Cemented carbides

Venkataraman B. - Dept. of Metallurgical Engineering, AUCE(A).
M.Tech.
By

Venkataraman B. - Dept. of Metallurgical Engineering, AUCE(A). 2
Development of:
-Nano Carbides
-Coatings
Present

Tungsten Titania Tantalum Pentoxide Cobalt Oxide
Mixed with Carbon Mixed with Carbon & Briquetted Mixed with Carbon Reduction in H₂
Carburized Carburized Carburized
WC Powder TiC Powder TaC Powder Co Powder
Carbide Solid Solutions Screened
Crushed & Screened
Ball Mill(Wet)
Filtered
Dried
Re-Reduced
Screened
Lubricant added
Pressing
Pressing into final shape Pre-sintering in H2
Shaping by cutting & Grinding
Final Sintering in H2
Sintered Cemented Carbide Components
Inspection & Control Finished Product
[1, 3]

These compounds are produced from either the Wolframite[(Fe,Mn)WO₄] or
Scheelite (CaWo₄) ores.
i) Most Carbide producers purchase Tungsten Trioxide, Tungsten acid or
Ammonium Para-tungstate from Chemical industry and reduce them.
ii) Purchase Tungsten powder directly.
2) Reaction of metal oxide with Carbon:
- Used for Titanium and Tantalum
- final Carbon content is difficult due to combination of reduction and
carburization reactions.
- necessitating crushing and re-carburization.
1) Direct reaction of the metal with Carbon:
- used for Tungsten and Molybdenum.
- Close control of ‘C’ is easily attained.
- expensive.
MO + H₂ = M + H₂O
MO + CO = M + CO₂
Tungsten Carbide(WC) is produced by the carburization of metallic Tungsten powders prepared by the reduction
of Tungsten Trioxide(WO₃), Tungsten acid (H₂WO₄), Ammonium Para-tungstate [5(NH₄) ₂ 0.12WO₃.xH₂O, where
x=5 for transparent laminated plates and x=11 for white acicular needles].
Reduction of Tungsten compound:
Furnaces used: Continuous electrically or gas-heated, push type furnaces.
Rotary F/Cs: Production of very fine Tungsten powders.
In which oxide, Tungsten acid or Ammonium Para-tungstate is spread into
thin layers in Ni boats and moved through the F/C in the direction opposite
to ‘H₂’ flow at 800-1100⁰C.

Tungsten Carbide(WC):
- First dry blending of Tungsten(W) powder and Carbon (6.9-7.2%)
- Excess ‘C’ avoids under-carburization.
- Mixture is charged either loose or pre-compacted into Graphite boats.
- Continuous carburization in a high-frequency induction F/C in a ‘H₂’ atm. between 1400 - 1800⁰C, depends on the particle size.
- Agglomerated carbide lumps obtained by carburization.
- Crushed by Gyratory or Jaw crushers, Roll crushers, stud mills and hammer mills.
- After crushing carbides are pulverised in eddy or ball mills and screened by vibratory sieves
- Carbon deficiency in the Tungsten Carbide(WC) powder results in the formation of brittle carbide phases such as ƞ-phase(Co₃W₃C₄),
Ѳ-phase(Co₃W₆C₂) and K-phase (Co₃W₁₀C₄) during sintering.
Titanium Carbide(TiC):
- Obtained from the rutile or ilmenite ore (69%TiO₂) is dry or wet blended with lamp black and dried.
- Mixture is then briquetted and carburized at 2100 - 2300⁰C.
- F/Cs: High-Frequency Vacuum F/Cs or Carbon tubes.
Tantalum Carbide(TaC):
- Mixture of Tantalum pentoxide and Carbon.
- Carburized at 1550⁰C for 4hr.
- Crushed, screened and re-carburized.
- Finally, a solid solution (more than one carbide) of TaC-NbC obtained by acid treatment.
Cobalt(Co):
- Reduction of oxide or oxalate by ‘H₂’ at 600 - 700⁰C.

