A Guide to Duplex Stainless Steel

The development and application of duplex stainless steel have experienced the first, second, and third generations of the development process of duplex stainless steel. 7 for acid oil and gas Wells with the requirements of oil well pipe and line pipe, Sweden has developed the second generation of SAF2205 duplex stainless steel. It is in a neutral chloride solution and H2S stress corrosion-resistant performance is better than 304 l, and 306 l austenitic stainless steel. In addition, due to the nitrogen, hole corrosion resistance is also very good, but also has good strength and toughness, can be cold, hot working, and good weldability, therefore is one of most applications of all the duplex stainless steel material.

SAF2507 after SAF2205, Sweden and developed the third generation of the super duplex stainless steel, used in harsh medium containing chloride. The steel grade PREN (pitting equivalent) = 43, ferrite and austenite phase 50% of each steel Gao Ming, high alumina and high nitrogen balance in the composition design, the steel has a high resistance to stress corrosion cracking, pitting corrosion and crevice corrosion resistance performance. The steel grade of the submarine pipeline was used in the North Sea.

Duplex austenite and ferrite stainless steel to a certain extent with the characteristics of austenitic stainless steel and ferritic stainless steel, duplex stainless steel in the ideal organization is ferrite and austenite of 50% each with proper control of chemical composition and heat treatment process, can be the good toughness and weldability of austenitic stainless steel and ferritic stainless steel of high strength and resistance to chloride stress corrosion performance together.

Duplex stainless steel ferritic/austenitic best proportion of the problem is the key to its corrosion resistance. As Cr, Mo in high solid solubility of ferrite, and the tendency to M and N Yu Zaiao solid solution in the austenite, so relative to the best proportion of ferritic/austenitic, duplex stainless steel in the austenite phase increase will reduce the alloy elements such as Cr, Mo, Ni, the overall content of the solid solution and reduce the corrosion resistance of stainless steel; In addition, Cr, Mo content of ferrite is easy to precipitate σ and χ phase, reduced material toughness, stress corrosion sensitivity increase.

Increasing the proportion of ferrite is equal to the reduced content of Cr, Mo in ferrite, also can reduce corrosion resistance; Austenite phase reduction on the one hand can reduce the impact toughness of duplex stainless steel, on the other hand, will cause the nitride precipitation. As a result, the organization of the duplex stainless steel is not only related to the composition but also related to thermal and processing technology, control is not good to make the material’s mechanical properties and corrosion-resistant performance, to a certain extent, also affect the use of duplex stainless steel.

Duplex stainless steel stress corrosion of the main influence factor for the Cl ion concentration, temperature, H2S partial pressure, pH value, and stress level. Its mechanism is the passivation membrane and ultimately affects the behavior of the crack. From the Angle of material, the influence factors include austenitic/ferrite and component level, and the proportion of cold deformation. Ferrite austenite compared with high transverse crack sensitivity, brittleness of sigma phase will significantly increase the brittleness of duplex stainless steel; Big grain crack sensitivity is higher than the sketch.

H2S will increase significantly dual phase steel on the absorption of hydrogen atoms, on the other hand, will increase the dissolution of ferrite in the active zone, and the austenite activation/passivation. Once the ferrite on the surface of the passivation film is destroyed, it will be hard to repair, eventually leading to local corrosion and crack. There is, of course, Cl – catalysis, when no Cl – in the solution, won’t produce local corrosion or cracks.

NACE standard MR0175 / IS015156-3 in the United States the use of duplex stainless steel more stringent environmental restrictions: H2S partial pressure ≤ 20 kpa, temperature, pH value in any combination. The current regulations are controversial. Many experiments prove that: to a certain temperature, salinity, and Cl – concentration range, duplex stainless steel in a 0.1 MPa H2S environment doesn’t craze. Reports confirm the duplex stainless steel can be used for the lMPa H2S environment.

Duplex stainless steel in the thermoplastic is poorer, the reason is that it is hot working when the deformation behavior of austenite and ferrite phases is different. Due to two different phases of the softening process, hot working in two-phase produces a non-uniform distribution of stress and strain, which leads to interfacial crack nucleation and propagation. Therefore, the traditional hot extrusion process commonly used duplex stainless steel hot working.

Kawasaki steel company adopts Mannesmann piercing and successfully produced KLC – 22 cr duplex stainless steel (SAF2205). Reducing the S content in the steel, improve the deformation capacity of steel; By adding Ca, will work with soluble S fixed in the steel, so as to further improve the deformation capacity of steel. At the same time, according to the duplex stainless steel in the tandem rolling mill and the automatic pipe mill rolling groove roll gap in the pipe wall thickness is easy to pull thin even with perforation problems, to formulate the appropriate rolling schedule.

Duplex stainless steel has become an important engineering material, widely used in petrochemical, offshore and coastal facilities, oilfield equipment, papermaking, shipbuilding, and environmental protection. 2507 super duplex stainless steel is developed based on the second-generation duplex stainless steel 2205. At present, there are SAF2507, UR52N+, Zeron100, S32750, 00Cr25Ni7Mo4N, and other grades. The 2507 structure is composed of austenite and ferrite, and both austenite The dual characteristics of stainless steel and ferritic stainless steel have a lower thermal expansion coefficient and higher thermal conductivity than austenitic stainless steel.

