Microstructure of as‐quenched 3.5NiCrMoV rotor steel. Part I. General structure and retained austenite

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Abstract
The microstructural features have been examined for 3.5NiCrMoV steam turbine rotor steel, in the as‐quenched state and tempered at 500 °C. Quenching produces lath martensite, with bands of retained austenite at the lath boundaries and, to a lesser extent, at prior austenite grain‐boundaries. Autotempering occurs during the quench, resulting in loss of tetragonality of the martensite and extensive carbide precipitation in the matrix and to a lesser degree at prior austenite grain boundaries, but not at lath boundaries. Tempering at 500 °C leaves the lath structure largely intact, but causes retained austenite to transform to bands of ferrite and cementite. This transformation does not correlate with the reduction in stress corrosion crack velocity which occurs on tempering. The strength of 3.5NiCrMoV steel in the as‐quenched and 500 °C tempered conditions is most probably due to the combination of carbide precipitation strengthening and substructure strengthening.