Stainless steel is one of the most widely used materials in engineering and technology, due to its excellent combination of strength, corrosion resistance, and aesthetics. However, conventional stainless steels have some limitations, such as low toughness, poor weldability, and high cost. To overcome these drawbacks, a new class of stainless steel, called duplex stainless steel (DSS), was developed in the 1970s.
Among the various grades of duplex stainless steel, 2205 is the most widely used and studied one. It has a nominal composition of 22% Cr, 5% Ni, 3% Mo, and 0.15% N.
Chemical compositions of duplex stainless steel 2205
The chemical compositions of duplex stainless steel 2205 are crucial for determining its properties and performance. The main elements that constitute duplex stainless steel 2205 are chromium (Cr), nickel (Ni), molybdenum (Mo), and nitrogen (N).
Chemical Compositions | Content |
|
Cr | 22-23% | Responsible for the formation of ferrite phase and enhanced corrosion resistance. |
Ni | 4.5-6.5% | Responsible for forming the austenite phase and improving ductility and toughness. |
Mo | 3-3.5% | Helps enhance resistance to pitting and crevice corrosion. |
N | 0.14-0.2% | Responsible for the stabilization of the austenite phase and the increase of the pitting and crevice corrosion resistance. |
C | 0.03% | These elements have minor or negative effects on the properties and performance of duplex stainless steel 2205. Therefore, their contents should be kept as low as possible. |
Mn | 2% | |
Si | 1% | |
P | 0.03% | |
S | 0.02% | |
Cu | 0.5% |
There are different grades of duplex stainless steel 2205, such as S31803 and S32205, which have slightly different chemical compositions. The main difference between these grades is the nitrogen content. S31803 has a lower nitrogen content than S32205, which means that S31803 has a lower pitting and crevice corrosion resistance than S32205. However, S31803 has a higher ferrite content than S32205, which means that S31803 has a higher strength and hardness than S32205.
Mechanical Properties of Duplex Stainless Steel 2205
The mechanical properties of duplex stainless steel 2205 are among the most attractive features of this material, as they surpass those of conventional stainless steel and other alloys. The mechanical properties of duplex stainless steel 2205 include strength, ductility, hardness, fatigue, and fracture toughness.
- Strength
Strength is the ability of a material to resist deformation and failure under applied forces.
The yield strength of duplex stainless steel 2205 is about twice that of austenitic stainless steels, and comparable to that of high-strength low-alloy steels.
The tensile strength of duplex stainless steel 2205 is also higher than that of austenitic stainless steel, and similar to that of precipitation-hardened stainless steel.
- Ductility
Ductility is the ability of a material to deform plastically without breaking or cracking.
Duplex stainless steel 2205 has good ductility, due to the presence of the austenite phase, which provides plastic deformation and strain hardening.
The elongation and reduction of area of duplex stainless steel 2205 are higher than those of ferritic stainless steels, and lower than those of austenitic stainless steels.
- Hardness
Hardness is the resistance of a material to indentation or abrasion.
The hardness of duplex stainless steel 2205 is higher than that of austenitic stainless steel, and lower than that of martensitic stainless steel. The hardness of duplex stainless steel 2205 can be increased by cold working or aging, but this may reduce the ductility and corrosion resistance.
- Fatigue
Fracture toughness is the ability of a material to resist crack propagation and fracture under stress.
Duplex stainless steel 2205 has a high fatigue resistance, due to the balanced combination of ferrite and austenite phases, which provide a good balance of strength and toughness. The fatigue strength of duplex stainless steel 2205 is higher than that of austenitic stainless steel, and lower than that of ferritic stainless steel.
- Fracture toughness
Fracture toughness is the ability of a material to resist crack propagation and fracture under stress.
Duplex stainless steel 2205 has a high fracture toughness, due to the presence of the austenite phase, which provides ductile fracture and energy absorption. The fracture toughness of duplex stainless steel 2205 is higher than that of ferritic stainless steel, and lower than that of austenitic stainless steel.
2205 Duplex Stainless Steel Casting Process
The casting process of duplex stainless steel 2205 is a method of producing near-net shape components by pouring molten metal into a mold and letting it solidify into a desired shape.
- Mold making: The mold is the cavity that defines the shape and size of the casting.
- Melting and pouring: The melting and pouring process involves heating the duplex stainless steel 2205 to a liquid state and transferring it to the mold.
- Solidification and cooling: The solidification and cooling process involves the transformation of the liquid metal into a solid metal within the mold.
- Demolding and cleaning: The demolding and cleaning process involves removing the casting from the mold and removing the excess metal, sand, ceramic, or plaster from the casting.
- Inspection and testing: The inspection and testing process involves checking the quality and performance of the casting against the specifications and standards.
The casting process of duplex stainless steel 2205 also poses some challenges and solutions, such as:
- Hot cracking: Hot cracking is the formation of cracks in the casting during or shortly after solidification, due to the thermal stress, the segregation, or the phase transformation of the metal.
- Solution: Hot cracking can be prevented or reduced by optimizing the mold design, the casting geometry, the pouring temperature, the cooling rate, and the chemical composition of the metal.
- Phase transformation: Phase transformation is the change of the ferrite and austenite phases in the casting during or after solidification, due to the temperature, the composition, or the deformation of the metal.
- Segregation: Segregation is the uneven distribution of the alloying elements in the casting, due to the different solubility, diffusivity, or density of the elements in the liquid or solid metal.
Advantages of 2205 Duplex Stainless Steel Casting
Duplex stainless steel 2205 casting is a superior method of fabrication compared to other methods, such as forging, rolling, and machining.
- Near-net shape production
Duplex stainless steel 2205 casting can produce complex and large components with minimal material waste and machining. For example, a study by the American Foundry Society showed that casting a valve body from duplex stainless steel 2205 saved 40% of the material and 30% of the machining time compared to forging.
- High performance and corrosion resistance
Duplex stainless steel 2205 casting can achieve a balanced microstructure of ferrite and austenite, which provides high strength, ductility, hardness, fatigue resistance, and fracture toughness.
Duplex stainless steel 2205 casting can also resist various types of corrosion, such as pitting, crevice, stress corrosion cracking, and general corrosion, due to the high content of chromium, molybdenum, and nitrogen. These properties make duplex stainless steel 2205 casting suitable for demanding applications, such as oil and gas, chemical, and marine industries.
- Environmental, economic, and social benefits
Duplex stainless steel 2205 casting can reduce greenhouse gas emissions, resource consumption, and the environmental impact of the fabrication process, compared to other methods.
Applications of 2205 DSS Castings
- Aerospace:Landing gear, engine parts, and structural elements.
- Automotive:Exhaust systems, turbochargers, and fuel injectors.
- Biomedical:Implants, prostheses, and surgical instruments.
- Construction: Bridges, towers, and pipelines.
- Valves
- Pumps
- Impellers
- Fittings
Conclusion
Duplex 2205 stainless steel is an advanced quality duplex stainless steel alloy. It also has good fatigue strength and excellent resistance to stress corrosion cracking, crevices, pitting, erosion, and general corrosion in harsh environments.
