Co-Cr-W vs Co-Cr-Mo Alloys: Should a Cobalt-Chromium Alloy Use Tungsten or Molybdenum?

Cobalt-chromium alloys are used where ordinary steels struggle with adhesive wear, corrosion, elevated temperature or combinations of these mechanisms. Typical industrial applications include valve balls and seats, bushings, hot-working components, extrusion parts, hardfacing deposits and localized wear surfaces.

However, “cobalt-chromium alloy" is not a complete material specification.

The addition of tungsten, molybdenum, carbon, nickel, silicon and iron can substantially change the alloy’s microstructure and service behavior. Manufacturing route is equally important. A cast Co-Cr-W component, a Co-Cr-Mo powder-metallurgy part and a welded cobalt hardfacing deposit should not be expected to behave identically even when their chemical analyses appear similar.

One of the most important material-selection questions is whether tungsten or molybdenum should provide the principal supplementary strengthening effect.

Both elements can strengthen a cobalt-rich matrix and participate in carbide formation. Their effects are nevertheless not interchangeable.

This article explains:

• The roles of cobalt and chromium
• How tungsten changes Co-Cr alloys
• How molybdenum changes Co-Cr alloys
• Why carbon content influences both systems
• How casting, powder and welding routes change the result
• Which service data should guide alloy selection
• How to specify chemistry and inspection requirements
• What information to send with an RFQ

The Basic Architecture of a Cobalt-Chromium Alloy

Industrial wear-resistant cobalt alloys commonly contain a cobalt-rich matrix and dispersed hard phases.

Cobalt provides the base matrix. Chromium supports corrosion and oxidation resistance and also participates in carbide formation. Carbon controls the amount and type of carbide that can form. Tungsten or molybdenum may strengthen the matrix, alter carbide chemistry and influence elevated-temperature behavior.

The microstructure can include:

• Cobalt-rich face-centered cubic or hexagonal close-packed matrix phases
• Chromium-rich carbides
• Tungsten- or molybdenum-containing carbides
• Eutectic constituents in interdendritic regions
• Intermetallic phases under some compositions or thermal histories
• Segregated regions created during solidification
• Oxides or inclusions introduced through processing

ASM International emphasizes that cobalt-alloy structure, properties and wear behavior depend on both chemical composition and manufacturing route. Cobalt alloys may be cast, wrought, welded as hardfacing or consolidated through powder-metallurgy processes.
Source: ASM—Friction and Wear of Cobalt-Base Alloys.

What Tungsten Does in a Co-Cr-W Alloy

Tungsten is a heavy refractory element with a high melting point. In cobalt-chromium alloys, it can contribute through solid-solution strengthening and participation in hard carbide or complex phase formation.

Solid-Solution Strengthening

Tungsten atoms are much larger and heavier than cobalt atoms. When tungsten dissolves in the cobalt-rich matrix, the difference in atomic size creates lattice strain that makes dislocation movement more difficult.

This can support:

• Matrix strength
• Resistance to plastic deformation
• Hot hardness
• Resistance to adhesive wear and galling
• Load support beneath hard carbide phases

The useful effect depends on how much tungsten remains in solution rather than being concentrated in carbides or segregated regions.

Tungsten-Containing Carbides

When sufficient carbon is available, tungsten can participate in carbide formation. Depending on the complete chemistry and solidification conditions, the alloy may contain chromium-rich carbides, tungsten-rich carbides or complex mixed-metal carbides.

Hard carbides can improve wear resistance by resisting:

• Micro-cutting
• Surface ploughing
• Adhesive junction deformation
• Local indentation
• Abrasive particle penetration

However, carbides must be adequately supported by the matrix. A high carbide fraction in a weak or highly segregated matrix can lead to cracking and carbide pull-out.

Elevated-Temperature Strength

Tungsten has low diffusivity and can contribute to elevated-temperature strength and structural stability. This makes Co-Cr-W alloys attractive for hot wear, hot cutting and high-temperature valve or sealing applications.

The result must still be validated at the actual temperature. Oxidation, thermal cycling, creep, thermal expansion and contact stress can dominate performance even when room-temperature hardness is high.

Density and Processing Consequences

Tungsten is considerably denser than molybdenum. High tungsten additions can therefore increase component or deposited-layer density.

Tungsten may also increase:

• Melt handling difficulty
• Risk of compositional segregation
• Sensitivity to incomplete dissolution
• Machining difficulty
• Raw-material cost
• Need for careful process control

A nominal tungsten percentage is not sufficient evidence that the element is uniformly distributed through a large casting.

