The word duplex is based on the concept of austenite and ferrite formulated in the same material.
Combines improved resistance to sulphide and stress corrosion cracking, chloride environments, pitting, crevice corrosion and high strength when compared with other stainless alloys.
It has an Austenitic, Ferritic, Chromium (Content high19-28%), Nickel, (Content low compared with Austenitic) and Molybdenum ( up to 5%) composition
Has a ductile/brittle transition at approximately -50°C.
It is approximately twice the strength of standard austenitic stainless steels.
It has a mixed microstructure of Ferrite and Austenite, the aim of this being to produce a 50/50 mix, although in commercial alloys, the mix may be 40/60 respectively.The most used Duplex Stainless Steel are the 2205 (22% Chromium, 5% Nickel) and 2507 (25% Chromium, 7% Nickel) sometimes the 2507 is also called Super Duplex due to the higher corrosion resistance.
ADVANTAGES
Duplex is cost-effective in more than one way. The material costs less than many other materials used in a corrosive atmosphere. Duplex’s yield strength is double that of the 300 series stainless steel. This factor allows the use of a thinner material to support a like load.
The cost of fabricating duplex usually is lower than for other materials because of its comparative ease of machining and welding. When high-nickel alloys are welded, small stringer beads and low heat input are recommended. This is not the case with duplex. Higher heat input and a slight weave bead are recommended for this material. The ferrite content causes the welding to be less intimidating than when welding hig- nickel alloys. This factor alone saves significant weld time.
Duplex requires fewer machining labor hours than high-nickel alloys. Welding wire and gas costs are less with duplex, except when the corrosive atmosphere is extreme.
Forming duplex is not complicated. The material can be bent and rolled by almost the same methods as 50,000-PSI-yield-strength material. Conventional rolls and dies are sufficient for forming most shapes, which eliminates the added cost of purchasing special rolls and dies. The only significant requirement is that the dies and rolls must have a hardness of at least 34 Rockwell C to prevent carbon impregnation.
DISADVANTAGES
Welding duplex alloys is not particularly difficult, but the heat input and interpass temperature must be controlled to maintain the ferrite-austenite balance. Otherwise, the advantages of using the duplex material will be sacrificed. The time spent at 1,300 degrees F to 1,800 degrees F must be controlled. The heat-affected zone (HAZ) time-at-temperature is critical in controlling the intermetallic phases.
If the matching filler wires or electrodes (ER2209 or E2209) are used to join the material, there is a slight disadvantage versus using the more expensive ERNiCrMo-3 or ENiCrMo-3 welding materials in the area of crevice corrosion. The nickel content of E2209 and ER2209 is in the range of only 7 percent to 9 percent, while the nickel content of ERNiCrMo-3 and ENiCrMo-3 is 56 percent to 58 percent. The addition of columbium and niobium limits the carbide precipitation in the HAZ. This cost is worthwhile, but it slightly diminishes the duplex’s cost advantage.
Another disadvantage is in the rolling and bending process. Duplex’s tensile and yield strength and the work-hardening tendency cause springback and sometimes re-forming. This is not always the case, but is common in materials thicker than 3/4 in.
Super Duplex Stainless Steel
- Highly alloyed, good pitting resistance.
- Good resistance to crevice corrosion and corrosion fatigue
- Suited for use in aggressive environments.
- Good resistance to stress corrosion cracking in chloride and sour environments.
- Typical.
C. Cr. Cu. Mn. Mo. N. Ni. P. S. Si. W.
0.03% 24.00- 0.50- 1.00% 3.00- 0.20- 6.00- 0.035% 0.015% 1.00% 0.50-
max 26.00% 1.00% max 4.00% 0.30% 8.00% max max max 1.00%
Typical Mechanical Properties
- Tensile Strength N/mm2 730 – 930 MPa
- 0.2% Proof Stress N/mm2 530 MPa
- Elongation 25%
- Hardness (Brinell) 290 HB
Properties are for solution annealed super duplex at room temperature.
Thickness up to 160mm max. Hardness for guidance only.
Heat Treatment
AT = Solution annealed. 1040-1120°C. Water quench