A complete overview of Duplex Stainless Steel
Duplex stainless steels are becoming more common. All the major manufacturers make them of stainless steel - and there are several reasons for this:
- High strength to reduce product weight
- High corrosion resistance, especially to corrosion cracking
But despite all this interest, the share of DuplexSteel Flanges in the world market is, according to the most optimistic estimates, from 1 to 3%. The purpose of this article is to explain in simple words the features of this type of steel.
Duplex Stainless Steel Overview
The idea of creating duplex stainless steel arose in the 1920s, and the first smelting was done in 1930 in Avesta, Sweden. Nevertheless, a noticeable increase in the share of the use of duplex steels occurs only in the last 30 years. This is mainly explained by the improvement of steel production technology, especially the processes of regulating the nitrogen content in the steel.
Austenitic - low strength (conditional yield strength 0.2% in the state after austenitization 200 MPa), low resistance to corrosion cracking
Ferritic - low strength (slightly higher than austenitic: conditional yield strength 0.2% is 250 MPa), poor weldability at large thicknesses, low-temperature brittleness
Also, the high nickel content in Super Duplex Tubing leads to their cost increase, which is undesirable for most end users.
The phase composition (approximately the same amount of ferrite and austenite) provides the following advantages:
1) High strength - the range of the conditional yield strength of 0.2% for modern super duplex flanges grade is 400-450 MPa. This allows you to reduce the cross-section of the elements, and therefore their mass.
2) Good weldability of large thicknesses - not as simple as austenitic, but much better than ferritic.
3) Good toughness is much better than that of super duplexpipe fittings, especially at low temperatures: usually up to minus 50 degrees Celsius, in some cases up to minus 80 degrees Celsius.
The corrosion resistance of duplex steels
Due to the variety of super duplex round bar in determining corrosion resistance, they are usually given together with austenitic and ferritic grades of steel. A single measure of corrosion resistance does not yet exist. However, for the classification of steel grades, it is convenient to use the numerical equivalent of pitting corrosion resistance (PREN).
Preventing the spread of duplex steels
Such an advantage as high strength instantly turns into a disadvantage, as soon as it comes to manufacturability of material processing by pressure and machining. High strength also means lower plastic deformation than steel. Therefore, super duplex fasteners are practically unsuitable for the manufacture of products in which high flexibility is required. And even when the ability to plastic deformation is at an acceptable level, all the same, to give the necessary shape to the material, such as when bending pipes, more effort is required.
The process of smelting super duplex 2507 round bar is much more complicated than austenitic and ferritic steels. In violation of production technology, in particular heat treatment, in addition to austenite and ferrite in the duplex steel, several undesirable phases can form. The two most significant phases are shown in the diagram below.