Today, duplex steel is gaining in popularity. Almost all companies in the field of metallurgy are engaged in the production of this type of stainless alloy. You can read more about what duplex steel is and what advantages it has in this article.
General information
Duplex steel has gained worldwide recognition. What characteristics does it have? Firstly, the high strength of this material allows you to reduce the final weight of any product. Secondly, it is famous for its enormous resistance to corrosion. This is especially noticeable when considering resistance to corrosion cracking.
It is worth saying that at the moment, raw materials have not yet become familiar to manufacturers, and therefore, conferences are held every few years at which technical articles are devoted to all the features of duplex steel. So far, despite the rather high interest in this type of product, the global market share is only 1-3%.
History of occurrence
It is worth noting that the idea of creating duplex steel was born back in 1920. But the first material was born only in 1930 in Sweden. The widespread use of this type of raw material began only in the last decade. The main reason for this lies in the fact that during these years the production technology has been greatly improved. In particular, manufacturers were given the opportunity to more accurately control the nitrogen content.
To understand the advantages and the reason for the appearance of duplex stainless steel, you need to understand two other main types.
Austenitic alloys, which are represented by the brand AISI 304 or 08X18H10, as well as ferrite grades AISI 430 or 12X17 are quite simple to manufacture. As their name implies, they consist mainly of austenite or ferrite. Despite their widespread use, they have technical flaws.
The disadvantages of traditional steels
If we talk about austenitic steels, the disadvantages include a low strength index, as well as a low resistance to corrosion cracking. As for the ferritic material, its strength is slightly higher, but still "does not reach" the ideal. In addition, the weldability of steel deteriorates significantly with increasing thickness of the material, and at low temperatures it becomes quite brittle.
Another small drawback of the austenitic alloy is the nickel content in the composition. This leads to a higher cost of the product, which, of course, not one end user is happy.
Benefits of duplex
The idea of creating duplex stainless steel came from the desire to balance the ferritic and austenitic base in order to get a new, better material. Approximately the same amount of ferritic and austenitic steel led to the following advantages:
- High rate of durability. The yield strength range at 0.2% will be from 400 to 450 MPa, which is 150-200 MPa more than the indicator that can be observed in austenitic or ferritic alloys. This led to the fact that it is possible to reduce the thickness of the product, while not losing strength. A decrease in thickness caused a decrease in the final mass. This is very important in the field of building structures, tanks and vessels subjected to pressure.
- What are the advantages of duplex steel in addition to strength? The weldability of the metal is quite good even with a large thickness.
- High toughness. Much better than ferritic alloys. This is especially true in cases where the ambient air temperature drops very low to -50, and sometimes to -80 degrees Celsius.
- High resistance to corrosion cracking. Austenitic materials are very prone to this defect. This parameter plays an important role in such areas of production as brewing tanks, enrichment plants, and pool frames.
- Duplex steel boilers are much more reliable than austenite.
Cracking
Currently, ordinary steel is susceptible to defects such as corrosion cracking or SCC - Stress Corrosion Cracking. This type of corrosion usually occurs under certain conditions. The causative agent can be a strong tensile stress, elevated temperature (50 degrees Celsius above zero). And if we talk about pools, then due to the constant exposure to water, this type of corrosion can also occur at 25 degrees.
Austenitic grades are quite strongly affected by this defect. Ferrite alloy, as well as duplex stainless steel a890 3a according to ASTM and other grades, are more reliable in this regard. A large resistance coefficient allows the use of this material in the production of water heaters, brewing tanks, desalination plants. That is, where there is an increased temperature and contact with liquids.
It is forbidden to make pool frames from ordinary austenitic steels due to this defect. Previously, it was necessary to use an alloy with a very high nickel content, which led to an increase in the cost of the product. Today, duplex or super duplex steel can be used.
Super and hyper duplex material
It is worth saying that if nickel is added to ferritic chrome steel, then a mixed base structure can be obtained. That is, it will contain both austenite and ferrite. As it became clear, it was such a mixture that they began to call duplex material. The prefixes "super" or "hyper" duplex steel indicate that the raw material contains an increased amount of alloying components. This indicates even greater corrosion resistance than conventional materials.
