Stainless steel is a ferrous alloy that contains a minimum of 12% chromium. We can also consider them as plain carbon steels to which other metals (alloying elements) have been added in sufficient quantities to materially alter the properties of the steel.
Stainless Steel Composition
Most common alloying elements added to steel:
- Nickel – to refine the grain and strengthen the steel.
- Nickel steel does not rust easily.
- Chromium – to improve the response of the steel to heat treatment and corrosion resistance of the steel.
- Molybdenum – reduces temper brittleness and enables operate continuously at high temperatures without becoming brittle.
- Manganese – purifies steel and adds strength and toughness.
- Tungsten – added in the proper amount makes steel self-hardening.
Chromium (Cr)
This is the most important alloying element in stainless steels. It is this element that gives the stainless steels their basic corrosion resistance. The corrosion resistance increases with increasing chromium content. It also increases the resistance to oxidation at high temperatures. Chromium promotes a ferritic structure.
Nickel (Ni)
The main reason for the nickel addition is to promote an austenitic structure. Nickel generally increases ductility and toughness. It also reduces the corrosion rate and is thus advantageous in acid environments. In precipitation hardening steels nickel is also used to form the intermetallic compounds that are used to increase the strength.
Molybdenum (Mo)
Molybdenum substantially increases the resistance to both general and localised corrosion. It increases the mechanical strength somewhat and strongly promotes a ferritic structure. Molybdenum also promotes the formation secondary phases in ferritic, ferritic-austenitic and austenitic steels. In martensitic steels it will increase the hardness at higher tempering temperatures due to its effect on the carbide precipitation.
Passivity
The reason for the good corrosion resistance of stainless steels is that they form a very thin, invisible surface film in oxidising environments. This film is an oxide that protects the steel from attack in an aggressive environment. As chromium is added to a steel, a rapid reduction in corrosion rate is observed to around 10% because of the formation of this protective layer or passive film. In order to obtain a compact and continuous passive film, a chromium content of at least 11% is required. Passivity increases fairly rapidly with increasing chromium content up to about 17% chromium. This is the reason why many stainless steels contain 17-18% chromium.
Classification or Types of Stainless Steel
Stainless steel can be classified into following types:
- Austenitic
- Ferritic
- Duplex
- Martensitic
- Precipitation Hardening
Austenitic Stainless Steel
Austenitic steels are the most popular grades of stainless steels because of their ductility, ease of working and good corrosion resistance and are very commonly used in manufacture of piping components. Austenitic steels are non-magnetic and non-hardenable by heat treatment, however they can be hardened by cold working. The most commonly used stainless steel grades are Type 304, Type 316 and Type 321.
Stainless steel grades with suffix L have low carbon content. The low carbon content provides good weldability and good corrosion resistance after welding, however they have lower strength than the grades with higher carbon content. The dual certified grades of stainless steel are commonly used in the industry such as SS 304/304L or SS 316/316L. For e.g. the SS 304/304L dual certified grade has lower carbon content similar to SS 304L grade but higher mechanical strength of SS 304 grade.
Type 304 grade contains approximately 18% Chromium and 8% Nickel.