Effect of volume fraction and shape of sulfide inclusions on through-thickness ductility and impact energy of high-strength 4340 plate steels

Abstract

The effect of volume fraction and shape of sulfide inclusions on the tensile ductility and impact energy of high-strength AISI 4340 plate steels in the transverse and through-thickness testing directions was investigated for four tensile-strength levels of 930, 1210, 1410, and 1960 MPa (135, 175, 205, and 285 ksi). The volume fraction of sulfide inclusions was changed by varying the sulfur content from 0.002 to 0.022 pct. The shape of the sulfide inclusions was changed from lenticular MnS to spheroidal RE2O2S (RExSy) by rare-earth treatment. Axisymmetric tensile ductility and charpy impact energy [22 °C (72 °F)] decreased much more rapidly in the through-thickness testing direction than in the transverse testing direction when the volume fraction of sulfide inclusions was increased. Changing the shape of the sulfide inclusions from lenticular MnS to spheroidal RE2O2S (RExSy) by rare-earth treatment increased axisymmetric tensile ductility and impact energy in the through-thickness testing direction for tensile-strength levels at or below 1410 MPa (205 ksi), but not at 1960 MPa (285 ksi). Impact energy was also improved in the transverse testing direction but only for tensile strength levels at or below 1210 MPa (175 ksi). Changes in the volume fraction or shape of the sulfide inclusions had little or no effect on transition temperature. Plane-strain tensile ductility was much less affected than either axisymmetric tensile ductility or impact energy by changes in inclusion morphology or in testing direction because of the formation of macroscopic shear bands inclined at about 45 deg to the tensile axis. As a result impact energy correlated better with axisymmetric tensile ductility than with plane-strain tensile ductility. The results are discussed in terms of various models involving the formation of voids at inclusion sites and their growth and eventual coalescence by localized shear during plastic straining.