Cemented Carbide

Question 1

What is cemented carbide, and what are its uses?

Answer 1

Class of hard tool material based on tungsten carbide (WC)
using powder metallurgy techniques with cobalt (Co) as
the binder.

Used in lathes, milling machines of cast iron, non ferrous alloys and
non metallic materials.

Question 2

What are common insert shapes?

Answer 2

(a) Round
(b) square
(c) rhombus with 80 point angles
(d) hexagon with 80 point angles
(e) triangle
(f) rhombus with 55 point angles
(g) rhombus with 35 point angles

Question 3

What order increases in strength, power requirement and vibration tendency/ decreases in versatility and accessibility?

Answer 3

From lowest point angle to highest point angle.

Question 4

What is the general manufacturing process of cemented carbides and coatings?

Answer 4

Raw materials, mixing of raw materials, spray dry, powder mixture is then pressed, sintering, grinding, coating followed by inspection, final product.

Question 5

What are general properties of cemented carbides?

Answer 5

High compressive strength but low-to-moderate tensile strength (brittle)
High hardness (90 to 95 HRA) HRA: from Rockwell hardness tester
Good hot hardness
Good wear resistance
High thermal conductivity
High elastic modulus - 600 x 103 MPa
Toughness lower than HSS

Question 6

What is the structure like for non-steel cutting grades?

Answer 6

Straight carbides WC-Co (cast iron and non-ferrous alloy machining).

Due to high % WC, they have the greatest resistance to flank wear compared to other cemented carbides.

Question 7

What is the structure like for steel cutting grades?

Answer 7

Mixed carbides WC-(Ti,Ta,Nb)C-Co (steel machining).

TiC: superior heat resistant & less crater wear.

Adhesion strength of TiC & Co is less than WC & Co.

Mixed cemented carbides have lower fracture resistance than
straight.

Question 8

Grain size vs. Properties?

Answer 8

Increase in grainsize/Increase in % Cobalt = Increase toughness and decreased deformation resistance.

Disadvantage of WC-Co coarse grain is inferior hardness, wear resistance and strength.

Question 9

What are coated carbides, and what are their applications?

Answer 9

Cemented carbide insert coated with one or more layers of TiC, TiN, and/or Al2O3 or other hard materials. Coating is applied by chemical vapor deposition or physical vapor deposition.

Applications: cast irons and steels in turning and milling operations, best applied at high speeds where dynamic force and thermal shock are minimal.

Question 10

What is the chemical vapour deposition process?

Answer 10

Reaction gas is fed into a high temperature environment chamber. A chemical reaction takes place and the result of this chemical reaction is that a thin film of hardened particles is left on the surface of the carbide substrate.

1. reaction temperature around 900C-1100C
2. High coating toughness
3. Possible to coat large batches all at once
4. Coating of TiN and Al2O3 is possible, all that needs to be done is
   to change the reaction gas

Question 11

What are CVD-coated tools applications and uses?

Answer 11

Applications: turning, boring, threading, grooving, parting and milling.

Used in: machining of carbon, alloy and stainless steels; gray and ductile irons, and Ni-based alloys over a range of speeds and feeds.

Question 12

What are advantages of CVD method?

Answer 12

Offers high thermal stability - suitable for high speed machining

High adhesion between the substrate and the coating layer allows for forming a relatively thick coating - high wear resistance extends tool life.

By changing the gases a number of different coatings can be carried out during the same process.

Question 13

What are dis-advantages of CVD method?

Answer 13

Due to a high temperature (900C-1100C) process, there is a restriction of which substrate can be used. Therefore, CVD is not suitable for high-speed steels or brazed tools -

Question 14

What is the physical vapor disposition (PVD) process?

Answer 14

This is a process that is carried out at relatively low temperatures. The coating is conducted physically in the form of either vacuum evaporation, sputtering or ion plating. The result of this process is that a thin film of hardened particles is left on the surface of the carbide substrate.

1. Reaction temperature up to 700C
2. Applicable to high-speed steel and brazed tools
3. The coating time is relatively quick

Question 15

What are PVD characteristics?

Answer 15

Smooth, low-friction, fine-grained, and crack-free coating even over sharp edges with compressive residual stresses.

Lower cutting forces with sharp edged tools.

Reduced tool-tip temperatures.

Finer workpiece finish.

Particularly useful in threading, grooving, parting. finish-turning, milling and drilling operations.

Typical workpiece materials:
Carbon, alloy and stainless steels
Hardened steels
High-temp alloys
Titanium alloys

Question 16

What are advantages of PVD method?

Answer 16

Coating is possible at relatively low temperatures (under 700C), therefore placing little restriction on the type of substrate used - Consequently, high speed steels, brazed tools, cemented carbides, cermets, etc can be coated.

The cutting edge strength (toughness) is maintained - Suitable for sharp edged tools.

A compressive residual stress is generated in the coating layer - Offers high fracture and thermal crack resistance.

The surface of the coating is smooth - Suitable for finishing.

Question 17

What are dis-advantages of PVD method?

Answer 17

Coating of Al2O3, and other oxides (insulators), that do not conduct electricity cannot be coated.

In comparison with CVD the adhesion between the coating and the substrate is low and is unsuitable for thick coatings.

Generally PVD is carried out using a line of sight coating and therefore makes it difficult to coat the entire insert.

Question 18

What does the basic grain structure of cemented carbide consist of? What is the role of the individual phases within?

Answer 18

WC (α) Angular
Co (β) (Binder- Glue)

Question 19

What is the practicality of different inset shapes?

Answer 19

There is a trade off between strength, power requirements and vibration tendency and versatility and accessibility. Increasing one decreases the other.

Question 20

What effect does increasing grain size have on the materials toughness and deformation resistance? What effect does increasing the Co concentration have on the WC grain size?

Answer 20

The toughness increases and the deformation resistance goes down. For an increase of 5 – 12 % the WC grain size increases from 1 – 5 um, becoming more coarse.

Question 21

What is CVD & PVD? What are the influences of residual stresses in the formation of coatings by these two processes and how do they influence the fracture resistance?

Answer 21

Chemical Vapour deposition and Physical Vapour Deposition. CVD produces coatings with tensile stresses due to the mismatch between the thermal expansion coefficients of the coating and substrates, lowering the fracture resistance. PVD produces compressive stresses due to the same reason, however will show improved fracture resistance.