The particle size of the Tungsten powder produced depends upon:
 Particle size of the original compounds or oxide.
 Reduction temperature.
 Water content of the H₂.
 Speed of H₂ flow.
 Reduction time.
 Rate of passage of oxide through the furnace.
 Depth of the oxide bed.
[4]

Milling
Cemented Carbides produces maximum hardness and minimum
porosity (or high density), a very fine (1 -2μm), uniform distribution
of carbides or carbide solutions with ‘Co’ is required.
Wet ball milling is preferred and distilled water, alcohol, acetone,
benzene as liquids (270 – 400cc/kg).
Balls: Steel (½ - 1 ¼ “Ø), ‘C’ picks up (>0.5%) in the charge, Carbide
(cylinders or rectangle with rounded edges)
Milling times: Rotary(2-8days), Vibratory(1-2days)
Ratio of powder to balls: Rotary (1:1- 1:3), Vibratory (1:3.5 – 1:5.5)

Vacuum filtration or centrifugal treatment: Removal of liquid agent.
Drying and reduction treatment.
Sieving: removes agglomerates, flakes, impurities.
Addition of Lubricants (5 -10%): Paraffin wax dissolved in gasoline, alcohol, glycol solution.
Cold Pressing & Shaping: 70 – 100MPa.
Green density: 55 – 60% as of sintered density.
Hydraulic press: Blocks or plates, Intricate shapes (milling, slitting, turning, boring, surface
grinding)
Hot Isostatic pressing
About 1300 - 1350⁰C depends on ‘Co’,
in ‘Ar’ atm ‘C’ dies, 100 - 150MPa.
Produces high density, fine grain structure and
pore free parts.
[5]

Sintering practice
Zone1: Burn off/Pre-heat.
The green compacts entering in this zone, are heated slowly so as to
avoid rapid heating which may result in the expansion of entrapped air
and lubricants.
Zone2: High temperature sintering zone.
The sintering temperature & time depends on the material, particle size
and shape.
Zone3: Cooling zone.
i) A short insulated cooling zone, which allows the sintered parts to be
cooled slowly in order to avoid thermal stresses.
ii) Relatively longer water-jacketed cooling zone, to cool the sintered
parts without exposure to air to prevent oxidation.
Heat treatment: 400 - 650⁰C to remove the lubricant by volatilisation.
Pre-Sintering: 900 - 1150⁰C in ‘H₂’ atm. for 30min. Only solid state
(negligible shrinkage)
[2]

Processes during sintering:
 Removal of moisture, RT - 150⁰C and removal of plasticizers or lubricants during early
stages (200 - 400⁰C).
 Reduction of ‘Co’ oxides at intermediate temperatures (400 - 700⁰C).
 Reduction of Tungsten oxides (WO) and mixed carbide oxides (>1000⁰C).
 Bonding of individual particles.
 Formation of WC-Co solid solution.
 Melting of binder phase: 1300 - 1400⁰C.
 Dissolution of WC particles in liquid phase (1400 - 1500⁰C) and reprecipitaion.
Care during sintering about:
Decarburization/Carburization from F/C atm.
Burning leads to blisters, swelling or blow holes on the surface.
Cracks due to thermal expansion gradient.
Buckling due to non-uniform density of green part.
Sintering: To increase the strength & hardness of compact.
In ‘H₂’ atmosphere at 1350 - 1550⁰C, ½ - 2hr depends on the ‘Co’(1495⁰C) content.
Increasing ‘Co’ content decreases sintering temperature.

