Abstract
In this experiment the impact fractures of mild steel, high carbon steel and Ni-Cr steel were studied by electron microscopy. The impact fractures of mild steel were observed in the temperature range from −70° to 350°. At low temperature, the fracture surfaces are very flat and are divided into domains of large sige comparable to that of crystal grain size. In the temperature range from 100° to 250° the fracture surfaces exhibit very ductile behavior similar to that of pure iron. Above this temperature range the impact values slightly decrease and the domains in fracture surfaces become smaller and many small steps are sometimes seen. In the case of a quenched state of high crabon steel or Ni-Cr steel, fracture surfaces exhibit a very flat and small ductile deformation. And the domains in fracture surfaces are very large. Fracture surfaces of tempered Ni-Cr steel present very different aspects depending on the cooling velocity after tempering. That is the samples water-cooled after tempering show the familier ductile deformation, but those furnace-cooled after tempering show small ductility and stepwise deformation.
