Abstract: The early fracture failure of 40 Cr screw shaftoccurred in a water treatment station of a factory after service for 4 years.The fracture causes were analyzed by chemical composition analysis,mechanical properties detection,scanning electron microscope and optical microscope.The results show that the fracture originates from the root of second threads,and the multi pass welding is causing fatigue fracture.The microstructure of core part of the shaft is composed of pearlite+net-like ferrite,which come from original structure without quenching and tempering treatment.Its mechanical properties and fatigue strength cannot meet the requirements.The fatigue source exists hard brittle martensite structure,which is in combination with alternating stress due to eccentric movement of shaft causing the crack to rapidly expand to fracture.