Duplex steels and their types
It is important to note that this steel is called two-phase because it has austenitic and ferritic phases.
Duplex stainless steel is quite popular. Recently, it has gained even more popularity. Many manufacturers are engaged in its production. This is preceded by certain reasons:
- increased degree of strength, which allows to reduce the weight of manufactured products;
- resistance to corrosion processes, especially cracking.
Duplex conferences are held approximately every two or three years. They present interesting articles with deep technical content. Today, duplex steel is actively promoted in the market for goods and services. New brands launching this product regularly appear on the market.
Despite the high share of optimism, according to statistics, this product is about one to three percent of the market.
The idea of creating a product first appeared at the beginning of the last century. The first melting was carried out in the 30s. A noticeable increase in the share of material use has been noticeable in recent years. This is due to the improvement of production processes. Previously, the content of nitrogen in production was regulated.
Austenitic (AISI 304), ferritic (AISI 430) stainless steel are easy to manufacture. Easy to handle. They consist of one phase — austenite or ferrite. They are widely used in various fields. Each type has its own technical disadvantages:
1. Austenitic. They are characterized by low strength characteristics. The approximate yield point is 0.2% immediately after austenitizing 200 MPa. The product has a low resistance to stress corrosion cracking.
2. Ferritic. The following disadvantages are characteristic: the minimum strength, which is slightly higher than that of the previous type. The approximate yield point — 0.2% is 250 MPa. The product does not weld well at large thicknesses. Differs in fragility when the temperature drops.
Due to the high content of nickel in the composition, austenitic stainless steel is expensive. This makes the product less in demand.
The idea of creating the product under consideration is to select a suitable chemical composition, which forms a similar volume of ferrite, austenite. The phase composition gives such positive qualities:
1. Strength qualities. The approximate yield point is 0.2%. Manufacturers offer the product after austenitization — 400 — 450 MPa. This minimizes the cross-section of the elements. The weight of the product becomes less. Makes sense in areas like:
- pressure vessels, steel tanks;
2. The ability to weld large thicknesses.
3. High degree of impact strength. Higher than ferritic stainless steel. It is especially clearly seen at low temperature conditions: — 50 degrees Celsius. It can withstand up to — 80 degrees Celsius.
4. Resistance to stress corrosion cracking. Austenitic stainless steel is susceptible to this phenomenon. Modern materials make it possible to resist the process. The advantage is important in the production of such structures:
- tanks designed for heating hot water;
- brewing tanks;
- concentration plants;
- pool frames.
How is the balance of duplex stainless steels achieved?
You can understand how such a product is obtained by comparing the composition of two types: austenitic — AISI 304 and ferritic — AISI 430 stainless.
The main components of materials can be divided into ferritizing, austenitizing. Each of them helps to form a certain structure.
Ferritizing elements are Cr (chromium), Si (silicon), Mo (molybdenum), W (tungsten), Ti (titanium), Nb (niobium).
Austenitizing components: C (carbon), Ni (nickel), Mn (manganese), N (nitrogen), Cu (copper).
The AISI 430 brand has ferritizing elements. Its structure is ferritic. The AISI 304 grade is distinguished by its austenitic structure due to the presence of nickel in the composition in the amount of eight percent. To obtain a duplex with the presence of each phase of about fifty percent, a balance of austenitizing and ferritizing components is needed. This is the main reason why the concentration of nickel in such stainless steel is much lower than in austenitic.
|Brand||EN / UNS number||A type||Approximate content|
|Zeron 100||1.4501 /
|RDN 903||1.4482 /
|DX 2202||1.4062 / S32202||Low alloyed||23||2.5||0.3||0.2||1.5||-||-|
|LDX 2101||1.4162 /
Some brands that have recently appeared on the modern market use a compound of manganese, nitrogen to significantly minimize the presence of nickel. This has a beneficial effect on the formation of the cost of production.
To this day, the technology for the production of such stainless steel is gradually gaining momentum. Each manufacturer offers its own brand. There are a lot of brands that are engaged in the manufacture and sale of duplex steels. A little later it will be possible to see how leading manufacturers enter the market.
