Effects of high silicon contents on graphite morphology and room temperature mechanical properties of as-cast ferritic ductile cast irons. Part II – Mechanical properties

In this second part of the investigation, room temperature mechanical properties and hardness evolution of cast irons with silicon contents ranging from 2.29 wt% to 9.12 wt% have been studied and related to structural results from the first part. Increasing silicon content increases ultimate tensile strength and yield stress until a maximum value of 719 MPa at around 5.0−5.2 wt% silicon for the former and 628 MPa at 5.2−5.4 wt% silicon for the latter. Brinell hardness remains increasing with silicon content with a maximum value of 396 at 9.12 wt% silicon. Elongation at rupture shows an opposite evolution and gradually decreases to zero at 5.3 wt% silicon. This evolution is related to chemical ordering of the ferritic matrix (embrittlement effect). Chunky graphite shows apparently no significant effect on the ultimate tensile strength and yield strength in cast irons with silicon contents higher than 4.0 wt%. However, it has a negative effect on elongation. This result contrasts with the negative effect of chunky graphite on mechanical properties of ductile irons reported in the literature for alloys with silicon contents lower than 3 wt%. It is suggested that this difference is due to the matrix strengthening effect of high silicon contents which overtakes the detrimental effect of chunky graphite. This study suggests that cast irons with silicon content as high as 5.0 wt% could be considered for industrial applications when high resistance and some ductility are requested.