Meaning
Microstructural clustering of primary alloyed carbides occurs during steel solidification, creating localized bands of elevated hardness and brittleness within tool steel stock. In tooling used for injection moulding and extrusion, carbide segregation distorts mechanical uniformity across cavity blocks and core pins. High concentrations of chromium or vanadium carbides along grain boundaries diminish local fracture toughness under fatigue loading.
This metallurgical condition governs localized tool wear and polished surface quality, reaching its boundary where vacuum remelting or powder metallurgy processes homogenize the carbide distribution throughout the steel matrix.
Tooling Metallurgy
Heavy primary carbide networks form during slow ingot cooling in high-carbon tool steels such as D2 or H13. When these ingots are forged into tool blocks, carbide segregation forms directional bands aligned with the working direction of the mill. Core pins subjected to high cyclic injection pressures initiate fatigue cracks along these brittle carbide bands.
Tool life drops when severe banding aligns perpendicular to applied mechanical loads.
Cavity Degradation
Unbalanced mechanical wear appears across cavity surfaces containing uneven carbide networks. Standard abrasive polishing fails to produce uniform surface finishes across segregated steel blocks.
Polishing Response
Achieving optical mirror finishes on mould cavities for polycarbonate lenses demands uniform matrix hardness. Areas affected by carbide segregation exhibit micro-pitting during diamond compound polishing because hard carbide clusters dislodge from the softer surrounding steel matrix. Toolmakers select electro-slag remelted or powder metal steels for optical cavities to eliminate hard carbide bands and prevent scattering defects on moulded lens surfaces.
