Dual-phase stainless steel

General description
Dual-phase stainless steel, commonly known as Duplex stainless steel (DP steel or DSS)，is a low-Nickle (4.5-6.5) stainless steel, because of its metallurgical structure containing two phases, ferrite (body centered cubic lattice) and austenite(face-centered cubic lattice) each with about 50% percentage respectively, it takes good characteristic from both austenitic and ferritic, ie. has good mechanical properties, corrosion resistance, and also weldability.

Historical development and its advantage
The DSS was first produced in the USA in the 1940s and is in its 3^rd generation as of 2020 now. the biggest advantage of DSS is high yield strength up to 400-550Mpa, almost double of usual single-phase stainless steel. Hence, it can save the usage of steel, therefore reducing general costs.
The 2^nd biggest advantage is its good performance in anti-corrosion, especially in the bad circumstance such as seawater, Chloridion rich area, compared to usual austenite stainless steel, DSS has much better resistance in pitting corrosion, crevice corrosion, and stress corrosion, and it also its fatigue of corrosion is very low, that only high alloy austenite stainless steel can compete with it.
Other advantages of DSS are:
l  High ultimate tensile strength(UTS, enabled by the martensite), with low initial yield stress (provided by the ferrite phase)
l  Low yield to tensile strength ratio (yield strength / tensile strength = 0.5) which is good to prevent sudden failure of metal components.
l  High initial strain hardening rates
l  Good uniform elongation
l  A high strain rate sensitivity(the faster it is crushed the more energy it absorbs)
Disadvantages of DSS:
Compared to austenite stainless steel(ASS)
1      DSS is not as universally suitable as ASS, eg, can not be used under 250℃
2      Plasticity and toughness are lower than austenite ASS, therefore its cold-forming and hot-forming performance is somehow inferior
3      There exists a mid-temperature brittleness area, which need to be careful to generate harmful phase during heat treatment or welding procedure
Nevertheless, thanks to its excellent advantages described before, DSS has become more and more popular to be used in heat exchangers and pipes for the industries of oil, natural gas, paper making, fertilizer, marine engineering, and chemical, and has also become widely used to make structural components in the industries of the bridge, airplane, ship, automotive, etc.
Nowadays, there are mainly four types of dual-phase stainless steel (DSS) classified by alloy content, each country has a different grade, below listed are that of six countries:

We just finished a custom machining job-- end rings for an environment protection machine that is used in chloride circumstances.
The rings are made of Duplex stainless steel SAF 2205 with a thickness of 16mm.

Here below is the chemical composition for SAF2205:
C≤0.030 Mn≤2.00 Si≤1.00 p≤0.030 S≤0.020 Cr 22.0～23.0 Ni 4.5～6.5 Mo3.0～3.5 N0.14～0.20

Compared with single-phase stainless steel 304 and 316L, DSS SAF2205 has better performance, especially on anti-chloride corrosion.

It is reported that a Seamless pipe made of SAF 2205 has been used for an 850km long pipeline to pump the LNG across China from the western to the eastern, thanks to its excellent mechanical properties and weldability.