Influence of thermal exposure on microstructure evolution and tensile fracture behaviors of compacted graphite iron

The mechanical properties of compacted graphite iron (CGI) may undergo degradation when it exposes at high temperature environment for a long period of time. In this study, the influences of thermal exposure conditions on microstructure evolution, tensile properties, especially in-situ tensile fracture behaviors of CGI were investigated. The experimental results show that the cementite surrounding the vermicular graphite (VG) gradually decomposes into ferrite and graphite with increasing the thermal exposure time. The increase of ferrite content with lower strength but higher ductility around VG particles, and the size and content of VG particles can lead to the easier initiation of crack, decrease of strain hardening rate, and the retard of the crack propagation rate in the matrix. All these factors often cause the decrease of strength but increase of elongation to fracture for CGI. Finally the relationship among yield strengths of CGI and matrix, and graphite content was discussed.