Duplex Stainless Steels

Question 1

What are duplex stainless steels?

Answer 1

• A class of stainless steels that are intermediate between Ferritic
  and Austenitic stainless steels.
• Called Duplex because they have a two-phase microstructure
  consisting of ferritic and austenitic stainless steel.

Question 2

What are characteristics of duplex stainless steels?

Answer 2

➢ Offered as an alternative to austenitic or ferritic SS:
* Higher strength leading to weight saving (i.e < thickness)
- Pressure Vessels/ Storage tanks
* Better price stability (Ni content) & lower price
- Ni expensive; higher in Austenitic SS
* Greater corrosion resistance; stress corrosion cracking
- Brewing tanks/ swimming pools structures

Question 3

What do duplex stainless steels contain?

Answer 3

Typical content:
➢ Duplex S.S. contains 18 – 30 % Cr to provide overall corrosion
resistance.
➢ 3 – 9 % Ni which improves the toughness and workability.
➢ Mo which again improves the corrosion properties, particularly
pitting corrosion.
➢ C contents are low (< 0.03wt%) to help reduce intergranular
corrosion (caused by Cr23C6 precipitation at grain boundaries).
* This also helps promote Cr2O3 formation
* Increases corrosion resistance

Question 4

What type of corrosion occurs in DSS? When does it occur.

Answer 4

Intergranular Corrosion:
* Heating 482-760°C (welding)
* Cr form with C at grain boundaries (𝐶𝑟23𝐶6)
* Creates Cr depleted zones, decreasing corrosion from protective
passive film