DUPLEX STEEL – CHARACTERISTICS
Duplex steel (austenitic-ferritic) has a balanced two-phase structure, with roughly equal amounts of austenite and ferrite. The steel has a high chromium content – from 19 to 32% – and molybdenum – up to 5%.
The history of creating duplex steel can be divided into several key stages:
- Initial research: Early work on developing duplex steel began in the 1930s, when researchers started experimenting with different proportions of the austenitic and ferritic phases to obtain a combination of the beneficial properties of both phases. The aim was to combine the mechanical strength of ferritic steel with the corrosion resistance of austenitic steel.
- Introduction of duplex alloys: In the 1960s, commercial production of duplex alloys began. Initially, alloy 329 (F51) was used, which was one of the first alloys with a composition close to today’s duplex steel. This alloy was characterized by a balance between the austenitic and ferritic phases, which allowed for better mechanical properties and corrosion resistance compared to other types of steel.
- Evolution of alloys: In subsequent years, duplex alloys continued to develop. Chemical composition was improved, heat treatment processes were optimized, and mechanical properties and corrosion resistance were enhanced. New alloys emerged, such as type 2205, which became one of the most popular steels used across various industries.
The strength properties of duplex steel and its resistance to acids or chlorides are higher than those of austenitic steels. It is also characterized by good resistance to pitting, crevice, and stress corrosion. Additionally, it features increased ductility, impact resistance and hardness.
PROPERTIES OF DUPLEX STEEL
Duplex steel has high mechanical parameters. It shows, above all, great resistance to general corrosion, intergranular corrosion, stress corrosion and pitting corrosion. It maintains very good resistance to corrosive cracking, wear and erosion. Duplex steel is magnetic, has good weldability and machinability, and also low thermal expansion. It does not harden as a result of heat treatment.
MARKET SHARE
Duplex-grade steels account for about 2% of the world’s stainless steel production, but their use shows a steady upward trend.
WHEN TO USE DUPLEX STEEL?
Duplex steel is used where a combination of high mechanical strength and corrosion resistance is required. It is most often used in desalination plants, the mining industry and the paper industry. There are also numerous bridge structures made entirely of duplex steel.
DUPLEX STEEL – APPLICATIONS
Duplex steel is used in many fields – in the chemical, construction and medical industries. It is used, among other things, in the manufacture of pressure equipment, in the food industry for building storage tanks, and in pulp processing. It is used to create large structural constructions (bridges, dams) and other architectural elements that require durability in heavily corrosive atmospheres. It is also used in the construction of oil and gas production platforms.
It is a high-alloy steel grade. Compared to duplex steel, it exhibits increased resistance to pitting, crevice and stress corrosion when in contact with chlorides and acids. It is used in aggressive environments.
Like duplex steel, it has a two-phase structure, being a combination of ferritic and austenitic structures. It is characterized by greater strength than austenitic and ferritic steels. It is an alternative to the aforementioned grades due to lower production costs of the material, without compromising quality and mechanical properties. An example of super duplex steel is grade 1.4410 with a content of 25% chromium, 4% molybdenum and 7% nickel.
Super duplex steel finds applications in the mining, energy and chemical industries. It is used for building production platforms, pipelines, in structures used in marine environments, for manufacturing heat exchangers and other elements exposed to harsh conditions.
