Duplex Steel

Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. They are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as Type 304 or 316.

The main differences in composition, when compared with an austenitic stainless steel is that the duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen.Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits.

Chemical Composition of Duplex Steel Alloys

Grade   C Mn Si P S Cr Mo Ni N
2205 (S31803) Min - - - - - 21.0 2.5 4.5 0.0
Max 0.030 2.00 1.00 0.030 0.020 23.0 3.5 6.5 0.20
2205 (S32205) Min - - - - - 22.0 3.0 4.5 0.14
Max 0.030 2.00 1.00 0.030 0.020 23.0 3.5 6.5 0.20
2304 (UNS S32304) Min 0.03 2.5 1.0 0.04 0.03 21.5 0.05 3.0 0.05
Max - - - - - 24.5 2.0 3.5 2.0
Alloy 329 (UNS S32900) ≤ 0.05 ≤ 2.0 ≤ 1.0 ≤ 0.035 ≤ 0.030 25.5 1.5 5 0.08

Physical Properties of Duplex Steel Alloys

Grade Density (kg/m3) Elastic Modulus (GPa) Mean Co-eff of Thermal Expansion (μm/m/°C) Thermal Conductivity (W/m.K) Specific
Heat
0-100°C (J/kg.K)		 Electrical
Resistivity
(nΩ.m)
0-100°C 0-315°C 0-538°C at 100°C at 500°C
2205 782 190 13.7 14.2 - 19 - 19 850
2304 7.8 - 13 13.5 14 15 - 500 0.8
329 7.8 200 13.0 14.0 - 15 500 0.8

Mechanical Properties of Duplex Steel Alloys

Grade Tensile Str (MPa) min Yield Strength 0.2% Proof (MPa) min Elongation (% in 50mm) min Hardness
Rockwell C (HR C) Brinell (HB)
2205 621 448 25 31 max 293 max
2304 87(600) 58(400) 25 31^J 293
329 620-800 450 20 - 260 max

Characteristics of Duplex Steel

Characteristics of Duplex Steel
Very good resistance to uniform corrosion Very good resistance to pitting and crevice corrosion High resistance to stress corrosion cracking and corrosion fatigue
High mechanical strength Good sulfide stress corrosion resistance Good abrasion and erosion resistance
Good fatigue resistance high energy absorption Low thermal expansion Good weldability

Grades of Duplex Steel


Alloy - 2205

Corrosion Resistance Properties of Alloy 2205

Grade 2205 stainless steel exhibits excellent corrosion resistance, much higher than that of grade 316. It resists localized corrosion types like intergranular, crevice and pitting. The CPT of this type of stainless steel is around 35°C. This grade is resistant to chloride stress corrosion cracking (SCC) at temperatures of 150°C. Grade 2205 stainless steels are apt replacements to austenitic grades, especially in premature failure environments and marine environments.

Heat Resistance Properties of Alloy 2205

The high oxidation resistance property of Grade 2205 is marred by its embrittlement above 300°C. This embrittlement can be modified by a full solution annealing treatment. This grade performs well at temperatures below 300°C.

Processing Alloy - 2205
Heat Treatment The best suited heat treatment for this grade is solution treatment (annealing), between 1020 - 1100°C, followed by rapid cooling. Grade 2205 can be work hardened but cannot be hardened by thermal methods.
Welding Most standard welding methods suit this grade, except welding without filler metals, which results in excess ferrite.
Machinability The machinability of grade 2205 is low due to its high strength. The cutting speeds are almost 20% lower than that of grade 304.
Fabrication The fabrication of grade 2205 is also affected by its strength. Bending and forming of this grade requires equipment with larger capacity. Ductility of grade 2205 is lesser than austenitic grades; therefore, cold heading is not possible on this grade. In order to carry out cold heading operations on this grade, intermediate annealing should be carried out.

