Titanium in the Rapidly Cooled Hypereutectic Gray Iron

Abstract

The effect of titanium on the structure and properties of a rapidly cooled hypereutectic cast gray iron has been studied on the example of permanent mold (PM) casting. A microstructure study showed that titanium is a relatively strong element in controlling solidification structure by increasing undercooling and thus promoting type D graphite. The effectiveness of titanium additions depends on the base iron carbon equivalent (CE) with more pronounced changes in iron with a lower CE. The undercooling ability of the titanium decreases after exceeding a certain level. Increasing titanium from 0.09–0.12% slightly increases undercooling in iron with lower CE, but this effect was reduced in a more strongly hypereutectic iron. Alloying with titanium generally improves tensile strength, but the effectiveness of titanium additions also depends on the base iron CE range. Scanning electron microscope (SEM) studies revealed that most of the titanium-containing compounds were located in the metallic matrix: titanium carbides have been found in pearlite, while titanium nitrides and carbonitrides were located in ferrite. The presence of large amounts of extremely hard titanium-containing compounds, which often appear with steadite in a relatively high phosphorous content PM gray iron, amplifies the negative effect of titanium on machinability. This study suggests that for optimal combination of tensile strength/microstructure with good machinability, the titanium content in PM gray iron should not exceed 0.075%.