Impact Strength and Transmission Electron Microscopy Investigations of Aged and Carburized Alloy 800H

The effect of carburization on the impact strength and microstructure of the commercial Fe-32 Ni-20 Cr alloy 800H has been investigated in the 20 to 800°C temperature range. The properties and microstructure of test pieces carburized at 850°C for up to 500 h in an argon-10 vol% methane atmosphere and of specimens heat treated at 850°C in an inert atmosphere for the same times were compared. The results showed that aging at 850°C reduced the impact strength at 20 to 800°C. With an increasing degree of carburization, the impact strength was progressively reduced to ∼50 J at a bulk carbon content of 0.6 wt%. Heat treatment after carburization caused a further decrease in impact strength as the depth of carbon penetration increased. Microstructural examination by optical and transmission electron microscopy (TEM) of broken test specimens showed precipitation of M₂₃C₆ carbides on grain and twin boundaries and intragranular fine precipitation of TiC and M₂₃C₆ as well as the presence of primary titanium carbonitrides. The TEM investigations using extraction replica and thin foil techniques established that the M₂₃C₆ carbides at grain boundaries retained a crystallographic orientation to one grain and grew into the adjacent grain. Lamellae of M₂₃C₆ carbides precipitated on noncoherent twin boundaries grew into the grain parallel to the twin plane, whereas M₂₃C₆ on coherent twin planes grew as plates along the twinning plane.