Rotary Drilling Carbide Bits — Grade & Rock Selection Guide



Why the Wrong Grade Destroys Rotary Drilling Bits Faster Than Wear

A foundation drilling contractor running a Bauer rig in mixed ground: sandstone interbedded with granite boulders switched to a high-hardness carbide button to reduce wear on the gauge row. Within 80 hours of drilling, buttons on the inner cone began spalling. The grade was HRA 91 or higher, chosen for abrasion resistance. The failure mode wasn’t wear — it was fracture from impact loads the grade couldn’t absorb.

For a system-level diagnosis before changing carbide, continue with the rod blanks for end mills and drills rotary drilling bits.

This is the most common mistake in rotary drilling carbide bits selection: choosing for one failure mode while the application punishes the other. In a drill string rotating at 25–60 RPM with up to 30 tons of weight on bit, the carbide insert experiences compression, bending, and impact simultaneously. The grade that resists abrasion best is often the grade that fractures first under impact.

The root variable that determines success or failure isn’t price, brand, or even button diameter: it’s the cobalt-to-grain-size ratio matched to rock compressive strength and impact frequency.

The Technical Variables That Determine Rotary Drilling Carbide Bits Performance

Three interdependent variables control how a rotary drilling carbide bits grade performs in a given formation: hardness (HRA), cobalt binder content (%), and tungsten carbide grain size (µm). Each one shifts the wear-toughness balance in a predictable direction.

Hardness (HRA) Sets the Abrasion Ceiling

The HRA scale for cemented carbide grades runs from approximately 87.0 to 93.0. For rotary drilling, the useful range is narrower: 88.0 to 91.5. Below HRA 88.0, the carbide matrix wears rapidly in any formation with quartz content above 20%. Above HRA 91.5, impact toughness drops below what most drilling applications can tolerate.

Ruixin SR7X at HRA 91.0 ± 0.5 sits at the upper end of this range. It resists abrasive wear longer than any other grade in our lineup, but it also has the lowest tolerance for impact. The trade-off is measurable: SR7X’s hardness comes from its fine grain structure (1.0–1.2 µm) and cobalt content (approximately 6%), producing a dense matrix that resists particle erosion, but at the cost of flexural reserve.

Cobalt Content Controls Impact Absorption Capacity

Cobalt is the ductile binder that holds tungsten carbide particles together. When an impact load hits the cutting face, the cobalt phase deforms plastically and absorbs energy. Higher cobalt means higher toughness, but lower hardness.

The relationship is inverse and roughly linear: increasing cobalt from 6% to 10% drops HRA from approximately 91.0 to 88.0, while flexural strength climbs from ≥ 2,000 MPa to ≥ 2,200 MPa. For rotary drilling carbide bits, the practical implication is straightforward:

• 6–7% cobalt (SR7X range): Hard, wear-resistant, low impact tolerance. Best for continuous cutting in abrasive non-impacting formations.
• 8% cobalt (SR8C range): Balanced wear-toughness profile. Handles moderate impact with acceptable abrasion resistance.
• 10% cobalt (SR10C range): Maximum toughness. Absorbs shock from fractured ground, boulders, and hard rock inclusions.

Grain Size Determines the Wear Mechanism

Grain size is the least discussed but most operationally relevant variable in rotary drilling carbide bits performance. At 1.0–1.2 µm (SR7X), the carbide structure is fine and dense. The wear surface stays sharp longer because individual WC particles are harder to dislodge. At 2.0–3.0 µm (SR8C, SR10C), the coarser structure provides more path length for cracks to travel, improving fracture resistance but accelerating matrix erosion in highly abrasive slurry.

The threshold worth remembering: if the rock’s Cerchar Abrasivity Index (CAI) exceeds 4.0, a grain size above 2.5 µm will cause measurable cobalt washout within 100 drilling hours, regardless of HRA. For formations above CAI 4.0, a fine-grain grade like SR7X is the safer starting point, provided impact frequency is low.

For rotary drilling carbide bits operating in medium-hard formations, grain size and cobalt content are the two levers Ruixin customizes most frequently in custom grade formulations.

