Cobalt vs HSS vs Carbide Drill Bits

Walk into a tool supplier and ask for drill bits, and the first question back is usually "what material?" Not what you're drilling, but what the bit itself is made from. High-speed steel, cobalt, carbide. Three materials that look similar at a glance but differ fundamentally in composition, manufacturing, and how they behave under stress.

The choice between these materials isn't about quality tiers. Each represents different engineering compromises between hardness, toughness, heat resistance, and cost. Understanding what these materials actually are and how they differ explains why identical-looking bits can have such different prices and performance characteristics.

High-Speed Steel Composition

High-speed steel isn't a single alloy but a family of tool steels containing tungsten, molybdenum, chromium, and vanadium in various proportions. The standard formulation, designated M2, contains about 6% tungsten, 5% molybdenum, 4% chromium, and 2% vanadium, with the balance being iron and carbon.

These alloying elements serve specific purposes. Tungsten and molybdenum form carbides within the steel matrix that provide hardness and wear resistance. Chromium adds corrosion resistance and hardenability. Vanadium forms extremely hard vanadium carbides that enhance wear resistance and allow the steel to maintain hardness at elevated temperatures.

The manufacturing process involves melting, casting into ingots, hot rolling to shape, then heat treating to develop final properties. Heat treatment includes austenitizing at around 1,200 degrees Celsius, quenching in oil or salt, then tempering at 550-600 degrees Celsius. This produces a hardness of 62-65 on the Rockwell C scale.

High-speed steel can maintain its hardness up to about 600 degrees Celsius. Beyond that temperature, the steel begins to soften as the tempered martensite structure breaks down. This temperature threshold determines how hard you can push a high-speed steel bit before heat generation causes rapid dulling.

The material costs less than cobalt or carbide because the alloying elements are relatively inexpensive and the manufacturing process is well-established. High-speed steel bits represent the baseline against which other materials are compared.

Cobalt Steel Difference

Cobalt bits aren't coated with cobalt. The cobalt is alloyed throughout the steel, typically at 5-8% by weight. This changes the fundamental properties of the material rather than just its surface characteristics.

The most common cobalt formulation is M35, which contains 5% cobalt along with tungsten, molybdenum, chromium, and vanadium similar to M2 high-speed steel. The cobalt addition changes how the steel behaves during heat treatment and at elevated temperatures.

Cobalt increases red hardness, the ability to maintain hardness at high temperatures. Where M2 high-speed steel softens around 600 degrees Celsius, M35 cobalt steel maintains hardness past 650 degrees Celsius. This 50-degree difference translates to significantly longer life in applications that generate substantial heat.

The metallurgical mechanism involves cobalt strengthening the iron matrix and stabilizing carbides at high temperatures. The result is steel that resists softening when the cutting edge heats during use. This matters most in hard materials like stainless steel or cast iron where cutting generates extreme temperatures.

Cobalt steel also machines differently during bit manufacturing. It's harder to grind and requires more aggressive grinding wheels. This increases manufacturing cost beyond just the material cost. The additional expense of cobalt as an alloying element combined with harder manufacturing makes cobalt bits typically cost 2-3 times what equivalent high-speed steel bits cost.

The visual appearance of cobalt bits varies. Some have a dull gray color. Others are bright silver. Some receive coatings that obscure the base material entirely. You cannot reliably identify cobalt content by appearance. The only certain way is manufacturer marking or material testing.

Hardness of cobalt steel after heat treatment reaches 65-67 Rockwell C, slightly higher than standard high-speed steel. The hardness difference is modest, but the heat resistance difference is substantial. This makes cobalt steel particularly valuable for continuous drilling where bits stay hot.

Carbide Material Properties

Carbide bits use tungsten carbide, a compound of tungsten and carbon with dramatically different properties from steel. The material has hardness around 1,500-2,000 on the Vickers scale, compared to high-speed steel at 800-900. This extreme hardness makes carbide highly wear-resistant.

Tungsten carbide forms through powder metallurgy. Tungsten carbide powder is mixed with a cobalt binder, typically 6-12% cobalt by weight. The mixture is pressed into shape, then sintered at around 1,400 degrees Celsius. During sintering, the cobalt melts and binds the carbide particles into a solid mass.

The cobalt binder in carbide serves a different purpose than cobalt alloyed in steel. In carbide, cobalt provides toughness. Pure tungsten carbide is extremely hard but brittle. The cobalt binder creates a tough matrix that prevents catastrophic fracture. Higher cobalt content makes carbide tougher but slightly less hard. Lower cobalt content maximizes hardness but increases brittleness.

