**2. Melting of HNS alloys**

Melting of HNS alloys requires special process techniques as the nitrogen content is above the solubility limit at atmospheric pressure. The equipment development has started in the 1960´s with pressurized induction furnaces (lab scale) and has finally led to the first PESR unit in 1980. Today, PESR is state of the art due to its high process capability, good productivity, large ingot sizes and a save H&S environment [Holzgruber, 1988].

#### **2.1 The PESR-process**

Today's biggest PESR unit is located at Energietechnik Essen GmbH, Germany. Its operating pressure is max. 40 bars and can achieve ingot weights up to 20 tons and 1030 mm diameter. The functional principle is shown schematically in fig.2.

Basically, the PESR process is a conventional remelting facility that works in a pressure tank. The process is designed to meet both, an ESR refining and nitrogen pick up. The metallurgical approach is similar to a standard ESR-process, i.e. refining, low segregation, no porosity or shrinkage, defined microstructure and solidification.

results have been published to date, common knowledge will summarized and topped up

Finally, some typical HNS grades will be discussed with regards to their industrial

Fig. 1. Mechanical properties in dependency of different nitrogen concentrations after

productivity, large ingot sizes and a save H&S environment [Holzgruber, 1988].

Melting of HNS alloys requires special process techniques as the nitrogen content is above the solubility limit at atmospheric pressure. The equipment development has started in the 1960´s with pressurized induction furnaces (lab scale) and has finally led to the first PESR unit in 1980. Today, PESR is state of the art due to its high process capability, good

Today's biggest PESR unit is located at Energietechnik Essen GmbH, Germany. Its operating pressure is max. 40 bars and can achieve ingot weights up to 20 tons and 1030 mm diameter.

Basically, the PESR process is a conventional remelting facility that works in a pressure tank. The process is designed to meet both, an ESR refining and nitrogen pick up. The metallurgical approach is similar to a standard ESR-process, i.e. refining, low segregation,

quenching at 1150°C, 2h in water. [Rashev et al.,2003]

The functional principle is shown schematically in fig.2.

no porosity or shrinkage, defined microstructure and solidification.

**2. Melting of HNS alloys** 

**2.1 The PESR-process** 

with own data and experimental results.

application.

Fig. 2. Schematic design of a pressure electro slag remelting furnace (PESR).

The physical fundamentals of nitrogen pick up are specified over the Sievert square root law, accordingly that the nitrogen solubility is a function of pressure and temperature:

$$\mathbb{E}\left[\%N\right] = k \cdot \sqrt{P\_{N2}} \tag{1}$$

With pN2: Nitrogen partial pressure over melt in bar, k: Material constant (temperature and alloy dependent)

In real systems, the actual solubility is additionally codetermined through the alloy composition. Thermodynamic activities are used to describe the effect of the individual elements.

$$\mathbb{E}\left[\%N\right]\_{\text{Fe}-X} = \frac{\left[\%N\right]\_{\text{Fe}}}{f\_N^X} \cdot \sqrt{p\_{N2}}\tag{2}$$

Corrosion Resistance of High Nitrogen Steels 59

Table 1. Activity coefficients of several elements with effect on the nitrogen solubility in steel

Due to the general alloy composition, the nitrogen solubility is accordingly larger in

The nitrogen pick up can occur through the gas phase and also as well from a solid nitrogen carrier. The choice of a solid body nitrogen pick up medium is down to the following

Characteristics of the slag or flux are not allowed to change (e.g. electrical conductivity,

In practice the standard Si3N4 is used, in exceptions CrN as well. A transfer of silicium respectively chromium in this case must be taken into consideration. The following table 2 provides a comparison about advantages and disadvantages of Si3N4 and gaseous nitrogen.

regarded.

achieved.

The selection of the slag takes place after metallurgical consideration and depends on the alloy. Above all, the slag composition has importance for the nitrogen pick up of the steel.

Silicium transfer in melt

must be known.

Nitrogen partial pressure: high enough to allow a dissociation at ~ 40 bar

**Advantage Disadvantage** 

Table 2. Advantages and disadvantages different nidriding mediums.

