A shearer drum works through 200 meters of coal seam, and the carbide tips are lasting half as long as they should. The grade specification on the purchase order says “cemented carbide” — which is exactly as informative as “metal.” Two tips sitting next to each other on the same drum can perform completely differently because one is HRA 91 with 1 µm grain size and the other is HRA 88 with 3 µm grain size. Same label. Radically different behavior under load.
Selecting the right carbide grade for coal mining is not complicated, but it requires understanding three variables: hardness (HRA), grain size (µm), and cobalt content (%). Get these right for your seam conditions, and tip life doubles. Get them wrong, and you’re replacing picks every third shift.
Why Most Carbide Tip Failures Trace Back to Grade Mismatches
Coal mining puts carbide tips under a specific combination of stresses that most other cutting applications don’t see simultaneously: abrasive wear from coal and mineral particles, impact loading from rock bands in the seam, thermal cycling from frictional heat followed by water-spray cooling, and corrosion from mine water.
A grade optimized purely for wear resistance — high hardness, low cobalt, fine grain — will crack on impact in banded seams. A grade optimized for toughness (lower hardness, high cobalt, coarse grain) will wear through too fast in highly abrasive formations. Neither failure is visible at the specification stage. It shows up two shifts into production.
The most common grade mismatches we see from incoming orders:
- Fine-grain, high-HRA grades in banded or stone-banded seams — tips chip and fracture rather than wear smoothly
- High-cobalt toughness grades in highly abrasive anthracite — wear rate doubles compared to the correct grade
- Specifying HRA alone — two grades at HRA 89 can have completely different fracture behavior depending on cobalt content and grain size
Each mismatch points to the same root cause: treating HRA as the only variable that matters.
The Three Variables That Determine Carbide Grade Performance
Hardness (HRA): What It Measures and What It Doesn’t
HRA measures the hardness of the cemented carbide composite. In mining grades, this typically ranges from HRA 86 to HRA 92. Higher HRA means harder material — more resistant to surface abrasion, more brittle under shock loading.
A coal tooth at HRA 91 will outlast an HRA 88 grade in a clean, uniform soft coal seam. That same HRA 91 grade will shatter in a seam with frequent calcite intrusions or sandstone bands.
Hardness is the most commonly specified variable — and the most commonly misunderstood. It is a result of grain size and cobalt content, not an independent parameter. You cannot meaningfully specify hardness without also specifying the other two. Any manufacturer who quotes a grade as “HRA 89” without providing grain size and cobalt content is giving you an incomplete specification.
Grain Size (µm): The Toughness-Wear Trade-Off Control
Grain size refers to the average diameter of WC (tungsten carbide) grains in the WC-Co composite matrix. Ranges used in mining applications run from fine (1.0–1.2 µm) through medium-coarse (2.0–3.0 µm).
Fine-grain grades (1.0–1.2 µm):
– Higher hardness at equivalent cobalt content
– Superior wear resistance in low-impact conditions
– Reduced fracture toughness — susceptible to chipping under impact
Coarse-grain grades (2.0–3.0 µm):
– Lower hardness at equivalent cobalt content
– Higher fracture toughness — better crack propagation resistance
– More forgiving under intermittent or high-impact loading
The presence of intercalated rock bands in your seam is the primary determinant of which grain range to use. Clean coal: fine grain. Mixed or banded seams: coarse grain.
Cobalt Content (%): The Binder That Controls Everything Else
Cobalt is the metallic binder in cemented carbide. It holds WC grains together and controls the balance between ductility and hardness.
The relationship is inverse: increasing cobalt from 6% to 12% drops HRA by approximately 3–4 points, but flexural strength rises from roughly 2,000 MPa to 2,800 MPa. More cobalt means tougher material. Less cobalt means harder, more wear-resistant material.
For coal cutter picks in soft-to-medium seams with some impact loading, cobalt content in the 8–12% range is standard. High-impact applications — banded seams, frequent rock encounters, mixed ground — typically require 10–15%.
Cobalt also has one vulnerability specific to underground environments: cobalt leaching. In high-moisture, acidic conditions, the cobalt binder phase dissolves from the surface, leaving a porous, weakened structure. That failure mode looks different from normal abrasive wear — the diagnostic is in the field diagnosis section below.
