Ex situ and in situ TEM investigations of carbide precipitation in a 10Cr martensitic steel

An ex situ approach combining fast quenching experiments in a dilatometer and postmortem microstructural observation in transmission electron microscopy (TEM) has been used to observe the dynamic microstructure change during differential scanning calorimetry (DSC) ramping of an Fe-10Cr-0.15C (wt%) alloy fabricated from high-purity components. The DSC measurements reveal two exothermic events at temperatures about 270 and 600 A degrees C in a heating process. The two events were discerned by TEM investigations on specimens interrupted during thermal ramping in a dilatometer. It is found that precipitation and growth of M3C carbide occurred first in a temperature range between 200 and 400 A degrees C, following the Bagaryatskii orientation relationship. Subsequently, M7C3 carbides precipitate on prior martensitic laths boundaries in a temperature range between 500 and 700 A degrees C at the expense of M3C. M23C6 carbides were found precipitating on the interface between M7C3 and matrix at approximately the same time with the precipitation of M7C3. The obtained results are also compared with an in situ TEM heating experiment, and differences between the two approaches are discussed.