcement, as it appeared in the 1915 New York Times, of the discovery of stainless steel.

Virtually all research projects into the further development of stainless steels were interrupted by the 1914-18 War, but efforts were renewed in the 1920s. Harry Brearley had left the Brown Firth Laboratories in 1915, following disagreements regarding rights, but the research continued under the direction of his successor, Dr. . It is Hatfield who is credited with the development, in 1924, of a stainless steel which even today is probably the widest-used alloy of this type, the so-called "18/8", which in addition to chromium, includes (Ni) in its composition (18wt% Cr, 8wt% Ni).

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.

Ferritic Stainless Steel

Composition and Formation

This group of stainless steel contains chromium content in the range of 10.5 % to 30 %. They have a ferritic body centred cubic crystal structure which cannot be heat treated as they remain ferritic at room and higher temperatures and therefore can only be hardened by moderate cold working. These steels are ferromagnetic while their ductility and toughness is moderate, but these two properties can be increased with an increase in chromium content.

The primary phase of the ferritic structure is formed by precipitation in ...

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Ferritic Stainless Steel

Composition and Formation

The primary phase of the ferritic structure is formed by precipitation in the temperature range of 350°C and 540°C. Titanium or Columbium is then added to the steel so a completely ferritic structure can be obtained even at room temperature. This can be also obtained by melting the steels at very low levels of carbon and nitrogen. Properties of ferritic stainless steels can be improved by the addition of certain elements such as molybdenum (which improves the pitting corrosion resistance) and silicon or aluminium (which improve the resistance to high temperature corrosion by oxidation).

This diagram shows a typical Ferritic grain structure of a free machining, low carbon steel with added sulphur to improve surface finishing.

Classification

Ferritic stainless steels are usually classified into three generations according to the time in which the specific stainless steels were developed. The first generation which was developed in the 20th century had relatively high carbon and chromium content due to the inefficient decarburising process which caused these steels not to be fully ferritic at room temperature. An example of this generation is AISI 430 (American Iron and Steel Institute.)

The introduction of elements such as Titanium and Niobium (which are carbon and nitrogen getter elements), caused the second generation of ferritic stainless steel to have lower carbon and nitrogen content. Titanium is also a ferrite-forming element which made the microstructure of this second generation completely composed of ferrite. AISI 409 and 439 which is widely used in the automobile exhaust industry are two examples of this generation.

After the year 1970, an improved decarburization process caused the third generation ferritic stainless steels to contain a lower percentage of carbon and nitrogen (less than 0.02 weight percentage). AISI 444 is a typical example of this generation. This Third generation also includes super ferritic stainless steels which have a chromium content greater than 25 weight percentage and are formed by carbon-reducing metallurgical processes such as VOD (Vacuum Oxygen Decarburisation) and AOD (Argon Oxygen Decarburisation) and the addition of molybdenum which is a strong ferrite former. Nickel can be added to these super ferritic steels to increase the pitting, toughness and crevice corrosion resistance without destabilising the ferrite. AISI 442 is a typical super ferritic stainless steel.

Inter-granular Corrosion

After the ferritic steel is heated to temperature above 950°C, precipitation of chromium carbides and chromium nitrides occurs during cooling which decreases the toughness and corrosion resistance of the material. This type of corrosion can be reduced or completely eliminated by the addition of titanium or columbium to bind again the carbon and nitrogen and therefore stabilising the steel. Lowering the level of carbon and nitrogen to a minimum also aids the reduction of this type of corrosion.

This diagram shows the effect of inter-granular corrosion.

The 475°C Embrittlement

In super ferritic stainless steel (which are mainly made up of Iron and chromium) at a temperature of 475°C or slightly below, exhibit under certain conditions clustering of body-centred cubic atoms which leads to spinodal decomposition. Spinodal decomposition is a process in which a solution made up of more than one material separates into different regions having different element content throughout the whole solution. In this case the solution undertakes spinodal decomposition into regions rich in chromium and other regions rich in iron. The hardness of the ferritic stainless steel increases with the time spent at the embrittlement temperature while impact toughness and ductility is greatly reduced. The addition of nickel accelerates the spinodal decomposition while also increasing the maximum temperature at which this process can be observed.

