Strength, microstructure and chemistry of ingot mould grey iron after different cycles of low frequency high temperature loads

• The tensile strength, matrix microstructure, fracture morphology and chemistry of HT200 grey cast iron after various periodic pretreatment have been studied.
• The micro and macro-mechanism of thermal fatigue cracks in large ingot mould made up of HT200 has been revealed.
• Some feasible measures for extending mould service life have been suggested.

A kind of grey iron for producing large steel ingot moulds has been analyzed by thermal simulation experiment, and its tensile strength, matrix microstructure, fracture morphology and chemistry after different cycles of low frequency high temperature loads with oxidizing atmosphere have been studied. The micro and macro-mechanisms of large mould fatigue cracking have been discussed. As the period of low frequency high temperature pretreatments increases, the carbon content in grey iron decreases, resulting in the decrease of pearlite fraction and also the tensile strength. A new kind of graphite flake in short and small size appears in the matrix after heating and cooling treatments, leading to the rise of graphite exposure rate in the fracture under low frequency high temperature periodic loads. Although the decarburization rate in the specimens with ferrite matrix is much lower than that with pearlite matrix, the carbon content has more influence on tensile strength, induced by the following grains coarsening and grain boundary weakening. Accordingly, some feasible measures for extending mould service life have been proposed.