Carbide maintains its properties at temperatures exceeding 1,000 degrees Celsius. This extraordinary heat resistance means carbide bits can operate at cutting speeds that would destroy steel bits. The limitation isn't temperature softening but thermal shock. Rapid heating or cooling can crack carbide due to thermal expansion differences between the carbide and cobalt phases.

Most drill bits aren't solid carbide. Instead, they use carbide tips brazed onto steel shanks. Solid carbide bits exist but primarily for specialized applications. The carbide tip provides the cutting performance while the steel shank provides economy and shock resistance. Brazing carbide to steel requires careful temperature control to prevent thermal stress that could crack the carbide.

Carbide's extreme hardness creates manufacturing challenges. Carbide cannot be machined with conventional tools. It must be ground with diamond wheels, and even then it wears grinding wheels rapidly. This makes carbide bit manufacturing expensive. Carbide bits typically cost 5-10 times what equivalent high-speed steel bits cost.

Toughness vs Hardness Trade-off

Hardness measures resistance to deformation and wear. Toughness measures resistance to fracture and shock. These properties exist in tension. Materials that maximize one typically compromise the other.

High-speed steel balances these properties reasonably well. It's hard enough to cut most materials effectively but tough enough to withstand the shock loads and vibration that occur during drilling. A high-speed steel bit flexes slightly under load rather than fracturing.

Cobalt steel shifts the balance slightly toward hardness and heat resistance while maintaining reasonable toughness. The material can withstand higher operating temperatures but is slightly more prone to chipping if impacted hard.

Carbide maximizes hardness at the expense of toughness. A carbide bit will maintain its edge far longer than steel but can chip or fracture from impact that a steel bit would survive. Dropping a carbide bit on concrete can chip the cutting edge. Side loading during drilling can fracture carbide tips.

This trade-off determines where each material works effectively. High-speed steel suits general-purpose drilling where bits encounter varying conditions. Cobalt steel suits production drilling in hard materials where heat is the limiting factor. Carbide suits operations where extreme hardness is necessary and conditions can be controlled to prevent shock loads.

Heat Resistance Comparison

The temperature at which these materials maintain cutting ability differs substantially. High-speed steel softens around 600 degrees Celsius. Cobalt steel maintains hardness to about 650-700 degrees Celsius. Carbide remains hard past 1,000 degrees Celsius.

These temperature differences translate to dramatically different operating speeds. A high-speed steel bit drilling steel might run at 200 surface feet per minute. A cobalt bit could run at 250-300 surface feet per minute. A carbide bit could run at 400-600 surface feet per minute.

The relationship between speed and heat generation isn't linear. Doubling speed more than doubles heat generation because both cutting frequency and friction increase. Materials that maintain hardness at higher temperatures enable faster cutting, which increases productivity but requires understanding the thermal limits.

Heat resistance also determines how quickly bits dull in abrasive materials. Materials like fiberglass or composite decking generate heat through abrasion rather than cutting. The abrasive particles heat the bit surface as they pass. Materials that maintain hardness at these temperatures dull more slowly.

Thermal conductivity differs between materials too. Tungsten carbide conducts heat about three times better than steel. This means carbide bits can dissipate heat faster, helping maintain lower operating temperatures even when generating heat rapidly. The combination of heat resistance and thermal conductivity makes carbide particularly effective in continuous operations.

Wear Pattern Differences

High-speed steel bits wear gradually. The cutting edge rounds over through abrasion. The bit gets progressively duller but continues functioning, just less efficiently. The wear is predictable and sharpening can restore performance.

Cobalt steel wears similarly to high-speed steel but more slowly in high-heat applications. The wear rate in room-temperature wood drilling shows little difference between cobalt and standard high-speed steel. In hot operations drilling stainless steel or titanium, cobalt bits last significantly longer because they resist the heat-accelerated wear that destroys high-speed steel.

Carbide exhibits different wear mechanisms. In abrasive materials, carbide wears through attrition where individual carbide grains are pulled from the surface. This creates a gradually dulling edge similar to steel but at a much slower rate. In impact situations, carbide can chip catastrophically, removing chunks of cutting edge instantly.

The wear patterns determine maintenance requirements. Steel and cobalt bits can be sharpened multiple times, extending their useful life significantly. Carbide bits are difficult to sharpen properly because they require diamond grinding. Many users treat carbide as disposable despite the higher initial cost.

