**3. Microstructrual characteristics of HNS**

Nitrogen stabilizes the -area in a very clear way. It is undesired in low alloyed steels due to the formation of brittle phases; however it is very beneficial in terms of strengthening and corrosion resistance for high alloyed steels. It is considered to be the most efficient solid solution strengthening element. [Pickering, 1988] has reported that nitrogen is approximately twice as efficient as carbon.

[Bernauer & Speidel, 2003] et al. has published results, whereas nitrogen can improve the stability by adjusting a carbon/nitrogen ratio. The high interstitial steels, i.e. containing carbon and nitrogen show a higher thermodynamic stability compared to common Cr-Mn-N-steels.

[Pickering, 1988] has investigated the influence of nitrogen within various steelbased alloys with regards to their microstructure. Therefore, nitrogen has a higher solubility in the lattice than carbon. Its presence is related with the formation of nitrides (or carbon nitrides). These nitrides tend to precipitate as small particles and – this is particular of interest for any hot forming applications – grow significantly slower than carbides. It is obvious that this will have an impact on recovery, grain growth and heat resistance. The nitrides are thermodynamically more stable than the corresponding carbides, i.e. have a lower solubility.

At first sight, the considered HNS- alloys are not significantly distinguishable than the nitrogen free varieties. The following fig. 4 & 5 show exemplary micrographs of a Mnaustenite with approx 0.65% nitrogen as well martensite with approx. 0.4% nitrogen.

Fig. 4. Microstructure of a unformed and solutions annealed austenite with ~ 0,6 % N.

Nitrogen stabilizes the -area in a very clear way. It is undesired in low alloyed steels due to the formation of brittle phases; however it is very beneficial in terms of strengthening and corrosion resistance for high alloyed steels. It is considered to be the most efficient solid solution strengthening element. [Pickering, 1988] has reported that nitrogen is

[Bernauer & Speidel, 2003] et al. has published results, whereas nitrogen can improve the stability by adjusting a carbon/nitrogen ratio. The high interstitial steels, i.e. containing carbon and nitrogen show a higher thermodynamic stability compared to common Cr-Mn-

[Pickering, 1988] has investigated the influence of nitrogen within various steelbased alloys with regards to their microstructure. Therefore, nitrogen has a higher solubility in the lattice than carbon. Its presence is related with the formation of nitrides (or carbon nitrides). These nitrides tend to precipitate as small particles and – this is particular of interest for any hot forming applications – grow significantly slower than carbides. It is obvious that this will have an impact on recovery, grain growth and heat resistance. The nitrides are thermodynamically more stable than the corresponding carbides, i.e. have a

At first sight, the considered HNS- alloys are not significantly distinguishable than the nitrogen free varieties. The following fig. 4 & 5 show exemplary micrographs of a Mn-

austenite with approx 0.65% nitrogen as well martensite with approx. 0.4% nitrogen.

Fig. 4. Microstructure of a unformed and solutions annealed austenite with ~ 0,6 % N.

**3. Microstructrual characteristics of HNS** 

approximately twice as efficient as carbon.

N-steels.

lower solubility.

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

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 of the alloy is mandatory to maintain the alloy characteristic.

#### **3.1 Atomic structure of nitrogen alloyed steels**

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

Corrosion Resistance of High Nitrogen Steels 63

In dependency to the carbon content and the tempering time, austentic steels tend to precipitate M23C6-Carbides at the grain boundaries. Through this the ductility and corrosion resistance of the material significantly declines. However, the strength properties have no mentionable change. The susceptibility for intercrystalline corrosion clearly increases.

The precipitation behavior of this carbide can only be prevented through a quick quench in the critical temperature range. Fig.8 shows the location of the precipitation with relation to the alloy composition. Fine carbides are beneficial with regards to the corrosion resistance as the local chrome depletion is less in comparison to coarser carbides. The Cr depletion can be balanced out through an extended homogenisation (i.e. holding time) within the

Carbon content %

Fig. 8. Influence of the carbon content in location of grain decay in unstable austentic steels

annealing time in h

For 12% Cr-steels, containing nitrogen it has been observed by [Pickering, 1988] that nitrogen lowers the martensite start temperature MS; 1% nitrogen lowers MS by 450 °C.

