The structure–property relationships of a vintage ASTM A7 steel is quantified in terms of stress state, temperature, and strain rate dependence. The microstructural stereology revealed primary phases to be 15.8% ± 2.6% pearlitic and 84.2% ± 2.6 ferritic with grain sizes of 13.3 μm ± 3.1 μm and 36.5 μm ± 7.0 μm, respectively. Manganese particle volume fractions represented 0.38–1.53% of the bulk material. Mechanical testing revealed a stress state dependence that showed a maximum strength increase of 85% from torsion to tension and a strain rate dependence that showed a maximum strength increase of 38% from 10−1 to 103 s−1at 20% strain. In tension, a negative strain rate sensitivity (nSRS) was observed in the quasi-static rate regime yet was positive when traversing from the quasi-static rates to high strain rates. Also, the A7 steel exhibited a significant ductility reduction as the temperature increased from ambient to 573 K (300 °C), which is uncommon for metals. The literature argues that dynamic strain aging (DSA) can induce the negative strain rate sensitivity and ductility reduction upon a temperature increase. Finally, a tension/compression stress asymmetry arises in this A7 steel, which can play a significant role since bending is prevalent in this ubiquitous structural material. Torsional softening was also observed for this A7 steel.
Stress-State, Temperature, and Strain Rate Dependence of Vintage ASTM A7 Steel
Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received June 3, 2018; final manuscript received July 20, 2018; published online October 18, 2018. Assoc. Editor: Curt Bronkhorst. This work is in part a work of the U.S. Government. ASME disclaims all interest in the U.S. Government's contributions.
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Brauer, S. A., Whittington, W. R., Rhee, H., Allison, P. G., Dickel, D. E., Crane, C. K., and Horstemeyer, M. F. (October 18, 2018). "Stress-State, Temperature, and Strain Rate Dependence of Vintage ASTM A7 Steel." ASME. J. Eng. Mater. Technol. April 2019; 141(2): 021002. https://doi.org/10.1115/1.4041388
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