For extremely close tolerances in finished materials.
Finishing: Metal bonded diamond wheels
Electro-spark machiningRough grinding: SiC grinding wheel
Ultrasonic machining
Higher degree of surface finish: Lapping and
polishing with fine diamond powders/pastes.
Machining, Grinding and polishing

CONVENTIONAL PRODUCTION OF ULTRAFINE WC POWDER
Ultrafine tungsten powders are made up of extremely fine metal grains of about 20-50nm obtained by the ‘H₂’ reduction of
the Tungsten oxides have a pseudo orphic (sponge-like) appearance in the oxide raw material.
The carburization of the ultrafine tungsten carbide powders is carried out at low temperatures, around 1200-1450°C, in order to
restrict severe particle coarsening during the W => WC transition, limit lies in the range of 50-150nm.
THE SPRAY CONVERSION PROCESS (SCP)
The SCP has been under development by Nanodyne since 1992.
The essential process comprises of three unit operations namely:
1) An aqueous solution mixing operation, in which the final composition of
the powder is established;
2) A spray drying operation, which converts the aqueous precursor solution
to an easily fluidized solid solution powder having good flow
characteristics;
3) A gas/solid reaction operation that converts the solid powder solution to
nanostructured WC-Co powder in a high temperature fluid bed reactor.
[6]

MILLING
During the course of milling stresses are induced in the Tungsten carbide(WC) grains and the cobalt(Co) phase changes from a
predominantly cubic to a hexagonal close packed structure.
 Shorter milling times can also result in inhomogeneous mixing which leads to discontinuous grain growth.
 Longer milling times result in increase coercivity and hardness.
High Energy Ball Milling:
Balls: Stainless steels, Tungsten Carbide (WC, expensive)
Milling atms: (Ar, air, N) or vacuum. N – may react with ground powders to form
interstitial solid solutions or nitrides.
Ball-to-Powder ratio: 10:1 – 20:1
Extent of Container filling: 50% max.
Hard milling: 10mmɸ balls
Soft milling: 3mmɸ balls
Milling speed: 200 – 300rpm.
Milling time: ½ hr, 1hr, 2hr, 4hr, 8hr, 24hr ….
Dry milling: Extended solid solutions.
Wet milling: Cold welding of nano particles.
Contamination: As negligible.
Mill about 10-20 g of the powder speed (500 - 1000 rpm) of the clamp motion, the ball velocities are high (on the order of 5 m s-1)[4][6]

Spark Plasma sintering
The pulsed DC directly passes through the graphite die, as well as the powder compact, 5 -10min, fast heating & cooling rates >500⁰C/min.
 Axial pressure is simultaneously applied from the beginning of the sintering cycle.
 Joule-heating and the sparking among the particles of sintered material leads to the faster heat and mass transfer instantaneously.
 After the sintering, the power is turned off and the sample is allowed to cool.
Advantages:
 Heat generated within the particle itself.
 Perform high speed consolidation of powder.
 Enhancing densification over grain growth.
Hot Isostatic Pressing (HIP): Pressure about 30 – 100MPs about 1300 - 1350⁰C depending upon the ‘Co’ content by an inert gas.
Pre-sintering: In ‘H₂’ at gradually rising from room temperature to 800°C.
14

Metal Strg MP⁰C Sp.Gr.
Carbon(C) HCP 3527 2.27
Tungsten(W) BCC 3410 19.35
Tantalum(Ta) BCC 2996 16.65
Niobium(Nb) BCC 2468 8.57
Vanadium(V) BCC 1890 6.11
Chromium(Cr) BCC 1857 7.19
Titanium(Ti) HCP 1660 4.54
Cobalt(Co) HCP 1495 8.90
Nickel(Ni) FCC 1453 8.90
Mercury(Hg) RH - 39 13.55
WC,
%
TiC, % TaC(NbC), % Co,
%
Density,
g/cm³
Hardness
,RA
TRS, lb/in² Thermal Conduct,
cal/cc.sec.deg.C
97 3 15.2 92 155,000 0.21
75 25 12.9 83 350,000 0.16
94 1 5 14.6 90.5 212,000 0.19
75 16 9 11.0 90.5 180,000 0.07
61 32 7 8.9 92.5 130,000 0.04
85 4 1 10 13.3 90 250,000 0.13
50.5 38 5 6.5 8.6 91.5 145,000
15 15Ni70Cr₂C₂ 6.5 87.5 100,000
1) Density:
Increasing the additions of ‘Co’ and TiC, reduces Cemented Carbides Density.
Mercury used as buoyancy medium: WC-Co grades sink in Hg, WC-TiC-Co & WC-TiC-Ta(Nb)C-Co grades will float.
2) Hardness:
Used to measure of wear performance.
Rockwell ‘A’ is used for speed values, but Vickers method is used for accurate values as high hardness.
Increasing ‘Co’ content decreases and Increasing TiC content increases hardness.
Abrasive or Scratch hardness about 9 on Mohr's scale.

