Carbide Composition: WC-Co Ratios, Carbon Balance & Eta Phase

Target Audience & Operational Background

This technical metallurgy guide assists mechanical design engineers, tooling specialists, and procurement leads analyzing chemical compositions, cobalt binder percentages, and phase diagrams for sintered tungsten carbide wear parts.

Stoichiometric Carbon Window & Eta-Phase Embrittlement

Cemented carbide is a metal-matrix composite produced via liquid-phase sintering at 1,400 ℃ to 1,450 ℃ under 5 to 10 MPa argon pressure. Because tungsten and carbon form stable stoichiometric mono-carbide (WC), the metallurgical balance during sinter-HIP determines whether the microstructure remains a clean two-phase system (WC + cobalt binder) or precipitates brittle secondary phases.

A definitive microstructural review in the International Journal of Refractory Metals and Hard Materials (doi:10.1016/j.ijrmhm.2018.12.004) confirms that the stable two-phase carbon window spans a narrow margin of approximately 0.05 to 0.10 wt% carbon. When carbon content drops below the lower stoichiometric threshold during sintering, carbon-deficient ternary carbides known as eta (η) phase (Co3W3C and Co6W6C) precipitate across the binder interfaces.

Eta-phase formation consumes ductile cobalt from the binder matrix, resulting in localized microhardness spikes (>1,600 HV30) while reducing transverse rupture strength (TRS per ASTM B406) by over 40%. Conversely, excess carbon shifts the alloy across the upper threshold, precipitating free graphite flakes (ASTM B276 C-type porosity) that lower compressive strength and accelerate abrasive erosion.

Grain Size Classification & Binder Alloying Mechanics

Beyond the nominal cobalt ratio, carbide performance depends directly on tungsten carbide mean grain size, classified per ASTM B390 and ISO 4499:

1. Fine-Grain Structures (0.5 to 1.2 μm): Providing maximum hardness (89.5 to 92.5 HRA) and high compressive strength (>4,200 MPa), fine-grained WC-Co alloys (Ref. YG6X class, ~6% Co) represent the primary grade class for high-velocity slurry orifices, mechanical seal faces, and precision wire drawing dies.
2. Medium-to-Coarse Grain Structures (1.5 to 3.5 μm): Exhibiting higher mean free paths between cobalt binder pools, coarser grain alloys provide superior impact resistance and thermal crack resistance in heavy-load metal forming and stamping tooling.

Carbide Composition & Phase Balance Matrix

Prohibited Metallurgical Conditions & Operating Limits

• Prohibited Deployment of Eta-Phase Microstructures in Impact Tooling: Components exhibiting eta-phase precipitation must never be installed in cyclic impact, stamping, or fluid pulsation environments. Eta-phase crystals act as sharp stress risers that propagate through the brittle matrix under cyclic shear stresses exceeding 800 MPa.
• Prohibited Specification of Binderless WC for Structural Wear Parts: Sintered tungsten carbide without metallic binders (pure WC) exhibits extreme brittleness with low fracture toughness. Carbidea does not supply binderless carbide; mechanical assemblies require ductile cobalt binder phases to absorb operational tensile spikes.

Overseas Print Conversion & Sourcing Traps

Western engineering prints often state vague metallurgical callouts such as "Carbide 94/6" without defining density tolerances, porosity limits, or carbon phase windows. Sub-tier manufacturers attempting to hit nominal cobalt percentages may use uncontrolled sintering atmospheres, generating unseen internal eta-phase layers.

Purchase specifications must mandate non-destructive magnetic saturation testing (verifying 4πσ within 88% to 96% of theoretical cobalt values) and coercivity testing to prove tight carbon window control. Critical ground surfaces must specify finish diamond grinding to ±0.005 to ±0.01 mm, surface roughness Ra ≤0.1 μm, and as-sintered blank allowances of ±0.10 to ±0.25 mm.

As an engineering sourcing partner for severe-service wear parts, Carbidea collaborates with ISO 9001:2015 audited partner mills to verify powder metallurgy chemistry, conduct ASTM B276 porosity cross-sections, and deliver engineering quotations within 1–2 business days.

FAQ

Q1: What is eta phase in tungsten carbide composition?

A: Eta (η) phase refers to brittle ternary cobalt-tungsten-carbide compounds (Co3W3C and Co6W6C) that precipitate when the total carbon content drops below the narrow stoichiometric balance. Eta phase consumes ductile cobalt, making the material brittle and vulnerable to impact shattering.

Q2: How does cobalt binder percentage affect carbide properties?

A: Cobalt content typically ranges from 6% to 15%. Lower cobalt content (e.g., 6% Co in the YG6X class) maximizes hardness (89.5–92.5 HRA) and abrasive wear resistance, whereas higher cobalt content (12%–15% Co) improves transverse rupture strength and fracture toughness at the expense of hardness.

Q3: How do partner mills verify carbon balance without destructive testing?

A: Carbon balance is non-destructively verified using magnetic saturation and magnetic coercivity testing. Because dissolved tungsten in the cobalt binder alters its ferromagnetic response, magnetic saturation measurements precisely indicate whether the alloy is in the two-phase window or trending toward eta phase or free carbon.