Strain Ageing of Steel: Part Two

---

Overview

Strain ageing has been shown to negatively impact low carbon structural steels. Numerous studies have investigated the effects of various parameters on the strain ageing characteristics of these materials.

In the research conducted by S. Gündüz, the ageing behavior of a carbon steel with a volume fraction of 40% martensite and a microalloyed steel with 20% martensite was studied. The variation of mechanical properties, particularly the increase in yield strength (YS), was assessed through tensile tests. Specimens were pre-strained in tension by 2%, 4%, and 6%, and aged at temperatures of 25, 100, 150, 200, and 250°C for 30 minutes, followed by restraining.

The steels used in this investigation included a commercially produced carbon steel (without alloying elements) and a microalloyed steel, with their chemical compositions detailed in Table 1. All specimens underwent an annealing treatment at 900°C for 30 minutes, followed by air cooling to homogenize the microstructure.

Chemical Composition of the Investigated Steels

	%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

Experimental Procedure

As previously mentioned, the specimens were pre-strained in tension by 2%, 4%, or 6%. After pre-straining, they were unloaded and aged at temperatures of 25, 100, 150, 200, and 250°C for 30 minutes. Following the ageing process, tensile tests were conducted at ambient temperature with a crosshead speed of 2 mm/min. At least three specimens were tested for each ageing temperature, and average values were calculated.

The increase in flow stress resulting from restraining was denoted as the strain ageing (ΔY2), illustrated in Figure 1. For samples pre-strained in tension, ΔY2 was determined by the difference between the 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).

Stress-Strain Behavior

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

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).

Both the dual phase carbon steel and the microalloyed steel exhibit continuous yielding prior to any ageing. This behavior is attributed to the mobile dislocations introduced during cooling from the intercritical annealing temperature. Multiple dislocation sources activate at low strain, initiating simultaneous plastic flow throughout the specimen, which suppresses discontinuous yielding.

Key Findings

The main conclusions from this study are as follows:

1. Both steels exhibited significant changes in appearance with increasing ageing temperature for the pre-strains studied, indicating that static strain ageing occurs in both dual phase carbon steel and microalloyed steel.
2. Contrary to the negative effect of pre-strains on the change in ΔY2 produced by subsequent ageing, increasing pre-strain markedly enhanced the change in UTS for both dual phase carbon steel and microalloyed steel. This suggests that the ΔY2 value is less sensitive to dislocation density and primarily dependent on solute segregation per dislocation.
3. Both steel types showed significant increases in YS, UTS, and ΔY2; however, the percentage elongation to fracture decreased as the ageing temperature rose from 25 to 100°C for the pre-strains studied. This behavior is due to atmosphere formation at dislocations and precipitation of carbonitrides during strain ageing.
4. Further increases in ageing temperature to 150, 200, and 250°C resulted in a reduction in YS but an increase in percentage elongation. This indicates signs of over-ageing, likely due to the tempering effect of martensite and coarsening of precipitates on the dislocations.
5. Ageing in the dual phase microalloyed steel occurred more slowly than in the dual phase carbon steel. This is associated with the chemical composition of the dual phase microalloyed steel, which contains nitrogen and carbide-forming elements such as titanium, vanadium, and aluminum, in addition to carbon atoms.