Shield Machine Carbide Tips — Soft & Mixed Ground Guide



Why Soft Ground and Mixed-Face Tunneling Destroy the Wrong Shield Machine Carbide Tips

Shield machine tunneling conditions vary more widely than any other excavation method, from saturated clays in EPB mode to boulder-filled mixed face and full hard rock. Using the wrong shield machine carbide tips in these conditions doesn’t just shorten tool life; it causes unpredictable failure that stops a ring-building cycle and costs hours of downtime per changeout.

The most common mistake is treating all shield machine picks as interchangeable. A grade that survives 200 meters in soft clay might fracture within 10 meters when the cutter head encounters a boulder seam. The root cause is never “bad carbide” — it’s a mismatch between the grade’s cobalt content and the actual impact load the tip receives.

In EPB tunneling through soft ground, abrasive wear from sand and silt particles is the primary failure mode. The tip erodes gradually until it loses cutting efficiency, requiring a planned pick change. In mixed-face conditions, where the cutter head simultaneously encounters soft soil and rock inclusions, the failure mode shifts to impact fracture. The tip chips or shatters on contact with a cobble or boulder, and replacement becomes reactive rather than scheduled.

A metro tunneling contractor in Southeast Asia contacted us after their existing supplier’s picks lasted only 40 meters in mixed ground with 30% cobble content. The grade they were running had HRA above 91, optimized for pure abrasion resistance — but in those ground conditions, tips fractured before they could wear. The failure wasn’t random; it was the predictable result of selecting a wear-optimized grade for an impact-dominated environment.

The Technical Variables That Determine Shield Machine Carbide Tips Performance in Tunneling

Selecting the right shield machine carbide tips comes down to three interconnected variables: hardness (HRA), cobalt binder content, and grain size. These three numbers define whether a tip will wear slowly or fracture early in your specific ground conditions.

Hardness (HRA) — The Wear Resistance Ceiling

For tunneling applications, the relevant hardness range is HRA 88–92. Higher HRA means better resistance to abrasive wear from sand and silt particles, which is critical in EPB tunneling through sandy soils. But hardness comes at a cost: higher HRA grades are more brittle and fracture more easily under impact loads.

Ruixin SR7X at HRA 91.0 ± 0.5 delivers the highest wear resistance in our tunneling range, but its 6% cobalt content and 1.0–1.2 µm grain size make it unsuitable for any application with impact risk. SR8C at HRA 89.0 ± 0.5 trades some hardness for impact resistance. SR10C at HRA 88.0 ± 0.5 prioritizes toughness for high-impact conditions.

Cobalt Content (%) — The Impact Resistance Lever

Cobalt is the metallic binder in cemented carbide. Higher cobalt content increases the grade’s ability to absorb impact energy without crack propagation. The relationship is inverse: increasing cobalt from 6% to 10% drops HRA by approximately 3 points but raises flexural strength from 2,000 to 2,200 MPa.

For shield machine picks, cobalt content between 8% and 10% covers the vast majority of underground conditions. Below 8% cobalt, the grade becomes too brittle for any face with rock inclusions. Above 10% cobalt, the trade-off in wear resistance is usually too steep for tunneling economies.

Grain Size (µm) — The Microstructure Factor

For tunneling carbide, grain size is often the deciding variable — even when cobalt content is matched. At 1.0–1.2 µm (SR7X), the fine microstructure produces maximum hardness and edge retention, but cracks propagate easily through the dense structure. At 2.0–3.0 µm (SR8C, SR10C), the coarser grains create a tougher matrix that arrests crack growth under impact.

A grade with the same cobalt content but finer grain size will always be harder and more wear-resistant, and always more impact-sensitive. For soft ground tunneling where impact loads are minimal, finer grain sizes work well. For mixed ground tunneling with unpredictable rock content, 2.0–3.0 µm grain size is the standard.

Exclusive Data Point — Ruixin Field Observation: In a comparative test across 1,200 meters of mixed-face tunnel in silty clay with 15% sandstone inclusions, Ruixin SR8C (HRA 89.0, 8% Co, 2.0–3.0 µm) recorded a tip survival rate of 94% through the sandstone zones, versus 61% for a finer-grain HRA 91.5 grade from the same cobalt class. The grain size differential, not hardness, was the deciding variable in this case.

Shield Machine Carbide Tips: Grade Options and Performance Trade-offs

The decision between available grades isn’t about finding a “best” grade — it’s about matching the grade’s spec profile to your dominant ground condition and secondary failure risk.

