Abstract
The influence of thermal holding at elevated temperature 620 °C for times from 100 up to 5000 hours on mechanical properties of the 9Cr-1Mo steel at ambient temperature was investigated. The structural changes in experimental material due to secondary phase particles precipitation during thermal holding after primary heat treatment (austenizing, quenching and high-temperature tempering) occured. The presence of secondary phase particles of M₂₃C₆, MX and Laves' phase (FeMo₂) in microstructure of the experimental steel by means of TEM/EDX analyses was confirmed. The changes in microstructure of the 9Cr-1Mo steel during ageing can influence its mechanical properties at ambient temperature and change failure mode of the steel.

Key words
9Cr-1Mo steel, thermal ageing, secondary phase precipitation, mechanical properties, ductile fracture, splitting

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