Nitrogen in Steels
Nitrogen in Steels
All steels contain some nitrogen which can enter the steel as an impurity or as an intentional alloying addition. The quantity of nitrogen in steels normally depends on the residual level arising from the steelmaking processes or the amount aimed in case of deliberate addition. There are significant differences in residual levels of nitrogen in steels produced from the two main steelmaking processes. Basic oxygen furnace (BOF) process generally results into lower residual nitrogen in steels, typically in the range of 30 to 70 ppm while electric arc furnace (EAF) process results into higher residual nitrogen, typically in the range of 70 to 110 ppm. Nitrogen is added to some steels (e.g. steels containing vanadium) to provide sufficient nitrogen for formation of nitride to achieve higher strength. In such steels nitrogen levels can increase to 200 ppm or higher.
Nitrogen in the liquid steel is present in the form of solution. During the solidification of steel in continuous casting, three nitrogen related phenomena can happen. These are
- Formation of blow holes
- Precipitation of one or more nitride compounds
- Solidification of nitrogen in interstitial solid solution.
The maximum solubility of nitrogen in liquid iron is around 450 ppm, and less than 10 ppm at ambient temperature (Fig 1). The presence of significant quantities of other elements in liquid iron affects the solubility of nitrogen. Mainly the presence of dissolved sulfur and oxygen limit the absorption of nitrogen because they are surface active elements.
Fig 1 Solubility of nitrogen in iron
Nitrogen is generally considered as undesirable impurity which causes embrittlement in steels and affects strain aging. However nitrogen produces a marked (intersititial solid solution) strengthening when diffused into the surface of the steel, similar to the strengthening observed during case hardening (Nitriding). Combined with aluminum, it produces a fine grain size.
Sources of nitrogen in steels
There are many sources of nitrogen which are present during the production of steel, the processing of steel in the ladle and the process of continuous casting. Nitrogen pick up can take place from charge materials and during different stages in the production and casting of steel. Major sources which cause nitrogen pick up in steels are given below.
- Nitrogen which is present in the charge/feed materials.
- Nitrogen in the stirring gases.
- Nitrogen pick up from ladle addition such as carburizers and ferroalloys etc.
- Nitrogen which is absorbed from the atmosphere during reblows in BOF steel making.
- Nitrogen which is absorbed from the atmosphere during tapping of steel.
Typical nitrogen content levels in some of the sources of nitrogen are given in Tab 1.
Tab 1 Nitrogen content levels of some of the nitrogen sources
|Bottom stirring gas (N2)|
|Bottom stirring gas (Ar)|
Nitrogen and steel properties
Nitrogen can influence steel properties either in a detrimental or beneficial way depending on the presence of other elements in the steel, the form and quantity of nitrogen, and the required behavior of the steel.
Normally most of the steels need nitrogen at the minimum level. High nitrogen content may result in inconsistent mechanical properties in hot rolled steels, embrittlement of the heat affected zone (HAZ) of welded steels, and poor cold formability. In particular, nitrogen can result in strain ageing and reduced ductility of cold rolled and annealed low carbon aluminum killed (LCAK) steels.
- Effect on hardness of steel – Hardness is the resistance of a material to surface indentation. Hardness of steel increases linearly with increasing nitrogen content (Fig 2). Nitrogen picked up during steelmaking goes into the strengthening of the interstitial solid solution and grain refinement, both of which increase hardness. Further, the figure shows that nitrogen picked up during the steelmaking process has a more significant impact than that picked up during annealing in a nitrogen rich atmosphere.
Fig 2 Influence of nitrogen on hardness (left) and impact value (right)
- Effect on impact strength – The ability of steel to withstand impact loading is known as its toughness. It is quantified by measuring the amount of energy that is absorbed by a test piece of known dimensions prior to its fracture. It is also analyzed by determining the fracture mechanism upon impact over a range of temperatures. With the decrease of temperature, the type of fracture changes from fibrous/ductile to crystalline/brittle. This arbitrary temperature is termed the ‘ductile to brittle’ transition temperature. The lower is the transition temperature the better are the impact properties since the failure due to a ductile fracture is less catastrophic than a brittle failure. Fig 2 shows that as the free nitrogen increases the transition temperature also increases which means that toughness decreases. This is due to the strengthening of the solid solution. Small amount of nitrogen present as precipitates have a beneficial effect on impact properties. Nitrides of aluminum, vanadium, niobium and titanium result in the formation of fine grained ferrite. Finer grain size lowers the transition temperature and improves the toughness. Hence it is necessary not only to control the nitrogen content but also the control its form in order to optimize impact properties.
- Effect on mechanical properties – The effect of nitrogen on mechanical properties is the result of (i) interstitial solid solution strengthening by the free nitrogen (ii) precipitation strengthening by aluminum and other nitride and (iii) grain refinement due to the presence of nitride precipitates. Fig 3 shows that the strength of LCAK steels decreases slightly and then increases with increasing nitrogen. Conversely, the elongation decreases and the r-value increases with increasing nitrogen. The r-value is the average ratio of the width to thickness strain of strip tensile specimens tested in various orientations. It is an inverse measure of formability. Hence, high nitrogen content leads to poor formability of LCAK steels.
Fig 3 Influence of nitrogen on mechanical properties
- Effect on strain ageing – Strain ageing occurs in steels due to interstitial atoms (mainly nitrogen) after they have been plastically deformed. After deformation, the nitrogen segregates to dislocations causing discontinuous yielding when further deformed. Strain ageing not only results in increased hardness and strength with reduction in ductility and toughness, but it also results in the appearance of ‘stretcher strains’ on the surface of deformed material. Duckworth and Baird have developed a measure of strain ageing termed ‘strain ageing index’. This is based on an empirical equation to calculate the increase in yield stress when deformed material is held for 10 days at room temperature. Fig 4 shows that increasing nitrogen results in a higher stain-ageing index, and therefore greater propensity for surface defects.
Fig 4 Effect of nitrogen on strain ageing index of steel
- Effect during welding – Nitrogen generally affects the toughness of the heat affected zone (HAZ) of welded steel. Since the weld metal should not be weaker in a welded structure, the role of nitrogen is important. The loss of toughness is normally known as HAZ embrittlement. It is thought this occurs when the nitrides present in the HAZ are dissociated as a result of the elevated temperatures that exist during welding. The absence of precipitates results in grains of larger diameter. Also, the metal cools quickly producing low toughness martensite or bainite, which contain high levels of free nitrogen further exacerbating the loss of toughness. Using lower heat input and several passes to prevent dissociation of the nitrides can prevent this.
Nitrogen as an alloying element in steel
Nitrogen as an alloying element in steel is being used since 1940s initially to produce stainless steels as a substitution for nickel. The use of nitrogen in high alloy steels has a number of advantages. These advantages as given below make nitrogen as an interesting alloying element.
- Significant increase in strength without restricting ductility
- Improvement in the corrosion resistance.
- Increase in the high temperature tensile strength
- Extended/stabilized austenite form
- No formation of tension induced martensite with high cold working rate
- Inhibits the discharge of inter metallic phases