Evolution of microstructure and high temperature tensile strength of gray cast iron HT250: The role of molybdenum

Abstract

The present work investigated the influence of molybdenum (Mo) content on the microstructure and high temperature tensile strength (HTTS) of gray cast iron (GCI) to achieve improvement of the HTTS of GCI brake drum for heavy-duty trucks and extension of their service life. The specimens of GCI with Mo (0.024%, 0.22%, 0.51%, 0.8%) have been prepared by smelting in a 25 kg vacuum induction furnace and casting in a standard mold. The microstructure was observed by scanning electron microscopy and optical microscopy, while physical properties tested by microcomputer-controlled testing machine and micro hardness tester. The results show that increasing the Mo content from 0.024% to 0.8%, the length of graphite flake decreased from 400 μm to 288 μm; the pearlite interlayer spacing decreased from an average of 619 nm–439 nm. However, the pearlite content also decreased with the increase of the Mo content. A portion of the Mo is in solution in the matrix; this contributes to solution strengthening of the matrix. The remainder of the Mo precipitates in the austenite grains, or the grain boundaries, as carbides. This results in the refining of the pearlite grains. Eutectic cell density increased from 217 cm−2 to 286 cm−2. Both room temperature tensile strength and HTTS increased with increasing Mo content. The findings suggest adding Mo to GCI is an effective way to improve the microstructure and increase its HTTS.