Blast-Hole Drilling in India's Open-Pit Mines
India is the world's second-largest coal producer, and the overwhelming majority of that output comes from vast open-pit mines. Coal India Ltd (CIL) and its subsidiaries run some of the largest surface drilling fleets on earth — Mahanadi Coalfields (MCL) at Talcher, South Eastern Coalfields (SECL) at Korba, Northern Coalfields (NCL) at Singrauli, BCCL and ECL in the Jharia belt, and Western Coalfields (WCL) — alongside Singareni Collieries (SCCL) in Telangana. The same blast-hole technology drives India's iron-ore sector: Odisha's Keonjhar and Barbil belt, Jharkhand, the Bailadila deposits in Chhattisgarh, and the Bellary–Hospet region of Karnataka.
Blast-hole drilling is the first production step in every open pit. Vertical holes — typically 152–311 mm in diameter, and up to roughly 380 mm on the largest benches — are drilled on a designed pattern, each hole reaching some 10–25 m down to the bench floor. Those holes are then charged with explosives such as ANFO or bulk emulsion and fired to fragment overburden and ore so that shovels and excavators can load it. Everything in that cycle depends on the bit reaching depth on time, hole after hole. The drill bit's carbide cutting elements take the full punishment of the rock: high weight-on-bit (WOB), continuous rotation, percussion energy, abrasion and the heat generated at the cutting face.
Why Tungsten Carbide?
Cemented tungsten carbide — usually written WC-Co — is a composite of extremely hard tungsten-carbide grains held together by a tough metallic cobalt binder. The carbide grains give the material a hardness far beyond hardened steel, so a carbide cutting element keeps its edge under the constant abrasion of rock that would round off a steel tooth in a fraction of the time. The cobalt fraction is what stops the brittle carbide from simply shattering: more cobalt buys toughness and impact resistance, but trades away some hardness. That single trade-off — hardness versus toughness — sits at the heart of every grade decision in mining.
In drilling tools, carbide appears as pressed inserts and buttons set into tricone and fixed-cutter bit bodies, and as the button bits screwed onto down-the-hole (DTH) hammers. These carbide buttons are the only parts that actually touch the rock, so their grade and shape decide how fast the bit penetrates and how long it lasts before it has to come out of the hole. Get the carbide right and a bit drills more metres at a lower cost per metre; get it wrong and you either wear buttons flat or break them off. Explore our range of tungsten carbide buttons & inserts to see the elements behind these tools.
Carbide Grades for Mining
Mining carbide is sold by grade class, and the practical shorthand for a grade is its cobalt content and its hardness measured on the Rockwell A scale (HRA). The table below summarises the three broad families a driller chooses between.
| Grade Class | Cobalt % | Hardness (HRA) | Property Bias | Typical Use |
|---|---|---|---|---|
| High-cobalt | 12–16% Co | 85.5–88.0 | Maximum toughness / impact resistance | High-impact mining, hard fractured rock, DTH in hard ground |
| Medium-cobalt | 8–12% Co | 88.0–90.0 | Balanced hardness & toughness | General blast-hole, tricone inserts, mixed ground |
| Low-cobalt | 6–8% Co | 90.0–92.0 | Maximum hardness / abrasion resistance | Highly abrasive rock, gauge protection, low-impact |
Grade choice in Indian ground follows the rock. The Deccan basalt and the granitic, banded formations of many iron-ore deposits are both hard and abrasive, which pushes the selection toward a harder, lower-cobalt grade for wear resistance — but with enough cobalt left in the binder to resist breakage in fractured zones. Coal-measure sandstones tend to be abrasive but only medium-hard, sitting comfortably with a balanced medium-cobalt grade. The failure modes are mirror images of each other: spec too hard a grade in high-impact, blocky ground and the buttons chip or snap; spec too soft a grade in abrasive rock and they wear flat far too quickly. This is why Coal India tenders frequently specify the carbide grade and minimum hardness for blast-hole tooling rather than leaving it open.
