Duplex stainless steel is a popular material in many industries. However, there are different grades with different chemical compositions. What the differences and advantages are, will be explained in the following.
Duplex Stainless Steel – Production and Alloys
The designation “Duplex” comes from the biphasic structural constitution of a ferritic and an austenitic phase, both to almost equal portions. A balanced ratio of 50:50 grants optimal characteristics of Duplex steels.
The content of chromium is between 19.5 and 33 percent. In comparison to austenitic steels, duplex has a content of nickel between one and nine percent. Furthermore, approximately 0.05 to 0.6 percent nitrogen for welding improvements is added. Furthermore, adding 0.1 to 5 percent molybdenum creates better durability in chloric media. An addition of one to six percent manganese on the other hand increases the strength. In some duplex grades, copper and tungsten are also added to the alloy.
To reduce the risk of intergranular corrosion, the content of carbon does not exceed or is equal to 0.03 percent.
History of Duplex Stainless Steel
The beginning of Duplex production goes back to 1930 when a Swedish mill melted the first commercial Duplex alloy with 26 percent of chromium, 5 percent nickel and 1.5 percent molybdenum. This alloy also known as 1.4460. With the increasing demand for oil and natural gas in the 1980s and the related increased demand for new conveyor systems, the need for stainless steels with high corrosion resistance, high strength and good workability rose as well. At the time, the most used duplex stainless steel grade was “2205”. It is also known as 1.4462 (X2CrNiMoN 22-5-3) according to EN 10088-2 and VdTÜV-material datasheet 418 or UNS S31803 or UNS S32205, according to ASTM/ASME (ASTM A240, ASTM A789/A790, ASTM A928).
Today 1.4462 is still the most used Duplex stainless steel grade covering approximately 75 to 80 percent of the entire Duplex production.
Further Development of Duplex Stainless Steel Grades
Thanks to the development of new Duplex grades, nowadays one can choose from five different Duplex families:
- Lean Duplex Stainless Steel: lower alloyed than the first version 1.4462/2205
- Standard Duplex Stainless Steel: Chromium content between 19.5 and 23 percent, nickel content between 3 and 6.5 percent
- Nomen est Omen: Duplex grades with 25 percent chromium often also have a higher content of nickel and molybdenum than the standard Duplex
- Super Duplex Stainless Steel: the chromium content is between 24 and 30 percent, nickel between 3.5 and 8 percent, molybdenum between 0.8 and 5 percent. Furthermore, copper (up to two percent) and tungsten (up to 2.5 percent) are added to the alloy
- Hyper Duplex Stainless Steel: is an even more advanced version with a content of chromium of up to 33 percent, nickel up to 9.5 percent, molybdenum up to 5 percent. The alloy also includes manganese and copper
Characteristics of Duplex Stainless Steel
Due to the high content of chromium in the alloy, Duplex stainless steel has an excellent corrosion resistance.
All Duplex steels have high strength and ductility values at an optimal ratio of ferrite to austenite.
Advantages of Duplex Stainless Steel
Duplex is characterised by its high PRE value. The PRE, also known as Pitting Resistance Equivalent, is the measurement for the consistency of the pitting corrosion of a stainless steel grade. The higher the PRE value is, the more corrosion resistant is the alloy. With the value of 32 or above, the material gets declared as seawater resistant.
For calculating the PRE value, one must use this following formula: PRE = 1 x %Cr + 3.3 x %Mo + 16 x %N
The advantages of a steel with the microstructure of Duplex are as following:
- Pitting corrosion
- Stress corrosion
- Fatigue corrosion
The particular advantages of Duplex are high strength values and good weldability. Other stainless steel grades do not perform as good as. Furthermore, some Duplex steels come in addition with a high resistance to frictional wear stress.
In addition, it is important to mention the little heat expansion. And a fundamental aspect is the benefit of big cost savings potential that is given by the possible weight reduction due to its high yield.
