Commonly Available Tool Materials:

1] Carbon tool steels:

• Carbon tool steels contain carbon in amounts ranging from 0.90 to 1.20 percent.

• These steels are relatively cheap and the tools are relatively easy to make and harden.

• With proper heat treatment these steels can attain hardness as much as any of the high speed alloys but they begin to lose their hardness at around 300°C.

• Cutting tools of Carbon steels are limited to low speed and light duty work.

• Carbon steels are used for machining soft materials like wood and for hand tools like files and chisels.

2] High speed steels:

• High speed steels were so named because they could cut at speeds higher than those for carbon steels.

• The name is misleading because the speeds at which these materials cut are actually much lower than those used for many other materials like carbides and satellites that are now available.

• High speed steels have excellent harden ability and can retain their hardness upto 650°C.

• They are relatively tough and moderately priced.

• They can be shaped easily.

• High speed steels are commonly used for drills, reamers; counter bores, milling cutters and single point tools.

• One of the oldest and the most common variety of high speed steels is 18-4-1 H.S.S. it contains 18 percent tungsten, 4 percent chromium, 1 percent vanadium and about 0.5 to 0.75 percent carbon. It is considered to be one of the best all purpose tool steels.

• Many high speed steels use molybdenum to replace tungsten partially or completely because one part of molybdenum can replace two parts of tungsten.

• Molybdenum high speed steels such as 6-6-4-2 containing 6 percent tungsten, 6 percent molybdenum, 4 percent chromium and 2 percent vanadium with about 0.6 percent carbon have excellent toughness and cutting ability.

• Cobalt is sometimes added to high speed steels to improve their red hardness. These super high speed steels are used for heavy cutting operations involving higher cutting pressures and temperatures on the tool but are too costly for general purpose work.

• One composition of these super high speed steel alloys contains 20 percent tungsten, 4 percent chromium, 2 percent vanadium and 12 percent cobalt.

• Tungsten in High speed steels provides hot hardness and form stability, it also improves harden ability. Chromium increases strength, hardness and wear resistance. Molybdenum and vanadium help in maintaining the cutting edge. Cobalt improves hot hardness and wear resistance.

• High speed steels have one major disadvantage in that they require lot of care in heat treatment. Rather complex heat treatment cycles are used to develop the favorable properties.

3] Cast non-ferrous alloys:

• These are alloys containing principally chromium, cobalt and tungsten with small percentages of one or more carbide forming elements like tantalum, molybdenum and boron but no iron. They also contain 1 to 4 percent carbon.

• A typical alloy of this type known as satellite contains 30 to 35 percent chromium 43 to 48 percent cobalt, 17 to 19 percent tungsten and about 2 percent carbon.

• Cast non-ferrous alloys are able to maintain good cutting edges upto 900°C.

• Compared with high speed steels they can be used at twice the cutting speeds.

• They have a good resistance to crate ring.

• They can take good polish which helps metal from sticking on the tool face and forming the built-up edge. They are also corrosion resistant.

• But they are brittle, can be machined only by grinding and do not respond to heat treatment, intricate tools can only be made by casting and grinding.

4] Carbides:

• Carbide cutting tool inserts principally consist of tungsten Carbide particles held together by cobalt or nickel as binder.

• Straight tungsten Carbide tools containing about 95 percent tungsten Carbide and 6 percent cobalt are used for machining cast iron and most other materials.

• They cannot be used for machining steel because the chips tend to stick to the tool.

• Tantalum, titanium or columbium Carbides are added in steel cutting tungsten Carbide grades in addition to increasing their cobalt content to overcome this difficulty.

• A typical analysis of a steel grade may consist of 82 percent tungsten carbide, 10 percent titanium carbide and 8 percent cobalt.

• Such a carbide has very low coefficient of friction and thus has less frequency for sticking.

• Carbide tools are made by powder metallurgy techniques.

• They have a high initial cost but can be used at speeds which are two-to-three times those for cast nonferrous alloys. They can retain their cutting edges upto 1200°C. they are very hard and have a high compressive strength but they are brittle and cannot withstand impact loading. Grinding is difficult and can only be done with silicon carbide or diamond wheels. Because of these reasons Carbide tools are generally used as brazed or throw-away inserts. Even then they have to be rigidly clamped.

• The need to provide high rotational speeds and yet assure extreme rigidity has led to considerable improvement in the design of machine tools used with these inserts.

5] Ceramics:

• Ceramics sintered oxides, or cemented oxides are essentially aluminium oxide powder along with additives of titanium, magnesium or chromium oxide with a binder processed by powder metallurgy in the form of tool inserts.

• These inserts are either clamped into a tool holder or bonded to it.

• Ceramics are harder than other materials discussed so far and retain their hardness upto 1100°C.

• They have a low coefficient of friction and a good resistance to crate ring.

• The surface finish produced by Ceramics is comparable with that produced by carbides but Ceramics consume about 20 percent lesser power.

