Advantages of duplex stainless steel pipe and selection of welding rod

2018-01-18

What are duplex stainless steels?

Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. The picture shows the yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic grains transform to austenitic grains (“islands”). The result is a microstructure of roughly 50% austenite and 50% ferrite.

fig duplex - Advantages of duplex stainless steel pipe and selection of welding rod

Duplex stainless steels have a two-phase microstructure of austenite and ferrite grains.

The duplex structure gives this family of stainless steels a combination of attractive properties:

Strength: Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels.

Toughness and ductility: Duplex stainless steels have significantly better toughness and ductility than ferritic grades; however, they do not reach the excellent values of austenitic grades.

Corrosion resistance: As with all stainless steels, corrosion resistance depends mostly on the composition of the stainless steel. For chloride pitting and crevice corrosion resistance, their chromium, molybdenum and nitrogen content are most important. Duplex stainless steel grades have a range of corrosion resistance, similar to the range for austenitic stainless steels, i.e from Type 304 or 316 (e.g. LDX 2101) to 6% molybdenum (e.g. SAF 2507) stainless steels.

Stress corrosion cracking resistance: Duplex stainless steels show very good stress corrosion cracking (SCC) resistance, a property they have “inherited” from the ferritic side. SCC can be a problem under certain circumstances (chlorides, humidity, elevated temperature) for standard austenitics such as Types 304 and 316.

Cost: Duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts of similar corrosion resistance. Due to the lower alloying content, duplex stainless steels can be lower in cost, especially in times of high alloy surcharges. Additionally, it may often be possible to reduce the section thickness of duplex stainless steel, due to its increased yield strength compared to austenitic stainless steel. The combination can lead to significant cost and weight savings compared to a solution in austenitic stainless steels.

They tend to be divided in three sub groups:
Grade 1.4462 which has been the grade initially developed. and reperesents the “mid-range” of properties and isperhaps the most used today
Super-duplex grades which were developed later to meet specific demands of the oil& gas as well as thos of the chemincal industries. They offer a superior corrosion resistance but are more difficult to process.Faulty processing will result in poor performance and users are advised to deal with reputable suppliers/processors
Lean Duplex grades, typically grade EN 1.4362, have been developed more recently for less demanding applications, particularly in the building & construction industry. Their corrosion resistance is closer to that of the standard austenitic grade EN 1.4401 (with a plus on resistance to stress corrosion cracking) and their mechanical properties are higher. This can be a great advantage when strength is important. This is the case in bridges, pressure vessels or tie bars.

Alloy Designations

Due to excellent corrosion resistance and high strength, the most widely used duplexstainless steel is 2205. The name 2205 comes from a typical composition of 22% Chromium and 5% Nickel. Other designations for 2205 are given in the table below.

UR52N+ is a popular “super duplex” grade that has high strength and exceptional corrosion resistance. This grade differs in composition to 2205 stainless steel in that it contains an addition of copper. This gives greatly improved resistance to hot chlorides and strong reducing acids, like sulphuric acid.

UR52N+ is the widely used registered trade name of the Usinor group. Other designations it is known by are given in the table below.

There are many other duplex stainless steel most of which are proprietary grades developed and produced by individual manufacturers.

Table 1. Some standard designations for duplex stainless steel alloys

Euronorm	UNS	BS	En	Grade
1.4462	S31803 S32205	318S13	–	2205
1.4507	S32520 S32550	–	–	UR52N+

Chemical Composition of Duplex stainless steel

Table 2. Typical chemical composition for two duplex stainless steel alloys

%	2205	UR52N+
C	0-0.03	0.03 max
Mn	2.0	1.5
Si	1.0	0.8
P	0-0.03	0.035
S	0-0.02	0.02
Cr	21-23	24-26
Mo	2.5-3.5	3.0-5.0
Ni	4.5-6.5	5.5-8.0
N	0.08-0.2	0.2-0.35
Cu	–	0.5-3.0

Properties of Duplex stainless steel

Mechanical Properties of Duplex stainless steel

Table 3. Typical mechanical properties of two duplex stainless steel alloys

Grade	2205	UR52N+
Tensile Strength (MPa)	620	770
Proof Stress 0.2% (MPa)	450	550
Elongation A5 (%)	25	25

Physical Properties of Duplex stainless steel

Table 4. Typical physical properties of two duplex stainless steel alloys

Property	2205	UR52N+
Density (g.cm3)	7.805	7.810
Modulus of Elasticity (GPa)	200	205
Electrical Resistivity (Ω.m)	0.085×10-6	0.085×10-6
Thermal Conductivity (W/m.K)	19 at 100°C	17 at 100°C
Thermal Expansion (m/m.K)	13.7×10-6 to 100°C	13.5×10-6 to 200°C

Corrosion Resistance of Duplex stainless steel

Duplex stainless steel are extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and sulphide environments, Duplex stainless steelexhibit very high resistance to stress corrosion cracking.