Grade Selection Table for Shield Machine Tunneling

Application Scenario	Recommended Grade	Key Parameters	Why This Grade
Soft ground EPB tunneling (sands, clays, silts, no rock)	Ruixin SR8C	HRA 89.0 ± 0.5, 8% Co, 2.0–3.0 µm grain, ≥2,200 MPa flexural strength	Abrasive particles in EPB slurry wear tips gradually; SR8C’s 8% cobalt provides enough toughness for occasional gravel without sacrificing the wear resistance needed for 300+ meter runs between changes
Mixed-face tunneling (soft soil + cobbles/boulders up to 30%)	Ruixin SR8C	HRA 89.0 ± 0.5, 8% Co, 2.0–3.0 µm grain, ≥2,200 MPa flexural strength	The balanced spec handles both abrasive soil wear and intermittent boulder impacts; 2.0–3.0 µm grain arrests crack propagation from cobble strikes
High-impact mixed face (>30% boulders, hard rock inclusions)	Ruixin SR10C	HRA 88.0 ± 0.5, 10% Co, 2.0–3.0 µm grain, ≥2,200 MPa flexural strength	Higher cobalt binder (10%) absorbs shock loads from large boulders; trade-off is ~1 point lower HRA, which is acceptable when impact fracture is the primary failure mode
Hard rock TBM (continuous rock, minimal soil)	Ruixin SR7X	HRA 91.0 ± 0.5, 6% Co, 1.0–1.2 µm grain, ≥2,000 MPa flexural strength	Continuous rock cutting is pure abrasion; SR7X’s fine grain and high HRA maximize wear resistance; only appropriate where impact frequency is near zero

When the Wrong Grade Costs You

Selecting the wrong grade is not theoretical; it has measurable financial consequences. Here are the specific penalties:

1. Tip life drops 30–50% in mixed ground when using a high-HRA, low-cobalt grade (HRA > 90, Co < 7%) instead of SR8C. The tips chip rather than wear, and effective cutting stops when even one tip in four is fractured.

2. Replacement frequency doubles in EPB tunneling when using an overly tough grade (Co > 10%) in purely abrasive soft ground. The tips don’t fracture, but they wear out 40–60% faster than a balanced grade, requiring a mid-ring change that costs 2–4 hours of production time.

3. Cost per meter rises 20–35% when grade mismatch forces reactive pick changes. Planned changes during scheduled maintenance cost only labor and parts. Unplanned changes during ring building require stopping the advance, retracting the head, and performing the swap under compressed conditions. Each event costs materials plus 3–6 hours of machine time.

4. Gauge zone failure accelerates when the wrong grade is used at the cutter head periphery. Gauge picks experience the highest cutting forces and the most abrasive particle flow. A grade too brittle for the impact load at gauge will fail within 20 meters, and changing gauge picks is the most labor-intensive maintenance operation on a shield machine.

Which Shield Machine Carbide Tips to Use, and Under What Conditions

The selection logic for shield machine carbide tips follows a simple conditional structure. Classify your ground, and the grade choice narrows immediately.

Decision Filter: Ground Condition → Carbide Grade

If your tunnel is soft ground with no rock inclusions (EPB mode in sands, clays, or silts):
Use Ruixin SR8C at HRA 89.0. The 8% cobalt content provides adequate toughness for occasional gravel seams, while the 2.0–3.0 µm grain delivers predictable wear rates that allow you to schedule pick changes. In purely abrasive soft ground without impact risk, SR8C delivers 300–500 meters between changes depending on sand content.

If your tunnel is mixed face with 10–30% cobbles or boulders:
Use Ruixin SR8C, but verify your pick arrangement. The critical spec here is the 2.0–3.0 µm grain size, which prevents crack propagation from intermittent boulder strikes. For mixed-face conditions with cobble content above 20%, some operators move to SR10C at the gauge zone while keeping SR8C in the center.

If your tunnel has high boulder content (>30%) or irregular hard rock inclusions:
Switch to Ruixin SR10C at HRA 88.0 with 10% cobalt. The additional cobalt binder absorbs higher impact energy. The trade-off is a measurable reduction in wear life in the soil zones, but preventing fracture is the priority when replacement is driven by breakage rather than wear.

If your TBM operates in continuous hard rock (not a shield machine application, but related):
Use Ruixin SR7X at HRA 91.0. This is a wear-optimized grade and should not be used in any ground with impact risk. It belongs in full-face hard rock TBM applications where the cutting action is pure abrasion.

The threshold here is HRA 90: grades above this are wear-optimized and will fracture in any ground with rock inclusions. Grades at HRA 88–89 with 8–10% cobalt (SR8C, SR10C) are the working range for shield machine picks. If your ground has any uncertainty, err toward SR10C. A slightly faster wear rate is cheaper than an unplanned fracture stop.

For most shield machine tunneling setups, SR8C is the starting point. See the full shield machine carbide tips product page for available dimensions, brazing specifications, and lead times.

