*Surface Hardening of Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.105036*

nitriding at 380°C and 550°C. At 550°C, the diffusion zone is heavily darkened due to a severe etching by Villela's reagent, but when nitriding is carried out at 380°C, the diffusion zone practically remains unchanged compared to the tempered martensite matrix. The darkening of the nitrided layer denotes loss of corrosion resistance due to the nitriding process, while the unetched nitrided layer indicates that the corrosion resistance is maintained in low-temperature nitriding [33].

In **Figure 15**, the X-ray diffraction maps show AISI 420 steel before and after plasma nitriding [33]. After quenching and tempering, only the peaks referring to the tempered martensite (α<sup>0</sup> ) are observed. When nitriding is carried out at 380°C, the

**Figure 15.** *XRD spectra for AISI 420 steel before and after nitriding [33].*

tempered martensite peaks give way to the expanded tempered martensite peak (α<sup>0</sup> N). Peaks corresponding to iron nitrides, Fe3N, and Fe4N are also observed. This result shows that this temperature is low enough to inhibit the diffusion of chromium, preventing the precipitation of CrN and Cr2N nitrides. Avoiding the precipitation of chromium nitrides at low temperatures is responsible for maintaining corrosion resistance. The CrN and Cr2N chromium nitrides diffraction peaks that appear after nitriding at 550°C show intense precipitation of chromium compound and chromium depletion of the matrix, responsible for the decrease in the corrosion resistance of the nitrided surface.

**Figure 16** shows the corrosion rate of a 380°C nitrided AISI 420 steel specimen when subjected to an immersion test in an aqueous solution with 10% HCl for 120 h. In the quenched and tempered condition, the corrosion rate after nitriding is lower than the non-nitrided material due to the higher nitrogen concentration on the nitrided surface.

While hardening occurs due to the precipitation of chromium nitrides, in the 550°C plasma nitriding treatment and in the 380°C nitriding treatment, the nitrided surface hardens due to the formation of expanded tempered martensite (γ<sup>0</sup> N), which induces compressive residual stresses.

**Figure 17** shows that, compared to the quenched and tempered matrix, with 590 HV, the low-temperature nitriding plasma treatment (380°C) promotes hardening near 1000 HV. For the 550°C nitriding, the hardening nearly reaches

### **Figure 16.**

*The corrosion rate of AISI 420 steel in aqueous solution with 10% HCl for 120 h before and after plasma nitriding at 380°C for 20 h. Author: Unpublished.*

**Figure 17.** *Maximum hardness after plasma nitriding of AISI 420 steel [32].*

1300 HV. Despite the lower hardening in the nitriding treatment at 380°C, this condition should be preferentially used, as it combines hardening and good corrosion resistance.

**Figure 18.** *Transverse hardening profiles after plasma nitriding of AISI 420 steel at 380°C and 550°C [32].*

**Figure 19.**

*Scratches made under constant load on the surface of an AISI 410 steel in the (a) quenched and tempered and (b) after plasma nitriding at 400°C conditions [57].*

Another important factor related to the hardening characteristic is the transverse hardness profile obtained in these two conditions, **Figure 18**. For the 550°C nitriding treatment, the transverse hardening profile shows a maximum hardness level throughout the diffusion zone with an abrupt drop at the matrix interface [32, 55, 56]. A very steep hardness gradient is not appropriate to withstand mechanical shear stresses found during sliding. Furthermore, exposing the steel to high nitriding temperatures causes a decrease in core hardness by an over-tempering effect [32]. When low-temperature nitriding is carried out, despite the lower maximum hardness, the transverse hardening profile is diffuse, with no decrease in core hardness, and suitable for most different applications.

The surface hardening promoted in the low-temperature plasma nitriding treatment is responsible for increasing the tribological properties [57]. **Figure 19** compares the scratch resistance of an AISI 410 martensitic stainless steel: (a) nonnitrided, quenched, and tempered to a 40 HRC hardness; (b) plasma nitrided at 400°C. The scratch path in the non-nitrided condition is thicker and more profound than in the nitrided condition and presents deformation in its surroundings. **Table 3** shows that the scratch severity is at least half of the non-nitrided condition for the scratch track's depth and thickness in the nitrided condition.

**Figure 20** compares the cavitation resistance of non-nitrided and 400° plasma nitrided AISI 410 stainless steel in a test [58]. One can see that the mass loss of the low-temperature plasma nitrided specimens lost 40 times less mass than the non-nitrided specimen.

Martensitic stainless steels can also be nitrocarburized or carburized [59–62]. Nitrocarburization of 420 martensitic stainless steel carried out at 450°C for 4 h can achieve a surface hardening close to 1280 HV with a layer composed of nitrogen and


### **Table 3.**

*Scratch width and depth for non-nitrided and 400°C plasma nitrided AISI 410 stainless steel.*

### **Figure 20.**

*Mass loss during cavitation tests of an AISI 410 steel in the quenched and tempered and 400°C plasma nitrided conditions [58].*

**Figure 21.** *Hardness profile for a 450°C (4 h) plasma nitrided AISI 420 martensitic stainless steel [61].*

carbon expanded martensite (γ<sup>0</sup> NC) and Fe3C/Fe2–3(CN) type precipitates. Nitriding at lower temperatures avoids these precipitates in the layer. **Figure 21** shows the hardness profile of the martensitic stainless steel after plasma hardening at 450°C for 4 h, with a maximum hardening potential of 800 HV and a hardening depth in the diffusion zone close to 0.040 mm [61].
