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Strain Ageing of Steel: Part OneAbstract:Strain ageing of steel was first observed during the 19th century when the maximum load carrying capacity of a test piece was increased after it had been retested after a previous series of testing in the plastic range.General acceptance has been recognized that strain ageing is due to the diffusion of carbon and/or nitrogen atoms in solution to dislocations that have been generated by plastic deformation however the resulting characteristics are the result of a complex series of metallurgical reactions.

The behavior of steel is significantly affected by its chemical composition and the method of manufacture. Bauschinger (1887) observed an increase in the maximum load-carrying capacity of a piece of steel after it was tested in the inelastic range, left for some time and then retested. This observation is now known as strain-ageing. Physical basis of this phenomenon can be interpreted by time dependent diffusion of small elements such as carbon and nitrogen to crystal flaws (e.g. dislocations). Plastic deformation (yielding) is triggered by the movement of dislocations through the crystal, whereas the presence of small atoms (e.g. carbon and nitrogen) can markedly increase the sliding resistance between the atoms and hence increase the yield strength.

Over the last 50 years a vast range of research has shown that all structural steels are potentially susceptible to strain-ageing. Strain-ageing is known to cause an increase in yield strength and ultimate tensile strength of reinforcing steel bars, while tensile elongation capacity will be less in strain-aged steel bars.

When steel has been strained (deformed plastically) and then allowed to age, it has been subjected to what is known as strain ageing. Figure 1 shows the stress-strain curve for mild steel (low-carbon steel) compared with that for the same steel pre-strained up to point B and then subjected to ageing. As a result of the strain ageing, the yield strength and the tensile strength were increased by ΔY and ΔU respectively, and the elongation was decreased by amount Δe. In general, strain ageing increases strength to some extent but results in a loss of some ductility. It is known that strain ageing is accompanied by a decrease in fracture toughness, which is usually reflected by an increase in the brittle transition temperature (BTT).

Figure 1: A schematic representation of the influence of strain ageing on the stress-strain curve for mild steel (UTS =ultimate tensile strength)

Mechanism of Strain Ageing

It is generally accepted that strain ageing is due to the diffusion of carbon and/or nitrogen atoms in solution to dislocations that have been generated by plastic deformation. Initially, an atmosphere of carbon and nitrogen atoms is formed along the length of a dislocation, immobilizing it. Extended ageing, however, results in sufficient carbon and nitrogen atoms for precipitates to form along the length of the dislocation.

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These precipitates impede the motion of subsequent dislocations, and result in some hardening and loss in ductility. The extent of strain ageing, which is a thermally activated process, depends primarily on ageing time and temperature. In general, extended ageing results in a saturation value above which further ageing has no effect.

In fact, ageing at very high temperatures can result in over-ageing, which is accompanied by a loss in hardness and a gain in ductility. Repeated straining and ageing usually produce a greater increase in strain ageing. For strain ageing to manifest itself, free carbon and nitrogen atoms in solid solution have to be available, but very little carbon and nitrogen are required. As little as 0.01 per cent, for example, is sufficient to produce.

Metallurgical Causes for Strain Ageing

The alloying elements in the steel are dispersed into their characteristic microstructural constituents, predominantly iron and iron carbide. In the case of nitrogen and some of the carbon that is not absorbed in iron carbide, they are in the iron-rich phase as small individual atoms in interstices in the crystal structure. After the steel cools from rolling, over time, the carbon and nitrogen atoms migrate through the structure to the dislocations due to the distortion they create in the crystal lattice.

The motion of these (small) interstitial atoms to the dislocations produces a stabilizing effect which increases the force necessary to cause the dislocation to allow slip. It now takes a greater force to deform the steel, raising its strength. If both carbon and nitrogen are present, iron-carbon-nitrogen compounds (carbonitrides) can form that also restrict the motion of the dislocations and raises the strength of the steel.

The effect of temperature is important on the “ageing” phenomenon in structural steels. Structural steels are more complex that sheet steels in that they contain relatively more carbon and alloy and have a more complex microstructure. As a result, the ageing of the steel is measured by increases in strength and loss in toughness which does not occur at room temperature. In general, temperatures in the 300°F-700°F range for periods of 1-5 h are necessary to develop ageing effects.

A second strengthening mechanism occurs when cold deformation (alone) is applied to steels. When dislocations break away for their pinning interstitial atoms and begin the movement causing slip they begin to intersect with each other. A complex series of interactions between the dislocations occurs, causing them to pin each other, decreasing their mobility. The decreased mobility also results in higher strength, lower ductility and lower toughness. As a result, cold deformed steels already have lowered ductility and toughness before any strain ageing occurs and when heating follows cold deformation, the loss in ductility and toughness is greater. It is this combination of events that is the most damaging to the toughness of structural steels.

