**4. Duplex diffusion surface treatments**

Duplex stainless steels can also be subjected to a duplex nitriding treatment consisting of a combination of high-temperature and low-temperature diffusion treatments. Firstly, an HTGN—high-temperature gas nitriding is carried out at 1200°C, followed by a 400°C LTPN—low-temperature plasma nitriding aiming for a better load-bearing capacity. In the high-temperature nitriding treatment, nitrogen is introduced on the surface of the steel, shifting the phase equilibrium so that ferrite stringers are transformed to austenite, thus forming a 100 μm thick fully γ

**Figure 32.** *Microstructure of duplex treated 2205 duplex stainless steel (HTGN + LTPN) [11].*

**Figure 33.** *Mass loss during a cavitation-erosion test. 2205 duplex stainless steel non-nitrided, HTGN, and duplex nitrided (HTGN + LTPN) [11].*

layer, raising the hardness from 280 to 330 HV. Subsequently, the LTPN—lowtemperature plasma nitriding diffusion treatment leads to a continuous and homogeneous layer of expanded austenite (γN), 1200 HV hard, on top of the austenite layer [11]. **Figure 32** shows the microstructure of the duplex nitrided 2205 steel. This microstructure grants greater load-bearing capacity than the single plasma nitriding treatment, and the alloy's performance under cavitation-erosion is much better, as shown in **Figure 33**.

Duplex treatments (HTGN + LTPN) can considerably increase the tribological properties of the surface. **Figure 33** shows the mass loss results during a 2205 duplex-stainless-steel cavitation-erosion test. The mass loss decreases after (HTGN) high-temperature gas nitriding, forming a thick high-nitrogen austenite layer. In the duplex treatment, the mass loss is almost null for testing times up to 64 h due to forming a 1300 HV hard expanded austenite layer sustained by a harder substrate [11].
