Thermal fatigue failure mechanism of gray cast iron

The thermal fatigue resistance of gray cast iron can be characterized by its tensile strength at corresponding temperature, which has been widely recognized by researchers. The experimental results show that the tensile properties of gray cast iron deteriorate gradually under the condition of periodic high-temperature oxidation, which corresponds to the thermal fatigue failure of cast iron equipment in this service environment.

Schematic illustration of cracking mechanism of grey cast iron under thermal cyclic loads

The fatigue failure mechanism of gray cast iron under low frequency and high temperature thermal cycle load is described in the figure. Generally speaking, the original structure of gray cast iron parts is pearlite, as shown in figure (a); After repeated service at high temperature, the sample is gradually oxidized and decarburized. When the carbon content is less than 1.55%, the matrix is basically transformed into ferrite. Because the resistance of pearlite matrix to crack initiation and propagation is much greater than that of ferrite, the tensile strength and thermal fatigue resistance of the sample decrease significantly. At the same time, short and fragmented new graphite continues to form, destroying the continuity of the matrix, as shown in figure (b); Although the decarburization rate decreases after ferrite matrix, the effect of carbon content on tensile strength is much greater than that of pearlite matrix; After that, the grain gradually coarsened and the secondary cementite precipitated, weakening the grain boundary and further reducing the tensile strength and thermal fatigue resistance of the sample, as shown in figure (c). In addition, the flocculent oxides of silicon, manganese and iron formed around the graphite sheet at high temperature increase the discontinuity of the matrix, which is conducive to the formation and propagation of fatigue cracks under the action of external force, as shown in figure (d).

It can be seen that the thermal fatigue properties of gray cast iron are closely related to high temperature, oxidation, load cycle and chemical composition, and are affected by graphite morphology, matrix phase, grain size and precipitates.

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