Grade Options and Performance Trade-offs

The following table maps Ruixin’s three standard grades against the working conditions most common in rotary and DTH drilling. Each grade is designed for a specific window of rock compressive strength and impact severity.

Application Scenario	Recommended Grade	Key Parameters	Why This Grade
High-abrasion sandstone, shale, or limestone (CAI 3.0–5.0, low impact, < 80 MPa UCS)	SR7X	HRA 91.0, 1.0–1.2 µm grain, ≥ 2,000 MPa flexural strength, 14.70 g/cm³	Fine grain and high HRA resist quartz abrasion; the low cobalt content minimizes cobalt washout in aerated drilling fluids; typical bit life extension of 30–40% over HRA 89 grades in this formation
Medium-hard granite or diorite (CAI 2.5–4.5, moderate impact, 80–150 MPa UCS)	SR8C	HRA 89.0, 2.0–3.0 µm grain, 8% cobalt, ≥ 2,200 MPa flexural strength	Balanced HRA and 8% cobalt absorb intermittent impact loads while maintaining acceptable wear progression; standard choice for Bauer, Liebherr, and Soilmec rotary rigs in mixed ground
Hard fractured rock, boulders, or quartzite (CAI 4.0–5.5, high impact, > 150 MPa UCS)	SR10C	HRA 88.0, 2.0–3.0 µm grain, 10% cobalt, ≥ 2,200 MPa flexural strength	Maximum cobalt content provides the highest impact absorption; designed for formations where SR8C would chip within 50 drilling hours; preferred for DTH applications in 150+ MPa granite

When SR7X Underperforms

SR7X is not a universal grade. In a documented case from a Middle Eastern drilling operation, a contractor used SR7X spherical buttons in granite with a compressive strength of 160 MPa. Buttons began chipping after 30 hours. The failure was not material quality: it was a grade-to-formation mismatch. SR7X at HRA 91.0 lacks the cobalt reserve to absorb the impact energy of a 150 MPa rock face. Switching to Ruixin SR10C on the same rig extended button life to 95 hours before the first measurable wear ring appeared.

When SR10C Is Over-Specified

Conversely, specifying SR10C for a low-abrasion sandstone formation at 60 MPa gives away wear life for toughness the application does not need. In pure abrasion conditions, SR10C at HRA 88.0 wears measurably faster than SR7X, typically 20–35% shorter service life, because the softer cobalt matrix erodes more readily, exposing WC grains to premature dislodging.

The right choice depends on which failure mode your actual drilling conditions punish more: abrasion or fracture. Ruixin’s grade selection process for rotary drilling carbide bits always begins with this question.

Wrong Grade Consequences: What Poor Selection Actually Costs

A mismatch between grade and formation doesn’t just reduce bit life; it changes the economics of the entire drilling operation. When you select the wrong rotary drilling carbide bits grade, here are the specific, quantified consequences:

1. Tip life drops by 30–50% when a high-toughness grade (SR10C) is used in abrasive sandstone. The soft cobalt matrix erodes faster than the wear rate the formation demands, forcing bit changes 1.5 to 2 times more frequently than necessary.

2. Replacement frequency doubles when a high-hardness grade (SR7X) is used in impact-prone fractured ground. Chipping and spalling begin within the first shift, and by hour 60 the gauge row buttons are no longer cutting: they’re grinding on a fractured face.

3. Cost per meter rises 20–35% because the cost of the carbide button itself is only 40–50% of the total bit cost. The real expense is rig downtime: mobilizing a crane, changing the bit, re-entering the hole. A 50% reduction in bit life doubles the downtime cost per meter of hole drilled.

4. Drilling rate penalty of 15–25% occurs when a worn or chipped grade forces the operator to reduce weight on bit and RPM to prevent further damage. The drill string runs slower, penetration rate drops, and the project schedule extends.

The failure isn’t random: it’s the predictable result of a cobalt-HRA-grain size combination that does not match the formation’s compressive strength and abrasivity index.

