Stainless steel is an iron alloyed with a minimum of 10.5% chromium. Other alloying elements are added to enhance the formability, strength and toughness. These include metals such as Nickel, Molybdenum, Titanium and Copper. Non-metal additions are also made, the main ones being Carbon and Nitrogen.
Stainless steel is divided into 5 types:
Ferritic
These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. Chromium is the main alloying element, typically 10.5-17%. They are usually 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 such as low cost food and catering equipment. They are not advisable for use in a very corrosive environment. Ferritic steels are sometimes chosen for their resistance to stress corrosion cracking.
They are magnetic.Typical grades are 409 and 430.
Typical use is low cost food and catering equipment.
Austenitic
These are the most common commercially available stainless steels. Low Carbon, Chromium contents typically 16-20%. Nickel contents typically 7-13%. Their microstructure is derived from the addition of Manganese and Nitrogen also. They have a good combination of weldability and formability. Corrosion resistance is very good and can be enhanced by adding Molybdenum. They are normally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.Typical grades 304 (for internal) and 316 (for external or corrosive environs).Typical use is general architectural metalwork and industrial and commercial.
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. So called “lean duplex” steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. “Superduplex” 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.
Typical grade 2205.Typical use is structural stainless steel and architectural metalwork, where high strength and high corrosion resistance are required.
Precipitation Hardening (PH)
These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the steel. With a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts strength. 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. This is in contrast to conventional hardening and tempering in martensitic steels where distortion is more of a problem. Corrosion resistance is comparable to standard austenitic steels like grade 304.
Typical grade 17/4PH.Typical use is machined components with high tolerance, high strength, and high corrosion resistance.
Martensitic
These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic.Typical grade 420.Typical use is machined components where surface hardening is required.Equivalent Grade Designations:AISI Code EN Code
409 1.4512
430 1.4016
304 1.4301
316 1.4401
2205 1.4462
420 1.4021
17/4PH 1.4542
