Duplex Stainless Steel – All You Need to Know
Introduction to stainless steel types
Stainless steel is most commonly divided into 5 basic branches – Austenitic, Ferritic, Martensitic, PH (Precipitation hardening), and Duplex.
Austenitic stainless steel (SS304, SS316) is the most widely used type of the alloy thanks to its strength and ductility. As the case is with all stainless steel types, austenitic alloys are corrosion-resistant under normal atmospheric conditions.
The addition of nickel (standardly 8-10%) makes the structure of austenitic stainless steels strong and durable.
Depending on their grade, austenitic stainless steel alloys are, among other things, used to build pipes, storage tanks, and aircraft parts.
However, this stainless steel type is also characterized by its vulnerability to stress corrosion cracking under increased temperatures and the presence of humidity.
Ferritic stainless steel has a carbon-like microstructure and contains no nickel in its composition.
Ferritic alloys are susceptible to cracking after welding due to grain growth in the heat-affected zone. They are also highly limited when it comes to ductility and thus cannot be used for formation of larger and thicker steel structures.
This type of steel is commonly used to build interior and exterior components of household appliances (e.g. fridges and washing machines).
The most common version of ferritic stainless steel is SS430.
Martensitic stainless steel can be hardened through heat treatment and contains carbon in addition to chromium. Due to its ability to withstand corrosion in humid conditions, martensitic steel is widely used in production of cutlery, automotive parts, cutting equipment, and medical instruments.
Precipitation hardened stainless steel is heat treated for increased strength and durability. In addition to nickel and chromium, PH steel may contain aluminum, copper, titanium, niobium, and molybdenum. This makes its structure extra strong.
Anti-corrosion properties of precipitation hardened steel rival those of austenitic 304 grade alloys. Additionally, this type of steel is less susceptible to stress corrosion cracking.
Due to these characteristics, PH stainless steel is strongly represented in aerospace, automotive, and nuclear industry branches.
Stainless steel types described above possess obvious advantages and shortcomings in terms of tensile strength and ductility – two important factors to consider when choosing materials for construction and manufacturing.
In addition to these factors, corrosion-resistance levels are also extremely important.
Any given stainless steel grade or type may be advantageous over other types in one way or another. However, the very same type may also provide inferior performance in other areas.
Thus, manufacturers are often compelled to select one important characteristic at the expense of other useful ones.
The introduction of Duplex stainless steel to the industry solved this problem to a great degree since it displays positive characteristics in all important areas – strength, hardness, ductility and corrosion-resistance.
We explain the composition and characteristics of Duplex stainless steel below.
What is a Duplex Stainless Steel?
The microstructure of Duplex stainless steel consists of 50% austenitic and 50% ferritic steel grains with different ratios being employed as needed.
As a combination of two types of steel, Duplex took the best features of both while eliminating some of their prominent flaws.
This is why Duplex outperforms most of austenitic and ferritic alloys when they are used individually.
Origins of Duplex can be traced back to Sweden where this steel type was developed during 1930s. However, the material started gaining popularity at the very end of the century with the advancement of metal processing technologies.
The structure of Duplex has also been significantly optimized over the years with new grades being developed constantly.
Addition of certain chemical components and different production methods can produce four basic Duplex forms:
- Lean Duplex usually contains no molybdenum and has lower amounts of nickel. It is also possible to produce lean Duplex with molybdenum added (S32003 grade). Lean Duplex is widely used for construction of storage tanks.
- Regular Duplex possesses standard amounts of both nickel and molybdenum – grade 2205 is a good representative of this type. Among other industries, regular Duplex is used widely in the pharmaceutical sector.
- Super Duplex has high amounts of molybdenum and chromium (24-26%) – grade 2507 would be an excellent example. The material is often used for production of tubes and pipes for oil, gas, and chemical industries.
- Hyper Duplex contains even higher volumes of molybdenum and chromium (27-32%) and is represented by the grade S32707. This Duplex type is used in energy and oil sectors and provides excellent results with subsea applications.
All Duplex grades contain nitrogen which, in combination with molybdenum and chromium, provides excellent protection against corrosion.
This is only one of many benefits associated with using Duplex – see more below.