[Pickering, 1988] has investigated the influence of nitrogen on the carbide morphologies. The main type is as previously described the M23C6 type. In Nb-containing alloys, M4X3 have been observed where nitrogen can occupy interstitial dislocations. It also can be solutioned

HNS martensitic steels are also characterised with good high temperature strength and show an according hot forming behavior. Under circumstances these steels are found in thermo mechanical forging and rolling applications. The last forming step will effectively increase the dislocation density so that adequate nucleation for a desired precipitation exists. For example, the precipitation of carbides, nitrides as well as carbon nitride could be finely distributed. This

with circa 18 % Cr und 8 % Ni. Examination in Strauß-Test. [Thyssen, 1989]

**3.2 Carbides** 

precipitation area.

annealing temperature i

n °C

within M6C.

nitrogen delays the precipitation of carbides as seen in fig. 6 & 7 , the likeliness of a local Cr depletion is limited.

Fig. 6. Schematic of a short range order. Nitrogen increases the concentration on free electrons. Thereby forming a non-directional bonding and an equal distribution of the atom in crystal lattice. [Berns, 2000]

Fig. 7. Schematic of a cluster formation. Carbon decreases the concentration of free electrons. Thereby forming a directional bonding of non equal distribution of the atom in crystal lattice. [Berns, 2000]

#### **3.2 Carbides**

62 Corrosion Resistance

nitrogen delays the precipitation of carbides as seen in fig. 6 & 7 , the likeliness of a local Cr

Fig. 6. Schematic of a short range order. Nitrogen increases the concentration on free electrons. Thereby forming a non-directional bonding and an equal distribution of the atom

Fig. 7. Schematic of a cluster formation. Carbon decreases the concentration of free electrons. Thereby forming a directional bonding of non equal distribution of the atom in crystal

depletion is limited.

in crystal lattice. [Berns, 2000]

lattice. [Berns, 2000]

In dependency to the carbon content and the tempering time, austentic steels tend to precipitate M23C6-Carbides at the grain boundaries. Through this the ductility and corrosion resistance of the material significantly declines. However, the strength properties have no mentionable change. The susceptibility for intercrystalline corrosion clearly increases.

The precipitation behavior of this carbide can only be prevented through a quick quench in the critical temperature range. Fig.8 shows the location of the precipitation with relation to the alloy composition. Fine carbides are beneficial with regards to the corrosion resistance as the local chrome depletion is less in comparison to coarser carbides. The Cr depletion can be balanced out through an extended homogenisation (i.e. holding time) within the precipitation area.

Fig. 8. Influence of the carbon content in location of grain decay in unstable austentic steels with circa 18 % Cr und 8 % Ni. Examination in Strauß-Test. [Thyssen, 1989]

For 12% Cr-steels, containing nitrogen it has been observed by [Pickering, 1988] that nitrogen lowers the martensite start temperature MS; 1% nitrogen lowers MS by 450 °C.

[Pickering, 1988] has investigated the influence of nitrogen on the carbide morphologies. The main type is as previously described the M23C6 type. In Nb-containing alloys, M4X3 have been observed where nitrogen can occupy interstitial dislocations. It also can be solutioned within M6C.

HNS martensitic steels are also characterised with good high temperature strength and show an according hot forming behavior. Under circumstances these steels are found in thermo mechanical forging and rolling applications. The last forming step will effectively increase the dislocation density so that adequate nucleation for a desired precipitation exists. For example, the precipitation of carbides, nitrides as well as carbon nitride could be finely distributed. This

Corrosion Resistance of High Nitrogen Steels 65

For prevention of such brittle phases the precipitation area of the hot forming must be followed through fairly quick. The nitrogen level has obviously an impact on the precipitation kinetics of Cr2N, see figure 11 for details. Best corrosion resistance can be

As seen in fig. 11 the precipitation depends on both, the alloy composition and holding time. The Cr2N-formation has been reported by [Pickering, 1988] to be a major issue to high Cr

> precipitation of Cr2N in low dislocations

> > limit temperature

recrystallization cold forming structure

structure

by

Fig. 11. Precipitation of Cr2N in 18Mn18Cr at various nitrogen levels. [Uggowitzer, 1991]

**annealing time [h]** 

Fig. 12. TTT-diagram for the beginning of Cr2N-precipitation at different nitrogen contents.

Curve is based on X5CrMnN18-12. [Rashev et al., 2003]

achieved if all nitrogen is in solid solution, i.e. no nitrides are precipitated.

and/or high Ni-alloys

**annealing temperature in °C** 

could be of interest to the high temperature strength. The previously mentioned effects of fine carbides concerning the dissolution and corrosion resistance are also valid here.