What Molybdenum Does in a Co-Cr-Mo Alloy

Molybdenum is also a refractory alloying element. It can strengthen the cobalt matrix, participate in carbide formation and influence corrosion behavior.

Matrix Strengthening

Molybdenum dissolved in the cobalt-rich matrix can impede dislocation movement and improve matrix strength.

Research comparing powder-metallurgy CoCrNi, CoCrW and CoCrMo alloys found that nickel, tungsten and molybdenum produced different phase balances, hardness values and high-temperature wear responses. The study also found that coefficient of friction did not directly predict wear rate.

Because the specimens were produced by powder metallurgy under specific compositions and test conditions, the ranking should not be transferred directly to sand castings or welded deposits.
Source: Effect of Ni, W and Mo on CoCr Matrix Alloys.

Carbide Formation

Molybdenum can enter complex carbides and modify their distribution, morphology and stability. As with tungsten, the effect depends strongly on carbon and chromium.

Molybdenum does not create wear resistance independently. Its benefit depends on whether the heat treatment or solidification route creates:

• An adequately strengthened matrix
• A useful carbide population
• Acceptable carbide morphology
• Limited brittle-phase formation
• Suitable matrix–carbide bonding

Corrosion and Passivity

Chromium is normally the principal passivating element in cobalt-chromium alloys. Molybdenum can modify the behavior of the passive film and may improve resistance in some chloride-containing or reducing environments.

A study of Co-30Cr-6Mo in a chloride- and bicarbonate-containing solution found that chromium and molybdenum suppressed cobalt dissolution and produced chromium-like passive behavior.

The study used a simulated physiological solution, not an industrial slurry or acid process. Its results demonstrate a mechanism but do not qualify an alloy for mining, chemical or oilfield service. Actual process fluid testing may still be necessary.
Source: Passivation and Corrosion Behaviours of Cobalt and Co-Cr-Mo Alloy.

Ductility and Matrix Support

Some Co-Cr-Mo systems can provide a favorable balance between matrix strength, ductility and carbide support. This can reduce the tendency for hard particles to detach under sliding contact.

However, Co-Cr-Mo is not automatically tougher than every Co-Cr-W alloy. Carbon, casting defects, heat treatment, section thickness and test temperature can reverse an apparent ranking.

Co-Cr-W and Co-Cr-Mo Selection Tendencies

These are engineering tendencies, not guaranteed rankings.

A 2020 study found different friction and wear behavior among powder-produced CoCrW and CoCrMo alloys from room temperature to 1000°C. A separate study of specific Co-Cr-Mo and Co-Cr-W-Mo dental alloys found the tested Co-Cr-Mo material had better wear resistance under its particular conditions.

Conversely, research on CoCrMo and CoCrW welding-wire deposits on a superalloy substrate reported condition-dependent differences in fatigue and high-temperature wear.

These results reinforce one central principle: performance rankings depend on the exact composition, manufacturing route, microstructure and test method.
Sources: CoCr Matrix Alloy Study, Co-Cr-Mo and Co-Cr-W-Mo Precipitation Study, and CoCrMo vs CoCrW Welding Deposits.

Carbon May Matter More Than the W-or-Mo Label

Tungsten and molybdenum cannot be evaluated without carbon.

Carbon controls the potential amount of carbide formation. A lower-carbon alloy may rely more heavily on solid-solution strengthening and matrix behavior. A higher-carbon alloy may contain a larger carbide fraction and higher hardness but reduced ductility.

The correct carbon content depends on the component and failure mechanism.

A valve seat exposed to adhesive wear, corrosion and sealing contact does not require the same matrix–carbide balance as a hardfacing layer exposed to mineral abrasion.

Co-Cr-W Is Not the Same as WC-Co Cemented Carbide

The term “tungsten cobalt alloy" can create confusion.

A cast or deposited Co-Cr-W alloy contains cobalt as the principal matrix with chromium, tungsten, carbon and other additions.

WC-Co cemented carbide is a different material system. It consists mainly of tungsten-carbide particles bonded by a smaller amount of cobalt. It is generally produced by powder-metallurgy and sintering processes.

The materials differ in composition, phase fraction, processing, toughness, repairability and cost. They should never be treated as interchangeable because both contain cobalt and tungsten.

