| Charpy impact testing was performed to investigate fracture behavior of AISI-4140 steel from viewpoint of variation in the intrinsic toughness of martensitic matrix with alloying additions of Ni(4140-Ni) and Si(4140-Si). In 4140-Si steel, transgranular fracture becomes the primary fracture mode as temperature deceases. When coarse carbides exist at grain boundaries, transition from intergranular fracture to transgranular fracture occurs as testing temperature decreases. In 4140-Ni steel, transgranular fracture occurs at very low temperature when there is few carbides at grain boundaries, while fracture mode is predominantly intergranular type in the presence of coarse carbides at grain boundaries. The different fracture behavior of these steels is attributed to alloying elements; since Si-addition decreases the intrinsic toughness of the matrix, transgranular fracture becomes easier to occur with decrease in temperature in 4140-Si steel; since Ni-addition increases the fracture toughness, transgranular fracture becomes more difficult to occur in 4140-Ni steel. |
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