Abstract
Microstructural characterisation has been carried out on X38CrMoV5-1 steel parts manufactured by gravity investment casting. Optimal heat treatment was applied to the parts: annealing at 780 °C (1 h) and cooling in the furnace; heating at 1020 °C (1 h), cooling in oil and double tempering at 580 °C (2 h). Hillert and Stefansson thermodynamic principles were used to calculate the fraction precipitated of V(C, N) and AlN and the percentages in solution of elements (V, Al, C, N). Calculations showed considerable V precipitation while SEM micrographs showed numerous V(C, N) precipitates of 400 nm average size. Small fraction of mixed (V, Mo)C and (V, Mo, Cr)C precipitates of approximately 5 µm was observed preferentially located close to the initial austenite grain boundaries. A small number of Al2O3 and MnS-type inclusions, with round and rectangular shapes, respectively, were also identified. Abundant pores of up to 165 µm in length and with an average size of 25 µm were located in the interdendritic regions. Comparison of the sizes and fractions of secondary phases (V(C, N), mixed precipitates), defects (pores, inclusions), and their consideration in Griffith’s equation lead to the conclusion that pores are the defects which could cause catastrophic cleavage fracture.
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