Abstract
The fracture behavior of hardened 52100 steel containing between 0.002 and 0.057 pct P has been investigated as a function of proeutectoid carbide allotriomorph networks produced by cooling after homogenizing at 1150 °C, air cooling after austenitizing at 960 °C or isothermal holding at 750 °C after 960 °C austenitizing. A range of fracture toughness, from 15.64 MPa · m112 (14.24 ksi · in112) to greater than 22.6 MPa · m112 (20.6 ksi · in112), was measured in compact tension specimens with different P contents and carbide distributions but otherwise identical quench and tempered microstructure of hardness between Rc 62 to 64. High fracture toughness correlated with networks of thin, discontinuous carbides while low fracture toughness correlated with the absence of carbide networks or networks composed of coarse, closely spaced proeutectoid carbides. Scanning electron micrographs of the fracture morphology associated with each type of microstructure are presented and discussed. Although phosphorous contributes to intergranular fracture, its major effect appears to be the stimulation of proeutectoid carbide allotriomorph growth. Even the 0.057 pct P heat showed high fracture toughness provided welldeveloped networks of thin, discontinuous carbides were present in the hardened microstructure.
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