High nitrogen steel is a good fit for many applications where reliability is paramount.
In this post, Todd Chavanne, Business Development at Sullivan Steel, discusses the benefits and applications of nitrogen as a component of steel. “Nitrogen gives steel some really desirable properties,” says Todd, “particularly for high-performance applications, like hardness, fatigue strength, and corrosion resistance.”
High nitrogen steel is simply steel with a nitrogen as an alloying element. The nitrogen steels available from Sullivan Steel are CRENs (Corrosion Resistant Nitrogen), often referred to as “nitrogen stainlesses.” The addition of nitrogen (which is a difficult and complex process) influences steel properties right at the microstructural level.
CRENs can exhibit both near-perfect stainless properties and bearing-quality hard surfaces at the same time. It is a unique combination, as most stainless grades either achieve high corrosion resistance or high hardness; one is usually sacrificed for the other.
What’s more, incorporating nitrogen also improves other properties:
The chromium component of stainless steel provides its corrosion resistance. In standard stainless grades, some of the chromium content gets trapped in carbides during heat-treatment as chromium-carbides, which leaves just the un-trapped, free chromium to impart corrosion resistance.
Adding nitrogen encourages the formation of nitrides for hardness rather than carbides. And, because nitrides do not trap chromium, nearly all the chromium remains free, strengthening corrosion resistance.
The carbon content of a standard stainless provides material hardness yet also creates corrosion susceptibility.
However, “once you get nitrogen into the steel matrix,” says Todd, “you don’t need to rely on carbon because it’s the nitrides that provide the hardness, rather than just carbides.” That’s how high nitrogen stainless steels can exhibit both high corrosion resistance and high hardness.
Steel grades with nitrogen typically have a more dispersed and finer grain structure.
“This smooth, even structure strengthens the properties that we love to see for the high-performance markets we’re selling to, like fatigue and wear resistance,” says Todd.
High nitrogen steels tend to resist softening and maintain their properties at higher temperatures than other grades.
There are two options for heat-treating CREN steels. One is for standard operation, and the other is for higher-temperature operations.
“You do sacrifice some of the corrosion resistance when you temper a CREN steel at that higher temperature,” Todd points out, “but it’s still much, much more corrosion resistant than a standard stainless steel (like 440C) would be.”
A high nitrogen steel is much more wear-resistant during bearing contamination situations, where foreign matter gets between the rolling elements and the race. The superior cleanliness of a nitrogen-steel’s processing gives it impressive resistance relative to other grades.
Choosing a high nitrogen stainless steel may be appropriate in flight-critical or other applications where reliability is essential. However, these steels are intensively processed and, hence, more costly.
Even if corrosion resistance isn’t necessary for your application, CRENs can have other benefits. For example, if you’re currently using a re-melted steel, XD16N® (an air-melted CREN) has high cleanliness, a smooth microstructure, and comes in at a significantly lower price than a re-melt grade with similar performance.
A high nitrogen steel is worth considering if your application requires fatigue strength, hardness, and corrosion resistance. Sullivan stocks nitrogen stainless steels, including:
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