The Fine Grain Effect on a New Carbide Fe₄C₃ Formed in Pipeline Steel X80

Pipeline steel is normally used in petroleum and natural gas transportation over long distances. Because of the special rolling technology used in its making, the pipeline steel possesses the finest grains, which endow it with very good mechanical properties, particularly very high impact toughness. In this research, a new carbide, Fe₄C₃, has been found in the island-like microstructure of this steel, which possesses a spinel structure with a lattice constant of 0.81 nm. The carbide has dimensions of 5–15 nm and is in coherence with the matrix, which produces a large coherent force near the boundary and makes the image exhibit an irregular dark blur in the bright field of the transmission electron microscope (TEM). The carbide is stable below 200°C and transforms to cementite above 400°C. To satisfy the formation conditions of Fe₄C₃, the grain size of austenite must be ultrafine, which makes normal pearlitic, bainitic, and martensitic transformations difficult with air cooling. The carbide displays a stronger corrosion resistance and makes the island-like microstructure etching more difficult than that of the ferrite matrix during metallographic and TEM sample preparation.