It is rather hard to say that high nitrogen stainless steels (HNS) are the new generation of stainless steels, because they have been produced since the 1940s. During the Second World War, nickel became a strategic element to produce austenitic stainless steels. The shortage of nickel led to the complete or partial replacement of nickel by other elements. It was the first stage of paying attention to nitrogen as an austenite stabilizing element.
The second stage was when the allergenic effect of nickel and nickel-containing alloys, on both human and animals, had been accepted. Scientists were looking to substitute high nickel stainless steels in biomaterial with a material with low nickel content.
In low alloy steels, nitrogen is known as an undesirable impurity that causes embrittlement by formation nitride precipitates and strain aging. Due to this fact, the content of nitrogen in low alloy steels is kept less than 100 ppm.
However, in high alloy steels like stainless steels, the story is different. Similar to nickel, nitrogen in stainless steels stabilizes the austenite phase at room temperature. Additionally, it can increase the strength of stainless steels by accommodating at interstitial sites of solid lattice of austenite. Therefore, it is allowable to add up to 0.5% (500 ppm) nitrogen to stainless steels without any problem.
In the following, we describe how nitrogen influences different properties of stainless steels.
Nickel as a substitutional austenitic element has a negative effect on strengthening austenitic steels (Figure 1). However, nitrogen is the most effective interstitial element for increasing the strength of austenite, while stabilizing the austenite phase. The powerful effect of nitrogen alloying on increasing yield and tensile strength is illustrated in Figure 2.
|Figure 1 - Solid solution strengthening effects by alloying in austenitic stainless steels|
|Figure 2 - Effects of nitrogen on strength and ductility of type 304 stainless steel|
|Composition (%wt.)||PREN|| UTS
|Nitronic® 60†||16.0-17.0||8.0-8.5||7.5-8.5||0- 0.75||0.1-0.18||0.06-0.08||45||106|
|316N||16.0-18.0||10.0 -14.0||= 2.0||2.0-3.0||0.1-0.16||=0.08||27||90|
|304||18.0 - 20.0||8.0 - 12.0||= 2.0||--||=0.1||=0.08||19||90|
|17-4 PH††||15.0 - 17.5||3.0 - 5.0||= 1.0||--||--||=0.07||16||198|
|Zeron® 100*||24.0 - 26.0||6.0 - 8.0||= 1.0||3.0-4.0||0.2-0.3||=0.03||40||109|
| † Also contains 3.7 - 4.2 % Si
†† Also contains 3.0 - 5.0 %Cu and 0.15 - 0.45 %(Ta + Nb)
*Also contains 0.5 - 1.0 %Cu and 0.5 - 1.0 % W (it is super duplex stainless steel)
Localized Corrosion Resistance
Equation 2: PREN = %Cr + 3.3 %Mo + 16 %N
In the above equation -- which is a general equation for austenitic stainless steels to determine and compare pitting corrosion resistance of stainless steels with different compositions -- a factor of 16 has been considered for nitrogen. However, there are various investigations indicating that this factor should be more than this value. Some of them have suggested 25 or even 32.
Furthermore, it is believed that a new PREN formula is required for high nitrogen stainless steels containing molybdenum and manganese; the concurrent presence of nitrogen and molybdenum has a synergetic effect on increasing resistance against both pitting and crevice corrosion. Additionally, nitrogen eliminates the deleterious effects of manganese on pitting corrosion (also stress-corrosion cracking). Therefore, the following PREN equation has been proposed for high nitrogen manganese and molybdenum containing stainless steels:
Equation 3: PREN = %Cr + 3.3 %Mo + 51 %N + 6 %Mo%N - 1.6 (%N)2
It has to be mentioned that molybdenum is a well-known alloying element that increases the pitting corrosion resistance of stainless steels. However, adding molybdenum to increase the pitting resistance is effective only in environments containing chloride ions. In other words, in environments containing other halides (such as iodide or bromide), molybdenum has no effect or is sometimes addressed to the negative effects of molybdenum. Conversely, nitrogen can increase localized corrosion resistance regardless of the type of halide ions.
Besides PREN, there is another concept that is used to show the susceptibility of stainless steels to the localized corrosion of austenitic in chloride-containing solutions, which is called "Measure of Alloying for Resistance to Corrosion" (MARC).
Equation 4: MARC = % Cr + 3.3 %Mo + 20 %N + 20 %C - 0.5 %Mn - 0.25 %Ni
MARC has been shown to be far better than the PREN formula, especially for high alloyed, high nitrogen stainless steels. The MARC formula applies only to alloying elements in solid solution.
Equation 4 shows the positive effect of nitrogen on the localized corrosion. On the other hand, both manganese and nickel, which are generally considered as austenite stabilizers (similar to nitrogen), have a negative influence on pitting and crevice corrosion.
There are several approved mechanisms to illustrate how nitrogen retards pitting in stainless steels. In the following, we summarize those that are generally accepted.
Pit initiation step: It is approved that the content of nitrogen in the passive layer is at least seven times more than balk material. In these circumstances, a highly stable protective nitride layer (Ni2Mo3N) is formed on the surface, which can protect the surface against localized corrosion and results in a decrease of dissolution of the passive layer.
Pit growth step: In case a pit is initiated at high potentials, nitrogen dissolves in the pit area and reacts with protons (H+) to produce an ammonium ion (according to the following reaction), which is an alkaline buffering substance that can control the localized pH in the pit. Thus the pH in the pit will not reduce to the acidic values. This means that the autocatalytic effect of pitting corrosion would be eliminated.
Equation 5: N + 4H++3e-? NH4+
High nitrogen stainless steels are not new types of stainless steels, but their consumption is increasing. Nitrogen can be a suitable substitute for nickel; the latter has a high relative price, deleterious effect on mechanical properties, and allergenic and probably carcinogenic features. Not only does nitrogen not have any of these issues, but it can also improve corrosion resistance and the strength of stainless steels.