Crack geometry indentation for hard metal, Palmqvist profile outlined with dotted.
Vickers hardness indentation on the surface of a sample
The hardness of cemented carbide: 1000 - 2000HV
by changing cobalt(Co) content and WC grain size.
3) Hot Hardness:
Importance for high speed machining.
6%Co grade has hardness of 100VPN at 750⁰C which is higher than that of
High Speed Steel at room temperature.
4) Impact Strength:
A measure of toughness.
WC-Co grades higher impact strength than WC-TiC-Co grades.
[7]

5) Transverse-Rupture Strength:
A good measure of toughness user for bar specimens supported
by Cemented Carbides.
WC-Co material’s strength increases with increasing ‘Co’ content,
higher ‘Co’ contents and decreases sintering temperature.
7) Thermal conductivity:
Larger amounts of heat generated due to cutting action, by conduction
through the tool leads to damage or failure.
The thermal conductivity of WC-Co is 2-3 times higher than the High
Speed Steels and considerably higher than the WC-TiC-Co grades.
Thermal conductivity lowers with increasing TiC and Co contents.
6) Compressive Strength;
Both WC-Co, WC-TiC-Co grades are outstanding at elevated temperatures.
Compressive strength decreases with in increasing ‘Co’ and TiC content.
Maximum compressive strength at 4%Co content.
8) Coefficient of Thermal expansion:
Importance for the joining of cemented carbide components.
Tips to other materials by brazing or shrink fitting.
Addition of TiC and ‘Co’ increases slightly the coefficient of thermal expansion.

9) Cutting performance:
Influenced by hardness, density, toughness, composition, microstructure etc.
Performance evaluated by turning, milling, shaping, drilling tests.
T-V curves(T-service or tool life in minutes and V-cutting speed in meters/min.
10) Coercivity:
Coercivity is the measure of the force required to move the magnetic domain
walls in the cobalt phase.
11) Corrosion and Oxidation resistance:
Importance for applications in wire drawing and chemical industry.
Corrosion resistance of WC-TiC-Co grades against acids and alkalis at room temperature, WC-Co grades at the boiling points.
TiC bonded with Ni-Cr alloys has been successfully employed in corrosive environments.
Oxidation resistance of cemented carbides in air is outstanding in hot pressed tools, turbine buckets, nozzles, but also for the
cutting performance with edge temperatures of 700-1000⁰C.
The Oxidation resistance of WC-Co grades considerably lower than the WC-TiC-Co grades

Porosity:
Pores are difficult to avoid during the production of the hard metals even when the correct carbon balance is maintained
Impurities:
In milling, powders can absorb iron, chromium or nickel from the stainless steel cladding of the mill itself or from the milling balls.
While ‘Ni’, does not have any influence on the hardness and the strength, chromium and iron reduce the strength.
Free Carbon (Carbon content):
Carbon deficiency causes the appearance of the eta-phase, a double carbide of variable composition (e.g. W3C03C), and excess
carbon results in the presence of free carbon.
The Vickers hardness decreases linearly with increasing carbon content while the TRS drops rapidly with increasing decarburisation.
To avoid embrittlement by the eta phase, commercial alloys usually contain a certain amount of excess free carbon.
Grain growth:
Vanadium, as a grain growth inhibitor(VC, Cr3C2 NbC and TaC) during the sintering process of cemented carbides.
VC are more effective during sintering of micron sized powder compared to sub-micron and nano-grained powders.

