18 Main Types of Cutting Tool Materials | Industrial Engineering

The following points highlight the eighteen main types of cutting tool materials used in industries. The types are: 1. Plain High Carbon Tool Steel 2. Low Alloy Carbon Tool Steel 3. High Speed Steel 4. Cast Cobalt Base Alloy Tools (Stellites) 5. Cemented Carbides 6. Ceramics 7. Non-Ferrous Alloys 8. Non-Tungsten Materials (Titanium Carbides and Titanium Nitrides) and a Few Others.

Type # 1. Plain High Carbon Tool Steel:

Before 1900, all types of tools were made of carbon tool steel. The chief characteristics of plain carbon tool steel are low hot hardness and poor hardenability. They are usually quenched into brine and even then only a thin layer can be fully hardened with the attendant risk of developing quenching cracks. The carbon steels are limited in use to tools of small section which operate at relatively low speed.

The typical composition of the plain carbon steel is:

C = 0.8 to 1.3%, Si = 0.1 to 0.4%, Mn = 0.1 to 0.4%

Carbon tool steels may be broadly classified into two categories—water-hardening steels and oil-hardening steels.

The higher the carbon content, the greater will be wear resistance of the tool. Actually when a hardened plain carbon steel tool having a tempered marten-site structure is heated, the smaller particles of cementite will dissolve and a corresponding amount of cementite will precipitate on the larger particles as soon as carbon in the vicinity of these particles has time to migrate into position.

The net result is fewer and coarser carbide particles dispersed in the ferrite matrix and hence a softer structure. This is overcome in high speed steels by adding tungsten and molybdenum which combine with iron carbide to form complex carbides.

Carbon tool steels are easy to machine. Keen cutting edge can be easily provided. It has high surface hardness with a fairly tough core. These lose their hardness above 200°C and do not regain it even after cooling. These are used for manufacture of milling cutters, twist drills, turning and form tools for wood, magnesium, brass and aluminium. Cutting speeds with carbon steel tools are limited to about 0.15 m/sec using huge coolant supply.

Type # 2. Low Alloy Carbon Tool Steel:

In order to increase hardness of tools, simple addition of carbon content makes it brittle. Small amounts of chromium and molybdenum are frequently used to improve harden ability of tool steels. Upto 4% of tungsten is sometimes added to these steels in order to improve their wear resistance.

These types of materials are used where wear resistance is required. These steels are widely used for drills, taps and reamers. Their hot hardness is about the same as that of the carbon steels and are not satisfactory for high speed turning and milling.

Type # 3. High Speed Steel:

The introduction of high speed steel made possible a significant increase in machining speed, which accounts for its name. The chief characteristics of these steels are superior hot hardness and wear resistance. These can retain their cutting edge hardness at temperatures upto 600°C but soften rapidly at higher temperatures. Cutting speeds are limited upto 0.75 to 1.8 m/sec beyond which they fail rapidly. The high speed steels are of two major types, viz., tungsten and molybdenum type and cobalt type.

Under the first category, the most common type of high speed steel is 18-4-1 tool steel which is typical of the high tungsten class of high speed steels. 18-4-1 means that this tool steel contains 18 parts of tungsten, 4 parts of chromium and one part of vanadium.

During World War I, a shortage of tungsten was faced, and realising that tungsten and molybdenum behave in the same general way, attempts were made to substitute tungsten by molybdenum; thus giving rise to two more types of HSS in the first category. It may be noted that only half as much molybdenum as tungsten is required on a weight basis to achieve the same effect.

The composition of three popular high speed steels of the first category is given below:

All the alloys in above table contain about 0.025% sulphur as an impurity and 0.25% manganese which will combine with sulphur and form manganese sulphide and thus prevent embrittlement.

Sometimes cobalt (in 4, 8 or 12% ratio) is added to any of these three types of high speed steels; the addition of which results in increase of hot hardness of the steel even beyond that of 18-4-1.

All the three elements—tungsten, molybdenum and cobalt help in achieving high hot hardness; the first two do so by forming complex carbides and the cobalt forms an alloy by going into solid solution in the ferrite matrix and thus raises the recrystallization temperature so that the material can retain the hardness it obtains as a result of strain hardening at a higher temperature.

The pronounced superiority of high speed steel tools containing cobalt, particularly on high roughening cuts, is due to their tendency to strain harden at the surface when abraded or scratched.

