Cutting Tool Materials

Basic properties that cutting tools must possess are:

• Tool material must be at least 30 to 50% harder than the workpiece material.
• Tool material must have high hot hardness.
• High toughness
• High wear resistance
• High thermal conductivity
• Lower coefficient of friction
• Easiness in fabrication and cheap

Different elements used in cutting tool materials and their properties are

Element	Properties
Tungsten	Increases hot hardness Hard carbides formed Abrasion resistance
Molybdenum	Increases hot hardness Hard carbides formed Improving resistance
Chromium	Depth hardenability during heat treat hard carbides are formed Improving abrasion resistance Some corrosion resistance
Vanadium	Combines with carbon for wear resistance retards grain growth for better toughness
Cobalt	Increases hot hardness, toughness
Carbon	Hardening element forms carbides

Different cutting tool materials used for cutting operations in practice are high carbon steel, high speed steel, non -ferrous cast alloys, cemented carbides, ceramics and sintered oxides, cermets, diamond, cubic boron nitride, UCON and sialon.

1. High Carbon Steel tools

• Its composition is C = 0.8 to 1.3%, Si = 0.1 to 0.4% and Mn = 0.1 to 0.4%.
• It is used for machining soft metals like free-cutting steels and brass, and for chisels, etc.
• These tools lose hardness above 250°C.
• The hardness of the tool is about Rc = 65.
• Used at a cutting speed of 5m/min.

2. High speed steel (H.S.S)

• General use of HSS is 18-4-1.
• 18- Tungsten is used to increase hot hardness and stability.
• 4 – Chromium is used to increase strength.
• 1- Vanadium is used to maintain the keenness of the cutting edge.
• In addition to these, 2.5% to 10% cobalt is used to increase red-hot hardness.

Rest iron

• H.S.S. is used for drills, milling cutters, single-point cutting tools, dies, reamers, etc.
• It loses hardness above 600°C.
• Sometimes tungsten is completely replaced by Molybdenum.
• Molybdenum-based H.S.S. is cheaper than tungsten-based H.S.S. and also has slightly greater toughness but less water resistance.

3. Non–ferrous cast alloys

It is an alloy of Cobalt – 40 to 50%, Chromium – 27 to 32%, Tungsten – 14 to 29%, Carbon – 2 to 4%.

• It can not heat treated and is used in cast form.
• It loses its hardness above 800°C
• It will give better tool life than H.S.S. and can be used at slightly higher cutting speeds.
• They are weak in tension and, like all cast materials, tend to shatter when subjected to shock load or when not properly supported.

4. Cemented carbides

• Produced by powder metallurgy technique with sintering at 1000°C.
• Speed can be used 6 to 8 times that of H.S.S.
• Can withstand up to 1000°C.
• High compressive strength is greater than tensile strength.
• They are very stiff, and theirYoung’ss modulus is about 3 times that of steel.
• High wear resistance.
• High modulus of elasticity.
• Low coefficient of thermal expansion.
• High thermal conductivity, low specific heat, low thermal expansion.

According to ISO, the various grades of carbide tool materials are grouped as

• For cutting CI and non-ferrous metals are designated as K10 to K50
• For cutting steel are designated as P10 to P50
• For general purpose application are designated as M10 to M50.

The advantages of carbide tools are

• They have high productivity capacity.
• They produce surface finish of high quality.
• They can machine hardened steel.
• Their use leads to a reduction in machining costs.

5. Ceramics and sintered oxides

• Ceramics and sintered oxides are basically made of Al₂O₃, These are made by the powder metallurgy technique.
• Used for very high speed (500m/min).
• Used for continuous cutting only.
• Can withstand up to 1200°C.
• Has very high abrasion resistance.
• Used for machining CI and plastics.
• Has less tendency to weld metals during machining.
• The generally used ceramic is sintered carbide.
• Another ceramic tool material is silicon nitride, which is mainly used for CI.

6. Cermets

• Cermets are a combination of ceramics and metals and are produced by the powder metallurgy process.
• When they are combined, ceramics will give high refractoriness,s and metals will give high toughness and thermal shock resistance.
• For cutting tools, a usual combination is Al₂O₃ + W + Mo + boron + Ti, etc.
• Usual combination: 90% ceramic, 10% metals. An increase in % of metals reduces brittleness to some extent and also reduces wear resistance.

7. Diamond

• Diamond has extreme hardness, low thermal expansion, high thermal conductivity, very low coefficient of friction.
• Cutting tool material made of diamond can withstand speeds ranging from 1500 to 2000m/min.
• On ferrous metals, diamond ar, diamondsitable because of the diffusion of carbon atoms from the diamond tothee workpiece
• Can withstand temperatures above 1500°C.
• A synthetic (man-made) diamond with a polycrystalline structure is recently introduced and made by the powder metallurgy process.

8. Cubic Boron Nitride (CBN)

• The trade name is Borozone.
• Consists of atoms of Nitrogen and Boron and is produced by the powder metallurgy process.
• Used as a substitute for diamond during machining of steel.
• Used as a grinding wheel on H.S.S. tools.
• An excellent surface finish is obtained.

9. UCON

• UCON is developed by Union Carbide in the USA.
• It consists of Columbium 50%, Titanium 30 %, and Tungsten 20%.
• This is a refractory metal alloy that is cast, rolled into sheets, and slit into blanks. Though its hardness is only 200 BHN, it is hardened by diffusing nitrogen into the surface, producing a very hard surface with a soft core. It is not used because of its higher cost.

10. Sialon (Si-Al-O-N)

• Sialon is made by powder metallurgy with milled powders of Silicon, Nitrogen, aluminium, and oxygen by sintering at 1800°C.
• This is tougher than ceramics, so it can be successfully used in interrupted cuts. Cutting speeds are 2 to 3 times compared to ceramics.
• At present, this is used for machining of aerospaclloys, nickel based gas turbine blades with a cutting speed of 3 to 5 m/sec.