Cemented carbide (WC-Co and WC-Ni grades) is sintered at 1,380–1,500°C in vacuum, passing through a liquid-phase sintering stage where the cobalt or nickel binder melts and flows to fill inter-particle gaps. During this stage, the graphite tooling — setter plates, boats, cassettes and pushers — is in direct contact with the carbide compacts at their most chemically reactive state. The wrong graphite grade contaminates the binder, causes surface defects, or bonds the part to the setter, resulting in scrap.
What Happens Between Carbide and Graphite at Sintering Temperature
At temperatures above 1,100°C, liquid cobalt reacts readily with carbon. The cobalt binder in WC-Co carbide wants to dissolve carbon from the graphite setter to reach its thermodynamic equilibrium carbon content. If it dissolves too much, the binder composition shifts and the sintered part shows:
- Brittle eta-phase (Co₃W₃C or Co₆W₆C) at grain boundaries — detected by microstructure analysis
- Reduced transverse rupture strength and hardness
- Dark surface discolouration on the contact face
- Part bonding to the setter if cobalt migrates into graphite pores
The goal of graphite tooling specification is to provide enough carbon activity to prevent eta-phase formation (carbon-lean binder) while not oversupplying carbon to the extent that the carbon window is exceeded (free carbon in the microstructure).
Graphite Grade Requirements for Carbide Sintering
Purity
Use graphite with ash content below 100 ppm for carbide sintering — preferably below 50 ppm. Higher ash grades contain silicon, sulphur, iron and other impurities that contaminate the cobalt binder. Even trace contamination at this scale can be detected in EDX analysis of the sintered microstructure and may cause batch rejection in aerospace or cutting tool applications.
Grain Size
Fine-grain isostatic graphite (grain size below 4–8 µm) is preferred over medium or coarse-grain grades. Finer grain means:
- Smoother contact surface — lower risk of part surface damage and bonding
- More uniform pore size distribution — more predictable carbon activity at the surface
- Better dimensional stability under load at sintering temperature
Density
Higher density graphite (1.80–1.90 g/cm³) has lower porosity, which reduces binder infiltration into the setter surface. Open-pore graphite at lower density allows liquid cobalt to wick into the surface and mechanically bond the part — this is the most common cause of part-to-setter adhesion in carbide sintering.
Recommended Grades
| Grade Type | Ash (ppm) | Density (g/cm³) | Grain Size | Application Fit |
|---|---|---|---|---|
| Standard isostatic (ISO-63) | <300 | 1.75–1.80 | 8–15 µm | General carbide sintering |
| Fine-grain isostatic (ISO-88) | <100 | 1.80–1.85 | 4–8 µm | Precision carbide, aerospace grade |
| High-purity (TTK-87, HPG-99) | <20 ppm | 1.85–1.90 | 2–4 µm | Ultra-clean carbide, medical device |
Setter Plate Design
Thickness
Setter plates for carbide sintering are typically 10–20 mm thick. Thinner plates reduce thermal mass (faster ramps) but flex under load at sintering temperature. A minimum flatness specification of 0.3 mm across the setter is needed — sagging setters cause non-uniform shrinkage in the parts sitting on them.
Surface Finish
The contact surface of the setter should be finished to Ra 0.8–1.6 µm. Rougher surfaces increase mechanical interlocking between part and setter. Some sintering operations lightly sand-blast the setter surface before use to open pores uniformly — this creates a consistent carbon activity interface rather than an irregular one.
Release Layer
Despite using correct grade graphite, many producers apply a thin layer of alumina (Al₂O₃) powder or BN spray between the part and the setter. This acts as a physical barrier to cobalt migration and eliminates adhesion, at the cost of a minor surface texture on the bottom face of the sintered part. Many manufacturers accept this as preferable to the risk of bonding or contamination.
Graphite Boats and Cassettes
In continuous-push vacuum sintering furnaces, parts are loaded into graphite boats or multi-tier cassettes that are pushed through the furnace on graphite rails. Key specifications:
- Wall thickness: 8–15 mm to resist deformation at sintering temperature under part weight
- Rail contact surface: Ra 3.2 µm or smoother to reduce friction and prevent boat sticking on rails
- Dimensional tolerance: ±0.5 mm on all critical dimensions — boats that are out of tolerance cause jams in the push mechanism
- Grade: same as setter plates — fine-grain isostatic, high density, low ash
Service Life and Replacement
Graphite setter plates in carbide sintering typically last 200–800 cycles depending on temperature, load weight and carbide grade. Signs that require replacement:
- Cobalt infiltration layer visible on setter surface (shiny metallic patches)
- Surface erosion exceeding 0.5 mm depth
- Flatness deviation exceeding 0.5 mm
- Cracking or chipping at edges
Expo Advanced Materials supplies cemented carbide sintering tooling in ISO-88 and high-purity isostatic grades, machined to customer drawings with flatness inspection reports on request.