Ease of operation and storage

 Continuous allowance possible Simple regulation over the

 High equal distribution in ingot Appropriate for Si-critical steel

Ease of dissociation

pressure

grades

Reduction of N-Solubitly

Increase of N-Solubility

 Very abrasive (joints and gaskets, valves) Kinetic of dissociation of N3- -ion must be

Sievert´sches law at high pressure not ideally

Diffusions conditions in system slag-metall

Non-continiuous allowance on slag

Slag composition very important

**Element Coefficient eN**

+ 0.125 + 0.065 + 0.01


austenitic as in ferrite or martensitic steels.

metallurgical properties, etc)

C Si Ni

W Mo Mn Cr V Nb Ti

at 1 bar.

boundary conditions:

**Si3N4** Nontoxic

**N2 - Gas** 

With [%N] Fe-X: Nitrogen solubility in multi-component systems, [%N]Fe= 0,044% (equilibrium constant in pure Fe at 1600 °C and 1 bar)

The activity coefficient f is thereby defined as

$$\log f\_N^X = e\_N^X[\%X] \tag{3}$$

With : *<sup>X</sup> eN* interaction coefficient, [%X]: Concentration of the elements X in %

Fig. 3. View of the industrialized PESR –process at Energietechnik Essen GmbH for ingot weight up to 20 t und 1030 mm.

It is obvious as per table 1 that specific elements will increase the nitrogen solubility (e.g. manganese), while others will reduce the solubility (e.g. silicon). This has not only an impact on the nitrogen pick up at remelting but also on the precipitation of inter-metallic phases in the solid state.

With [%N] Fe-X: Nitrogen solubility in multi-component systems, [%N]Fe= 0,044%

*eN* interaction coefficient, [%X]: Concentration of the elements X in %

Fig. 3. View of the industrialized PESR –process at Energietechnik Essen GmbH for ingot

It is obvious as per table 1 that specific elements will increase the nitrogen solubility (e.g. manganese), while others will reduce the solubility (e.g. silicon). This has not only an impact on the nitrogen pick up at remelting but also on the precipitation of inter-metallic phases in

log [% ] *X X fN N e X* (3)

(equilibrium constant in pure Fe at 1600 °C and 1 bar)

The activity coefficient f is thereby defined as

weight up to 20 t und 1030 mm.

the solid state.

With : *<sup>X</sup>*


Table 1. Activity coefficients of several elements with effect on the nitrogen solubility in steel at 1 bar.

Due to the general alloy composition, the nitrogen solubility is accordingly larger in austenitic as in ferrite or martensitic steels.

The nitrogen pick up can occur through the gas phase and also as well from a solid nitrogen carrier. The choice of a solid body nitrogen pick up medium is down to the following boundary conditions:


In practice the standard Si3N4 is used, in exceptions CrN as well. A transfer of silicium respectively chromium in this case must be taken into consideration. The following table 2 provides a comparison about advantages and disadvantages of Si3N4 and gaseous nitrogen.


Table 2. Advantages and disadvantages different nidriding mediums.

The selection of the slag takes place after metallurgical consideration and depends on the alloy. Above all, the slag composition has importance for the nitrogen pick up of the steel.

Corrosion Resistance of High Nitrogen Steels 61

Fig. 5. Tempered microstructure of a nitrogen alloy martensite (1.4108). (M 1000:1)

of the alloy is mandatory to maintain the alloy characteristic.

**3.1 Atomic structure of nitrogen alloyed steels** 

However – and this is a difference to the conventional nitrogen free alloy variations – one should consider that HNS-alloys have a specific precipitation behavior. This must be kept in mind so that potential difficulties at hot forming at heat treatment can be avoided. Additionally, any precipitation will affect the corrosion resistance so a good understanding

Much effort has been put into place to understand the beneficial effect of nitrogen in stainless steels over the past years. A major step was the calculation of the atomic structure within the d-band of Fe-C and Fe-N carried out by [Rawers, 2003], [Gravriljuk & Berns, 1999] and [Mudali & Raj, 2004]. Therefore, nitrogen increases the state density on the Fermi surface whereas carbon leads to a decrease of state density. Consequently, a higher concentration of free electrons can be found in austenitic nitrogen alloyed steels – this result in a metallic character of interatomic bonds. This also explains the high ductility in HNS, even at high strengthening. Contrary, interatomic bonds in carbon austenites show a covalent characteristic. This is due to the localization of electrons at the atomic sites [Rawers, 2003]. The preference for different atoms to be nearest neighbors is defined as short range order and is mainly driven by the degree of metallic character of an intermetallic bond. A metallic interatomic bond supports a homogenous distribution as single interstitials, whereas a covalent bond results in clustering of atoms. These clusters can then potentially precipitate secondary phases such as carbides, nitrides etc. A cluster is to be realized as local accumulation of approx. 100 atoms [Berns, 2000]. The high thermodynamic stability of nitrogen stabilized austenites can also be led back on the hindered clustering of atoms [Rawers, 2003]. In summary, the electron configuration is therefore the main driver for an increased corrosion resistance. Due to nitrogen, the allocation of Cr-atoms within the lattice is homogenous so that Cr- clustering and formation of M23C6-carbides is reduced. Since
