Effect of Stress on Types of Fracture Surface of Reheat Cracking : Study of Reheat Cracking of Cr-Mo Steels, Report 7

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The effect of stress on the types of fracture surfaces of reheat cracking was investigated on three grades of heat resisting steels : 1Cr-1/2Mc, 21/4Cr-1 Mo and 1/2Mo steels. The modified implant test method was employed under a constant load condition. Two types of fracture surfaces were recognized : an intergranular-and a transgranular types, depending on the magnitude of stress. With a higher stress, the crack initiated in an earlier stage of reheating and it became the intergranular type. At this stage, the grain boundary of prior-austenite was weakened by the phosphorus segregation. With a lower stress, the crack initiated after a long lapse of time at 873K. In this stage, the percentage of intergranular fracture surface decreased with an increasing lapse of time because phosphorus, which had formerly been segregated at the grain boundary by the reheating in a short time, diffused back into the grain-interior. The stress increased the phosphorus concentration in the grain boundary both in the earlier and in the later stage of reheating.
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1988-04-01