How to Inspect Hardmetal Microstructure, Porosity, and Graphite

Hardmetal Types and Structures

Powder-metallurgy alloys such as WC-Co and WC-TiC-Co are commonly called cemented carbides or hardmetals. Major families include tungsten-cobalt (YG), tungsten-titanium-cobalt (YT), tungsten-titanium-molybdenum-niobium (YW), steel-bonded (YE), titanium-nickel-molybdenum (YN), and cast tungsten carbide (YZ). WC-Co generally offers higher strength and impact toughness, while WC-TiC-Co provides greater wear resistance, hot hardness, and permissible cutting speed.

YG hardmetal normally contains light WC alpha phase and a dark binder beta phase. Fine-grained grades include YG3X (K01), YG (K20), and YG6X (K10); YG6X is especially fine. YG8 and YG8N (K30) lie between fine and medium. Coarse-WC, high-binder grades such as YG11C and above have lower hardness but better toughness for impact-loaded dies and mining tools. This classification follows ISO 4499-1:2008.

YT hardmetal normally contains blocky WC alpha phase, binder beta phase, and rounded orange-yellow to brown TiC-WC gamma phase. Color varies with oxidation conditions. Its WC grains are commonly medium and may become coarse as gamma-phase content increases.

TaC and NbC additions refine WC and TiC-WC grains and form hard, fine, irregular complex carbides. Steel-bonded hardmetal may use TiC or WC in a heat-treatable steel matrix; its carbide particles remain stable while the steel binder can be annealed, normalized, quenched, and tempered.

Specimen Preparation

Because surface and core porosity can differ, a fracture cross-section is often selected. Cut by wire EDM or fracture mechanically. For manual preparation, grind with water cooling on a 150–250 μm silicon-carbide wheel, then lap with 10 or 12 μm boron-carbide or silicon-carbide powder on a cast-iron disk at about 450 r/min. Polishing may use a solution of 1,000 mL water, 10 g potassium ferricyanide, 10 g potassium hydroxide, and about 40 g chromium oxide or alumina, or synthetic diamond paste.

Mechanical polishing uses coarse and fine stages. Coarse-mounted specimens can be fixed with sulfur in an approximately 80 mm ring. Fine polishing may use an 85% alumina and 15% rubber disk with saturated potassium permanganate solution for about 20 minutes.

Etching and Phase Identification

Fresh equal-volume mixtures of potassium ferricyanide and potassium hydroxide solutions at 20%, 10%, or 5% reveal WC and TiC-WC phases; lower concentrations can reveal eta phase from orange-yellow to orange-red. Alternatively, heat a polished, dry specimen in air at 450–500°C for about 10–15 minutes until pale yellow. Overoxidized dark-blue surfaces must be repolished.

Porosity and Graphite Assessment

Porosity appears as sharply bounded black circular holes below 40 μm. Examine an unetched polished specimen at 100× or 200× over multiple fields from edge to center, and report both general and severe porosity, emphasizing the severe value. ISO 4499-4:2016 may be consulted as an international reference method; no equivalence or acceptance claim is implied.

Excess carbon produces fine nest-like or point-like graphite. At about 2% volume fraction in WC-Co it becomes flake-like; in WC-TiC-Co, about 1.5% can produce a finer nest-like form. Evaluate unetched polished specimens by graphite quantity, size, morphology, and distribution. Graphite, porosity, and eta phase reduce strength, toughness, and wear resistance, so their identification supports powder, mixing, pressing, and sintering process control.