It is these two effects, the increased strength and reduction in ductility and toughness from cold strain followed by an additional strength increase and toughness loss through ageing, that are the primary elements in the process of strain ageing.

Strain Ageing of Steel: Part TwoAbstract:Strain ageing can have a serious detrimental effect on low carbon structural steels and so two material examples are examined to see how different pre-strain and ageing conditions affect material mechanical properties.A carbon steel (40% martensite) and a microalloyed steel (20% martensite) were both treated under the same parameters and then the UTS and stress strain curves were evaluated to gain some valuable conclusions.

Strain ageing has been found to cause a detrimental effect in low carbon structural steels. A lot of studies have been made on the effect of different parameters on strain ageing characteristics of theses steels.

In the work of S. Gündüz, the ageing behaviour of a carbon steel with 40% martensite volume fractions and a microalloyed steel with 20% martensite volume fractions were studied. The variation of mechanical properties, especially the increase in YS was measured by tensile tests. The specimens were pre-strained in tension by 2, 4 and 6%, aged at 25, 100, 150, 200 and 250°C for 30 min followed by restraining.

The steels used in this investigation are commercially produced a carbon steel (without alloying elements) and a microalloyed steel with a chemical composition shown in Table 1. All the specimens were first subjected to an annealing treatment at 900 °C for 30 min followed by air cooling to homogenize the micro-structure.

Samples used for the annealing treatment are of dimensions approximately 170mm × 30mm × 4.5 mm. A Carbolit furnace capable of operating up to 1200°C was used. The temperature in the heat

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treatment furnace was measured using a K-type thermocouple and temperature variation during heat treatment did not exceed ±3°C.

  %C %Si %S %P %Mn %V %Ti %Al %Nb

Microalloyed steel 0.11 0.10 0.010 0.020 1.20 0.10 0.05 0.020 0.070

Carbon steel 0.22 0.40 0.020 0.025 1.40 - - 0.015 -Table 1: Chemical composition of the investigated steels

As mentioned above, the specimens were pre-strained in tension by 2, 4 or 6%. After this, they were unloaded and aged at 25, 100, 150, 200 and 250°C for 30 min. After ageing of the specimens, they were subjected to a tensile test at ambient temperature at a crosshead speed of 2 mm/min. At least three specimens were tensile tested for each ageing temperature and average values were calculated.

The increase in flow stress as a result of restraining was taken as the strain ageing, ΔY2, which is illustrated in Figure 1. For samples, pre-strained in tension, ΔY2 was determined with single specimen by the difference between lower yield stress after ageing and the flow stress at the end of the pre-straining.

Figure 1: Stress–strain curve for low carbon steel strained to point A, unloaded, and then restrained immediately (curve a) and after ageing (curve b).

Figures 2 and 3 show the stress–strain diagrams of the dual phase carbon steel and the microalloyed steel pre-strained in tension by 2, 4 or 6%, aged at different temperatures, and restrained.

As shown, the dual phase carbon steel and the microalloyed steel, prior to any ageing, exhibits continuous yielding which has been commonly attributed to mobile dislocations introduced during cooling from the intercritical annealing temperature. Many dislocation sources come into action at low strain and plastic flow begins simultaneously through the specimen, thereby suppressing discontinuous yielding.

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Figure 2: Variation of stress–strain curves of the dual phase carbon steel at different ageing temperatures for the pre-strains of 2% (a), 4% (b) and 6% (c).

Figure 3: Variation of stress–strain curves of the dual phase microalloyed steels at different ageing temperatures for the pre-strains of 2% (a), 4% (b) and 6% (c).

The main conclusions from this study are as follows: 1. Both steels displayed significant changes in appearance as the ageing temperature was increased for the pre-strain in the range studied. This indicated that static strain ageing takes place in both dual phase carbon steel and microalloyed steel.2. In contrast to the negative effect of pre-strains on the change in ΔY2 produced by subsequent ageing, it was found that increasing pre-strain markedly increased the change of UTS of both dual phase carbon steel and microalloyed steel. This indicated that ΔY2 value is insensitive to dislocation density and is principally dependent on the solute segregation per dislocation.3. Both dual phase carbon steel and microalloyed steel showed significant increases in YS, UTS and ΔY2, however the percentage elongation to fracture decreased as the ageing temperature was raised from 25 to 100°C for the pre-strain in the range studied. This is due to atmosphere formation at dislocation and precipitation of carbonitride on dislocations during strain ageing.4. Further increase in ageing temperature to 150, 200 and 250°C caused a reduction in YS, but an increase in percentage elongation. These are signs of overageing probably due to tempering effect of martensite and coarsening of the precipitates on the dislocations.5. The ageing in the dual phase microalloyed steel occurred more slowly than the dual phase carbon steel. This was associated with the chemical composition of dual phase microalloyed steel which, in addition to carbon atoms, contained nitrogen and carbide forming elements such as titanium, vanadium and aluminum.