Thermal fatigue degradation progress in SiMo ductile cast iron under oxidation conditions

Abstract

Thermal fatigue tests were conducted on a high Si low Mo ductile cast iron (DCI) under oxidation conditions at 600 °C. The study aimed to understand the evolution of surface degradation, focusing on complex composed graphite nodules, pearlite islets, and their impact on crack initiation and growth in relation to oxidation processes, etc. Degenerated graphite nodules, that include ferrite particles, and areas of increased graphite nodule density, along with pearlite islets, were examined for their oxidation behavior. At lower thermal cycles numbers cracks primarily initiated of at the graphite-matrix interface and grew faster in the case of successively arranged graphite nodules and their higher local number density. Graphite degradation involved debonding between graphite and matrix, followed by complex oxidation processes. Degenerated nodules exhibited higher oxidation rates due to the presence of ferrite which acts as oxidation pathways. Areas with increased graphite density and larger sizes facilitated accelerated oxidation via crack formation. Additionally, pearlite degradation started with cracked cementite lamellae, followed by oxidation. Strategies to enhance thermal fatigue resistance included reducing graphite nodule diameter, ensuring their uniform distribution, preventing of formation of degenerated nodules and eliminating porosity. These findings improve understanding of thermal fatigue behavior in DCI that has proven to be even more complex than previously thought. The findings will guide the development of advanced materials for demanding applications.