The thermal fatigue behaviour of steel 5CrMnMo after various heat treatments has beenexamined by means of Coffin-model and self-strained thermal fatigue testing machines.Thethermal fatigue crack initiation and propagation have been observed under SEM and TEM.The majority of thermal fatigue cracks of the quenched samples initiated firstly at the grainboundaries and that of the quenched-tempered samples at the breakage interface of the car-bide and matrix.The thermal fatigue cracks of either quenched or quenched-tempered steelpropagated mainly along the breakage interface of carbide and grain boundaries.
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