Cutting Tool Materials – Properties and list of materials
Cutting tool materials are the materials of the cutting tool present in the machines which are used in different machining processes like turning, shaping, slotting, etc. Here we are not talking materials of cutting tools like knives.
Properties of cutting tool material:
1) Hot or Red Hotness:
It is the ability of the cutting tools to withstand high temperatures without losing its cutting edge.
2) Wear Resistance:
It is the ability of the cutting tools to resist wear due to adhesion and abrasion.
A cutting tool material should have high toughness to handle impact load, shacks, and vibrations during machining.
4) Thermal Conductivity:
Thermal conductivity is the ability of a material to conduct heat. The cutting tool should have high thermal conductivity to conduct heat away from the chip tool interface.
5) Co-efficient of friction:
The coefficient of friction of the cutting tool material should below so that there will be no friction between the tool and the chip or between the tool and the workpiece. If the friction is high between the tool and the chip then chips will stick to the tool and this will affect the quality of the machining process.
6) Thermal Stability:
Thermal stability is defined as the ability of a material to resist breaking down under heat stress. The thermal stability of the cutting tool should be high so that it wouldn’t breakdown at high temperatures.
The cost of the cutting tool should be optimum. One should not buy a cheap cutting tool and ruin the machining process. These cheap tools prove much costlier in the long run compared to one which has a high initial cost but can not be operated for a longer time at high speed.
List of different cutting tool materials available for different machining processes:
1) Carbon Tool Steel:
It is one of the cheapest material used in cutting tools for different machining processes. This material is also easy to make.
These carbon tool steel has a composition of 0.6-1.5 % carbon and very small amount of Mn and Si which are less than 0.5%.
These material looses their hardness rapidly at a temperature of about 250°C, so these are not suitable for high temperature applications and are not used at high cutting speed. These are only used at low cutting speed. The hardness of this material is about 65 Rc.
It is generally used for machining soft materials like aluminum, brass, magnesium, etc.
2) High Speed Steel (H.S.S) –
These materials were named high speed steel because these materials can cut at speed higher than carbon steels.
But in today’s world, we have materials that can be used to cut at a higher speed than high speed steel.
High speed steel is high carbon steel with a significant amount of alloying elements such as tungsten, chromium, molybdenum, etc are present to improve hardenability, toughness, and wear resistance.
These materials can retain their hardness up to 650°C.
High speed steel comes in two series:-
i) Tungsten Series
In the tungsten series, the most famous material is 18-4-1 HSS, it means it has 18% tungsten, 4% chromium and 1% vanadium. Apart from these there is 0.5% to 0.75% carbon is also available in this material.
ii) Molybdenum Series:
In the molybdenum series, the most used material 6-6-4-2 HSS. In this material, there is 6% tungsten, 6% molybdenum, 4% Chromium and 2% vanadium. It also has 0.6% carbon.
3) Cast non-ferrous alloys:
It is named so because it is nor-ferrous alloy and made using casting.
A typical alloy of this type is stellite which has 30-35% chromium, 43-48% cobalt, 17-19% tungsten, and nearly 2% Carbon.
Cast non-ferrous alloys can maintain their hardness up to 900°C.
Cast non-ferrous alloys can cut at double speed than the speed of the high speed steel.
These materials are corrosion resistant. But these are brittle which is a disadvantage.
These are in the form of inserts and are brazed to tool shank.
Carbides are the cutting tool that mainly consists of tungsten carbides particles that are held together by cobalt and nickel.
Straight tungsten carbide tools contain about 94% tungsten carbide and 6% cobalt. Straight tungsten carbides are used for machining cast iron.
It is not used for machining steel because if we use it for machining steel then the chips tend to stick to the tool. To overcome this problem tantalum, titanium is added to the carbides.
These carbides are made by powder metallurgy techniques.
The initial cost of carbides is high but it can cut at a speed which is 4 to 5 times the speed of High speed steel (H.S.S).
Carbides can retain their cutting edges up to 1200°C. They are very hard and they also have very high compressive strength.
The disadvantage with carbides is that they are brittle in nature and cant withstand impact loading.
5) Ceramics or Cemented Oxides:
These are essentially aluminum oxide powder. Apart from aluminum oxides additives like titanium, magnesium, or chromium oxide are also used.
Ceramics are harder than all the materials discussed above.
Ceramics can retain their hardness up to 1100°C.
Cermets are the combination of ceramics and metals and they are produced using powder metallurgy technique.
In the combination of metal and ceramics, ceramics will give high refractoriness, and metals will give high toughness and thermal shock resistance.
For cutting tools made with cermets, the usual combination is Al2O3 + W + Mo + born + Ti etc. It contains 90% cermaic, 10% metals.
As the quantity of metal is increased in cermets, the brittleness reduces to some extent, and the wear resistance also reduces.
7) Cubic Boron Nitride (CBN):
Cubic Boron Nitride is the second hardest material available after diamonds to be used as a cutting tool material. It was introduced in the early 1960s.
CBN tools are used in two ways:-
1) It is used as a small solid tip.
2) It is used as a 0.5 to 1mm thick layer of polycrystalline boron nitride sintered into a carbide substrate under pressure.
In the second case, the CBN layer provides high wear resistance and cutting edge strength and the carbide provides high shock resistance.
CBN is mainly used for machining alloy and tool steel with a hardness of 50 Rc or higher.
It is the hardest material of all discussed so far. Its main disadvantage is that its cost is very high.
Since its cost is very high it is used to machining only such materials which are very hard and cannot be cut using any of the tool material mentioned above.
Diamond is also used as cutting tool material in such applications where high accuracy and surface finish are required.
UCON consists of 50% columbium, 30% titanium and 20% tungsten.
Its hardness is only 200 BHN but it is bis hardened by diffusing nitrogen into surface producing a very hard surface. It has a very soft core.
It is used very rarely because of its high cost.
10) Sialon (Si-Al-O-N):
Sialon is named by combining the name of elements from which it is formed. It contains Silicon, Aluminium, Oxygen and Nitrogen by sintering them at 1800°C.
The cutting speed of this material is 2 to 3 times compared to ceramics. The hardness of this material is more than that of ceramics so it can be successfully used in interrupted cuts.
The main application of Sialon is that it is used in the machining of aerospace alloys. nickel based gas turbine