WHAT ARE CUTTING TOOL ALLOYS ?

Cutting alloys are used on tools that operate at very high cutting speeds with high temperatures up to 1000*C to 1100*C developing on the cutting edges. Since they cannot be machined by ordinary methods, they are applied to cutting tools either by means of hard - facing techniques (as a weld deposit or tips brazed to the tool shank or body).

As to the method by which they are produced, cutting alloys are classified as cemented carbides and cast cutting alloys (Stellites).

Cemented Carbides :

They are made up of powdery mixture of tungsten and titanium carbides and metallic cobalt which is first compacted and then sintered, thus employing the well-known powder metallurgy process.

In their finished form cemented carbides consist of extremely fine grains of tungsten and titanium carbides with a cobalt binder. These alloys are extremely hard with Rockwell hardness number up to 85 and even higher and they retain this hardness at temperatures up to 1000*C.

The one most widely used grade of cemented carbides contains 2 to 15 per cent cobalt and the remainder is tungsten carbide; and the other grade contains a few per cent cobalt and 5,14,30 and 60 per cent titanium carbide respectively, the remainder again, being tungsten carbide.

The cutting properties of cemented considerably exceed those of high - speed steel. They are extremely resistant to wear and for this purpose they are valuable for wire - drawing dies and other similar applications.

Stellites :

They contain large amounts of metals like cobalt and tungsten, have high hardness ( Rhn = 60 to 65 ) and melt at very high temperatures. Tips or rods from 5 to 10mm thick , cast of these alloys, are used in the hard - facing of tools by welding techniques to increase the life of the cutting edges. Dies and wear surfaces of various machine parts are hard - faced in the same manner.

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THROTTLE GOVERNING OF STEAM TURBINES

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the turbine constant at part load and hence this method of governing is called "Throttle Governing". Construction : Throttle governing system consists of a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber. Working : When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movement of the

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Mechinopedia For Mechanical Engineers

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