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Static strain aging in low carbon ferrite-pearlite steel : forward and reverse loading Mojtaba, Mansouri Arani


The combination of static strain aging and plastic strain reversal is important to understand for both the forming of components and also analysis of in service performance, for example, in the case of fabrication of pipeline, motor shafts or structural components in buildings and ships. Static strain aging phenomenon has been experimentally studied for the cases of forward and reverse re-straining after aging on a low carbon steel (0.16 wt% C) with a ferrite-pearlite microstructure. Torsion tests on hollow tubular samples were used for the mechanical tests. The shear strain on the surface of the sample was measured with the digital image correlation. The influence of the amount of pre-strain, aging time and temperature, and the strain path reversals on the stress-strain response after aging has been measured experimentally. A maximum increase of 46 MPa was obtained in the yield stress of the samples re-strained after full aging in the same direction as the initial straining. This maximum increase in yield stress as well as the rate of increment in yield strength during aging was almost independent of the amount of pre-strain and the increase in the flow stress occurred without a significant variation in the work hardening behavior. Further, it was shown that a yield point phenomenon was absent if the direction of re-straining after aging was reversed and the increase in the flow stress level after aging was proportional to the amount of pre-strain and increased with extended aging time. In this case, the absence of a sharp yield point after prolonged aging time led to the speculation that the activation of dislocations sources, rather than unpinning of locked dislocations in re-straining after aging was the controlling mechanisms although proof of this requires further investigation. Although it is difficult to unambiguously identify all of the underlying physical mechanisms, nevertheless, a comprehensive set of experimental results has been measured which can be used by the design engineer when considering cases where static strain ageing and strain path reversals are relevant for a ferrite-pearlite steel with 0.16 wt% carbon.

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