How to Implement the Right Grade in Your Operation

Selecting the correct grade is step one. Ensuring it performs as expected in your specific machine and ground conditions requires attention to implementation.

Pick Arrangement and Grade Zoning

In mixed-face conditions, consider grade zoning. Use SR10C at the gauge zone (peripheral picks) where cutting forces peak and impact risk is highest; use SR8C in the center and face picks where abrasion from soil particles dominates. This hybrid approach balances cost and performance — you pay for the higher-cobalt grade only where you need it.

Brazing and Attachment Quality

The interface between the carbide tip and the steel pick body is a common failure point. SR8C and SR10C both braze well with standard silver-based filler alloys, but ensure your braze temperature stays below 700°C to avoid thermal degradation of the cobalt binder. Ruixin can provide brazing recommendations for each grade upon request.

Batch Consistency Verification

When ordering shield machine carbide tips in production volumes, request a Material Test Report with every batch. Each batch should show density (g/cm³), HRA hardness, and flexural strength (MPa). Batch-to-batch variance in density should stay within ±0.05 g/cm³. If it exceeds this, the sintering process has drifted and the mechanical properties have changed.

For a deeper understanding of how grade parameters interact, read our TBM carbide cutting tools guide, which covers formation hardness matching with additional field data. If your conditions involve intermittent hard rock, our coal tooth carbide tips page covers similar impact-tough grades for shearer and roadheader applications.

If your conditions fall outside the parameters above — a specific Cerchar abrasivity index, unusual boulder size distribution, or a tunneling method that differs from standard EPB — a custom carbide for roadheader tunneling grade formulation may be needed. Ruixin can adjust cobalt content by ±2% and grain size across the 0.8–4.0 µm range to match your specific ground profile.

Frequently Asked Questions

How do I choose the right carbide grade for shield machine tunneling applications?

Start by classifying your ground conditions: soft ground (sands, clays, silts) demands high toughness grades like Ruixin SR10C at HRA 88.0 with 10% cobalt; mixed face conditions require balanced grades like SR8C at HRA 89.0 with 8% cobalt and 2.0–3.0 µm grain size. The limiting factor is the hardest material your cutter head will encounter. Match your tip grade to that ceiling, not the average formation.

What is the difference between SR8C and SR10C for tunneling applications?

SR8C delivers HRA 89.0 with 8% cobalt and 2.0–3.0 µm grain size, balancing wear resistance and impact toughness for mixed-face conditions. SR10C uses 10% cobalt at HRA 88.0 with the same grain structure, prioritizing impact resistance over abrasion resistance. In soft ground EPB tunneling, SR10C resists chipping from boulder impacts; in mixed ground with sandstone or gravel, SR8C provides longer service life between changes.

Which grade performs best under high-impact conditions in shield machine tunneling?

For high-impact conditions with boulders, cobbles, or hard rock inclusions, Ruixin SR10C at HRA 88.0 with 10% cobalt and flexural strength above 2,200 MPa is the recommended choice. Its higher cobalt binder content absorbs impact energy without propagating cracks. Under severe impact cycles with intermittent hard rock, using SR8C (HRA 89.0, 8% cobalt) instead of SR10C increases tip fracture rates by 30–50%.

How does cobalt content affect carbide performance in shield machine picks?

Cobalt content is the primary lever for toughness in cemented carbide. Higher cobalt (10% in SR10C) increases flexural strength and impact resistance but lowers HRA hardness. Lower cobalt (6% in SR7X) maximizes wear resistance but reduces shock tolerance. The relationship is inverse: raising cobalt from 6% to 10% drops HRA from 91.0 to 88.0 while flexural strength rises from 2,000 to 2,200 MPa. For shield machine picks, 8–10% cobalt is the standard range for most underground conditions.

What causes premature carbide tip failure in shield machine cutter heads?

The most common cause is grade mismatch: using a wear-optimized grade (high HRA, low cobalt) in high-impact ground results in chipping or catastrophic fracture rather than gradual wear. Other causes include uneven loading from poor pick arrangement, abrasive particles at the gauge zone accelerating edge wear, and thermal fatigue from interrupted cutting in mixed ground. Ruixin recommends sending wear pattern photos and ground condition data to identify the specific failure mechanism.

Get a Custom Grade Recommendation

Every tunnel is different. Ground conditions vary by meter, and the optimal carbide grade for your project depends on formation data, machine parameters, and operating pressure. Send us your application details: tunnel diameter, ground conditions (with soil type, cobble percentage, and rock strength if known), machine model, and current tip specifications. Our engineers will confirm the correct Ruixin grade and available dimensions within 24 hours.

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

Factory-direct pricing. Custom grade formulation available. Batch Material Test Reports included with every shipment.