Over the last decades, the duplex stainless steels entered the relevant industrial sectors. Today, an advanced understanding of the effect of different processing and service conditions on the mechanical behaviour, the fracture mechanics and the corrosion behaviour of duplex stainless steels is available.
Since 1979, BUTTING has been continuously producing longitudinally welded pipes and components made from all kinds of duplex stainless steels for industrial purposes.
As a result of over 30 years of experience, this article gives a lot of practical advice for manufacturing longitudinally welded pipes from all kind of duplex stainless steels.
It was in the year 1927 when the chemists Bain and Griffith first described the binary austenitic/ferritic structure of the nowadays so commonly used duplex stainless steels. The typical ferritic/austenitic structure is achieved by alloying carbon steel with ferrite forming elements such as chromium and austenite forming elements such as nickel, manganese and nitrogen in a certain range of concentration (Figure 1). During hot forming and solution heat treatment of the alloy, the austenitic areas in the microstructure are frozen within the ferritic matrix.
In the optimum condition, the material shows an austenite/ferrite ratio of 50 % to 50 %. Duplex stainless steel is much more than a compromise between austenitic and ferritic materials but the ideal combination of strengths: the high mechanical properties of ferritic materials and the good corrosion behaviour of austenitic materials with increased stress corrosion resistance.
A comparison of the yield strengths of different duplex, superduplex and lean duplex stainless steels with austenitic stainless steels is given in Figure 2. This picture gives an idea of what wall thickness engineers might save during the design phase by using the advantage of the higher strength of the ferritic austenitic materials.
Depending on the particular price of raw elements important cost advantages can be obtained as shown in Figure 3.
Properties of Duplex Stainless Steels
The designations "superduplex stainless steel", "lean duplex stainless steel" and "hyperduplex stainless steel" are not clear alloy definitions or material designations. Duplex stainless steels are commonly understood as ferritic/austenitic materials, where the Pitting Resistance Equivalent (PREN = % Cr + 3.3 x % Mo + 16 x % N) is between 32 and 35, superduplex stainless steels as higher alloyed ferritic/austenitic materials with a PREN > 40, and lean duplex stainless steels show a typical PREN between 27 and 32.
An overview of the typical chemical composition of different duplex stainless steels is given in Table 1:
The combination of properties of duplex stainless steels explains the success of this group of materials in the process industry:
- Higher strength (0.2 % yield strength > 450 MPa) than austenitic CrNi-steels
- Good toughness and ductility
- Hot workability (temperatures 1,000 - 1,100 °C, rapid cooling)
- Good erosion resistance
- Corrosion resistance comparable with superaustenites, 6 moly stainless steels, (1.4529, 1.4547...)
- High resistance against stress corrosion cracking (ferrite)
- High resistance against pitting corrosion (Cr, Mo, N)
- Good weldability
- Cost effectiveness
Table 2 shows the chemical composition of higher alloyed superduplex stainless steels.
The superduplex stainless steels compete with superaustenitic materials and find a growing acceptance in corrosive applications where significant amounts of chlorides, oxidizing conditions and low pH values are present. Table 3 shows the chemical composition of lean duplex stainless steels.
Risks during Manufacturing Duplex Stainless Steels
In order to enjoy the above described properties of duplex stainless steels during the industrial service, several precautions should be taken:
The phase equilibrium of the binary structure of duplex stainless steels is mandatory for the properties. An imbalance of austenite and ferrite could generate the loss of toughness, a reduced corrosion resistance, a loss of the mechanical strength or the stress corrosion cracking resistance of the material.
Whenever there are higher temperatures involved, maybe during hot forming or welding, the sensitivity of duplex stainless steels concerning intermetallic phase precipitations needs to be considered. These can be σ-Phase (Cr- und Mo-rich intermetallic phases) precipitations, which are followed by reduced toughness or corrosion resistance.
Between 280 and 525 °C the so called "475 °C embrittlement" is observed and normally detrimental to the toughness and ductility of duplex stainless steels.
Therefore, the recommended practice for final solution annealing after welding, hot forming or high degrees of cold forming is a solution annealing at temperatures above 1,000 °C followed by a rapid cooling.
Relevant standards summarizing the chemical composition and typical properties of duplex stainless steels are the DIN EN 10088 and the ASTM standards ASTM A240, ASTM A928 and ASTM A790.
Read about the "Weldability of Duplex Stainless Steels during Pipe Production" in our next newsletter...