Which Grade to Use: Matching by Rock Condition

Selecting rotary drilling carbide bits for a specific rock formation requires matching grade variables to the formation’s compressive strength and abrasivity. The following conditional logic applies:

If rock compressive strength is below 80 MPa and CAI is below 3.0 (soft shale, claystone, weathered sandstone):
Use SR7X at HRA 91.0 with spherical button geometry. Impact loads are minimal in these formations, so the hardness ceiling of SR7X delivers maximum footage per bit. The fine 1.0–1.2 µm grain structure resists the fine abrasion typical of clay-rich drilling mud.

If rock compressive strength is 80–150 MPa with intermittent impact (granite, diorite, basalt, moderate jointing):
Use SR8C at HRA 89.0 with 2.0–3.0 µm grain and 8% cobalt. This is the standard recommendation for most rotary drilling applications in medium-hard rock. SR8C’s flexural strength of ≥ 2,200 MPa handles the impact of an occasional boulder or hard inclusion. For most foundation drilling rigs (Bauer, Liebherr, Soilmec), this is the starting grade.

If rock compressive strength exceeds 150 MPa with high impact frequency (massive granite, quartzite, fractured chert):
Use SR10C at HRA 88.0 with 10% cobalt. Impact resistance, not wear resistance, is the limiting factor in these conditions. Spherical button geometry is mandatory: ballistic or chisel profiles will fracture on initial contact. SR10C maximizes impact cycles before failure, even at the cost of accelerated wear.

If conditions are mixed and unpredictable (riverbed drilling with boulders, glacial till, urban construction ground):
Use SR8C for the cone and gauge rows, with SR10C confirmed for the gauge row if boulder strikes exceed 5 per hour. A mixed-grade bit, where inner buttons use a different grade than gauge buttons, can extend total bit life by 15–20% compared to a single-grade layout.

For most operations, SR8C is the starting point because its balanced HRA-cobalt profile covers the widest drilling window. Ruixin’s standard recommendation for first-time buyers of rotary drilling carbide bits is SR8C, with a review after the first 100 drilling hours to confirm wear patterns match expectations. See our carbide cutter bits for rotary drilling page for available dimensions and button configurations.

How to Implement This in Your Operation

Grade selection is only one half of the equation. The button geometry, seating design, and brazing process all affect how your rotary drilling carbide bits perform on the rig.

Button Geometry by Formation

• Spherical buttons: Highest impact resistance. Use in hard rock (> 100 MPa UCS) and when impact frequency is moderate to high. Ruixin recommends spherical geometry for all SR7X and SR10C applications in hard formation drilling.
• Parabolic / semi-ballistic buttons: Faster penetration but lower impact tolerance. Use in soft to medium formations (< 80 MPa UCS) where penetration rate is the primary KPI.
• Flat-top / conical buttons: Specialized for very soft ground. Not recommended for rock drilling.

Seating and Brazing Compatibility

The carbide button must be press-fit or brazed into a steel bit body. Thermal expansion mismatch between carbide (approximately 5 × 10⁻⁶ /K) and steel (approximately 12 × 10⁻⁶ /K) means that excessive brazing temperatures above 700°C can induce micro-cracks in the carbide. These cracks propagate during drilling and shorten button life regardless of grade quality. Ruixin verifies batch thermal tolerance for every grade shipment and can provide brazing parameter recommendations with each order.

Batch Consistency Verification

B2B procurement of rotary drilling carbide bits carries a hidden risk: the sample batch performs well, but the production batch drifts on critical specs. The root cause is typically raw material variance (WC powder source changes) or sintering parameter drift. Every Ruixin shipment includes a material test report with density, HRA, and flexural strength measured per batch. If a supplier cannot provide these three values per lot, the incoming quality variance alone can cost 15–25% in service life spread across a procurement run.

For a deeper explanation of how cobalt content and grain size interact across the full cemented carbide spectrum, read our education guide on cemented carbide grade selection.

If your drilling conditions fall outside the parameters above (softer rock requiring a lower-cobalt formulation, non-standard button diameter, or a volume commitment requiring batch-level consistency across 12+ months), a custom grade formulation may be needed. Ruixin can adjust cobalt content by ±1.5% and grain size to 0.5 µm resolution within standard production cycles. As a factory-direct ISO-certified carbide manufacturer with 500 tons annual capacity, we can deliver custom formulations with the same batch-level QC as our standard grades.

Frequently Asked Questions

How do I choose the right carbide grade for rotary drilling bits?

Rock abrasiveness and impact frequency are your two inputs. For abrasive rock with low impact (sandstone below 100 MPa), use a high-HRA grade like Ruixin SR7X (HRA 91.0). For high-impact hard rock (granite above 150 MPa), use a tougher grade like Ruixin SR8C (HRA 89.0, 8% cobalt) or SR10C (HRA 88.0, 10% cobalt). The correct match is determined by whether your primary failure mode is wear or fracture.

What is the difference between SR7X and SR8C for drilling applications?

Ruixin SR7X has a hardness of HRA 91.0 and grain size of 1.0–1.2 µm, optimized for high wear resistance in low-impact abrasive formations. SR8C has HRA 89.0 with 8% cobalt and grain size 2.0–3.0 µm, offering better impact toughness. In a typical medium-hard rock application, SR8C resists chipping where SR7X would fracture under repetitive impact loading.

Which grade performs best under high-impact conditions in rotary drilling?

Ruixin SR10C with HRA 88.0, 10% cobalt, and 2.0–3.0 µm grain size is the best choice for high-impact rotary drilling conditions. The higher cobalt content provides the flexural strength needed to absorb shock loads without fracture. SR10C is recommended for formations with intermittent hard rock inclusions, boulders, or fractured ground where impact cycles exceed 500 per shift.

How does cobalt content affect carbide performance in rotary drilling bits?

Cobalt content directly controls the toughness-versus-wear-resistance trade-off. Increasing cobalt from 6% to 10% drops HRA from approximately 91.0 to 88.0, but flexural strength rises from 2,000 to 2,400 MPa. For rotary drilling bits, higher cobalt (8–10%) provides impact resistance in hard rock, while lower cobalt (6%) extends bit life in abrasive but non-impacting formations like sandstone or shale.

What causes premature carbide tip failure in rotary drilling?

The most common cause is grade-to-formation mismatch. Using a high-hardness low-cobalt grade in impact-prone hard rock causes chipping and spalling, the most frequent failure in rotary drilling carbide bits used above their impact threshold. Using a tough high-cobalt grade in highly abrasive sandstone accelerates wear by 30–50%. Other causes include incorrect button geometry (spherical vs. ballistic), excessive rotation speed generating heat above 600°C causing cobalt washout, and batch-to-batch consistency issues in the carbide structure.

How does rock formation abrasiveness affect carbide grade choice?

Rock abrasiveness measured by the Cerchar Abrasivity Index (CAI) directly dictates the minimum HRA required. For CAI below 2.0 (low abrasion like clay or shale), HRA 88.0 grades are sufficient. For CAI 2.0–4.0 (medium abrasion like limestone or sandstone), HRA 89.0–91.0 is required. For CAI above 4.0 (highly abrasive like quartzite or granite), a fine-grain grade like Ruixin SR7X at HRA 91.0 with 1.0–1.2 µm grain is typically needed to resist rapid matrix erosion.

What is the difference between spherical and ballistic carbide buttons for drilling?

Spherical buttons provide the highest impact resistance and are preferred for hard abrasive rock where fracture risk is high. They distribute stress more evenly across the button face. Ballistic (chisel) buttons offer faster penetration rates in soft to medium formations but are more susceptible to breakage in hard rock. For rotary drilling bits, Ruixin recommends spherical carbide buttons in SR8C or SR10C grade for rock above 100 MPa compressive strength.

Get a Custom Grade Recommendation

Send us your application details: rig model, rock type (UCS and CAI if available), current bit configuration and button diameter, and the failure mode you’re seeing (wear, chipping, or both). Our engineers will recommend the matching rotary drilling carbide bits grade and confirm available dimensions within 24 hours.

Email: info@ruixintungstencarbide.com
WhatsApp: +86-15253178777

We manufacture in-house on a 14,200 m² production floor with up to 500 tons annual capacity. Every batch is ISO-certified and shipped with a material test report covering density, HRA, and flexural strength. Factory-direct means the people setting the sintering parameters are the same people answering your email.