In-situ SEM observation on fracture behavior of austempered silicon alloyed steel

Crack initiation, propagation and microfracture processes of austempered high silicon cast steel have been investigated by using an in-situ tensile stage installed inside a scanning electron microscope chamber. It is revealed that micro cracks always nucleate at the yielding near imperfections and the boundary of matrix-inclusions due to the stress concentration. There are four types of crack propagations in the matrix: crack propagates along the boundary of two clusters of bainitic ferrite; crack propagates along the boundary of ferrite-austenite in bainitic ferrite laths; crack propagates into bainitic ferrite laths; crack nucleates and propagates in the high carbon brittle plate shape martensite which is transformed from some blocky retained austenite due to plastic deformation. Based on the observation and analysis of microfracture processes, a schematic diagram of the crack nucleation and propagation process of high silicon cast steel is proposed.