Coating wear interacts with base material wear. A coated high-speed steel bit loses coating at the cutting edge during use, exposing bare steel that then wears faster. A coated carbide bit retains coating longer because the hard substrate doesn't wear as quickly. The coating effectiveness depends on how long the base material maintains its geometry.

Cost and Value Relationships

High-speed steel bits cost least because material and manufacturing expenses are lowest. A basic twist bit might cost a few dollars. This makes high-speed steel economical for occasional use or applications where bits are likely to be damaged.

Cobalt bits typically cost 2-3 times what equivalent high-speed steel bits cost. The material itself costs more, and cobalt steel is harder to machine during manufacturing. The value proposition depends on usage. For occasional drilling in wood or soft materials, the extra cost provides minimal benefit. For production drilling in hard metals, cobalt bits can last 3-5 times longer than high-speed steel, making them cheaper per hole despite higher initial cost.

Carbide bits cost 5-10 times what high-speed steel bits cost, sometimes more for specialized designs. The material is expensive and manufacturing is complex. Carbide bits make economic sense primarily in production environments or for drilling extremely hard materials like concrete where steel bits would destroy themselves rapidly.

The cost comparison becomes murky when considering bit life. A $5 high-speed steel bit that drills 50 holes costs $0.10 per hole. A $30 carbide bit that drills 1,000 holes costs $0.03 per hole. The carbide bit saves money if you drill enough holes, but the upfront investment is substantial.

Market pricing doesn't always reflect manufacturing costs. Some manufacturers charge premium prices for cobalt or carbide bits beyond what material and process costs justify. Consumer perception of "better" materials allows higher markups. Industrial pricing tends to track actual costs more closely because buyers are more cost-sensitive.

Material-Specific Characteristics

High-speed steel has reasonable corrosion resistance from the chromium content but will rust if stored in damp conditions. The rust is primarily cosmetic unless it pits the cutting edges. High-speed steel is magnetic, which can be useful for retrieval but also means it attracts ferrous dust and chips.

Cobalt steel has similar corrosion resistance to high-speed steel. The cobalt addition doesn't significantly change oxidation behavior. The material is magnetic like all steel-based materials. Weight is essentially identical to high-speed steel because cobalt and iron have similar densities.

Tungsten carbide has excellent corrosion resistance. The material doesn't rust in any practical sense. It can develop surface discoloration from oxidation at extreme temperatures, but this doesn't affect performance. Carbide is much denser than steel, about twice the density. A carbide bit feels noticeably heavier than an equivalent steel bit.

The magnetic properties differ significantly. Tungsten carbide itself is non-magnetic. However, carbide bits with cobalt binder show weak magnetic attraction because cobalt is magnetic. Solid carbide bits or carbide with nickel binder show no magnetic attraction. This can serve as an identification method, though it's not definitive.

Thermal expansion coefficients differ between materials. Steel expands about 11-13 parts per million per degree Celsius. Tungsten carbide expands about 5 parts per million per degree Celsius. This difference creates problems in brazed carbide tips where differential expansion during heating or cooling can crack the carbide. It's why carbide bits shouldn't be quenched in water for cooling.

Manufacturing Process Impact

High-speed steel bits are machined from rod stock or made from rolled and formed blanks. The flutes can be milled or ground. Heat treatment happens after forming, which creates some distortion that must be corrected by grinding. The process is highly automated and relatively fast, contributing to lower costs. Bits are manufactured in the complete range of standard sizes from micro diameters to over an inch.

Cobalt steel undergoes similar processing but requires harder grinding wheels and takes longer to machine. The material is more abrasion-resistant even before heat treatment, increasing tool wear during manufacturing. Quality control is more critical because the higher material cost makes scrapping defective bits more expensive.

Carbide bits follow a completely different manufacturing path. The carbide tips are formed through powder metallurgy, then brazed onto steel shanks. Brazing requires precise temperature control and appropriate filler metals. After brazing, the cutting edges are ground using diamond wheels. The entire process is more manual and time-consuming than steel bit manufacturing.

Solid carbide bits are ground from carbide blanks. The grinding is slow because carbide wears grinding wheels rapidly. A single carbide bit can take 10-20 times longer to manufacture than an equivalent steel bit. This labor intensity drives much of the cost difference.

Quality variations within each material category can be substantial. Premium high-speed steel uses finer grain structure and more precise heat treatment. Budget high-speed steel may have inconsistent composition and heat treatment. The difference shows in bit performance and life, though both are technically "high-speed steel."

