What is Austenitic Stainless Steels?
Stainless steel is a broader term covering austenitic, ferritic, martensitic, ferritic-austenitic (Duplex) and precipitation hardened stainless steels. Each of its types have unique properties and applications.
Types of Stainless Steels
Stainless steels are divided into 5 main types:
- Ferritic–steels are Chromium based with small amounts of Carbon usually maximum 0.10%. They have a microstructure similar to carbon and low alloy steels. Ferritic stainless steels are limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels, but they are magnetic. Ferritic stainless steels along with Duplex stainless steel are prone to 885 °F 475 °C Embrittlement.
- Austenitic- steels are the most common. Their stable microstructure is obtained from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures i.e. above lower transformation temperature. This structure gives these steels their characteristic combination of weldability and formability. Continue reading this article for detailed information on the Austenitic Stainless Steels.
- Martensitic- These steels are similar to ferritic steels having primary alloying element as Chromium but have high Carbon contents as high as 1% to suit various metallurgical properties. This allows martensitic stainless steel to be hardened and tempered much like carbon and low-alloy steels. They are used when high strength and moderate corrosion resistance is required. They have generally low weldability and formability. They are magnetic in nature and can be differentiated from ASS with this properties.
- Duplex– These steels have a microstructure which is approximately 50% ferritic and 50% austenitic,. This gives them a higher strength than either ferritic or austenitic steels. They are resistant to stress corrosion cracking. The “lean duplex” steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. Super Duplex steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.
- Precipitation hardening (PH)– These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the steel. Following a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts strength to the alloy. These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment makes them suitable for shaft material for aerospace, oil & gas etc. Corrosion resistance of PH is comparable to standard austenitic steels like 1.4301 (SS304).
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, 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 having high carbon, manganese and nitrogen compare to 300 Series & lower Ni to compensate 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 provide the improved creep strength at elevated temperature. Nitrogen is added to certain grades as strength enhancer mainly at ambient and cryogenic temperatures. Nitrogen added types are designated with suffix N e.g. 304LN. Manganese increase the solubility of nitrogen in the austenitic phase due to this higher N is added to grade having higher Mn% especially in 200 series. Further ASS grades are suffix with L (nominal value of 0.03 wt% C) or H (upto 0.1 wt% C) which mean low carbon or high carbon respectively. Grade 316 substitutes approximately 2% molybdenum (Mo) to nearly equal amount of Cr to enhance pitting corrosion resistance. Some grades contain 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 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 upto 1% of Ti & Nb effectively reduce the matrix carbon content thus preventing M23C6 types formation. This also reduce the sensitization effect in ASS and hence prevent intergranular corrosion (IGC).
Austenitic stainless steels being ductile types exhibits significant elongation in a tensile test. The presence of Cr as highly oxidizing elements provides a protective surface layer for resistance against the corrosion and further oxidation. They are not an appropriate choice in some common environments such as sea water, chloride containing media or in highly caustic environments due to their susceptibility to stress corrosion cracking (SSC).
Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. Austenitic stainless steels cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard grades of austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. Austenitic stainless steels are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.