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The influence of tempering process for properties of boron steel
Doležal, Luděk ; Kubíček, Jaroslav (referee) ; Slováček, Marek (advisor)
This bachelor thesis deals with the problem of the occurrence of residual austenite in the crossbar of a harvester forestry tracks during tempering after a longer period of time. The chosen material is boron steel 27MnCrB5-2. The experiment is carried out by comparing three samples, the first is tempered immediately after quenching, the second is tempered 48 hours after quenching and the third is not tempered. According to preliminary calculations of cooling rates and Mf temperature, the presence of residual austenite is unlikely. On all metallographic smears, an identical martensitic-bainitic structure can be observed by optical microscopy. On a scanning microscope, both tempered materials show lower bainite with carbides within the ferritic lamellae. Only the quenched sample has carbides that are difficult to distinguish from the martensitic structure, but no evidence of residual austenite. Based on the hardness measurements, the quenched crossbar has the highest hardness. According to the corresponding curve in the CCT diagram, the hardness corresponds to the bainitic structure. The occurrence of residual austenite was definitely ruled out by X-ray diffraction at less than 2% resolution of the equipment used
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