Application Patterns

High-speed steel dominates general-purpose drilling. Wood, plastic, soft metals like aluminum and brass, and occasional steel drilling all work well with high-speed steel. The material's balance of properties and low cost make it suitable for the majority of drilling operations in home shops and light commercial use.

Cobalt steel appears primarily in metalworking, particularly for drilling stainless steel, cast iron, titanium, and other materials that generate substantial heat during cutting. Production environments doing repetitive metal drilling often use cobalt bits as standard because the longer life offsets higher cost.

Carbide dominates masonry drilling because masonry materials fracture rather than cut, creating extreme abrasion that destroys steel bits rapidly. Carbide's hardness withstands the abrasion that grinds away steel. Carbide also appears in production metalworking where high speeds and long production runs justify the investment.

Different bit types show different material preferences. Twist bits are available in all three materials. Spade bits are almost always high-speed steel because the application doesn't generate enough heat to justify cobalt or carbide. Masonry bits are almost always carbide-tipped because steel won't survive the application.

The pattern reflects economic optimization. Materials get used where their properties provide sufficient benefit to justify their cost. High-speed steel fills the large middle ground where its properties are adequate. Cobalt and carbide target specific applications where high-speed steel fails or performs inadequately.

Identification Methods

Distinguishing between these materials by appearance alone is unreliable. Coatings obscure the base material. Surface finishes vary by manufacturer. Color varies by heat treatment and finishing processes.

Manufacturer markings provide the most reliable identification. Quality bits are marked with material designation like "HSS," "HSS-CO," "M35," or "C" for carbide. The markings appear on the shank, though they can wear off with use.

Weight provides a clue for carbide versus steel. Carbide's higher density makes bits noticeably heavier. A carbide-tipped bit has a heavy tip on a lighter steel shank, creating an obvious weight distribution difference from solid steel bits.

Magnetic testing separates carbide from steel but doesn't distinguish cobalt from regular high-speed steel. A strong magnet will attract steel and cobalt steel bits but not pure carbide. Carbide with cobalt binder shows weak attraction.

Spark testing, where the bit is touched to a grinding wheel, produces different spark patterns for different materials. High-speed steel produces long, branching yellow sparks. Cobalt steel produces similar but slightly redder sparks. Carbide produces virtually no sparks or very short orange sparks. This method risks damaging the bit and isn't practical for most users.

Price provides a rough indicator. Bits priced significantly higher than standard high-speed steel are likely cobalt or carbide. However, brand premium, coatings, and marketing can distort this relationship.

Performance in Specific Materials

Wood drilling shows minimal difference between high-speed steel and cobalt. The cutting forces and temperatures are low enough that cobalt's heat resistance provides negligible benefit. Carbide bits work well in wood but are typically unnecessary except for extremely abrasive materials like particleboard or MDF with high glue content.

Aluminum drilling favors high-speed steel or cobalt. Aluminum's tendency to adhere to cutting edges causes problems with all materials, but carbide's extreme hardness makes it particularly prone to aluminum gumming. The lower cutting temperatures in aluminum mean cobalt's heat resistance is less beneficial than in harder metals.

Stainless steel drilling strongly favors cobalt or carbide. Stainless work-hardens rapidly and generates extreme heat. High-speed steel bits dull quickly. Cobalt bits provide substantially longer life. Carbide bits work well if cutting conditions can be controlled to prevent chipping from the material's toughness.

Cast iron drilling benefits from cobalt or carbide. Cast iron is abrasive and hard, wearing high-speed steel relatively quickly. Cobalt provides better life. Carbide excels but requires appropriate speeds and feeds to prevent the material's tendency to fracture carbide from sudden load changes.

Titanium drilling requires cobalt at minimum, preferably carbide. Titanium combines high strength with poor thermal conductivity, creating extreme heat at the cutting edge. High-speed steel softens almost immediately. Only materials that maintain hardness at very high temperatures survive extended titanium drilling.

The Material Selection Matrix

No single material is universally superior. Each excels in specific conditions and fails in others. High-speed steel provides adequate performance at minimal cost for general work. Cobalt extends capability into harder materials and longer production runs. Carbide enables extreme performance in abrasive materials and production metalworking but requires careful handling and appropriate conditions.

The manufacturing differences create cost structures that determine economic viability. The wear patterns create different maintenance and replacement schedules. The physical properties create different strengths and weaknesses.

Understanding what these materials actually are - their composition, how they're made, how they behave under stress - provides the context for evaluating them. The differences aren't subtle or marketing hype. They reflect fundamental material science and engineering trade-offs that determine where each material makes sense and where it wastes money or fails entirely.