ADVANCED CASTING TECHNOLOGIES/DUPLEX STAINLESS STEELS

One of the significant advantages of duplex stainless steels over other stainless steels is their higher strength. This advantage makes these materials stand out in the production of parts with thin sections. For this reason, they are preferred in structural applications such as bridges, as well as in applications such as pressure vessels and storage tanks, and in petrochemical, marine, and pipeline industries.

SUPER DURABLE

As can be understood from the term “duplex” in the name of the material, the structure of duplex stainless steels consists of two phases: ferrite and austenite. In addition to the ferrite phase, which is stable at room temperature, the other half of the structure consists of the austenite phase, which is normally expected to be stable at high temperatures but can also be found at room temperature with various alloying elements. As a natural result of this mixture, we see that the resulting material is able to synthesize the physical properties of these two phases within its own structure.

The term “stainless steel” is a general term that refers to steels that contain low carbon but high chromium (at least 11%) and, as the name suggests, exhibit a certain degree of corrosion resistance. Although the term “stainless” appears in the name, it is important to note that these steels can still rust under certain conditions, and a more accurate term would be “corrosion-resistant steel.” To increase corrosion resistance, chromium is added to these steels, and depending on the type of alloy, other alloying elements such as molybdenum and nickel may also be added. When examining the microstructural properties of these materials, we can see that they are produced to exhibit one of five different structures: martensitic, ferritic, austenitic, precipitation hardening, and duplex (two-phase).

First production in Sweden

When we take a look at the history of this type of steel, we realize that it is not a very young material: The idea for these steels began to emerge in the 1920s and the first production trials date back to the 1930s in Sweden. We can see in the literature that these materials, which were developed with the idea that they could stand against the intergranular corrosion problem seen in high carbon austenitic steels, were produced by casting in Finland in the 1930s.

From this perspective, it may come as a bit of a surprise that, although it is an older material than spheroidal graphite (ductile) cast iron, which emerged in the early 1950s, it took much longer to find serious industrial use. The reason behind this delay lies in the inadequate level of technology available at the time to enable the production of the material: Considering that in the 1930s, steel production technologies, especially those that would make it possible to control the amount of nitrogen, were not yet at a level that would produce the desired results, it is understandable why the widespread use of this material was delayed.

We said that duplex stainless steels are a mixture of austenite and ferrite phases. What kind of material does the mixture of these two phases produce? Let's start with the effects of the different components: The ferrite phase in the structure gives the material mechanical strength and resistance to stress corrosion cracking, while the austenite phase provides ductility and general corrosion resistance. 

Durability advantage

Another important advantage of duplex stainless steels compared to other stainless steels is their higher strength. This advantage makes these materials stand out especially in the production of parts with thin sections. With yield strengths ranging between 400-550 MPa, these materials are preferred in structural applications such as bridges, as well as in applications such as pressure vessels and storage tanks. In addition to these areas, duplex stainless steels can also be used in petrochemicals, shipping and pipelines.

Duplex stainless steels, which we can call the first generation, performed poorly in terms of weldability due to their low structural stability. The high rate of ferrite formation in the heat-affected zone during welding both negatively affected the toughness of these zones and led to a worse corrosion resistance compared to the base material. However, the argon oxygen decarburization (AOD) method developed after 1968 opened a new page in stainless steel technology. By adding nitrogen as an alloying element and precisely controlling the nitrogen level, this technology can prevent the reduction in toughness in the heat-affected zones and provide corrosion resistance close to that of the base metal.

Divided into 5 main groups

Just like austenitic stainless steels, duplex stainless steels are divided into groups that exhibit varying levels of corrosion resistance depending on the alloying elements they contain. Although there is no clear idea in the technical literature on how to define these groups, we can evaluate these materials within five main groups as follows:

• Lean duplex: Duplex steels with no molybdenum added.
• Lean duplex with molybdenum: Duplex steels containing molybdenum.
• Standard duplex: These steels, which contain about 22% Cr and 3% Mo, account for about 60% of duplex steel use.
• Super duplex: Duplex steels containing about 25% Cr and 3% Mo and exhibiting high corrosion resistance (Pitting corrosion resistance equivalent (PREN) between 40 and 45).
• Hyper duplex: Steels with higher Cr and Mo content and higher corrosion resistance than super duplex steels. (Pitting corrosion resistance equivalent (PREN) above 45.)