• The use of Ceramics tools is limited only by their brittleness and the lack of rigidity and speed range available on the conventional machine tools.

6] Diamond:

• Diamond is the hardest known material and can be used for machining at very high cutting speeds upto 25 m/s.

• Because of its high cost diamond is justified only when machining hard materials which are difficult to machine with other tool materials or for applications where very high accuracy and surface finish are desired.

• Diamond is also brittle, does not conduct heat well and take only light cuts.

• Typical applications are precision boring of holes and machining of highly abrasive materials like fiberglass.

• Diamonds are also used for dressing grinding wheels and in finishing operations like lapping, honing and superfinishing.

• When used as cutting tools Diamonds must be held very rigidly to avoid shock loading.

7] Cubic Boron Nitride (CBN):

• Cubic Boron Nitride consist of atoms of boron and nitrogen with a structural configuration similar to that of diamond.

• It is the hardest tool material next to diamond.

• Boron Nitride exists in three polymorphic forms:

• Hexagonal graphite like structure.

• Ultra hard cubic structure.

• Ultra hard hexagonal structure.

• It is the hexagonal form of boron nitride that is converted into cubic boron nitride with sufficiently high pressure and temperature in the presence of a catalyst.

• Cubic Boron Nitride has a high hardness, high thermal conductivity and high tensile strength.

• Unlike diamond that reacts with oxygen and burns away at temperatures above 800°C, CBN in chemically inert.

• When made in the form of indexable inserts CBN cana be used with standard tool holders like those used for carbides. When made in the form of blanks of standard form and shape the blanks can be brazed on steel shanks andform ground. In some applications a thin layer (0.5 mm) of CBN is applied on cemented carbide tools to combine the high wear resistance and hardness of CBN and shock resistance and toughness of tungsten carbide.

• CBN is capable of machining hardened tools steels, chilled cast iron, high strength alloys and heat resistant alloys.

• On ferrous materials CBN compacts perform better when machining materials harder than RC 45. The surface finish obtained is often as good as that obtained with grinding thus eliminating the need for subsequent grinding in many cases.

• Since CBN tools cut cool, grinding defects like burns and thermal cracks are not produced.

• CBN is used successfully in grinding of high speed steel and satellite tools as well as for machining stainless steel, mnemonic alloys and titanium. It is also used in manufacture of ball and roller bearings, machining of hardened steel screws and slide ways of cast iron beds.

8] Coated Carbides :

• Coated Carbide tools consist of cemented carbide tools on which a microscopic, chemically stable, shock resistant, refractory coating is applied to obtain good wear resistance as well as toughness.

• The refractory coatings consist of TiC, Ti N and $Al_2O_3$. They are applied using Chemical Vapour Deposition (CVD) techniques.

• The coating makes the insert two to three times stronger for wear resistance allowing higher feed rates to be used.

Titanium carbide coating:

• This coating is applied by passing hydrogen and titanium tetrachloride vapours over the hot carbide inserts.

• Carbon diffusing from the substrate reacts with titanium in the furnace atmosphere to form titanium carbide. The coating is metallurgically bonded to the substrate.

• Titanium carbide coating has the following advantages:

1] A uniform coating thickness of about 4 to 8 microns.

2] Micro hardness of the tool about 50 percent higher than that with the hardest cemented carbide.

3] Very low affinity for most ferrous alloys.

4] Goof flank wear resistance.

• TiC coated tools have a wide range of applications compared to conventional cemented carbide tools, cutting forces are reduced by about 20 percent and temperatures by about 50°C. feed rates can be considerably increased reducing machining time.

• TiC coatings are, however, not recommended for milling and other interrupted cutting operations because of the sensitivity of the coating to flanking or cracking. They are also not recommended for high alloys or super alloys where sharp cutting edges are required.

TiN coating:

• In this case a layer of about 7 to 8 micron thick titanium nitride is chemically bonded to a tough carbide base.

• This coating has a lower coefficient of friction compared to TiC. As such it has far greater resistance to cratering thus giving a much higher tool life.

• This coating is useful for general metal removal applications as well as for finish machining of steels.

• TiC coatings however do not have much adhesive strength to the substrate. This drawback is overcome by multi-coating.

• In multi-coating, next to the tough carbide layer, a layer of titanium carbide is placed which guarantees strong adhesives by diffusion into the base metal. This layer is followed by an interpenetrating carbonitride layer and then finally a titanium nitride coating. The resulting material combines the good flank wear resistance properly of titanium carbide and crater resistances of titanium nitride.

$Al_2 O_3$ coating:

• In this case a controlled layer of 7 to 8 micron thickness aluminum oxide is deposited on a steel cutting grade carbide substrate.

• This material has found wide applications in machining of cast iron and other ferrous metals beyond the range upto where ceramics can be used.

• Wear resistance of this material is high because of good heat dissipation. The tools can operate at about twice the speed used for other coated carbides.

• $Al_2 O_3$ coated tools are also useful in higher speed range where titanium carbide and titanium nitride coated tools fail because of oxidation.