Duplex Stainless Steel in Comparison to Austenitic Steels: Some Examples
|S32101||1.4162||0,04||21,0-22,0||1,35-1,7||0,1 – 0,8||0,20-0,25||4,0-6,0||0,1-0,8||–||26|
Duplex with 25% Cr
Using Duplex Stainless Steel in Many Ways
Duplex is a very successful stainless steel grade, for use in many different applications in various industry branches. Mainly duplex stainless steel is used in the following area:
- storage tanks, container or vessels, in pulp and paper, as well as in chemical industry
- tanks and containers in the food and beverage industry
- pressure vessels in all kinds of industries
- pipelines for oil and gas industry
- pipelines which are exposed to seawater and aggressive chemicals
- heat exchanger in petrochemical plants
- flue gas cleaning plants
- desalination plants
- components for vehicle construction – for example for trains, tramway or cars
- structural elements for bridges and architectural projects – especially near seawater
- all kinds of profiles which are needed for above mentioned range of use
Duplex suits for a temperature range between minus 40 degrees Celsius and plus 280 degrees Celsius.
Welding of Duplex Stainless Steel
Skilled welders can weld Duplex stainless steel sheets or duplex stainless steel profiles using the common welding methods for high-alloyed steels.
Thereunder one can find: submerged welding, tungsten inert gas welding, plasma welding, active gas metal arc welding, laser welding and electro-beam welding.
Where to Pay Attention
The cooling process after welding takes significant influence in the austenite-ferrite ratio of the material at room temperature. Like this, a delayed cooling results in a higher austenite content and in general a quenching to a lower austenite content.
The choice of the suitable welding process, as the correct seam geometry, ensures ease in adjusting quite precisely the austenite-ferrite ratio. For that matter, it is important to ensure a slow cool down rate between the temperatures of 1.100 degrees Celsius and 1000 degrees Celsius. The weld’s temperature shall pass from 950 to 700 degrees Celsius within two minutes, in order to avoid unwanted precipitations. A fast cooling speed should be kept until the temperature of 300 degrees Celsius.
Super Duplex is also weldable similar to a high-alloyed steel, therefore with good results.
Processing of Duplex Stainless Steel
The main product in Duplex, but also Lean Duplex as well as Super Duplex, are hot rolled sheets and bar steel. From this, we produce different productsd: Duplex pipes, stainless steel profiles like duplex beams, channels as well as rectangular hollow sections and square hollow sections.
Some Duplex steels are suitable for cold forming, but due to the high strength, that needs great forces. Besides, it is important to consider the high resilience properties.
Duplex stainless steel is also suitable for hot forming or bending, this has to occur in a temperature range between 1000 degrees Celsius to 1100 degrees Celsius and a fast cool-down rate is mandatory.
Processing of Super Duplex Stainless Steel
Super Duplex is also suitable for cold forming. However, due to the even higher strength, the resulting resilience properties have to be carefully monitored.
Hot forming of Super Duplex has to occur at temperatures between 1080 degrees Celsius und 1280 degrees Celsius. The range between 1000 degrees Celsius to 700 degrees Celsius must be avoided as precipitation of an unwanted σ-phase could take place. σ-phases are intermetallic phases that negatively influence the characteristics of steel. The disadvantage is: they are hard, brittle and have low corrosion resistance. It is advisable to continue cooling to a temperature of 300 degrees Celsius as fast as possible to avoid further unwanted phases.
Surface Treatment of Duplex Strainless Steel
High-alloyed steels have an optimal corrosion resistance but only at blank surfaces. A final chemical treatment allows to achieve this surface, this is done either by bath pickling or by so-called spray-pickling. Removing possible oxidation layers is another method to achieve a clean and corrosion-resistant surface. For this operation grinding, blasting or brushing are suitable methods. Pickling is supposed to be the most suitable process for stainless steels. A blasting process can eliminate adherent scale prior to final pickling.