We know that the resistance of stainless steels to localized pitting corrosion is largely determined by the alloying elements they contain. When we look at the alloying elements that increase this resistance, we see that chromium (Cr), molybdenum (Mo) and nitrogen (N) are mentioned in the literature. Although tungsten (W) is not a widely used element, it can create a similar effect, albeit half as much as molybdenum. To evaluate the resistance of stainless steels to pitting corrosion in chloride solutions, we can make an assessment based on the composition of the alloy. We can evaluate the pitting corrosion resistance with the PREN (pitting corrosion equivalent number) using the following equation.

PREN = %Cr + 3,3(%Mo + %0,5W) + 16%N

Alloying elements found in duplex stainless steels

When adjusting the composition of duplex stainless steels, the Fe-Cr-Ni phase diagram is instructive. Below is a cross-section of 68% Fe in this ternary phase diagram. As can be seen on the diagram, we observe that duplex steels first solidify in the ferrite structure and then partially transform into austenite during cooling, depending on the alloying elements. The presence of nitrogen in the structure increases the transformation temperature of ferrite to austenite, positively affecting the structural stability of the material. This is especially important for the weldability of duplex steels and the prevention of ferrihydrification in the heat-affected zone.

As can be seen in the diagram, slight variations in composition can dramatically change the ratio of ferrite to austenite that we expect to see in the structure of duplex stainless steels. Since the elements, taken individually, have either a ferrite-forming or austenite-forming effect, a slight excess of one of the alloying elements can lead to an increase in the amount of either the ferrite or austenite phase in the structure.

Let us now consider the effects of these basic alloying elements one by one.

Chrome (Cr)

As is well known, chromium is an essential element in stainless steels. The minimum 11% chromium added to steels enables the formation of a thin passive film on the steel surface, thereby enhancing the steel's resistance to atmospheric corrosion. In duplex steels, the minimum chromium content required increases to over 20%. As the chromium content rises, we observe an increase in the corrosion resistance of stainless steels. Due to chromium's ferrite-forming properties, higher levels of chromium in duplex stainless steels require an increasing amount of nickel (Ni), a ferrite-forming element, to maintain the ferrite-austenite balance. Additionally, higher chromium content helps improve the steel's resistance to high-temperature corrosion.

Molibden (Mo)

Molybdenum increases the resistance of stainless steels against pitting corrosion. When the chromium content of stainless steels exceeds 18%, molybdenum begins to increase corrosion resistance 3 times more effectively than chromium. Since molybdenum, which is a ferrite-forming element like chromium, causes the formation of some harmful intermetallic phases, it is generally used at a maximum of 7% in stainless steels and at a maximum of about 4% - 5% in duplex stainless steels.

Nitrogen (N)

Nitrogen is an element that increases the resistance of austenitic and duplex stainless steels to both pitting and crack corrosion. In addition to this positive effect on corrosion resistance, nitrogen also has the ability to create solid solution hardening and enhance the strength of steels. Due to its relatively low cost and austenite-forming properties, nitrogen is often used as a substitute for some of the nickel used in the production of austenitic steels. The increased toughness of nitrogen-containing duplex stainless steels is achieved not only by the formation of the austenite phase but also by a reduction in the amount of certain harmful intermetallic phases. Nitrogen does not completely prevent the formation of these harmful intermetallic phases. However, by delaying their formation, it ultimately results in such an effect. In particular, in austenitic and duplex stainless steels containing high amounts of chromium and molybdenum, nitrogen is utilized to prevent the formation of the sigma phase.

Niquel (Ni)

Nickel is used in duplex steels to maintain the ferrite-austenite balance due to its austenite-forming properties. To form austenite against ferrite-forming chromium and molybdenum, a certain amount of nickel and nitrogen must be added to the alloy. In ferritic stainless steels, where the formation of the austenite phase is undesirable, nickel is not used. However, in duplex steels, where we want some austenite to form, we need to add nickel in the range of 1.5% to 7%. In austenitic stainless steels, the nickel content must be at least around 6%. Nickel, like nitrogen, has an effect that delays the formation of harmful phases observed in austenitic stainless steels. However, it is important to note that nickel's effectiveness in this regard is significantly lower compared to nitrogen.