Duplex stainless steel

Ferrite and austenite each account for around 50% of duplex stainless steel (often referred to as DSS), but typically less phase content also needs to achieve at least 30% of stainless steel. In low C cases, the Cr content ranges from 18% to 28% while the Ni ranges from 3% to 10%.Additional alloying elements like Mo, Cu, Nb, Ti, and N can be found in some steels.

This type of steel combines the properties of austenitic and ferritic stainless steel. When compared to ferritic, it exhibits higher plasticity, toughness, no-room-temperature brittleness, intergranular corrosion resistance, and significantly improved welding performance, while still retaining the 475°C brittleness and high thermal conductivity of ferritic stainless steel, as well as other properties. It is stronger and more resistant to chloride stress corrosion and intergranular corrosion than austenitic stainless steel.Duplex stainless steel is a nickel-free stainless steel with good pitting resistance.

Historical development

Since its invention in the 1940s in the United States, duplex stainless steel has advanced to the third generation.Its primary advantage is that it has a yield strength that is twice as high as standard stainless steel (400–550 MPa), allowing for material savings and lower manufacturing costs.The resistance to pitting, crevice corrosion, stress corrosion, and corrosion fatigue performance of duplex stainless steel is significantly better than that of regular austenitic stainless steel, and can be comparable with high-alloy austenitic stainless steel in terms of corrosion resistance, especially under the condition of harsh medium environment (such as seawater, high chloride ion content).

Material introduction

Performance characteristics

The duplex stainless steel has the benefits of ferritic stainless steel and austenitic stainless steel due to the characteristics of the two-phase structure, proper control of chemical composition, and heat treatment process, which combines the excellent toughness and weldability of austenitic stainless steel with the high strength and chloride stress corrosion resistance of ferritic stainless steel.Due to its excellent qualities, duplex stainless steel has quickly evolved into a weldable structural material and, since the 1980s, has joined the martensitic, austenitic, and ferritic stainless steel classes as a separate steel class.Duplex stainless steel exhibits the following functional traits:

(1) Duplex stainless steel with molybdenum exhibits good resistance to chloride stress corrosion under low stress.In neutral chloride solutions above 60°C, 18-8 austenitic stainless steel is typically susceptible to stress corrosion fracture, and heat exchangers, evaporators, and other equipment made of this type of stainless steel have a propensity to produce stress corrosion fracture in trace chloride and hydrogen sulfide industrial media. Duplex stainless steel has good resistance.

(2) Duplex stainless steel with molybdenum offers good pitting resistance.The critical pitting potential of duplex stainless steel is comparable to that of austenitic stainless steel with the same pitting resistance value (PRE=Cr%+3.3Mo%+16N%).Duplex and austenitic stainless steels have pitting resistance that is equivalent to AISI 316L.25%Cr, especially high chromium duplex stainless steels containing nitrogen, has greater resistance to pitting and crevice corrosion than AISI 316L.

(3) It has strong fatigue and wear corrosion resistance.It can be used to create pumps and valves that are used in some corrosive media conditions.

(4) Strong overall mechanical qualities.It has greater yield strength than austenitic stainless steel of the 18-8 type and greater strength and fatigue resistance.The toughness value AK (V-notch) is 100J, and the elongation of the solid solution state is 25%.

(5)Good weldability, no tendency for hot cracks, often no requirement for preheating before welding, no heat treatment required after welding, and can be welded with dissimilar metals like carbon steel or austenitic stainless steel of the 18-8 type.

(6) Direct rolling rather than forging is used to produce steel plates in dual-phase stainless steel with low chromium (18%Cr) that has a larger hot working temperature range than 18-8 austenitic stainless steel and less resistance.Duplex stainless steel with a high chromium content (25%Cr) can be processed to create plates, tubes, and wires but is a little trickier to work with than austenitic stainless steel.

(7) Because of the significant work hardening impact of cold processing on austenitic stainless steel of the 18-8 type, it is necessary to apply more stress during the initial deformation of tubes and plates.

(8) Compared to austenitic stainless steel, the material's thermal conductivity is high and its linear expansion coefficient is low, making it ideal for usage in the manufacture of composite panels and as a liner for machinery.It can be used to create the heat exchanger's tube core and has a heat transfer efficiency that is higher than austenitic stainless steel.

(9) High chromium ferritic stainless steel nevertheless has a number of brittleness tendencies and shouldn't be used at temperatures above 300°C.The less detrimental the brittle phase is, the lower the chromium concentration of duplex stainless steel is.

Use

used in cold showers, seawater resistant high temperature concentrated nitric acid heat exchangers, fertilizer, paper manufacturing, petroleum, and chemical industries.

Structure and type

Due to its austenitic + ferritic biphase structure, duplex stainless steel possesses the traits of both austenitic and ferritic stainless steel, and the content of the two phase structures is essentially equal.The yield strength is twice as strong as standard austenitic stainless steel and can reach 400Mpa to 550MPa.Duplex stainless steel is more durable, has a lower brittleness transition temperature, is more resistant to intergranular corrosion, and is easier to weld than ferritic stainless steel.In addition, it preserves some properties of ferritic stainless steel, including magnetism, high heat conductivity, low linear expansion coefficient, and brittleness at 475°C. Duplex stainless steel has a higher strength than austenitic stainless steel, notably in terms of yield strength, and it also performs better in terms of pitting corrosion resistance, stress corrosion resistance, and corrosion fatigue resistance.

