Specimens made from spring steel 54SiCr6 were nitrided using various parameters of plasma nitriding in order to achieve a layer of variable thickness. The microstructure, phase constitution, hardness and case hardness depth were investigated. Subsequently notch toughness was measured by the instrumented Charpy impact test on 10x10x55 mm test specimens with U-notch 5 mm depth. Instrumented Charpy impact test was carried out at -40 °C, +21 °C and +70 °C. Results of notch toughness were correlated with the structure, chemical composition and the depth of the nitrided layers both on the surface and especially in U-notch of specimens. In the end the fractographic analysis of fracture surfaces were carried out. The notch toughness is negatively influenced as a result of presence of a nitrided layer on the surface of root of U-notch and decrease again as the thickness of nitrided layer increases and with decrease in temperature. It turned out, if the nitrided layer in root of U-notch was “sufficiently” thick, the crack spread had unstable course. The change of the character of the crack spread was also influenced by temperature.Keywords: Plasma nitriding, Notch toughness, U-notch, Case depth
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