Effects of cobalt on mechanical properties of high silicon ductile irons

Abstract

High silicon ductile irons are being developed due to their advantages relating to pearlitic-ferritic grades (high ductility, fully ferritic structures, good machinability, etc.). Recent studies reported that silicon contents higher than 5.2 wt.% originates drastic embrittlement due to chemical ordering. For improving the mechanical properties, addition of other alloying elements becomes an interesting way of work. This study focuses on the cobalt effect on as-cast microstructures and mechanical properties of ductile irons with silicon contents that maximize ultimate tensile strength. The results obtained show that addition of 4 wt.% cobalt increases the ultimate tensile strength by about 10% and decreases the silicon content at the maximum in this property respecting the unalloyed alloys because cobalt enhances ordering as does silicon.

Effect of Cobalt in Thin Wall Ductile Iron and Solid Solution Strengthened Ferritic Ductile Iron

“…All the cobalt bearing alloys resulted in higher levels of degenerate graphite (Table 5.5). This finding was also reported by González-Martínez et al who observed that cobalt deteriorates the graphite shape and increases the tendency to CHG formation [67]. Early crack initiation can start from degenerated graphite nodules, given the notch effect on graphite nodules in ductile iron, which results in lower fracture toughness [125], [126].…”

“…A study of Fe-Si-Co alloys has shown that in addition to the B2 and D03 ordered phases that were discussed in Chapter 2, a two-phase region (B2+D03) exists between these phases, which cobalt shifting the transition between disordered (A2-phase) to ordered (B2-phase) to lower silicon values [128]. This behavior was observed by González-Martínez et al, who found that the maximum tensile strength for cobalt-bearing alloys was around 4.3-4.4 wt.% Si versus 5.2 wt.% Si for alloys without cobalt, attributing the brittleness of these alloys to the presence of ordered phases [67]. For this reason, x-ray diffraction (XRD) was performed on the 4 wt.% Co sample to look for the presence of ordered phases.…”

“…The increase in yield strength is only significant when compared to the base SSF heat, and the percent elongation is not substantially increased. Nonetheless, González-Martinez et al [67] reported one alloy that has a similar chemical composition to the 4 wt.% Co heat (4.14 wt.% Si and 3.82 wt.% Co), which resulted in mechanical properties similar to the ones observed for this heat. Hence, this data set could potentially mean that the maximum static properties have been reached with 3 wt.% Co.…”

“…• González-Martínez, Sertucha, and Lacaze recently determined that high silicon and high cobalt contents lead to the embrittlement of ferrite decreasing the ductility of these grades. They attributed this reduction in tensile properties to the presence of ordered phases [67].…”

“…In contrast, Okunnu reported an increase in the nodule count with 2 wt.% and 4 wt.% Co in the 600-10 grade[63]. Moreover, González-Martínez et al found a slight increase in the nodule count of high silicon alloys only with 4 wt.% Co[67].…”