Its pitting corrosion coefficient (PREN) is greater than 40, and it has high resistance to pitting and gaps. Corrosion, chloride stress corrosion cracking resistance, high strength, high fatigue strength, low temperature, and high toughness at the same time, is a widely used duplex stainless steel. In recent years, with the continuous expansion of the application fields of duplex stainless steel, the demand for welding technology has increased, which has accelerated the development of welding technology. Therefore, summarizing and discussing the research results on the weldability of 2507 super duplex stainless steel at home and abroad has important engineering practical significance for the application of 2507 super duplex stainless steel.

The very low C content in the chemical composition of 2507 duplex stainless steel can improve the weldability of the steel and reduce the precipitation tendency of carbides at the grain boundary during heat treatment, increase the intergranular corrosion resistance, high chromium, high molybdenum, and higher nitrogen content, It can improve the corrosion resistance, so that it has good resistance to uniform corrosion such as formic acid, acetic acid, nitride, etc., resistance to pitting corrosion, and resistance to stress corrosion.

Nitrogen is added as an alloying element to stainless steel, which can improve the stability of austenite, balance the phase ratio of dual-phase steel, increase the strength of steel without affecting the plasticity and toughness of the steel, and partially replace Ni in stainless steel and reduce Cost, N in the duplex stainless steel has the effect of delaying the dispersion and precipitation of intermetallic compounds and stabilizing austenite.

The structure of 2507 super duplex stainless steel is composed of ferrite and austenite. The austenite is distributed on the ferrite matrix in strips. At higher magnifications, the interface between austenite and ferrite is not smooth and appears to be jagged. This shows that during the cooling process after rolling, austenite is formed by nucleation and growth at the ferrite interface. The presence of austenite in the structure of duplex stainless steel can reduce the brittleness and grain growth tendency of high chromium ferrite, improve weldability and toughness, and chromium-rich ferrite can increase the yield strength of austenite in stainless steel.

Resistance to intergranular corrosion and stress corrosion, that is, the ferrite dual-phase structure has high strength and high toughness but also maintains high resistance to stress cracking, pitting, and crevice corrosion, especially chloride and sulfide It has high resistance to stress corrosion cracking, so it can effectively solve the long-standing failure problem of austenitic stainless steel caused by local corrosion.

The 2507 super duplex stainless steel welding method has a wide range of applicability. It can be welded in a variety of ways. The welding heat input and cooling rate affects the phase balance of ferrite and austenite and the performance of the welded joint. To ensure that the weld has a suitable structure Comparative example and good mechanical properties and corrosion properties.

Too small or too large heat input should be avoided during welding, and the heat input should be controlled within 5～20kJ/cm. The lower limit should be removed when welding thin-walled parts, and the heat should be increased appropriately when welding thick-walled parts. Enter, the temperature between tracks should not exceed 100°C.

1. Assess Corrosion Resistance Requirements:

   • For marine environments, super or hyper duplex grades are recommended.

2. Consider Mechanical Strength:

   • Applications with high loads may require standard or super duplex.

3. Evaluate Cost Constraints:

   • Lean duplex can be a cost-effective choice for non-critical applications.

4. Review Fabrication Needs:

   • Ensure the selected grade has appropriate weldability and formability for the intended application.

In duplex stainless steel, the main elements are Cr, Si, Ni, Mn, Mo, C, rare earth elements, and so on. These alloying elements mainly affect the structure of the dual-phase steel, and then have an effect on the performance of the dual-phase steel. Some of them promote each other, and some restrict each other. Therefore, in each dual-phase steel, the determination of alloying elements and their content can meet reasonable conditions such as corresponding mechanical properties, process properties, and corrosion resistance. In this experiment, the influence of different addition amounts of C on the microstructure, phase comparison, mechanical properties, and corrosion resistance of 00Cr22Ni5Mo3N duplex stainless steel was compared to optimize the best C content in the duplex stainless steel.

It is smelted in an intermediate frequency furnace and poured into plum blossom samples. In the smelting process, decarburization, deoxidation, and desulfurization cannot be carried out, so high-quality raw materials are used. This test uses 00Cr22Ni5Mo3N duplex stainless steel with different C content, the chemical composition is (mass fraction, %): Cr22, Ni5, Mo3, N2.5, Cu0.6, Si0.6, Mn0.9, RE0.2, P≤ 0.040, S≤0.020, C content is 0.012, 0.022, 0.032, 0.042, 0.052, respectively.

Carry out solution treatment on the sample with water cooling at 1070℃×4h. The microstructure was observed and photographed with an OLYMPUSGX71 electron microscope. The tensile test was carried out on the WDW3300 micro-control electronic universal testing machine. The hardness test is carried out on the HB-3000C electronic Brinell hardness tester. The room temperature impact test is carried out on the JBN-300B impact testing machine, and the impact sample is a Charpy V-notch standard sample. Use CS350273A potentiostat for the electrochemical corrosion test. The stress corrosion test is carried out by GB/T17898-1999 “Test Method for Stress Corrosion of Stainless Steel in Boiling Magnesium Chloride Solution”. The sample is processed into a standard stress corrosion sample with a wire-cutting machine.

The results show that as C increases from 0.012% to 0.052%, austenite slightly increases, ferrite gradually decreases, yield strength and tensile strength change little, elongation after fracture continues to increase, and the reduction of area is first After increasing, it fluctuates in a small range, the impact absorption energy is significantly reduced, and the hardness gradually decreases. When the C content is 0.012%, the performance required for electrochemical corrosion resistance and stress corrosion fracture resistance is the best. When the C content is 0.012%, 00Cr22Ni5Mo3N duplex stainless steel has the best overall performance.