Nitrogen in Steels: Part Two

Sumário:

The effect of nitrogen on mechanical properties is the result of interstitial solid solution strengthening by the free nitrogen; precipitation strengthening by aluminum and other nitrides; and grain refinement due to the presence of nitride precipitates.
Nitrogen absorbed during steelmaking results in interstitial solid solution strengthening and grain refinement, both of which increase hardness.

Effect of Nitrogen on Formability

Figure 1 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, even after annealing.

Figure 1: Effect of nitrogen on yield strength, tensile strength, r-value and elongation of LCAK steel in the annealed condition

The effect of nitrogen on mechanical properties is the result of interstitial solid solution strengthening by the free nitrogen; precipitation strengthening by aluminum and other nitrides; and grain refinement due to the presence of nitride precipitates.

Effect of Nitrogen on Hardness

Hardness is the resistance of a material to surface indentation. The Figure 2 shows that hardness increases linearly with increasing nitrogen content. Nitrogen absorbed during steelmaking results in interstitial solid solution strengthening and grain refinement, both of which increase hardness. Further, the diagram shows that nitrogen absorbed during the steelmaking process has a more significant impact than that absorbed during batch annealing in a nitrogen-rich atmosphere, although both have a measurable effect.

Figure 2: Increase in hardness of aluminum-killed sheet steel with respect to increasing nitrogen content

Strain ageing occurs in steels containing interstitial atoms, predominantly nitrogen, after they have been plastically deformed. After deformation, the nitrogen segregates to dislocations causing discontinuous yielding when further deformed. Not only does strain ageing result in increased hardness and strength, and reduced ductility and toughness, but it may also result in the appearance of "fluting" or "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. Figure 3 shows that increasing nitrogen results in a higher stain-ageing index, and therefore greater propensity for surface defects.

Figure 3: Effect of nitrogen on strain ageing in mild steels with varying manganese content

Effect of Nitrogen on Impact Properties Including Welded Material

The ability of a material to withstand impact loading is commonly known as toughness. It is sometimes quantified by measuring the amount of energy that is absorbed by a test piece of known dimensions prior to fracture. It is further analyzed by determining the fracture mechanism upon impact over a range of temperatures.

As temperature is decreased, the fracture type will change from fibrous/ductile to crystalline/brittle. This arbitrary temperature is termed the "ductile-to-brittle" transition temperature. The lower the transition temperature the better the impact properties, since failure via ductile fracture may be less catastrophic than that via brittle failure.

Figure 4 demonstrates that as free nitrogen increases, the transition temperature increases, and therefore toughness decreases. This is attributed to solid solution strengthening.

Figure 4: Effect of free nitrogen on impact properties

Conversely, limited amounts 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. Further, the smaller the grain size the lower the transition temperature, hence improved toughness. Therefore, it is necessary to carefully control, not only the nitrogen content, but also the form in which it exists, in order to optimize impact properties.

Nitrogen is known to affect the toughness of the heat-affected zone (HAZ) of welded steel. This is important, since the weld metal should not be a point of weakness in a welded structure. This loss in toughness is often referred to as HAZ embrittlement.

It is thought that 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 may prevent this.

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