What is Stainless Steel?
Stainless steel is a broader term covering austenitic, ferritic, martensitic, ferritic-austenitic (Duplex) and precipitation hardened stainless steels. Each of its types has unique properties and applications & unique microstructures. Some stainless steel responds to heat treatment while some do not.
Naturally iron (Carbon steel) is reactive to the oxygen. Iron reacts with oxygen and converts to iron oxide as rust. But when carbon steel is added with minimum of 10.5% chromium, chrome forms a passive layer on the material surface to protect it from rust and corrosion as explained in the below picture.
Types of Stainless Steels
Stainless steels are divided into 5 main types:
- Ferritic Stainless steel
- Austenitic Stainless Steel
- Martensitic Stainless Steel
- Duplex Stainless Steel
- Precipitation hardened (PH) Stainless Steel
What is Austenitic Stainless Steel?
This article focused on 18/8 or 18Cr-8Ni steels generally called Austenitic Stainless steel. They are having tensile strength equivalent to mild steels, with approximately 210 Mpa (30 ksi) minimum yield strength at room temperature. They resist to hardenable by thermal transformation and offer very good sub-zero impact properties and weldability. They can be hardened by cold working and strength can be enhanced.
Types of Austenitic Stainless Steel
Austenitic stainless steels are classified as:
- 200 series and
- 300 series alloys by American Iron and Steel Institute (AISI).
200 Series has high carbon, manganese, and nitrogen compared to 300 Series & lower Ni to compensate for these elements. 200 series used in special applications e.g. application required galling resistance (Galling is a form of wear caused by adhesion between sliding surfaces e.g. wear at the bolt threads).
In Austenitic stainless steels, nickel is the main element promoting the austenitic structure stability at the room temperature (generally over 8%) inter-alia to C, N, and Cu which also work as austenite stabilizers. Carbon also provides improved creep strength at elevated temperatures.
Nitrogen is added to certain grades as a strength enhancer mainly at ambient and cryogenic temperatures. Nitrogen added types are designated with suffix N e.g. 304LN. Manganese increases the solubility of nitrogen in the austenitic phase due to this higher N is added to grades having higher Mn%, especially in the 200 series.
Further ASS grades are suffix with L (nominal value of 0.03 wt% C) or H (up to 0.1 wt% C) which mean low carbon or high carbon respectively. Grade 316 substitutes approximately 2% molybdenum (Mo) to a nearly equal amount of Cr to enhance pitting corrosion resistance. Some grades contain a higher amount of silicon and/or aluminum with carbon to maintain oxidation and/or carburization resistance and strength respectively.
The stabilized grades of ASS such as 321 and 347, contain a small amount of Ti and Nb, which are having higher carbide (MC-type) formation affinity to carbon compare to Cr at elevated temperature, thus preventing Cr-carbide (M23C6) precipitation. Austenitic stainless steels having up to 1% of Ti & Nb effectively reduce the matrix carbon content thus preventing M23C6 types formation. This also reduces the sensitization effect in ASS and hence prevents intergranular corrosion (IGC).
Austenitic stainless steels being ductile types exhibit significant elongation in a tensile test. The presence of Cr as a highly oxidizing element provides a protective surface layer for resistance against corrosion and further oxidation. They are not an appropriate choice in some common environments such as seawater, chloride-containing media or in highly caustic environments due to their susceptibility to stress corrosion cracking (SSC).
In stainless steel materials, the addition of Chromium, Molybdenum, and Nitrogen improves the corrosion resistance for different types. Austenitic stainless steels cannot be hardened by heat treatment due to their non-responsive microstructure to heat. Although, austenitic stainless steel properties can be changed by work hardening phenomena without any loss in material toughness and ductility. Being an austenitic microstructure, austenitic stainless steel is a non-magnetic material. One of the main alloying elements i.e. Nickel improves the resistance to SCC or stress corrosion cracking failure mechanism. All the above super properties of austenitic stainless steel make it one of the very important engineering materials for various service environments.