General and Stress Corrosion Resistance

Ferritic stainless steels have a high resistance to chloride (which is a type of inter-granular corrosion) and caustic (environments having high pH) corrosion stress cracking. The same high resistance is demonstrated in strong oxidising environments such as nitric acid and in inorganic acids.

Examples of Ferritic Stainless Steels:

AISI 430 – It is a general purpose ferritic stainless steel which is widely used for decorative purposes and as the main material for the production of vessels used in the food and chemical industry.

AISI 409 – This has only 12% Chromium which makes it relatively cheap while having decent formability, weldability and resistance to atmospheric corrosion properties. It is used in making automobile exhaust equipment, catalytic reactors and radiator tanks.

AISI 444 – The lower carbon content in this type of ferritic stainless steel gives good toughness, weldability and stress corrosion cracking. Only thin sections of AISI 444 should be used as toughness decreases with increase in thickness. This type of ferritic stainless steel is used in heat exchanger tubes.

Martensitic Stainless Steels

Composition and Formation

Martensitic stainless steels were the first stainless steel commercially developed. Martensitic stainless steels are essentially alloys of chromium and carbon with a composition of chromium varying from 12% to 18% and that of carbon from 0.1% to 1.2%. The chromium and carbon contents are balanced to ensure a martensitic structure after hardening. This type of stainless steel possesses a distorted body – centred cubic (BCC) or body – centred tetragonal (BCT) martensitic crystal structure in the hardened condition.

The following diagram shows the microstructure of a Martensitic Stainless Steel:

The composition of such steel is such that the austenite in these steels is able to transform into martensite. This allows a degree of control on the mechanical properties by exploiting the phase change. Typical heat-treatments consist of austenitisation at a temperature high enough to dissolve carbides followed by quenching to obtain martensite. Given the high hardenability inherent in such alloys, the quench rate required to achieve martensite is not high; oil and water quenching are used only when dealing with thick sections.

As with other martensitic steels, a balance must be sought between hardness and toughness. An untempered martensitic structure is typically strong but lacks toughness and ductility to an extent which depends on the carbon concentration. As a conseqnece, the martensite is in many cases tempered between 600 and 750°C to optimise the mechanical properties.

The chromium and carbon contents are balanced to ensure a martensitic structure after hardening. Martensitic stainless steels are chosen for their good tensile and fatigue strength properties, in combination with moderate corrosion resistance and heat resistance. They are ferromagnetic, subject to an impact transition at low temperatures and possess poor formability. Their thermal expansion and other thermal properties are similar to conventional steels.

Molybdenum can be added to improve mechanical properties or corrosion resistance. When higher chromium levels are used to improve corrosion resistance, nickel also serves to maintain the desired microstructure and to prevent excessive free ferrite. The limitations on the alloy content required to maintain the desired fully martensitic structure restrict the obtainable corrosion resistance to moderate levels.

Welding martensitic stainless steel also has its drawbacks. Martensitic stainless steels are considered to be the most difficult of the stainless steel alloys to weld. Higher carbon contents will produce greater hardness and, therefore, an increased susceptibility to cracking.

In addition to the problems that result from localized stresses associated with the volume change upon martensitic transformation, the risk of cracking will increase when hydrogen from various sources is present in the weld metal. A complete and appropriate welding process is needed to prevent cracking and produce a sound weld.

Due to their high strength in combination with some corrosion resistance, martensitic steels are suitable for applications where the material is subjected to both corrosion and wear. An example is in hydroelectric turbines. Other uses are for bearings, molds, cutlery, medical, industrial components, instruments and aircraft structural parts requiring hardness and corrosion resistance. Type 420 is used increasingly for molds for plastics

Examples of Martensitic Stainless Steels

Austenitic stainless steel

Composition and Formation

The most basic and widely used definition for austenitic steels is “ Steels containing high percentages of certain alloying elements such as manganese and nickel which are austenitic at room temperature and cannot be hardened by normal heat-treatment but do work harden.” It is to be noted that they are also non-magnetic.

stainless steels comprise over 70% of the total stainless steel production in the world. They contain 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 melting point of the alloy. This is termed as “retained austenite”. So though from Physical Metallurgy we know that austenite is an unstable phase and should change to one of the stable phases (such as Martensite or Bainite) as it cools down the addition of alloying elements nickel and/or manganese shift the nose on the TTT diagram to the right and the Martensite finish line to temperatures well below 0º Celsius, sometimes even down to the cryogenic region at minus 150º Celsius and below. This of course depends on the amount of these alloying elements. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower . A high chrome and nickel content suppress the transformation where Nickel maintains the austenite phase on cooling and the Chrome slows the transformation down so that a fully austenitic structure can be achieved with only 8% Nickel.

