Stainless steel is not a specific metal but rather an umbrella term used to classify a number of types of steel. All forms of stainless steel are iron alloys with a minimum of 10.5% chromium. The chromium is an essential part of the alloy, as it reacts with oxygen to form a thin layer of chromium oxide (i.e., passive layer) – which prevents corrosion of the steel’s surface.
Chromium levels in stainless steel can be increased to provide greater resistance to corrosion. Carbon, manganese, nickel, and molybdenum can also be used to give stainless steel specific properties, such as enhanced formability, weldability or toughness.
In general, stainless steel can be categorized into four main types. They include:
Austenitic steels – Austenitic steels are the most common type stainless steel. The addition of nickel, manganese and nitrogen gives them the same microstructure that’s present in ordinary steel at higher temperatures, making them weldable and pliable. While they cannot be hardened by heat treatment, they can be work hardened to high-strength levels whilst retaining ductility and toughness. They are generally non-magnetic and vulnerable to corrosion.
Ferritic steels – Ferritic steels usually have a relatively low carbon content, giving them similar microstructure to carbon and low-alloy steels. They are not conducive to welding, which can limit their use to thin sections. The high levels of chromium and molybdenum in ferritic steel, however, make it ideal in applications exposed to aggressive stress corrosion environments. Ferritic steel is magnetic and cannot be hardened by heat treatment.
Martensitic steels – Martensitic steels are similar to ferritic steels in chromium content; however, with carbon levels are as high as 1%, they can be hardened and tempered like carbon or low-alloy steels. They are magnetic, with low weldability and formability, but are strong and have moderate corrosion resistance.
Duplex steels – Duplex steels have a microstructure between that of ferritic and austenitic, but possess higher strength and stress corrosion cracking resistance. These two qualities can also be further enhanced in “lean duplex” or “Superduplex” steels. They are weldable under certain conditions, and are moderately formable and magnetic.
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