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Research Papers

Stress-State, Temperature, and Strain Rate Dependence of Vintage ASTM A7 Steel

[+] Author and Article Information
S. A. Brauer

Mechanical Engineering,
Mississippi State University,
Mississippi State, MS 39762
e-mail: sbrauer@cavs.msstate.edu

W. R. Whittington

Center for Advanced Vehicular Systems,
Mississippi State University,
Starkville, MS 39759
e-mail: wrw51@cavs.msstate.edu

H. Rhee

Center for Advanced Vehicular Systems,
Mississippi State University,
Starkville, MS 39759
e-mail: hrhee@cavs.msstate.edu

P. G. Allison

Department of Mechanical Engineering,
University of Alabama,
Tuscaloosa, AL 35406
e-mail: pallison@eng.ua.edu

D. E. Dickel

Center for Advanced Vehicular Systems,
Mississippi State University,
Starkville, MS 39759
e-mail: doyl@cavs.msstate.edu

C. K. Crane

Geotechnical and Structures Laboratory,
U.S. Army Engineer Research and
Development Center,
Vicksburg, MS 39180
e-mail: charles.k.crane@usace.army.mil

M. F. Horstemeyer

Mechanical Engineering,
Mississippi State University,
Mississippi State, MS 39762
e-mail: mfhorst@cavs.msstate.edu

1Corresponding author.

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.

J. Eng. Mater. Technol 141(2), 021002 (Oct 18, 2018) (9 pages) Paper No: MATS-18-1159; doi: 10.1115/1.4041388 History: Received June 03, 2018; Revised July 20, 2018

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.

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Figures

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Fig. 1

Historical timeline of ASTM A7 and successor ASTM A36

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Fig. 2

Optical microscope images of A7 steel polished (left) and etched (right). Second phase particles consisting of manganese are readily identifiable in the polished sample. The etched sample revealed the grain boundaries as well as the two primary phases, ferrite (light grains) and pearlite (dark grains).

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Fig. 3

Representative specimens used for obtaining simple stress state behavior of ASTM A7 steel. Specimens for low (quasi-static) and high rate tests are nearly identical in cross-sectional area for their respective stress state with the exception of the high rate tensile specimen.

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Fig. 14

Void density and nearest neighbor distance quantified for each quasi-static tensile test type. A decrease in void density of large voids at 573 K (300 °C) is indicative of a change in failure mechanism from ductile to brittle.

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Fig. 13

Area fraction and area of a single void quantified for each quasi-static test type. Large voids were not as prevalent in the specimens tested at 573 K (300 °C) due to the change in failure mode from ductile to brittle.

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Fig. 12

Sample fractographic images of ASTM A7 failed in tension at (a) ambient temperature and (b) 573 K (300 °C) at quasi-static strain rates. Triaxiality dominated regions are enclosed by the white dashed lines that decreased in size (52%±7% to 14%±3%) as temperature increased. Expanded micrographs of the shear lip regions also showed a decrease in void density as the temperature increased.

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Fig. 11

Quasi-static stress–strain behavior of ASTM A7 steel tested in tension and compression at 573 K (300 °C)

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Fig. 10

Direct comparison of the stress–strain behavior between relative strain rates for ASTM A7 steel. Strain was restricted to 0.6 mm/mm to more easily compare all three stress state behaviors.

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Fig. 9

Torsion stress–strain behavior of ASTM A7 steel tested at ambient temperature. Quasi-static and high strain rate behavior is provided in von Mises equivalent values for direct comparison to compression and tension behavior.

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Fig. 8

Varying strain rate tensile behavior of ASTM A7 steel at ambient temperature showing the dynamic strain aging affect at a strain rate of 0.1 s−1

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Fig. 7

Comparison of tensile behavior of ASTM A7 steel at ambient and 598 K (300 °C) for two quasi-static strain rates. An inversion of strain rate sensitivity occurs as the strain rate increases from 0.001 s−1 to 0.1 s−1.

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Fig. 6

Stress–strain response in compression for ASTM A7 steel tested at various strain rates at ambient temperature

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Fig. 5

Stress–strain response of ASTM A7 steel tested in compression at ambient and 598 K (300 °C) for two quasi-static strain rates

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Fig. 4

Stress–strain behavior for ASTM A7 steel tested in compression with specimens cut from two planar sections. The material behavior is nearly the same in both directions for each strain rate indicating an isotropic mechanical behavior.

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Fig. 15

Stress values for tension, compression, and torsion determined at 0.2 mm/mm strain at various strain rates. Strain rate sensitivity for compression and torsion both behave in a positive sense, while the tension exhibits a negative than positive sensitivity.

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Fig. 16

Binding energy of a C interstitial solute in the vicinity of an edge dislocation in Fe as a function of C solute position. Positive energies indicate binding, or lower total energy. The high binding energy gradient across the dislocation core indicates a strong cross-core diffusion [14].

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