Seam Conditions and What They Demand from Your Carbide Grade
The coal seam itself is the specification sheet for grade selection. Using the Protodyakonov hardness scale (f-value) common in coal and mining engineering:
| Seam Classification | f-Value | Typical Lithology | Grade Priority | Starting Approach |
|---|---|---|---|---|
| Soft coal, clean seam | f 1–3 | Pure coal, no stone bands | Wear resistance | Fine grain, 6–8% Co, HRA 90–92 |
| Medium coal, minor bands | f 3–6 | Occasional calcite or shale | Balanced | Medium grain, 8–10% Co, HRA 88–90 |
| Hard coal, mixed ground | f 6–9 | Frequent rock intrusions | Toughness | Coarse grain, 10–12% Co, HRA 87–89 |
| Banded / highly abrasive | f 9–12+ | Alternating coal and hard rock | High toughness | Coarse grain, 12–15% Co, HRA 86–88 |
This is a starting framework. The Cerchar abrasivity index of intercalated rock, seam temperature, and water pH all modify the optimal selection. When in doubt, bias toward toughness (higher cobalt, coarser grain): premature fracture is more disruptive to operations than accelerated wear.
Longwall Shearer vs. Roadheader: Different Impact Profiles
Longwall shearers operate at consistent cutting angles with continuous coal engagement. The primary wear mode is abrasive, not impact. Grade selection skews toward wear resistance — fine-to-medium grain, moderate cobalt (8–10%).
Roadheaders cut a wider range of formations and encounter more impact events per cutting cycle. Grade selection skews toward toughness — medium-to-coarse grain, higher cobalt (10–12%). Roadheader picks also see more varied engagement angles than shearer drum picks, which increases bending stress on the tip shoulder.
Ruixin Grade Specifications for Coal Mining Applications
We manufacture three grades used in coal mining, roadheader, and related underground applications:
| Grade | Density (g/cm³) | Hardness (HRA) | Flexural Strength (MPa) | Grain Size (µm) | Best Application |
|---|---|---|---|---|---|
| SR7X | 14.70 ± 0.05 | 91.0 ± 0.5 | ≥ 2,000 | 1.0–1.2 | Clean soft-to-medium seams; high abrasion, low impact |
| SR8C | 14.65 ± 0.05 | 89.0 ± 0.5 | ≥ 2,200 | 2.0–3.0 | Mixed ground, roadheader picks, balanced conditions |
| SR10C | 14.45 ± 0.05 | 88.0 ± 0.5 | ≥ 2,200 | 2.0–3.0 | High-impact; banded seams, hard stone cutting |
SR7X — The Wear-Dominant Grade
SR7X is our hardest mining grade at HRA 91.0, with a fine grain size of 1.0–1.2 µm and density of 14.70 g/cm³. It performs best where abrasive wear is the primary failure mode and impact loading is low or absent. Clean coal seams with f-values below 4, minimal rock intrusion, and consistent cutting conditions are its domain.
Do not specify SR7X for seams with frequent stone bands. The fine grain structure and lower cobalt content make it susceptible to chipping under repeated impact loading.
SR8C — The Roadheader Standard
SR8C is the grade we most commonly recommend as a starting point for roadheader applications and longwall mining in mixed or medium-hard seams. At HRA 89.0 and 2.0–3.0 µm grain size, it balances wear resistance and fracture toughness. Flexural strength at ≥ 2,200 MPa provides meaningful impact resistance for periodic rock encounters.
Most procurement teams selecting coal tooth carbide tips for mixed-formation roadheader applications converge on SR8C after comparative trials. It sits at the practical center of the wear-toughness trade-off for typical underground coal conditions.
SR10C — The Impact-First Grade
SR10C shares the grain size range of SR8C (2.0–3.0 µm) but has lower hardness (HRA 88.0) and equal flexural strength (≥ 2,200 MPa) at lower density (14.45 g/cm³). The density difference reflects higher cobalt content in the binder phase: more impact resistance, less wear performance.
