Carbon Graphite Grades Guide: Impregnation Code Meanings and How to Select a Grade by Service Conditions

1. How a Carbon Graphite Grade Is Built

A carbon graphite sealing grade typically has two parts: the base graphite and the impregnation code. Take M106D: M106 is the fine-grain carbon graphite base, D means antimony impregnation. Baked graphite leaves the furnace with 10%-20% porosity and must be impregnated to seal the pores (below 2% after impregnation) before it can become a seal — so the impregnant largely sets the grade's temperature, corrosion and strength class.

Huahao Sealing's impregnation code rules:

• F = phenolic resin: general corrosion resistance, lowest cost, rated to about 180°C
• H = epoxy resin: similar class to phenolic, resists water, oil, weak acids/alkalis, rated to about 180°C
• K = furan resin: broader acid/alkali resistance; food-grade resin-impregnated grades (such as M106K) serve FDA-relevant hygienic services, rated to about 220°C
• D = antimony: metal impregnation, rated to 500°C, with markedly higher strength and hardness

The same base with different impregnants gives different grades: the difference between M106H and M106D is not the graphite but the material filling its pores.

2. Real Grades and Performance Data

The following data come from Huahao Sealing's technical parameter table and can be used directly for selection comparisons.

2.1 M106H (Epoxy-Resin-Impregnated)

• 180°C, max media pressure 1.5 MPa, max peripheral speed 25 m/s
• Compressive strength 92 MPa, flexural 38 MPa, thermal conductivity 45 W/(m·K)
• Uses: mechanical seal rings and bushings for centrifugal, chemical and clean-water pumps in water, oil and weak acids/alkalis below 180°C

2.2 M106D (Antimony-Impregnated)

• 500°C, max media pressure 2.0 MPa, max peripheral speed 25 m/s
• Measured density 2.20 g/cm³, compressive strength 190 MPa, flexural 72 MPa, Shore hardness 92 HS (third-party tested), thermal conductivity 70 W/(m·K)
• Uses: high-temperature thermal-oil pumps, molten-salt pumps, steam valves, high-temperature oven conveyor bushings, 220-500°C service

2.3 M106K (Food-Grade Resin-Impregnated)

• 220°C, max media pressure 1.5 MPa, max peripheral speed 20 m/s
• Compressive strength 90 MPa, flexural 36 MPa, thermal conductivity 42 W/(m·K)
• FDA and EU food-contact compliant, completely oil-free self-lubricating; used in chocolate mixers, filling machines and pharmaceutical agitator shafts

2.4 M191T (Carbonized)

• 600°C, max media pressure 2.5 MPa, max peripheral speed 20 m/s
• Compressive strength 100 MPa, flexural 40 MPa, thermal conductivity 55 W/(m·K)
• Runs at 500-600°C without metal impregnation; used in solar-thermal molten-salt pumps and metallurgical furnace bearings

2.5 204B and 59U

• 204B: special corrosion-resistant impregnation, resists seawater galvanic corrosion, DNV certified, 200°C and 95 MPa compressive, for marine stern shaft seals and seawater cooling pump bushings
• 59U: babbitt-inlaid structure compensating shaft runout, 180°C, 88 MPa compressive, max speed 15 m/s, for chemical pump and agitator seals under pressure fluctuation

General carbon graphite base reference: carbon content 99.9%, bulk density 1.75-1.85 g/cm³, Shore hardness 78-82 HS, ash 0.002%, thermal expansion 4.0-5.0×10⁻⁶/°C, porosity before impregnation 10-15%, 600°C in inert atmosphere.

3. A Four-Step Method for Selecting by Service Conditions

3.1 Step 1: Media Chemical Compatibility

First exclude media that attack both the impregnant and the graphite: strongly oxidizing media (concentrated nitric acid, chromic acid, fuming sulfuric acid) rule out conventional resin and metal-impregnated grades. Seawater and chloride-bearing media take 204B. In H₂S-bearing hydrocarbon gases the graphite base is unaffected, but the impregnant must be checked item by item. Food and pharmaceutical contact surfaces take food-grade grades such as M106K.

3.2 Step 2: Temperature Ceiling

• Up to 180°C: M106H suffices
• 180-220°C: M106K or corrosion-resistant impregnated grades
• 220-500°C: M106D
• 500-600°C: M191T
• Keep air service separate from inert atmosphere: carbon graphite starts oxidizing in air from about 450°C, and grade temperature ratings refer to non-oxidizing media or protective atmosphere

3.3 Step 3: Media Pressure and Structure

A grade's maximum media pressure (1.0-2.5 MPa) is a boundary value calculated from face loading. Above 1.5 MPa use a balanced mechanical seal to reduce face pressure; for higher pressures or strong pressure fluctuation (agitators, reciprocating compressors), prefer inlaid structures such as 59U that compensate shaft runout.

3.4 Step 4: P-V Value Check

The product of face pressure P (MPa) and sliding speed V (m/s) sets the frictional heat load. A grade's maximum speed of 15-25 m/s and maximum pressure are linked constraints — you cannot take both to the limit. For high heat load, prefer grades with higher conductivity: M106D at 70 W/(m·K) outperforms M106H at 45 W/(m·K), used together with flush or quench plans.

4. Summary

The key to reading a grade is the second half: the impregnation code sets the temperature and corrosion class. The F/H/K resin family covers general chemical service below 180-220°C, D (antimony) covers 220-500°C high-temperature high-strength service, and special treatments (M191T carbonizing, 204B seawater-resistant impregnation) close the two ends: 600°C extreme heat and marine corrosion. The selection order is fixed: media, then temperature, then pressure, then P-V value — a grade qualifies only after passing all four. Grade tables give typical values; for a real project, send the media composition sheet, temperature, pressure, shaft diameter and speed to the manufacturer for review — more reliable than matching numbers in a table.