Effect of hot-rolling reduction on shape of sulfide inclusions and fracture toughness of AISI 4340 ultrahigh strength steel |
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Authors: | Yoshiyuki Tomita |
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Affiliation: | (1) Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, 4-804 Mozu-Umemachi, Sakai, 591 Osaka, Japan |
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Abstract: | Commercial AISI 4340 ultrahigh strength steels with hot-rolling reductions of 80 to 98 pct have been studied to determine
the effect of the shape of sulfide inclusions on plane-strain fracture toughness(K
IC
) of the ultrahigh strength low alloy steels. The significant conclusions are as follows: decreasing the hot-rolling reduction
from 98 to 80 pct for the steels modified the shape of sulfide inclusions from the stringer (average aspect ratio = 17.5)
to the ellipse (average aspect ratio = 3.8). This improved theK
IC
in the longitudinal testing orientation by about 20 MPa · m1/2 at similar strength levels. This could be due to the fact that the ellipsed sulfide-inclusions separate from the matrix during
plastic deformation, producing large voids. During testing these act to blunt and arrest cracks propagating across the specimen
which would normally cause failure. The decrease in the hot-rolling reduction also developed theK
IC
in the transverse testing orientation by about 17 MPa · m1/2 at increased ductility and Charpy impact energy levels. This can be attributed to the fact that lamellate fracture, which
occurs in a brittle manner along the interfaces of the sulfide-inclusion/matrix at the crack tip, is considerably suppressed
by modifying the shape of the inclusions from the stringer to the ellipse. |
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Keywords: | |
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