OTHER COMMONLY USED STAINLESS STEELS
There are three other more recent categories of stainless steels commonly found on the market, these were created to cover those areas where classical stainless steels did not provide adequate mechanical and/or chemical/physical properties.
PRECIPITATION-HARDENED STAINLESS STEELS, OR PH STEELS (PRECIPITATION HARDENING)
Precipitation hardening stainless steels, or Precitation Hardening, hence the PH acronym by which they are identified, have the possibility of significantly increasing their mechanical properties due to special heat aging treatments, which allow the precipitation of hard intermetallic phases in the austenitic or martensitic matrix, in order to increase the mechanical properties and fill the gaps of other classes of stainless steels, 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 strength and high toughness thanks to the addition of elements capable of forming precipitates during ageing heat treatments: Aluminium, Copper, Titanium, Molybdenum, Niobium, Vanadium, Nitrogen; these steels also have a corrosion resistance comparable to that of classic austenitic stainless steels, with the same amount of chromium and molybdenum.
BIPHASIC AUSTENITIC-FERRITIC STAINLESS STEELS, OR DUPLEX STAINLESS STEELS
The biphasic austenitic-ferritic stainless steels, also known as Duplex, have a structure of mixed crystalline grains of austenite and ferrite. They are steels with a hybrid microstructure: the chromium content ranges from 18% to 32% and tends to stabilise the ferritic microstructure, while the nickel content ranges from 4.5% to 7% and is insufficient to determine a microcrystalline structure that is completely austenitic and therefore remains partly ferritic.
Almost all Duplex variants contain between 2.5% and 4% Molybdenum but there are some forms of so-called "poor Duplex" which do not contain Molybdenum and have Nickel contents of less than 4.5%.
The basic properties are:
- peculiar microcrystalline structure known as duplex, austenitic and ferritic, which gives more resistance to stress corrosion cracking;
- higher degree of passivation due to the higher chromium content (and the presence of molybdenum) and therefore better resistance to point corrosion than 18/8 austenitic stainless steels;
- Good weldability and forgeability;
- high tensile and yield strength.
The most common uses are: heat exchangers, material handling equipment, tanks and vats for liquids with a high concentration of chlorine, seawater chillers, desalinators, food brine plants and groundwater rich in aggressive substances. It is also used for the construction of turbines in hydroelectric power stations.
MN-N OR NICKEL-FREE AUSTENITIC STAINLESS STEELS
Austenitic stainless steels with Manganese-Nitrogen arise from the need to replace Nickel in the metal alloy, as it is very expensive and an allergen.
Manganese and Nitrogen are elements capable of stabilising the austenitic microstructure. In detail, Manganese allows an increase in the solubility of Nitrogen in the ferrous matrix, while Nitrogen provides the real austenitising contribution and strongly increases the mechanical characteristics and resistance to corrosion. In order to guarantee the austenitic microstructure, Manganese must be present in large quantities in the alloy, up to 23% and Nitrogen up to 1%.
The good mechanical properties and sufficient machinability combined with the biocompatibility of these two elements make these steels ideal for medical applications and good substitutes for conventional austenitic stainless steels.