SR10C is the right call when impact loads dominate: banded seams, mixed coal-rock formations, cutting environments where picks encounter hard inclusions regularly. If your operation is replacing tips due to fracturing rather than gradual wear-through, trial SR10C before specifying anything else.
For applications requiring custom grade formulation beyond these three — specific cobalt percentages, tailored grain size distributions, or seam-specific performance targets — send us the application brief. Our engineers work directly from your service conditions.
How to Diagnose Grade Mismatch in the Field
The failure mode tells you which direction to move.
Signs You Need a Tougher Grade (Higher Cobalt, Coarser Grain)
- Tips chip or fracture rather than wearing gradually
- Failure surface shows irregular fracture faces, not smooth wear flats
- Failures concentrate at the tip shoulder rather than the cutting face
- Tips fail inconsistently — some wear, some shatter in the same session
Corrective action: Move down one HRA point. Increase cobalt content and grain size. If on SR7X, trial SR8C.
Signs You Need a Harder Grade (Lower Cobalt, Finer Grain)
- Tips wear through faster than expected with smooth, polished wear flats — no fracturing
- Wear rate is consistent across the drum but tools are lasting fewer cutting meters
- Formation is confirmed soft coal with no stone bands or rock inclusions
Corrective action: Move up one HRA point. Reduce cobalt content and grain size. If on SR8C in clean coal conditions, trial SR7X.
Cobalt Leaching: The Failure Mode That Looks Like Wear
Cobalt leaching is distinct from abrasive wear. Acidic mine water and high moisture cause the cobalt binder to dissolve from the surface layer, leaving a visibly porous, matte texture. The WC grains are still present but unbound — they shed rapidly under load.
If leaching is suspected:
1. Inspect the wear surface under magnification — look for surface porosity and exposed WC grains, not a polished flat
2. Check water spray pH in the cutting zone (target neutral; below pH 6 accelerates leaching)
3. Investigate seal integrity on water-cooling systems — pooling water near picks worsens leaching significantly
Switching grades is secondary. Fixing spray chemistry usually resolves leaching faster than any material change.
Four Procurement Mistakes That Increase Your Cost Per Cutting Meter
1. Specifying HRA only.
Without grain size and cobalt content, the specification is incomplete. Two suppliers can deliver HRA 89 material with radically different fracture behavior. Always request all three parameters.
2. Copying a previous supplier’s grade name without translating specs.
Proprietary grade designations (YG8, K30, and equivalents) do not map directly across manufacturers. What one producer calls YG8 may differ in cobalt content and grain size from another’s. Request the actual HRA, grain size (µm), and cobalt % — then compare those.
3. Evaluating by piece price instead of cost per cutting meter.
A tip priced 15% lower that lasts 30% fewer meters costs more per meter cut. The right metric is: unit price ÷ meters cut per tip. That number requires a field trial to establish, but it’s the only one that reflects real operating cost.
4. Not telling the manufacturer your seam conditions.
Any supplier who quotes a grade without asking about seam hardness, rock band frequency, and cutting equipment type is guessing. We ask. If they don’t, that’s diagnostic information about their engineering capability.
The same three variables — HRA, grain size, and cobalt — govern performance across all cemented carbide applications, not just coal mining. Our cemented carbide properties guide covers the underlying WC-Co metallurgy for readers who want the full picture.
For carbide wear components used elsewhere in mining equipment — liners, strips, rod blanks — see our tungsten carbide wear parts for mining guide.
Get a Grade Recommendation for Your Seam
Selecting the right carbide grade for coal mining is a three-variable problem: HRA hardness, grain size, and cobalt content, matched to your seam’s f-value and impact profile. SR7X for clean, soft-to-medium seams where abrasion dominates. SR8C for mixed ground and roadheader applications. SR10C when fracture is your primary failure mode. Custom formulation when none of the three fits.
Send your application details: seam classification or f-value, rock band frequency, current grade and failure description, and cutting equipment type. Our engineers confirm grade selection and available dimensions within 24 hours.
Email: info@ruixintungstencarbide.com
WhatsApp: +86-15253178777
Ruixin Tungsten Carbide is an ISO 9001:2015 certified manufacturer based in Jinan, Shandong. 14,200 m² production floor. 500 tons/year capacity. We manufacture, not trade.
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