Alloy - 2304

Corrosion Resistance Properties of Alloy 2304

Due to its high chromium content of 23%, the corrosion resistance properties of Duplex 2304 are practically equivalent to those of Alloy 316L Its duplex microstructure and low nickel and high chromium contents allows Duplex 2304 to have improved stress corrosion resistance properties compared to the 304L and 316L standard austenitic grades. More resistant to pitting and crevice corrosion resistance that Alloy 316L Outperforms Alloys 304L and 316L in stress corrosion cracking resistance in chloride containing aqueous solutions Its corrosion rate in boiling nitric acid (65%) is higher than that of Alloy 316L Its high yield strength allows Duplex 2304 to perform well in abrasion/corrosion applications

Processing Alloy - 2304
Hot Forming Hot forming must be performed in the 1150°/900°C (2100°/1650°F) temperature range.After forming, a new solution annealing treatment is recommended in the 950°/1050°C (2100°/1650°F) temperature range to fully restore corrosion resis- tance properties and mechanical properties. Parts formed with 2304 must be supported carefully during heating to avoid creep deformation.
Cold Forming Alloy 2304 may be cold formed without any problem. The same equipment as used for the cold forming of 304L and 316L grades can be used. Due to its higher mechanical properties, including the yield strength, higher stresses are required for cold forming. A final solution annealing heat treatment is also recommended after cold forming in order to restore the mechanical and corrosion resistance properties, as described in ‘hot forming.’
Weldability Can be successfully welded by TIG manual and automatic, PLASMA, MIG, SMAW, SAW, FCAW Duplex microstructure renders the alloy less sensitive to hot cracking Pre-heating and post welding is not required Filler metal should be a balanced ferrite/austenitic type
Machinability Exhibits improved machinability properties particularly when considering drilling Low speeds and high feeds will minimize this alloys tendency to work harden

Alloy - 329

Corrosion Resistance Properties of Alloy 329

Alloy 329 shows very good corrosion resistance particularly in chloride-bearing environments. Its resistance to chloride caused attacks such as pitting, crevice corrosion, stress corrosion and corrosion fatigue is much better than that of fully austenitic stainless steels of EN 1.4404 type. Also, in most cases EN 1.4460 is much better than EN 1.4404 type with regard to general corrosion resistance in reducing and oxidizing acids. Besides the two-phase microstructure and the low carbon content render EN 1.4460 better resistance to intercrystallite corrosion after sensitization within the temperature interval 500 - 900º C. When extremely high demands are imposed on resistance to pitting and crevice corrosion it is recommended that machining of the steel surface be followed by pickling or passivation.

Processing Alloy - 329
Heat Treatment Solution annealing 1020 - 1100ºC. Holding time at solution annealing temperature approx. 30 min, followed by rapid cooling in water. Stress relief treatments can in special cases be performed at 550ºC - 600ºC.
Welding Alloy 329 possesses good weldability and can be welded in the same manner as austenitic material. Welding should be carried out without preheating and with small weld beads, i.e., with as little heat supply as possible. It is not necessary to use welding consumables of the same steel grade but also austenitic ones can be used. Welding of EN 1.4460 and subsequent application in highly corrosive environments could lead to a reduction in corrosion resistance. After annealing at 980° C and quenching in water the corrosion resistance of the weld will be just as high as that of the base material.
Machinability Alloy 329 is a machinability improved Duplex stainless steel, which has considerably better machinability properties then EN 1.4462 It is not a ” stainless free cutting steel ” but a high class norm steel. It is an” easy to machine steel”, considered for parts where extensive machining is required.
Fabrication Hot and cold forming Hot forming should be carried out in the temperature range 1200-950° C. It should however, be observed that the strength of the duplex material is low at high temperatures. Hot working should normally be followed by solution annealing. Due to the high proof strength of duplex material, greater working forces than those required for austenitic steel are usually needed for cold forming of duplex steel. The spring back is relatively high because of the high yield point. Solution annealing is normally recommended after more than 10 % cold deformation.