Duplex and Super Duplex ferritic-austenitic steels
In other words - Cr-Ni-Mo two-phase steels, containing 21-29% chromium, 3.5-8% nickel, 0-4.5% Molybdenum and addition of Nitrogen.
Duplex steels are acid-resistant grades that retain high corrosion resistance - atmospheric, pitting, intercrystalline, stress, maintaining high strength properties of ferritic stainless steels, and relatively low coefficient of thermal expansion compared to austenitic stainless steels, with less tendency to grain growth. The term "biphasic steel" is synonymous with the occurrence of a "mixture" of austenitic and ferritic structure in a grade in a generally accepted 50/50 ratio. If to the standard austenitic stainless steel 18/8, which is 18% Chromium and 8% Nickel, will be added chromium by up to 20%, the austenitic structure will transform into a ferritic-austenitic mixture.
The addition of molybdenum, vanadium, titanium, aluminum and silicon together with the basic chromium element contributes to the widening of the α phase. In turn, the increased content of nickel acts the other way round - it broadens the γ phase range. Nickel reaches a concentration of 12% in two-phase steel with a range of 20% Chrome, resulting in the formation of austenitic structure in acid-resistant steel. When the sum of the contents of the elements extending the α phase ranges exceeds the equivalent content, the steel acquires a ferritic-austenitic structure. Another interesting fact is that the continuous structure of the ferrite structure, due to the appearance of ferrite, is magnetic.
Duplex and superduplex steels characteristics
Duplex and super duplex grades maintain the appropriate mechanical properties in the 250 - 300 ℃ range. Longer operating temperatures result in a brittleness at a temperature of 475 ℃. Impact restore at room temperature is similar to austenitic grades, but temperatures below 0 ℃ test results are evenly decreasing. At -50 ℃ the duplex steel goes into brittle condition.
Two-phase steels have twice the yield strength of acid-resistant austenitic stainless steels, exhibiting excellent weldability and resistance to cracking, wear resistance, abrasion and erosion resistance. They also exhibit resistance to environments containing chloride ions, hydrogen sulphide, ammonium carbamate, phosphoric acid, naphthalene, pentanes, sodium hypochlorite, potassium hydroxide, formic acid, acetic acid, hydrochloric acid, sodium carbonate, depending on the working temperature and concentration of the substance. Duplex products are delivered in a supersaturated condition, which is carried out in the temperature range 1000 - 1150 ℃.
First Dual-phase steels
Duplex grade prototypes were patented in France in the first half of the 19th century, where they were used in the form of heat exchangers and chemical containers. High resistance to various types of salt has prompted manufacturers to introduce duplex materials to designs exposed to the marine environment. In the 1970s was constructed the first ship to transport chemicals in the Dunkerque Shipyard, which construction consisted of, among others, made of duplex steel As a result of the discovery of new properties and characteristics of two-phase steels, duplex grades have been introduced in the petrochemical, paper, shipbuilding, cryogenic, oil, gas, food, chemical and pharmaceutical industries.
Structure of austenitic-ferritic steels
The share of austenite in Duplex and Super Duplex steels is usually in the range of about 40-60%, where the rest of the structure is ferrite. Duplex types are considered to be permanently resistant to stress corrosion, with corrosion resistance at the grain boundary, which is due to the formation of chromium carbide. Their main strengths are one of greater plasticity limits and resistance to intergranular corrosion. They are characterized by very good mechanical properties, plasticity, resistance to stress corrosion, resistance to cracking and good weldability. Welds made using electrodes of duplex steel are particularly clean and less susceptible to cracks than welds of acid resistant austenitic steels.
Structural division - 50/50 is not always uniform and varies depending on the species. Many properties of duplex grades depend precisely on the proportion of ferrite and austenite in the alloy structure. Where the relative volume of austenite is greater in the grade, duplex materials exhibit greater impact resistance, ductility and corrosion resistance. For alloys where the volume of ferrite surpasses the austenite, duplex products are characterized by increased strength properties - hardness, yield strength and tensile strength.
In addition to α ferrite and γ austenite duplex steels differ depending on the separation processes:
- high-chromium ferrite α'BCC which is a metastable phase that is generated at temperatures of 400-550 ℃. It is divided into isomorphic zones with high and low chromium content. It exhibits brittleness at 475 ℃ and significantly reduces the ductility of steel.
- secondary austenite γ2 BCC resulting from the non-diffusion isothermal transformation below 650 ℃ has a similar chemical composition to the secondary ferrite. At a temperature of 650-800 ℃, the austenite exhibits a Widmanstäten structure, and at the temperature of 700-900 ℃ the resulting eutectoid reaction is formed by α = σ + γ2.
- Phase χ BCC, meaning Fe36Cr12Mo10 is a phase of A48B10 type, which increases the brittleness of steel and significantly reduces corrosion resistance. It is produced at temperatures of 700-900 ℃ with long soaking. Contains significantly more molybdenum than the σ emitting itself at the boundaries of α and γ phases.
