In this thesis, tempering diagrams of 34CrAlNi7-10 steel were created from an
industrial forging with a diameter of 340 mm. The forging was heat treated by
quenching and tempering: quenching was carried out from 890 °C, followed by
tempering at 400 °C. Samples for investigation were taken from three depths of the
cross-section, namely 12.5 mm, 85 mm (D/4), and 170 mm (core), and tempered at
different temperatures between 400 °C and 650 °C. The purpose of the research was
to compare the influence of different cooling rates and microstructures on the
mechanical properties and the course of tempering diagrams with respect to the cross
sectional depth.
The analyses included hardness measurements, tensile tests, determination of impact
toughness, and metallographic characterization. The results showed that all tempering
diagrams exhibit a characteristic course for low-alloy steels, with increasing tempering
temperature leading to a decrease in strength properties and an increase in toughness.
However, significant differences occur between the individual depths: the surface,
which has a fully martensitic structure, achieves the highest strength properties and
impact toughness, while the core, due to a higher content of ferrite and pearlite, shows
lower values. The D/4 region represents a transitional zone with intermediate
mechanical properties, and the tempering effect is less pronounced.
The results confirm that, due to cooling rates and microstructural heterogeneity, the
mechanical properties of industrial forgings often deviate, and therefore laboratory
tempering diagrams are not always representative of massive industrial products.
Keywords: structural steel 34CrAlNi7-10, tempering diagram, heat treatment,
microstructure, mechanical properties, forging.
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