A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless, is often used in . Similarly, 18/0 and 18/8 are also available. Superaustenitic stainless steels, such as alloy and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking.

The higher alloy content of superaustenitic steels makes them more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications.

The low carbon version of the Austenitic Stainless Steel, for example 316L or 304L, is used to avoid corrosion problem caused by welding. The "L" means that the carbon content of the Stainless Steel is below 0.03%. An exampole is 316L, a stainless steel used very widely in the watch industry and in many marine operations due to it’s excellent resistance to corrosion and the cost savings it provides over superaustenitic steels.

Austenitic stainless steels have high ductility, low yield stress and relatively high ultimate tensile strength and high resistance to brittle fracture when compared to a typical carbon steel.

Austenitic steels have a F.C.C atomic structure which provides more planes for the flow of dislocations, combined with the low level of interstitial elements (elements that lock the dislocation chain), gives the material its good ductility. This also explains why it has no clearly defined yield point. Austenitic steels have excellent toughness down to true absolute (-273°C), with no steep ductile to brittle transition.

A weak point to be noted is that if any part of austenitic stainless-steel is heated in the range of 500º to 800º Celsius for any reasonable time there is a risk that the chrome will form chrome carbides (a compound formed with carbon) with any carbon present in the steel. This reduces the chrome available to provide the passive film which inhibits corrosion and leads to preferential corrosion, which can be severe. This is often referred to as sensitisation. This is particularly to be noted when welding the material. Although sensitisation of modern low carbon grades is unlikely unless heated for prolonged periods, the “L” following certain grades of the steel (such as the 316 and 304 mentioned above) avoids this problem as the carbon content is not high enough for sensitization to occur. Small quantities of either titanium (321) or niobium (347) are added to stabilise the material and will further inhibit the formation of chrome carbides.

Typical Carbon content for austenitic steels is always < 0.1 %Carbon and if the steel has > 0.08 %Carbon it is termed to be High Carbon (austenitic) steel. The higher the carbon content the greater the yield strength. From Physical Metallurgy we know that plain carbon steels are still termed low carbon when the Carbon content is <0.3 %Carbon. This difference is to be noted.

Examples of Austenitic Stainless Steel

All of these stainless steel grades are basically variations of a 304 steel. Superaustenitic steel typical alloy content is 20 %Chromium, 25 %Nickel and 4.5 %Molybdenum. Superaustentic steel with this composition would be graded as 904L. Superaustentic steels are also sometimes referred to as Nickel alloys. As the name readily implies the Nickel content is higher than for standard austenitic steels. This pushes the price up but offers increased corrosion resistance and also higher resistance to acid attack. Rolex the most widely known watch brand in the world is the only manufacturer to use this kind of steel over the widely used 316L by almost all other manufacturers (even higher end brands). The only setback apart from the initial price is that the high Nickel content can be the cause of skin allergies. The higher Nickel content in superaustenitic also gives the material a particular sheen that is whiter in colour than other stainless steels.

Precipitation Hardening Stainless Steels

Composition and Formation

Precipitation hardening stainless steels are steels that contain chromium and nickel. These provide the optimum combination of the properties of martensitic and austenitic stainless steels. These alloys have the ability to gain high strength through heat treatment from the martensitic stainless steels and they also have the corrosion resistance of austenitic stainless steels. The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is achieved through the addition of one or more of the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum. The most well known precipitation hardening steel is 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steels grade 630.

The advantage of precipitation hardening steels is that they can be supplied readily machineable. After machining or another fabrication method, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age-hardening. As it is carried out at low temperature, the component undergoes no distortion.

Characteristics of Stainless Steels

Precipitation hardening stainless steels are characterised into one of three groups based on their final microstructures after heat treatment. The three types are: martensitic (e.g. 17-4 PH), semi-austenitic (e.g. 17-7 PH) and austenitic (e.g. A-286).

Martensitic Alloys:

Martensitic precipitation hardening stainless steels have a predominantly austenitic structure at annealing temperatures of around 1040 to 1065°C. Upon cooling to room temperature, they undergo a transformation that changes the austenite to martensite.