## **3.3 Nitride and nitrogen perlite**

In the case of the austenitic steels it should be considered, that in the temperature range of approx. 500- 900 °C and in connection with the alloy composition a precipitation of nitrides (Type Cr2N) occurs. This nitrogen perlite identified microstructure raises significantly the susceptibility to cracking of the steel but can also support intergranular cracking. Depending on the alloy composition, the precipitation window for nitrogen perlite or other nitrides are adjusted to higher or lower temperatures. The figures 9 & 10 show exemplary an austenitic structure with beginning and advance nitrogen perlite precipitation. Clear to recognize at what speed that the precipitation occurs.

Fig. 9. Beginning of precipitation of nitrogen perlit cold worked austenitic structure 1.3816. 800 °C/15 min.

Fig. 10. Advanced precipitation of nitrogen perlit cold worked austenitic structure 1.3816. 800 °C/30 min.

could be of interest to the high temperature strength. The previously mentioned effects of fine

In the case of the austenitic steels it should be considered, that in the temperature range of approx. 500- 900 °C and in connection with the alloy composition a precipitation of nitrides (Type Cr2N) occurs. This nitrogen perlite identified microstructure raises significantly the susceptibility to cracking of the steel but can also support intergranular cracking. Depending on the alloy composition, the precipitation window for nitrogen perlite or other nitrides are adjusted to higher or lower temperatures. The figures 9 & 10 show exemplary an austenitic structure with beginning and advance nitrogen perlite precipitation. Clear to recognize at

Fig. 9. Beginning of precipitation of nitrogen perlit cold worked austenitic structure 1.3816.

Fig. 10. Advanced precipitation of nitrogen perlit cold worked austenitic structure 1.3816.

carbides concerning the dissolution and corrosion resistance are also valid here.

**3.3 Nitride and nitrogen perlite** 

what speed that the precipitation occurs.

800 °C/15 min.

800 °C/30 min.

For prevention of such brittle phases the precipitation area of the hot forming must be followed through fairly quick. The nitrogen level has obviously an impact on the precipitation kinetics of Cr2N, see figure 11 for details. Best corrosion resistance can be achieved if all nitrogen is in solid solution, i.e. no nitrides are precipitated.

As seen in fig. 11 the precipitation depends on both, the alloy composition and holding time. The Cr2N-formation has been reported by [Pickering, 1988] to be a major issue to high Cr and/or high Ni-alloys

Fig. 11. Precipitation of Cr2N in 18Mn18Cr at various nitrogen levels. [Uggowitzer, 1991]

Fig. 12. TTT-diagram for the beginning of Cr2N-precipitation at different nitrogen contents. Curve is based on X5CrMnN18-12. [Rashev et al., 2003]

Corrosion Resistance of High Nitrogen Steels 67

Nitrogen does not show any influence on the thickness of the oxide layer, investigations of various alloys with different nitrogen contents have confirmed an average thickness of 12-22

A synergism of nitrogen and molybdenum is suggested by many authors [Mudali & Rai, 2004], [Pickering, 1988], [Pedrazzoli & Speidel, 1991]. Molybdenum shifts the metal dissolution to higher potentials which will consequently lead to an increased enrichment of nitrogen at the metal/oxide-interface. In this case, nitrogen can lower the current density

Fig. 13. Breakdown potential of HNS and commercial stainless steels in various electrolytes.

It has been suggested that Molybdenum and nitrogen support the formation of highly mobile ions that interact with the passivation film. Addtionally, nitrogen seems to have a

4 []4 3 *N H e NH* (4)

repassivation and reconditioning of the base material [Mudali & Rai, 2004]. It also has impact on the depronotation, which might explain the good performance in acids and halide

[Truman, 1988].

+ - ions helps to increase the pH value which results in an improved

buffer effect by reacting as follows in oxidizing corrosive media:

Å [Mudali & Rai, 2004] within all varieties.

[ETE-11]

The formation of NH4

containing liquids such as Cl-, Br- and I-

below the critical value for pitting corrosion [Mudali & Rai, 2004].

Other intermetallic phases, such as Laves-phase, Z-phase and -phase have been investigated with regards to nitrogen alloying within literature. Generally, nitrogen seems to delay the formation of intermetallic phases. The underlying mechanism has been discussed controversially; the enhanced solubility of Chromium and Molybdenum due to nitrogen or its influence on Gibbs free energy for phase formation [Mudali & Rai, 2004]. The following table will provide an overview about the influence of nitrogen on some intermetallic phases [Mudali & Rai, 2004], [Heino et al., 1998].


Table 3. Overview about the role of nitrogen alloying on the precipitation behavior of some intermetallic phases. [Mudali & Raj, 2004], [Heino et al., 1998]
