Stainless ferritic-austenitic Cr-Ni-Mo steel with nitrogen addition.
Duplex 31803 is characterized by:
- High corrosion resistance, especially against stress cracking in solutions containing chloride
- Good strength properties
- Resistant to intergranular corrosion in the temperature range up to 300°C (575°F)
- Heat treatment subsequent to welding not necessary
Additional information about Stainless Steel Duplex 31803
Duplex 2205 is a stainless steel that is Nitrogen-enhanced. It was developed to take on the common corrosion problems encountered in stainless steel. The standard duplex stainless steel 31803 has been refined over the years by many steel suppliers, which resulted in a restricted composition range known as UNS S32205 in 1996.
Duplex stainless steel actually dates back to the 1920s. The first cast was made in Sweden in 1930. It has only been in the last 30 years, the very end of the 20th century, that duplex steels have begun to rise in popularity. This is because advances in steelmaking techniques have perfected the Nitrogen content.
During the 1960s and early 1970s, several factors led to the creation of duplex steels. First to come along was the introduction of vacuum and Argon-Oxygen Decarburization (VOD and AOD) processes. These led to the productions of modern duplex grades. Duplex stainless steel was developed using low Carbon content in combination with high Chromium content, high Nitrogen content, and an approximately equal balance between austenite and ferrite.
The results were materials that had extremely good qualities. The alloy content was the answer everyone was looking for when it came to the local and uniform corrosion, which had been such a problem. The duplex microstructure contributed to the higher strength of the material. This newer, modern duplex steel also had good weldability.
These duplexes, such as stainless steel S31803, made their appearance at about the same time as increased activity in the offshore industries. This particular industry required a stainless steel that could withstand aggressive environments. Because a Nickel shortage at that time drove up the costs of austenitic steels, the oil industry began taking a second look at duplex steels.
The subject of duplex stainless steels seems to come up with great regularity every 2-3 years, and there is a lot of marketing activity surrounding duplex 31803 stainless steel and others.
The term “duplex” refers to a family of stainless steels. One side is austenitic, which means the steel is a metallic, non-magnetic allotrope of Iron or a solid solution of Iron, with an alloying element. The other side is ferritic, which is steel that is high in Chromium content, and magnetic stainless steels that have a low Carbon content.
Standard austenitic steels – such as the 304 – and ferritic steels – such as 430 – are easy enough to make and to assemble. Even though they are fine for a wide range of purposes, there are technical weaknesses in each one.
Austenitic weaknesses include:
- low strength
- low resistance to stress corrosion cracking
Ferritic weaknesses include:
- low strength ( slightly higher than austenitic)
- where sections are thick there is poor weldability
- poor low-temperature rigor
It is worth noting that the high Nickel content of the austenitic types leads to price unpredictability, which does not always make for happy customers.
This is why the basic idea of a duplex stainless steel has been so popular. A chemical composition had to be created that combined an approximately equal mixture of ferrite and austenite.
The structure of 2205 duplex stainless steel consists of austenite pools that are surrounded by a continuous ferrite phase. There is high strength in 2205 stainless steel and it is often referred to as a work-horse grade. In the duplex family of stainless steels, 2205 is the most widely-used grade.
Stainless Steel Duplex 31803 is characterized by:
- High corrosion resistance, especially to Chloride stress corrosion cracking. Standard austenitic steels are notorious for this type of corrosion. This is why stainless steel duplex S31803 is perfect for such applications as hot water tanks, brewing tanks, process plants and swimming pool structures.
- Higher strength. The range of 0.2% PS for the current duplex grades is from 400 – 550 MPa. Section thicknesses are reduced, which leads to a reduced overall weight. This is very good news for applications such as: pressure vessels and storage tanks as well as structural applications, such as bridges.
- Excellent general corrosion resistance, up to 300°C (575°F).
- Good weldability in thick sections. Not as uncomplicated as austenitic, but definitely better than ferritic steel.
- Good toughness. When at low temperatures, toughness is much better than ferritic steel, especially down to minus 50° C, drawing out all the way to minus 80°C.
Duplex stainless steel, such as the stainless steel duplex 31803, is part of a growing industry with very strong prospects in today’s markets. They continue to gain in popularity and rightly so, as they are very cost effective, especially when industries start to take a hard look at potential cost savings. Duplex usage across the board can lead to not only lower maintenance costs, but also longer life cycles.
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