Ref: [AGREN, J., BRANDT, J., HAGLUND, S. and UHRENIUS, B. (1996) Modelling of
solid state sintering of cemented carbides. In: GERMAN, R.M. and MESSING, G.L.,
(eds.). Proceedings of sintering technology, New York, 1996, pp.149-156].
Ref: [R. Spiegler and H. F. Fischmeister, Prediction of crack paths in
WC-Co alloys, Acta metal. Material. 40 (1992) 1653-1661].

Ref:[Analysis of the material behaviour of cemented
carbides (WC-Co) in grinding by single grain cutting tests,
F. Klockea, C. Wirtza,*, S. Muellera, P. Mattfelda],
[Laboratory for Machine Tools and Production Engineering
(WZL), RWTH Aachen University, Steinbachstrasse 19, 52074
Aachen, Germany].

[11]

W:C atomic ratio < 1 = carbon precipitates in the form of graphite.
W:C atomic ratio > 1 = brittle η-phase precipitates.
These both phases reduces the Transverse Rupture Strength of the alloy.
[8]

Metallographic Examination:
Light Optical Microscopy (LOM)
The physical properties of the Cemented Carbide tools are controlled also by the microstructure (Structure and Fracture toughness).
Fractures surface examination at 30X magnification, information regarding porosity, graphite, ƞ-phase, grain size.

Electron backscatter diffraction (EBSD):
EBSD maps of 180 min milled WC-10Co after sinter-
HIP, showing :
(a) diffraction pattern quality map;
(b) orientation colour map of the WC phase;
(c) pattern quality map of WC phase (red) and
cubic-Co (yellow);
(d) pattern quality map of cubic-Co and misindexed
Phase.

The microstructure is examined 1500X-2000X under electron microscopy to reveal the
carbide grain size, distribution of carbide and binder phase and the presence of
secondary carbide phases.
The microstructure of WC-Co alloy contains only two phases α-phase(WC) and Ƴ-phase
(Co with WC in solid solution).
The α-phase (WC grains) appear as regular shapes such as triangles of trapeziums.
The microstructure of WC-TiC-Co grades, a second carbide phase as β-phase(TiC-WC
solid solution).
Heat tinting electrolytic etching consist in heating the metallographic specimens in air at 400-500⁰C which
leads to colouring of different phases.
After examining the fractured surface, the test piece is ground to flat within 0.025mm, lapped and polished with diamond powder
(replace by Boron Carbide powder as economy).
The polished test piece is etched for 3-4 minutes in a solution of 10%NaOH and K₃Fe(CN)₆ for WC-Co, an electrolytic etching by nitric-
hydrofluoric acid mixture for WC-TiC (TaC, NbC )-Co grades.
Scanning Electron Microscopy:
[1]

[9]
Excess ‘Co’Pores Pores Pores
Pores Pores Pores Non uniform size distribution
30

Pores
[9]
Uniform distributionFine distribution
Non uniform size distribution Non uniform size distribution ‘Co’ poolingPores

[9]

[9] [10]

93.5 -97.5%WC, 2.5 -6.5%Co:
Finishing – Medium roughing cuts: Cast Iron, Non – Ferrous metals, Austenitic-alloys, Super-Alloys
86-92%WC, 8-14%Co:
Rough cuts: Cast Iron etc., particularly on planers
76-80%WC, 12-14%TiC, 8-10%Co:
Machining with Average cutting speeds: Steel and Cast steel
48-77%WC, 20-40%TiC, 0-5%Tac, 3-7%Co:
Light high speed finishing: Steels
63-80%WC, 10-20%TiC, 3-7%TaC, 7-10%Co:
Medium Cuts at speeds: Steels
66-79%WC, 3-7%TiC, 10-17%TaC, 8-10%Co:
Heavy cutting and general: Steels
Where resistance to wear, cratering/abrasion wear by scale is required.

