Fracture Analysis of Fit Screws of a Tractor

The failure of fit screws used in a tractor was analyzed.A comprehensive study on composition,microstructure,fracture surfaces and cracks indicated that surface of screws underwent decarbonization,which led fatigue damnification at the root of the spiral burr and resulted in cracking.It is suggested to strictly control the quality of heat treatment to avoid decarbonization or substitute the screws with bolts or to use bigger screws.