Nitrogen in Steels
Nitrogen in Steels
Nitrogen exists in steel in two forms namely (i) in the atomic form as interstitial nitrogen, or as unstable and easily dissolved nitrides, e.g. Fe4N etc., and (ii) in the form of stable nitrides. In atomic form, it is known as the active or free nitrogen in steel. In micro-alloyed steels e.g. high strength low alloy (HSLA) steels, some or all of the interstitial nitrogen combines with alloying elements (V, Ti or AI) and forms stable nitrides in the steel. Both forms of nitrogen have a strong influence on the properties of steel.
Nitrogen as an alloying element in iron-based alloys is known since the beginning of this century having been profoundly studied during the last few decades. However, nitrogen steels are so far not widely used. The reason for the comparatively narrow industrial application lies in the old customer skepticism in relation to nitrogen as an element causing brittleness in ferritic steels, some technical problems involved with introducing nitrogen into steel and, the insufficient knowledge of the physical nature of nitrogen effects in iron and its alloys.
The role of nitrogen in steel was virtually ignored for many years. Steels produced by Bessemer converters, where air was blown through the liquid steel, steel had significant content of nitrogen. With the introduction of oxygen steelmaking, the effects of nitrogen on the steel became apparent and this led to various major investigations into the roles of carbon and nitrogen on the steel being carried out in the 1950s and 60s.
Nitrogen is present in all commercial steels. Since the contents of nitrogen are normally small and its analysis is complex and expensive, its existence is generally ignored even in the steel specifications given in the standards. However, whether present as a residual element or added deliberately as an alloying element, the effects of nitrogen in steel are significant. Along with carbon, it is responsible for the discontinuous yield point which characterizes the stress-strain curve for low carbon steels. The dislocation pinning responsible for this yield point also contributes to the characteristic fatigue limit of these steels.
Nitrogen is normally considered as undesirable impurity which causes embrittlement in steels. Nitrogen was considered for long period to be in the same category as certain undesirable residual elements in the steel, which are normally harmful for the properties of the steel. It was believed that the high nitrogen contained steel is subject to strain ageing with deterioration of its plasticity with time. Recently, it was noticed that nitrogen has significant effect on mechanical properties, phase stability, corrosion behaviour and oxidation resistance. Nitrogen can also react with titanium and aluminum in liquid steel and generate nitride inclusions, which can damage the surface of the steel and decrease the quality of the final product. Nitrogen produces a marked (interstitial 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.
Absorption of nitrogen during steelmaking
The nitrogen content of steel can be derived from several sources. The major source of nitrogen depends upon the steelmaking process. The several sources of nitrogen which exist during the steelmaking process include the hot metal, scrap, pig iron, DRI /HBI, lime, coke/coal, ferro alloys, impurity nitrogen in oxygen, and the nitrogen used as a stirring gas. Nitrogen pick-up from the atmosphere can occur during various stages of the steelmaking. 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 input materials|
|Sl. No.||Nitrogen source||Unit||Value|
|3||DRI / HBI||ppm||20-30|
|5||Coke / coal||ppm||5,000 -10,000|
|8||Bottom stirring gas|
|8a||Nitrogen||%||Greater than 99.9|
|8b||Argon||ppm||Less than 30|
The factors which affect the nitrogen content of steel are (i) the composition of the melt, (ii) the partial pressure of nitrogen in the gases in contact with the melt, or the nitrogen potential of the slag, (iii) the duration of contact between the atmosphere and the liquid steel, (iv) the temperature of the liquid steel, and (v) nitrogen additives.
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 steelmaking process normally results into lower residual nitrogen in steels, typically in the range of 30 ppm to 70 ppm while electric steelmaking process results into higher residual nitrogen, typically in the range of 70 ppm 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 can exist in steel, either as uncombined ‘free’ nitrogen (sometimes called lattice nitrogen), or chemically combined with other elements in the form of nitrides or carbo-nitrides. The strain ageing effects are due to free nitrogen which is why these can be removed from low nitrogen steels by the addition of strong nitride formers such as titanium which ties up any free nitrogen, preventing its migration to sites around dislocations. However, this is not a straight forward phenomenon. In coarse grained low nitrogen mild steel strained in the temperature range 200 deg C to 300 deg C, new dislocations form at such a rate that yielding, as evidence by a drop in the stress without a previous rise, occurs repeatedly but this phenomena does not occur in a similar steel with a high free nitrogen content. This is since in the low nitrogen steel, there is insufficient nitrogen to lock the newly forming dislocations immediately, whereas in the high nitrogen steel the dislocations are locked as they form and remain locked. This is reflected in the greater capacity for work-hardening in the high nitrogen steel.
Fig 1 Solubility of nitrogen in iron
Nitrogen is a strong austenite stabilizer, and the yield and tensile strengths of nitrogen containing steels increase with increasing nitrogen content with no adverse effects on ductility. The rate of fatigue-crack growth decreases with increasing nitrogen content, while the creep strength is enhanced by the addition of nitrogen.
Nitrogen in the liquid steel is present in the form of solution. During the solidification of the steel in continuous casting, three nitrogen related phenomena can happen. These are (i) formation of blow holes, (ii) precipitation of one or more nitride compounds, and (iii) 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 sulphur and oxygen limit the absorption of nitrogen because they are surface active elements.