Manufacturing Route Changes the Alloy

Investment or Sand Casting

Casting creates a dendritic solidification structure. Tungsten, molybdenum, chromium and carbon may segregate between dendrite cores and interdendritic regions.

Important casting controls include:

• Charge-material identity
• Melt chemistry
• Deoxidation practice
• Pouring temperature
• Mold material
• Solidification rate
• Feeding and shrinkage control
• Section-thickness transitions
• Heat treatment
• Machining allowance

Large section changes can create different cooling rates and carbide morphologies within one component.

Powder Metallurgy

Powder routes can provide:

• Finer starting particles
• Different segregation behavior
• Greater control of near-net geometry
• Potentially uniform distribution
• Different residual porosity risks
• Different oxide exposure

A result obtained from a powder-metallurgy research sample should not be assumed for a conventional casting.

Welding and Hardfacing

Hardfacing introduces additional variables:

• Base-metal dilution
• Cooling rate
• Deposit thickness
• Number of layers
• Preheat and interpass temperature
• Residual stress
• Heat-affected zone
• Shielding gas
• Cracking tolerance
• Post-weld treatment

AWS A5.21/A5.21M covers classification of bare electrodes and rods for surfacing. The current 2024 edition includes cobalt-base classifications. Filler-metal classification describes the consumable or undiluted deposited chemistry; it does not by itself guarantee the chemistry of the final diluted overlay.
Source: AWS A5.21/A5.21M:2024.

Thermal Spray, HVOF, PTA and Laser Cladding

A cobalt alloy powder may be used for different deposition processes, but particle size, morphology and chemistry must match the equipment.

The final coating depends on:

• Powder size distribution
• Sphericity and flow
• Oxygen content
• Heat input
• Particle velocity
• Dilution
• Porosity
• Oxide content
• Bond strength
• Finishing method

A powder suitable for one process should not automatically be used in another.

Matching the Alloy to the Failure Mechanism

Adhesive Wear and Galling

For metal-to-metal sliding, ask:

• What is the counterface material?
• Is lubrication available?
• What is the contact pressure?
• Is sliding continuous or oscillating?
• Is seizure the primary failure?
• Does the surface need to maintain a seal?

A Co-Cr-W alloy may be attractive for hot hardness and galling resistance, but the final selection must include surface finish and counterface compatibility.

Abrasive Wear

For mineral or particle abrasion, record:

• Abrasive hardness
• Particle size and shape
• Sliding speed
• Contact pressure
• Impact energy
• Wet or dry condition

A high-carbide cobalt alloy may resist abrasion but can become vulnerable if the carbide population is too coarse or poorly supported.

Corrosion-Wear

Corrosion-wear cannot be predicted by a dry hardness test.

Provide:

• Fluid chemistry
• pH
• Chloride concentration
• Sulfur-containing species
• Temperature
• Solids concentration
• Flow velocity
• Shutdown and cleaning conditions

Molybdenum may be useful in some corrosive environments, but laboratory testing should reproduce the actual fluid where failure consequences are significant.

High-Temperature Wear

For hot service, include:

• Normal and maximum metal temperature
• Heating and cooling rate
• Atmosphere
• Oxidizing or reducing condition
• Sulfur, carbon or halogen exposure
• Contact stress
• Thermal-cycle frequency

High tungsten content does not eliminate oxidation, thermal fatigue or creep.

Inspection Requirements for Cobalt Alloys

The inspection plan should define test locations and acceptance criteria before production.

Questions to Include in a Co-Cr-W or Co-Cr-Mo RFQ

Provide:

• Component name and part number
• Equipment model
• Dimensioned drawing
• Required manufacturing route
• Current alloy designation
• Current chemical-composition report
• Desired generic standard or customer specification
• Normal and maximum temperature
• Wear mechanism
• Contact pressure and sliding speed
• Impact level
• Corrosive medium and concentration
• Counterface material
• Lubrication condition
• Required hardness range
• Machined surface finish
• Dimensional and geometric tolerances
• NDT requirements
• Chemistry sampling locations
• Quantity
• Delivery destination
• Photographs of the failed component
• Current service data and failure mode

Avoid requesting “the highest tungsten alloy" or “maximum hardness" without operating data. More alloying content can increase cost and brittleness without solving the actual failure.