"Super" and "hyper" duplex steel in gas production equipment is used quite actively. In addition, raw materials are used in food, chemical, construction and even medical industries.
Welding material
Despite the good weldability of this product, certain rules should be followed. Before starting welding work, pay attention to the following:
- in order to ensure good penetration of the material, a clearance at the root of the joint should be made, and the angle of cutting the edges should be slightly larger than for ordinary steel;
- the junction, as well as the metal around this place, is recommended to be thoroughly cleaned of any contaminants;
- in this case, you can only use brushes made of metal wire with corrosion resistance;
- welding electrode must be dry.
During welding of duplex steel, the following rules should be followed:
- It is very important to monitor such a parameter as heat input. It should not be too low or, conversely, too high. For ordinary duplex steel, a range of 0.5–2.5 kJ / mm should be adhered to during welding. In addition, the inter-pass temperature should be no more than 200 degrees Celsius.
- There should be no scale on the back of the weld. It is very important to choose the right gas that will protect the root of the seam. Most often, a mixture of gases such as high-purity argon and hydrogen or nitrogen is used for protective purposes.
- Welding should only be carried out by a highly qualified specialist. This is due to the fact that in places of burn-through, the metal will greatly increase the tendency to corrosion, as well as the likelihood of cracking.
- During welding, do not make wide transverse vibrations of the electrode. This can lead to too much heat input, which rereads the rules.
Work after welding
There are certain recommendations that should be followed even after welding.
- To ensure high corrosion protection of the weld metal, it should be thoroughly cleaned. It is necessary to completely remove the oxide film and remove all residual slag.
- Stripping work is carried out only by hand and only by wire brush with corrosion resistance properties. If you use mechanical devices, this can lead to the appearance of micro-tears in the weld area, which will reduce its strength.
- Most often, no heat treatment of the seam after welding is no longer required.
Disadvantages of raw materials
Despite the distribution of the material and its seemingly obvious and significant advantages, it still has not received universal recognition and, most likely, will never occupy the leading lines in the market. This is due to several disadvantages that are worth knowing about. Because of them, such an alloy will always be "niche."
Immediately it is worth starting with high strength material. It would seem a significant advantage, but it is it that becomes a huge disadvantage when it becomes necessary to treat metal mechanically or by pressure. Another disadvantage of high strength is a strong reduction in the possibility of plastic deformation. For this reason, duplex raw materials are practically not suitable for the production of any products that must have high ductility.
Even in those cases when the plasticity, it would seem, is at a level acceptable for the work, you still have to make great efforts. For this reason, duplex steels and alloys for oil and gas fittings are not used very often.
The next disadvantage is the great technological complexity of smelting duplex type stainless steel. Austenitic and ferritic material is much easier to melt. If you violate the manufacturing technology, especially during the heat treatment, other phases will form in the material in addition to austenite and ferrite, which is completely undesirable. Most often, a sigma phase or 475-degree fragility is formed.
Undesired phases in the feed
The sigma phase is formed in such a product at 1000 degrees Celsius and above. Typically, these temperatures occur when the cooling process after welding or during the production process is not fast enough. In addition, the more alloying elements will be contained in the composition, the higher the likelihood of such a phase. In other words, it is extremely difficult to create super duplex or hyper duplex steel.
As for the 475-degree fragility, it appears in the case of the formation of a phase called alpha stroke. As the name implies, the most dangerous temperature is 475 degrees Celsius, but the problem can also occur with much lower rates, about 300 degrees. Because of this, a maximum temperature limitation is imposed on the use of products from this type of raw material. Naturally, because of this, the range of steel application is narrowed even more.
Based on the foregoing, we can draw this conclusion. Duplex steel grades are a good solution and a replacement for standard materials, but in a very narrow area.
Summary
Duplex alloy is much less widespread than other types of metals. Nevertheless, it is popular in certain areas. Most often it is used in the petrochemical field, automotive industry and in the manufacture of pools. Excellent weldability, corrosion resistance and high wear resistance make this material a real find for many.