Duplex Stainless Steel – Main Benefits
As stated above, Duplex is normally performing better than individual steel types found within its microstructure. Better said, the combination of positive characteristics coming from austenite and ferrite elements provides a better overall solution for a great number of different production situations.
Here is how Duplex compares to austenitic and ferritic alloys when it comes to several important production and performance factors:
Anti-corrosive properties – The effect of molybdenum, chromium, and nitrogen on corrosion resistance of Duplex alloys is immense. Several Duplex alloys can match and exceed anti-corrosive performance of popular austenitic grades including 304 and 316. They are especially effective against crevice and pitting corrosion.
Stress corrosion cracking – SSC comes as a result of several atmospheric factors – temperature and humidity being the most evident ones. Tensile stress just adds to the problem. Normal austenitic grades are highly susceptible to stress corrosion cracking – Duplex stainless steel is not.
Toughness – Duplex is tougher than ferritic steels – even at lower temperatures while it does not actually match the performance of austenitic grades in this aspect.
Strength – Duplex alloys can be up to 2 times stronger than both austenitic and ferritic structures. Higher strength means that metal remains firm even with reduced thickness which is especially important for reducing weight levels.
Ductility – Duplex is fairly more ductile than ferritic steel. However, its ductility cannot rival that of the austenitic type.
Weldability – Austenitic steels are easier to weld than Duplex. However, this material is still more weldable than the ferritic steel. Duplex welding requires careful process planning to control hydrogen, nitrogen, oxygen, and ferrite levels for optimal corrosion resistance and structural stability. Low thermal expansion of Duplex compensates for some of its shortcomings when it comes to welding.
Price – With less nickel in its composition, Duplex is certainly a more affordable solution than austenitic grades that are similar in all other aspects. Additionally, the ability to use Duplex in thinner forms (due to its strength) decreases the need for extra weight and adds to its cost-effectiveness.
Is There a Catch?
Several positive characteristics of Duplex stainless steel might also count as negatives while also explaining why this material is not getting the attention it deserves.
Due to their strength, most of Duplex alloys are not easily formable and machinable. This is why operators need to apply more force and be prepared for a slower process since austenitic steel is machined much faster.
Consequently, this may result in an increase in the number of cutting consumables used.
Limited ductility (compared to austenitic steel) means that Duplex is far less usable for precise formation of smaller and more delicate structures.
Furthermore, there are couple of unwanted consequences that appear if duplex processing is not carefully handled and controlled.
If inappropriately cooled down or affected by unusually high or low temperatures, duplex will show signs of embrittlement (cracking).
Functional temperatures for duplex alloys are between -80°C (-112°F) and 300°C (572°F) which negatively affects the prospects for its use.
Duplex Weld Cleaning with Cougartron Machines
When compared to other stainless steel types, Duplex displays fantastic anti-corrosive properties.
However, this only means that it’s less susceptible to corrosion – not completely immune.
Different types of fabrication can damage the structure of Duplex to a greater or lesser extent and thus make it more vulnerable when facing harsh working environments.
Welding is no exception in this regard even though Duplex has a high tolerance to heat-caused cracking – more so in the weld area than in the heat-affected zone.
To optimally protect the steel surface against formation of rust and other impurities, it is important to thoroughly clean the area before and after welding.
Duplex does not differ from other stainless steel types when considering proper post-welding cleaning methods.
Oxidation and heat tinting are just some of the common side-effects that appear after stainless steel welding.
When compared to the austenitic type, heat tints (discoloration) can be quite stubborn when they appear on Duplex welds.
Cleaning the affected area is only part of the solution – a passive layer that provides stainless steel with its anti-corrosive properties has to be fully restored.
Electrochemical cleaning is the only method that guarantees this process. Manual grinding and chemical pickling are also used fairly often but are faced with numerous shortcomings.
Cougartron offers a variety of useful and smart machines for electrolytic stainless steel weld cleaning – all capable to clean Duplex.
However, we recommend our Cougartron ProPlus for the most demanding jobs. The machine is especially effective when used on larger steel structures (e.g. storage tanks) and can withstand heavy-duty use over larger periods of time.
If you are faced with a specific challenge related to Duplex fabrication and cleaning, give us a call – we are always happy to help.
Tel: +45 43 20 19 00