Relevant EB Castings Pages

For cast cobalt-alloy components, see:

• Cobalt-Chromium Alloy Valve Balls and Ring Seats
• Custom Cobalt-Base Alloy Castings

For deposited wear surfaces, see:

• Cobalt-Base Alloy Powder for PTA, HVOF and Laser Cladding
• Cobalt-Alloy Hardfacing Wire

For additional background, read:

• Cobalt Chromium Molybdenum Alloy
• Performance and Wear Resistance of Cobalt-Based Alloys

These pages describe different products and processes. The alloy and process must be confirmed from the customer’s drawing, service condition and acceptance requirements.

Engineering and Safety Boundaries

This article provides general material-selection guidance. It does not establish that Co-Cr-W or Co-Cr-Mo is universally superior.

Final material selection remains the responsibility of the equipment owner, responsible engineer, purchaser and manufacturer under an agreed specification.

Casting, grinding, machining, thermal spraying and welding cobalt-containing materials can generate cobalt-bearing dust or fume. NIOSH identifies inhalation, ingestion and skin or eye contact as exposure routes for cobalt metal dust and fume and lists the respiratory system and skin among target organs.

Use a documented industrial-hygiene assessment, process enclosure or local exhaust ventilation, appropriate housekeeping and task-specific personal protective equipment. Respirator selection must be based on measured exposure and an applicable respiratory-protection program.
Sources: NIOSH Pocket Guide—Cobalt Metal Dust and Fume and NIOSH Local Exhaust Ventilation for Welding.

Frequently Asked Questions

Is Tungsten Always Better for High-Temperature Wear?

No. Tungsten can support hot hardness and matrix strength, but oxidation, carbide morphology, thermal fatigue and manufacturing route also control performance.

Is Molybdenum Always Better for Corrosion?

No. Molybdenum may benefit passivity in some environments, but chromium content, fluid chemistry, temperature and mechanical film removal remain important. Test the actual process medium when corrosion risk is significant.

Can Tungsten and Molybdenum Be Substituted by Weight?

No. They have different atomic mass, carbide behavior, partitioning and processing effects. Substitution requires alloy redesign and qualification.

Does Higher Hardness Mean Better Wear Resistance?

Not necessarily. Wear also depends on carbide support, toughness, friction mode, temperature, corrosion and counterface material.

Can an XRF Analyzer Confirm the Complete Alloy?

XRF can help identify cobalt, chromium, tungsten and molybdenum, but it generally cannot provide the carbon result needed for carbide control. Use a suitable carbon-analysis method.

Are Cast Parts and Hardfacing Deposits Equivalent?

No. Castings solidify as complete components. Hardfacing deposits are affected by dilution, layer thickness, cooling and the base metal.

Should We Specify a Trade Name or Chemical Composition?

Whenever possible, specify a recognized generic classification or an agreed chemistry and acceptance plan. Trade names can cover different forms or historical variants.

Can EB Castings Recommend W or Mo from a Drawing Alone?

The drawing defines geometry, but service data are also necessary. Provide wear mechanism, temperature, medium, load, counterface and current failure information.

Conclusion

Tungsten and molybdenum are both valuable additions to cobalt-chromium alloys, but they solve different metallurgical problems.

Tungsten can contribute strong solid-solution strengthening, hot hardness and tungsten-containing hard phases. It can also increase density, segregation sensitivity and machining difficulty.

Molybdenum can strengthen the cobalt-rich matrix, participate in carbide formation and support corrosion behavior in certain environments. Its benefit still depends on chromium, carbon, heat treatment and manufacturing route.

The correct engineering question is not:

“Is tungsten better than molybdenum?"

It is:

“Which combination of matrix chemistry, carbide structure and manufacturing process best addresses the documented wear, corrosion and temperature conditions?"

Send drawings, current chemistry, failed-part photographs and operating data through the EB Castings contact page or email cast@ebcastings.com.

All alloy, process and inspection requirements should be confirmed before quotation and manufacturing.

References

1. ASM International. Cobalt and Cobalt Alloys.
2. ASM International. Friction and Wear of Cobalt-Base Alloys.
3. Li et al. Effect of Ni, W and Mo on CoCr Matrix Alloys.
4. Jenko et al. Influence of Precipitation Hardening on Co-Cr-Mo and Co-Cr-W-Mo Alloys.
5. Li et al. Microstructure and High-Temperature Properties of CoCrMo and CoCrW Welding Deposits.
6. AWS. AWS A5.21/A5.21M:2024—Bare Electrodes and Rods for Surfacing.
7. NIOSH. Cobalt Metal Dust and Fume.