Tools: Nozzles, Carbide inserts for mining, circular saws, brazed tools, glass & ceramic forming.
Dies: Extrusion dies, Wire drawing, tube drawing and bar drawing.
Rolls: Hot rolling wires, rods and plates.
Wear resistant surfaces & coatings.
Balls: As grinding media.

Coated Carbides
Micro thin layer, Coating materials:
Titanium nitride (TiN)
Titanium carbide (TiC)
Titanium carbonatite (TiCN)
Aluminium oxide (Al₂O₃)
Zirconium nitride (ZrN)
Titanium aluminium nitride (TiAlN)
Methods of Coating:
 Chemical Vapour Deposition (CVD)
 Physical Vapour Deposition (PVD)
Advantages:
 High hot hardness
 Chemically stable
 High cutting speeds 150 – 250m/min
 High tool life, 2-3 times higher than carbide

Toxicity of powders:
• Leads to undesirable health effects like eye, skin irritation, vomiting, respiratory problems, blood poisoning etc.
• powder like lead, nickel are highly toxic & Al, iron are less toxic
Precautions:
Use of protective gloves, respiratory masks, protective clothing etc.; use of well ventilated storage, workplace;
careful handling, disposal of wastes.
flammability & reactivity data is required
Health effects: Inhalation – disturbs the respiratory track; remedial measures include moving the person to fresh air.
Artificial breathing is required if patient not breathing properly.
Skin, eyes – Brushing, washing skin and eyes with water and soap. Clean eyes with fresh water for 15min.
Recycling

Venkataraman B. - Dept. of Metallurgical Engineering, AUCE(A). 02-09- 39
[5] Henderson, R.J., Chandler, H.W., Akisanya, A.R., Barber, H., Moriarty, B. (2000): Finite Element Modelling of Isostatic Pressing. Journal of
the European Ceramic Society, 1121-1128.
[6] Porat eta/., Proc. Euro Powd. Metall. Conf., European Powder Metallurgy Association, Shrewsbury, 101 (1996).
[7] B. Roebuck, et al., Measurement Good Practice Guide No. 20: Mechanical Tests for Hard metals, National Physical Laboratory, 1999.
[8] L. Åkesson, An experimental and Thermodynamic Study of the Co-W-C System In the Temperature Range 1470-1700 K, Science of Hard
Materials, p. 71-82, 1982.
[9] J.D. Kim and S.J.L.Kong. Formation of Grain Boundaries in Liquid phase sintered WC-Co Alloys. Journal of American Ceramic society,
88:500 503, 2005.
[10] V.Kumar, Z.Z.Fang, S.I.Wright, and M.M.Nowell. An analysis of Grain Boundaries and Grain Growth in Cemented Tungsten Carbide using
Orientation Imaging microscopy. Metallurgical and Materials Transactions A,37A:599 607,2006.
[11] Designation and properties of WC-10Co for different WC grain sizes (Richter, Ruthendorf and Drobniewski 1995).
[4] Powder Metallurgy & Nano Composites - HIP, Spark Plasma Sintering, High Energy Ball Milling lectures – 2016 by Dr. Baburao Jinugu,
Professor in the Dept. of Metallurgical Engineering, AUCE(A), Visakhapatnam, India.
[1] Powder Metallurgy – A.K.Sinha, Dhanpat Rai Publications, 2000.
[2] Powder Metallurgy Science, Technology and Applications – P.C.Angelo, R.Subramanian, Prentice Hall India (P) ltd, 2008.
[3] P.Schwartzkopf and R.Kieffer, Cemented Carbides. The MacMillan Company, New York, 1960ed.

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