Nitrogen and steel properties
Nitrogen can influence steel properties either in a detrimental or beneficial way depending on (i) the presence of other elements in the steel, (ii) the form and quantity of nitrogen, and (iii) the required behaviour of the steel. Normally most of the steels need nitrogen at the minimum level. High nitrogen content can result in (i) inconsistent mechanical properties in hot rolled steels, (ii) embrittlement of the heat affected zone (HAZ) of welded steels, and (iii) 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 the steel has a linear relationship with the nitrogen content. It increases with increase of the 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. Nitrogen, like carbon, when in solution as an interstitial in steel results in increases in hardness and yield strength in the temperature range 100 deg C to 200 deg C and a corresponding decrease in toughness.
Fig 2 Effect of nitrogen on properties of steel
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 which 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 (DBTT). 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.
Nitrogen increases the ‘impact transition temperature; (ITT) in Charpy tests and high levels of uncombined nitrogen can result in a change in the fracture energy to occur above room temperature with a resultant change from ductile to brittle behaviour. In pure body centred iron it has been shown that nitrogen segregates to the grain boundaries and that this segregation can result in inter-granular embrittlement. This mechanism probably occurs in steels, as killed steels where nitrogen is tied up by silicon or aluminum show improved impact properties compared to rimming or semikilled steels. It has been shown that additions of titanium and aluminum, in 8 % manganese steel, reduced the DBTT presumably by tying up free nitrogen but also reduced the hardness in both the air cooled and the water quenched condition.
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 content. 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 samples 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 fracture toughness – Nitrogen can play a distinctively destructive role in the fracture toughness of structural steels. Small changes in nitrogen content produce significant variations in the fracture mode transition temperature of these steels. These variations are complicated by consequential changes in precipitated nitrides, associated changes in grain size, and the interaction between nitrogen and manganese.
Effect on strain ageing – Strain ageing is a yield related phenomenon and caused by nitrogen at temperatures below 150 deg C and by carbon above this temperature. The effectiveness of carbon and nitrogen in producing strain ageing is a function of (i) their solubilities in ferrite, (ii) their diffusion coefficients, and (iii) the severity with which each locks dislocations. The main difference between carbon and nitrogen arises from their widely differing solubilities in ferrite.
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 mild steel
It is the effect on the yield which for many commercial steel applications has resulted in nitrogen being simply regarded as an ‘undesirable residual’ due to the phenomenon of strain ageing. Strain ageing is the reappearance of a yield point in steel which has previously been deformed beyond the yield point into the plastic region. The current interpretation for this phenomenon was first put forward Cottrell and Bilby in 1948. They specifically dealt with carbon but pointed out that the arguments could ‘with very little modification’ be applied to nitrogen. Nitrogen, and to a lesser extent carbon, gradually diffuse to the preferential sites around the new dislocations which have formed when the steel initially yielded This leads to the reappearance of the yield phenomenon and the associated problems it causes when attempting to produce smooth cold formed shapes. Normally, it only occurs after the steel has been allowed to stand at room temperature for a period of several weeks or months but even a small rise in temperature can considerably speed up the diffusion and so shorten this time. As a result, a lot of work has been carried out on producing ‘interstitial free’ steels, such that bulk steels with less than 20 ppm of nitrogen are now routinely produced for use in the automobile sector, for pressed body and chassis components.
Nitrogen generally causes more of a problem with strain ageing than carbon as a result of its higher solubility in ferrite, the carbon being precipitated out on the existing carbides whilst the nitrogen is still free to migrate to new dislocations. At temperatures above ambient temperature but below around 400 deg C the return of the yield point occurs much more rapidly and yielding becomes a continuous event known as dynamic strain ageing as nitrogen (and some of the carbon) rapidly migrates to the preferential sites around new dislocations as they form. This results in an increase in the tensile strength of the steel, and a drop in the ductility and fracture toughness. These effects tend to peak at temperatures around 250 deg C. This was explained (for carbon) by Cottrell and Bilby as the formation of saturated atmospheres around new dislocations which only require a carbon level of 0.003 % (or a similar level of nitrogen).
However, Gladman has pointed out that the interstitial levels associated with strain age hardening and strain age embrittlement are well in excess of this level. The suggested explanation being that carbide (and nitride) precipitation occurs on the dislocations giving an additional precipitation strengthening effect. Work initially by Baird and MacKenzie and later by Baird and Jamieson showed that whilst nitrogen alone in pure iron gave a high rate of strain hardening (a symptom of dynamic strain ageing) upto 225 deg C, a manganese and nitrogen addition to iron continued this effect upto 450 deg C. It was suggested that this effect was due to pairs or small clusters of manganese and nitrogen atoms where the presence of the manganese restricted the mobility of the nitrogen atoms around moving dislocations.
Effect during welding – Nitrogen generally affects the toughness of the heat affected zone (HAZ) of welded steel. Since the weld metal is not to 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 high temperatures which exist during welding. The absence of precipitates results in grains of larger diameter. Also, the steel 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 make nitrogen as an interesting alloying element.
Nitrogen as an alloying element has been known and used in technical applications since the 1940s, initially under the premise for nickel substitution in stainless grades. Nitrogen in low alloy steels is undesirable due to the formation of brittle nitrides. However, the use of nitrogen in high alloy steels has an array of advantages which makes it appear interesting as an alloying element. The most important points in this respect are (i) significant increase of strength without restricting ductility, (ii) improvement of corrosion resistance, (iii) increasing the high temperature tensile strength, (iv) extended / stabilized austenite form, (v) no formation of tension induced martensite with high cold working rates, and (vi) Inhibits the discharge of inter-metallic phases. These high nitrogen steels as a specific material group are characterized through an interesting material profile, i.e. a combination of strength and corrosion resistance.