- Phase ε BCC s dispersed at temperatures of 500-600 ℃. Containing a large amount of copper and affects the stability of the passive layer as a result of which the surface of the steel becomes corrosion-resistant. Occurs in α hardening it.
- Regular G phaseduring silicon and nickel treatment, it occurs in the areas along with the elements at the α and α' boundaries. The formation of this phase causes the material to heat up at 300-400 ℃. Its appearance is accompanied by the occurrence of the high chromium α' phase. owej.
- Rhombic τ phasemeans that Fe, Cr, Mo, Ni are formed with long soaking at the α limit at temperatures of 550-650 ℃
- HexagonalR phase(Fe28Cr13Mo12) and A2B phase(Fe2Mo) which by the molybdenum reduces the impact restore and resistance to pitting corrosion. They arise as a result of the temperature of the range 550-650 ℃ in the areas α and α'.
- Tetragonalσ phase- Fe-Cr-Mo increases the brittleness of steel, reduces the resistance to intergranular corrosion and pitting corrosion. It affects the impoverishment of places where there is Chromium and Molybdenum. It is produced at temperatures of 650-1000 ℃.
- Nitrides: Cr2N trigonaland CrNBCC reduce the occurrence of chromium in ferrite as a result of prolonged soaking at 700-900 ℃, or sudden cooling which is relatively common during welding. As a result of the low solubility of nitrogen in ferrite, their presence at the ferrite grain boundary is increased.
- Carbides: M7C3 rhombicand M23C6 BCC cause intercrystalline corrosion by separating them from the solid solution by precipitation of chromium adhering to the grain boundaries. M23C6 carbides are formed at a temperature of 600-1000 ℃, and M7C3 at the α and γ borders at temperatures of 950-1050 ℃.
Production and implementation
The chemical composition of duplex steel differs slightly from acid-resistant austenitic steels. Two-phase steels contain an average of about 25% chromium, about 5% nickel, and about 2% molybdenum. This helps to lower the cost of production due to the high price of nickel, while increasing mechanical properties and corrosion resistance. Concentration of nickel allows the addition of nitrogen in the chemical composition. Nitrogen helps to maintain a stable biphasic structure while reducing the amount of nickel in the composition.
As a relatively difficult operation we can count the introduction of nitrogen in a proportion of over 0.2% to the duplex steel composition, and to make the process successful, the production must be carried out by pressure die-casting or powder metallurgy. In addition, the dissolved nitrogen in the austenite increases the mechanical properties of the duplex grades that are near the ferritic stainless steel structure, improves the steel's resistance to pitting corrosion, and improves the weldability of the materials. The content of nitrogen in super duplex grades, due to the higher content of alloying additives (Cr, Mn, Mo) is on average 0.3%. In the case of ordinary duplex steel, the ratio is 0.15%.
In addition to damaged and worn out components, machine parts can be repaired without problems, which increases the service life of the components. Unfortunately, with duplex steel compared to austenitic steels, there are some disadvantages. Their plastic processing is significantly impeded - forging and rolling are one of the more difficult treatments, and the cold drawing process was practically impossible to achieve. The same applies to deep drawing.
Chemical composition of duplex and super duplex steel - influence of individual elements
Duplex steels, unlike a large group of martensitic stainless steels, have a significantly reduced carbon content. In most alloys, this level does not exceed 0.03%. This significantly improves the corrosion resistance of the duplex steels group. Low carbon limits the release of chromium carbides, which deplete chrome areas adjacent to the grain boundaries, resulting in deterioration of the intercrystalline corrosion resistance.
Nickel in duplex steels increases the resistance to organic and inorganic acids, and positively affects steel passivity. Manganese, as in most species, improves the wear resistance of the products, reduces the ductility of the steel and increases the wear resistance, but its excessive concentration can contribute to the reduction of the critical pitting temperature. The tungsten additionally enhances the steel's resistance to pitting and stress corrosion while stabilizing ferrite (eg 1.4501, UNS S32760). Copper in high concentrations decreases the plasticity and weldability of steel, and its concentration is limited to < 2%. It should be noted that its presence positively increases the steel's resistance to corrosion in non-oxidizing environments and tendency to stress corrosion. The addition of molybdenum in the range of 1-3% increases the steel resistance to acetic and sulfuric acid.
Bars, sheets and tubes from duplex steel - general use
The use of duplex steel is partly described in the text above, but there were only a mentions. The duplex steels are used in structures and equipment used in seawater, in installations, in oil and gas extraction installations, for elements working in hydrogen sulfide contaminated atmospheres, in the shipbuilding industry in the construction of ships transporting aggressive chemicals, f.ex. from duplex plates are made of special bulkheads, inner bottoms, decks and tanks for storing these substances.
Other uses include heat exchangers in the form of seamless pipes through which flows oil and gas, off-shore installations, machinery parts and industrial machinery, tanks as well as pipelines in the petrochemical industry, rotors fans and corrosion-resistant rollers, water desalination plants, agitators for the sewage treatment plants and agitators for the food industry, pipelines transporting liquids containing chlorides, tanks and pressure equipment in which many aggressive chemicals are processed.