Other stainless steels

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Three more recent categories of stainless steel have emerged in the market to address areas where traditional stainless steel lacked sufficient mechanical and/or chemical/physical properties.

PRECIPITATION-HARDENED (PH) STAINLESS STEELS

Precipitation-hardened stainless steels, also known as PH steels, have the unique ability to significantly enhance their mechanical characteristics through special heat treatments for ageing. These treatments allow hard intermetallic phases to precipitate into the austenitic or martensitic matrix, thereby increasing their mechanical properties and addressing the limitations of other stainless steel classes, such as the poor mechanical properties of ferritic and austenitic stainless steels and the poor corrosion resistance of martensitic stainless steels.

Precipitation-hardened stainless steels achieve high mechanical strengths and toughness through the addition of elements capable of forming precipitates during ageing heat treatments, including Aluminium, Copper, Titanium, Molybdenum, Niobium, Vanadium and Nitrogen. Moreover, these steels exhibit corrosion resistance comparable to that of classic austenitic stainless steels, provided they contain the same amounts of chromium and molybdenum.

AUSTENITIC-FERRITIC DUPLEX STAINLESS STEELS

Austenitic-ferritic duplex stainless steels, often referred to simply as Duplex, have a mixed crystalline grain structure comprising both austenite and ferrite. These steels feature a hybrid microstructure: the Chromium content ranges from 18% to 32% and tends to stabilise the ferritic microstructure, while the Nickel content, between 4.5% and 7%, is insufficient to achieve a fully austenitic microcrystalline structure, resulting in a partially ferritic composition.

Most variations of Duplex steel contain between 2.5% and 4% Molybdenum, however, there are certain types known as "poor Duplex" that do not contain Molybdenum and have Nickel contents below 4.5%.

The basic properties are:

  • special microcrystalline structure known as duplex, comprising both austenitic and ferritic, which provides greater resistance to stress corrosion cracking;
  • special microcrystalline structure known as duplex, comprising both austenitic and ferritic, which provides greater resistance to stress corrosion cracking;
  • good weldability and forgeability;
  • high tensile and yield strength.

Common applications for duplex stainless steels include heat exchangers, material handling machines, tanks and vats for liquids with high chlorine concentrations, seawater chillers, desalinators, plants for food brine and groundwater rich in aggressive substances. They are also used in the construction of turbines for hydroelectric power stations.

MN-N OR NICKEL-FREE AUSTENITIC STAINLESS STEELS

Manganese-Nitrogen austenitic stainless steels have come from the need to replace Nickel in the metal alloy due to its high cost and potential to cause allergies.

Manganese and Nitrogen serve as stabilising elements for the austenitic microstructure. More specifically, Manganese increases the solubility of Nitrogen in the ferrous matrix, while Nitrogen contributes to the formation of austenite, significantly boosting the mechanical and corrosion resistance properties. To maintain the desired austenitic microstructure, these alloys require substantial amounts of Manganese (up to 23%) and Nitrogen (up to 1%).

The excellent mechanical properties and adequate machinability combined with the biocompatibility of these two elements make these steels ideal for medical applications. Moreover, they serve as effective substitutes for conventional austenitic stainless steels.

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