The super duplex grades are even more resistant to corrosion.

Heat Resistance of Duplex stainless steel

The high chromium content of Duplex stainless steel that p rotects against corrosion, causes embrittlement at temperatures over about 300°C.

At low temperatures duplex stainless steels have better ductility than the ferritic and martensitic grades. Duplex grades can readily be used down to at least -50°C.

Fabrication of Duplex stainless steel

Fabrication of all stainless steel should be done only with tools dedicated to stainless steel materials. Tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steel by easily corroded metals that may discolour the surface of the fabricated product.

Heat Treatment of Duplex stainless steel

Duplex stainless steel cannot be hardened by heat treatment. They can however be work hardened.

Solution treatment or annealing can be done by rapid cooling after heating to around 1100°C.

Machinability

Although machinable, the high strengths of Duplex stainless steel makes machining difficult. As an example, machining of 2205 is around 20% slower than for 304.

Machining can be enhanced by using the following rules:

Cutting edges must be kept sharp. Dull edges cause excess work hardening.
Cuts should be light but deep enough to prevent work hardening by riding on the surface of the material.
Chip breakers should be employed to assist in ensuring swarf remains clear of the work
Low thermal conductivity of austenitic alloys results in heat concentrating at the cutting edges. This means coolants and lubricants are necessary and must be used in large quantities.

Welding of Duplex stainless steel

Duplex stainless steel have good weldability. All standard welding processes can be used. They are not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding. The recommended filler material for 2205 stainless steel is 2209.

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Applications of Duplex stainless steel

Duplex stainless steel are typically used in:

Chemical processing, transport and storage
Oil and gas exploration and offshore rigs
Oil and gas refining
Marine environments
Pollution control equipment
Pulp & paper manufacturing
Chemical process plant

Supplied Forms

Aalco typically supplies 2205 duplex stainless steel in the following forms:

Because of the duplex stainless steel pipe in austenite and ferrite each accounted for about half of that belongs to the ferritic stainless steel, 304 stainless steel pipe, the duplex stainless steel pipe with austenitic stainless steel and 304 stainless steel pipe common features.
Compared to the 304 stainless steel, duplex stainless steel pipe not only resistant to high stress corrosion ability, and cold forming and fabrication process and other localized corrosion resistance is better than that of ferritic stainless steel. The duplex stainless steel pipe itself has excellent plasticity and toughness, and compared to ferritic stainless steel, it will not be sensitive to the fragility, and the mechanical properties of the duplex stainless steel pipe are better than that of the 304 stainless steel pipe.
We all know that 304 stainless steel pipes in the production process, in order to improve the corrosion resistance, wear resistance and other properties, to carry out heat treatment, but the duplex stainless steel pipe with excellent welding performance, so no preheating before welding, also need for heat treatment after welding so, the application scope in ferrite stainless steel pipe.
The selection of duplex stainless steel pipe electrode is also more selective. For example, E2209 welding rod is made of titanium calcium type duplex stainless steel electrode, and its welding performance and thermal crack resistance are better. It is mainly used in welding duplex stainless steel with approximately 22% chromium, and can also be used for welding and repair welding of other components of stainless steel. It is commonly used in pressure vessel and petrochemical industries.
There is also a A1002 duplex stainless steel welding rod, it is also a titanium calcium type coating duplex steel electrode, because the deposited metal contains about 40% ferrite, so the crack resistance is not, has good welding performance, usually used for welding of high carbon steel, tool steel, high temperature steel, steel armor the other, dissimilar steel.
In addition to 2215 duplex stainless steel welding rod, it is ultra-low carbon duplex stainless steel electrode of an alkaline coating, deposited metal containing 40%-50% ferrite, mechanical properties and corrosion resistance is excellent, especially with reliable performance and high resistance to chloride corrosion pitting resistance. DC reverse welding, all position welding. Its main purpose is welding ultra low carbon, 00Cr22Ni5Mo3N, SAF2205 and other duplex stainless steel.
Finally, also called a S2216 duplex stainless steel electrode, which is ultra-low carbon duplex stainless steel welding titanium calcium type coating, deposited metal containing 40%-50% ferrite has excellent mechanical properties and corrosion resistance, especially the resistance to chloride corrosion is reliable and high corrosion resistance. Good welding performance. Can be AC and DC dual-use, mainly used to weld ultra low carbon, 00Cr22Ni5Mo3N, SAF2205 and other duplex stainless steel pipe.

Source: China Duplex Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References:

http://www.imoa.info/molybdenum-uses/molybdenum-grade-stainless-steels/duplex-stainless-steel.php
https://www.azom.com/article.aspx?ArticleID=2870
https://en.wikipedia.org/wiki/Duplex_stainless_steel
https://www.metallicsteel.com/the-welding-technology-of-2205-duplex-stainless-steel.html
https://www.yaang.com/advantages-and-disadvantages-of-duplex-stainless-steel.html

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Advantages of duplex stainless steel pipe and selection of welding rod