The analysis of an abnormal crack of a forging plate is presented in this work. The crack
was found after forging process. Macro-analysis, SEM, composition inspection, metallo-graphic analysis, inclusion analysis, EPMA and EDS were implemented. SEM shows that
cleavage fracture is the main feature of the fracture surface, according to which it can be
decided that the fracture is brittle fracture. XRF and carbon and sulphur analyzer indicate
regular composition conditionof the plate. EPMAand EDS suggest phosphorus segregation
spread a lot around the crack. The results indicate that: brittleness caused by phosphorus
segregation around crack zone is the main reason that cracks generate in the 12Cr13 steel
during forging. As all we know, phosphorus has deleterious effect on toughness of steel
because of its enrichment on grain boundaries, which can weaken the bond strength of
grain boundary. Its existence should be avoided and microstructure of the steel should be
homogeneous when good plasticity and toughness are wanted.