Different machining applications require different tool materials. In those conditions along with general properties of cutting tools they require some unique properties.
Basic properties that cutting must posses are:
§ Tool material must be at least 30 to 50% harder than the work piece material.
§ High temperature stability i.e., Tool material must show high red hardness
§ High toughness
§ High wear resistance and long life
§ High thermal conductivity
§ Lower coefficient of friction
§ Chemically inert to the working material and lubrication fluid.
§ Flexibility in fabrication and low cost
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 ormed
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 we use high carbon steel, high speed steel(HSS), non-ferrous cast alloys, cemented carbides, ceramics and sintered oxides, diamond, and cubic boron nitride(CBN).
1. High Carbon Steel tools:
§ Carbon tool steel is one of the inexpensive metal cutting tool.
§ It’s composition is C = 0.8 to 1.5%, Si = 0.1 to 0.4% and very small amount of Mn = 0.1 to 0.4%. and also other materials like Cr, V are added to change properties like hardness and grain size.
§ It is used for low-speed machining & soft metals like free cutting steels, aluminium and brass and used as Twist drills, chisels etc.
§ These tools possesses good machinability.
§ These tool loose hardness above 250°C so these can’t be used for high temperature applications.
§ Hardness of tool is about HRC = 65.
§ Used at cutting speed of 5m/min.
2. High speed steel (H.S.S):
High speed steel is an alloy element. The main constituents are Tungsten, molybdenum and chromium etc., Generally used HSS tools composition 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 keenness of cutting edge.
In addition to these 2.5% to 10% cobalt is used to increase red hot hardness.
Rest of the content is iron
High speed steel is an alloy element. The main constituents are Tungsten, molybdenum and chromium etc., Generally used HSS tools composition 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 keenness of cutting edge.
In addition to these 2.5% to 10% cobalt is used to increase red hot hardness.
Rest of the content is iron
§ H.S.S is used for drills, milling cutters, single point cutting tools, broaches, dies, reamers etc.
§ It looses it’s hardness above 600°C temperature.
§ Some times tungsten is completely replaced by Molybdenum.
§ These tools are two types they are T-type : Tungsten predominant metal & M-type : Molybdenum predominant metal
§ Molybdenum based H.S.S is cheaper than Tungsten based H.S.S and also slightly greater toughness but less water resistance.
§ Coolant should be used for better tool life.
3. Non – ferrous cast alloy tools:
It is an alloy having composition of Cobalt – 40 to 50%, Chromium – 27 to 32%, Tungsten – 14 to 29%, Carbon – 2 to 4%
It is an alloy having composition of Cobalt – 40 to 50%, Chromium – 27 to 32%, Tungsten – 14 to 29%, Carbon – 2 to 4%
§ It can not heat treated and are used as cast form.
§ It looses it’s 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:
§ Cemented carbide cutting tool is Produced by powder metallurgy technique with sintering at 1000°C.
§ Cutting speed range is 60-200 m/min, this is 6 to 8 times that of H.S.S.
§ They doesn’t loose their hardness up to 1000°C.
§ Compressive strength is more than tensile strength.
§ They are very stiff and their young’s modulus is about 3 times that of the steel.
§ High wear resistance.
§ High modulus of elasticity.
§ Low coefficient of thermal expansion.
§ High thermal conductivity, low specific heat, low thermal expansion.
§ Hardness is upto HRC- 90.
5. Ceramics and sintered oxides:
Most common Ceramics and sintered oxides are basically made of Al2O3 and SiN. These are made by powder metallurgy technique with sintering.
§ Used for very high speed (300-600m/min). These are 10 times faster than HSS.
§ Used for continuous cutting only.
§ Can withstand up to 1800°C.
§ These tools are chemically inert and Have very abrasion resistance.
§ Having high compressive strength.
§ Used for machining CI and plastics.
§ Has less tendency to weld metals during machining.
§ No coolant is required.
§ Generally used ceramic is sintered carbides.
§ Another ceramic tool material is silicon nitride which is mainly used for CI.
6. Cubic Boron Nitride (CBN):
§ This is the second hardest material after diamond.The trade name is Borozone.
§ Consists of atoms of Nitrogen and Boron and produced by power metallurgy process.
§ Used as a substitute for diamond during machining of steel.
§ Used as abrasive grinding wheels.
§ Excellent surface finish is obtained.
§ Hardness HRC- 95.
§ Speed 600-800m/min
7. Diamond:
Diamond is the hardest metal known in the world and also costly.
§ Diamond possesses
Extreme hardness
Low thermal expansion.
High thermal conductivity.
High melting point
Very low coefficient of friction.
§ Diamond tools give good surface finish and dimensional accuracy.
§ Speeds ranging 1500 to 2000m/min.
§ These are not recommended for ferrous metals because of the diffusion of carbon atoms from diamond to work-piece.
§ Can withstand above 1500°C.
A synthetic (man made) diamond with polycrystalline structure is recently introduced and made by powder metallurgy process.