Duplex corrosion resistance
Due to the wide variety of materials, when identifying corrosion resistance, they are brought together with austenitic, ferritic grades. Corrosion resistance is not the same measure. The Pitting Resistance Equivalent (PREN) can be used to classify manufacturers.
|Brand||EN / UNS number||A type||Indicative PREN|
|6% Mo||1.4547 /
|Ferrinox 255 /
|Zeron 100||1.4501 /
|DX2202||1.4062 / S32202||Duplex||27|
|2101 LDX||1.4162 /
|316L 2.5 Mo||1.4435||Austenitic||26|
|RDN 903||1.4482 /
When choosing the type of steel, you need to pay attention to how suitable it is for operation in a particular corrosive environment.
Stress corrosion cracking
SCC is a type of corrosion that appears when exposed to a whole range of external factors:
- corrosive environment;
- tensile stress;
- high temperature conditions, up to +50 degrees Celsius.
Duplex steels such as austenitic steels such as AISI 304 and AISI 316 are susceptible to stress corrosion cracking. The following materials are more resistant to cracking:
- duplex steels;
- austenitic steels characterized by a high concentration of nickel in the composition.
Resistance to stress corrosion cracking makes it possible to use duplex in many types of processes carried out at high temperature conditions:
- used for the manufacture of water heating structures;
- used for the production of tanks for brewing beer;
- used for the manufacture of desalination plants.
Steel pool frame structures are also known for their tendency to stress corrosion cracking. The use of simple austenitic stainless steels for welding is strictly prohibited. Suitable for this procedure are austenitic steels known for their high nickel content. Super Duplex Steel is an excellent alternative to this material.
Reasons behind the slow ubiquity of duplex steels
High strength, a wide range of values of resistance to corrosion phenomena, an average degree of weldability — all this should increase the level of production of this material. But, you need to understand that such steels have certain disadvantages. They prevent duplex steels from fully covering the market for these products.
The increased degree of strength is considered one of the disadvantages when it comes to the manufacturability of processing raw materials by pressure. This indicates a lower deformation capability than austenitic steels. For this reason, the material is unsuitable for the manufacture of products that require a high degree of plasticity. When the ability for this type of deformation is at an acceptable level, sufficient force is required to give the raw material a suitable shape, for example, when bending pipes. There is one exception to the rule regarding unsatisfactory cutting machinability: LDX 2101 from Outokumpu.
The process of melting duplex steels is rather complicated and laborious. If the production technology is violated, more unwanted phases may appear.
The formation of the sigma phase is noted when there is a deficit in the cooling rate during production and welding. If a large number of alloying elements prevail in steel, then the probability of the formation of this phase increases. In this case, the most vulnerable are super duplex steels.
Typically, 475-degree brittleness is noted during the formation of a phase called α '(alpha bar). This temperature regime is dangerous. It should be noted that brittleness can appear even at temperatures of 300 degrees Celsius. This provokes the appearance of certain restrictions on the maximum temperature for using this raw material. Often this restriction further narrows the range of possible uses.
It is important to note that even with the minimum permissible temperature regime for the use of such a material, there are certain restrictions. Duplex steels have a brittle-tough transition when tested for impact resistance. The permissible temperature regime for testing products that are used for the manufacture of structures for offshore oil and gas production is 46 degrees Celsius.
Overview of the main properties of the duplex
They are as follows:
1. The strength of the material is several times greater than that of other types.
2. It has an impressive range of corrosion resistance values. This makes it possible to choose the right brand depending on the purpose.
3. High impact strength is up to -80 degrees Celsius. This significantly limits the scope of use in cryogenic environments.
4. High degree of resistance to stress corrosion cracking.
5. Excellent weldability of large sections.
6. The appearance of some difficulties in machining, stamping.
7. The maximum application temperature of the material in question is limited to 300 degrees Celsius.
The price of this material is affordable, so it can be bought in our country at a good price. It has many benefits. For example, the cost of two-phase raw materials is significantly lower than the price of other stainless metals. The strength characteristics are significantly higher than that of the AISI 300 class product. This makes it possible to use much less raw materials for the same type of equipment.
Duplex steel is actively used in the automotive industry. Many businesses use this material. This allows you to create a model of transport that meets the basic requirements of protection, without increasing the weight of the product. With the use of modern technologies and materials, as a rule, the weight of a car becomes about a third more. That is why it is worth using two-phase steels to significantly reduce vehicle weight. At the same time, the security problem is being addressed. Duplex allows the production of vehicles that meet all basic safety standards. At the same time, no increase in the price of the final product due to the use of this type of raw material occurs.