Semi-austenitic Alloys:

Unlike martensitic precipitation hardening steels, annealed semi-austenitic precipitation hardening steels are soft enough to be cold worked. Semi-austenitc steels retain their austenitic structure at room temperature but will form martensite at very low temperatures.

Austenitic Alloys:

Austenitic precipitation hardening steels retain their austenitic structure after annealing and hardening by ageing. At the annealing temperature of 1095 to 1120°C the precipitation hardening phase is soluble. It remains in solution during rapid cooling. When reheated to 650 to 760°C, precipitation occurs. This increases the hardness and strength of the material. Hardness remains lower than that for martensitic or semi-austenitic precipitation hardening steels, austenitic alloys remain nonmagnetic.

General Stress, Heat and Corrosion Resistance

Yield strengths for precipitation-hardening stainless steels are 515 to 1415 MPa. Tensile strengths range from 860 to 1520 MPa. Elongations are 1 to 25%. Cold working before ageing can be used to facilitate even higher strengths.

The key to the properties of precipitation hardening stainless steels lies in heat treatment. After solution treatment or annealing of precipitation hardening stainless steels, a single low temperature "age hardening" stage is employed to achieve the required properties. As this treatment is carried out at a low temperature, no distortion occurs and there is only superficial discolouration. During the hardening process a slight decrease in size takes place. This shrinking is approximately 0.05% for condition H900 and 0.10% for H1150.

Typical mechanical properties achieved for 17-4 PH after solution treating and age hardening are given in the following table. Condition designations are given by the age hardening temperature in °C.

Table 1: Physical Properties

Table 2: Properties for Differently Annealed Steels

Table 3: Typical Chemical Composition for 17-4PH

Table 4: Typical Mechanical Properties

Precipitation hardening stainless steels have moderate to good corrosion resistance in a range of environments. They have a better combination of strength and corrosion resistance than when compared with the heat treatable 400 series martensitic alloys. Corrosion resistance is similar to that found in grade 304 stainless steels. In warm chloride environments, 17-4 PH is susceptible to pitting and crevice corrosion. When aged at 550°C or higher, 17-4 PH is highly resistant to stress

corrosion cracking. Better stress corrosion cracking resistance comes with higher ageing temperatures. Corrosion resistance is low in the solution treated (annealed) condition and it should not be used before heat treatment. 17-4 PH has good oxidation resistance. In order to avoid reduction in mechanical properties, it should not be used over its precipitation hardening temperature. Prolonged exposure to 370-480°C should be avoided if ambient temperature toughness is critical.

Fabrication and Heat Treatment

Fabrication of all stainless steels should be done only with tools dedicated to stainless steel materials or tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steels by easily corroded metals that may discolour the surface of the fabricated product. Cold forming such as rolling, bending and hydroforming can be performed on 17-4PH but only in the fully annealed condition. After cold working, stress corrosion resistance is improved by re-ageing at the precipitation hardening temperature. Hot working of 17-4 PH should be performed at 950°-1200°C. After hot working, full heat treatment is required. This involves annealing and cooling to room temperature or lower. Then the component needs to be precipitation hardened to achieve the required mechanical properties.

In the annealed condition, 17-4 PH has good machinability, similar to that of 304 stainless steels. After hardening heat treatment, machining is difficult but possible. Carbide or high speed steel tools are normally used with standard lubrication. When strict tolerance limits are required, the dimensional changes due to heat treatment must be taken into account.

Precipitation hardening steels can be readily welded using procedures similar to those used for the 300 series of stainless steels. Grade 17-4 PH can be successfully welded without preheating. Heat treating after welding can be used to give the weld metal the same properties as for the parent metal. The recommended grade of filler rods for welding 17-4 PH is 17-7 PH.

