Nitrogen in Steels: Part One

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Introduction to Nitrogen's Role in Steel

All steels contain nitrogen, which plays a crucial role in improving mechanical and corrosion properties when it remains in solid solution or precipitates as very fine and coherent nitrides. In austenitic steels, nitrogen addition simultaneously enhances fatigue life, strength, work hardening rate, wear resistance, and localized corrosion resistance.

High nitrogen martensitic stainless steels demonstrate superior resistance to localized corrosion (including pitting, crevice, and intergranular corrosion) compared to their carbon-containing counterparts. This improved performance has led to high nitrogen steels being recognized as a promising new class of engineering materials.

Nitrogen Solubility in Steel Manufacturing

The nitrogen solubility in steel follows specific patterns that can be expressed through several key equations:

½N₂=[N]        (1)

The equilibrium constant for nitrogen absorption is:

K=[ppmN] / (p_N2)^½        (2)

The reaction constant relates to free energy:

ΔG = ΔG₀ + RT ln K_N        (3)

At equilibrium:

       (4)

Assuming Henry's law applies, the nitrogen concentration can be calculated as:

[%N]=K_N(p_N2)^½        (5)

Sources of Nitrogen in Steelmaking

Numerous sources of nitrogen exist during the melting, the ladle processing and the casting operations. Sources of nitrogen in oxygen steelmaking include the hot metal, the scrap, the impurity nitrogen in oxygen and nitrogen used as a stirring gas.

Nitrogen pickup from the atmosphere can occur during reblows in which case the furnace fills up with air, which is then entrained into the metal when the oxygen blow restarts. Also, during the tapping of steel, air bubbles are entrained into the steel where the tap stream enters the bath in the ladle. Other sources may include atmosphere (through ladle slag), coke (carburizers) and various ferro-alloys. Ladle additions often contain moisture. To get an impression of the sources of nitrogen during the melting process, Table 1 shows the amount of nitrogen present in each of the feed materials typically used in the EAF.

Table 1. Nitrogen content of feed materials used in EAF steelmaking

Feed Material	Nitrogen Content
Scrap	30-120 ppm
HBI/DRI	20-30 ppm
Liquid iron from the BF	60 ppm
Cold pig iron (CPI)	20-30 ppm
Hot heel	10 ppm
Coke	5000-10000 ppm
Oxygen	30-200 ppm
Carbon Carrier Gas (Air)	78%
Bottom stirring gas(N2)	> 99,9%
Bottom stirring gas (Ar)	< 30 ppm
CaO	400 ppm

Nitrogen Behavior During Steel Processing

In liquid steel, nitrogen exists in solution. During solidification, three primary phenomena may occur:

1.   Blowhole formation
2.   Nitride compound precipitation
3.   Interstitial solid solution formation

The maximum nitrogen solubility in liquid iron reaches approximately 450 ppm, reducing to less than 10 ppm at ambient temperature. Dissolved sulfur and oxygen, being surface-active elements, limit nitrogen absorption - a characteristic utilized during steelmaking to prevent excessive nitrogen pickup.

Figure 1: Solubility of nitrogen in iron for temperatures of 600-2000°C

Effects on Steel Properties and Performance

Effect of Nitrogen on Formability

In Low-Carbon Aluminum-Killed (LCAK) steels, nitrogen content significantly influences mechanical properties. As shown in Figure 2, steel strength initially decreases then increases with rising nitrogen levels. The elongation demonstrates an inverse relationship, decreasing as nitrogen content rises. The r-value, which represents the ratio of width to thickness strain in strip tensile specimens, increases with nitrogen content. This relationship indicates that higher nitrogen levels lead to reduced formability in LCAK steels, a characteristic that persists after annealing.

These mechanical property changes result from three key mechanisms: interstitial solid solution strengthening by free nitrogen, precipitation strengthening through aluminum and other nitrides, and grain refinement from nitride precipitates.

Figure 2: Effect of nitrogen on yield strength, tensile strength, r-value and elongation of LCAK steel

Effect of Nitrogen on Hardness

Surface indentation resistance, or hardness, increases linearly with nitrogen content. This relationship stems from nitrogen absorption during steelmaking, which promotes both interstitial solid solution strengthening and grain refinement. Nitrogen absorbed during the steelmaking process shows more pronounced effects compared to absorption during batch annealing in nitrogen-rich atmospheres, though both processes measurably affect final hardness values.

Figure 3: Increase in hardness of aluminum-killed sheet steel with respect to increasing nitrogen content

Effect of Nitrogen on Strain Ageing

Strain ageing manifests in steels containing interstitial atoms, particularly nitrogen, following plastic deformation. During this process, nitrogen migrates to dislocations, causing discontinuous yielding during subsequent deformation. This phenomenon increases hardness and strength while reducing ductility and toughness. Surface defects known as 'fluting' or 'stretcher strains' often appear as a result.

Duckworth and Baird developed the strain ageing index to measure this effect, calculating yield stress increases in deformed material held at room temperature for 10 days. Figure 4 demonstrates that higher nitrogen content correlates with increased strain-ageing index values, indicating greater susceptibility to surface defects.

Figure 4: Effect of nitrogen on strain aging in mild steels

Effect of Nitrogen on Impact Properties

Material toughness, measured by energy absorption before fracture, varies significantly with nitrogen content. As temperature decreases, fracture behavior transitions from fibrous/ductile to crystalline/brittle. This transition temperature increases with higher free nitrogen content, resulting in decreased toughness through solid solution strengthening.

Interestingly, controlled amounts of nitrogen present as precipitates can enhance impact properties. Aluminum, vanadium, niobium, and titanium nitrides promote fine-grained ferrite formation. Smaller grain sizes lower the transition temperature, improving overall toughness. Therefore, optimizing impact properties requires careful control of both nitrogen content and its form within the steel.

Figure 5: Effect of free nitrogen on impact properties

Nitrogen Effects in Welded Zones

The heat-affected zone (HAZ) of welded steel presents particular challenges. During welding, elevated temperatures can dissociate nitrides in the HAZ, leading to increased grain size. Rapid cooling then produces low-toughness martensite or bainite containing high levels of free nitrogen, further compromising toughness.

This HAZ embrittlement occurs primarily due to the dissociation of nitrides present in the HAZ during the high-temperature welding cycle. The absence of precipitates results in larger grain diameters, while rapid cooling produces microstructures with reduced toughness. The combination of these factors, along with increased free nitrogen levels, significantly impacts the mechanical properties of the welded region.

To mitigate these effects, welding procedures can be modified through lower heat input and multiple welding passes. These techniques help prevent complete nitride dissociation and maintain more favorable microstructural characteristics in the HAZ.