**2. High-temperature diffusion surface treatment**

Berns [1] proposed, in the first half of the 1990s, carrying out a hightemperature nitriding process by exposing austenite to an N2 atmosphere. Nitrogen dissolves in austenite up to the solubility limit during the high-temperature nitrogen treatment. Nitrogen solubility in austenite is much greater than in the BCC phases. Then, by maintaining stainless steel in a furnace containing a pure N2 gas atmosphere, the nitrogen equilibrium between the furnace atmosphere and the alloy can be attained. According to Sieverts' law [2], nitrogen can reach contents up to 1 wt.% in solution. Thermocalc [3] phase diagrams considering the N2 gas phase were calculated to predict the N2 content in equilibrium in austenite as a function of temperature and partial pressure, as shown in **Figure 1**.

The high-temperature nitriding treatment consists of a case hardening that enriches the stainless steel's surface with nitrogen contents up to 1 wt.%, to a depth of 1–2 mm. Berns [5] named this process solution nitriding (SN). After this pioneer proposal, several research works have followed on studying this solid-state route for introducing high N2 contents in solution in austenite, being called hightemperature-gas-nitriding—HTGN [6] or high-temperature-solution-nitriding HTSN [7].

The amount of nitrogen dissolved in austenite, in equilibrium with pure N2 gas atmosphere, increases with decreasing temperature and pressure, as shown in **Figure 2** [6].

Berns [5] envisaged different possibilities of obtaining tailored engineered stainless steels depending on the composition and surface treatment. Therefore, austenitic stainless steels can be HTGN, obtaining a fully austenitic case with an *Surface Hardening of Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.105036*

### **Figure 1.**

*TPT diagram relating nitriding parameters (nitrogen temperature and partial pressure) with microstructure, nitrogen content, and martensitic layer depth for 3 h gas nitriding treatments at high temperature for an AISI 410S steel [4].*

### **Figure 2.**

*Fe—13% Cr—N isopleths (a) not considering the gas phase as an equilibrium one, with N2 isobars overlaid and (b) considering the N2 gas phase as an equilibrium one [6].*

outermost N content of 0.48 wt.%N and case depths of up to 1 mm, with a hardness variation from 1.95 GPa in the low N core to 3.17 GPa in the 0.48 wt.%N case [8]. Martensitic stainless steels can be HTGN, obtaining a much harder 0.4 wt.%N martensitic cases 725 HV hard [9]. Extra-low carbon (0.017 wt.%C) dual-phase stainless steel (α + Martensite) may form a fully martensitic case 550 HV hard, after HTGN [10]. Finally, an UNS 31803 ferritic/austenitic duplex stainless steel can be hardened by HTGN, achieving a fully austenitic layer near the surface due to enrichment in austenite stabilizer element (N), as shown in **Figure 3** [11]. Excess of diffused nitrogen causes a solid solution hardening effect, proportional to its content, reaching a maximum value of 330 HV at maximum concentration, as shown in **Figure 4** [12]. It is worth noting that the N absorption and diffusion on the surface during the HTGN process induce phase transformations, resulting in microstructural gradients from the surface to the core and corresponding microhardness gradients.

**Figure 3.** *UNS S31803 duplex stainless steel HTGN at 1200°C [11].*

### **Figure 4.**

*Microhardness gradient from the low nitrogen duplex ferritic-austenitic core toward the fully austenitic 0.8 wt. %N surface [12].*

Tschiptschin [13], using this concept, proposed a Powder Metallurgy route to enrich a ferritic stainless steel powder (0.02 wt.%C, 16.2 wt.%Cr, and 0.81 wt.% Mo), exposing the powder particles at high temperatures (1100°C and 1200°C) to N2 gas atmosphere. The N enriched austenitic powder transforms during quenching to martensite, becoming very hard. One of the main challenges in this HNS production route is obtaining fully dense components with uniform nitrogen content in volume and excellent surface properties. A uniform nitrogen distribution leads to a more homogeneous microstructure and better mechanical properties. **Figure 5** shows the amount of nitrogen as a function of temperature. According to Sieverts' law [2], increasing temperature decreases the amount of nitrogen content of the obtained alloy. High temperatures are necessary to grant that all the nitrogen is dissolved in austenite, avoiding the precipitation of chromium nitrides.

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

### **Figure 5.**

*Nitrogen content as a function of temperature and N2 pressure for an AISI 434L ferritic stainless steel [13].*

### **Figure 6.**

*Cyclic polarization curves for a 0.66 wt.%N martensitic stainless steel in different stages of fabrication. Solution 0.5 M H2SO4 + 3.5% NaCl. S/N: Sintered/nitrided, HIP: Hot isostatic pressed, HT: Heat treated (quenched and tempered at 200°C) [13].*

In this route, high-nitrogen (0.66 wt.%N) martensitic stainless steel could be obtained by high-temperature gas nitriding an AISI 434L ferritic stainless steel powder, compressing the high nitrogen powder to near net shape parts, followed by hot isostatic pressing and proceeding with a 1200°C quenching and a 200°C tempering treatment. As a result, the obtained hipped material showed high hardness and much better corrosion resistance, measured in potentiodynamic polarization tests carried out in 0.5 M H2SO4 + 3.5% NaCl, as shown in **Figure 6**.