Applications and Properties

Austenitic Stainless Steel:

Properties:

Excellent corrosion resistance in organic acid, industrial and marine environments.
excellent weldability (all processes)
excellent formability, fabricability and ductility
excellent cleanability, and hygiene characteristics
good high and excellent low temperature properties (high toughness at all temperatures)
non magnetic (if annealed)
hardenable by cold work only (These alloys are not hardenable by heat treatment)

Applications:

computer floppy disk shutters
Computer keyboard key springs
Kitchen sinks
Food processing equipment and kitchen utensils- It is one of the most hygienic surfaces for the preparation of foods and very easy to clean, as its unique surface has no pores or cracks to harbor dirt, grime or bacteria. It is very attractive and requires minimal care, since it won't chip or easily rust. It will not affect flavor, as it does not react with acidic foods during food preparation or cooking. With proper care, it has a useful life expectancy of over 100 years, and it is totally recyclable

Architectural applications- stainless steel is chosen for this application due to its unique combination of properties, versatility and due to it being cost competitive. In architecture pollution, salt exposure, weather patterns, design and cleaning frequency must be considered. Stainless steel is used because of:

Aesthetic appearance
The bright, easily maintained surface of stainless steel provides an attractive and contemporary appearance, ideal for a myriad of architectural applications.
Corrosion resistance
Lower alloy grades resist corrosion in normal atmospheric and potable water environments, while the more highly alloyed grades can resist corrosion in many acids and alkaline solutions, and some chloride bearing environment, properties which are widely utilised in process plants.
Strength
Stainless Steels have high tensile strengths and excellent fatigue properties. Austenitic grades work harden with cold working, and duplex steels allow for reduced thicknesses over traditional grades. Substantial cost savings therefore result as well as increased competitiveness with alternative materials.
Low maintenance costs
Stainless steel normally requires only a periodic wash with soap and water to maintain its original finish.
Long term value
When the total life cycle costs are considered, stainless steel is often the least expensive material option.

Pedestrian bridge at Cheung Kong Center, Hong Kong, Type 316 stainless steel, Cambric.

chemical plant and equipment
Marine applications- this type of material is used in marine applications due to the high corrosion resistance since it is one obvious environment where pitting corrosion is of concern. Type 316 austenitic steel (18Cr-12Ni and 2-3 wt% Mo) is often chosen in this case. When the marine requirements are particularly severe, as with offshore platforms, alloys with molybdenum concentrations up to 5 wt% are used. for example, call for heavily alloyed steels with up to 6 wt% Mo.
Used in water distribution- There are millions of miles of pipelines, worldwide, used for the transmission of drinking water. These pipelines must offer corrosion resistance to the water itself, soil chemistries, and treatment chemicals in order to provide both a long service life and hygienic delivery of drinking quality water. Type 316 (2-3% Mo) stainless steel has been shown to provide the necessary corrosion resistance for most applications, at a cost competitive with other piping materials. Where additional corrosion protection might be needed, as in shoreline installations, the duplex SS (3.0-3.5% Mo) can be used to achieve the required hygiene, cost and service life.

Ferritic Stainless steels:

Properties:

moderate to good corrosion resistance increasing with chromium content
not hardenable by heat treatment and always used in the annealed condition magnetic
weldability is poor
formability not as good as the austenitics

Applications:

computer floppy disk hubs
automotive trim
automotive exhausts
colliery equipment
hot water tanks

Storage tank Automotive gaskets

Martensitic Stainless Steels:

Properties:

moderate corrosion resistance
can be hardened by heat treatment and therefore high strength and hardness levels
can be achieved
poor weldability
magnetic

Applications:

Knife blades
Surgical instruments – these types of steels are well-suited for surgical instruments because they are very easy to clean and sterilize and also because they are strong and corrosion-resistant. Instruments made out of this type of steel are easier to keep sharp.

shafts
spindles
pins

Precipitation Hardening Stainless Steels:

Applications:

Gears
Valves and other engine components
High strength shafts
Turbine blades
Moulding dies
Nuclear waste casks

References:

http://www.ssina.com/overview/history.html
http://www.stainless-steel-world.net/basicfacts/ShowPage.aspx?pageID=464

http://steel.keytometals.com/Articles/Art58.htm
http://en.wikipedia.org/wiki/Stainless_steel
http://www.madehow.com/Volume-1/Stainless-Steel.html
http://www.outokumpu.com/files/Group/HR/Documents/STAINLESS20.pdf
http://www.msm.cam.ac.uk/phase-trans/2005/Stainless_steels/stainless.html
George E. Totten, Steel heat Treatment – Metallurgy and Technologies, Second Edition, Taylor & Francis Group.
Callister W.D., Materials Science and Engineering, An Introduction, 6th edition, John Wiley & Sons Inc.