Vanadium in high speed steels forms very hard carbides (vanadium-iron-carbide being the hardest constituent in HSS) and thus increases the wear resistance of the tool at all operating temperatures. Vanadium also helps to inhibit grain growth at the high temperatures required in heat treatment.

Due to above reason, increased vanadium content in tools is used for machining a highly abrasive material such as a high carbon high chromium die steel containing chromium and vanadium as the major constituents. These are most desirable tools for machining highly abrasive stock.

As the cobalt and molybdenum have a tendency to promote decarburisation, steels containing these elements should be ground to a greater depth in finishing to remove the decarburised layer which will not become fully hardened. Such steel should also be packed in carbonanceous material when being heat treated.

The chromium and cobalt have the tendency to promote retention of austenite which has further tendency to transform into martensite at a low temperature when the tool is subjected to shock or cold work as by grinding or in use of cutting.

Due to this, large internal stresses are set up which frequently cause cracks to develop in the tool, resulting in the premature breakdown of the cutting edge in use. Such steels should, therefore, be tempered twice, or treated at very low temperature in order to reduce the amount of retained austenite to about 5%.

In case of conventional HSS, carbide segregation can take place which induces local variations in chemical composition and structure. Such problems can be overcome by manufacturing HSS by powder metallurgy which facilitates finer and more uniform distribution of carbide particles and better homogeneity of alloying elements.

HSS is also produced by electroslag refining process and this tool has uniform carbide distribution and is free from inclusions. The properties of HSS are affected by heat treatment and as such it should be done as per recommendation of manufacturer. Further hardening of surface is possible by work hardening treatment. HSS cutting tools are nowadays coated with layers of refractory metal carbide or nitride by chemical vapour decomposition technique.

Fig. 23.1 shows the effect of temperature on hardness of various tool-materials. It is interesting to note that carbon tool steel is harder than high speed steel at temperature below 250°C. Thus from economic consideration, HSS tools should be used for high speed operation where tool tip temperatures will be more than 250°C: otherwise carbon- tool steel should be used (for slow cutting processes where the cutting temperatures are relatively small). Nowadays indexible HSS inserts are manufactured which can be clamped, brazed to a low alloy steel body.

Type # 4. Cast Cobalt Base Alloy Tools (Stellites):

A number of non-ferrous alloys (usually known as stellites) high in cobalt have been developed for use as cutting tool materials. These materials cannot be heat treated and are used as cast at a temperature of about 1260°C. These contain about 40-50% cobalt, 27-32% chromium, 14-29% tungsten and 2-4% carbon.

Though the cast alloys are not as hard as the tool steels at room temperature, but they retain their hardness at higher temperatures. Under certain conditions, the cast alloy tools gives somewhat better life than the high speed steels, but they are not in wide use, due to their fragile nature. Like all cast materials, these alloys are relatively weak in tension and hence tend to shatter when subjected to a shock load or if not properly supported.

The coefficient of thermal expansion of cast alloy is same as of steel and as such two can be brazed or welded without problem of thermal stresses.

From Fig. 23.1 it will be seen that stellite is harder than HSS at temperature above 500°C. It maintains its hardness and cutting edge even at red heat (750°C). Thus stellite tools are ideal for rapid machining of hard metals. These are used for making form tools.

Stellite is available in the form of bars of round or square section for manufacturing cutting tools; and as inserts to attach to tough steel milling cutter bodies.

Type # 5. Cemented Carbides:

Though it had been discovered long back that tungsten carbide is very hard material but difficulties were faced in joining fine crystals of tungsten carbides into tool bits by sintering (prolonged heating of the compressed material just below the melting point) because the required temperature was so high that the material decomposed.

Later on it was found that tungsten carbide crystals when mixed with cobalt powder could be sintered at a temperature near the melting point of cobalt (1980°C) to provide a strong material for use in certain machining operations.

Thus cemented carbide is a typical powder metallurgy product. Cemented carbides are very effective in machining cast irons and certain abrasive non-ferrous alloys, but as such are not good for cutting steel because wear craters are developed on the face of the tool.

This can be avoided by adding titanium and tantalum carbides to the mixture before sintering, and accordingly there are two general grades as follows:

(а) The C-grade consisting of tungsten carbide with cobalt as a binder, for use in machining cast iron and non- ferrous metals. In this grade, cobalt concentration is varied from 3-16%. Higher is the cobalt content, greater is the resistance to shock.

