Characteristic : The characteristics of the ideal material are.  

1.Hot hardness. The material must remain harder than the work material at elevated operating temperatures.

2. Wear resistance. The material must withstand excessive wear even though the relative hardness of the tool-work materials changes.

3. Toughness.  The term 'toughness' actually implies a combination of strength and ductility ne material must have sufficient toughness to withstand shocks and vibrations and to prevent breakage.

4. Cost and easiness in fabrication. The cost and easiness of fabrication should have within reasonable limits. 

Tool Materials : The selection of proper tool material depends on the type of service to which the tool will be subjected. No material is superior in all respects, but rather each has certain characteristics which limits its field of application.

The principal cutting materials are :

I . Carbon steels.

2. Medium alloy steels.

3. High-speed steels.

4. Stellites.

5. Cemented carbides.

6. Ceramics.

7. Diamonds.

8. Abrasives.

1. Carbon steels : Carbon steels contain carbon in amounts ranging from 008 to 15 per cent. A disadvantage of carbon 1001 steels is their comparatively low-heat and wear-resistance. They lose their required hardness at temperatures from 2000 to 250'C. Therefore, they may only he used in the manufacture of tools operating at low cutting speeds (about 12m/min) and of hand operated tools. But they are comparatively cheap, easy to forge, and simple to harden.

2. Medium alloy steels : The high carbon medium alloy steels have a carbon content akin to plain carbon steels, but in addition there is, say , up to 5 per cent alloy content consisting of tungsten, molybdenum, chromium and vanadium. Small additions of one or more of these elements improve the performance of the carbon steels in respect of hot hardness, wear resistance, shock and impact resistance and resistance to distortion during heal treatment. The alloy carbon steels, therefore, broadly occupy a midway between plain carbon and high speed steels. They requited temperatures from 250" to 350°C.

3. High-speed steels : High-speed steel (hss) is the general purpose metal for and medium cutting speeds owing to its superior hot hardness and resistance to wear. High-speed steels operate at cutting speeds 2 to 3 times higher than for carbon steels and retain their hardness up to about 900C. It is used as a popular operations of drilling, tapping, bobbing. milling, turning etc. There are three general types of high-speed steels; high tungsten, high molybdenum, and high cobalt. Tungsten in hss. provides hot hardness and form stability, molybdenum Or vanadium maintains keenness of the cutting edge, while addition of cobalt improves hot hardness and makes the cutting tool more wear resistant. Three general types of high-speed steels arc as follows
a. 18-4-1 high-speed steels (T-series). This steel containing 18 per cent tungsten, 4 cent chromium and 1 per cent vanadium, is considered to be one of the best of all purpose tool steels. In some
steels of similar the percentage of vanadium is slightly increased to obtain better results in heavy-duty work.
b. Molybdenum high-speed-steel (M-series). This steel containing 6 per cent molybdenum, 6 per cent tungsten, 4 per cent chromium and 2 per cent vanadium have excellent toughness and cutting There are other molybdenum high speed steels now marketed, having various tungsten-molybdenum ratios, with or without cobalt, or with variations in percentages of the minor alloys chromium and vanadium

c. Cobalt high-speed steels : This is sometimes called super high- speed steel. Cobalt is added from 2 to 15 per cent to increase hot hardness and wear resistance. One analysis of this steel contains 20 per cent tungsten, 4 per cent chromium, 2 per cent vanadium 12 per cent cobalt.

4. Stellites: Stellite is the trade name of a nonferrous cast alloy composed of cobalt, chromium and tungsten. The range of elements in these alloys is 40 to 48 per cent cobalt, 30 to 35 per cent chromium, and 12 to 19 per cent tungsten. In addition to one or more carbide forming
elements, carbon is added in amounts of 1.8 to 2S per cent. They can not be forged to shape, but may be deposited directly on the tool shank in an oxy-acetylene flame, alternately, small tips of cast stellite can be brazed into place. Stellites preserve hardness up to and can be operated on
steel at cutting speeds 2 times higher than for high-speed steel. These materials are not widely used for metal cutting since they are very brittle, however, they are used extensively in some non-metal cutting application, such as in rubbers, plastics, where the loads are gradually applies and the support is firm and where wear and abrasion are problems.

5. Cemented carbides : Cemented carbides are so named because they are composed principally of carbon mixed with other elements. The basic ingredient of most cemented carbides is tungsten carbide which is extremely hard. Pure tungsten powder is mixed under high heat, at about
1500C, with pure carbon (lamp black) in the ratio of 94 per cent and 6 per cent by weight. The new compound, tungsten carbide, is then mixed with cobalt until the mass is entirely homogeneous. This homogenous mass is pressed, at pressures from 1,000 to 4,200 kg/cm-, into suitable blocks and then heated in hydrogen. Boron, titanium and tantalum are also used to
form carbides. The amount of cobalt used will regulate the toughness of the tool. A typical analysis of a carbide suitable for steel machining is 82 per cent tungsten carbide, 10 per cent titanium carbide and 8 per cent cobalt. Carbide tools are made by brazing or silver-soldering the formed
inserts on the ends of commercial steel holders. The most important properties of cemented carbides are their very high heat and wear resistance. Cemented carbide tipped tools can machine metals even when
their cutting elements are heated to a temperature of I,OOO°C. They can withstand cutting speed 6 per cent or more than 6 times higher manufactured material and has extremely high compressive strength However, it is very brittle, has low resistance to shock, and must be very rigidly supported to prevent cracking.

The two types of cemented carbides are the tungsten and titanium tungsten varieties. The tungsten-type cemented carbides are less brittle than the titanium-tungsten type; they contain 92 to 98 per cent tungsten
carbide and from 2 to 8 per cent cobalt. These cemented carbides are designed chiefly for machining brittle metals such as cast iron, bronze, but they may also be used for non-ferrous metals and alloys, steel, etc. The
titanium-tungsten type are more wear-resistant. They contain 66 to 85 per cent tungsten carbide, 5 to 30 per cent titanium carbide and 4 to 10 per cent cobalt. These cemented carbides are designed for machining tougher
materials chiefly for various steels.

6. Ceramics : The latest development in the metal-cutting uses aluminium oxide generally referred to as ceramics. Ceramics tools are made by composing aluminium oxide powder in a mould at about 280
kg/cm- or more. The part is then sintered at 2200C. This is known as cold pressing. Hot pressed ceramics are more expensive owing to higher mould costs. Ceramic tool materials are made in the form of tips that are to be
clamped on metal shanks, Other materials used to produce ceramic tools include sillicon carbide, boron carbide, titanium carbide and titanium  boride. 

These tools have very low heat conductivity and extremely high compressive strength. But they are quite brittle and have a low bending strength. For this reason, these materials can not be used for tools operating in interrupted cuts, with vibrations as well as for removing a heavy chip. But they can withstand temperatures up to 1200 C and can be used at cutting speeds 4 times that of cemented carbides, and up to about 40 times that of high-speed cutting tools. They are chiefly used for single- point tools in semi-finish and finish turning of cast iron, plastics, and other work, but only when they are not subject to impact loads. To give them increased strength often ceramic with a metal bond known as "cermets" is used. Because of the high compressive strength and brittleness the tips are given a 5 to 80 negative rake for steel and zero rake for cast iron and for non-metallic materials to strengthen their cutting edge and are well supported by the tool holder. Heat conductivity being very low the tools are generally used without a coolant.

This article was taken by  ELEMENTS OF WORKSHOP TECHNOLOGY 


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