Button Shapes & Geometry
After grade, the second lever is the shape of the button itself. The geometry sets the balance between penetration rate and durability, and different formations reward different profiles:
- Spherical / hemispherical (dome): the most durable profile, with the carbide carried in a rounded dome that resists wear and impact. Ideal for hard, abrasive rock, at the cost of a lower penetration rate.
- Ballistic / conical (also semi-ballistic or parabolic): a pointier profile that concentrates load to bite harder, giving higher penetration in medium rock. It is the all-round workhorse for most blast-hole drilling.
- Chisel / wedge: an aggressive edge that maximises rate of penetration (ROP) in soft-to-medium formations, but wears faster than rounded shapes in abrasive ground.
Buttons are also placed by role across the bit. Face-row buttons do the bulk of the cutting on the bit face, while gauge-row buttons sit at the outer edge and protect the hole diameter so the bit drills a full-gauge, straight hole. Insert exposure (how far the button stands out of the body) and insert density (how many buttons, how closely spaced) are both tuned to the formation — more, blunter buttons for hard abrasive rock, fewer, sharper ones for soft fast-drilling ground. These choices map directly onto our conical inserts and thrust (gauge) buttons.
Air-Flush Tricone vs FDC vs DTH
India's open-pit benches are drilled by three main blast-hole methods, and each presents the carbide in a different way.
| Method | Cutting Action | Best Rock | Hole Size | Carbide Element |
|---|---|---|---|---|
| Air-flush tricone | Rotary crushing | Soft to hard, versatile | Large (200–311 mm+) | Pressed-in inserts |
| FDC (fixed cutter) | Shearing / scraping | Soft to medium | Small to medium | PDC / diamond cutters |
| DTH hammer | Percussion + rotation | Hard fresh rock | Small to medium | Button bit (carbide) |
The air-flush tricone is the bench standard for large production holes. Three roller cones studded with carbide inserts crush the rock as the bit rotates under high WOB, with enlarged air channels clearing cuttings up the hole. It is the most versatile choice across soft to hard ground, and the cones are rebuildable. FDC (fixed-cutter) mining bits use fixed PDC or diamond cutters with no bearings to fail and deep junk slots for fast cuttings evacuation; they deliver high ROP in soft-to-medium rock but are less suited to very hard, abrasive formations. The DTH (down-the-hole hammer) drives a carbide button bit by percussion as well as rotation, making it the strongest performer in hard, fresh rock such as Deccan basalt and granitic ore, and the best at drilling straight holes — which is why it is common for smaller production holes and for pre-split lines.
Choosing per bench is a matter of rock and hole size: large holes in mixed coal overburden lean tricone, soft fast benches favour FDC, and hard basalt or fresh iron-ore benches reward DTH percussion. Browse our mining & blast-hole bits to match the method to your formation.
Getting the Most from Carbide in the Pit
Carbide rewards attention. Start by matching both grade and button shape to the specific bench rather than running one configuration everywhere. Then watch the way the buttons wear, because the wear pattern tells you whether the grade is right: a smooth, polished "snakeskin" abrasion means the grade is doing its job and may even be a touch hard, while chipping or outright breakage is the signature of an impact failure — a grade that is too hard or too brittle for that ground. Regrind DTH buttons before they flatten so the hammer keeps biting, and keep gauge buttons sharp so every hole stays full diameter.
Drilling parameters matter just as much as the steel. Controlling WOB, RPM and air volume keeps the carbide working in its design window and stops both heat checking and premature impact damage. Finally, treat the data as part of the tool: track metres-per-bit and cost-per-metre across grades and shapes, because that is the number that proves a carbide choice. CIL subsidiaries and private miners alike are tightly cost-aware and tender-driven, so the right carbide is the one that lowers cost-per-metre on your rock. When in doubt, consult the VBM India team for grade selection before the tender closes.
Ready to specify? See our full range of tungsten carbide buttons & inserts and mining & blast-hole bits, and if you are also running rotary tricones, read our IADC tricone bit code chart to translate formation into the right insert programme.