(b) The S-grade consisting of tungsten carbide, titanium carbide and tantalum carbide with a cobalt binder for use in machining steel (Tantalum carbide: 0-10%, TiC: 0-16%). TiC reduces the tendency of chips to weld to tool, increases hot hardness. Tantalum carbide helps improve resistance to crater wear and make the structure fine grained. In this steel grain size also exerts great influence on properties of steel. The coarser grain produces soft metal but more resistance to shock.

The cemented carbides have high hardness over a wide range of temperature; are very stiff (Young’s modulus is nearly three times that of steel); exhibit no plastic flow (yield point) even on experiencing stresses of the order of 33300 kg/cm², have low thermal expansion compared with steel; relatively high thermal conductivity: and a strong tendency to form pressure welds at low cutting speeds. However, these are weak in tension than in comparison. Their high hardness at elevated temperatures enables them to be used at much faster cutting speed (3-4 m/sec with mild steel).

These unusual properties of the cemented carbides call for special consideration in the design of carbide tipped tools. Due to the very high stiffness of the cemented carbides, they should be well supported on a shank of sufficient thickness. The tool should be so proportioned that tensile stresses are kept small.

The relatively small coefficient of expansion of the cemented carbides, makes it necessary to use a relatively thin layer of braze metal so that the braze will not crack upon cooling as a result of large tensile stresses which arise from differential contraction of the carbide and the braze metal. In view of the adverse- pressure welding characteristics of cemented carbide tools, they should be operated at speeds considerably in excess of those used with high speed tools.

It may be mentioned that carbide grade with different properties is required for machining cast iron and steel because the wear characteristics of cast iron machining are quite different from those of steel. No single grade can satisfy all the maximum values of three important desirable properties (edge wear resistance, crater resistance and shock). Thus compromise in selection has to be made.

The properties of carbides are governed by the tungsten-carbide grain size and the greater the cobalt, the lower the hardness and impact resistance. For longest tool life, the tool with the finest grain size and the lowest cobalt content, which just prevents chipping and fracture, should be used. For cutting steel, addition of titanium carbide improves the crater-wear resistance, but reduces the abrasive wear resistance. For increasing abrasion resistance, tantalum carbide is added.

The material removal capacity (accelerated metal removal with no loss in tool life) of convention carbides can be considerably increased by using coated inserts obtained by metallurgical bonding of a thin coating (0.005 mm thick) of titanium carbide to a tough carbide core.

This results in lowering of the coefficient of friction between tool and chip with consequent reduction of cutting forces (by 35% approximately) and temperature reduction of the order of 70°C. Crater wear is also practically eliminated.

Type # 6. Ceramics:

Ceramics consist mainly of sintered oxides (A1₂O₃) and are prepared in the form of clamped tips and as throw away inserts. These can be used at very high speed (beyond carbide tools), resist built-up edge and produce good surface finish. These are extremely brittle, so their use is limited for continuous cuts. Friction at rake face is usually lower as compared to carbide tools but temperature is higher because these are poor conductors of heat. To strengthen the cutting edge, a small chamfer or radius is often stoned on the cutting edge and negative rake of about 15-20° is provided.

Type # 7. Non-Ferrous Alloys:

These consist of varying percentages of cobalt, chromium, tungsten and carbon and are used for machining hard metals at speeds slightly in excess of those used with HSS tools. These find wide application in drilling operation. Their properties are superior to HSS but inferior to tungsten carbide.

Type # 8. Non-Tungsten Materials (Titanium Carbides and Titanium Nitrides):

Shortage of tungsten has led to the introduction of titanium carbides and nitrides with nickel and molybdenum as bonding materials. Their life is high. These exhibit higher hot hardness and do not form built-up edge on their rake faces.

Type # 9. Coated Carbides:

An important development of cemented carbides has been to improve wear resistance and at the same time retain high toughness. This is possible by coating carbides (titanium carbide and titanium nitride) in microscopic layers over tough carbide substrate.

Type # 10. Micro-Grain Carbides:

Micro grain carbides have very high transverse rupture strength and are very well suited for severe metal cutting operations.

Type # 11. Cast Carbides:

In cast carbides, a hard carbide alloy is dispersed in a high strength refractory binder by electrode arc melting and spin casting of metal into graphite moulds. It is highly resistant to crater formation and plastic flow at high temperatures.

Type # 12. Cemented Oxides (Ceramics):

These are mixtures of oxides of aluminium, and small quantities of other oxides. These may consist of almost pure aluminium oxide (approx 97%), or they may contain 80% Al₂O₃ with titanium, magnesium and tungsten oxides and carbides. These are hard and have good resistance to abrasion wear and cratering. These can retain cutting hardness upto 700°C and have high wear resistance. These are good only for uninterrupted cuts.

