Crystal structure and morphology of the carbide precipitated from martensitic high carbon steel during the first stage of tempering

Abstract

A martensitic steel containing 1.13 wt. %C was tempered at 120°C for 1–100 days and the structure and morphology of precipitated carbide were studied by electron microscopy and selected area diffraction. The carbide is orthorhombic with lattice parameters a = 4.704 ± 0.016, b = 4.318 ± 0.005 and c = 2.830 ± 0.006 Å. The space group is Pnnm and the atom positions are 4Fe at 4 g with x = 2 3 , y = 1 4 and 2C at 2 a. The carbide, named η-carbide of iron or η-Fe 2C, is isomorphous with Co 2C and Co 2N. The structure is similar to but different from the structure of hexagonal ε-carbide, which has long been believed to precipitate at the first stage of tempering. The orientation relationship between the η-carbide and the tempered martensite α“ was determined as (110) η ∥(010) α” and [001] η∥ [100] α. The carbide has a plate-like shape with thickness of 30–50 Å and precipitates periodically along dislocations with an interval of about 100 Å.