According to the classification of its chemical makeup, duplex stainless steel can be split into four types: Cr18 type, Cr23 (without Mo) type, Cr22 type, and Cr25 type.Duplex stainless steel of the Cr25 type can be classified into ordinary and super duplex varieties, with Cr22 type and Cr25 type seeing the most application.The exact grades of biphase stainless steel that are used in China are 3RE60 (Cr18), SAF2304 (Cr23), SAF2205 (Cr22), and SAF2507 (Cr25).

Classification

Duplex stainless steel

The first kind is a low-alloy type that corresponds to the steel grade UNS S32304 (23Cr-4Ni-0.1N), which has a PREN value of 24–25 and can be used in place of AISI304 or 316 in terms of stress corrosion resistance. This steel does not contain molybdenum.

The second category, which represents the medium alloy UNS S31803 (22Cr-5Ni-3Mo-0.15N), has a PREN value of 32–33 and has corrosion resistance that falls between AISI 316L and 6%Mo+N austenitic stainless steel.

The third class is a high alloy type, with a standard grade of UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), a PREN value of 38–39, and a corrosion resistance that is higher than 22%Cr duplex stainless steel. It typically contains 25%Cr, as well as molybdenum, nitrogen, and in some cases copper and tungsten.

The fourth category is super duplex stainless steel, which is a type with high levels of molybdenum and nitrogen. Its standard grade is UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), and some varieties also contain tungsten and copper. It has good corrosion resistance and mechanically comprehensive properties, making it comparable to super austenitic stainless steel.

Stainless steel

There are numerous varieties of stainless steel, each with a unique set of qualities, and over time, different categories have developed.

According to the structure, there are four categories: Martensitic stainless steel (including precipitation hardening stainless steel), ferritic stainless steel, austenitic stainless steel and austenitic ferrite duplex stainless steel.

Steel is classified into chromium stainless steel, chromium nickel stainless steel, chromium nickel molybdenum stainless steel, and low carbon stainless steel, high molybdenum stainless steel, according to its primary chemical composition or some distinguishing elements in steel;

Steel is separated into nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, pitting resistant stainless steel, stress corrosion resistant stainless steel, high-strength stainless steel, etc. based on the performance characteristics and uses of the material.

Steel is separated into low-temperature stainless steel, non-magnetic stainless steel, free-cutting stainless steel, super-plastic stainless steel, and so on based on its functional properties. According to the properties of the steel's structure, its chemical composition, and a combination of the two ways, the generally used categorization method is categorized. Typically split into the following categories: martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, duplex stainless steel, precipitation hardening stainless steel, or divided into the two categories of chromium stainless steel and nickel stainless steel.

Welding characteristic

In comparison to ferritic stainless steel and austenitic stainless steel, duplex stainless steel has good welding properties. It is neither like the welding heat affected zone of ferritic stainless steel, where severe grain coarsening and plastic toughness are greatly reduced, nor is it like austenitic stainless steel, which is more vulnerable to welding hot cracks.

Due to its unique benefits, duplex stainless steel is frequently used in petrochemical equipment, equipment for treating wastewater and seawater, oil and gas pipelines, paper machinery, and other industrial fields. In more recent years, it has also been researched for use in bridge bearing structures, which offers promising growth potential.

Duplex steels are frequently plagued by weldability issues. No matter the method, welding normal duplex steel is not a challenge because there are acceptable welding materials for these purposes.In terms of metallography, welding 2101 (1.4162) presents no issues at all; in fact, it is even simpler to weld than standard grade duplex steel because this material can be welded using the acetylene welding process, whereas standard duplex steel materials must always be avoided using this method.The wettability of welding 2101 is a little bit worse due to the viscosity of the molten pool being different.Because of this, welders are forced to utilize more arc welding, which is the source of the issue. We frequently prefer to select a matching welding material, even if this might be offset by selecting superalloyed welding materials.

More favorably than in 2304, 2205, or 2507, there is also a HAZ interaction in 2101 between the microstructure in the low and high HAZ.A different form of "tempering color" with more nitrogen and manganese is created in experiments with 2101 due to the reduced nickel concentration, which influences the corrosion characteristics.This lecture will go into greater detail about this new issue for biphase steel grade materials, which is the compositional loss in the arc and molten pool caused by the evaporation and deposition of nitrogen and manganese.

Welding characteristics

The welding characteristics of duplex stainless steel are as follows:

The steel comprises roughly 50% to 60% austenite and 50% to 40% ferrite structure following the standard solid solution treatment (1020°C to 1100°C heating and water cooling).The ratio of the two phases remains relatively constant as the heating temperature rises.

Duplex stainless steel has excellent low temperature impact toughness; for example, a 20 mm thick plate transverse sample at -80°C can absorb more than 100J of impact energy.In most media, its uniform corrosion resistance and point corrosion resistance are good, however it should be noted that this sort of steel will have much worse stress corrosion resistance when heated below 950°C due to the precipitation of the phase. Its resistance to stress corrosion will be much lower as a result of the precipitation of the phase.The steel has good intergranular corrosion resistance because it has an appropriate ratio of Cr equivalent to Ni equivalent, which leaves a significant amount of primary austenite structures after high temperature heating and the potential for secondary austenite to form during cooling. This austenite phase makes up at least 30% to 40% of the total mass of the steel.

Furthermore, welding this steel has a very low tendency to crack, therefore neither preheating nor post-welding heat treatment are necessary.Due of the high N concentration in the base material, there will typically be at least 30% austenite present during welding operations close to the seam zone.Both electrode arc welding and argon tungsten arc welding are applicable welding techniques. In general, low line energy welding should be employed as much as possible while welding in order to prevent grain coarsening in the vicinity of the seam.