They are brittle and have poor shock- resistance. These can’t be brazed to steel shank, but can be cemented by epoxy resins. These are best suited for maintaining close dimensional tolerances and surface finish when machining long shaft because these have very low tool wear.

Type # 13. Diamond Tools:

It is a well-known fact that diamond is the hardest known substance which burns to CO₂ when heated to about 810°C. In addition, it has lowest thermal expansion (12% that for steel), high heat conductivity (2 times that for steel), very low coefficient of friction against metals and is poor electrical conductor. Due to these properties, diamond tipped tools are sometimes used for special applications such as the production of surfaces of high finish on soft materials that are difficult to machine. These produce finish of 0.05 to 0.08 μm on non-ferrous metals like copper, aluminium etc.

Since very high hardness is always accompanied by brittleness, a diamond tool must be cautiously used to avoid rupturing to the point. This usually limits the use of diamond tools to light continuous cuts in relatively soft metals, and low values of the rake angle are normally used to provide a stronger cutting edge.

The very low coefficient of friction and high heat conductivity provide a low operating temperature and make it possible to use high speeds (> 150 surface metre per minute) inspite of the fact that a diamond will decompose in air at temperature above 810°C.

The relatively high cost of diamonds that are satisfactory for tools are partially responsible for their limited use. Diamonds are commercially divided into four classes: carbons, ballas, boarts and ornamental stones. The first two of these categories comprise stones which are poly-crystalline.

These are less dense, less hard than perfect diamonds and are satisfactory for some industrial uses but not for cutting points. Diamond cutting tools are usually made from boarts, which are single crystals, less clear and fault free.

Synthetic polycrystalline diamonds are available as mechanically clamped cutting tips. These are used for machining abrasive aluminium-silicon alloys, fused silicon and reinforced plastics. The random orientation of their crystals gives them improved impact resistance making them suitable for interrupted cutting.

Type # 14. Abrasives:

The abrasive grains are used in grinding wheels, abrasive belts, sand papers, sand blasting and other similar operations. These operations actually involve cutting in which the abrasive grains produce tiny chips from the work material.

The abrasive commonly used may either be natural or artificial (manufactured). Natural abrasives include corundum, emery, quartz, garnet and diamond. Manufactured abrasives include aluminium oxide, silicon carbide and boron carbide.

Aluminium oxide and silicon carbide are by far the most widely used for all grinding abrasives. Silicon carbide is harder than aluminium oxide but is, in general more friable. Hardness is of importance chiefly in the grinding of very hard materials. The choice between silicon carbide and aluminium oxide lies in balancing the attrition resistance with the body strength which determines the ability to fracture when dulled.

Type # 15. UCON:

It is a nitrided refractory metal alloy having composition of 50% columbium, 30% titanium and 20% tungsten with no carbide. It has excellent thermal shock resistance, high hardness, and toughness. It exhibits excellent resistance to diffusion and chip welding. It is available in the form of throwaway inserts having 3-5 times more edge life than conventional carbides. It operates in the speed range of 250-500 m/min on steels of 200 BHN.

Type # 16. CBN (Cubic Boron Nitride):

It consists of atoms of nitrogen and boron, with a special structural configuration similar to diamond. It has high hardness and high thermal conductivity. It is chemically inert. It is used as a grinding wheel for HSS tools and stellite. These are available in the form of index-able insert and are capable of machining hardened tool steel, chilled cast iron, high strength alloys. It is hardest material next to diamond.

Type # 17. Sialon:

The word Si AI ON stands for silicon nitride-based materials with aluminium and oxygen additions. It is produced by milling Si₃N₄, aluminium nitride, alumina and yttria. The mixture is dried, pressed to shape and sintered at 1800°C. This tool material is tougher than alumina and thus suited for interrupted cuts. Aerospace alloys and nickel-based gas turbine discs can be machined using sialon tool bit at a cutting speed of 200-300 m/mt.

Type # 18. Polycrystalline Diamond (PCD):

High speed machining operations make use of PCD and CBN tools for best results. In motor vehicle and aircraft manufacturing industries, component parts of non-metallic materials are being used extensively. PCD cutters achieve exceptional standards of surface quality at high feed and material removal rates with such materials. PCD tools are extensively used for machining aluminium components.