While there are thousands of different alloys of stainless steel,they are all commonly classified by their crystalline structure into four broad categories. Each grade is used for specific purposes and comes with its own advantages and disadvantages:

martensitic stainless steels:

Martensitic stainless steels is the least common category of stainless steel alloy. They may contain some Ni (Type 431) which allows a higher Cr and/or Mo content, thereby improving corrosion resistance and as the Carbon content is also lower, the toughnessis improved. Grade EN 1.4313 (CA6NM) with a low C, 13%Cr and 4%Ni offers good mechanical properties, good castability, good weldability and good resistance to cavitation. It is used for nearly all the hydroelectric turbines in the world, including those of the huge “Three Gorges” dam in China. At the same time,martensitic stainless steel can be nondestructively tested using the magnetic particle inspection method, unlike austenitic stainless steel. Martensitic stainless steels, depending upon their carbon content can be seen as:

– corrosion resistant engineering steels

pumps
boat shafts

-wear resistant and corrosion resistant applications

Cutlery
razor blades
medical tools (scalpels, razors and internal clamps)
bearings (ball bearings)
injection molds for polymers

ferritic stainless steels:

The second most common form of stainless steel after austenitic alloys. By compared with austenitic stainless steels, ferritic stainless steels have poor hardenability in cold working and poor weldability, and are generally not recommended for use at low temperatures.Some types, like the 430, have excellent corrosion resistance and are very heat tolerant. They also tend to be less expensive because of their reduced nickel content.

austenitic stainless steels:

Obviously ,austenitic stainless steels is the most commonly used types of stainless steels. It was named after Sir William Chandler Roberts-Austen, an Englishman known for his studies of the physical properties of metals. Austenitic stainless steels is used extensively in many industries including medical applications, automotive applications, aerospace applications, and industrial&consumer applications.

duplex stainless steels:

The main differences in composition, when compared with an austenitic stainless steel is that the duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen. Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. Duplex stainless steels is used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc. and in the petrochemical industry in the form of pipelines and pressure vessels. In addition to the improved corrosion resistance compared with the 300 series stainless steels duplex steels also have higher strength.
In addition to being divided into four categories according to their crystal structure, stainless steel are available in the form of:

stainless steel plate
stainless steel sheet
stainless steel strip
stainless steel foil
stainless steel bar
stainless steel wire
stainless steel pipes
stainless steel tubes

Stainless steels are a iron-based alloy containing at between 10.5% to 30% Cr. Stainless steel achieve its stainless characteristic through the formation of an invisible and adherent chromium-rich oxide surface film.

Other alloying elements added to improve the characteristics of the stainless steel include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, Sulphur, and selenium.

Carbon is normally in amounts from 0.03% to more than 1.0% in some martensitic grades.

Selection of stainless steels are in general based on

corrosion resistance
fabrication characteristics
availability
mechanical properties for specific temperature ranges
product cost

Since stainless steel resists corrosion, maintains its strength at high temperatures, and is easily maintained, it is widely used in items such as automotive and food processing products, as well as medical and health equipment. The most common US grades of stainless steel are:

TYPE 304

The most commonly specified austenitic (chromium-nickel stainless class) stainless steel, accounting for more than half of the stainless steel produced in the world. This grade withstands ordinary corrosion in architecture, is durable in typical food processing environments, and resists most chemicals. Type 304 is available in virtually all product forms and finishes.

TYPE 316

Austenitic (chromium-nickel stainless class) stainless steel containing 2%-3% molybdenum (whereas 304 has none). The inclusion of molybdenum gives 316 greater resistance to various forms of deterioration.

TYPE 409

Ferritic (plain chromium stainless category) stainless steel suitable for high temperatures. This grade has the lowest chromium content of all stainless steels and thus is the least expensive.

TYPE 410

The most widely used martensitic (plain chromium stainless class with exceptional strength) stainless steel, featuring the high level of strength conferred by the martensitic. It is a low-cost, heat-treatable grade suitable for non-severe corrosion applications.

TYPE 430

The most widely used ferritic (plain chromium stainless category) stainless steel, offering general-purpose corrosion resistance, often in decorative applications.

According to the use, the classification of steel can be divided into: