Why Underground Face Drilling Destroys the Wrong Carbide Grade
Underground jumbo drill bits for face drilling fail differently than surface DTH or rotary bits. Using the wrong carbide grade turns a single bit into a recurring replacement cost that compounds across every blast round.
The fundamental difference is mechanical. In underground face drilling, the drill string (rods) transmits impact energy from a hydraulic drifter (Sandvik HF1500, Epiroc COP 1838, or a Boomer rock drill) to a bit that can be 10–20 meters from the power source. The rod string flexes. The bit collars at angles on uneven faces. Flushing is typically water at moderate pressure, not high-pressure air. These conditions create a stress profile that is distinct from surface drilling: higher cyclic impact variation, more bending load on the gauge buttons, and greater risk of thermal shock when water flushing hits a hot bit face.
A mine operator in Western Australia running Sandvik DT820 jumbos through dolerite (UCS 180–220 MPa) learned this the hard way. The in-service bit used a high-hardness carbide grade, approximately HRA 91, optimized for wear resistance in abrasive formations. On paper it should have lasted. In practice, button shear started appearing shift two, and by shift four, every bit had lost at least two gauge buttons. The root cause wasn’t the carbide quality. It was a grade designed for pure abrasion being run in an application where impact toughness was the limiting constraint.
For a system-level diagnosis before changing carbide, continue with the tungsten carbide rod blanks for jumbo drill bits.
The failure isn’t random. It’s the predictable result of mismatching cobalt content and grain size to the specific impact cycle of underground face drilling.
The Technical Variables That Determine Grade Performance for Jumbo Bits
Carbide grade selection for underground jumbo drill bits comes down to three interdependent variables: cobalt content, grain size, and hardness (HRA). Every Ruixin grade is engineered at a specific intersection of these three values, and shifting one shifts the others.
Cobalt content is the toughness governor. Increasing cobalt from 6% to 10% raises flexural strength (Ruixin SR10C at 10% cobalt achieves ≥ 2,200 MPa) but drops HRA from roughly 91 to 88. In jumbo drilling terms, this means a grade that survives repeated impact loads from misaligned collaring and fractured rock is a grade with cobalt at 8% or higher. Below that threshold, the carbide matrix has insufficient ductility to absorb the cyclic shock that underground face drilling generates.
Grain size is the parameter that gets the least discussion but controls the ceiling for both wear resistance and toughness. Ruixin SR7X uses 1.0–1.2 µm grain size for maximum hardness (HRA 91.0). That works in pure abrasion applications like wear parts. But in face drilling, where the bit sees impact every percussion cycle, grain size needs to shift to 2.0–3.0 µm. Both SR8C and SR10C use this range. At this grain size, the WC-Co structure has enough free path in the cobalt binder to absorb crack propagation without allowing catastrophic fracture.
Hardness (HRA) is the outcome of cobalt and grain size together. A higher HRA number means better abrasion resistance but lower impact tolerance. The threshold for underground face drilling is approximately HRA 90: grades above this (like SR7X at HRA 91.0) wear slowly but chip under the compound loading of a jumbo drifter. Grades near HRA 88–89 (SR8C at HRA 89.0, SR10C at HRA 88.0) trade some abrasion resistance for the toughness required to survive a full shift of face drilling without losing buttons.
For underground jumbo face drilling, grain size is the limiting constraint. A grade with sub-micron grain will have excellent edge retention but insufficient toughness for the impact cycle. SR7X at 1.0–1.2 µm is the wrong starting point regardless of its wear resistance advantages in other applications.
Grade Options and Performance Trade-offs for Underground Face Drilling
The table below maps Ruixin’s grades to the specific conditions of underground jumbo drill face drilling. These are not general-purpose ratings. They reflect how each grade performs under the stress profile of hydraulic drifters in production face drilling.
| Grade | Hardness (HRA) | Cobalt (%) | Grain Size (µm) | Flexural Strength (MPa) | Best For | Key Limitation |
|---|---|---|---|---|---|---|
| SR7X | 91.0 ± 0.5 | 6 | 1.0–1.2 | ≥ 2,000 | High-abrasion wear parts; low-impact surface drilling | Too brittle for jumbo impact cycle; button shear risk in fractured rock |
| SR8C | 89.0 ± 0.5 | 8 | 2.0–3.0 | ≥ 2,200 | Medium-hard massive rock (UCS 100–150 MPa); good balance of wear + impact | Wears faster than SR7X in highly abrasive rock; not optimal for blocky fractured ground |
| SR10C | 88.0 ± 0.5 | 10 | 2.0–3.0 | ≥ 2,200 | Fractured/blocky ground; high-impact collaring conditions; UCS > 150 MPa | Lower abrasion resistance; shorter bit life in clean massive rock |
The trade-off explained: SR8C at HRA 89.0 with 8% cobalt sits at the balance point for most underground face drilling. It has enough cobalt to survive the collaring shock and rod whip that characterize jumbo drilling, while the 2–3 µm grain size provides adequate wear resistance for medium-hard rock. SR10C shifts the balance 2% more cobalt toward toughness — which is the right move when the rock is fractured, the face is uneven, or the mine has a history of button shear failures.
The choice isn’t “which grade is better” — it’s “which failure mode does your drilling cycle punish more: gauge button loss or face wear?”
Button Geometry Selection
Grade selection alone doesn’t complete the decision. Button geometry must match the rock type and the drifter’s energy output.
- Spherical buttons: Best for hard brittle rock (UCS > 150 MPa). The uniform contact surface distributes impact stress evenly. Used with SR10C in fractured granite and dolerite.
- Semi-ballistic buttons: A shallower profile that increases penetration rate in medium-hard rock. Used with SR8C for massive limestone or andesite at UCS 100–150 MPa.
- Conical buttons: Reserved for very soft or friable formations where maximum penetration speed is the priority. Rare in jumbo face drilling.
Ruixin manufactures spherical carbide buttons in both SR8C and SR10C grades, with dimensional tolerances designed to match Sandvik and Epiroc bit body specifications.
Which Grade to Use — and Under What Conditions
The decision filter for underground jumbo drill face drilling follows three conditions:
Condition 1: Massive medium-hard rock with consistent ground (UCS < 150 MPa, minimal fracturing)
→ Use SR8C at HRA 89.0, 8% cobalt, spherical or semi-ballistic button geometry.
→ Ruixin SR8C delivers the wear ceiling needed for full-shift production without excessive button loss. The 2–3 µm grain structure maintains a stable wear rate across the button face.
→ See Ruixin SR8C grade specs for spherical carbide buttons
Condition 2: Fractured, blocky, or seamy ground with frequent collaring on uneven faces
→ Use SR10C at HRA 88.0, 10% cobalt, spherical button geometry.
→ The extra 2% cobalt absorbs the spike loads from collaring on stone and passing through discontinuities. In a case at a mine with f=14–16 fractured granite, switching from a medium-cobalt grade to Ruixin SR10C reduced gauge button loss by 35% over a 6-week trial.
Condition 3: High-abrasion rock with low fracturing (quartzite, massive sandstone)
→ Evaluate SR7X only if the drifter setup runs low impact energy. Most COP and Sandvik drifters deliver percussion energy that exceeds SR7X’s impact tolerance. For these conditions, SR8C with a tighter 2.0 µm grain specification remains the safer starting point.
For most jumbo face drilling setups with Sandvik, Epiroc, or Boomer drifters, SR8C is the starting point. SR10C is the adjustment when ground conditions add impact severity.
How to Implement This in Your Operation
Grade selection is one part. Integration into the bit body and drifter setup determines whether the theoretical advantages appear in actual meters drilled.
Bit body compatibility: Ruixin SR8C and SR10C carbide buttons are manufactured with a dimensional tolerance of ±0.05 mm on button diameter and protrusion height. This ensures consistent interference fit in the bit body. Loose buttons fail by rotation or fall-out within the first 20 meters of drilling, regardless of the grade’s metallurgical quality.
Flushing optimization: Underground face drilling typically uses water flushing at 8–12 bar. Inadequate flushing at the button face causes thermal fatigue, especially in harder grades where heat conduction through the carbide is slower. SR10C with its higher cobalt content has marginally better thermal conductivity (about 15% higher than SR7X), which helps dissipate face heat in water-flushed applications.
Batch consistency: Ruixin ships every production batch with a material test report covering density, HRA hardness, and flexural strength. For a mine running 12 jumbos across three shifts, consistent carbide batch quality in cobalt content (±0.3%) and grain size distribution means every bit drills predictably with no sudden performance drop between reorders.
If your ground conditions include mixed rock types, water ingress at the face, or non-standard drifter configurations (hydraulic vs. pneumatic, high-frequency vs. low-frequency percussion), a custom grade formulation may be needed. Ruixin’s R&D collaboration with Central South University supports grade adjustment at the cobalt content and grain size levels. These are the same variables that determine performance in your specific drilling cycle.
For a deeper foundation on how cemented carbide grades work across applications, read the cemented carbide guide on cobalt content vs. grain size tradeoffs.
Frequently Asked Questions
How do I choose the right carbide grade for jumbo drill bits in underground face drilling?
Start by quantifying two variables: rock abrasiveness (Mohs hardness, Cerchar index) and rock fracturing (joint spacing, UCS). For medium-hard massive rock with UCS below 150 MPa and low fracturing, Ruixin SR8C at HRA 89.0 with 8% cobalt provides the best balance of wear resistance and impact toughness. For fractured ground, blocky rock, or UCS above 150 MPa, switch to SR10C at HRA 88.0 with 10% cobalt. Match button geometry to rock type: spherical for hard brittle rock, semi-ballistic for medium rock.
What is the difference between SR8C and SR10C for jumbo drill applications?
SR8C uses 8% cobalt binder with 2.0 to 3.0 micron grain size at HRA 89.0, flexural strength above 2,200 MPa. It works best in medium-hard rock with consistent ground conditions. SR10C uses 10% cobalt with identical grain size at HRA 88.0 and flexural strength above 2,200 MPa. The extra cobalt adds impact toughness, which matters in fractured or blocky ground where the bit encounters sudden stress spikes during collaring and drilling through discontinuities.
Which Ruixin grade performs best under high-impact conditions in face drilling?
For high-impact conditions typical of fractured or blocky ground in underground face drilling, Ruixin SR10C at HRA 88.0 with 10% cobalt is the recommended grade. The higher cobalt content absorbs the shock loads from misaligned collaring and passing through rock discontinuities. In a test case on a mine with f=14–16 fractured granite, SR10C showed button shear rates 35% lower than the medium-cobalt grade previously used.
How does cobalt content affect carbide performance in underground jumbo drill bits?
Cobalt content directly controls the hardness-to-toughness tradeoff. Higher cobalt (10%) increases flexural strength and impact energy absorption but lowers HRA hardness, accelerating wear in high-abrasion rock. Lower cobalt (6%) raises HRA for better wear resistance but reduces the grade’s ability to survive impact loads. For underground face drilling where rod whip and collaring misalignment add cyclic impact stress, 8% cobalt (SR8C) is the standard middle point, with 10% (SR10C) reserved for high-impact ground conditions.
What causes premature carbide button failure on jumbo drill bits?
The most common failure modes are button shear from misaligned collaring, skirt wear from rod whip, and thermal fatigue from inadequate flushing. Button shear typically occurs when the bit is collared at an angle, concentrating impact stress on one side of the carbide insert. Skirt wear accelerates when rod whip causes the bit to vibrate against the borehole wall. Ruixin SR10C with its HRA 88.0 and 10% cobalt content offers the best resistance to button shear in fractured ground, while SR8C provides better skirt wear resistance in consistent medium rock.
What button geometry works best with which carbide grade for jumbo drilling?
For spherical buttons, the most common geometry for hard rock face drilling, Ruixin SR10C provides the impact toughness needed for hard, fractured ground. Semi-ballistic buttons penetrate faster in medium rock and pair optimally with SR8C for UCS 100–150 MPa ground. The geometry choice matters because a spherical button on SR8C in medium rock may under-penetrate, while a semi-ballistic button on SR10C in hard rock may chip at the profile edge.
How does underground jumbo drilling differ from DTH in carbide requirements?
Underground face drilling with hydraulic drifters produces a different stress profile than DTH: shorter rod strings create more bending load on the bit, collaring on uneven faces generates asymmetrical impact loading, and the feed force (12–18 kN for COP drifters) combined with rotation produces a scrubbing wear mode on gauge buttons. For DTH drilling, see the full guide on carbide buttons for DTH drill bits.
Get a Custom Grade Recommendation
The right carbide grade for your underground jumbo drill bits depends on rock type, drifter model, and ground conditions. A grade that works in one mine’s face may underperform in another’s.
Send us your application details: rock type and UCS, machine model and drifter type (Sandvik HF1500, Epiroc COP 1838, or equivalent), current bit dimensions and button geometry, and photos of failed buttons if available. Our engineers will confirm grade selection (SR8C, SR10C, or a custom formulation), button geometry fit, and available dimensions within 24 hours.
Email: info@ruixintungstencarbide.com
WhatsApp: +86-15253178777
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