From Baking to Graphitization: How the Furnace Temperature Curve Determines Final Seal Ring Performance

Two carbon graphite blanks with the same formulation — one conducts 80 W/(m·K), the other 120 W/(m·K) — usually differ not in recipe but in furnace temperature curve. Baking and graphitization determine the ring's density, porosity, strength and conductivity. This article breaks down the key control points of the curve.

1. Two Curves: Baking and Graphitization

Heat treatment of carbon graphite happens in two steps.

1.Baking: about 1000-1300°C, carbonizing the binder pitch and giving the body mechanical strength. At this stage the material is still "carbon", not "graphite"

2.Graphitization: about 2400-3000°C in an electric furnace, converting amorphous carbon into the graphite crystal structure — this is where thermal conductivity, electrical conductivity and self-lubricity form

Seal ring grades normally pass through both steps. Baked-but-not-graphitized carbon is strong but wears poorly, and is mostly used where strength matters more than friction performance.

2. Baking: Heating Rate Sets Crack Risk

The core conflict in baking: the binder pitch decomposes violently between 400-600°C, releasing volatiles while the body shrinks.

2.1 Controlling the Heating Rate

• Typical heating rates are held at about 10-30°C/h, and slower still through the volatile release window
• Too fast: volatiles cannot escape, internal pressure builds, and the body cracks or blisters — scrap
• Too slow: the cycle stretches over weeks and energy cost rises, with no extra reduction in crystal defects

2.2 Baking Output Metrics

After baking, porosity is typically 10%-20% — the channels left by escaping volatiles are the pores. Density and porosity at this stage directly determine how much impregnation work follows: the higher the porosity, the harder it is to fill.

3. Graphitization: A Few Hundred Degrees Is a Performance Gap

Graphitization runs in Acheson or lengthwise-fed electric furnaces; peak temperature is the first variable.

• About 2400-2600°C: crystals are initially perfected; porosity and resistivity fall clearly, conductivity rises
• About 2800-3000°C: crystals approach full order, impurities vaporize off, purity improves, and conductivity reaches the upper end of the 70-150 W/(m·K) range
• Each step up in temperature grows crystallite size and reduces interlayer defects, improving both wear resistance and self-lubricity

Graphitization has costs too: the body shrinks about 10% at temperature, redistributing total pore volume, and strength drops slightly. High-strength grades therefore limit graphitization, while high-conductivity grades push the temperature up — this is how one base material becomes different grades.

4. How the Curve Reaches the Finished Ring

Four properties carry the curve through to the seal ring:

1.Density and porosity: determine whether impregnation can achieve the sub-2% impermeability — directly linked to leakage risk

2.Thermal conductivity: friction heat must leave the face through the ring; the upper end of 70-150 W/(m·K) comes from higher graphitization temperature

3.Friction coefficient: well-ordered material reaches the lower end of the 0.04-0.15 dry friction range — less heat, slower wear

4.Compressive strength: baking sets the skeleton, typically 150-300 MPa; excessive graphitization lowers it slightly and must be compensated in the formulation

5. Summary

Two questions reveal a supplier's process depth: how is the baking heating rate controlled, and what peak graphitization temperature is used. Huahao Sealing was founded in 2006 with its factory in Lu'an, Anhui, with full-process quality inspection from base material to finished ring. Our antimony-impregnated M106D measures 2.20 g/cm³ density and 190 MPa compressive strength, and we customize to customer drawings. A supplier with transparent process is the one whose batch consistency you can trust.