Further considerations on the inconsistency in toughness evaluation of AISI 4340 steel austenitized at increasing temperatures |
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| Authors: | Robert O Ritchie R M Horn |
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| Affiliation: | (1) Department of Mechanical Engineering, Massachusetts Institute of Technology, 02139 Cambridge, MA;(2) Department of Materials Science and Mineral Engineering, University of California, 94720 Berkeley, CA |
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| Abstract: | A study has been made of the influence of austenitizing temperature on the ambient temperature toughness of commercial AISI
4340 ultrahigh strength steel in the as-quenched (untempered) and quenched and tempered at 200°C conditions. As suggested
in previous work, a systematic trend ofincreasing plane strain fracture toughness(K)
Ic
anddecreasing Charpy V-notch energy is observed as the austenitizing temperature is raised while the yield strength remains unaffected.
This effect is seen under both static <slowbend> and dynamic (impact) loading conditions, and is rationalized in terms of
a differing response of the microstructure, produced by each austenitizing treatment, to the influence of notch root radius
on toughness. Since failure in all microstructures was observed to proceed primarily by a ductile rupture (microvoid coalescence)
mechanism, an analysis is presented to explain these results, similar to that reported previously for stress-controlled fracture,
based on the assumption that ductile rupture can be considered to be strain-controlled. Under such conditions, the decrease
in V-notch Charpy energy is associated with a reduction in critical fracture strain at increasing austenitizing temperatures,
consistent with an observed decrease in uniaxial and plane strain ductility. The increase in sharp-crack fracture toughness,
on the other hand, is associated with an increase in “characteristic distance” for ductile fracture, resulting from dissolution
of void-initiating particles at high austenitizing temperatures. The microstructural factors which affect this behavior are
discussed, and in particular the specific role of retained austenite is examined. No evidence was found that the enhancement
of fracture toughness at high austenitizing temperatures was due to the presence of films of retained austenite. The significance
of this work on commonly-used Charpy/KIc empirical correlations is briefly discussed.
formerly with Lawrence Berkeley Laboratory, Berkeley, CA |
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