A study has been made on the crystal structure of the precipitated carbide and the mode of precipitation from martensitic nickel steels tempered at 180°C. Like the case of plain carbon steel, the precipitates are orthorhombic η-carbide, but the lattice parameters are influenced markedly by nickel content in the mother steel. There are two types in the carbide. N-type carbide, precipitated from low nickel (below 9.1 wt%) steels, is similar to the η-carbide from the plain carbon steel and takes the orientation relationships [001]_η⁄⁄[100]_M and (100)_η⁄⁄(010)_M. R-type carbide from high nickel (above 9.1 wt%) steels takes a little different relationships (001)_η⁄⁄(211)_M and (110)_η2°(100)_M. The carbide particles are smaller than 100 Å and line up to make needle-like shapes. Needle directions are scattered, but for the N-type carbide the average needle direction is [531]_M and for the R-type carbide it is [532]_M.
The carbide contains nickel as well as iron. Its carbon content is considered to decrease with increasing nickel. Superstructure reflections do not appear in the R-type carbide. This is ascribed to a disorder in the carbon atom arrangement caused by the replacement of iron by nickel. The superstructure reflections from the N-type carbide show frequently an intensity anormaly. An explanation is given on the basis of an antiphase domain model with respect to the carbon atom arrangement in the carbide. This gives that in some carbide particles antiphase domain boundaries may be formed in every about three unit cells in the [001]_η direction.